THERMOFORMING STATION PROJECT WRITE-UP phuserweb.eng.gla.ac.uk/philip.harrison/Teaching/2007...

Thermoforming Station Project Gordon Pettigrew 1/86

PROJECT D4 PH 2: THERMOFORMING STATION PROJECT Gordon Pettigrew (0307206) Supervised by Dr Phillip Harrison


THERMOFORMING STATION PROJECT PROJECT D4 Gordon Pettigrew (0307206) Supervisor : Philip Harrison CONTENTS

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GANTT CHART 3 OBJECTIVES 4 INTRODUCTION AND OVERVIEW OF TASK 4 STARTING POINT (WORK BY OTHERS UP TO MY START) 5 TOOL ASSEMBLY AND ALTERATIONS 6 PRE-HEATER DESIGN 16 EXPERIMENTS AND FORMING TESTS 37 APPENDICES 58 APPENDIX 1: FAX TO SMC PNEUMATICS 59 APPENDIX 2: SUPPORT SHAFT DEFLECTION 60 APPENDIX 3: SCREW STRESS CALCULATION 63 APPENDIX 4: POST HOLDER STRESSES 65 APPENDIX 5: TABLE EXTENSION STRESSES 66 APPENDIX 6: RADIANT HEATER TEST RESULTS 69 APPENDIX 7: FORMING TEST RESULTS 73 APPENDIX 8: SOLID EDGE DRAWINGS 74 REFERENCES 85 NOTATION 86


PROJECT OVERVIEW I have been assigned, as my final year project, the task of designing and implementing a pre-heating system for a thermoforming machine for forming composite sheets. Thermoforming is a manufacturing process commonly used for making thin-walled objects from thermoplastics. This involves the controlled heating of the material being formed and the surface it is to be formed around. The thermoforming machine will be used to form a flat sheet of composite into a hemispherical shape. To make this possible, the following general procedure is followed:

1. The composite sheet must be raised to its forming temperature (just above the melting temperature of its resin)

2. The temperature must be maintained until the press is brought down, trapping the sheet between the male tool and female tool. This forms the sheet into the shape of the tooling (in this case a hemisphere)

3. The equipment and formed specimen must be cooled back to room temperature before the tooling is separated to release the specimen.

My project is part of a group of 3 closely related projects, all supervised by Dr Philip Harrison, which build to the production of a complete thermoforming machine, capable of forming various types of composite material. There are 2 other projects which produce(d) designs which my design must be compatible with.


• Laurent Maurel’s project. This project has now been completed. Its focus was

mainly on designing the tooling required to form the composite sheet. His project resulted in the design of the basic thermoforming machine which Gail Gemmel and myself intended to make improvements to.

• Gail’s project began at the same time as mine, but lasts 2 years. The main focus of her project is to create a more versatile blank-holder (the object which holds the composite sheet ‘blank’). She is designing it to include its own integrated heating system. This is intended to work alongside my design to make the operation of Laurent’s machine more efficient and easy to use.

The main objectives of my project are as follows:

Objective 1. Finalise, assemble, install and test the existing tooling in the press. Objective 2. Design a pre-heating addition to improve the operation of the

thermoforming machine Objective 3. Carry out and report on various experiments and forming tests


STARTING POINT

By the start of my part of the project, Laurent had already designed the tooling for forming the composite sheet. This tooling was designed to be able to be heated and cooled during its use to cater for the varying temperature requirements of different materials. This had been considered catered for by the following means.

1. To make heating possible the tools each had 5 holes drilled into their bodies to fit 4 cartridge heaters and 1 thermocouple (to measure the temperature within the tool during heating and cooling).

2. For cooling, the tools are hollow and are connected by hoses to a water supply. To cool the tools, cold tap water is passed through them until the heat is sufficiently lowered.

Male tool �

Blank-holder (to be

positioned by hand) � Female tool

�

Figure 1.1: Laurent’s Design

Laurent had produced Solid Edge drawings which could used as a basis for my design to ensure geometric compatibility. He had also carried out some thermal simulations using Abaqus software to predict how much power will be required and had used these to calculate the required heating power and purchased the appropriate heaters. At the time Laurent finished his project and left the university his drawings of his parts had been submitted to the workshop and were being manufactured by the technicians. This was the stage at which my project began.


OBJECTIVE 1: TOOL ASSEMBLY AND ALTERATIONS

Female tool The female tool (figure 1.2) was machined from aluminium in 2 main parts, the hollow tool and its base. The main features of the tool are:

• Inlet and outlet ports for cooling water • Large space inside to house cooling water • 4 drilled holes to house electric cartridge heaters • 1 hole to house a thermocouple for heat monitoring purposes • slot on bottom surface for o-ring seal • 12 M5 threaded holes around the base to fit the cap to

The other major part is a base (basically a cap) to close the bottom of this main part. Its main features include:

• 4 drilled holes to house electric cartridge heaters • 1 hole to house a thermocouple for heat monitoring purposes • o-ring slots machined around each hole to avoid leakage • 12 blank holes countersunk to house allen capped screws for attachment to the

bottom of the tool

TOOL

BASE

Figure 1.2: Exploded view of female tool Male Tool The male tool (figure 1.3 a), like the female tool, is made from 2 main parts. In this case, the top part is the cap and the bottom part is the actual shaped tool (see figure 1.3 b).


The main features of the tool are similar to those of the female tool. They include: • Half sphere shape with radius slightly smaller (1mm) than that of the cut-out in

the female tool. • 4 holes to house electric cartridge heaters • 1 smaller hole to fit a thermocouple • Hollowed inside to leave space for cooling water to be passed through • Groove around the top surface for o-ring seal • 8 M5 threaded holes for joining to the cap

The cap has:

• Drilled holes to allow the wires from the heaters and thermocouple to leave the tool.

• 2 ports for pipes to fit to (using specially made 3-screw flanges and an o-ring seal each)

figure 1.3 a: Exploded view of male tool figure 1.3 b: Main body of male tool (with o-ring)

The tasks which remained for me to complete were:

1. Make o-rings to fit the slots (resistant to high temperature) 2. Fit the copper pipes for connection to the cold water supply 3. To join the 2 parts of the tool and test for water tightness and drainage 4. Fit the electrical components to the tool and test its heating and cooling 5. Finalise the method of fitting to the Instron press ensuring there is space for the

heater and thermocouple wiring to leave from the underside of the base.


O-ring seals

From the seal kit Laurent had purchased, used the Ø3/32” rubber strip was used. Strips of this were cut and glued at their ends using superglue (cyano-acrylate) to create o-rings. Once a full set of these seals were made, they were fitted and the tooling was closed before carrying out water-tightness tests.

Initial Water-tight Tests Male Tool: The O-ring seals were sufficient to prevent leakage at the pressure given from the tap. However, when air was blown through the tool, it didn’t drain fully (around 10mm depth was left at the centre of the bowl shape) as the copper tube does not reach the bottom of the tool. To solve this, a small section of tubing was added to reach closer to the bottom of the tool and tested the drainage again. This time, only around 3mm depth was left at the bottom inside the tool. Slightly less could possibly be left over if the position of the bottom of the tube is altered slightly and a more constant faster flow of air is passed through (not breath). Note: all tubing referred to in this section is the ‘Fisherbrand® silicone platinum-cured 12.5mm id x 2.25mm wall thickness roll’ purchased by Laurent. This tubing is safe to use high temperature environment existing in the machine. Female tool: The outer o-ring seal sealed perfectly. The 5 small 3/32” o-rings leaked slowly under the full tap pressure. I used 5 standard pre-made o-rings the workshop supplied and these sealed the tool fully. These o-rings would not be suitable for the operating temperature of the tool so, using the kit, equivalent o-rings were produced. These were made using Ø3mm rubber. Using these, a perfect seal was achieved. During testing it was also found that the tooling virtually drained itself when the openings were unblocked. This indicated that it could be emptied without use of an air supply (assuming the tank is lower than the tool). Cooling water flow It had initially been planned that the water would be made to flow through the tools to remove their heat before flowing into a large tank. I noticed that this would lead to some serious difficulties:


• I was unsure how much water will be required to flow through the tooling to reduce it to a safe temperature. Experimentation was required for this but, due to time constraints, this was not feasible as I didn’t receive the heaters in a working state from the electrician until very late in the 2nd semester. For this reason, I didn’t know what size of tank would be required

• Once the tank is full (presumably with a large volume of water) it must be able to be drained. Without a pump, this would be a very time consuming process since the tank would be at floor level and the sinks are approximately 1m from the floor. Siphoning would not be possible so it may be necessary to empty the tank using jugs or buckets of water until it is light enough to lift to a sink or drain,

These problems would not be critical to the function of the machine (as long as the tank is not left to overflow) but would make its use far more awkward than is necessary. To overcome this, I came up with the idea of connecting water supply as shown in figure 1.4 a. The tools are connected as if in series (water flows through the top tool then bottom tool) and sending the water leaving the bottom (male) tool directly to the drain underneath the tap. There was some doubt as to whether or not there would be enough pressure from the tap to force the water through its complete route so I tested it in practice.

