Generally speaking, the structure of injection mould of thermosetting liquid silicone rubber (LSR) is similar to that of thermoplastic rubber, but there are many significant differences. For example, LSR compounds generally have a low viscosity and therefore a short filling time, even at very low injection pressure. In order to avoid air entrapment, it is very important to set up a good exhaust device in the mold.

In addition, LSR compound in the mold will not shrink as thermoplastic compound, they often encounter thermal expansion and slight contraction when cold. Therefore, the products are not always left on the convex surface of the mold as expected, but in the cavity with large surface area.

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

The position of the injection point is also worth considering, because the shrinkage in the direction of the flow of the compound is usually larger than that in the direction perpendicular to the flow of the compound. The shape and size of products also have an impact on their shrinkage. The shrinkage of thicker products is generally smaller than that of thinner ones. If the second vulcanization is needed, it may contract by 0.5% - 0.7%.