The injection mold structure of thermosetting liquid silicone rubber (LSR) is generally similar to that of the mold used for thermoplastic rubber, but there are also many significant differences. For example, LSR rubber is generally low viscosity, so the filling time is very short, even under very low injection pressure. In order to avoid air entrapment, it is essential to set up a good exhaust device in the mold.

In addition, LSR rubber will not shrink like thermoplastic rubber in the mold. They tend to expand when heated and contract slightly when cooled. Therefore, the products are not always left on the convex surface of the mold as expected, but in the mold cavity with large surface area.

1 Shrinkage
Although LSRs do not shrink in the mold, they often shrink 2.5% - 3% after demoulding and cooling. As for the actual shrinkage, to some extent, it depends on the formula of the compound. However, from the perspective of the mold, the shrinkage rate may be affected by several factors, including the temperature of the mold, the temperature of the rubber material when demoulding, the pressure in the mold cavity and the subsequent compression of the rubber material

2 Parting line
Determining the position of parting line is one of the first steps in designing silicone rubber injection mold. The exhaust is mainly realized through the groove on the parting line, which must pass through the area where the pressure injection rubber finally arrives. This is helpful to avoid internal bubbles and reduce the strength loss at the adhesive joint.

3 Exhaust
With the injection of LSR, the air trapped in the mold cavity is compressed when the mold is closed, and then discharged through the vent groove with the mold filling process. If the air cannot be completely discharged, it will stay in the rubber (which often causes the white edge of the product to be exposed). The width of ventilation trench is generally lmm-3mm, and the depth is 0.004mm-0.005mm
Some injection molding equipment allows operation under variable closing force, which enables the processor to close the mold under low pressure until 90% - 95% of the mold cavity is filled with LSR (making air easier to exhaust), and then switch to a higher closing force to prevent silicone rubber from expanding and overflowing.

4 Injection point
The cold runner system is used for molding LSR. The advantages of this compound can be maximized, and the production efficiency can be raised to the maximum. To process products in such a way, it is not necessary to remove the glue injection path, so as to avoid increasing the labor intensity of the operation, and sometimes avoid a large waste of materials. In many cases, the structure without glue injection can also shorten the operation time.

5 Demoulding
The vulcanized liquid silicone rubber is easy to adhere to the metal surface, and the flexibility of the product will make it difficult to demould. The high temperature tear strength of LSR can enable it to demould under normal conditions, even large products will not be damaged. The most common demoulding technologies include demoulding, demoulding pin demoulding and pneumatic demoulding. Other common technologies are roller scraping, demoulding of the guide plate and automatic die holding.

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