Figure 1.4 a: Cooling water flow

To test this set up, I simply connected the tubes to the appropriate pipes (with the tools mounted in position on the press) and ran water through them.


After a short time the water was flowing steadily through both tools and exiting the final tube into the drain. This was exactly what I was hoping to see so I was satisfied that this cooling system would be the most effective of the possibilities at hand. However, connecting the tools in this way made it difficult to drain the system after use (to prepare for the next test). Draining For adequate drainage, the flow direction of the water must be reversed and air allowed to flow in the highest entry to push the water out of the lowest. In the male tool, gravity is sufficient but a gush of air into the top tool will be required to start the flow (using the siphon effect). I attempted to evacuate the water from the 2 tools together by blowing into tube 3 but my breath was far too weak; compressed air would be required but may just create bubbles. Instead, I cut tube 2 and rejoined it using a short length of copper pipe and 2 hose (or jubilee) clips, one of which had a thumbscrew for convenience when splitting. Now I had 2 separate tools to drain (as shown in figure 1.4 b), just like in the previous water-tightness tests, so could easily predict how the tools would each drain:

• The female tool would drain by its self when the tube 2 is open to air and held above its top surface.

• The male tool will drain if the exit of the outlet pipe (now tube 1 due to reversed flow) is lower than its bottom and tube 2 has air blown into it until the water flows by its self. (Note: before doing this, tube 1 must be detached from the tap and put down into the drain)


Figure 1.4 b: Water Drainage Flow

Note: In this set up, the bucket only ever has to hold the volume of water held inside the female tool so there will be no risk of overflow as long as the bucket is emptied before every 3rd test.

I had considered buying in valves to allow the user to control flow of water through the tooling in each direction. I decided against this though as I didn’t believe the benefits would be worth the cost. It would only be worth doing if it made the system easier or safer to use. In my opinion, having valves would be likely to make it more confusing to operate and so increase the chance of making mistakes and spilling water. This would be dangerous as water could come in contact with electricity or even scald the user. Also, since the tools will only ever be drained once the tools are cooled close to room temperature and time is not a significant factor at that stage, splitting the hose will be safe and the extra time it will take over a valve system will cause no problem. The following instructions will be followed when cooling and draining the tools:

1. Connect the tubes as shown in figure 1.4 a and (figure 1.4 c) and open the tap. 2. Once the tools have cooled enough, close the tap 3. Move the end of tube 3 into the bucket, being careful not to spill any water


4. Remove tube 1 from the tap, blow through it with one breath until air can be seen to fill tube 2.

5. Position tube 1 in the drain. 6. Split tube 2 by unscrewing the thumbscrew on the jubilee clip and pulling its

lower section from the short section of pipe and point it into the bucket. 7. Hold the upper section of tube 2 as high as possible and blow into it until water

stops coming out into tube 1. 1 breath is usually sufficient since the volume of water inside the tool is so small.

8. The female tool should empty its self using the siphon effect when tube 3 is held above it. If this doesn’t start (because of air blocking the tube), simply blow into tube 3 and hold it up. The water will rush out into the bucket. When this stops, both tools are effectively empty.

9. Rejoin tube 2, connect tube 1 to the tap and place tube 3 back into the drain ready for the next test (shown in figure 1.4 c).

Figure 1.4 c: Photograph of tooling connected and ready for cooling flow


Electrical components I was responsible for the physical fitting of the heaters to the tooling. However, not being an electrician, I am not qualified to work with any wiring. So I met with the electricians in the workshop upstairs in the James Watt building. They were very helpful and organised the wiring for the heaters and controls for me. Before the cartridge heaters were connected to their power source, I had to fit them into the tooling and leave only their wires protruding:

• Male Tool There were no complications in installing the heaters or thermocouple in this tool. The cartridge heater and its wiring fitted perfectly into the tool and through the top cap respectively. Once the wiring had been organised by the electricians, I made, with the help of John Davidson in the Materials Lab, an aluminium holder for the small printed circuit board (PCB) made by the electricians. Shown below (figure 1.5 a):

Figure 1.5 a: PCB connecting heater power wires in parallel

The purpose of these circuit boards is to connect the heaters, in parallel, to the 230V power supply given from the control unit. The aluminium holder avoids putting stress on the wires to the heaters as it eliminates any movement in them. It also reduces the chance of short circuits since the wiring (particularly any bare wires at connection points) cannot move to come in contact with any other wiring or conductors.


• Female tool

The female tool was not so straightforward. The heaters did all fit perfectly into their holes in the mail body of the tool but the drilled holes for their wires were not wide enough to accommodate the wiring. The problem was the way the wires leave the back of each heater each have a metal connection to the section of wire with the thermal shielding. Since there are there 2 wires connected to each heater and their connectors are fitted at the same distance from the heater end, these create a cross sectional area too large to fit through the drilled hole in the cap. I considered 2 methods of overcoming this problem and to increase ease of assembly of the female tool.

1. Have one of the 2 wires cut shorter to avoid the connectors clashing 2. Bore the holes in the cap to a large enough size to accommodate the thick point in

the wires I decided to use option 2 for the following reasons:

• Reliability. Interfering with the original wiring of the heaters would increase the risk of flaws. This could cause short circuits and/or bad connections. Bad connections would be very difficult to identify and would cause the equipment not to heat evenly so any specimens formed would not be accurate and so the equipment would be virtually useless.

• Ease of assembly. By simply opening the holes wider, we would make assembly far easier. When the tooling is put together, 5 wires have to be fed through the cap simultaneously. This is a very difficult job to carry out, even with 2 people working together. Larger holes would make this less difficult.

We re-sized the holes from 6mm up to 8mm to allow the cables to pass through. This had no effect on the seal between the tooling parts as these holes are surrounded by the o-rings as described previously. This time, the parts slotted together with relative ease with the heaters installed so all that was left was to have the wires connected to a PCB then their power supply and control units (as in the male tool). I made another aluminium PCB holder to keep the wiring safely away from the steel base surface of the press (shown in figure 1.5 b).


Figure 1.5 b: PCB for female tool heaters

Heating and Cooling test This was carried out along with the first forming test. The male tool took just 6 minutes to settle at a temperature of 118°C when set to a target of 120°C. By trial and error it was found that the temperature would settle at 124°C if the target was 126°C (the controls are shown in figure 1. 6 The female tool took far longer to reach and settle at a certain temperature. By trial and error I found that it would have to be set o 130°C to settle at a temperature of 124°C. These settings were used in the appropriate experiments in order to maintain repeatability.

Figure 1. 6: Heater control units in protective casing


OBJECTIVE 2: PRE-HEATER DESIGN CONCEPTS In the beginning of this project, I sought to reach a workable solution where the blank holder, complete with composite sheet, would be heated in an oven before being quickly transported to the press and accurately dropped into position. This would be an automated version of the procedure carried out with the existing equipment. Some of my proposed solutions to this are as follows:

1. Rollers and gravity In this concept, the blank holder was to run on rails (sample shown below) which would have a small incline. When the desired temperature was reached in the oven, the blank holder would be released and travel down to the press.

Figure 2.1: Roller Rail concept

On further consideration we noticed:

• The Roller-rail (figure 2.1) would be expensive to produce since: 1. each roller would need its own individual bearing and axle. 2. the rollers themselves (made of a rubber or polymer material) would have

to be bought in. 3. a large number of rollers would be required to give smooth and reliable

motion without the risk of the blank-holder moving out of line and jamming.


• Actuation of the motion may be required in the form of a solenoid, spring release (figure 2.2) or push rod to give the blank-holder the momentum to overcome static friction and start on its way down the incline.

Figure 2.2: Spring actuator

• Due to the free running nature of the mechanism, it would be difficult to ensure reliable operation. The blank-holder may reach the press going too fast one time, but travel too slow the next and stop short of the press. External conditions could seriously affect its operation.

• The final positioning of the blank-holder would be difficult to control due to the large variations in the arrival speed. Some kind of brake or buffer system would be required as well as. Or integrated into the positioning mechanism.

For a mixture of these reasons, mainly the unpredictability, this concept wasn’t taken any further.

2. Chain or belt driven I considered using a chain or belt drive instead of gravity. This would give us far more control of the speed and positioning of the blank holder. The chain/belt would either be controlled by electric motors with electronic controls or by human hand via a cranking handle. The latter sounds primitive but would be very effective in the positioning stage of the operation. The following issues would possibly hinder the production and operation of this system:

• Due to high temperatures, a belt drive isn’t likely to be suitable. • A chain would have to be kept sufficiently tight, lubricated and clean to reduce

wear. Its long length may complicate this • Accurate but fast motor control will be required • The complete system will require a lot of space for both storage and use


• When heating the blank and its holder, the oven door must be closed to avoid losing the majority of the heat produced. This may pose a problem as the structure holding these parts will block the door.

