corporate training and planning introduction to plastics processing

CORPORATE TRAINING AND PLANNING

INTRODUCTION TO PLASTICS PROCESSING


1.0 INTRODUCTION

Plastics – The unique class of wonder materials – came into existence by virtue of their superior performance and cost effectiveness over to conventional materials.

Over the years the applications spectrum of plastics have been widened with the advent of new generation Polymers, blend alloys and composites

. Every day newer and newer application are being promoted in all the key sectors of Indian Economy viz, Automobiles, Agriculture, Aerospace. Building & Construction, Infrastructure, Telecommunication, IT, Medical & Bio Medical engineering, Packaging, etc.

This inturn necessitates the need for different types processing methods and machinery to produce quality plastics products at affordable cost

Today a host of processing methods and machinery are available to manufacture plastics products meeting stringent quality requirements and cost to performance balance.


1.1 DEFINITION:

Plastics Processing – in a simple layman’s language – can be defined as the process

of converting the plastic raw materials into Semi-finished or finished products.

Raw Materials(Powder/granules)

Semi-finishedor

Finished Products

Processing


In a more technocrate way we may say “Get the Shape and set the Shape”. Technique to get the shape and Set the shape varies depending on Process and Material employed.

Principles: Deformation of a polymer melt (ex) Injection, Extrusion, Blow Moulding etc Deformation of a polymer in Rubbery state

Ex: ThermoformingVacuum FormingPressure Forming

Deformation of a SuspensionEx: PVC Plasticsol Processing / Coating

Deformation of a Solution Ex : Solvent Casting of CN Film

Deformation of a low melt polymer / monomerEx: Acrylic Sheet Casting

Preparation of GR Laminates Machined Structures

Secondary fabrication operation


1.2 CLASSIFICATION OF PROCESSING METHODS

1. Primary Processing Methods:

More importance by virtue of

o Extent of utilisation for Varied applications

o Growth Potential

Ex: Injection Moulding, Extrusion, Blow Moulding, Compression / Transfer

Moulding,

2. Secondary Processing Methods

Lesser extent of utilisation

Acts as supplementary to primary operation

Ex: Roto Moulding, Thermoforming, Coating, Casting, FRP Fabrication

Methods, Calendaring, etc.,


Performance Requirements

Practical Approach

Engineering Approach

Material Selection

Properties Process Cost

Ideal Choice

Product Manufacture : A Simplified Flow Diagram

1.3 PROCESSING FUNDAMENTALS


The process selection depends on several interrelated factors:

(1) Designing a part to meet performance and manufacturing requirements at the

lowest cost;

(2) Specifying the plastic;

(3) Specifying the manufacturing process, which requires

Designing a tool ‘around’ the part,

Putting the ‘Proper Performance’ fabricating process around the tool,

Setting up necessary auxiliary equipment to interface with the main processing

machine

Setting up ‘Completely integrated’ controls to meet the goal of zero defects;

(4) Purchasing equipments and materials, and warehousing the materials.


COMPETITIVE PROCESSES

S.

No

Product Inj.

Moulding Extrusion

Thermo

forming Blow

Moulding

Comp./

Transfer Moulding

Roto

Moulding

1 Narrow neck

container

- - - 1 - -

2 Oil Barrels

Upto (200 ltrs)

- - - 1 - 1

3. Tanks (20000 ltrs)

- - - - - 1

4. Films,Profiles, Pipes

- 1 - - - -

5 Housing, Auto parts

1 - 2 2 2 -

6. Wider Neck parts

1 - - - - -

7. Hallow Containers

- - - - - 1

1. Best Process 2. Supplementary Process


1. Setting up specific performance requirements;

2. Evaluating material requirements and their processing capabilities;

3. Designing parts on the basis of material and processing characterstics, considereing

part complexity and size as well as a product and process cost comparison

4. Designing and manufacturing tools (Moulds, Dies, etc) to permit ease of

processing;

5. Setting up the complete line, including auxliliary equipment;

6. Testing and providing quality control, from delivery of the plastics, through

production, to the product

7. Interfacing all these parameters by using logic and experience and / or obtaining a

required update on technology.

Parameters that help one to select the right options are


1.4 PROCESSABILITY:

Processability means generally the ease or difficulty with which a plastic can be

handled during its fabrication into film, moulded products, pipe, etc.

