The structure of the thermosetting liquid silicone rubber (LSR) injection mold is generally similar to the structure of the mold used for thermoplastic rubber, but there are also many significant differences. For example, LSR compounds generally have low viscosities, resulting in very short mold filling times, even at very low injection pressures. To avoid air entrapment, it is crucial to have good venting in the mold.
In addition, LSR rubber compounds do not shrink in the mold like thermoplastic rubber compounds. They tend to expand when exposed to heat and shrink slightly when exposed to cold. Therefore, the product does not always stay on the convex surface of the mold as expected, but stays in the mold cavity with a larger surface area.
1 Shrinkage
Although LSR does not shrink in the mold, they often shrink by 2.5%-3% after demolding and cooling. As for the exact amount of shrinkage, it depends to a certain extent on the formula of the rubber compound. However, from a mold perspective, shrinkage may be affected by several factors, including the temperature of the mold, the temperature of the rubber material when it is demoulded, as well as the pressure in the mold cavity and subsequent compression of the rubber material.
The location of the injection point is also worth considering, because the shrinkage in the direction of the flow of the rubber is usually greater than the shrinkage in the direction perpendicular to the flow of the rubber. The outer dimensions of the product also have an impact on its shrinkage rate. The shrinkage rate of thicker products is generally smaller than that of thinner products. If secondary vulcanization is required, there may be an additional shrinkage of 0.5%-0.7%.