It is possible to overmold silicone rubbers on to thermoplastics, metals, electronics and other substrates. Silicone rubber is overmolded onto plastics to absorb shock, act as a seal, give a soft feel to touch or to separate the non-biocompatible internal components of a device with a bioinert silicone elastomer. One of the most common problems in rubber overmolding is protecting the insert or the substrate to be overmolded. The mold has to be designed carefully to keep the insert in place to withstand injection pressures but also avoid tight clearances which could crush the insert.
It is common to see dimensional variations within a sample lot of plastic parts, metal inserts, PCBAs or other substrates. Our incoming quality control inspection team detects parts which fit too tightly or loosely on the mold. These incoming samples out of specification are then quarantined. There are cases where inserts are expensive, but it is always a lot more expensive to fix a damaged tool.
One of the important aspects of silicone overmolding is to achieve the desired adhesion between the silicone and the plastic or metal substrate. This can be done is several ways, through mechanical interlocking, primers or self adhesive silicones. We always keep the inserts or substrates clean and avoid any chemicals which could inhibit the silicone cure chemistry. Pre-heating plastic and metal inserts is an option as it helps with achieving a stronger bond. We avoid preheating any materials which have a low melt temperature.
Mechanical interlocking: if you have the design freedom to adjust the insert or substrate design, add a few undercutting features which will lock and hold the silicone rubber in place.
Primers: If you do not have the option to modify the insert design, the next best option is to use a primer to treat the surface. Once applied, the primer dries off anywhere between 30 mins to 2hrs, depending on the primer used. Once ready we overmold the insert with silicone rubber and this process produces a good bond.
Surface Treatments: Plasma, Corona, Particulate Blasting are common techniques. Corona discharge is commonly used for plastics inserts. These surface treatment techniques increase the surface area and change the surface chemistry to enhance the bonding between the two materials.
Self Adhesive Materials: There are several self adhesive liquid silicone rubber (LSR) materials which bond to plastics without the use of primers and surface treatments. These silicone elastomers bond well with Polycarbonate (PC), PolyEthylene Terepthalate (PET), PolyButylene Terepthalate (PBT), PolyPhenylene Sulfide (PPS),PolyEther Ether Ketone (PEEK) and Nylons (PA).
Contact Konark Silicones today and one of our silicone molding experts will be happy to assist you with your silicone overmolding project.