Dos and don’ts for liquid silicone overmolding on thermoplastics

Hard or soft overmolding has become a basic technique for injection molders in the past few years, and the rising number of molders enhances the technology of thermoplastic combinations. In a few applications, liquid silicone rubber offers an edge over TPEs in heat resistance, extreme temperature elasticity, intrinsic lubricity, and chemical resistance. 

While durable adhesion can be a problem in overmolding TPEs on thermoplastic substrates, the problem is more with liquid silicone rubber on thermoplastics.

Developing the substrate surface with chemicals can help handle the adhesion challenges. To reduce the processing steps, LSR suppliers must choose self-adhesive grades that stick with thermoplastics. However, there are some rules to follow to get the best results. 

Make sure it is clean: Cleanliness is important regarding adhesion. So make sure the substrate is dry and tidy. If the substrates are molded with the LSR in a two-shot mold instead of molding separately, this doesn’t come up as an issue.

Keep it hot: It is important to keep the substrate part hot. The LSR bond to the thermoplastic is a chemical process. It needs perfect pressure, temperature, and timing. The hotter it is, the better. Usually, the mold temperature for LSR is 300 to 400 degrees Fahrenheit. The LSR should be in the internal temperature range to cure properly. 

If you go for silicone overmolding on a cold substrate, it will slow the curing procedure. It may not be a problem in two-shot overmolding. But, when you’re molding substrate individually, you may have to preheat it,

Look for additives and mold releases: Using a thermoplastic substrate with interior mold releases is not suggested. It is done so because sometimes they cause an adhesion issue. 

External mold releases are a big no. You should also prevent any additive with amines or sulfur, which hinders LSR curing. 

Demold softly: Of course, LSRs have great strength when they are out of the mold, but when the mold opens, the curing and adhesion may not have reached its final stage. Hence, never pull or stretch the LSR while demolding. 

Consistency matters: If you understand the pointers above and follow them, you will get better results with self-adhesive LSR, regardless of whether you opt for two-shot molding or separate molding. However, with two machines, you need to transfer from one machine to another. And you also need to maintain a constant substrate temperature for overmolding and prevent contamination.

Mechanical interlock can help: Even when using self-adhesive LSR, it is a great backup to use some mechanical interlock between the materials. Let LSR penetrate through holes on its back for a robust finish and bonding. A rough finish on the overmolding can help but is not needed with a self-bonding adhesive.

Go for a test first: To better understand how well the substrate material will stick to the LSR, you should do a sample test. Once you get the results for testing, you can go for mass work.

These are the pointers to keep when choosing an adhesive and bonding the substrate material with liquid silicone rubber for a perfect result. 

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