1.6 Thermoforming Machinery

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1.6 Thermoforming Machinery

Transcript of 1.6 Thermoforming Machinery

  • sophisticated these procedures become, the more difficult it is to adapt them tohigh-speed forming without exacting process controls. And yet many new packagingapplications, for example, are seeking just such designs. Thermoforming processinnovation remains a lively art.

    1.6 Thermoforming Machinery

    As noted, there are two general thermoforming categories. Typically, heavy-gagesheet is handled as discrete cut sections and the forming equipment are calledcut-sheet thermoformers. Thin-gage sheet is handled in continuous rolls and theforming equipment is usually called roll-fed thermoformers. The equipment in bothcategories includes: Some form of sheet handling device, A way of moving the sheet from one station to another, A means of controlling the various elements that allow the sheet to be heated,

    formed and moved from station to station, A sheet heating oven, A vacuum system, A forming press, and A formed part removal region.In addition, the equipment may include: Some form of prestretching such as:

    preblowing orplug assist,

    A pressure system, A trimming press, and Some form of trim removal.Certain guidelines pertain to both categories of forming equipment. Table 1.7 givesan overview for thermoforming equipment in general [20]. Some of these aresummarized below.

    Heating Source

    The various heating methods are detailed in Chapter 3. Sheet temperature should becontrolled to within + 5C or + 1O0F. During transfer to the forming station, thesheet temperature drop should not exceed 5 to 100C or 10 to 200F. Infrared heatingis most popular today. The various heating methods include [21]: Simple nickel-chrome heating wires, Metal resistance rods, sometimes called calrods,

    Previous Page

  • Table 1.7 General Specifications for Thermoformers1

    Platen size, W x L (in x in or mm x mm)Maximum depth of draw (in or mm)Forming process (vacuum, pressure, matched mold, plug assist, twin-sheet capability)Platen power drive (pneumatic, hydraulic, mechanical, electric)Indexing power drive (pneumatic, hydraulic, mechanical, electric)Floor space (ft2 or m2)Heater type (metal rod, quartz, ceramic, radiant, gas, nichrome wire)Heater controls (proportioning, percentage timers, timer controlled, zone controlled, programmable

    controlled, machine controlled)Maximum heater output (kW/ft2 or kW/m2)Special features (purpose, type such as shuttle or rotary, number of stations, number of ovens, etc.)1 [20] with permission of John Wiley & Sons

    Ceramic bricks or tiles, Quartz heaters in rod, spiral or square plate form, Direct gas-fired burners, Indirect gas-fired catalytic burners, Heat lamps, Quartz glass plates, Halogen bulbs, and Wire or rod heated metal plates.Heater surface temperature is usually measured with thermocouples, thermistors orinfrared pyrometric devices. Heating wires and resistance wires are inexpensive butoxidize rapidly and so lose heating efficiency. Quartz heaters are quite efficient, canbe turned on and off like light bulbs but are quite expensive and fragile. Quartz ispreferred for high temperature and "shaped heating" needs as described in Chapter3.

    Sheet is also heated by direct contact with a hot metal plate (trapped sheetheating), by placing the sheet in a hot air oven (convection heating), or by passingit through a high-frequency electromagnetic field (RF or microwave heating). In thelast case, the plastic must absorb the high-frequency energy. PVC is heated byradio-frequency energy in the flow-molding embossing process. Other polymers mustbe doped with "lossy" substances such as inorganic hydrates or even carbon black.

    There are certain elements that pertain to the ovens for all forming presses. Forexample: There must be a way of separating the sheet from the heater source at shut-down.

    Baffles and dampers are used for heavy-gage sheet and fly-open and extractingshuttles are used for thin-gage sheet.

    There should be adequate means for rapid replacement of burned-out heaterelements on both top and bottom heater banks.

  • Forming Platform

    The forming station should include all elements necessary to prestretch the sheet,form it, cool it and eject it from the mold. Some of these elements include: Substantial guide-rods are needed. For vacuum molds greater than 12 in or 300

    mm by 24 in or 600 in dimension and for all pressure forming applications, fourguide-rods are recommended.

    Clamp tonnage should be in proportion to mold size, that is:Typically >100 lbf/in2 or 0.7 MPa for straight vacuum forming, andTypically > 200 lbf/in2 or 1.4 MPa for pressure forming.

    The press frame pit should not be deeper than 4 ft or 1.3 m. In many areas, pitsdeeper than this must be entered by people wearing self-contained breathingapparatus.

