Seminar Model (1)

Semester: VII Branch: ME BLOW MOULDING

ABSTRACT

Blow moulding is a process used to produce hollow objects from

thermoplastic.The basic blow moulding process has two fundamental phases. First, a

parison (or a preform) of hot plastic resin in a somewhat tubular shape is created.

Second, compressed air is used to expand the hot preform and press it against mould

cavities. The pressure is held until the plastic cools.

Toc H Institute of Science & TechnologyArakkunnam – 682 313 Page No: 1


CONTENTS

CHAPTER DESCRIPTION PAGE NO

I INTRODUCTION 3

II BLOW MOULDING 6

III TYPES OF BLOW MOULDING 8

IV POLYMERS USED 15

V MATERIAL SELECTION 18

VI POLYPROPYLENE 22

VII ECONOMIC FACTORS 26

VIII IMPLICATION FOR DESIGN 28

IX CONCLUSION 30

X REFERENCE 32



Chapter I

INTRODUCTION

Introduction



Plastics are actually chains of polymers held together by a strong but fluid bond.

The reason a thin soda bottle is strong enough to withstand the pressure of carbonated

liquids is a phenomenon called 'biaxial orientation.' The chains in a plastic bottle form in

two directions, creating a very strong webbing effect. The plastic itself can be stretched

out without sacrificing strength.

Blow molding experts takes advantage of this property to make thin but strong

containers. Blow molding a two-liter soda bottle requires a preformed piece called a

parison. This parison is usually extruded from a plastic injection mold placed very close to

the blow molding machinery. The warm parison looks like a upside-down plastic test

tube, with a preformed collar and threads for the cap at the bottom.

The parison is mechanically loaded onto a stand and two sides of a bottle-

shaped metal mold come together around it. Before the parison cools down, a hollow

ramrod is injected into its center and pushed to the top of the mold, stretching out the

warm plastic preform as it goes. Compressed air is then forced out in controlled low-

pressure stages through the hollow ramrod. The plastic form is forced out to the sides of

the mold. Because the stretching is performed evenly, the plastic remains uniformly thin

and strong. The soda bottle assumes the shape of the mold and is dropped out of the

blow molding machine as the two mold halves separate. A new parison is extruded and

the entire blow molding process begins again. The actual manufacture of a soda bottle

takes only a few seconds.



Blow Molding is a highly developed molding technology developed back in the

late 1800's to produce celluloid baby rattles. It is best suited for basically hollow parts

(such as plastic bottles) with uniform wall thicknesses, where the outside shape is a

major consideration.

There are other forms of blow molding, but the general principle is the same.

The plastic acts much like a latex rubber balloon -- as long as the pressure from the

compressed air is controlled, the material will expand uniformly and form the shape of the

mold. This requires a fair amount of skill and experience on the part of the blow molding

engineers who design new pieces of equipment.



Chapter II

BLOW MOULDING



Blow moulding

It is the process of inflating a hot, hollow, thermoplastic preform or parison inside

a closed mould so its shape conforms to that of the mould cavity.

The Blow Molding Process

1. A thermoplastic resin is heated to a molten state.

2. It is then extruded through a die head to form a hollow

tube called a parison.

3. The parison is dropped between two mold halves,

which close around it.

4. The parison or preform is then inflated with high

pressure air.

5. The plastic solidifies as it is comes into contact with

the chilled blow mold.

6. The mold opens and the finished component is

ejected.



Chapter III

TYPES OF BLOW

MOULDING



There are basically four types of blow molding used in the production of plastic bottles, jugs and jars. These four types are:

1. Extrusion blow molding2. Injection blow molding3. Stretch blow molding4. Reheat and blow molding.



Extrusion blow molding is perhaps the simplest type of blow molding. A hot

tube of plastic material is dropped from an extruder and captured in a water cooled mold.

Once the molds are closed, air is injected through the top or the neck of the container;

just as if one were blowing up a balloon. When the hot plastic material is blown up and

touches the walls of the mold the material "freezes" and the container now maintains its

rigid shape. This process produces a container with a blow chamber and tail scrap

attached to the container. The blow chamber and tail must be removed from the

container through a secondary process. After removal, the blow chamber and tail are

ground into small particles referred to as regrind and are typically blended with virgin

resin to manufacture new bottles. Extrusion blow molding allows for a wide variety of

container shapes, sizes and neck openings, as well as the production of handle ware.

