On the surface these two moulding processes may seem very similar, however there are distinct differences that should be taken into consideration when making production decisions.

Injection Moulding

In the injection moulding process, the chosen material is introduced into the injection moulding machine via a hopper.  The virgin material starts the process in pellet form. The injection moulding machine consists of a heated barrel from which a reciprocating screw feeds the molten plastic into a temperature-controlled mould.

The screw melts the pellets and acts as a ram during the injection phase. The molten plastic is injected into a mould tool that has been manufactured to the specific design of the moulded part.

Once the part is moulded, the mould opens to release the part. This whole process is then repeated for the entirety of the production run.

After the moulding process, the parts are cooled, before being trimmed/finished if required.

Advantages and Disadvantages of Injection Moulding

Advantages

• Low cost per part on medium to large production runs
• Consistency across the full production run
• Quality control
• Complex and precision detail
• Large choice of material options

Disadvantages

• Higher price per part on small production runs
• Higher initial costs for moulds and set up processes
• Fixed designs
• Weight/thickness/size limitations
• Simple profile details can cause complex tooling issues
• Material shrinkage

Cast Moulding

Cast moulding is a manufacturing process where liquid polyurethane is poured into a heated mould. The material is thermoset and therefore the polymer is heat cured and, when solid, the moulded part is removed from the mould. Generally, moulds are open on one full side which means the poured face usually produces a flat surface. Therefore, some profile details may require additional machining/processing after moulding.

Due to the requirements of heat processing, moulds are warmed on a “hot plate” or within an oven system depending on the target temperatures of the mould/casting material and/or the mould size/profile. This setup/processing requirement is also dictated by the mould material;

• steel tools require higher temperatures and take longer to heat up but they then hold their heat for longer making them better suited to parts which require have longer pouring times and higher batch quantities
• aluminium tools are easier to handle and take less time to heat up but lose temperature more quickly making them better suited to closed cast parts and lower quantities.

Advantages and Disadvantages of Cast Moulding

Advantages         

• Ideally suited to metal bonding and overmoulding.
• More cost- effective tooling.
• Large/heavy parts can be produced.

Disadvantages

• General reduction in quality of surface finish than injection moulding.
• Limited material range in comparison to injection moulding.
• Better to suited to parts with at least one flat face.

Summary

Overall, neither manufacturing process is “better” than the other. The key is understanding each process to enable you to make an informed decision on which suits your specific needs the most.

If you would like any more information or advice, please find more resources on the PolyGlobal website or contact sales@polyglobal.co.uk