Rubber and other flexible parts... Should I Injection mould or Compression mould?

Flexible components are present in most assemblies. Whether you want a simple gasket, a seal, a more intricate connector, or a bellows, the common denominator is that the part is required to flex in its operation or assembly.

Material options

Materials that can flex are frequently referred to generically as rubbers. However, there are two basic categories simply differentiated by their processing technique, thermoplastic and thermoset.

Thermoplastic elastomers (TPE’s) are copolymers that are a blend of rubber and plastic, processed by injection moulding or extrusion. Within TPE’s there are six sub sets, which is where names like TPU and TPV come from.

Rubbers are thermosets, processed by compression moulding.

Silicones may be single or two part and processed by compression moulding or injection moulding (LSR).

The decision on whether to injection mould using TPE or to compression mould using rubber is usually a function of quantity and service environment.

Quantity is important

Quantity is important as the cycle times of the two processes are wildly different. Rubber (whether synthetic or natural) is a thermoset and as such must cross link during processing  – which is referred to as vulcanisation. Depending on a variety of parameters, vulcanisation can take from minutes to hours.

In contrast TPE’s are processed on conventional injection moulding machines with cycle times similar to moulding conventional thermoplastics.

Tooling is a requirement

Tooling is a requirement to process either option. A large range of rubbers including silicone, nitrile, neoprene, EPDM and Viton can be moulded in compression tooling. This is relatively simple compared to injection mould tooling and multiple cavities can be used to offset longer cycle times.

In contrast TPE’s require tooling designed and manufactured to injection moulding standards. Material is processed and injected in the same way as ABS or PC would be for rigid parts. It is more sophisticated than compression mould tooling and as a result more expensive.

Thus for a low call off of say 100 parts per year, the lower tool cost and greater cycle time of compression moulding is unlikely to be an issue. As the quantities rise, the number of cavities in a tool can be increased, so a ten, twenty or more cavity tool is possible (geometry permitting).

However, ultimately the additional expense of injection mould tooling and faster cycle time will pay off, and needs to be considered. (material properties being equal)

Environment and exposure

Service environment is also important and both maximum and minimum operating temperatures as well as exposure to solvents need to be taken into consideration. Some rubbers can be better at retaining there elastomeric properties, particularly in high temperature environments and silicones excel in situations where the material must be highly inert.

Sustainability

If you need your component to be recyclable, then thermoplastics are your only option. Rubbers being a thermoset means components are destined for landfill at the end of their life.

Prototyping flexible parts 

Prototyping flexible parts can be a challenge. Whilst it is possible to 3D print materials that demonstrate a level of elongation in a variety of harnesses’, their properties fall well short of production intent materials.* Vacuum Casting can offer a better solution, although more costly, again in a range of harnesses’.

However neither of these options will truly emulate a production TPE or rubber, especially in areas such as elongation, temperature resistance or stiction. Thus if the goal is to create anything more than a visual prototype there is little option but to manufacture a bridge tool and prototype in the production intent material.

Plunkett Associates can support both your prototyping requirements and production requirements, in either class of materials. Drawing on strengths in both tooling and moulding, we will ensure you get to market on time. 

*(Note flexibility can be achieved through additive techniques but tear strength, chemical resistance and elongation to break will be significantly reduced.)