The structure of the liquid silicone rubber (LSR) injection mold is generally similar to the mold structure used for thermoplastic compounds, but there are many significant differences.
For example, LSR compounds generally have a low viscosity, so the filling time is very short, even at very low injection pressures. In order to avoid air stagnation, it is important to provide 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, their products do not always stay on the convex surface of the mold as expected, but stay. In a cavity with a large surface area.
the shrinkage rate may be affected by several factors, including the mold temperature, the temperature at which the rubber is demolded, and the pressure in the mold cavity and the subsequent compression of the rubber.
The location of the injection point is also worth considering, because 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.
1. Shrinkage
Although LSR does not shrink in the mold, they often shrink by 2.5%-3% after demolding and cooling. As to how much shrinkage depends to a certain extent on the formula of the compound.
The external dimensions of the product also have an effect on its shrinkage. The shrinkage of thicker products is generally smaller than that of thinner products. If secondary vulcanization is required, it may shrink an additional 0.5%-0.7%
2. Parting line
Determining the position of the parting line is one of the first few steps in designing a silicone rubber injection mold. Exhaust is mainly achieved by grooves on the parting surface.
Such grooves must be in the area where the injection compound finally reaches. This helps to avoid internal bubbles and reduce the strength of the glued loss.
Due to the low viscosity of LSR, the parting line must be precise to avoid spillage. Even so, parting lines can often be seen on shaped products.
Demoulding is affected by the geometric dimensions of the product and the position of the parting surface. Designing the product to be slightly chamfered helps to ensure that the product has a consistent affinity for the other half of the cavity.
