The structure of the injection mold for thermoset liquid silicone rubber (LSR) is, in general, similar to the mold structure used for thermoplastic compounds, but there are many significant differences. For example, LSR compounds generally have lower viscosity, resulting in very short filling times, even at very low injection pressures. To avoid air entrapment, it is critical to have a good vent in the mold.

In addition, LSR compounds do not shrink in the mold like thermoplastic compounds do. They tend to expand when heated and shrink slightly when cooled. As a result, the product does not always stay on the convex surface of the mold as desired, but rather in the cavity with a larger surface area.

1. Shrinkage
Although LSRs do not shrink in the mold, they often shrink by 2.5%-3% after demolding and cooling. How much shrinkage depends to some extent on the formulation of the compound. However, from a mold perspective, shrinkage can be affected by several factors, including the temperature of the mold, the temperature at which the compound is released from the mold, and the pressure in the mold cavity and subsequent compression of the compound.

The location of the injection point is also worth considering, because the shrinkage in the flow direction of the compound is usually larger than the shrinkage in the direction perpendicular to the compound flow. The external dimensions of the product also affect its shrinkage rate, and the shrinkage rate of thicker products is generally smaller than that of thinner ones. Additional shrinkage may be possible if secondary vulcanization is required.

2. Parting line
Determining the location of the parting line is one of the first steps in designing a silicone rubber injection mold.

Due to the low viscosity of LSR, the parting line must be precise to avoid spillage. Even so, parting lines are often seen on shaped products. Release is affected by the geometry of the product and the location of the parting surface. Designing the part with a slight chamfer helps ensure that the part has a consistent affinity for the desired other half of the cavity.