This document discusses plastics and polymers. It begins by defining polymers as large organic molecules made of repeating units linked in chains. It then classifies polymers as thermoplastics, thermosets, or elastomers. The document describes common thermoplastic and thermosetting polymers and their applications. It also summarizes several common plastic processing methods like injection molding, extrusion, blow molding, and compression molding.

1. BY NORHAZLINA BT. AMON
MECHANICAL ENGINEERING
DEPARTMENT
Topic 6 : PLASTIC

2. INTRODUCTION
What are polymeric materials?
 Polymers are organic materials made of very large molecules
containing hundreds of thousands of unit molecules called
“mers” linked in a chain-like structure (repeated pattern)
 Polymers are characterized by:
 Low density materials (replace metals such as steel, aluminium
etc)
 Versatility in synthesis – processing – properties relationship
 Raw materials and processing are cost-effective
 Recycling is possible and practical

3. Applications of Polymers
PP
Polycabonate
roof
Bottles extrusion
blow molding

4. Classification of
Polymers
POLYMERS
ELASTOMER
(RUBBER)
NATURAL SYNTHETIC
THERMOPLASTICS
PE, PVC, PP, PS
THERMOSETS
Epoxy, phenolic
resins
e.g; wood, cotton, leather,
skin, hair

5. Chemical Composition of Polymers
 Polymers are classified into:
1. Homopolymers
 Only 1 type of repeat unit

6. Chemical Composition of Polymers
2. Copolymers
 At least 2 types of repeat unit

7. Molecular Chain
Thermostetting
polymers
Rubbers
Thermoplastics: PVC, acrylic,
polyethylene
Polyethylene
1.
Linear
2.
Branched
3. Cross-
linked
4. Network
Strength increases1 4
Structure of polymers consists of a large number of
repeating molecules to form a long flexible chain.

8.  Thermoplastics - Linear or branched polymers in
which chains of molecules are not interconnected to
one another.
 Thermosetting polymers - Polymers that are heavily
cross-linked to produce a strong three dimensional
network structure.
 Elastomers (Rubbers) - These are polymers
(thermoplastics or lightly cross-linked thermosets)
that have an elastic deformation > 200%.

10. Thermoplastics and Thermosettings
 Polymers are basically divided into:
1. Thermoplastics
 Solids at room temperature that are melted or softened by heating, placed
into a mould and then cooled to give the desired shape
 Can be recycled
2. Thermosets
 Can be either liquids or solids at room temperature that are place into a
mould and then heated to cure (set) or harden, thus giving the desired
shape and solid properties
 Thermosets cannot be reshaped by heating

11. Thermoplastic Uses Mechanical properties
Polyethylene(PE) Packaging bags, containers, piping,
chemical equipment, coating for
cables and wires.
Tough and flexible at temperatures.
Good dimensional stability. Easily
moulded. Good resistance to common
solvents.
Polypropylene(PP) Moulding for hospital and laboratory
equipment; chemical plant.
Similar properties to polythene,
better heat resistance to chemical
attack. Tough, rigid and light in
weight.
Polystyrene(PS) Refrigerator trays, boxes and many
articles of household hollowware.
Toys, display figures, etc.
A tough, dense plastic-hard and rigid.
Good dimensional stability. Moulds
with high surface gloss.
Thermoplastic materials
PE

12. Thermoplastic Uses Mechanical properties
Polyvinyl Chloride
(PVC)
Mouldings. Imitation leather cloth,
table cloths, raincoats; etc. Piping
and electrical cable covering.
Soft, flexible and rubbery. Good
dimensional stability. Good resistance
to water, acids, alkalis and most
common solvent.
Polymethyl
Methacrylate
(Perspex)
Lenses, dentures, telephones, knobs
and handles. Aircraft windows,
sinks, building panels.
Excellent transmission of light. Strong
and rigid, easily scratched. Attacked
and many organic solvents.
Polyamides (Nylon) Plastic gears, valves, bearings, cams.
Raincoats. Climbing ropes, fishing
lines and textiles.
Very strong and tough. Good
resistance to abrasion. Good
dimensional stability. Deteriorates
with long exposure to outdoor.
Thermoplastic materials

13. Thermoset materials
 Thermoset parts are made from polymer resins that are capable of forming
chemical crosslinks.
 As the number of crosslinks increases, the stiffness of the material also
increases. Thus, many thermosets are typically stiffer and more brittle than
thermoplastics
 The impact toughness can be increases by adding fillers or reinforcements
(also increase strength)
 Thermosets include:
 Phenolics (PF)
 Amino plastics (UF and MF)
 Polyester thermosets (TS)
 Epoxies (EP)
 Thermoset polyamides

