Injection molding of Liquid Silicone Rubber (LSR) is a common technology used for the production of different products for medical and electrical applications, among others. In addition to the innate properties of the material, the parameters of the process are critical too. LSR injection molding is a multistep process that is presented in Figure 1.
The first step is the preparation of the mixture. LSR usually consists of two components, pigment, and additives (fillers for example), depending on the desired properties of the final product. In this step, the ingredients of the mixture are homogenized and can be combined with the temperature stabilization system for a better control of silicone temperature (ambient temperature or silicone preheating).
Figure 1 – Schematic of Liquid Silicone Rubber (LSR)
The internal part of the product, called inserts, which are embedded in the silicone coating after the injection process, are placed in the mold. These inserts can be kept in ambient conditions prior to injection or preheating.
Liquid Silicone Rubbers are normally supplied in barrels. Because of their low viscosity, the material can be pumped through pipelines and tubes to vulcanization equipment.
The mold is usually heated by water flowing inside the mold or by electrical heaters. Additionally, a cold runner system can be used to prevent the premature curing of the flowing silicone in the runners or its vicinity.
In the second step, the mold is closed by a clamping machine, as shown in Figure 2, and the injection process starts.
Figure 2 – Clamping machine and mold used in silicone rubber injection molding
The injection can be made either under atmospheric pressure (an efficient ventilation system is required) or in vacuum conditions. The injection time depends on the injection pressure determined by the product to be manufactured and the silicone properties, such as viscosity. The latter depends on both the shear rate, increasing during the flow, and the temperature.
Once the mold is filled with silicone, the mold temperature is increased to accelerate the crosslinking process of the silicone rubber, resulting in the material transformation from a liquid to solid state; this step is called the curing stage. The time of the curing stage is specific for a given silicone grade and, of course, product. This is another critical step in the processing, because the material overheating and significant pressure increase inside the mold can be observed as a result of the exothermic effect of a curing reaction and high thermal expansion of silicone.
The final step of the injection molding process of silicone rubber is the cooling of the product inside the mold, followed by a product demolding and final cooling in ambient conditions.
All parameters mentioned above strongly affect the course of silicone molding. In consequence, all these aspects can be subjected to optimization to ensure the shortest production cycle time while simultaneously keeping the highest final product quality.
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