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Rubber injection moulding & compression moulding

The Difference Between Rubber Injection and compression moulding

When it comes to rubber moulding, there are 3 – 4 different approaches taken to it; one is compression moulding, the others are types of injection moulding. Each approach has its advantages and disadvantages.

Continuous Injection

When most people think of rubber injection moulding, this is what springs to mind. Key differences to other approaches include:

1. Uncured rubber is pulled into an injection cylinder automatically, the rubber is pre-heated and mixed ready for the next injection
2. The mould tool only consists of the moulding elements to the tool as injection is managed by the machine.
3. Normally, people will be used to remove the product and cured rubber in the sprues and channels within the tool, and to load any substrates required.

Single-Shot Injection (Trans-Injection)

This is being increasingly replaced with Transfer Injection. It is very similar to Continuous injection in that injection is handled by the machine and the mould tool only consists of the product related tooling plates, however:

A shot of uncured rubber, or a rubber blank, is dropped into the injection cylinder for each cure rather than rubber continuously and automatically feeding the machine.
No intentional pre-heating happens in the machine, but can happen before the shot is loaded

Normally, people are used to drop the rubber shot into the machine, to remove the product and cured rubber from the sprues, channels and the injection cylinder as well as to load any substrates required.

Continuous Injection and Trans-Injection moulding

Transfer Injection

Again, rubber is injected into the cavities, however:

1. The injection is handled by the mould tool not by the machine. A shot of uncured rubber, or a rubber blank, is inserted into an injection cavity within the mould tool, not in the machine.

2. As the mould tool is compressed the rubber is squeezed from the loading cavity into the product cavities.

3. Normally, people are used to remove products, to load the rubber blanks, remove cured blanks and to load new substrates. Note, this rarely has cured rubber in sprues and channels to clear cured rubber from.

Compression

There is no injection in this approach. Instead:

1. Uncured rubber is pre-loaded into the product cavity.

2. As the mould tool is compressed, the rubber is squeezed within the product cavity and fills it.

3. Normally, people are used to load the rubber, removed the products and to clear any excess rubber.

Product precision
Compression
Transfer Trans-Inject Continuous Injection
Product precision Med-High High High High
Rubber changeover time Fast Fast Fast Slow
Injection points No Yes Yes Yes
Tooling costs Low High Medium Medium
Cycle time Med Med-Fast Med-Fast Fast
Limitations Suited primarily to small rubber thicknesses Careful design required to maintain rubber pressure with larger shot sizes Limited shot size based on machine design, machines are generally being replaced High volumes required to justify start/stop costs, high machine purchase costs, moulding volume limited by shot size
Advantages Low cost tooling, no injection mark, easier to handle, low overhead costs
Can mould under vacuum, high rubber efficiency
Cycle time, low overhead costs, can mould under vacuum, cycle time, increased shot size flexibility Cycle time, low overheads cost Cycle time, low human interaction levels
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