With the growing popularity of electronic wearables and advances in medical diagnostic equipment, silicone molders are seeing an increased demand for parts molded onto electronic substrates and overmolding silicone processes. There are many additional considerations that a molder must be wary of when taking on an electronics over-molding project. We’ve highlighted three of the most important considerations in this post.
1. First, when designing overmolding silicone parts, be mindful of the dimensions. Allow enough clearance to avoid damaging the substrate, while simultaneously shutting-off on the substrate to prevent overflow of material onto unwanted areas. This can be something of a delicate balance and will likely be different, based on part geometry and material used.
2. Secondly, when overmolding silicone onto electronics, be mindful of temperature restrictions. For some electronic devices, there are components that have a lower operating temperature range than what is ideal for silicone over-molding (125°C – 175°C). Typically, this is true of battery’s capacitors. The normal operating range for most capacitors is -30°C to 125°C with nominal voltage ratings for a working temperature to be no greater than 70°C for plastic capacitors. If possible, it is best to avoid these components when over-molding electronics with LSR.
3. The last secret to overmolding silicone electronics is surface compatibility to improve adhesion to the substrate. When two materials are chemically dissimilar, certain options are available to improve bonding:
- It is always important to make sure that the substrate is clean prior to molding.
- Mechanical surface preparation; such as roughening via sanding and grit-blasting, can produce good adhesion. For further improved bond strength creating undercuts or thru-holes in the substrate.
- Chemical surface preparation; such as primers or other adhesion promoters, can be purchased with the suppliers of the liquid silicone such as Shin-Etsu and Wacker Silicones.
- Another type of surface modification process is applying plasma to change the surface tendencies of polymer. Plasma is compressed energized gas that is applied onto the polymer molecules to break up some of the bonds. When some of the molecules are broken apart, the surface of the material will create available binding sites. Atoms from the silicone material are then allowed to come in and fill up these binding sites.
- Corona discharge works very similarly to plasma treatment, but differs in that it uses an electrical arc to free up the bonding sights instead of gas and flame. Due to this corona treatment is also an effective means to improve adhesion with metallic substrates.
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