Plastic Rubber Overmolding
Plastic Rubber Overmolding, also known as Silicone Plastic Overmolding, Plastic Insert Overmolding, Plastic to Rubber Overmolding. Plastic Rubber Overmolding is an injection molding/ compression molding/ Transfer Injection Molding process that molds silicone rubber over, under, or through a substrate material to finish the final part. Over-molding onto plastic inserts can solve many design problems. The flexibility, toughness, solvent and chemical resistance of rubber is combined with rigidity, light weight, and cost savings of plastic. Plastic materials commonly used in the bonding process include nylon, polycarbonate, modified PPO, polysulfone, and polyphyenylene sulfide.
Plastic Rubber Overmolding Ways & Application
Silicone Rubber Bracelet Overmolding with PC Barb by Compression Mold
Generally, silicone rubber is high temperature cured material with temperature at about 170~220℃, so if overmolding with plastic part, it needs to be high temperature resistant, like PEEK, PPS, etc.
It should be noted that many plastic substrates have low softening and melting temperatures. Two common reference points for thermoplastics include Vicat softening temperature or alternatively Heat Deflection Temperature which may be an indicator that the material will not be rigid enough to overmold silicone using standard processes, like PC or PA material, they will melting or distort easily.
But if the material must be PC or PA, we can add GF (Glass Fiber) into it to increase the strength, meanwhile choose low temperature vulcanization silicone for the compression molding process, in this way, we can finish the part by lower down the curing temperature, but at the same time, the cycle time will be longer and the production efficiency will be decreased.
Generally, we can only add 8%-10% GF into plastic, if adding too much GF, there will be floating fiber on the surface.
For the low temperature vulcanization silicone, we need to add platinum vulcanizing agent into it, to speed up the forming time.
Before Overmolding, the plastic part need to be coated with glue, glue primer is another keypoint in rubber to plastic overmolding process. During overmolding, glue adhered silicone and plasic together and meanwhile silicone will cover or encapsulate the plastic inserts.
Silicone Baby Toothbrush Overmolding with PA Rod by LSR Injection Mold
One of the main reasons that an overmolding design is used is that it improves the overall aesthetic design and functionality of a product. Numerous color options can be created through plastic overmolding. Substrate molding also allows crucial details to be added to products for a specific consumer use. For example, a baby silicone toothbrush can be fitted with a colorful and comfortable handle through the overmolding design process. Other reasons to use plastic overmolding, include:
Provides a soft grip surface around a part of the separate material.
Break up color (aesthetic impacts).
Adds flexible areas to a rigid part.
Eliminates assembly line time. Instead of manufacturing a metal tool and a plastic hand grip separately and then joining the two together manually or with automation, you can just overmold the metal tool with a plastic hand grip and eliminate the need for assembly altogether.
Captures one part inside of another without having to use fasteners or adhesives.
LSR Injection Molding can provide a rather low curing temperature than silicone rubber compression molding (110~160℃), so Liquid Silicone Injection Overmolding is a good choice for rubber to plastic bonding.
Overmolding Silicone Full Face Mask
Silicone Plastic Overmolding Nasal Pillows
Plastic Rubber Overmolding Main Material Specification
Plastic Rubber Overmolding Main Problem
Poor Silicone Filling and Much Bubble at runner income area, appears that improper glue was applied to PBT part, or it was treated by some chemicals to increase adhesion.
Solution a: Change Proper plastic treatment – Chemical reaction of glue (or whatever you used) leads to bubbles, extra friction between injected LSR and part may produce much higher inner pressure which causes extra deformation and elongation of overmoulded part.
Solution b: Adjusting the heating device so that the heating is uniform. Mold uneven heating temperature also lead to bubble.
Solution c: Adjusting the injection parameters.
Overmolded plastic part deformation and elongation are inevitable (necessarily will happen) due to mold high temperature and inner pressure. Normal elongation is up to 0.15-0. 35 mm.
Solution a: Precisely tune the amount of injected LSR. Too much extra amount also produces higher inner pressure, which deforms plastic part.
Solution b: Modify the mechanical positioning design to refuse rigid support of overmolded parts.
Solution c: Adding Glass Fiber into the PBT part to increase the strength.
Extra Material Leaking from the Edge
Solution a: Making an overflow groove on the edge ofthe parting surface
Solution b: Reduce the amount of LSR material, make good sealing surface, increase the mold temperature and decrease the mold injection pressure.