• The oven would need to be placed very close to the press. This is not possible since the blank holder’s size dictates that the largest oven must be used. This oven is installed far from the press and neither it nor the press are moveable.

This concept was not developed further since it would be expensive to manufacture and the problem with the oven would render it useless. Moving Heater Concept After considering the difficulties in heating and carrying the bulky blank-holder to the press I decided to concentrate on devising a method of heating it in position on the press. This idea is one half of an overall design intended to eliminate the need for use of extra heating equipment (i.e. the oven). The other part is the focus of Gail’s project and will be described later in this section. The perceived advantages of a moving heater instead of a moving blank holder are:

1. Speed. The heater will only have to move a short distance. This will take a very short time compared to the time taken to move the blank from the oven into position in the press.

2. Efficiency. By reducing the time the specimen and blank holder are left in the open air without heating, less heat will be lost so the process should provide better results.

3. Simplicity. There will be only a simple straight movement required. This will increase accuracy and reliability

4. Cost. The cost of a system will be less since less parts and materials will be required

The heater is required to be moved in a horizontal plane from a position clear of the moving press to its operating position above the sample. The main design requirements of the moving heater concept are:

• SPEED: The heater must be able to move quickly to a safe position, clear of the moving press, so the tooling can be brought down to form the composite sheet. Heating times must also be kept to a reasonable minimum

• ADJUSTABILITY: Since different types of sheets will be formed using the same equipment, it is likely that the distance between the heater and the sheet during heating will have to be variable. It is also likely that, in the future, different tooling will be used to form different shapes. For these reasons, the pre-heating system must be adjustable to cater for alterations.

• STIFFNESS: Due to the moving parts of the system, the structure must be built to be very stiff to avoid oscillation during and after movement of the heater.


To reach these requirements, I first designed and had the machine built as a working prototype. As problems or obstacles presented themselves I, with the assistance of the technical staff in the workshops, devised alterations to improve the function of the machine. USER AND WORKING ENVIRONMENT Before designing anything, it is important to consider who will actually use the finished product. This enables the designer to focus on their perceived needs and so design a suitable product for them. For my design, the main users will be staff at the university. More specifically, it is likely that it will simply be a lab technician who will operate my machine. The main reason for this is that it is to be attached to and operated simultaneously with the Instron press in the materials laboratory which, as far as I’m aware, only one person (John Davidson) is able to operate. I came to the following conclusions regarding the intended user and working environment of my machine

• The user will already be competent and experienced in operating laboratory style automated equipment.

• The users will be far more interested in how well the machine operates than its appearance.

• There is limited space available for the machine since the workshop is often busy with student labs and other experimental work being carried out. The parts and activities should be confined to as small an area around the Instron press.

FREQUENCY OF USE In the process of each forming test, the heater will be required to once in each direction. It will begin back and clear of the press then be moved into position over the centre of the specimen for heating. Once the heating is complete, it will be moved back to its starting position ready for the next test. If successful, the machine will be used to aid forming of a large number of specimens for testing. A machine requiring manual movement of the heater would become tedious and perhaps tiring to use. For this reason I opted to use a pneumatic circuit to automate the motion of the heater. PNEUMATICS By using pneumatics, I have allowed the user to move the heater into position by simply posing a small hand lever. This is, due to my positioning of the lever, done from a safe position a small distance (about 1m) from the heater and its supporting components, thus reducing the risk of injury from and/or damage to the hot moving parts. The diagram of the pneumatic circuit I used is shown on the following page


Figure 2.3: PNEUMATIC CIRCUIT DIAGRAM


SELECTION OF PNEUMATIC COMPONENTS The components I was to purchase had to be compatible with the air compressor made available to me from the workshop in the basement of the James Watt building. It can supply up to 200 litres per minute of air at a pressure of 6 bar. The target speed of the heater movement is: 0.16 m/s (Based on a stroke distance of 320mm and target time of 2 seconds) The mass of the heater was measured to be 1.17 kg and this will be mounted on the end of the piston. To over compensate for the aluminium parts which will be used to support the heater, I assumed the mass of the heater with components mounted on the piston end to be 2kg I intended to fit the heater to the piston as shown in figure 2.4.

Figure 2.4: original heater configuration

However, this would exert a moment on the piston at the point where it exits the main body of the cylinder. This would be at its maximum when the piston is in its most extended position. From the equation: � = Fr where: � refers to the moment F is the force exerted r is the length of the lever arm I calculated that the maximum static moment exerted on the piston would be 0.86 Nm I then searched on the internet (using www.Globalspec.com) for a pneumatic cylinder that would be capable of meeting these requirements. I found a selection of large cylinders which I hoped would satisfy my requirements but only 1 non-rotating cylinder. This was another important requirement for use in my design as the heater must be kept in a horizontal plane to ensure even heating and avoid accidental clashing with other objects when being moved. The cylinder I found was manufactured by SMC and had the manufacturer’s part number: CP95KDB50-320. This cylinder uses a piston of diameter 50mm and rod diameter of


20mm. These lead to effective piston areas of 7.854E-3 m^2 and 6.6E-3 m^2 for outstroke and return-stroke respectively. Delivery rate = 200 l/min = 3.333 E-3 m^3/s So Piston velocity (at full delivery rate) = Delivery rate Effective area = 0.424 m/s (Will be faster but exert less force on return stroke) Hence, using the compressor we have in the university, this cylinder could move the heater at a speed of up to 0.42 m/s. This is faster than my target of 0.16m/s so any cylinder with annulus area � 50mm will be easily capable of my intended operating speed. Since the cylinder is only operating in the horizontal plane (i.e. light action and raising no mass) only a relatively small force will be required to move the heater. I was at this stage working on the assumption that the 20mm rod would be sufficient to support the mass of the heater and its holder. I could not find anything to confirm or negate this assumption in the manufacturer’s literature about the product so I decided to contact them. I sent a fax (see appendix 1) to them to get a professional opinion.


The response I received was helpful, but not the answer I hoped for. I was told that it was not safe to use the piston in the way I had intended. I was informed that use in this manner would risk bending the piston rod and cause excessive wear on the rubbing parts within the cylinder. They suggested that a larger piston could be used. I decided this would not be beneficial to my design as the larger piston would be more expensive would also require a greater air delivery rate to operate at speed. I researched the cylinders produced by the other leading manufacturers, namely Parker Hanafin and Norgren but found they had similar specifications. They would all be unsuitable for the same reasons as the SMC cylinder. My next idea was to use a ‘guided cylinder.’ A guided cylinder is a combination of a pneumatic actuator and a guidance unit. A variety of these guided cylinders produced by Parker is shown in the photograph below (figure 2.5)

Figure 2.5: Promotional photograph of Parker Guided Cylinders

Each Guided cylinder had 2 additional guiding rods to ensure straight linear movement of the piston rod.


The main advantage of these cylinders is that the piston rod is supported by one rod on each side so it will only be require to have strength in the direction of its motion (what they are designed for). Due to this fact, smaller cylinders could be used in my machine. I was unable to easily find a price for these guided cylinders so I checked for alternative products from the other main manufacturers. I did find Norgren’s equivalent to the Parker HBB base slide (the model I initially looked for) which would have suited the requirements for use in my machine. It is shown below (figure 2.6) along with a graph (figure 2.7) showing its load capability.

Figure 2.6: Norgren guide block (QA/8032/51/320)

Figure 2.7: Graph from Norgren handbook, modified to show maximum load for my specific use

Unfortunately, it turned out to be very expensive (the combined cost of the cylinder and the guide unit was £326.13 excluding VAT) so I decided to use these as inspiration and design my own cylinder supporting unit. The general idea was to position a beam above the piston rod which would provide the strength to keep the heater level and, most importantly, protect the piston rod from ‘off-axis’ loads. By ‘off-axis’ loads, I’m referring to any forces which act in any direction other than the direction the piston travels. These ‘off-axis’ forces would damage the cylinder and could cause excessive friction and wear so it is a critical design requirement that the cylinder is not exposed to this type of loading.


I selected a smaller bore cylinder than the previous one since:

• It will cost less • It will weigh less • It should be able to move faster (more movement for the same amount of air

supplied) • Being smaller, it will require less space

The chosen cylinder is a Parker P1A-S025DS-0320 double acting cylinder. It has a 320mm stroke, 25mm bore and the piston rod diameter is 10mm. From these dimensions, and a pump delivery rate of 200 l/min I calculated (in the same way as for the 50mm bore cylinder) that the piston could move at up to 1.7 m/s. In practice, due to inertia effects and energy losses, the piston will never actually reach this velocity but may still move too fast. To give the user the option of adjusting the speed of the piston, I included a flow regulator which can be adjusted by hand without the need for tools. This can be seen on the pneumatic circuit diagram. The selection of pneumatic components and their prices from our supplier are shown in the following table.

