What is injection molding?

Injection Molding Definition:

Injection molding is a manufacturing process where melted thermoplastic is forced into a mold cavity under pressure. A mold cavity is essentially a negative of the part being produced. The cavity is filled with plastic, and the plastic changes phase to a solid, resulting in a positive. Typically injection pressures range from 5000 to 30,000 psi. Because of the high pressures involved, the mold must be clamped shut during injection and cooling. Clamping forces are measured in tons, and the max shot/part weight measured in ounces. Scotts Tool has molding machines as small as 15-ton clamp with a 1oz shot, up to 300-ton clamp with a 20oz shot.

Precision:

The injection molding process 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 relatively high tooling costs, ($5,000-$100,000) as the molds must be built to high levels of precision and must be robust enough to withstand the high pressures of the process. Our low cost nameplates are molded in quick change insert molds. With this method, we can offer custom nameplate injection molds at a fraction of the cost of a dedicated mold.  Molds are usually constructed of hardened tool steel, but may be constructed of aluminum or other soft materials when tooling life is not an issue. 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. Soft tooling may have a longer life on purely structural parts where injection pressures are low and materials flow easily.

Capacities:

Injection molding is a high capacity process. Cycle times range from a few seconds to several minutes depending upon the configuration of the part being molded. Our quick change insert nameplate molds have 1 to 4 cavities and run on a 10 to 15 sec cycle. 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 with an increasing loss of precision as cavities are added.

Economics:

Injection molding is well suited to applications requiring 10 or 20 pieces to billions of pieces. Sometimes there is no other way to produce parts from certain materials except via injection molding. When alternate manufacturing methods, and materials are an option, injection molding usually becomes economically viable at around 1000 pieces. Injection molding is a process with large numbers of variables. A steep learning curve is involved with certain materials or when high precision is desired.