Liquid silicone overmolding, which is also known as two shot molding, is a process in which uncured silicone rubber is applied to a base material or substrate and formed to shape, while in direct contact with the substrate. Liquid silicone is typically bonded to metal, thermoplastics and silicone. Silicone overmolding takes advantage of either mechanical bonding or chemical bonding, or both. A mechanical silicone bond takes place by physically creating undercuts and interlocks for the liquid silicone to "grab" hold of the substrate. Chemical silicone bonding is typically stronger and more reliable than mechanical bonding, since it takes place on a molecular level. The resulting chemical bond welds the cured silicone to the substrate material.
In order to determine the best liquid silicone and substrate match for bonding, testing should be performed. This is typically done using lap shear tests, tensile strength tests and cyclic fatigue tests. It should also be noted that many plastic substrates have glass transition temperatures lower than that required by silicone to properly cure. When possible, it is best to select a substrate material with a glass transition temperature above 300°F. However, Albright has completed process development work on liquid silicone overmolding at low temperatures for temperature sensitive substrates.
Typically applications in silicone overmolding are for electronics, wearable devices, reinforcing or mechanical structure improvement, gaskets, seals, fluidics, medical catheters and medical implants. Overmolding reduces assembly costs and creates the opportunity to design silicone devices that could otherwise not be created. Silicone acts as a protective cover against dust, water, impact, heat and electrical shock. For implantables, silicone creates a bio-friendly barrier for implantables between the medical device and the patient. Whatever your silicone overmold application, select Albright’s engineering and process experts for your next product. Contact us today at 978.466.5870 or via email. Feel free to send your silicone overmolding part designs to us via our RFQ page to ensure your design is suitable for production manufacturing and to receive a quote.
Approaches using primers, adhesion promoters and treatments can be used to encourage the liquid silicone to bond to a metallic substrate; however these approaches are not typically required as liquid silicone bonds to metal quite easily.
Metals allow for mechanical silicone bonds to be created very easily. The metal substrate can be shot peened, sand blasted and surface abraded. These processes all create more surface area for the silicone to grab onto during the silicone overmolding process. When working with metals it is important to be aware of inhibitors. Some metals contain sulphur, which can inhibit liquid silicone curing and can slow or even prevent the crosslinking of the liquid silicone matrix.
This process is known as two shot silicone molding. The existing silicone part will have no issues with the temperatures necessary to process the second shot of silicone unlike thermoplastics. As long as the silicone substrate is clean, the new liquid silicone will bond to the original part without primers, adhesion promoters, or plasma treatment. If the original silicone part has not been post-baked for full cross-linking, then it should be post-baked after the second shot. Post-baking the silicone part after silicone overmolding will promote the best bond.
When choosing a thermoplastic to overmold to, one must consider the glass transition temperature of the plastic they select. The higher melting point of the plastic, the more options you will have for liquid silicone materials to overmold with. We have had a high rate of success overmolding silicone on to Polycarbonate (PC), Polybutylene Terephtalate (PBT), Polyphenylene Ether/Oxite (PPO), Polyetheretherketone (PEEK) and Polyphenylsulfone (PPSU).
There are many methods of bonding silicone to thermoplastics available. Primers can be used to bond the silicone to the thermoplastic, self bonding silicones can be utilized, or the thermopastic substrate can be treated with a plasma or corona treatment prior to overmolding silicone.
The primer is applied prior to the application of the liquid silicone. The amount of time and technique for application will determine which primer is used. Most of our silicone suppliers offer platinum primers to support their line of liquid silicones. The problem with using primers is that it hinders productivity and efficiency in the manufacturing process. In medical silicone applications, the primer can affect biocompatibility.
We can also use self bonding silicones, which are also called primerless liquid silicones. The adhesion promoters (bonding agents) are mixed into the liquid silicone material. There are adhesion promoters manufactured for different thermoplastics. The issue with self bonding silicones is that the material tends to stick to everything, including the mold tooling. Another issue with self bonding silicones is that the adhesion promoters can affect biocompatibility.
The third method is to pretreat the thermoplastic part with a plasma or corona treatment. This method of overmolding liquid silicone is new and is gaining acceptance. The reason for this is that expensive platinum based primers or self-bonding silicones are not required. Also, the bond between the treated thermoplastic and liquid silicone is even stronger than primed or self bonded materials. Plasma or corona treatment provides a more repeatable option, but still leaves the concern of high temperatures in molding. Plasma and corona treatment for bonding liquid silicone to thermoplastic substrates is very favorable for medical silicone applications as there are no added chemicals.