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What is Rubber Overmolding
Rubber overmolding is the process of encapsulating or bonding a substrate, with other rubber material into a single part. To some extent, it can also be called as rubber insert molding. The substrate can be various materials, like metal, plastic, fabric or rubber, while the subsequent overmold material generally is silicone, liquid silicone or other industrial rubber like nitrile, EPDM, SBR, CR etc. That is two materials or multi-materials are molded and formed into a finished product. Rubber overmolding manufacturing process is achieved by compression molding, transfer molding, or liquid silicone injection molding.
Rubber to plastic overmolding is realized by liquid silicone injection to the plastic substrate. Liquid silicone injection molding can provide a rather lower curing temperature (about 110°C - 160°C) compared with rubber compression molding, so LSR injection overmolding is a good choice for rubber to plastic bonding.
Rubber Overmolding Application
Rubber overmolding parts are widely used in different fields, like auto, electronics, medical, and baby care fields.
Electronic: silicone USB overmolding cables, SIM card slot, LSR gaskets to plastic housing
Auto: rubber vibration absorber , rubber hand grips with metal inserts, screw drivers, rubber to metal bumpers
Baby Caring: baby bottles, silicone spoons by LSR overmold to PPSU, silicone pacifier with plastic inserts, liquid silicone toothbrush
Plastic PPS Ring Bonded to Silicone Bottle
Liquid Silicone to Electronics Overmolding
LSR Overmolded Toothbrush
Automotive Rubber Dampers with Inserted Metal Screws
Rubber to Plastic Overmolding Process and Bonding Types
In rubber overmolding process, there are two joint methods to connect silicone part with plastic or metal piece robustly, mechanical bonding and chemical bonding. To create a stronger joint, mechanical bonding and chemical boning can be used at the same time.
Mechanical bonding is that plastic insert is with some interlock design to better connect with rubber material. When liquid silicone is injected, silicone flow into the interlock, hole and grip firmly with plastic piece. So surface adhesive can be omitted to speed up production and save cost.
Chemical bonding is surface treating on overmold substrates which is considered when different materials are not compatible, and when plastic inserts are without interlocks, which will cause week bonding effect.
Surface treatments, like primer, glue, or surface activations are pre-coated the plastic insert before overmolding. Surface treating act as a bridge to connect silicone with plastic.
Chemical Rubber to Plastic Bonding Process - Glue Adhesive with Overmold
During liquid silicone to plastic overmolding process, careful preparation of plastic insert should be done before overmolding.
Plastic substrate should be cleaned to make sure it without contamination of dust, dots and burrs which will affect the connection.
Then coat the primer or other adhesive agents to plastic insert surface where plastic part will contact with overmolded rubber.
If necessary to get a high-quality durable product, plastic inserts with primer should be dried to enhance adhesive effect before molding. Adhesion of dried primer is better because preheat can boost the combination of plastic and silicone rubber.
Without surface treating, plastic parts will drop from overmold material easily, as they are not tightly bonded together.
Put the pre-coated plastic inserts to mold cavities, and then inject liquid silicone to cover the plastic. Primer react to be adhesive in mold heat temperature to join different overmolding materials.
Silicone Rubber Overmolding to Plastic Manufacturing Ways and Guide
Silicone to plastic bonding can be done by compression molding, transfer molding, and liquid silicone molding. But one important point that whether the overmold material can be compatible with molding temperature need to be take into consideration first. Some plastic materials become soft and melted when meet with high temperature. So here is the heat distortion temperature chart of different plastic to show the overmold material compatibility.
Overmold Material Compatibility Chart
ASTM D 648
ASTM D 648
Flame Retardant PP
High Heat Resistance PP
Flame Retardant PP
High Heat Resistance PP
Compression mold development cost is cheap, but production cost is high. In overmolding production, raw material silicone is directly placed on the inserts, plastic or metal inserts will move away and cause bonding deviation when mold plates clamp. When silicone is cured and solidified, plastic part may not be overmolded to the right location. So rejection rate is high. Compession molding temperature is about 170°C, so overmolded plastic heat resistance should be higher than this temperature.
Transfer molding is a cost-effective way and more high-precision way to produce rubber to plastic bonding parts. Raw material is placed to mold pot to avoid squeeze with substrates. Transfer molding temperature is similar to compression molding, and both of their rubber materials can be different rubber, like rubber, nitrile, EPDM, SBR etc.
Though liquid silicone injection molding has a limitation to overmolding subsequent material (only can be liquid silicone), it accepts more types of plastic substrates, because LSR injection overmolding is low-temperature curing with platinum vulcanization agent, about 90-120°C. LSR overmolding parts are more accurate with tight tolerance and beautiful appearance. Moreover, liquid silicone injection curing time is short, which can reduce production time and it is suitable for large volume production.