Pneumatic Parts cost list

www.rswww.com ORDER CODE QUANTITY UNIT COST (BEFORE VAT) COST

parker cylinder P1A-S025DS-0320 456-6582 1 £41.33 £41.33 Locknut for cylinder,20/25mm dia 456-6699 2 £0.89 £1.78 Piston rod clevis for cylinder,25mm dia (p1a-4jrc) 389-5378 1 £3.87 £3.87 Foot mount for cylinder,20/25mm dia (p1a-4hmf) 389-5429 1 £2.50 £2.50 Rear hinge mount for cylinder,20/25mmdia (p1a-4hmt) 389-5441 1 £5.28 £5.28 Pneumatic plastic silencer,G1/4 454-7395 2 £1.45 £2.90 G1/4 P3A miniature filter w/manual drain 389-6753 1 £13.03 £13.03 Male parallel thread elbow,G1/4x6mm 265-5551 6 £2.36 £14.16 Male parallel elbow fitting,G1/8x6mm 812-314 2 £2.21 £4.42 Supply type flow regulator,G1/4x6mm tube 492-8586 1 £9.54 £9.54 NATURAL STD NLYON TUBE (30m) 483-5557 1 £19.03 £19.03 Parker G1/4 5/2 lever/lever valve 389-6680 1 £59.39 £59.39 Flange MS3 - Mini ISO Cylinder 20,25 389-5429 1 £2.50 £2.50 TOTAL £179.73 TOTAL INC VAT £211.18

These parts were all paid for by Don & Low.


DESIGN AND MANUFACTURE OF THE GUIDE SYSTEM The main design requirements for the piston guide mechanism were:

• Minimal deflection. The support beam/shaft must not sag more than 3mm at full extension.

• Smooth in operation. The guide system must not hinder the smooth movement of the piston.

• Range of 320mm to match the stroke of the cylinder To meet these requirements I devised a construction where the cylinder and a short piece of rectangular hollow section steel were mounted on a steel plate as shown below

Figure 2.8: cylinder and mountings

The block on top is to hold a linear bearing which will constrain the shaft and ensure it runs parallel to the piston rod. The diameter of this shaft had to be calculated to ensure adequate stiffness. This was calculated. See appendix 2 I decided to use a 16mm diameter steel shaft as it was the smallest diameter which was calculated to be stiff enough. I also searched for a linear bearing (shown in figure 2.10) to support this size of shaft. RS Components stocked both parts. The prices of these are shown in the following table


www.rswww.com ORDER CODE QUANTITY UNIT COST (BEFORE VAT) COST

��

�� 217-9704 1 £12.85 £12.85 LJM linear bearing shaft,0.8m Lx16mm dia 285-0380 1 £20.35 £20.35 TOTAL £33.20

TOTAL INC VAT £39.01

Figure 2.9: Significant details of the selected linear bearing

The above details show that the maximum dry operating speed is 91.4 m/min. This translates to 1.53 m/s (higher than the target speed). It also shows that the maximum load for the bearing is far higher than is required for this application. I intended for this to give a large factor of safety.


Figure 2.10: Significant details of the bearing shaft

This shaft (figure 2.11, above) matches the requirements for use with the previously shown linear bearing. I designed a part to hold the heater at the end of the piston rod. This part was to have a ‘pillow block’ attached to its top surface (connected to the bearing shaft) as shown in figure 2.12.

Figure 2.11: Heater, holder and shaft construction


The red object in figure 2.12 is the Watlow Raymax 1010 radiant heater. This was purchased by Laurent along with the heaters to fit inside the tooling. I originally designed the heater holder to be made from aluminium with a wall thickness of 5mm (see figure 2.13). I had intended to over-engineer this part to compensate for any dynamic loading and weakening caused by the changing temperature it would be exposed to.

Figure 2.12: Dimensioned draft of original heater holder

However, when I spoke to Brian Robb about having it made, he made me realize that the way I had considered manufacturing this part would be very wasteful in terms of both time and material. My design would have had to be machined from a block of aluminium in the workshop. Instead, I was advised to use 3mm thick aluminium plate for the main construction (shown in figure 2.14) as this could be folded into shape then have strengthening plates added if required after testing. The drilled lug was to be welded or screwed on. Since its purpose is to connect to the end of the piston rod, the most sensitive component of the design, I opted to use mechanical fixings (15mm long M3 allen capped screws) as these would allow removal for any possible required alterations to be made without any detrimental effects to the strength.


Figure 2.13: Actual heater holder in position on the machine

After testing, I concluded that this construction was sufficiently strong and stiff enough so no further strengthening was required. I now had a means of moving the heater back and forth in the horizontal plane. The next step was to create a means of supporting this mechanism. The design requirements of this support structure were as follows:

• There must be accurate tilt adjustment. This is required to compensate for deflection in the beam and heater support, to keep the heater perfectly horizontal above the specimen for even heating

• The height must be quickly adjustable. This will make the machine flexible for use with new and altered equipment.

• It must be strong enough to support the weight of all the parts of the system and not fail under the dynamic load created by the moving mass of the heater and piston.

• It must also be stiff enough to not deflect significantly in either of the extreme positions or under the dynamic loading described above

I started by trying to think of a tried and tested device which has a similar function in an unrelated application. One of these items was a camera tripod. I considered the use of a large camera tripod. This would be suitable as it would give very accurate adjustment of height, tilt and rotation. However, they did tend to be expensive and are also designed to hold cameras with no significant moving parts. With the weight of the heater, its holder and the piston accelerating and stopping, a standard tripod is unlikely to be stable.


I then considered a mechanism I was familiar with from my hobby and part time job. I work in a bike shop so have become very familiar with practically all things to do with cycling. I noticed that a seat-post form a bike would be a very useful part for my design. The benefits I noticed in using one of these were:

• Tried and tested design. As a well developed consumer product, seat-posts are proven to be effective and reliable in a wide range of extreme conditions.

• Large design safety margin. Seat-posts are made to withstand extreme loads far greater than could be produced in normal use of my machine. For example, they are designed to be strong enough not to fail when landed on by a heavy person in the event of an accident. Even the static load of a person sitting on the saddle (which the seat-post holds) would be far greater than the load experienced in my application.

• Length. These seat-posts are available in lengths of up to 400mm. This would provide more adjustment than is perceived as necessary. This being said though, the extra length of post would not be detrimental in any way to the design.

Photograph showing me racing my downhill bike, BUSA student championships 2007

By coincidence (and having a lot to do with price) I am using exactly the same make and model of seat-post in the photograph above as the one I selected for my design. I can say from experience (unfortunately) that they can withstand a substantial amount of force without failing. The post I selected was a BBB Skyscraper, named after its particularly large size. Its details are shown below in figure 2.15


Figure 2.14: Online listing of the product

This extract was taken from www.bikedock.com, an online bicycle store. I selected a 30mm diameter post as it was a midrange size and would be plenty stiff (as any of the available sizes would be). This size was not particularly significant at this stage but would be when designing a clamping system for the post. Bicycle saddles have a standardized fitting they have 2 parallel rails (shown in figure 2.16), each 7mm in diameter, which the seat-post clamps tightly against to hold it in position.

Figure 2.15: Typical saddle rail configuration for a bicycle

I measured the spacing between the rails to be 43mm (centre to centre). To use the seat-post effectively in my design, I had to create an interface to allow the seat-post to connect strongly with the cylinder platform. The most effective way of doing this, in my opinion, was to manufacture a pair of mounts (shown in figures 2.17 a and 2.17 b) which each incorporated a counterpart rail. The idea of this was to enable the


seat-post to be put into use with n need for any alterations, thereby maintaining its strength and reliability.

Figure 2.16 a: Rail replacements (upright) Figure 2.17 b: Rail replacements (upside down)

The back parts shown in the above images are M7 allen cap screws. These screws are to be clamped by the seat-post clamp in the same way it would clamp the sadly rails. Since these are a very unusual size of screw, and the 7mm diameter was critical to the design, I had them made individually in the workshop. The holes shown in the image (above right) were to be threaded with an M6 tap so the cylinder platform could be bolted on to the top. Screw diameter calculation: See appendix 3 The next part on my list to be designed was the part required to hold the post up in its vertical position and to provide height adjustment. This part was to be mounted on a flat surface with material removed to allow the post to slot through (so as not to stop it being able to be set to a low height). The design I came up with is shown in figure 2.18. For stress calculations, see appendix 4.

Figure 2.17: Post Holder


The main features of this part are:

• 30mm bore through the centre • Slot cut at top of the clamping tube section to allow clamping of the post • Large flange with 4 equally spaced 6mm drillings (for mechanical fixing to the

support surface). • Thick wall at base, tapering towards the top • Made from aluminium bar by CNC machining

I used a quick-release seat-post clamp (also a standard bike component) to give a simple, user friendly adjustment mechanism. These are easy to use with a simple screw adjustment and a locking lever.

