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Plastics: Structure and Moulding

Processes

Thermoplastics

Thermosetting

plastics

Process

Addition polymerisation (Ethylene)

·

A catalyst or free radical is added to

break the bonds with the carbon atoms.

·

Carbon atoms now have a free contact point

where they can join with other ethylene mers.

·

A large chain is created (polymer).

·

The reaction finishes when there are no

ethylene molecules or catalyst.

Condensation polymerisation (Phenol

formaldehyde)

·

Formaldehyde is used to join two phenol

molecules.

·

The new bonds cause two hydrogen atoms and

an oxygen atom to be released. This is given off as water vapour.

·

As with addition polymerisation long chains

are created, with the addition that this time the structure is 3D.

Bonding

Covalent

bonding

Secondary

bonds with weak Van der Waals forces. It is these Van der Waals forces which

allow thermosetting plastics to be reformed. When heated these bonds are

temporarily weakened allowing the molecules to move.

Covalent

bonding

Primary

bonds. The structure is held together by 3D cross links. As a result

thermosetting plastics cannot be reformed.

Structure

Properties

Properties

·

Low melting

point

·

Allows for easy

moulding (reformability/fluid)

·

Easily disrupted

by heat

·

Low tensile

strength (Branched structures have higher tensile strength than linear)

·

Ideal for

recycling

Properties

·

High melting

point

·

High tensile

strength

·

Good thermal

insulation

·

High temperature

resistance (very little loss of rigidity)

·

Stiff and less

flexible

Phenol Formaldehyde

Ethylene

Copolymerisation

Copolymerisation uses two

different types of mers which can allow for a huge range of properties. It does

not appear that we need to know the process. An example of a copolymer:

Polyvinylchloride

Compression moulding

Procedure:

1.

Mould is filled or charged with powder/granules.

2.

Embedded heating elements in both halves of the

moulds produce the heat required to melt the charge.

3.

As the two halves of the mould are brought

together the liquid charge is forced fill the mould.

4.

The component is ejected from the mould. Any

excess material formed at edges is removed (flash).

Products: Electrical

plug tops, game controllers

Plastics:

Urea formaldehyde, phenol formaldehyde

Transfer moulding

Procedure:

1.

The powdered charge is poured into the

compartment above the mould. Heating elements surrounding the compartment melt

the charge.

2.

A plunger forces the liquid charge into the mould.

3.

The plastic solidifies and the finished product

is removed.

Products:

Electrical plug tops

Extrusion and Injection moulding

Extrusion

moulding diagram

Injection Molding

Extrusion

Injection

Continuous process used to produce solid and

hollow products.

Non-continuous

process used to inject molten plastic into a mould.

Thermoplastic granules are fed from the hopper via

rotating screw through the heated chamber.

Powdered plastic is

poured into a hopper. The hopper gravity feeds the powder into the barrel. A

rotating screw begins moving the charge to the end of the barrel.

The tapered screw compacts the plastic as it

becomes elasticized.

Heating

elements surrounding the barrel heat the plastic is it forced towards the

mould.

The interchangeable die is fitted at the end of

the extruder barrel determines the cross section of the extrusion.

When enough melted

plastic has accumulated a hydraulic ram forces the screw forward, which

forces the molten plastic into the mould.

Produces 9-60 meters of extruded product per

minute. The thicker the cross section the slower the process.

Pressure

is kept on the mould until the plastic is sufficiently cooled.

As the extrusion leaves the die it is cooled by

passing through cold water.

Note the screw is

called a "barrel" and is usually steel

Product: Window frames, hose pipe

Product:

Toy saxophones, bottle caps

Plastics:

Elastomer (TPE), polythene (LDPE, HDPE), PVC, nylon, polypropylene and

polystyrene

Plastics used:

Polystyrene, ABS, polypropylene,

Nylon, polycarbonate,

Vacuum forming

Step 1

A mould is clamped to a platen.

Thin sheet of plastic is clamped under a heater.

The heater softens the plastic to the point where it

is very flexible.

Step 2

When the plastic has been heated for long enough air

is blow in to raise the sheet slightly.

The mould is forced up into the plastic.

Once at the correct height the mould locks in place.

Step 3

The vacuum pump is turned on and removes the air

pocket underneath the sheet.

Atmospheric pressure forces the plastic to surround the mould.

After the sheet has cooled and hardened the finished

product is removed.

Products:

Garden pond liners, food trays

Plastics:

Polystyrene (HIPS)

Property additives

Lamination

The manufacturing

of a material in multiple layers. Can also increase a material's toughness by

making it more shatter resistant.

Flame retardant

A chemical used to reduce the flammability

of a substance.

Degradation

A decline to

a lower condition, quality or level, or break down or deteriorate chemically

Stabilizers or Antioxidants

A substance which prevents the breakdown of

emulsions. Prevents the degrading effects of heat, UV and oxidation.

Plasticizer

A substance

(typically a solvent) added to a synthetic resin to produce or promote

plasticity and flexibility and to reduce brittleness.

Filler

A filler is a substance added to change a

polymer's properties or reduce the amount o polymer required.

Colour pigments

Give the

plastic the desired colour.

Lubricants

Used to make the plastic easier to mould

(wax and soap are commonly used). Very little can be used or it will damage

the material's properties significantly.

List of Fillers:

Wood fibre - Adds strength and mouldability

Cloth fibre - Adds impact resistance/strength

Whiskers of metal - Adds very high "strength"

(Because that's a fucking useful description)

Mica - Adds electrical insulating properties

Blow Moulding

Procedure:

1.

An extruder feeds a plastic tube called a

"parison" into two halves of a mould.

2.

The mould closes. One of the ends has a piece to

allow airflow into the parison.

3.

Compressed air fills the parison and it cools on

contact with the mould.

4.

The mould opens. The finished product is removed

and the flash is cut off.

Products:

Bottles

Plastics:

High density polyethylene (HDPE), Low density polyethylene (LDPE). Plastics

used must be viscous.

Name: Rotational molding

Product: Road

cones

Define the following

terms:

Thermosetting plastics:

Phenol formaldehyde - Crosslinked

Thermoplastics:

Linear or branched structure

Irrelevant useful

diagram: