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Process Overview

Injection molding is the most common process for manufacturing plastic parts. An injection molding machine pushes melted plastic, under pressures of up to 30,000 psi, into a mold. The mold, usually made from steel or aluminum, has the shape of the desired part machined into it. The mold holds the shape of the plastic as it cools and solidifies; the plastic part is then removed.

Some important parts of an injection molding machine are the hopper, barrel, screw, and mold (see Figure 1). The hopper contains the raw plastic material before it has been processed. The electrically heated barrel is a hollow cylinder that holds the melted plastic before it is formed. The screw, residing inside of the barrel, feeds the material and pushes it into the mold. The mold, cooled with water, holds the molten plastic in the desired shape as it is cooled.

Several steps comprise the injection molding process: feeding, melting, injecting, cooling, and ejecting. In the feeding step, the injection molding machine rotates the screw, pumping plastic from the hopper towards the front of the barrel, creating a reservoir of melted plastic (see Figure 2). During screw rotation, electric barrel heaters and friction melt the plastic pellets. Next, in the injection phase, the screw moves forward, pushing the molten plastic into the mold. The cooling water in the mold draws away the plastic’s heat during the cooling phase. After the plastic has been held long enough to solidify, the injection molding machine opens the mold and ejects the newly formed plastic part.

Materials

Nearly all thermoplastic materials can be injection molded.

Summary

Injection molding is the most common plastic manufacturing process. It is used to produce high volumes of parts.

Advantages of injection molding

· Complex geometry and fine features are easily produced, because very high pressures are possible

· Cycle times are relatively low, and many parts can be made from a single mold, making extremely high volumes (millions per year) possible

· Injection molding is commonly automated. Many machines can be run by a single operator

Disadvantages of injection molding

· Large undercuts cannot be formed (such as bottles)

· Mold cost is high, so low part volumes are not recommended (usually less than 1000 parts is considered low; most volumes for injection molded parts are well over 10,000 pieces per year)

Some characteristics of injection molding are:

· The parts are thin-walled (walls greater than 0.125" are not common)

· Thermoplastic materials are used in this process (thermoplastics can be remelted, as opposed to thermosets)

· All exterior surfaces of the part touch the mold (as opposed to blow molding, where air forms the inside)