Figure: Quick release clamp for height adjustment

The support for the complete system is basically an extension to the base of the press. The base has an array of 15mm threaded holes which are intended for the purpose of securing test equipment such as this extension. Bolts were supplied with the press so I simply had to have holes drilled through the extension to match those on the base. I considered various shapes of table extension before settling on a T-shape. This shape would minimize material wastage and, if calculated correctly, could be sufficiently strong. Calculation of the strength and dimension requirements of this part can be seen in appendix 5


Figure: Base with extension fitted (bolts indicated by black rings)

Stresses had to be considered when deciding the dimensions of critical components of my machine. I considered 3 areas of maximum stress:

Figure 2. 18: Stress points in the machine


There other points of high stress but the ones indicated in figure 2.18 were perceived as the most critical. These were the positions for which the predicted stresses were calculated before dimensions were set to give reasonable factors of safety. Note: I was working on the assumption that, to compensate for dynamic loads in the system, a factor of safety > 5 would be ample to avoid failure. The stress calculations for these area are included in appendices 3, 4 and 5 To evaluate the stiffness of the parts, I simply assembled the machine and demonstrated the action of the piston. On doing these tests, I observed that there was excessive flex in the extension. This caused the machine to oscillate and sway rather violently when the heater was moved, especially when the piston reached full extension and stopped. To stiffen the table extension, a structure was welded to its underside with a hand-screw adjustable stabilizing rod. This can be seen in figure 2.18. The cylinder platform was also stiffened by welding fins to its underside. On testing the machine after these improvements, it proved itself to be adequately stiff. NOTE: Dimensioned Solid Edge Draft images can be found in appendix 8 of this report for reference.


OBJECTIVE 3: EXPERIMENTS AND FORMING TESTS I carried out several types of tests and experiments throughout the course of this project. These included:

1. Tests of the heating behaviour of the radiant heater 2. Heating and cooling tests on the male and female tools 3. Forming tests on pre-consolidated composite sheets 4. Forming tests on un-consolidated ‘fabric’ sheets 5. Functional tests of the pneumatic system and support structure

In this section, I will explain the findings of all of these tests except number 5 (since these have already been covered in the design section of this report (objective 2).

1) Tests of the heating behaviour of the radiant heater The initial test I carried out on the radiant heater involved placing a sample composite sheet (the ‘blank’) between the plates of the blank holder. For these experiments I used pre-consolidated composite sheets. These are made of self-reinforcing poly-propylene and are supplied as relatively rigid flat sheets. A sheet of this material is shown below in figure 3.1

Figure 3. 1: surface of pre-consolidated sheet.

Radiant heater experiment 1 Aims

1. To test the heater and prove it is effective


2. To give an impression of how close or far the heater must be from the surface to provide sufficient heating

3. To test the accuracy of the heat sensor within the heater. Method

1. The heater was mounted onto it’s holder on the end of the pneumatic system (set at full extension).

2. The composite sheet was sandwiched between the 2 aluminium blank holder plates and placed under the heater

3. The heater was set to be 50mm above the composite sheet. 4. Thermocouples were attached to the composite sheet using thermal tape at 3

different positions (1 at the centre and 2 on the sheet just close to the edge of the circular cut-out of the blank holder at 90° from each other (shown in figure 3.2 below)

5. The heater was set to a target temperature of 160° and temperatures were taken at 1 minute intervals at all 4 thermocouples (3 external and 1 in the heater)

The set-up of this experiment can be seen in figure 3.2

Figure 3. 2: Positions of thermocouples on top surface of the sheet

Results Time (minutes) PV(°C) T1(°C) T2(°C) T3(°C)

0 21 21 21 21 1 25 26 27 37 2 48 83 60 58 3 75 115 90 91 4 111 154 114 117 5 183


In the table above, PV refers to the ‘present value’ given from the heater control unit. This temperature is measured by a thermocouple inside the radiant heater. I created a graph (figure 3.3) of these results, plotting the temperatures against time so as to allow easy visual comparison of each temperature.

Temperature/time

020406080

100120140160180200

0 1 2 3 4 5

time (minutes)

Tem

pera

ture

(°C

)

PV (°C)T1 (°C)T2 (°C)T3 (°C)

Figure 3. 3: Graph of results of experiment 1

The experiment was stopped after 5 minutes as the sheet suddenly deformed and wrinkled. This was to be expected at such a high temperature since I had been told that the forming temperature of the material is supposed to be around 117°C. The force the sheet produced when deforming pushed the blank holder apart (see figure 3.4). Since I hadn’t expected this to happen, I had simply placed the plates together.


Figure 3. 4: failed experiment (sheet lifted the blank holder plate)

Conclusions From this experiment I came to the following conclusions:

1. The PV reading on the control unit is not an accurate monitor of the heater temperature during the initial heating process. Its response to heating appears to be very slow (it read only 111°C when the object it was radiating heat to exceeded 180°C).

2. The heater can heat the composite sheet extremely fast; after just 3 minutes the centre of the sheet had reached forming temperature.

3. When heating the specimen at this speed, the temperature distribution across the sheet was very wide. This would lead to unpredictable results when forming and reduce repeatability

4. The temperatures at the outside edges (T2 and T3) seemed to follow a very similar trend to the PV reading. This may be significant or simply coincidence. More experiments would be useful to quantify this.

Points to improve on in future tests

• Vertical temperature distribution. The temperature readings were all taken from the top surface in this case. This gave me no indication to how much (or little) the temperature throughout the sheet and on the underside was affected.

• Initial set up. The plates should be held together to avoid movement if the sheet deforms. This would remove the danger of the plate falling and potentially causing damage or injury.


Radiant heater experiment 2 Aims

1. To find a relationship between the distance between the heater and sheet and the temperature through the sheet

2. To find a relationship between the temperature of the blank holder (very close to the edge of the hole) and the sheet

3. To test the effectiveness of a new infra-red thermometer Gail had purchased for use with her project’s experiments.

Method

The equipment was set-up as in the previous experiment except for the following alterations:

• Thermocouple 2 was positioned on the underside of the sheet in the centre (directly under 1)

• Thermocouple 3 was positioned on the top surface of the blank holder as can be seen in figure 3.5

• The plates were clamped together with g-clamps at 2 of the corners to prevent movement in the event of the sheet deforming

• The control was set to a lower temperature of 90°C • The vertical gap between the heater surface and the sheet was set to 25mm,

50mm and 75mm (to give 3 sets of comparable results) • Surface temperature readings were taken using the infra-red device at several

times in 2 of the 3 experiments.

Figure 3. 5: Experiment 2 during test


Results I compiled the results in 3 spreadsheets (See appendix 6). From these spreadsheets I created graphs to show the temperature/time relations for each experiment

Heating (d=25mm, Tset=90 C)

0

20

40

60

80

100

120

140

160

0 1 2 3 4 5 6 7 8 9 10 12 14 16 18 20 22 24 26 28 30 33 35 39

time (minutes)

Tem

pera

ture

(C)

T(heater)T(top)T(bottom)T(plate)

Graph 3. 1: temperature vs time (gap = 25mm)


Heating (Tset = 90, d=50)

0

20

40

60

80

100

120

140

0 1 2 3 4 5 6 7 8 9 11 13 15 17 19 21 23 25 27 29 32

time (minutes)

Tem

pera

ture

(C)

T(heater)T(top)T(bottom)T(plate)


Heating (Tset = 90, d=75mm)

0

20

40

60

80

100

120

0 1 2 3 4 5 6 7 8 9 11 13 15 17 19 21 23 25 27 29 31 33 35

time (minutes)

Tem

pera

ture

(C)

T(top)T(bottom)T(plate)T(heater)



Comparison of overall results

Gap (mm)

Tpeak (°C)

T1 final (°C)

T2 final (°C)

Tmean final (°C)

time to reach Tfinal (minutes)

Temperature drop (°C)

Temperature drop (%)

25 152 69 63 66 21 24 27 50 129 59 53 56 19 34 38 75 110 48 45 46.5 22 43.5 48

I also made a graph comparing the temperature reading from the infra-red thermometer to those obtained from thermocouple 1 at the same time. See graph 3.4

Thermocouple Vs Infra-red

0

10

20

30

40

50

60

20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35

time

Tem

pera

ture

Thermocouple TemperatureInfra-red reading

Graph 3. 4: comparison of thermocouple and infra-red thermometer readings

(Taken from 75mm gap experiment)

Conclusions

1. The radiant heater heats to a significantly higher temperature than the set value before cooling back to it.

2. The heater takes approximately 20 minutes to settle at a set temperature of 90°C. I expect that further testing would show that the time taken to settle at a temperature is not directly proportional to the target temperature. i.e. to heat to twice that temperature, it will take probably take significantly less than twice the time.


3. If the gap is larger, the peak temperature the top surface reaches is less extreme. This will be an important factor since overheating is likely to damage the composite sheet and invalidate any forming results obtained.