A plastic with good processability possesses the properties necessary to make it

easy to process the plastics into desired shapes.

The main characterstics or properties which determine a plastic’s processability

are molecular weight, uniformity, additive type and content, and plastic feed rates.


1.5 PROCESSING METHODS:

The type of process to be used depends on a variety of factors, including product

shape and size, plastic type, quantity to be produced, quality and accuracy

(Tolerances) required, design load performance, cost limitation, and time schedule.

Each of the processes provides different methods to produce different products. As

an example, extrusion with its many methods produces films, pipe,sheet, profile,

wire coating, etc.

Almost all processing machines can provide useful products with relative ease, and

certain machines have the capability of manufacturing products to very tight

dimensions and performances. The coordination of plastic and machine facilities

these processes.


PROCESSING METHODS – An Overview:

Machine Operation Terminology

Terminology in the plastics industry regarding the operation of machinery is as follows:

Manual Operation

Each function and the timing of each function is controlled manually by an operator.

Semiautomatic Operation

A machine operating semi automatically will stop after performing a complete cycle of programmed moulding functions automatically. It will then require an operator to start another complete cycle manually.

Automatic Operation

A Machine operating automatically will perform a complete cycle of programmed moulding functions repetitively; it will stop only for a malfunction on the part of the machine or mould, or when it is manually interrupted.

Depending upon the configuration of the part, economic viability and the part tolerance, etc the process can be selected.


An extruded parison tube of heated thermoplastic is positioned between two halves of an

open split mould and expanded against the sides of the closed mould via air pressure. The

mould is opened and the part ejected. Low tool and die costs, rapid production rates, and

ability to mould fairly complex hollow shapes in one piece.

BLOW MOULDING :

Description :

Limitations:

Generally limited to hollow or tubular parts; some versatile mould shapes, other than

bottles and containers.


BLOW MOULDING


Very widely used. High automation of manufacturing is standard practice. Thermoplastic

or thermoset is heated to plasticate in cylinder at controlled temperature, then forced

under pressure through a nozzle into sprue, runners, gates, and cavities of mould. The

resin undergoes solidification rapidly. The mould is opened, and the part ejected,

Injection Moulding is growing in the making of glass-reinforced parts. High production

runs, low labour costs, high reproducibility of complex details, and excellent surface

finish are the merits.

INJECTION MOULDING :

Description :

Limitations:

High initial tool and die costs; not economically practical for small runs.


INJECTION MOULDING


Widely used for continuous production of film, sheet, tube, and other profiles; also used in conjunction with

blow moulding. Thermoplastic moulding compound is fed from a hopper to a screw pump where it is heated

to plasticate then pumped out through the shaping orifice (die) to achieve desired cross section. Production

lines require input and takeoff equipment that can be complex. Low tool cost, numerous complex profile

shapes possible, very rapid production rates, can apply coatings or jacketing to core materials (Such as wire).

EXTRUSION :

Description :

Limitations:

Usually limited to sections of uniform cross section.


EXTRUSION


Thermoset compound, usually preformed, is positioned in a heated mould cavity; the mould is closed

(heat and pressure are applied) and the material flows and fills the mould cavity. Heat completes

polymerization and the part is ejected. The process is sometimes used for thermoplastics, e.g. Vinyl

phonograph records. Little material waste is attainable; large, bulky parts can be moulded; process is

adaptable to rapid automation.

COMPRESSION MOULDING :

Description :

Limitations:

Extremely intricate parts containing undercuts, side draws, small holes, delicate inserts,

etc.; very close tolerances are difficult to produce. Time consuming process.


Widely used to produce Thermoset products with part complexity. Thermoset moulding

compound is fed into transfer chamber where it is then heated to plasticate; it is then fed

by a plunger through sprues, runners, and gates into a closed mould where it cures; mould

is opened and part ejected. Good dimensional accuracy, rapid production rate, and very

intricate parts can be produced.

TRANSFER MOULDING

Description :

Limitations:

High mould cost; high material loss in sprues and runners; size of parts is somewhat

limited.


Dough-consistent thermoplastic mass is formed into a sheet of uniform thickness by

passing it through and over a series of heated or cooled rolls. Calenders are also utilized

to apply plastic covering to the backs of other materials. Low cost, and sheet materials are

virtually free of moulded-in stresses.

