What is the difference between extrusion and injection molding?

Extrusion is a process where molten plastic is pushed through a two-dimensional die opening. After the molten plastic exits the die opening, it passes through a series of sizers or forms that hold the shape of the part as it cools. A puller or conveyor grips the cooled extrudate and moves it through the sizing equipment. The extrusion process is capable of producing two dimensional profiles or shapes that are continuous in length, yet can be either coiled or cut to length at the end of the process.  Injection molding is a process where molten plastic is injected into one or more cavities in a three-dimensional mold. This mold opens and closes as parts are formed and purged from the mold cavity. Unlike extrusion, this process is capable of producing three-dimensional shapes.

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