Chapter - 5- PLASTIC PROCESSING

5.1. Introduction :

Plastics belongs to the family of organic materials. The organic materials are those materials which are derived directly from the carbon. They consist of carbon chemically combined with hydrogen, oxygen etc. This large organic group consist of natural and synthetic materials. The plastics and synthetic rubbers are termed as polymers. The term polymer is derived from two Greek words : ‘poly’ and ‘meros’. Poly means many and meros means units. Thus polymers are composed of a large number of repeating units called monomers. The monomers are joined with each other by reaction known as polymerization. Thus the polymers are made up of thousands of monomers joined together to form large molecules.

Plastic is the name given to a family of materials made up of repultions of molecules of high molecular weight in the form of flexible chain. These materials are soft and mouldable during manufacturing and can be formed into various shapes through the application of heat and pressure. So, the plastics are the synthetic organic materials which can be moulded into any desired shape when subjected to heat and pressure. In general, plastics are compounds of carbon with other elements such as oxygen, nitrogen, hydrogen etc.

The plastics are being used in one form or other in almost all fields of engineering. The great variety of various physical, mechanical and chemical properties of plastics have found wide application in various fields.

The plastics exhibit some properties as mentioned below.(i) Plastics are light in weight. The specific gravity of plastics ranges between 1 to 1.24.(ii) Plastics act as good thermal and electrical insulators. These materials have

high resistivity and low dielectric constant which made them better insulators.(iii) Plastics have good machinability as compared to metals. (iv) These are highly corrosion resistant materials.

(v) Plastics are highly chemically inert.(vi) Plastics have low ther

mal expansion coefficient.

(vii) Some plastics have high transparency. So they can substitute glass, lens etc. (viii) Plastics can be easily moulded into the desired shape.(ix) The plastics are less brittle than glass but are equally transparent. (x) Some plastics have good dimensional stability.(xi) Some plastics have good shock absorbing capacity.(xii) Plastics are attractive in nature. They can take a variety of colours which do not fade easily. (xiii) Plastics of certain special category are tough. For example, fiber reinforced plastics are tough.

However, following are some limitations of plastics :(i) Generally, the plastics are softer as compared to metals. The tensile and

compressive strength of plastics is lower than the metals.(ii) Plastics have low melting point and have low heat resistance. (iii) Plastics can deform under load easily.(iv) Plastics have poor ductility.

5.1.1. Additives in Plastics

Additives are substances mixed with monomers to obtain desire properties in polymers. Following are some commonly used additives :

S.No. Additives Examples

1 . 2. 3 . 4. 5 .

Fillers Plasticizers DyesBinders Catalyst

Wood, mica, glass powder Camphor, resins, organic solvents Metallic oxidesCellulose derivativesBenzoyl peroxide, calcium phosphate

1. Fillers : These are the materials which are added to plastic to dilute them and reduce their consumption, thus reduces material cost. Fillers are added to improve strength dimensional stability, heat resistance and finish of plastics. These reduces brittleness and shrinkages. Wood, cotton fibres, mica, paper and pulp are some common examples of fillers.

2. Plasticizers : The properties like plasticity, flexibility and toughness are improved by adding plasticizers. These act as internal lubricants. Oleic acid and camphors are the examples of plasticizers.

3. Dyes and Pigments : To provide colour to the plastics, the dyes and pigments are used. The dyes and pigments must be able to mix uniformly throughout the molten plastics and they should be stable at high temperature.

4. Binders : These are natural or synthetic resins to bind various additives of plastics. Cellulose derivatives are good binders.

5. Catalysts : Catalysts are added for faster polymerisation. These are also called accelerators and hardners.

5.1.2. Classification of Plastics

On the basis of thermal characteristics, the plastics are classified as thermoplastics and thermosetting plastics.

(1) Thermoplastic MaterialsThe plastics which can be softened again and again by repetitive heating are called

thermoplastics. Such plastics can be heated and reshaped many times. On cooling, they become hard. These plastics are also called cold setting materials. Thermoplastics are linear polymers. They contain linear or branched long chain molecules which are not interconnected. The long molecular chains are not bonded together. They have high inherent plasticity. Plasticity increases as the temperature increases. They do not undergo chemical changes when they are heated. So, they can be reshaped while in softened state and can be rehardened. Polyethyline, P.V.C. nylon, polypropylene are the examples of thermoplastics.

