What is Silicone Overmolding?
Silicone Overmolding, also named Rubber Overmolding, Liquid Silicone Rubber(LSR) Overmolding, is a multi-step injection molding process where two or more components are molded over top of one another. Overmolding is sometimes referred to as two-shot molding because it is a two-step process.
The common secondary molding material concludes PC, PPSU, PA + GF, PBT, etc., All are thermoplastic materials with high-temperature resistance. While common overruled material is silicone (high-temperature vulcanization silicone or low-temperature vulcanization silicone) or liquid silicone rubber (LSR). Compared with the assembly or bonding parts, the silicone overmolding parts have better quality.
How is Silicone Overmolding Done?
Silicone Overmolding is a multi-step injection molding process where two or more components are molded over top of one another. Overmolding is sometimes referred to as two-shot molding because it is a two-step process.
(1) First, a base component (otherwise known as a substrate) is molded and allowed to cure. Overmolded substrates are often made of plastic.
(2) Then the second layer is molded directly on top of the first to create a single solid piece.
Overmolding is commonly used to manufacture plastic parts that feature a rubber handle. The two-shot process of overmolding a toothbrush, for example, consists of forming a base layer for the plastic handle and a top layer of rubber (to make the toothbrush less slippery to hold).
LSR Overmolding is widely used in health care, high-precision seals& gaskets, and automotive parts.
Two Options for Materials in Silicone Overmolding
1) Self-adhesive silicone rubber. This material requires that the product structure is conducive to sealing, if there is glue overflow in the production process, it is difficult to clean up.
2) Liquid silicone rubber. Brushing the primer on the substrate material before the overmolding of liquid silicone rubber. There will be adhesion where the substrate is brushed, and there is no adhesion where the substrate is not brushed. Therefore, It is better to clean up the overflow glue than the self-adhesive silicone rubber.
Liquid silicone rubber overmolding generally uses a needle, valve-type cold runner. Theoretically, the mold cavity can achieve a complete vacuum for glue sealing. This advantage can avoid the poor appearance of parts& products for the reason that the gas in the mold cavity cannot discharge. Most liquid silicone rubber products have a greater risk of glue overflow during the process of silicone overmolding, so the mold cavity should keep a vacuum and use a needle valve runner.
The Benefits of Silicone Overmolding
Silicone Overmolding offers many benefits. One of the benefits of liquid silicone over-molded components is that the manufacturing process allows for using the non-plastic substrate. Aside from thermoplastic materials and liquid silicone rubber, it is the only way to incorporate the material into the injection molding.
You can manufacture products with various material and customized the color. Overmolding is labor and cost saving compared to other manufacturing methods where a part would need to be transferred to a completely different mold or machine.
• Compatible with more plastic materials, such as medical cryogenic PC, PA, PPS, or other high-temperature resistant plastic
• The curing time is short
• The injection molded parts without burrs
• The size control of LSR plastic parts with strict tolerance
• Better quality control
Multi-shot Overmolding & Insert Molding
There are two methods to achieve silicone overmolding: Multiple-shot molding or Insert molding. In double-shot molding, the first material is a substrate, the second one is the mold. The pressure from multiple barrels to press shooting two different resins into the same injection molding tool. The entire operation uses one machine in the whole process, instead, double-shot molding is more suitable for large volumes because the mold and machine are more expensive.
Insert Molding usually place the substrate, like metal, or plastic into the mold, then oversold the partial parts or the whole parts. For better adhesive strength, the insert part will be preheated, and the insert parts will be coated with a coating. Insert molding is more sufficient for low volume, however, insert molding needs a substrate and a cavity mold for overmolded material.
