Picking Out the Right Rubber Moulding Process

Before picking out the ideal moulding process offered by different rubber moulding companies, you need to consider a few key factors. These include:

  • The size and shape of the part you’re creating
  • The weight of the finished part
  • The total volume of your parts
  • The material requirements

Rubber Compression Moulding

Compression moulding is the oldest, most refined mode of production available for working with rubber. This process is best suited to low to medium-volume applications; parts which are frequently created this way include gaskets, O-rings, seals, and parts which are large and bulky. Because compression moulding is economical, efficient, and widely available, it’s a smart choice for use in creating small runs of larger parts and for products that require higher-cost materials.

Compression moulding can be the cost-effective choice if one or more of the following cases apply:

  • The total number of parts required is small
  • Part size is large
  • Material hardness is a requirement
  • The final application of the parts involves rubber to metal bonding

Rubber Transfer Moulding

Transfer Moulding is a “hybrid” process which offers many of the same advantages as both compression moulding and injection moulding. Rubber transfer moulding is especially suited for use when the parts being created are intricate, have multiple cavities, require rubber to metal bonding, or are shaped such that mould cavities trapping air needs to be avoided.

Transfer moulding usually offers the following advantages over compression moulding:

  • Because a single pre-form can fill multiple cavities, the overall pre-form requirements are simpler.
  • Dimensional tolerances are tighter. This is because the mould is closed, precluding the possibility of letting excess material escape through cavity parting lines. The finished geometry of the parts produced is more exact and parting lines are less noticeable on the finished part.

Injection Moulding

Injection moulding was originally developed for plastics; it made the jump to rubber parts in the early 1960s. The process needed to be adjusted for the unique characteristics of the material: rubber heats up while it’s being moulded where plastics cool down, and rubber also requires a significantly larger amount of pressure on the moulding cavity surface.

Rubber injection moulding processes fall into 3 categories:

Organic Rubber Moulding

The organic rubber injection process starts with efficient material preparation. After mixing, the moulding material is converted into continuous strips and conveyed into a loading barrel through a screw. After the mould is closed, the barrel’s contents are injected into the cavities in the mould.

Benefits of organic rubber injection moulding:

  • Operators do not need to place pre-forms by hand.
  • Cycle times are somewhat quicker than transfer or compression moulding.
  • The material is pre-heated in the injection screw before injection. Making the material less viscous leads to faster, more even flow into the moulding cavities.

Liquid Injection Moulding (LIM or LSR)

In this moulding process, a two-part silicone compound is combined in a static mixture. This LSR material is run through a platinum cure system before being delivered to the injection unit and injected into the mould’s runner and gate system. The production cycle ends once the material is cured and the newly-formed parts are ejected from the moulding cavities.

The benefits of LIM moulding include:

  • Automated, closed systems minimize contamination.
  • Cycle times are optimized.
  • Parts are produced with minimal flash.
  • Even complex components can be produced in large quantities affordably.
  • LIM is particularly suited to creating medical products.

Thermoplastic Rubber Injection (TPR or TPE)

This moulding process uses thermoplastic elastomers, or TPE, as the materials of choice for creating parts. The advantage of TPE as a material is that its finished properties are very similar to rubber but it can be handled and processed like plastic during the manufacturing process.

Benefits of TPE compared to thermoset rubber:

  • Processing is simpler – no mixing or vulcanization required.
  • Lower density and thinner wall sections translate to lower part costs.
  • Scraps and parts are recyclable.
  • TPEs can be coloured