(2) Thermosetting PlasticsThe plastics which acquire a permanent shape when heated and pressed, and can not

be softened by reheating are called thermosetting or heat setting plastics. Thermosetting plastics are made from chains which have been linked together. They have three dimensional network of molecules. The long molecular chains are not only interlinked but also bonded together by additional covalent bonds. Hardness, rigidity and heat resisting properties are due to cross linked network structure. Under the influence of heat and pressure, these plastics undergo chemical changes of condensation and polymerisation to form a rigid final shape, which is unaffected by heat. These plastics are practically insoluble, fire proof and usually hard and brittle. These plastics can not be reused. Phenol formaldehyde, melamine formaldehyde are the important thermosetting plastics.

S.No. Plastics Method of Manufacturing Characteristics

1 .

2.

3 .

4.

5 .

Thermoplastics : Polyethylene

Polyvinyl chloride

(P.V.C.)

Polystyrene

Polyvinylidine

Nylons

Addition polymerization of ethylene monomer.

Addition polymerization of vinyl chloride monomer.

Polymerization of styrene with butadine.

Addition polymerization of vinyl chloride.

Condensation of amine and organic acids.

Low density polymer

Low tensile strength

High dielectric strength

Chemically resistant

Alathon, Hi-fax are the trade names

Used for packing material, bags and wire coating.

Low strength

Can be made into sheets

Good insulating properties

Water resistant

Used for making pipes, rain coats, table clothes, tanks etc.

Tygon, saran are the trade names.

Transparent

Good insulating properties

Good mouldability

Resistance to chemicals and moistures

Resolute, styron, myler are the trade names

Better strength

Temperature resistant than PVC

Acid resistant tubes, gaskets are made from this

Floor coverings, seat covers are made from this

Good abrasion resistance

Tough, strong but flexible also

Resistant to moisture

Good mechanical properties

Trade name zytel, plaskon

Used in bearings, handles, gears etc.

COMPARISON OF THERMOPLASTICS AND THERMOSETTING PLASTICSFollowing table shows the comparision of properties of thermoplastic and thermosetting plastics.

Difference Between Thermoplastic and Thermo-setting Plastics.

S.No. Properties Thermoplastic Thermosetting Plastics

1 . 2.

3 .

4.

5 .

Strength Polymer structure

Effect of heat

Re-usability

Supplied formExamples

Use :

Softer and less strongLinear Structure

Softned between 70 to 90°C Can be reused after remelting

Granular material Polythenes,Polystyrene Polyvinyl chloride,

Hoses, Electrical Insulation, toys, insulating tape

More strong and hard. Cross linked or three dimension structure. Do not soften easily.Can not be reused and recycled.

Partially polymerised form Phenol formddehyde, Polysters, melamina silicones.Telephone receivers, electric plug, radio, T.V. cabinets, camera body, switch pannels etc.

5.1.3 Plastics as an Engineering MaterialNow a days, plastics are widely used for industrial and domestic applications. In building industry, they are used for manufacturing handles for doors, windows and outer sculptures. In electrical field- switches, insulation, telephone receivers, radio and television receivers are made of plastics. Common household items as soap case, furniture, combs, buckets etc. are made of plastics ; rope and brushes are made of nylons. Toothbrushes, typewriter keys, handles are made of cellulose. Gears, bushings are made of nylon. Pressure vessels are made of polyactals. Pumps and grills are made of polyphenyline

sulphide. Pipings, sheets, cable insulation and footwears are made of polyvinyl chloride. Fuse boxes and jump holders are made of phenolics. Pipe fitting made of cast iron are replaced by P.V.C. Jute made ropes are being replaced by nylon rope. Old canes made of tin plates are also replaced by plastics.

S.No. Plastics Method of Manufacturing Characteristics

6.

7 .

8 .

9 .

10.

Polyteta fluoroethylee

(PTFE)

Thermosettings:

Phenolics (Phenol formaldehyde)

Expoxies

Melamine

Melamineformaldehye

(M.F.)

Addition polymerization of tetrafluoroethylene in the presence

of oxygen as catalyst

Condensation polymerization of phenol and formaldehydes.

Condensation polymerization epichlorohydrine & bisphenol.

Condensation polymerization of dicyanidmide with formal- dehyde.

Copolymerization of melamine and formaldehyde.