14. Thermoset Uses Mechanical properties
Phenol
formaldehyde
Electrical equipment, handles buttons,
radio cabinets, engine ignition
equipment, cheap jewellery,
ornaments.
Filled with fibrous materials to
increase strength. Absorbs water but
resistant to alcohol, oils and most
common solvents.
Epoxides Sold as resins and syrup. Used as
adhesives for gluing metal. Laminates
(with glass fibre): boat hulls, table top.
Commonly used with glass fibre for
increased strength. Good adhesion to
metals. Good resistance to water and
most solvents.
Melamine
formaldehyde
As syrups and resins : surface coating
of papers and textiles , laminates.
Mouldings : electrical equipment, table
ware.
Properties are generally similar to
those of urea formaldehyde but with
improved resistance to heat.
Thermoset materials

15. The advantages of using plastics as
engineering materials:
 Good electrical insulation
 Excellent heat resistance
 Shock – resistant
 Good resistance to attack by acids and alkalis.
 Self-lubrications.

16. The disadvantages of using plastics:
 Low range temperatures operation (150-600 oF).
 Adversely affected by prolonged exposure to
ultra-violet light.
 Not suitable for outdoors.
 Ductile in low temperatures.
 Deteriorate with atmospheric moisture and
temperatures.

17. Processing of Plastics
 Plastics can be formed from granular and pallets into
shape product such as sheet, rods, extruded sections,
pipe, or finished moulded parts.
 Process used depends on whether the plastic is a
thermosetting or thermoplastic.
 Among of the processes:
 Injection moulding
 Extrusion moulding
 Compression moulding
 Transfer moulding
 Blow moulding

18. Injection Moulding
 Widely used process of forming thermoplastic
materials.
 The basic process involves six major steps in the
moulding cycle:
The hopper is loaded with granular plastic
materials.
Heat is applied to the plastic until it becomes soft
enough to flow.
The softened plastic is forced through a nozzle into
the mould cavity.
When cool, the halves of the mould are separated.
The part is ejected from the mould.
Gates connecting the product to the runner system
are removed.

20. Processing Plastics - Molding
 Compression and transfer molding
 thermoplastic or thermoset
Adapted from Fig. 15.23,
Callister 7e. (Fig. 15.23 is from
F.W. Billmeyer, Jr., Textbook of
Polymer Science, 3rd ed.,
John Wiley & Sons, 1984. )

21. Compression Moulding
 This process was the first to be used to form plastics. It involves four steps:
1. Pre-formed blanks, powders or pellets are placed in the bottom section of a
heated mould or die.
2. The other half of the mould is lowered and is pressure applied.
3. The material softens under heat and pressure, flowing to fill the mould. Excess
is squeezed from the mould. If a thermoset, cross-linking occurs in the mould.
4. The mould is opened and the part is removed.
 When thermoplastics are used, the mould is cooled before removal so the part
will not lose its shape.
 When thermosets are used, they may be ejected while they are hot and after
curing is complete. This process is slow, but the material moves only a short
distance to the mold, and does not flow through gates or runners. Only one part
is made from each mold.

22.  This process makes parts of constant cross section like pipes and rods. liquid
polymer goes through a die to produce a final shape. It involves four steps:
1. Pellets of the polymer are mixed with colouring and additives.
2. The material is heated to its proper plasticity.
3. The material is forced through a die.
4. The material is cooled.
Extrusion Moulding

23. Blown-Film Extrusion
Adapted from Fig. 15.26, Callister 7e.
(Fig. 15.26 is from Encyclopædia
Britannica, 1997.)

24. Transfer Moulding
 Transfer moulding is most generally used for
thermosetting plastics.
 Method is like compression moulding in that the
plastic is cured into an infusible state in a mould
under heat and pressure.
 In transfer moulding the plastic is heated to a point of
plasticity before it reaches the mould and forced into
a closed mould by means of a hydraulically operated
plunger resulting in lower cavity pressure.

25. Blow Moulding Process
 The blow moulding of thermoplastic materials
began during World War 11.
 Blow moulding is intended for use in
manufacturing hollow plastic products.

26. Blow Moulding Process
 The basic steps of blow moulding process are:
 Melt the material
 Form the molten resin into a tube or parison
 Enclose the hollow parison in the blow mould
 Inflate the parison inside the mould
 Cool the blow-moulded part
 Remove the part from the mould
 Trim flash, as needed