Self-Adhesive Silicone Overmolds Hard-to-Bond Substrates

Multimaterial molding is supposed to take advantage of the efficiencies in forming two different materials into a single molded part. However, one firm’s planned production of a thermocontroller got complicated when the self-bonding overmolding material designers had specified couldn’t bond to the unit’s polycarbonate housing.

Overmolding rigid plastics with liquid silicone rubber (LSR) can improve grip, dampen vibrations, and protect against weather and impacts. And a soft feel with numerous color and texture options contributes to aesthetics.

But many LSRs require priming, pretreatment, or secondary operations to strongly fasten the overmolded layer to the substrate. This appeared to be the case with material specified by designers at a client of custom molder Amerimold Tech Inc., Marlboro, N.J.

So the designers decided to look into overseas production, but this generated an unacceptable 60% scrap rate. And parts often came back to the U.S. with the supposedly permanent self-bonding LSR peeling off the thermocontroller keypad.

The next idea was to buy variable-frequency light-pumping equipment to enhance surface-substrate bonding with a UV pretreatment. The equipment cost about $17,000, and adding production time and manpower boosted costs further. Worse yet, testing the UV process showed that silicone still failed to adhere reliably to the substrate.

Amerimold engineers then tried skipping the pretreatment and using KE2090-40 Select-Hesive liquid injection molding silicone from Shin-Etsu Silicones of America instead of the specified self-adhesive LSR.

The engineering team conducted three trial runs of two-stage molding with KE2090-40 and no pretreatment. Initial physical inspection showed that KE2090-40 parts were strongly bonded from the first injection-molding shot. The new material also passed other tests ordered by the client. The result was a go-ahead for production with a quantity increase from 10,000 to 25,000 pieces.