Teflon is the trade name Completely inert

. Extremly stable It does not soften at high temperature Low coefficient of friction Good electrical insulation properties Used for non stick coating, anticorosive

seals and high temperature electronic parts

Bakelite is the trade name Strong and rigid plastics Dimensional stability Used for auto distributors, telephones

Araldite is the trade nameUsed as adhesives Excellent adhesive properties Superior toughness High bond strength

Resistant to chemicals and water Heat resistant Used for distributor housing electrical

items as plugs, switches etc.

Melmac, resimena are the trade names. Good tensile strength Used for crockery and automobile parts Heat and moisture resistant

Various Trade Names of Engineering Plastics And Their Applications

S.No. Chemical Name

Trade Name Type Applications.

1 .

2 .

3 .

4 .

5 .

Poly ethylene

Cellulose acetate

Cellulose nitrate

Polyvinyl chloride(P.V.C.)

Polyvinyl butyle

Polythene, Alathon

Fibe stos, plastacele, Lumanth

Celluloid, Nitron, Pyralin

Geon, Koro seal, Tygon

Butacite, Saflex.

Thermoplastic

Thermoplastic

Thermoplastic

Thermoplastic

Thermoplastic

Nursing bottles, ice cube trays, developing trays.

Packing, light cases, radio panels, toys.

Fountain pens, tooth brush,handles.

Cable jackets, insulation, fabric coatings etc.

Rain coats, tanks and flexible moulded articles.

S.No. Chemical Name

Trade Name Type Applications.

6 .

7 .

8 .

9 .

.10

11.

Acrylates

Polysters

Melamine formaldehyde

Urea formal dehyde

Phenol formaldehyde

Phenol furfural

Plexiglas, Acryloid, Lucite

Duraplex, Beckosol

Resimene, Ufromite, Melmac

Plakson, Beetle

Bakelite,Durite, Resinox, cataline.

Durite

Thermosetting

Thermosetting

Thermosetting

Thermosetting

Thermosetting

Thermosetting

Showers, doors, gauge covers, transparent model, doors.Can bodies, surface coating castings.

Telephone parts,enamel, circuit breakers

Buttons, light fixtures.

Grinding wheels, rotating parts, castings, laminated products.

Brake linings, binder for wheels,etc.

5.2. Industrial Use of Plastics :

In modern days, plastics are replacing metals and alloys due to their extraordinary properties. Following are some important applications of plastics :(i) Electrical insulation(ii)Floorings. (iii) Pipes(iv) Sheets(v) Paints and varnishes(vi) Decorative mouldings(vii) Wall tiles and floor tiles.

The plastics possess a combination of properties which makes them preferable to other materials. The plastics are extensively used in industries such as automotive, electrical appliance etc.

The industrial uses of plastics can be explained as under—(i) Polyethylene is used for packaging, paper coating, wire coatings etc.(ii) Polyvinyl chloride (PVC) is used for flexible tubes, wall coverings, wire

insulation, pipes etc. (iii) Polypropylene is chemically resistant and is used for battery cases. (iv) Polyesters are used for making textile fibres.(v) Polyamides (nylons) have good mechanical properties, so they are used for making gears,cams, bearings etc.

(vi) Polyvinyl acetate have adhesive properties and film forming properties. These are used as adhesives.

(vii) Bakelite have excellent thermal stability, chemical and electrical insulating properties. It is used for making electrical components.

(viii) Polyesters have excellent thermal stability. These are used for making fiber glass, helmets, automobile body components etc.

(ix) Polytetra Fluoro Ethylene (PTFE) is chemically inert and have good insulating properties.These are used for making high temperature electrical components.

(x) Polycarbonates have good toughness and impact strength. These are used for structural applications.

(xi) Polyvinylidene is used for making molded parts because it has very good mouldability.

5.3. Processing of plastics :

The plastics are generally available in the form of powder, granules etc. They are processed to form useful shapes by using various methods as compression moulding, injection moulding, extrusion and casting. The method selected depends upon the shape and size of the part to be formed, the number of pieces to be made and type of material to be moulded. Compression moulding is most commonly used for thermosetting plastics. The injection moulding is widely used for thermosetting plastics. Die casting, extrusion, blow moulding calendering are some other important processes.

