Why is silicone rubber ideal for
direct injection molding?
Manufacturers in various industries, from fire prevention to healthcare, via automotive and other transport sectors, require easy-to-mold (and demold) low or high-viscosity rubber silicones to produce parts efficiently and cost-effectively. They are looking for mold fillings to ensure batch stability in direct injection systems and other automated processes. They are also in need of products with specific features, such as fast curing or 'flashless' technology achieved by the inclusion of additives to adapt to a wide range of molding processes.
To meet all these requirements, Elkem Silicones offers a wide range of silicone rubbers: LSR (Liquid Silicone Rubber) are supplied in tubes and drums that can be pumped directly through pipelines and tubes to the vulcanization equipment, whereas HCR (High Consistency Rubber) are supplied in boxes including various shapes. These molding compounds can cover a temperature range from -50°C to +300°C. Productivity is optimized through our know-how in curing systems and our offer of additive ingredients in multiple colors and finishes, including high transparency.
Liquid Silicone Rubbers are made of
two components that are pumped through a static mixer by a metering pump. One
of these components contains the catalyst, typically platinum-based. A coloring
paste as well as other adjuvants can also be added by the metering pump. In the
static mixer the components are mixed properly and are transferred to the
cooled metering section of the injection molding machines. The static mixer
renders a very homogeneous material that results in products that are not only
very consistent throughout the part, but also from part to part.
Because of their very low viscosity index, LSRs require perfect seals in the parting line and mold cavity to guarantee a burr-free finished product. As injections are carried out at high temperatures, steel distension and natural shrinkage of materials must also be considered at the design stage of the LSR injection tooling. From the metering section of the injection-molding machine, the compound is pushed through cooled sprue and runner system at a high pressure into a heated cavity where vulcanization takes place. This results in no loss of material in the feed lines and enables production of LSR parts with near-zero material waste, eliminating trimming operations and yielding significant savings in the cost of materials.
High Consistency Rubber materials can be purchased as master batches or pre-mixed compounds ready for vulcanization. HCR can be processed by transfer molding or compression molding, especially for large parts such as electrical insulators. Injection molding for HCR is nearly the same process as for LSR.
Due to the higher viscosity, runner cross-section should be larger.
Feeding is made with preformed strips cut from sheets.
Loading and de-molding has to be operator-made.
HCR also has slower cure.
Injected HCR parts require de-flashing.
Elkem Silicones works with you to define which compounds are most cost effective and best suited to your processing conditions, to achieve the performance levels you require. Our technical team has expertise on both of these materials and can advise you in your choice and all along your development.