There is little difference between liquid and high consistency silicones in terms of physical properties. Nowadays Liquid Silicone Rubbers are used by many fabricators due to their technological advantages presented earlier.

Liquid silicones are used mainly in injection moulding of high quality finished parts, for the automotive, medical and electronic industries. On the following this presentation focuses the key parameteres of LSR processing: meter-mixing, injection and tool design.

The major advantage of the liquid silicone rubber system is that it is designed to be used in highly automated, closed systems, with very little labor required once the system has been put into operation. For that reason all the steps of the production cycle must be carefully designed to minimize cycle time.

Meter Mixing of LSR

The objective of the meter-mix process is to introduce equally proportioned and mixed LSR to the injection molding machine. The equipment associated with this process, though independent of the actual injection molding machine, definitely affects the processability of the LSR.

The two LSR components are delivered at a rate of one to one from the 20 liters pails or 200 liters drums by a pneumatically or hydraulically driven reciprocating pumps through flexible pipes to a static mixer. The two main components are then delivered via the static mixer, which consists of staggered mixing vanes inside, in to the barrel of the injection moulding machine.

Special attention should be given to the non-return valve, which helps to prevent variations in the final volume caused by the leakage flow. Such an occurrence can influence final part size: with the additional volume of material in the cavity, the part tends to shrink less than under normal circumstances.

If one fails to deliver the two components at a rate of 1:1, in most cases, "off-ratio" material will fully cure, but parts may stick to the mold or exhibit physical properties outside the acceptable specification range.

Also a colour aditive can be added to the main two components via an additive pump linked to the static mixer.

Injection of LSR

The injection molding machine is the equipment used to convert LSR into finished parts. The injection process is also very important for the quality of the finished parts. During this process three factors: injection pressure, vulcanization temperature and vulcanization time have to be managed in the right way in order to obtain quality finished parts.

Shrinkage of finished parts is related usually to injection pressure. In general, higher injection pressure results in lower part shrinkage.

In general, vulcanisation time is the time in which the part is vulcanized sufficiently to allow for easy removal from the mold without any deformation. As an average a vulcanisation time of 3-6 s/mm of wall thickness is suficient in most cases.

To facilitate demoulding in automated operations, it is often desirable to ensure that the part sticks to the surface with the lower temperature. For this reason one can vary the temperature between the two plattens with 3-5sC.

Tool design

Even in the case of a succesful filling it is still possible that the part may not meet the specifications and additional work on tool may be necessary.

Often, after a few trials it becomes apparent that the cavity won’t fill completely. This problem can be counteracted by a proper venting of the cavity.

Also, in order to obtain quality products it is compulsory to maintain a uniform temperature profile across the mold surface, poor heat distribution affecting the physical properties of the molded part.

Another important factor in the tool design is the position of the gate and the runner system. The whole runner system should be designed in such a manner that all the cavities are filled evenly.

The mould, if properly tooled, produces minimal flash, eliminating the need for trimming, finishing operations being no longer necessary.

Also because Liquid Silicone Rubber uses the additon-cure mechanism a postcure is not required, but it may be performed to enhance properties of the parts.

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