5.3.1. Injection Moulding

Injection moulding is used for thermoplastics. In this operation, the plastic material in the form of granules or powder is fed into the heating chamber through hoper in a measured amount at definite time interval. The material is heated and melted in this zone. The softened plastic material is pushed through nozzle by hydraulically driven piston or ram operated by screw, into the mould. The mould is made in two sections so that the finished component may eject easily. One part of mould is movable and another is stationary. The mould is cooled by circulating water to solidify the molten metal into final shape. The pressure on the material is maintained until it is solidified. After some time, the mould is opened and the solidified component is ejected outside. The injection moulding machine is as shown in fig. 5.1.

Fig. 5.1. A ram or plunger type injection molding machine

The injection moulding machine have following components.(i) Feed hoper (ii) Heating chamber(iii) Nozzle (die) (iv) Ram(v) Torpedo

Th plastic material which has to be injected is first heated to remove moisture. Then it is fed into hoper. From hoper, it is transferred to the heating section by a freeding device, where the temperature is raised to 150°C–370°C. After closing the mould, the ram is moved forward applying pressure behind the plastic material. The softened material is fed into the water cooled mould through the nozzle. Torpedo is used in order to push and spread the plastic material uniformly inside the mould. The cool water circulating around the mould abstract heat from molten plastics material which is forced into the mould. After the material has been cooled and sufficiently hardened in the mould, the mould is opened and the produced part is ejected out from the mould.

Advantages : Following are the main advantages of injection moulding :(i) This method is suitable for mass production as the manufacturing rate is high.(ii) By the use of injection moulding machine, wide range of shape can be made

and different plastic materials can be molded.(iii) The accuracy of the produced component is high. (iv) The material losses are less.

5.3.2. Compression Moulding

Compression moulding is used for thermosetting materials. The mould have two halves known as upper mould and lower mould. The lower half have cavity of specific shape in which the upper half sets in forming the gap which is the shape of the component to be made. In compression moulding, the plastic material is placed in the lower mould. It is subjected to heat and pressure simultaneously. The polymer material is compacted between the two moulds. The material is placed and forced inside the cavity and excess material is squeezed out. After specific curing time, the mould is opened and the component is removed. The compression moulding machine is as shown in fig. 5.2.

Fig. 5.2. Compression moulding machine

It consist of following parts : (i) Hydraulic plunger(ii) Mould base(iii) Mould with heating arrangement(iv) Guide pins(v) Mould plunger

The measured amount of plastic material in the form of powder is placed in the heated lower mould and is compressed at suitable pressure. The lower mould is continuously heated by electricity. The mould is raised so that the lower mould contacts the upper mould. The moulding temperature and pressure can be upto 180°C and 535

kgf/cm2 respectively. The combined effect of temperature and pressure causes the plastic material to flow into the mould cavity. Hydraulic press is used to provide pressure. The press stays closed while the heat hardens and sets the plastic material. This is called curing. The curing time depend upon material and design of components to be produced. Handles, container caps, electrical components, electronic components etc. are made by this process.

5.3.3. Transfer Moulding

Transfer moulding is a modified form of compression moulding. This method is developed to avoid the disadvantages of compression moulding. More intricate shapes and components of better quality (as compared to compression moulding) can be made by this method.

In this method, the heat and pressure are applied to the plastic material separately outside the mould in the transfer chamber. When the plastic material becomes fluid, it is transferred to the mould under pressure.

Fig. 5.3. Transfer molding

Following are some advantages of transfer moulding :

(i) The plastic materials acquire uniform temperature in the transfer chamber prior to transfer to the mould cavity.

(ii) The intricate mould cavities can be filled properly. (iii) The plastic materials acquire uniform properties.

5.3.4. Blow Moulding

This method is used for making hollow components as bottles, toys etc. In blow moulding process, a hot softened thermoplastic tube known as parison is placed between two piece hollow

mould. Blowing pin is placed at top internal side of parison. The parison is blown by passing compressed air through the blowing pin. The hot parison forms the shape of the mould cavity with hollow interval cavity as shown in fig. 5.4.

Fig. 5.4. Blow-moulding processAir is injected into parison to force the plastic against the wall of mould. The air

pressure ranges from 0.7 to 10 kg/cm2.The thermoplastic materials such as polyethylene, polycarbonates, PVC, nylon,

ABS etc. can be blown moulded. This method is widely used for making soft drink bottles.

