PLASTIC

Learning Outcomes;

Define what is plastic

Define the polymeric structure

Differentiate between thermoset and thermoplastic

List advantages and disadvantages of plastics application

Explain the making of plastic products processes

What is Plastic?

Organic polymer (synthetic / semi synthetic)

Moldable – easy to be shaped when heated

Categorized into two;

i) Thermoset plastic

ii) Thermoplastic

Terms in Polymeric Structure

Monomer – single / small molecule

Polymer – many monomer that held together by covalent bond in long chain.

Polymerization- chemical reaction process that produces a polymer from its monomer.

Amorphous – non-crystalline

Types of Polymer Chain

There are 4 types of polymer chains;

a. Linear chain polymers

- in the form of a long molecular chains

b. Branched chain polymers

- the linear chain with a series of branched

c. Cross-linked chain polymers

- with short links which connected the closer chains together

d. Network chain polymers

- molecular structure in 3 dimension networks

Plastic

Thermoplastic

-Polyethylene (PE)

-Polypropylene (PP)

-Polysytrene (PS)

-Polyvinyl chloride (PVC)

- Perspex

-Polymide (Nylon)

Thermoset

- Phenol formaldehyde

-Epoxy

-Polyester

-Urea formaldehyde

-Elamine formaldehyde

Thermoplastics Can be softened every time they are

heated

Can be reshaped / remolded

No chemical changes (curing) takes place during the molding operations

Thermoset

Can be heated and formed only once

Can never again be softened by heating

Undergo chemical changes (curing) during molding

Advantages of Plastic

1. Cheap

2. Easily molded / shaped

3. Corrosion resistance

4. Light weight

5. Transparent

6. Water resistant

7. Reusable & recyclable

Disadvantages of Plastic

1. Brittle at low temperature

2. Produce toxic fumes when its burnt

3. Low heat resistance

4. Softer and less elastic than metal

5. Not biodegradable

Plastic Processes

Injection Molding

Extrusion Molding

Blow

Molding

Compression Molding

Injection Molding Process Work Principles;

i) Resins inserted into hopper.

ii) Resins goes into injection channel while softened by heater.

iii)Softer resin injected into nozzle to the mould cavity by plunger / screw.

iv) Product will be solidify under water cooling system.

v) Lastly, ejector pins will ejected the product from the mould.

Injection Molding Machine

Injection Molding

ADVANTAGES DISADVANTAGES

Good surface finish Sink = caused by molding temperature or the pressure are higher

Lower cost (mold & labor) Flashing = caused by unfitted mould

Various shape, intricate/thinner products can be produced

Bubble = caused by cold mould or the pressure are too low

Process can be highly automated

Weld line = caused by cold mould

Extrusion Molding Process Work Principles;

i) Resins are inserted into hopper.

ii) Resins fed into the heated cylinder by rotating screw.

iii)When resin become softer, it will be forced continuously by the rotating screw ram.

iv)Product are formed into continuous shape.

v) Product cooled by air / water system.

Extrusion Molding Machine

Extrusion Molding

ADVANTAGES DISADVANTAGES

Economical for mass production

Dies are easily wear out

Accurate measurement

Need to be closely controlled

Good surface finish

Thermoset plastic are not suitable

Blow Molding Process Work Principles;

i) Resin inserted into hopper.

ii) Resins goes into injection / extrusion channel while softened by heater.

iii) Softer resin injected / extruded into nozzle to the mould cavity by plunger / screw.

iv) Air then blowed into the parison through a cavity at the top of the mould.

v) Product will be solidify under water cooling system.

vi) Lastly, ejector pins will ejected the product from the mould.

Blow Molding Diagram

Blow Molding

ADVANTAGES DISADVANTAGES

Various shape, intricate /thinner products can be produced

Not economical for small production

Doesn’t need the finishing / tidying

Certain mould only for certain product

Compression Molding Process

Work Principles; i. Resin inserted into the mould as

preformed powdered. ii. The preheated resin is loaded into a hot

mould containing the cavity. iii. The upper part of the mould is forced

down on the plastic resin. iv. Pressure and heat are then applied,

forces the softened resin to fill the cavity.

v. Lastly, product ejected from the mould.

Compression Molding Diagram

Compression Molding

ADVANTAGES DISADVANTAGES

Mould costs are low Complicated shape difficult to make

Economical for mass production

Moderate dimension accuracy

More compact product are possible

Need further finishing (trim the flash)