Extrusion blown containers can also have their gram weights adjusted over a fairly wide

range by altering the extruder output. Extrusion blow molds are generally much less

expensive than injection blow molds and can also be produced in a much shorter period

of time. There are 2 types

a. Continuous Parison Type

This is the simplest and most common type of blow moulding. A hollow tube

called parison is continuously extruded from an annular die. When the desired

length of the parison is reached, the mould closes around the parison. The parison

is cut above the mould and the mould is transferred to a separate station where the

parison is blown by compressed air, cooled and finally the article is ejected out of

the mould.

b. Intermittent Type

The parison is extruded between the two halves of the open mould. When the

desired length of the parison is reached, the mould closes around the parison.

Compressed air is blown through a bore in the mandrel. At the same time, the

article is externally cooled by water circulating in the mould. The article is then

ejected by compressed air or metal stripper.



Injection blow molding is part injection molding and part blow molding. With

injection blow molding, the hot plastic material is first injected into a cavity where it

encircles the blow stem, which is used to create the neck and establish the gram weight.

The injected material is then carried to the next station on the machine, where it is blown

up into the finished container as in the extrusion blow molding process above. Injection

blow molding is generally suitable for smaller containers and absolutely no handleware.

Extrusion blow molding allows for a wide variety of container shapes, sizes and neck

openings, as well as the production of handleware. Extrusion blown containers can also

have their gram weights adjusted through an extremely wide range, whereas injection

blown containers usually have a set gram weight which cannot be changed unless a

whole new set of blow stems are built. Extrusion blow molds are generally much less

expensive than injection blow molds and can be produced in a much shorter period of

time.



Stretch blow molding is a blow molding process that produces fairly

lightweight containers with very high impact resistance and, in some cases, superior

chemical resistance. This is brought about by aligning the molecules of the resin during

the stretching process. This process is perhaps best known for producing PET bottles

commonly used for water, juice and carbonated beverages. The process can also be

utilized to manufacture polypropylene containers but the processing parameters are more

stringent and therefore not widely practiced. There are two processes for stretch blow

molded containers.



The reheat and blow molding process (RHB) is a type of stretch

blow process. In this process, a preform is injection molded by an outside vendor. There

are a

number of companies who produce these "stock" preforms on a commercial basis.

Factories buy the preforms and put them into a relatively simple machine which reheats it

so that it can be blown. The value of this process is primarily that the blowing company

does not have to purchase the injection molding equipment to blow a particular container,

so long as a preform is available from a stock preform manufacturer. This process also

allows access to a large catalog of existing preforms. Therefore, the major expense is

now for the blow molds, which are much less expensive than the injection molds required

for preforms. There are, however, some drawbacks to this process. If you are unable to

find a stock preform which will blow the container you want, you must either purchase

injection molds and have your own private mold preforms injection molded, or you will

have to forego this process. For either type of stretch blow molding, handleware is not a

possibility at this stage of development. The stretch blow molding process does offer the

ability to produce fairly lightweight containers with very high impact resistance and, in

some cases, superior chemical resistance. Whether using the injection stretch blow

molding process or the reheat and blow process, an important part of the process is the

mechanical stretching of the preform during the molding process. The preform is

stretched with a "stretch rod." This stretching helps to increase the impact resistance of

the container and also helps to produce a very thin walled container.



Chapter IV

POLYMERS USED



Polymers used for Blow Moulding Process

Polyethylene

- Low Density Polyethylene

- Linear Low Density Polyethylene

- High Density Polyethylene

- High Molecular Weight, High Density Polyethylene

Polypropylene

- Homopolymer

- Random Copolymer

- Impact Copolymer

Polyvinyl Chloride

Polyethylene Teraphthalate

Polycarbonate

Material Considerations (Component)

Good Elasticity + Good Viscosity = Viscoelasticity



Material Considerations (Mould)

Since blow molding pressures are relatively low compared to other molding operations,

mold material strength is not as important and a large proportion of molds are made from

high strength aluminum alloys.

However, mold wear may become a problem. Plated steel and beryllium-copper are

alternative materials for molds or these more wear-resistant materials can be used for

various components of aluminum molds, e.g., inserts and pinchoffs.

Pressure Considerations

The Blow Moulding is a Low Pressure Process:

Normal Pressure : 0.5 to 1 mPa

With an Extreme of : 0.2 to 4 mPa

Large Pressure is required for thick walled mouldings

Cooling Considerations

Coolant flow channels are provided to accelerate part cooling and so reduce cycle time.