5.3.5. Extrusion Process

The extrusion process is used to make solid rods, pipes, tubes etc. from thermoplastic materials. The thermosetting plastics are generally not suitable for extrusion. The extrusion process is similar to injection moulding except the die assembly. A revolving screw is used to push the molten plastic material through the die. The material is fed from hopper through a port in the cylinder. The rotation of the screw imparts both axial and rotary motion to the plastic material. The heating elements provide heat to the plastic material. This molten material is forced by screw into a heated die. The continuous lengths of extruded sections can be produced by this method.

The simple extrusion machine is as shown in fig. 5.5.

Fig. 5.5. Simple extrusion machine

The main parts of extrusion machine are as under—(i) Main cylinder (ii) Hopper(iii) Screw (iv) Die assembly.The plastic material, in the form of granules or powder is fed into the main

cylinder through hopper. In the cylinder, the archimedean screw pushes the material into the heating zone. The plastic material is softened in the heating zone and is compressed by the working of the screw and then it becomes flowy in metering zone. The pressure build up in this zone pushes the plastic material to pass through die. Different shapes can be produced based on shape of the die assembly used. The extruded material emerging from the die assembly is quite hot and it is cooled immediately using cold water spray or cool air.

5.3.6. Casting

The casting of plastics involves only heating and pouring the molten metal into mould and no pressure is used. Thermosetting materials as phenolics, epoxy and polyesters and thermoplastics such as PVC, acrylic can be cast. Since the viscosity of the thermoplastic materials can not be reduced sufficiently by raising the temperature, so the gravity die casting is not very much suitable for plastic material. But nylon can be cast by this method. In die casting method, the plastic material in molten state is poured into a die of specific shape in order to obtain the components with the required shape. After cooling, the solid component is taken outside from die. Epoxies, phenolics and acrylics are used in die casting method. Various other casting methods are also used. The thinner plastic sheets are produced by using film casting equipment as shown in fig. 5.6.

Fig. 5.6. Film-casting equipmentIn this process, the liquid plastic material is allowed to fall in a controlled rate

over a metallic belt. The belt moves in the form of conveyer belt. The film is produced by the vapourisation of the solvent present with plastic materials.

Hollow shapes can be made by certrifugal casting of the molten plastic material which is another casting method very commonly used in potting. In this method, the plastic material is cast around an electrical component. The component gets embedded in the plastic material.

5.3.7. Calendering

Calendering is the process of producing thin sheet or film by squeezing a thermoplastic material between the rotating rolls. The rolls are kept at a specific spacing. The material (thermoplastic) is heated before it is fed into the rolls. The molten metal is compressed through these rolls. The thickness of sheet depend upon spacing between the rolls. Before the film is wound, it passes through water cooled rolls. The set up is as shown in fig. 5.7.

Fig. 5.7. Calendering machine

Polythene sheets, cellulose acetate films and sheets are made by this process.

5.3.8. Thermoforming or Vacuum forming process

The thermoforming process involves the conversion of plastic sheets to articles of desired shape. In this method, the thermoplastic sheet is heated to softening temperature (55° to 90°C). This sheet is then pressed between two halves of the mould. It is as shown in fig. 5.8.

Fig. 5.8. Schematic diagram explaining thermoforming process

The vaccum is also used through small holes in the die or mould. Due to the application of vaccum, the plastic sheet is pulled rapidly and tightly against the mould acquiring the shape of the mould. This is as shown in fig. 5.9.

A wide variety of products can be made by the use of this method. Containers, refrigerator inner panel and containers for packaging are made bythis process. The materials like PVC, ABS, polythenes are used in this method to form the components of the desired shape.

Fig. 5.9. Vacuum forming process

5.4. Potential and Limitations of use of Plastics

In modern days, plastics are replacing metals and alloys due to their extraordinary characteristics. There are a great number of special properties that plastics exhibit in comparison to metals and alloys. Plastics have replaced many metals and alloys due to their specific properties. Nylon is used as a substitute of aluminium, PVC is used as a substitute of cast iron, PP is used as a substitute of glass etc.

Following are some factors which have increased the potential of plastics as an engineering material : (i) The basic raw materials used for plastic materials are readily available in large quantity and

at low cost.(ii) The plastic materials are very easy to mould to get any desired shape and

they can be machined very easily.(iii) The mechanical, electrical and thermal properties of plastics can be controlled

easily by a using additives, binders, lubricants etc.(iv) Most of the plastic processes can be automated and are suitable for mass production.(v) The plastics have good chemical stability, when subjected to action of solvents

and certain oxidizing agents.(vi) In mass production, plastics may be used as a substitute of ferrous and non-ferrous materials.