CALENDERING :

Description :

Limitations:

Limited to sheet materials and very thin films are not possible.


A predetermined amount of powdered thermoplastic material is poured into mould; mould

is closed, heated, and rotated in the axis of two planes until contents have fused to the

inner walls of mould; mould is then opened and part is removed. Low mould cost, large

hollow parts in one piece can be produced, and moulded parts are essentially isotropic in

nature.

ROTATIONAL MOULDING

Description :

Limitations:

Limited to hollow parts; production rates are usually slow.


ROTATIONAL MOULDING


Heat-softened thermoplastic sheet is positioned over male or female mould; air is

evacuted between sheet and mould, forcing sheet to conform to contour of mould.

Variations are vacuum snapback, plug assist, drape forming, etc. Tooling costs are

generally low, large part production with thin sections possible, and often comes out

economical for limited part production.

THERMOFORMING

Description :

Limitations:

Limited to parts of simple configuration, high scrap, and limited number of materials

from which to choose.


THERMOFORMING


Liquid plastic which is generally thermoset except for acrylics is poured into a mould without pressure,

cured, and taken from the mould. Cast thermoplastic films are produced via building up the material

(either in solution or hot-melt form) against a highly polished supporting surface. Low mould cost,

capability to form large parts with thick cross sections, good surface finish, and convenient for low-

volume production.

CASTING

Description :

Limitations:

Limited to relatively simple shapes. Most thermoplastics are not suitable for this method.

Except for cast films, method becomes uneconomical at high volume production rates.


Reinforcement is placed in mould and is rotated. Resin distributed through pipe;

impregnates reinforcement through centrifugal action. Utilized for round objects,

particularly pipe.

CENTRIFUGAL CASTING: Description :

Limitations:

Limited to simple curvatures in single axis rotation. Low production rates.

COATING

Process methods vary. Both thermoplastics and thermosets widely used in coating of numerous materials. Roller coating similar to calendaring process. Spread coating employs blade in front of roller to position resin on material. Coatings also applied via brushings, spraying, and dipping.

Description :

Limitations:

Economics generally depends on close tolerance control.


Excellent strength-to-weight. Continuous, reinforced filaments, usually glass, in the form of roving are saturated

with resin and machine-wound onto mandrels having shape of desired finished part. Once winding completed,

part and mandrel are cured; mandrel can then be removed through porthole at end of wound part. High-strength

reinforcements can be oriented precisely in direction where strength is required. Good uniformity of resin

distribution in finished part; mainly circular objects such as pressure vessels, pipes, and rocket cases.

FILAMENT WINDING :

Description :

Limitations:

Limited to shapes of positive curvature; openings and holes can reduce strength if not

properly designed into moulding operations.


Material, usually in form of reinforcing cloth, paper, foil, metal, wood, glass fibre, Plastic

etc., preimpregnated or coated with thermoset resin (sometimes a thermoplastic) is

moulded under pressure greater than 1000psi (7Mpa) into sheet, rod, tube, or other simple

shapes. Excellent dimensional stability of finished product; very economical in large

production of parts.

LAMINATING :

Description :

Limitations:

High tool and die costs. Limited to simple shapes and cross sections.


A variation of the conventional compression moulding, this process employs two metal

moulds possessing a close-fitting, telescoping area to seal in the plastic compound being

moulded and to allow trim of the reinforcement. The mat or preform reinforcement is

positioned in the mould and the mould is closed and heated under pressures of 150 –

400psi (1-3MPa). The mould is then opened and the part is removed after curing.

MATCHED-DIE MOULDING :

Description :

Limitations:

Prevalent high mould and equipment costs. Part often require expensive surface finishing.


Liquid thermoplastic material (Plastisol) is poured into a mould to capacity; mould is

closed and heated for a predetermined time in order to achieve a specified buildup of

partially fused material on mould walls; mould is opened and excess material is poured

out; and semifused part is removed from mould and fully fused in oven. Low mould costs

and economical for small production runs.

SLUSH MOULDING :

Description :

Limitations:

Limited to hollow parts; production rates are very slow; and limited choice of materials

that can be processed.