In blow molding the general intent is to cool the part to a suitable ejection temperature

as quickly as possible.

In the production of preforms in injection blow molding the coolant may be heated to a

temperature lower than the melt temperature but high enough so that the preform can

be directly transferred to the blowing station with no, or little temperature conditioning.



Chapter V

MATERIAL SELECTION



Requirements for Blow Moulding Materials

From a processing technology standpoint, all materials with melts that have the following

properties are suitable for blow moulding applications :

· Sufficient thermal stability for the processing temperature range and, if necessary, for

repeated processing

· Sufficient flowability of the homogenous, plasticated melt.

· Sufficient stretchability of the tube (parison) even at high stretching speeds.

· Excellent repeatability of parison weight and length

· A smooth parison surface.

· Compatibility with additives such as masterbatches, pigments etc.

· A sufficiently wide processing range for the required finished part properties.

· Excellent lot to lot consistency.

Process Based Requirements for Material Suitability

Extrusion blow moulding

The various melt streams formed in the flow channels must be capable of reuniting into a

consistent parison. The vertically hanging extruded parison must have sufficient melt

strength to allow time for the mould to close. Good welding in the pinchoff area of the

blow mould is important.

Injection blow moulding / Stretch blow moulding

Because the parison is supported on a core in this process, It is possible to use materials

with a melt strength that would be too low for extrusion blow moulding. Multiple cavity



moulds, often with hot runners, dictate that a material with easy flow and low adhesion to

the core be utilized.

End Use Criteria for Material Selection

In addition to the general technological requirements, the requirements of specialized end

uses must be taken into account. There are as many of these as there are applications,

but some of the more common include :-

· Chemical resistance

· Permeation characteristics

· Environmental stress crack resistance (ESCR)

· Mechanical properties (eg. cold impact resistance)

· Physiological properties (eg. for food applications)

· Optical properties

· Weather resistance

· Long-term properties

Selection of Material for Packaging Application

Following requirements should be complied while selecting a raw material for both

moulding products used for packaging applications

· Stiffness under load

· Dimensional stability at high filling temperatures

· Good surface quality

· Good printability

· Good drop impact resistance even at low temperature

End Use Applications of Polypropylene Blow Moulded Products

· Cosmetic bottles

· Shampoo bottles

· Mineral water bottles



· Oil bottles

· Feeding bottles

· Containers for pain balms

· Pharmaceutical bottles

· Tablet containers

· Mouth wash bottles

· Antiseptic liquid bottles

· Chemical containers

· Containers for harsh liquid cleansers

· Barrier bottles (one layer)



Chapter VI

POLYPROPYLENE



Polypropylene offers following distinct advantages over other polymers due to

which it is becoming popular with processors and end-users

· Very low density

· High stiffness

· High surface hardness

· Very good chemical resistance

· Contact clarity and gloss

· Good resistance to high temperatures

· Very low water absorption and transmission

· Very good processing

· Very good toughness at low temperatures when alloyed with elastomers

As compared to homopolymers, random copolymers have more clarity and impact

resistance, both strong advantages for blow moulding product.

PROCESSING POLYPROPYLENE

Polypropylene presents no extreme difficulties for processors. Certain guidelines need to

be followed for ease in processing

(I) Screw and Barrel Design

Generally screws used for polyethylenes are widely used for processing polypropylenes.

But ideally a screw with depth compression ratio of about 3.5:1 is recommended. The

L/D ratio should be minimum of 20. Higher the ratio, better the homogeneity.

(II) Processing Temperatures



Polypropylene can be processed at temperature settings of about 170-190°C at feed

throat

and gradually increased in steps to about 200-205°C at the die. The melt temperature

should be between 180 and 240°C

(III) Tooling for Polypropylene

Polypropylene exhibits 70% of the die swell of HDPE hence it is recommended to tailor

the die ring / mandrel design to suit the product for processing of polypropylene. Highly

polished tooling will give better parison surface.

(IV) Moulds

(i)Finish : Highly finished moulds may be used with polypropylene to accentuate its

glossy appearance.

(ii) Venting : For effective mould venting it is recommended that radial polishing of

the mould cavity using 360 grit paste be done and to allow air to escape upto

parting line

(iii) Pinch off design : Generally pinch-off for polypropylene are sharper than that

used for HDPE or PVC. A pinch-off land of about 1.2 mm is recommended for

PP as compared to 2.4 mm for PE & PVC. It is recommended to use steel or

beryllium-copper alloy inserts for pinch-off inserts when making moulds for PP

as likely chances of wear-out are posed due to sharper pinch-off.