This reduces the production cost upto large extent.

(vii) From production point of view, the main factor which is responsible for good potential of plastics is relatively low melting point and their ability to flow into a mould.

Following are some limitations of plastics to be used as engineering material :— (i) Most of the plastic materials are not capable to withstand high temperature. (ii) The plastics can deform under load easily.(iii) Many thermoplastics are soft and cannot bear heavy load.(iv) Tensile and compressive strength of plastics is lower than the metals. It ranges

between 1.0 to 8.0 kg/mm2.(v) The plastics have poor ductility.These limitations can be overcome by providing specific properties to the

plastic materials. Some plastics are very tough and have less strength. The strength of plastics can be increased by cross-linking of monomers. Fibre reinforced plastics are used where high strength is required.

The plastics find important applications for commodity products such as textiles, tires, packaging etc. Thermosets are widely used for composite applications in automotive, marine, and aerospace industries. There is great potential of plastics in new technology also. Plastics have important applications as artificial organs.

Some plastic materials are made to be electrically conductive and offers great potential for the semiconductor industry and as lightweight components used in automotive and aerospace applications.

Examina t ion Ques t ions ( so l ved )

Q.1 . What are the polymers ?Ans. The term ‘polymer’ is derived from the two Greek words : poly, meaning

“many” and meros, meaning ‘parts’ or units. Thus polymer are composed of a large number of repeating units. The repeating units called monomers are joined by the reaction known as polymerization. Thus a polymer is made up of thousands of monomers joined together to form a large molecules. The polymers can be divided into three groups : plastics, fibres and elastomers.

Q.2.What do you mean by polymerization?

Ans. The process of linking together of monomers to obtain polymers is called polymerization.

When two or more chemically similar molecules are polymerized to form a long molecular chain, the process is called addition polymerization. When two or more chemically different monomers are polymerized to form a long molecular chain, the process is called co- polymerization. The process in which two or more chemically different monomers are polymerized to form a cross-link polymer, along with a byproduct as water or aminonia is called condensation polymerization.

Q.4. Give the names of different additives used in polymers.Ans. The additives are used in polymers to provide specific properties. Following

are some additives—(i) Fillers—Wood, mica, glass powder.(ii) Plasticizers—Camphor, resins, organic

solvents. (iii) Dyes—Metallic oxides.(iv) Binders—Cellulose derivatives. (v) Catalyst—Benzoyl peroxide.

Q.5. What are the functions of additives ? Ans. The main functions of additiveare :

(i) Fillers are used to dilute the polymers. These are added to improve strength and provide the dimensional stability.

(ii) Plasticizers are used to improve the properties like toughness, flexibility and plasticity. (iii) Dyes provide colour.(iv) Binders are used to bind various additives of plastics. (v) Catalysts are added to make the polymerization faster.

Q.6. What are the important properties of the plastics which make them suitable for large number of engineering uses ?

Ans. The plastics are widely used for large number of engineering applications due to some specific properties like—

(i) Plastics are light in weight.(ii) Plastics act as a good thermal and electrical insulator.(iii) Plastics are highly corrosion resistant as compared to metal. (iv) Plastics have resistance to chemical attack.(v) They can be manufactured and processed easily.

Q.7. Give some important applications of plastics.Ans. Some important plastic materials and their applications are as under :—

(i) Nylon (polyamides) are used for gears, bushings and some engine parts.(ii) Polyvinyl chlorides (PVC) are used for piping, sheets, cable insulation, flooring etc. (iii) Polyacetals are used for pressure vessels.(iv) Alkyds are used for circuit breakers.(v) Vinyl esters are used for tanks, ducts etc.(vi) Polyethylene is used for packaging and paper coating.(vii) Polytetra Fluoro Ethylene (PTFE) or teflon is used for high

temperature electrical parts.(viii) Phenolics or bakelite is used for electrical components.

(ix) Myler is used in magnetic recording tape. (x) Polyvinyl acetate is used as adhesive.

Q.8. Discuss some limitations of plastics.Ans. Following are some limitations of plastics :

(i) Plastics have low melting point or softening point. (ii) Plastics have poor ductility.(iii) Plastics can deform under load easily. (iv) Many thermoplastics are soft.

