Plastics and Injection Molding

Injection molding is a low-cost way of making thermoplastic parts in large quantities. Intricate parts can be made to close tolerance with no need for second operations. The single-stage reciprocating screw system is commonly used for the injection molding process it prepares material thoroughly for the mold and is general a faster system. Injection molding of thermosets average 25% faster than compression molding, but requires careful attention. Molds can also be costly and larger volumes are often required for economical operation.

In compression molding, a specific amount of material in a mold cavity is squeezed by a punch or force. The plastic material is heated until softened, and it flows to fill the space between the force and the mold. The mold is kept closed until the formed piece has had time to harden.

Transfer molding is a process where material is heated, loaded into a chamber called a pot, and forced through a sprue into mold cavities. With this, closer tolerances and more uniform density can be held since the mold is enclosed, rather than open like in compression molding.

Most common for hollow parts, in blow molding a die closes around a liquid plastic tube, is poured vertically, as air is forced into the hollow part of the tube. This forces the plastic to the outer part of the mold.

This process is often used with injection molding. A thin film of plastic, either on a reel (IMD) or a die cut piece (IML), is inserted into an injection molding die and integrated with the part through the injection process.

Hollow molds are filled with a resin powder and the molds are heated and rotated, allowing the plastic to stick to the walls of the mold and itself. This is most commonly used for hollow, large, and durable parts.

Typically used for parts that need two different types of plastic (soft/rubber and hard) or two different colors (clear and opaque). The first shot (substrate) is usually covered —partially or fully— by the second shot (overmold).

Used with metal inserts and injection molding, the insert is placed into the mold, then during the injection process the insert is encased by the plastic.

A sheet of plastic is placed over the male die mold. Through heat, the plastic is vacuumed to take the shape of the mold. This is most commonly used for large, simple parts.

Often used for shell parts, with thermoforming a sheet of plastic is heated and pressed between a male and female die. The plastic takes the shape of the space between the two parts.