4. The amount of heat radiated by the heater surprisingly didn’t seem to be at its highest when the heater was at its hottest. Instead, the surface of the sheet heated most dramatically when the power supply to it was on (indicated by a small light on the control unit). This brings in the risk of accidental overheating of specimens but, at the same time, gives us the opportunity to heat them very quickly.

5. Assuming 100% accuracy in the thermocouples, the infra-red thermometer is accurate to within 4.3% when measuring the surface temperature of this type of sheet.

Possible focus for future tests I intended to carry out more tests similar to this one but, due to time constraints and the installation and testing of the rest of the machine, this never worked out. I would suggest to anyone carrying out further testing that the following areas should take priority:

1. Testing to find a correlation between the (settled) temperature of the heater and the temperature of the sheet. Pre-heating the heater before putting it above the sheet should avoid the high peak temperature created in my tests.

2. Experiment to find the optimum spacing between the heater and the sheet. This will be the gap which gives the best balance between having as even a temperature distribution as possible and losing as little heat as possible


Forming test 1 Aim My aim was to form a hot sheet of the pre-consolidated, self re-enforcing polypropylene using room temperature tooling. From the start I did not expect this test to yield particularly successful results (since the tooling used in thermoforming is supposed to be hot during forming). I carried out this first test for 2 reasons:

• To have a grounds for comparison against future tests (with heated tooling) • At the time, the control units and heaters were being set up by the electricians but

I wanted to have a test done using the press to observe how it operated The pre-consolidated sheet has a forming temperature of 117°C and had to be heated in an oven. Due to the large dimensions of the blank holder, it will only fit in the largest oven in the materials laboratory. This oven is positioned quite for from the thermoforming station (press) (carrying the heated blank holder from the oven to the thermoforming station takes approximately 30 seconds) Method

1. Fit a sheet (30x30cm) into the blank holder 2. Put the blank holder into the oven (pre-heated to 120°C) 3. Heat the blank holder and sheet for 30 minutes 4. Remove the blank holder from the oven, carry it to the press and place it in

position. 5. Bring down the press to form the sheet 6. Leave the press down for 5 minutes to let the sheet cool before lifting the press

and removing the blank holder and formed sheet Results When the press was most of the way down forming the sheet, the sheet began to distort and crease. This forces the 2 blank holder plates apart (shown in figure 3.6), compressing the springs (round the bolts in the 4 corners of the blank holder). These springs were intended to put equal pressure across the surface of the sheet and prevent it creasing as it is formed. Evidently, this hadn’t worked in this case.


Figure 3. 6: Finished test (plates pushed apart by deformed sheet)

The resulting specimen was extremely poor quality. It had large creases across its whole body. Figure 3.7 shows the ‘formed’ sheet.

Figure 3. 7: Result of forming hot sheet over cold tooling


Forming Test 2 Aim To form a hot sheet of pre-consolidated, self re-enforcing polypropylene sheet using heated tooling. This would then be used for comparison with the previous test. 2 major changes were made to the set–up of the previous experiment:

• Heated tooling • The springs were removed and the bolts tightened to hold the sheet without

allowing any vertical deflection This test was also used to discover the effectiveness of the water cooling system. Method

1. Fit a sheet (30x30cm) into the blank holder and bolt the plates together 2. Put the blank holder into the oven 3. Pre-heat the oven to 120°C 4. Set the heaters in the male tool to 130°C and in the female tool to 126°C (this

makes both tools settle at a temperature of 124°C at their thermocouples) 5. Heat the blank holder and sheet for 30 minutes 6. Check that the tooling has settled at the desired temperatures. If not, wait until

they have 7. Remove the blank holder from the oven, carry it to the press and place it in

position. 8. Bring down the press to form the sheet 9. Leave the press down for 5 minutes, keeping the heaters on. 10. Switch off the heaters (by setting the temperatures down to below room

temperature) and start timing. 11. Raise the press when the temperature in each tool has fallen back down below

25°C Results

This time, there were no problems encountered during the time the press was moving into the specimen. The press was stopped at a peak load of 5000N approximately. Choosing this load was very difficult as it had to be done visually by watching the digital display on the press. The reading increased very quickly as the press moved down so it fast reactions were required to reach a specific target loading.

The result of the cooling part of the test was very successful. It showed that the tooling cools very quickly when the tap is opened fully. The tooling cooled from 124°C to room temperature (around 21°C) in just 5 minutes.


The sheet was formed with a far better quality finish than in the previous test. It had a deeper mould with less imperfection. However, it still was not up to the standard desired as it still had significant creasing (see figure 3.8).

Figure 3. 8: Result of forming test 2 (left) and forming test 1 (right)

It can be observed that the 2nd test result (left, above) has a better appearance and smaller (but more) creases. Conclusions

1. Removing the springs in favour of bolting the plates together had a positive affect on the test result. This indicated that more pressure was required to constrain the sheet against deformation as it is pulled into the mould shape. However, even with the plates bolted together, it appears that the pressure exerted on the sheet is still insufficient to prevent creasing. The blank holder may need modification in the future to yield more successful test results.

2. There are a few other contributing factors to the flaws I this result:

• The temperature settings may not be the optimum settings for forming the material. Further experiments would be required to find this value.


• The blank holder and composite sheet will have cooled down from the time they are leave the oven. By the time they are pressed, they may be too cold for effective forming.

Future tests and changes This was the last test carried out forming pre-consolidated sheet since the apparatus available was not adequate to create the required conditions for successful, high quality tests. This will change in the future:

• A heated blank holder (currently being designed by Gail) could be effectively used along with the radiant heater to heat the composite sheet to a relatively even temperature throughout before forming.

• The blank holder in existence now would need modification to be more effective. • The Instron press (used in all of my tests) is quite slow. It has a maximum speed

of just 200mm/second. For this reason the sheet would have a relatively long period of time to cool when the radiant heater is retracted and the press is moving towards the sheet. I have been informed that Dr Bezensek is in the process of buying a new high speed press. With slight modification to the ‘table extension’ support of my radiant heater should fit to this press.

• A combination of Laurent’s, Gail’s and my design should be successful in carrying out high quality, repeatable forming tests. Until that time though, it would not be very productive to carry out further forming tests on pre-consolidated sheets.


Forming test 3 Aim The aim of this test was to form and consolidate a single sheet of self re-enforcing polypropylene. This is the un-consolidated ‘fabric’ version of the sheet used in the 2 previous tests (see figure 3.9) It was also intended to measure the surface temperature of the tools when they are at a controlled temperature Method

1. Place a single sheet of the material in position in the blank holder and tighten

the plates together 2. Using heat resistant tape, attach a thermocouple to the centre of each tool. 3. Position the blank holder in the press 4. Bring the press down to form the sheet 5. Heat the tooling to forming temperature (around 117°C as with the pre-

consolidated material) 6. Keep the temperature at the forming temperature for 5 minutes 7. Cool the tooling 8. Lift the press and remove the blank holder and formed sheet.

Results Around half way through the heating stage a fuse blew in the heater control units. Fortunately, the thermocouples were still working and could be used to measure the temperature the tooling reached. The temperature of the top tool reached 81°C and the bottom reached 52°C. From this, I assumed the material reached 67°C (the mean of these 2 temperatures) The equipment was left to cool for 7 minutes before water was passed through to finally cool the tools before the press was lifted. The blank holder and specimen were removed. Figures 3.10 and 3.11 show the resulting formed sheet. On inspection of the formed sheet, it was observed that it held its shape but deflected under very small forces (i.e. by lightly squeezing it by hand). After small deflections, it sprung back to its original shape.


Figure 3. 9: Self re-enforcing polypropylene sheet in 'fabric' form

Figure 3. 10: Formed sheet in blank holder (1) Figure 3. 11: formed sheet in blank holder (2)

Conclusions Not much could be concluded from this text except for the fact that the ‘fabric’ formed into a better overall shape than the ‘pre-consolidate.’ The wires to the thermocouples caused there to be lines in the formed sheet. In subsequent tests, this should be avoided.


Forming test 4 Aims To form 4 sheets of the ‘fabric’ material together and compare 2 orientation configurations. In 1 of the tests, the sheets all face in the same direction (the direction of the weave in each sheet is the same) and in the other test the sheets were each placed 45° to the direction of its neighbour sheet. Method I originally planned to carry out 2 identical tests as follows:

1. Fit the sheets (30x30cm) into the blank holder in the appropriate orientation for each particular test.

2. Place the blank holder into position in the press 3. Lower the press to form the sheet, aiming for a load of 3000N (60% load

displayed on press when set to a maximum load of 5000N) 4. Set the male and female tool heaters to 126° and 130° respectively and leave in

position, heating for 30 minutes 5. Turn off the heaters and pass water through the tooling until it has cooled to room

temperature. 6. Raise the press and remove the blank holder.