Q.9. What is the difference between thermosetting plastics and thermoplastics ?Ans. The thermosetting plastics, when once hardened and set, do not soften with the

application of heat while the thermoplastics can be repeatedly softened by heat and hardened by cooling. The thermo-setting plastics are more stronger and harder than thermoplastics. The objects made by thermosetting resins can be used at comparatively high temperature. The thermoplastics are comparatively softer and less strong. The objects made by thermoplastics can not be used at comparatively higher temperature.

Q.10. Compression moulding is applicable to what kind of resin ? Give the valve of moulding temperature and moulding pressure for thermosetting materials.

Ans. Compression moulding is applicable to thermosetting materials. The moulding temperature

of thermosetting materials ranges from 150°C to 180°C and the moulding pressure up to535

kgf/cm2.

Q.11 . Define the following terms—(a) Calendering (b) Blow moulding.

Ans. Calendering is the process in which the molten metal is compressed through rotating rollers kept at a specific gap to produce thin films or sheets.Blow moulding is used to produce hollow components. In blow moulding process, a hot softened thermoplastic tube known as parison is placed between two piece hollow mould. The parison is blown by passing compressed air through the blowing pin which is placed at top internal side of parison.

Q.12. What do you mean by film forming ?Ans. In this process, the liquid polymer solution is allowed to fall in a controlled

rate over a polished metallic belt moving in the form of conveyer belt. The film is produced by vaporisation of the solvent present along with polymer. The polymer film is striped off and wounded up.

Q.13. What do you mean by laminating of plastics ?Ans. Laminating is the process of bonding together the variety of materials by heat

and pressure together. The materials which are to be bonded are first coated with thermosetting resin and then combined together under heat and pressure to form the commercial product. The laminated materials are hard, strong and impact resistant.

Q.14. Comment on machining of plastics.Ans. Most plastic materials can be machined by using metal-working tools and

machines. The cutting fluid is used to keep the heat produced during machining to minimum value. In machining plastics, the light cuts and high cutting speed with low feed rate should be used. Turning, drilling, reaming, tapping, threading etc. are some operations which can be done on plastic materials while machining.

Q.15. What do you mean by reaction injection moulding, solid state forming and cold forming of plastics.

Ans. Reaction Injection Moulding (RIM) : This method is different from conventional injection moulding. In this process, a mixture of two or more monomers is forced into the mould cavity instead of molten polymer. The chemical reaction takes place between the constituents of the mixture giving off heat to form a plastic polymer, which solidifies, producing a thermoset component.Solid State Forming : In this process, the temperature of the polymer is just 10° to 20°C below the melting point of polymer. The operations involved are sheet metal techniques as bending, stretching, deep drawing etc.Cold Forming : Cold forming, involves operations like cold drawing, cold pressing, cold rolling etc. Filaments and fibres are produced by cold drawing. Rolling is also used for producing fibres. In cold pressing, the thermosetting materials are put into the mould and pressed to shape at room temperature. The formed part is removed from the mould and cured in an oven.

Q.16. Give the names of various joining processes used to join plastics. Ans. The methods used to join plastics are :

(a) Hot gas welding. (b) Friction welding. (c) Ultrasonic welding.

(d) Hot sealing.Fill in the blanks

1. Bakelite is the trade name of________2. Thermoplastics have structure as under________3. The specific gravity of plastics varies from_______4. Bottles are made by the process_______5. Mylar is the trade name of_______6. The process of making plastic products without application of pressure is

called______7. Compression moulding is most suitable for_______8. The method used to increase the strength of plastics is ________9. The process of linking of monomers together is called________10. Thermoplastics are the plastics which can be________

ANSWERS

Fill in the blanks 1. Phenolics 2. Linear 3. 0.9 to 0.3 4. Blow moulding 5. Polysters 6. Injection Moulding 7. Thermosetting mateials 8. Cross linking of manomers 9. Polymerization 10. Re-moulded

True/False questions

1. Bottles are made by the process blow moulding.2. Thermoplastics have structure as under cross-linked.3. Mylar is the trade name of polysters.4. Bectle is the trade name of PVC5. Araldite is the trade name of vinyls.6. Thermoplastics are the plastics which can be re-moulded.7. Thermosetting plastics are materials which can be re-moulded.8. The ratio of molecular weight of polymer and molecular weight of monomer is

called degree of polymer.9. The plastic sheets or films are made by process of extrusion.10. Plastic material are bio-degradable.

