Commercial processes for polymers

Injection moulding forces molten thermoplastic into a closed metal mould under pressure. The shaped space inside the mould is called the mould cavity. It determines the form of the component, allowing products such as bottle caps, clips, toys and electrical casings to be made repeatedly.

The process follows a repeating cycle:

1. Polymer granules enter the heated barrel from a hopper, a container that feeds material into the machine.
2. A rotating screw moves the granules along the barrel as they melt.
3. The molten polymer is injected through a nozzle into the closed mould cavity under pressure.
4. The polymer cools and hardens inside the mould, keeping the cavity’s shape.
5. The mould opens and ejector pins push the cooled component out. The mould then closes ready for another cycle.

Ejector pins release the moulding without an operator having to pull it out by hand. Notice that the mould stays closed while the polymer is injected and cooled: it opens only when the component is ready to be removed.

Why injection moulding suits large production runs

The metal mould is expensive tooling: it must be made for the required component. Injection moulding therefore suits mass production, where large numbers of the same product are needed. The cost of the mould can be spread across many components rather than being carried by just a few.

A bottle cap is a useful example. Each cap is a separate three-dimensional component, so a mould cavity can form its shape and the machine can repeat the cycle to produce many matching caps. Other applications include chairs and electrical fittings.

Consistent quality depends on controlling temperature, pressure and cooling time. These settings help the cavity fill properly and the component harden into the intended shape. Poor control can produce defects such as a short shot, where the cavity is not completely filled; sink marks, which are unwanted depressions in the surface; or warping, where the component becomes distorted.

Extrusion: making a continuous profile

Extrusion continuously forces softened polymer through a shaped die. Unlike a mould cavity, a die is an opening that the material passes through. Its profile determines the product’s cross-section — the shape seen if the product is cut across its length.

1. Granules feed into a heated barrel.
2. A screw softens the polymer and pushes it forwards.
3. The polymer passes through the die, taking the required profile.
4. The continuous section is cooled so that it holds its shape, then cut to the required length.

A pipe illustrates the result: it can be cut into different lengths, but its hollow cross-section remains the same along each length. Extrusion is also used for window-frame profiles, rails, sheet, film and cable insulation.

The process is efficient for long, continuous products. Its central limitation is that the cross-section must remain constant along the length; extrusion alone cannot produce a shape that becomes wider, narrower or otherwise changes profile from one point to another.

The die shape, temperature, screw speed and cooling must be controlled to achieve the required dimensions and surface finish.

Choosing between the two processes

Start with the product’s geometry. A separate shaped component, such as a clip or casing, suits injection moulding. A long product with an unchanged profile, such as a pipe or window-frame section, suits extrusion.

Then consider production. Injection moulding repeats a filling, cooling and ejection cycle, and its expensive mould is best justified by making many identical components. Extrusion produces material continuously through the die and is efficient when long lengths of the same profile are required. Both need controlled processing conditions to produce consistent results.

• Separate components made by forcing molten thermoplastic into a closed metal mould under pressure.
• Sequence: hopper → heated barrel and screw → nozzle → mould cavity → cooling → mould opens → ejector pins release component.
• Uses: bottle caps, clips, toys, casings and chairs.
• Expensive mould tooling suits mass production.
• Control temperature, pressure and cooling time to reduce short shots, sink marks and warping.

Extrusion

• Continuous lengths with a constant cross-section made by forcing softened polymer through a shaped die.
• Sequence: granules → heated barrel and screw → die → cooling → cutting to length.
• Uses: pipes, window-frame profiles, sheet, film and cable insulation.
• Efficient for long continuous products; the profile cannot vary along the length.
• Die shape, temperature, screw speed and cooling affect dimensions and surface finish.

Key distinction

Injection moulding: closed cavity, separate mouldings, repeating cycle.

Extrusion: shaped opening, continuous profile, constant cross-section.