(iv) Mould cooling : Mould cooling should be very effective and uniform for

polypropylene to get faster cycles and better clarity since PP has lower thermal

conductivity.

(V) Deflashing

Polypropylene bottles should be deflashed immediately after moulding otherwise it will

give problems in deflashing due to stiffness and good flexural properties. When using

automatic deflashing as done with I.V.Fluid bottles punching off the pinch flashes is

recommended than deflashing by pulling cylinders to obtain sharp cut

(VI) Parison Transfer

When using extrusion blow moulding for polypropylene, hot wire cutter is required for

PP processing before parison transfer to blowing station to obtain a sharp cut.



RESIN PROPERTIES

For blow moulding applications, polypropylene with MFI between 1 to 3 g/10min are

used resin upto 6.5% ethylene copolymer exhibit more clarity and impact stiffness than

homopolymer

Additives

- Polypropylenes need a moderate level of antioxidant for better thermal stability

and colour stability

- Lubricants may be added for better mould release

- Nucleating agents may be added to a low level to enhance stiffness and reduce

cycle times

- Clarifying agents may be added to improve clarity

Shrinkage

Polypropylenes exhibit a moulding shrinkage of 1.2-2.2% which is similar to shrinkage

value of HDPE 1.2-3% due to rigidity and stiffness of containers, it is difficult to eject the

products from the undercuts which may be stripped off in case of HDPE



Chapter VII

ECONOMIC FACTORS



Economic Factors

For blow moulding tooling costs are moderate to high with lead times that are usually only

a few days. The flexibility of the process is limited by dedicated dies and there are short

set-up and tool change over times. Full automation of the process is viable.

The process production rate is 100–2500 components/hour and production volumes are

generally 1000 but the process is well suited to high volume production.

Extrusion blow moulding allows a continuous operation but increases the waste material

as the complexity of the mould also increases; material utilisation is generally good.

Although some trimming is required the cost is minimal.

Comparitive Equipment And Tooling Cost


Process Equipment cost Tooling CostBlow Moulding Med to high Low to med.casting Medium --------------------

Injection moulding Med to very high High to very high

Rotational Moulding Med to high Low

Machining Med to high Low


Chapter VIII

IMPLICATION



Implication For Design

The complexity of the components is limited by its hollow and rounded structure that has

a relatively thin section and does not allow a large degree of asymmetry. Inserts, threads,

Bosses, undercuts, ribs and lettering are possible.

The section range for blow molding components is 0.25–6mm, those components with a

thick section may need cooling aids e.g. carbon dioxide or nitrogen gas. The parting line

should not interfere with critical dimensions and corner radii should be as generous as

possible, certainly more than 3mm.

Draft angles are not required for this process which can produce components of volumes

up to 3m.



Chapter IX

CONCLUSION



Conclusion

Blow molding is the only process through which bottles are made, there is no alternative

for this process. Recent advancements in machine design allow for insertion of pre-

moulded, pre-sterilized components to be moulded into the container creating additional

design options to create multi-use and injectable product containers. Furthermore, the

blow-fill-seal process flow is normally impacted by only two raw materials, product and

polymer, that are each processed inline, thereby making the process amenable to large

uninterrupted batch sizes, some in excess of 500,000 units, and fill durations of up to 120

hours. The net effect is routinely an increase in production efficiency and a subsequent

decrease in operational costs for the user.



Chapter X

REFERENCE



Reference

1. Practical Extrusion Blow Molding, Samuel L. Belcher, Sabel

Plastechs, Inc., Moscow, Ohio, ISBN: 0-8247-1997-2

2. Blow Molding Handbook, 2nd Edition, Dominic Rosato, Andrew

Rosato, David Dimattia, ISBN: 1-56990-343-3

3. Blow Molding Design Guide, 2nd Edition, Norman Lee, ISBN: 978-

1-56990-426

4. PT-2-3 Blow Molding Practice Guide Manual, Version 3, July 2007,

SPDC, Ltd.

5. CT-1-5 Beginning Plastics, Version 3, July 2007, SPDC, Ltd.

6. CT-1-6 Introduction to Plastics Fabrication Technology, Version 3,

July 2007, SPDC, Ltd.

7. CT-2-2 Plastics Materials I, Version 3, July 2007, SPDC, Ltd.

8. CT-3-4 Plastics Materials II: Formulation Technology, Version 3,

July 2007, SPDC, Ltd.


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