However, the fact that this size of sheet was smaller than the minimum theoretical required sheet size of 350mm had been overlooked. For the second test, I used 350x350mm sheets in cross ply (0°-45°-0°-45°) orientation. Results The first test merely served a provisional test to give an indication of how well the forming process would work. Due to its smaller size it would not be particularly useful for comparison of different forming test settings but can be used for visual comparison of the quality of results. Figure 3.12 shows the result of this test. The edges are uneven as the original sheets were quickly, but not very accurately. The formed section of the sheet had a smooth stiff but not very strong body. When squeezed, the specimen felt quite hard before making creasing noises and deflecting. It was also wrinkled around the entry area to the female tool.


Figure 3. 12: First test 30x30cm in-line

Figure 3. 13: first test result from other side


Figure 3.14 shows the result of the 2nd test. In this test, 4 accurate 35x35mm sheets were used accurately for in-line orientation. The results were very similar to the previous test. This raised doubt as to whether the minimum dimension calculated by Laurent is accurate and required for forming this material. Further tests with a modified blank holder would be helpful in clarifying this. On inspection of these 2 results it was noticed that the grid lines (drawn orthogonally on the flat sheet before forming) had remained relatively square. This indicated that the shear strain in the material (at least the outside layer) is very small. This is a positive result as excessive shear stresses and strain could lead to the material tearing and specimens being wasted.

Figure 3. 14: 4 sheets 30x30cm in-line

Because the first 2 test results were so similar in their stiffness, strength and appearance the conditions of the final forming test were altered in the hope of gaining a different result. The change that was made was that the tools were preheated to close to the forming temperature of the material before the blank holder was placed and the press brought down. Unfortunately, the samples had already been cut before this decision was made and so were in cross-ply formation. This was not altered as there was insufficient


time left to cut new samples so these were used. The test result should be useful as comparison of how consolidated the sheets were after heating and forming. Figure 3.15 shows the result of this 3rd test.

Figure 3. 15: Result of the final test

As it can be seen, this test result was very similar aesthetically to the previous tests. The physical properties also seemed very similar although, without controlled testing of these, it is very difficult to compare. Conclusions From these limited experiments I reached the following conclusions:

1. Preheating the tooling did not make a significant difference to the finished quality of the specimen.

2. The creasing in the formed specimen appeared to be a result of mechanical flaws in the equipment (rather than heating) as they could be observed taking shape as the sheets were being pushed into the female tool by the male tool. A better blank holder design, in my opinion, would alleviate this problem.


3. The sheets in all of these experiments did not seem to be very well consolidated. I say this because they do not offer great resistance to peeling apart (see figure 3.16)

Figure 3. 16: Result from 3rd test being peeled apart

Further details of these tests can be found in appendix 7


APPENDIX


APPENDIX 1 Fax to SMC Pneumatics


APPENDIX 2 Support Shaft Deflection Calculation

Using the equations: REIM

Z

z 1= and 64

4dI z

π=

Where: Mz = moment about point z E = Young’ Modulus Iz = 2nd moment of area z d = diameter of the shaft The available bearing shafts are made from stainless steel. Its properties, significant to these calculations, are: The Young’s modulus, E = 190 GN/m^2 Its density, � = 8.1 Mg/m^-3

Figure A 1: Free body diagram of heater and rod From the free body diagram (figure A 1)

)*5.0( heatershaftz massmassgLM +=

Where: g = gravitational constant = 9.81 N/kg And L = length of shaft


I put these equations into a spreadsheet to calculate the radius of curvature of the deflected beam. Shaft deflection Calculations � = 8100 kg/m^3 E = 1.9E+11 N/m^2 � = 3.14159 Moment due to heater and holder 1.68 Nm Shaft Diameter (m)

Mass of shaft (kg)

Mz (Nm) Iz (m^4) 1/R R(m)

0.01 0.81 17.76 4.90873E-10 0.19 5.25 0.011 0.99 18.03 7.18688E-10 0.13 7.57 0.012 1.17 18.32 1.01788E-09 0.09 10.56 0.013 1.38 18.64 1.40198E-09 0.07 14.29 0.014 1.60 18.99 1.88574E-09 0.05 18.87 0.015 1.83 19.36 2.48505E-09 0.04 24.39 0.016 2.08 19.75 3.21699E-09 0.03 30.94

At this stage, it was obvious that the larger shafts are stiffer (as expected) but it is hard to judge what radius of curvature was acceptable. To solve this difficulty, I drew up a sketch using Solid-Edge CAD software. For each radius given by the spreadsheet, I made a diagram to work out its vertical deflection (figure 2.9).

Figure 2.19: Method of finding vertical deflection using Solid-Edge software (not to scale)


Resulting predicted shaft deflection: Shaft Diameter (m) R (m)

Deflection (mm)

0.01 5.25 16.83 0.011 7.57 11.66 0.012 10.56 8.36 0.013 14.29 6.17 0.014 18.87 4.67 0.015 24.39 3.62 0.016 30.94 2.85

Errors will exist in these calculations but these will not be significant since:

• The results will pessimistic as the length of the arc will be very slightly longer than 420mm (the length of the beam) so the vertical deflection would in reality be slightly smaller.

• There will be movement and small amounts of twist allowed by the linear bearing(s) used to support the shaft


APPENDIX 3 Fixing screw size calculation

Figure A 2: Diagram showing significant points for calculation

Using figure A 2 (above), the moment about point O was calculated. For simplicity and to give a pessimistic result (leading to an over-strong product) I only calculated the moment using over-estimated values of the 2 main masses which would lead to a positive moment (using the sign convention shown in the diagram). For the calculation, I assumed a pinned joint at point O. This again led towards a pessimistic result (i.e. giving an over-estimate of the stress created). This also simplified the problem greatly as the equation (using S.I. units)

0080.0)075.0594(. =−+=� tensionRHSheaterO FMassMassgM now applies where: g = 9.81 N/kg heaterMass = 3 kg RHSMass = 3 kg Note: [In this case, heaterMass refers to the mass of the heater and its supporting components RHSMass refers to the RHS steel part and the pillow block on top]


Manipulation of the above equation gave the result: 1.246=tensionF N

Now A

Ftension=σ where A = the cross sectional area of the screw

� = tensile stress in the screw So, for an M5 screw (cross sectional area would be 1.96E-5 m^2), the tensile stress would be: � = 12.5 MPa The screws will be made from mild steel which has a yield stress of 240 MPa so M5 (or larger) screws will give a factor of safety of almost 20 when the machine is in the demonstrated, and highest stress, position IF the stress was taken by only 1 screw. Since there are 2 rail replacement parts (1 hidden behind the other in figure A 2) the cross-sectional area is, in reality, twice as large and so is the factor of safety


APPENDIX 4 Post holder stress calculation This part is basically a hollow tube with a bending moment applied to it.

Bending moment, )115.0634(. RHSheater MassMassgM += = 22 Nm (by using equation from appendix 3, but moving point O 40mm to the right)

)( 44 IDODI −= π where OD and ID are the outside and inside diameters respectively I created a spreadsheet to calculate the bending stress for various wall thicknesses: Bending stress calculations Pi = 3.14159 Internal diameter = 0.03 m External diameter > 0.03m Bending moment 22 Nm Wall thickness (mm) External Diameter I (Nm^4) Stress (Pa)

0.5 0.031 5.57E-09 6.12E+07 1 0.032 1.17E-08 3.01E+07

1.5 0.033 1.85E-08 1.97E+07 2 0.034 2.58E-08 1.45E+07

2.5 0.035 3.39E-08 1.14E+07 3 0.036 4.27E-08 9.28E+06

3.5 0.037 5.22E-08 7.79E+06 4 0.038 6.26E-08 6.68E+06

4.5 0.039 7.38E-08 5.81E+06 5 0.04 8.59E-08 5.12E+06

5.5 0.041 9.89E-08 4.56E+06 6 0.042 1.13E-07 4.09E+06

6.5 0.043 1.28E-07 3.69E+06 7 0.044 1.44E-07 3.36E+06

7.5 0.045 1.62E-07 3.06E+06 8 0.046 1.80E-07 2.81E+06

8.5 0.047 2.00E-07 2.59E+06 9 0.048 2.21E-07 2.39E+06

9.5 0.049 2.43E-07 2.22E+06 10 0.05 2.67E-07 2.06E+06

This part will be made from aluminium (yield strength = 40 MPa) so the results show that the outside diameter at the base of the holder must be at least 32mm (when using a 30mm seat-post). For extra strength, large safety margin and aesthetics purposes, I decided to have the post holder manufactured with a thick base and tapering wall.