    The overhead press frame structure should be robust enough to support suchelements as:

    The entire weight of the mold, if it is desired to mold in an "up" position,In-mold trimming components including cylinders and framework,Ancillary mold elements such as:

    Plug assist cylinders and frames,Ejector cylinders and frames, andCavity isolator cylinders and frames, andPressure boxes.

    The press should allow for easy mold removal and maintenance. The press platens should allow for easily adjustable mold daylight. There should be adequate headroom for overhead ancillary equipment. There should be adequate space around the presses for vacuum lines, vacuum and

    air pressure lines, and adequate mold temperature hoses and manifolds. There should be adequate provision for prestretching and billowing, sheet lock-

    down and stripping and ejecting. There should be allowance for free surface temperature control, including chilled

    air. The frame should be adequately reinforced and gusseted to carry heavy molds,

    ancillary hardware, and day-to-day vibration and shaking. There should be adequate provision for part removal and trim or web takeaway.

    The drive system that raises and lowers the platens is the key to forming stationperformance. Depending on the application, the drive unit can be as inexpensive asa simple air cylinder or as complex as the hydromechanical clamps used on injectionmolding presses. Table 1.8 [21] rates some of the drives used in thermoformingpresses. Many high-speed presses use electric toggle clamps and cams. For high-speedpressure forming of polypropylene, double toggle clamps are used. Some straighthydraulic clamps are used in high pressure applications. Electrically driven clampsare being developed that are touted to be more accurate with less maintenance thanconventional clamp systems. Most vacuum forming drive systems are designed toclose and clamp at maximum pressures of 20 to 40 lbf/in2 or 0.15 to 0.30 MPa. For

  • 1 From [21: Table 8] by permission of Carl Hanser Verlag

    Key: 0 = Low or poor; 3 = Moderate or average; 5 = High or excellent

    pressure forming, the drive system must clamp against the forming air pressure.Safety factors of 3 to 4 are recommended. If 30 lbf/in2 or 2 MPa air pressure is usedin forming, the drive system should be designed to remain closed against 90 to120 lbf/in2 or 6 to 8 MPa pressure. For a typical mold forming area of 20 x 10 in or500 x 250 mm, the drive system clamping load is 9 to 12 T or 32 to 107 Mkgf.Platens and guide-rods must be designed to accommodate high bending forces. Forforming pressures in excess of 100 lbf/in2 or 0.7 MPa, forming station designs beginto resemble those used in thermoplastic structural foam injection molding [22]. Forforming pressures in excess of 200 lbf/in2 or 1.4 MPa, the presses begin to resemblethose used in compression molding.

    Vacuum System

    The vacuum systems for both categories of forming equipment are quite similar. Forstand-alone shuttle and rotary formers, the vacuum pump, surge tank and plumbingis usually an integral portion of the machine (Fig. 1.23). For installations of severalforming presses, regardless of the sheet gage, a centrally located vacuum system isfrequently used.

    Even the least expensive vacuum forming press must have adequate means ofrapidly drawing the sheet against the mold surface. One critical factor in efficientvacuum draw-down lies in an unencumbered, adequately sized line between the

    Characteristic

    UniformityStroke limit controlTiming controlRepeatabilitySpeed controlForming forceClamping forceStabilitytrackingEnergy consumptionTrouble-free natureMaintenanceNoise levelConstruction qualityCostPreblowing capacity

    Total

    Pneumaticaircylinder

    01203/4330/3034015/25

    30/31

    Gearedmotor-cogwheel

    55550/2/5333/4555534/15

    61/65

    Hydraulic

    45445550/31003105

    42/45

    Pneumaticdriventoggle

    031010540220530

    26

    Eccentricmotor-cams

    332301543434510

    41

    Motor-drivenspindle

    554525452331510

    50

    Table 1.8 Comparison of Forming Table Drive Units1

  • Figure 1.23 Single station cut sheet shuttle pressDrypoll/Zimco

    vacuum surge tank and the mold cavity. Proper vacuum system design requires avacuum pump capable of drawing down to 710 to 735 mm Hg vacuum1 in the surgetank prior to the beginning of the forming cycle. The path between the surge tankand the cavity between the hot sheet and the mold should have as few constrictionsas possible. Long pipes, flow constrictors, quick-disconnects, restrictive valves andlarge L/D vent holes should be eliminated. Fast-acting rotary ball valves arerecommended for vacuum shut-off [23]. Section 6.5 details a method for determiningpressure drops through each of the constrictions from the mold cavity to the vacuumpump inlet. A good estimate of the time required to evacuate a mold cavity isobtained from:

    where 0 is the pump-down time, V is the total volume of the system to be evacuated,P1 is the ini