Plastic Process

AS Graphics

Injection Moulding

Injection Moulding

• Description– A highly automated production process for

producing large quantities of identical items– For producing 3D shapes (can be complex)

Injection Moulding

• Materials– HIPS (High Impact Polystyrene) – ABS– HDPE– LDPE

Injection Moulding

• Keywords– Hydraulic system– Mould– Heater– Hopper– Ejector pins

Injection Moulding• Stages– Plastic powder or granules are fed from a hopper into a

hollow steel barrel which usually contains a rotating screw.– The barrel is surrounded by heaters which melt the plastic

material as it is carried along the barrel by the screw towards the mould.

– The screw is forced back as the melted plastic collects at the end of the barrel.

– Once a sufficient charge of melted plastic has accumulated a hydraulic ram forces the screw forward injecting the thermoplastic through a sprue (small channel) into the mould cavity.

– Pressure is kept on the mould until the plastic has cooled sufficiently for the mould to be opened and the component ejected (ejector pins can be used)

Injection Moulding

Advantages• Complex 3D shapes• Excellent surface finish• Variety of materials• Quick• Good for batch and

mass production

Disadvantages• No inverse angles• Expensive set up

Extrusion

Extrusion

• Description– A continuous process used to produce both solid

and hollow products that have a constant cross-section.

Extrusion

• Materials– PVC– PET

Extrusion

• Keywords– Die– Coil– Heater– Hopper– Tubes

Extrusion• Stages– Thermoplastic granules are fed from a hopper by a

rotating screw through a heated cylinder.– The tapered shape of the screw compacts the

plastic as it becomes plasticised. The softened material is allowed to flow out through a die in a continuous stream.

– The die which is fitted to the end of the extruder barrel determines the cross-section of the extrusion.

– As the extrusion leaves the die it is cooled by passing through a cooling trough containing cold water.

Extrusion

Advantages• The length of the part is

potentially infinite• Quick• Good for batch and

mass production

Disadvantages• Parts must be the same

all the way along

Vacuum Forming

Vacuum Forming

• Description– Vacuum forming is usually, though not always,

restricted to forming plastic parts that are rather shallow in depth.

Vacuum Forming

• Materials– HIPS • It is relatively cheap, comes in a wide range of colours

and is easy to form. Typical thicknesses of sheet material used range from less than 1mm up to 6mm.

– Acrylic– PET– PVC– Polypropelene

Vacuum Forming

• Keywords– Platen– Heater– Bow– Air– Vacuum– Draft angle– Vent holes– Rounded corners– Mould (MDF / stainless steel / aluminium)

Vacuum Forming• Stages

– A mould is attached to a platen (bed).– The platen and mould are then lowered and rigid thermoplastic

sheet material is clamped onto an air tight gasket and heated from above.

– Once the thermoplastic sheet is softened enough (i.e. reaches a plastic state) then air is blown in to raise the sheet in a slight bubble before the platen is raised bringing the mould into contact with the plastic.

– Any trapped air remaining between the platen and the heated plastic sheet is then evacuated by a vacuum pump.

– Once the plastic sheet has cooled down to below it's freeze (solid) point the air flow is reversed to lift the forming off the mould. If this is not done quickly the forming tends to grip onto the mould and attempts to prise them apart often result in damage to the forming.

– Trim and finish the edges.

Vacuum Forming

Advantages• Can be inexpensive to

set up (mould can be MDF)

• Good for batch production (stainless steel / aluminium mould)

Disadvantages• Limited to hollow

objects• Thin sheets only

Blow Moulding

Blow Moulding

• Description– an automated process that is used extensively to

make bottles and other lightweight, hollow parts from thermoplastic materials.

Blow Moulding

• Materials– High density polyethylene (HDPE)– low density polyethylene (LDPE) – Polypropelene

Blow Moulding

• Keywords– Parison– Extruded down– Flash– Cold mould– Air

Blow Moulding• Stages– The cycle starts with the mould open. A hollow

length of plastic, called a parison, is extruded down between the two halves of the mould.

– The mould closes and compressed air is blown into the inside of the parison which inflates it, pushing the soft plastic hard against the cold surfaces of the mould. The plastic is cooled by the mould, causing it to harden quickly.

– The mould is then opened (below), the moulding ejected and the waste (called flash) is trimmed off with a knife.

Blow Moulding

Advantages• Quick• Good for batch and

mass production• Complex bottle shapes

can be produced

Disadvantages• Bottles only• Expensive setup cost• No inverse angles

Rotational Moulding

Rotational Moulding

• Description– Rotational moulding is a process used mainly to

manufacture hollow shaped products such as footballs, road cones and storage tanks

Rotational Moulding

• Materials– 90% of rotational mouldings are made from

polyethylene

Rotational Moulding

• Keywords– Mould– Rotated– Plastic powder– Coating

Rotational Moulding• Stages

– Moulds are attached to the end of each arm and are rotated continuously.

– Open mould is filled with plastic powder– Mould is closed and begins to rotate– Mould is heated and the plastic melts, coating the inside (As the mould

rotates its inner surface passes continually through the mass of powder at the bottom of the mould. The powder in contact with the hot metal fuses and sticks to the mould surface. As the mould rotates more plastic melts and sticks to the inner surface finally producing an even layer up to 10mm thick).

– Mould is cooled to set the plastic (Still rotating, the mould moves into a cooling area or chamber where it is cooled by air or water jets).

– Mould is opened and the product removed

– Mould materials:• aluminium• Mild steel • copper (producing very detailed surface finishes)

Rotational MouldingAdvantages• Great flexibility in terms of the shape

and size of product that can be produced. Both symmetrical and asymmetrical designs can be formed as well as complex,

• both tough rigid shapes and for flexible mouldings with deep undercuts such as inflatable rescue craft and toys.

• best suited to making medium to large mouldings.

• Rotational moulding does not involve high pressures and so moulds can be lighter and costs are lower.

• It is one of the few processes in which undercuts in the mould are allowable.

Disadvantages• Hollow shapes only• Not as fast as other

processes

Safety Features of all these processes

• Full guards (1) protecting cutting blades / moving parts (1)

• Light beam (1) which stops operation if the beam is broken (1)

• Emergency stop button (1) to isolate power/stop machine quickly (1)