APPENDIX 5 Table extension stress calculations

Figure A 3: Table extension stress diagram

I simplified the problem to a mass of 8.31 kg (found by weighing the individual components) at the end of a beam. In reality there will not be such a large moment around the base but, to give a worst case stress value, this will give a useful result. Using M = Fr where:

M = the size of the moment (Nm) F = the force exerted on the end of the beam = 81.5 N r = the length of the beam I calculated the moment to be: M = 31.1 Nm Now, using

RE

yIM

==σ

and 12

3bhI =


where: � = bending stress I = 2nd moment of area of the beam b = the breadth of the beam h = the height of the beam y = distance from neutral axis

R = radius of curvature of deflected beam E = Young’s modulus (= 200 GNm^-2 for mild steel)

(for b, h and y see figure A 4)

Figure A 4: diagram showing dimensions for calculation of 2nd moment of area

I calculated the required width of ¼” thick steel sheet (since there was plenty available in the workshop) would be required to withstand a moment of 31.1 Nm using a spreadsheet (shown below) Table extension stress calculations Bending moment

31 Nm

h= 0.00635 m y= 0.003175 m b I (Nm^4) � (Pa) 0.05 1.06687E-09 9.23E+07 0.06 1.28024E-09 7.69E+07 0.07 1.49361E-09 6.59E+07 0.08 1.70699E-09 5.77E+07 0.09 1.92036E-09 5.13E+07 0.1 2.13373E-09 4.61E+07 (= 46.1 MPa) 0.11 2.34711E-09 4.19E+07 0.12 2.56048E-09 3.84E+07 0.13 2.77385E-09 3.55E+07 0.14 2.98723E-09 3.29E+07


0.15 3.2006E-09 3.08E+07 0.16 3.41397E-09 2.88E+07 Using mild steel with these dimensions gives a yield factor of safety of just over 5. The radius of curvature of the deflected beam had to be calculated using the equation:

RE

yIM

==σ

re-arranged to give MEI

R =

This gave: R = 13.7m This will give a very small deflection which will be easily corrected by using the angle adjuster on the seat-post clamp.


APPENDIX 6 Radiant Heater test result spreadsheets Gap=25 Tset = 90° time (minutes) T(heater) T(top) T(bottom) T(plate)

Surface Temperature

(Infra-red Thermometer)

0 19 15 15 14 0.5 19 16 15 14 1 22 28 20 16 1.5 33 58 37 15 2 45 84 60 16 2.5 61 120 86 16 3 75 135 110 17 3.5 91 152 127 17 4 99 152 127 17 112.6 4.5 104 138 130 17 131.1 5 107 128 123 17 105.2 5.5 109 120 115 17 104 6 109 108 104 17 92.1 6.5 109 97 95 17 76.2 7 107 90 87 17 7.5 106 82 82 17 8 105 74 74 17 58.1 8.5 102 70 70 17 61.9 9 99 66 63 17 58.4 9.5 98 59 59 17 56.8 10 95 57 55 17 54.6 11 89 50 49 17 49.1 12 85 46 44 17 43.3 13 83 50 49 17 57.6 14 84 62 55 17 65.8 15 87 71 64 17 67 16 88 73 68 17 67.6 17 88 73 66 17 69.7 18 87 68 61 18 64.7 19 87 66 61 18 62.9 20 86 67 61 18 56.1 21 87 69 63 18 50.6 22 87 70 64 18 68.6 23 87 69 64 18 68.6 24 87 68 64 18 57.6 25 87 70 63 18 63.9 26 87 69 63 18 62.2 27 87 68 63 18 54.4


28 87 69 63 18 62.2 29 87 67 63 18 61.1 30 87 67 63 18 65.3 31 87 68 63 19 70.9 33 87 68 64 19 63.3 34 87 68 63 19 64.8 35 87 67 63 19 61.1 36 87 67 63 19 61.9 39 88 69 64 19 68.1 40 87 69 63 19 67

d=50

Tset = 90°

time (minutes) T(heater) T(top) T(bottom) T(plate)

0 17 17 17 21 0.5 21 19 18 21 1 27 28 24 21

1.5 37 46 36 21 2 90 70 54 21

2.5 63 94 75 21 3 78 120 93 21

3.5 92 129 105 21 4 98 125 109 21

4.5 103 116 104 21 5 105 106 97 21

5.5 106 95 89 21 6 106 85 82 21

6.5 106 78 75 21 7 105 72 68 21

7.5 103 66 62 21 8 102 61 58 21

8.5 99 56 53 21 9 97 53 49 22 10 92 46 44 22 11 88 42 39 22 12 84 42 39 21 13 84 49 43 22 14 86 56 49 21 15 87 61 55 21 16 89 63 57 22 17 88 60 55 22 18 87 58 53 22 19 87 58 53 22 20 87 58 53 22 21 87 58 53 22 22 87 58 53 22 23 87 58 53 22 24 87 58 53 22


25 87 59 53 22 26 87 59 53 22 27 87 58 53 22 28 87 59 53 22 29 87 59 52 22 30 87 58 53 22 32 87 59 53 22

d=75 Tset = 90°

time (minutes) T(top) T(bottom) T(plate) T(heater) Surface

Temperature 0 21 21 21 21

0.5 21 20 21 22 1 28 24 22 26

1.5 43 33 23 36 2 60 47 24 47

2.5 82 62 26 62 3 102 80 27 79

3.5 110 94 27 91 4 108 95 26 98

4.5 100 92 25 102 5 93 87 24 105

5.5 84 80 23 106 6 77 73 23 106

6.5 70 66 22 106 7 63 61 22 104

7.5 58 56 22 103 8 54 52 22 101

8.5 50 48 22 99 9 47 46 21 96 10 41 40 22 91 11 38 36 21 83 12 39 36 21 83 13 44 40 21 84 14 52 46 22 86 15 54 49 22 88 16 53 49 22 88 17 52 48 22 88 18 46 45 22 87 19 43 40 22 86 20 43 40 22 86 47.5 21 44 40 22 86 47.5 22 46 42 23 86 48.3 23 48 43 23 86 51.3 24 48 44 23 86 51.9 25 49 47 23 86 52 26 49 46 23 86 50.6


27 50 46 23 87 52.6 28 48 44 23 87 49.4 29 46 41 24 86 48.2 30 46 42 23 86 47.2 31 46 42 23 86 49.9 32 48 45 23 86 48.6 33 49 45 23 86 49 34 49 45 23 87 48.5 35 48 45 23 86 48.6

Mean value of temperatures after 20 minutes (inclusive) Thermocouple on surface: 50.47 Infra-red thermometer: 52.74 Error in infra-red reading ( assuming 100% accuracy in thermocouple): 4.31%


APPENDIX 7 Final 3 tests: More Detail 1st test: Press force: 70% of 5kN = 2930N Temperature of tooling at end of forming: Male tool temperature = 124°C Female tool temperature = 117°C By the end of the test, the load on the tooling had relaxed to almost zero The running water cooled the tools from this temperature back to room temperature in a time of 6 minutes 2nd test: Same as above except loading was 3500N at the start and relaxed to 2930N 3rd Test: Tools both at temperature of 124°C (at the thermocouples) at the start of forming and kept at this temperature throughout. Press force: 60.5% of 5kN = 3025N (beginning) This relaxed to 24.4% over the 10 minutes the press was down and still.


APPENDIX 8 Solid Edge Drafts of Machine Parts


REFERENCES Books Engineering Materials 1 Michael F Ashby & David R H Jones Engineering Mechanics , Statics

by J. L. Meriam + L. G. Kraige MFE Report: Thermoforming Station Project By Laurent Maurel Parker Pneumatic Actuators Handbook 2007 By Parker Hanafin SMC Pneumatics website: http://www.smceu.com/Prod/WebContent/main/index.jsp?lang=en&ctry=UK Pneu-store website (SMC distributor) http://www.pneu-store.co.uk/acatalog/Special_Packages.html Global-spec (engineering equipment search): http://www.globalspec.com/ Norgren website (including online shop) http://www.norgren.com/uk/default.asp RS Components http://rswww.com/cgi-bin/bv/rswww/home.do?cacheID=ukie&returningUser=N Handbook for Frelon Lined Linear Bushings http://docs-europe.electrocomponents.com/webdocs/0168/0900766b80168d64.pdf Bikedock (online cycle shop) www.bikedock.com


http://www.bikedock.com/posit/shop/productinfo.php?partno=PPSCLAMP&srcwind=%2Fposit%2Fshop%2Findex.php%3Fcategory%3D30%26group%3D3 http://www.bikedock.com/posit/shop/productinfo.php?partno=BBBSKYSCSP&srcwind=%2Fposit%2Fshop%2Fsearch.php University of Cambridge (for of 2nd moment of area of various common shapes) http://www.doitpoms.ac.uk/t/tlplibBD1/secondmoment.php NOTATION Calculations in this report were carried out using standard metric units. NOTATION PROPERTY UNITS L, h, b Distances m F Force N I 2nd moment of inertia Nm^4 T Temperature °C � Stress Pa (or Nm) � Density kg/m^3 m Mass kg M Moment Nm The meanings of this notation and any subscripts are given along with the equation and calculation of each property.

THERMOFORMING STATION PROJECT WRITE-UP phuserweb.eng.gla.ac.uk/philip.harrison/Teaching/2007...

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