The structure of the thermosetting liquid silicone rubber (LSR) injection mold is similar to the mold structure used for thermoplastic compounds in general, but there are many significant differences. For example, LSR compounds generally have a lower viscosity and therefore have shorter filling times, even at very low injection pressures. In order to avoid air stagnation, it is important to have a good exhaust device in the mold.
In addition, LSR compounds do not shrink like thermoplastic compounds in the mold. They tend to expand when heated and shrink slightly when cooled. Therefore, the product does not always stay on the convex surface of the mold as expected, but stays in the cavity with a large surface area.
Shrinkage rate
Although LSRs do not shrink in the mold, they often shrink by 2.5% -3% after demolding and cooling. Exactly how much it shrinks depends to some extent on the formulation 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 at which the compound is released, and the pressure in the mold cavity and subsequent compression of the compound.
The location of the injection point is also worth considering, as the shrinkage in the direction of flow of the compound is usually greater than the shrinkage in the direction of flow perpendicular to the compound. The external dimensions of the product also have an impact on its shrinkage. Thicker products generally have smaller shrinkage than thinner ones. If secondary vulcanization is required, it may shrink an additional 0.5% -0.7%.