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 What is Injection Molding

Definition

Injection molding is a manufacturing process where heated molten plastic is forced into a mold cavity under pressure.

A mold cavity is a negative of the part being produced, when the cavity is filled with plastic, it is cooled and the plastic becomes solid material resulting in a completed positive component.

Typically injection pressures range from 5000 to 20,000 psi. Because of high pressures involved, molds must be clamped shut during injection and cooling by clamping forces measured in tons.

Precision

Injection molding is capable of producing large numbers of parts to very high levels of precision. Holding tolerances of less than .001" (.0025 mm) is easily accomplished with the right combination of material, part design, and mold design. Even tighter tolerances can be held with additional effort.

Tooling

Injection molding has high tooling costs usually ranging form $5,000 to $100,000. Molds must be built to high standards of precision and must be sufficiently strong to withstand the very high pressures of the process. Molds are usually constructed of hardened tool steel, but may be constructed of aluminum or other soft materials when tooling life is not a consideration.

Typically, a hardened steel mold will withstand 500,000 to 2,000,000 molding cycles, without appreciable wear, depending upon the material and processing conditions. Aluminum and other soft molds will only withstand 1,000 to 100,000 molding cycles and may have a longer life where injection pressures are low and materials flow easily.

Capacities

Injection molding is a high volume production process with cycle times typically ranging from a few seconds to a minute or so depending upon the configuration and thickness of the part being molded. Single cavity molds offer the lowest tooling costs and highest precision at the penalty of higher unit costs. Multi-cavity molds can be utilized to increase capacity and lower unit costs but with signifcantly higher mold costs.

Economics

Injection molding is well suited to applications requiring thousands to many millions of pieces. Sometimes there is no other way to produce parts from certain materials except via injection molding. Typically injection molding starts to become economically viable at around 1000 pieces.

The molding process has a large numbers of variables and a steep learning curve is involved with certain materials or when high precision is desired. Injection molding machines require special plant services not required by other manufacturing equipment.