Insert molding is a more effective alternative to the assembly of discrete parts by means of soldering, adhesives or fasteners.
Some benefits of plastic insert molding include: lower costs, since multiple tasks are being done at once both labor and assembly time is saved; greater durability, the plastic resins mold to the insert more securely than other joining methods; and lighter components, plastic resin weighs less than metal connectors which would typically have joined the components. Insert molding serves industries such as electronics, medical, military, industrial, automotive, aerospace, food processing, commercial, residential, cosmetic and construction.
Typical insert injection molding applications include cable embedded electronics, medical devices, printers, thermostats, aerosol nozzles, electrical plugs, screwdrivers, needle hubs, bifurcations and threaded fasteners. Materials used during the insert molding process includes polystyrene, polyvinyl chloride, polyethylene, polypropylene, thermoplastic elastomers and many more engineering plastics.
Insert molding is done through an injection molding machine. On the top of the machine is a hopper, through which the plastic granules enter the barrel of the machine. In the barrel, the plastic granules are heated until they become molten plastic. A reciprocating screw inside of the barrel carries the plastic granules into the mold cavity by way of a nozzle. The nozzle leads directly into the mold cavity. The mold is attached to a moveable platen so that it can push into and pull back from the mold cavity. An insert, which can be metal or plastic, is placed into the mold cavity just before molding.
The molten plastic is then injected into the cavity through the nozzle and molds around the insert piece. Once the plastic has cooled, the mold is opened and the newly joined components are removed. Two types of bonding occur during insert molding; mechanical and molecular. Molecular bonding takes place by either shrinking the encapsulating plastic resin around the insert as it cools or surrounding the irregularities on the insert’s surface by the plastic resin.
Mechanical bonding, however, only takes place when the insert’s material is the same as or very similar to the plastic resin. In addition there are three types of insert molding: low volume, 100-1000 parts that the operator inserts all at once; medium volume, 1000-10,000 parts that the operator loads in multiples and is aided by a loading device; and high volume, 100,000 parts or more that are loaded automatically.