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Which Type of Moulding Process Is Suitable for Your Application?

>> Jan 11, 2018

When determining the most suitable type of moulding process to use, you need to take several factors into consideration. Some of the factors include:
  • The volume of parts needed
  • The material needed
  • The weight of your part
  • The geometry and size of your part

Rubber Compression Moulding

The original method of moulding rubber is compression moulding. It is suitable for low to medium production volume and a very useful process for moulding O-rings, seals, gaskets, and large, bulky parts. It is an efficient, commonly used, and economic method for producing many parts, especially low production volumes of medium to large parts as well as higher cost materials. It is only possible for compression moulding to be cost-effective if the following is true:
  • Larger parts are required
  • Extreme hardness or strong rubber bonding of materials is needed
  • It can be applied to bond rubber to metal
  • Only a low quantity is required

Rubber Transfer Moulding

Transfer combines the ease of compression moulding with the advantage of injection moulding. Rubber transfer moulding is a suitable process for moulding intricate parts that require a closed mould, parts that require multiple cavities, bonding rubber to metal parts and if the part’s geometry can cause the cavities in the mould to trap air. Why Transfer Moulding is Better Than Compression Moulding:
  • Simpler and fewer pre-forms since a single pre-form can fil multiple cavities.
  • Tighter control over dimensional tolerance since the mould is closed, which means that excess material spilling out of the cavity parting line during moulding does not hold it open. This ensures that the geometry of the part is kept exact and the parting lines are less noticeable and smaller.

Injection Moulding

Rubber injection moulding was initially an offshoot from the plastics industry back in the early ‘60s, once the initial issues temperatures, (plastics are cooled during moulding while rubber is heated), and pressure (rubber injection requires significantly higher amounts of pressure per square inch of the cavity surface) were overcome. Rubber injection moulding is of 3 main types:

1. Organic Rubber Injection

The first step in organic rubber injection is the efficient preparation of materials. The material is mixed and stripped into continuous strips then fed into a screw that charges a barrel as required with a pre-determined quantity of material. Once the mould is closed, the material within the barrel is injected into mould cavities then cured. Benefits of Injection Moulding:
  • Total elimination of operator placement of pre-forms
  • Moderately faster cycle times than transfer and compression moulding
  • The material is pre-heated by the injection screw before it is forced into the cavities thus reducing the viscosity of the material and letting it flow easily into the cavities

2. Injection or Liquid Injection Moulding (LIM)

It is a process where 2-part liquid silicone compounds are delivered at a constant ratio into a mixer that’s static. The LSR mixture then blends with a platinum cure system then delivered to the injection unit where it is injected via a gate and runner system into the closed mould until it is cured. Once the cycle is complete, the parts are ejected or removed from the cavities and the next cycle starts. Benefits of Liquid Injection Moulding:
  • Optimized cycle times
  • Parts are almost “flash-less”
  • Contamination is limited by automatic closed-loop systems
  • It is perfectly suited to the unique needs of the medical product industry
  • High-quality components that have complex shapes can be repeated cost-effectively in large quantities

3. Thermoplastic Rubber Injection

It is a process that utilises thermoplastic elastomers (TPE) with properties and performance similar to rubber but is processed like plastic. Benefits of using TPE as opposed to thermoset rubber:
  • Recyclable parts and scrap
  • TPEs can be coloured
  • Parts cost less because of thinner and lower density wall sections
  • Simplified processing since no vulcanization or mixing is involved.

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