The structure of the thermosetting 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 lower viscosity, resulting in very short filling times, even at very low injection pressures. In order to avoid air entrapment, it is crucial to have a good exhaust in the LSR molding die.
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.
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, LSR molding 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, as the shrinkage in the direction of flow of the compound is usually greater than that in the direction perpendicular to the flow of the compound. 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. If secondary vulcanization is required, an additional 0.5%-0.7% shrinkage may be possible.
Determining the location of the parting line is one of the first steps in designing a silicone rubber injection mold. Exhaust is mainly achieved through grooves located on the parting line, such grooves must be in the last area of the injection compound. This helps avoid internal air bubbles and reduces strength loss at the bond.
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.
With the injection of LSR, the air trapped in the mold cavity is compressed when the mold is closed, and then expelled through the vent grooves as the mold fills. If the air cannot be completely discharged, it will stay in the rubber compound (this often causes the white edge of the product to be exposed). The general width of the ventilation groove is 1mm-3mm, and the depth is 0.004mm-0.005mm.
Vacuuming inside the LSR forming mold creates the best venting effect. This is accomplished by designing a gasket on the parting line and using a vacuum pump to quickly evacuate all the mold cavities. Once the vacuum reaches the rated level, the mold is fully closed and injection begins.
Some injection molding equipment allows operation with a variable closing force, which allows the processor to close the mold at low pressure until the cavity is 90%-95% full with LSR (making it easier for air to escape), then switch to higher The closing force to avoid the overflow of silicone rubber due to expansion.