Effect of polymer properties on Process Technique


1.6 EFFECT OF POLYMER PROPERTIES ON PROCESS TECHNIQUE

(1) Water absorption of Raw materials,

(2) Physical form of raw material,

(3) Thermal stability of polymer,

(4) Flow properties,

(5) Adhesion of melt to metal,

(6) Thermal properties affecting, Heating and cooling of melt,

(7) Compressibility and shrinkage,

(8) Frozen in Orientation.

When processing thermoplastic melts the following factors should be taken into account in order both to process efficiently and obtain quality products.


Water / Moisture is the greatest enemy for processing of plastics.

Hygroscopic Materials

Absorption phenomena - Ex: Nylon, POM, PC.

Adsorption phenomena - Ex: HIPS, PS, ABS.

All these materials should be pre-dried.

Non-Hygroscopic material - Ex: PVC, Polyolefins, etc.

Need not be predried. Except when completely wet during monsoon.

WATER ABSORPTION


Actions Necessary

1. Use granules as soon as the bag is opened.

2. Pre-drying ovens, Hopper drier, Dehumidifying drier can be used.

3. For PC - Dehumidifying drier preferable

Physical form of Raw Material

Powder form, granular form, lumpy/slab form

Slab Form - Calendering,Compression Moulding

Granular Form – Preferred - Uniform pellet size ensures even and faster feeding.

Powder Form - Difficulty in feeding - But savings in cost because of the ability to

avoid pelleting stage - Special feeder attachment essential to ensure proper feeding.


Thermal stability of polymers PVC thermally sensitive material - Little higher melt temp. may lead to depredation - HCL is

released - This can leads to corrosion and harmful to human being. PID Temperature

controller can be used.

PMMA, POM upon depredation liberates MMA & formaldehyde respectively - MMA

volatilize and cause bubbles - Formaldehyde gas causes “eye-irritation”.

PVC & POM (acetal) should never be processed one after the other. This may lead to

explosion

Adhesion of melt to metal:

Wetting of the polymer melt against the metal wall of processing equipment can

lead to strong adhesion of polymer to metal. Ex: difficulty in removing PVC - Mix

from two roll mill.

PC has a strong adhesion to metal. It can take away the skin of the barrel if not

properly purged


THERMAL PROPERTIES AFFECTING HEATING AND COOLING

In the case of polymer melts the specific heat varies with temperature. For crystalline

polymers such as POM, NYLON etc. latent heat of fusion and sp.heat should be taken in

to account. i.e Total heat content (Enthalpy) =LH of fusion + sp.heat.

POLYMER PROCESS TEMP 0C ENTHALPY / KJ / KG

PS 200 310

LDPE 200 500

HDPE 260 810

PP 260 670

Because of higher enthalpy PP requires more cooling time than LDPE and PS.


COOLING SHRINKAGE AND COMPRESSIBILITY

When polymers are in molten stage the vibrations of the molecules results in the polymer

chain being pushed apart so that the volume occupied by a given polymer mass is higher

than when the material is solid.

POLYMERDENSITY AT 20

C(G/CC)DENSITY AT PROCESS

TEMP (G/CC)

LDPE 0.923 0.746(210c)

PP 0.905 0.765(210c)

PMMA 1.180 1.105(210c)

SPVC 1.48 1.390(190c)

Because polymer melts are compressible moulding shrinkage is much less than the above fig.


FROZEN-IN ORIENTATION

When polymer melts are being shaped by either injection moulding or Extrusion the

long polymer chains tend to be elongated or uncoiled in the direction of flow.

After shaping, the melt is usually cooled rapidly and there is seldom time for the

oriented molecules to return to a random coiled shape by the process known as

relaxation.

Some orientation is thus “Frozen-in” the product. Such stressed parts are very weak.

Hence annealing is must.


PROCESS SELECTION CRITERIA FOR PLASITC PRODUCTS

Introduction:

With the advent of New Generation Polymers, blends alloys and composites, over the last decade, the application spectrum of plastics has been widened.

Today with the result, the plastics have penetrated deeply in all the key sectors of economy which includes

Automobiles Telecommunication

Aerospace Defence

Biomedical Building & Construction etc

In the liberalized economy the survival of plastic industries largely depends upon timely delivery, quality, cost and cost / performance balance of plastics products.


With the availability of host of plastic raw materials and a wide range of

plastic processing methods and technologies, it is a difficult task for the

processor to select a suitable cost effective process for a specific product.

