Many liquid silicone manufacturers often encounter some problems when manufacturing liquid silicone molds or producing liquid silicone products. In general, the structure of liquid silicone molds is somewhat similar to that of thermoplastic molds, but there are many significant differences. For example, the viscosity of liquid silicone product materials is generally low, so the filling time is short, even under low injection pressure. To avoid air retention in the mold, a good exhaust device must be designed in the liquid silicone mold.
In addition, liquid silicone does not shrink in the mold like thermoplastic compounds. They are more prone to thermal expansion and do not contract slightly as expected, so their products do not stay on the convex surface of the mold as expected and are difficult to demould.
The following two points need to be noted when manufacturing liquid silicone molds:
1. Shrinkage rate
Although liquid silicone does not shrink during the molding process, it typically shrinks by 2.5-3% after demolding and cooling. The accuracy of shrinkage depends to a certain extent on the formula of the material. However, from the perspective of the mold, shrinkage may be influenced by several factors, including the temperature of the mold, the temperature of the material during demolding, the pressure inside the mold cavity, and so on.
The location of the injection point is also worth careful consideration, as the contraction in the composite flow direction is usually greater than the contraction perpendicular to the direction of the compound. The size and appearance shape of a product also have a certain impact on its shrinkage, with thicker products generally having a smaller shrinkage rate.
2. Mold closing line
The first step in designing a silicone liquid injection mold is to determine the position of the clamping line. The exhaust design is mainly achieved through grooves located on the parting surface, which must be located in the final flow direction of the injected liquid silicone, thereby avoiding the formation of bubbles during the molding process and reducing the strength loss of the bonding joint.
Due to the low viscosity of liquid silicone, it is necessary to design the clamping line accurately to avoid the overflow of raw materials. Even so, clamping lines are often seen on products (which is inevitable, as liquid silicon bonding lines are much thinner). Liquid silicone molds are influenced by the geometric dimensions of the product and the position of the parting surface. A slightly chamfered liquid silicone product design helps ensure that the product has a consistent affinity with the desired other half of the mold cavity.