ANSWERS

True/False Questions

1. True 2.False 3. True 4.False 5.False 6.True 7.False 8.True 9. False 10. False

Objective Type Questions

Multiple Choice Questions

1 . Thermoplastics are the plastics which can be(a) moulded only once (b) re-moulded(c) moulded twice only (d) None of the above.

2 . Thermoplastics have structure as under(a) linear (b) cross-linked(c) complex (d) None of the above.

3 . Thermosetting plastics are materials which can be(a) moulded once and cannot be remoulded(b) can be remoulded (c) not affected by heat (d) None of the above.

4. The specific gravity of plastics varies from(a) 0.9 to 3.0 (b) 3.0 to 6.0(c) 6.0 to 9.0 (d) None of the above.

5 . The process of linking of monomers together is called—(a) diffusion (b) polymerization(c) recrystallization (d) None of these.

6. The plastic sheets or films are made by process of(a) compression moulding (b) injection moulding(c) calendering (d) extrusion.

7 . Bottles are made by the process(a) compression moulding (b) injection moulding(c) blow moulding (d) None of these.

8 . Compression moulding is used for(a) thermoplastics (b) thermosetting(c) both (a) and (b) (d) None of these.

9. To increase the strength of the plastics, which of the following method is used ? (a) linear linking of monomers (b) deformation(c) cross linking of monomers (d) None of these.

10 . Bectle is the trade name of(a) urea formaldehyde (b) PVC(c) cellulose acetate (d) None of these.

1 1 . Bakelite is the trade name of(a) urea formaldehyde (b) PVC(c) polysters (d) phenolics.

12 . Araldite is the trade name of(a) phenolics (b) epoxies(c) vinyls (d) PVC.

1 3 . Mylar is the trade name of

(a) polysters (b) phenolics(c) nylons (d) None of these.

14 . The ratio of molecular weight of polymer and molecular weight of monomer is called

(a) degree of polymer (b) atomic number(c) atomic weight (d) None of these.

1 5 . Compression moulding is most suitable for—(a) thermosetting materials (b) thermoplastic materials(c) both (a) and (b) (d) None of these.

16 . The bottles from thermosetting materials are made by(a) compression moulding (b) injection moulding(c) blow moulding (d) extrusion.

1 7 . The process of making plastic products without application of pressure is called(a) moulding (b) extrusion(c) casting (d) None of these.

1 8 . The long plastic rods and tubes are made from the process of(a) moulding (compression) (b) extrusion(c) injection moulding (d) None of these.

19 . Parison is used in(a) compression moulding (b) blow moulding(c) injection moulding (d) None of these.

20. The sheets making procedure in which the thermoplastic material is squeezed between the rolls is called—

(a) laminating (b) blow moulding(c) calendering (d) None of these.

ANSWERS

Multiple Choice Questions :

1 . (b) 2 . (a) 3 . (a) 4. (a) 5 . (b) 6. (c) 7 . (c) 8 . (a) 9. (c) 10 . (a)

1 1 . (d) 12 (b) 1 3 . (a) 14 . (a) 1 5 . (a) 16 . (c) 1 7 . (c) 1 8 . (b) 19 . (b) 20. (c)

E x p e c t e d Q u e s t i o n s

1 . How the plastics are classified ? Differentiate thermoplastics from thermosetting plastics.2 . Discuss important characteristics of thermosetting and thermoplastic materials.3 . What are the commonly used fillers in plastics ?4 . Give the applications of plastics.5 . What do you mean by injection moulding ? Discuss some advantages of injection moulding.6 . Explain compression moulding of plastics.7 . Give the short note on

(a) Extrusion of plastics (b) Casting of plastics(c) Calendering.

8 . Explain following with neat sketch : (a) Injection moulding machine.(b) Compression moulding machine.(c) Calendering machine.

9 . Explain vaccum forming process and blow moulding process.1 0 . Explain compression moulding and transfer moulding.1 1 . Discuss plastics as an engineering material.1 2 . Discuss potential and limitations of use of the plastics.1 3 . Describe various types of thermosetting and thermoplastic materials.1 4 . Discuss the function of the following plastics—

(a) Fillers (b) Plasticisers(c) Stablisers (d) Accelerators.

1 5 . What are the advantages of injection moulding over the compression moulding process of plastics.