Although in some cases one or more processes may be suitable for producing a

specific plastics product A plastics processor can select a specific process

keeping in mind the key parameters such as cost, quality and cost/performance

balance.

In this presentation, a few selected case - studies were made for the benefit of

plastic processors.


Process Selection Criteria

The following parameters play a key role in selecting the best suited process to produce a specific product for a specific application.

1. Material Processibility - Limitations

2. Volume of production

3. Size & shape of the product (configuration)

4. Cost to performance balance

5. Quality


Material Processibility LimitationsIf a specific material is suitable for a particular application, then material processibility will decide the specific method of Processing / Production.

Case :1

Product : PET bottle

Process : Blow Moulding

Specific process : Injection stretch Blow moulding

Not possible by : Extrusion Blow moulding or Extrusion Stretch blow

moulding

Reason:

PET material used today does not have the required hot melt strength to hold/self support a parison.


Case: 2Product : PTFE sheets

Process : Compression moulding following by sintering process.-The sintered

sheets are subsequently machined to the required dimension.

Other Conventional Process:

Extrusion - Not possible.

Reason:

PTFE exhibits - very high melt viscosity above its melting temperature. Hence only

best suited process is compression moulding followed by sintering.


Configuration of Product- Size & shape governs the process selection.

Case 1:

Narrow neck containers like Shampoo bottles, Pharmaceutical

containers, soft drink bottles will have to be produced by Blow moulding only.

Case 2:

Hollow containers such as Tanks (20,000ltrs) will have to be produced by

Roto moulding inspite of higher initial cost on plant & machinery.


Case 3:

Product : FRP Boat

Raw materials : Fiber glass material, Polyester resin and other

additives

Process : 1. Hand lay-up 2. Spray up

Most suitable process:

If volume of production is less then hand lay up. If more then spray up

technique. Spray up technique calls for higher initial investment.


Cost / Performance BalanceIf more than one process is suitable for producing a product based on performance, then cost plays a crucial role in process selection.

Case: 1

Product : 200 litres chemical container

Material : HDPE

Process : 1. Blow Moulding 2. Roto Moulding


Sl. No.

Parameter Blow Moulding Roto Moulding

1. Specific Process

Accumulator Blow Moulding

Three arm rotomoulding

2. Volume of Production High Low 3. Initial investment on

plant & machine High Moderate

4. Performance Good Good 5. Cost/performance Best good

Hence accumulator type blow moulding process can be selected.


Case - 2Product : FRP pipe

Material : Glass fiber, Polyester resin & other additives

Process : 1. Limited lengths & high strengths - Filament winding

2. Continuous lengths & high strength - pultrusion

3. Centrifugal casting - Batch process

If medium strength and relatively stress free pipes are required with better optical properties, then centrifugal casting is the best method.


Case: 3Product : PVC Flexible sheet

Material : Compounded PVC with additives such as plasticizer, stabilizer,

colourants etc.

Process : 1. Calendering 2. Extrusion

Suitable process : Calendering

Reason : Wider width, cost effectiveness

Case: 4Product : Acrylic sheet

Material : PMMA

Process : 1. Casting 2. Extrusion


Sl. No.

Parameter Casting Extrusion

1. Mechanical Properties Lower Higher

2. Optical properties High Low

3. Initial investment Low High

4. Stress built-up Almost stress

free

Stress built-up due to

orientation.

Since optical properties are very essential based on cost/performance & quality casting process is best.


QualityMainly due to quality reasons, inspite of higher cost of manufacture, certain products are produced by a specific process only.

Case:1

PVC - gramaphone records - even today produced by compression moulding.

Reason: Stress-free moulding

Case:2

Product: PP Blown film

Process: Only downward extrusion process should be used instead of upward blown film process.

Reason:

PP, being a crystalline polymer,in order to get transparency, the film emerging out of die lips needs to be quenched.The best /economical way of quenching is to dip in water which is possible only in downward extrusion. So due to quality reason this process is selected.


Case:3

Product : Plastic float

Material : PP/HIPS

Processes:

1. Roto Moulding

2. Injection Moulding - Two halves and then joining by vibration welding.


Sl. No.

Parameter Roto Moulding Injection Moulding & Welding

1. Initial Investment Moderate High 2. Quality Good Moderate 3. Production rate Low High 4. Low volume / prototype Best - 5. High volume Production - Best

corporate training and planning introduction to plastics processing

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