In general, the structure of solid liquid silicone rubber (LSR) injection mold is similar to that of thermoplastic rubber, but there are some significant differences. For example, LSR adhesives tend to have a low viscosity and thus have a very short filling time, even at very low injection pressures. To avoid air retention, it is vital to have a good exhaust device in the mold.

In addition, LSR adhesives do not contract in the mold as thermoplastic adhesives do. They tend to expand when they are heated and contract slightly when they are cooled. As a result, the products do not always remain on the convex surface of the mold as expected, but remain in the cavity with a large surface area.

1. The shrinkage rate

Although the LSRS do not shrink in the mold, they often shrink by 2.5 to 3 percent upon release and cooling, depending in part on the formulation of the compound. However, from a mold perspective, the shrinkage rate can be affected by several factors, including mold temperature, the temperature at which the glue is released, as well as pressure in the mold cavity and subsequent compression of the glue. The location of the injection point is also worth considering because the shrinkage of the flow direction of the compound is usually greater than that of the perpendicular flow direction of the compound. The external dimension of the product also has an effect on its shrinkage rate, the shrinkage rate of thicker products should be thinner generally. If secondary vulcanization is required, an additional contraction of 0.5%-0.7% May be possible.

2. The parting line

Determining the position of parting line is one of the first steps in designing silicone rubber injection mold. Exhaust is mainly achieved by groove located on the parting surface, such groove must be in the final area of the injection of pressure rubber material, which helps to avoid internal bubbles and reduce the strength loss of glue joints. Due to the low viscosity of LSR, the parting line must be accurate to avoid glue overflow. Even so, parting lines are often seen on molded pieces. Demoulding is affected by the geometry of the product and the position of the parting surface. Design the part to be slightly chamfered to help ensure that the part has a consistent affinity to the desired half of the cavity.

3. The exhaust

With the injection of the LSR, air trapped in the cavity is compressed when the die is closed and then discharged through the vent groove as the die is filled. If the air can not be completely discharged, it will be stuck in the glue (this will often cause parts of the product exposed white edge). Ventilation grooves are generally L-3 mm wide and 0.004-0.005mm deep. Vacuum in the mold to create the best exhaust effect. This is achieved by designing a gasket on the parting line and using a vacuum pump to rapidly vacuum all cavity. Once the vacuum reaches the rated level, the die is completely closed and the injection begins.