The Plastic Injection Molding Process

Because of its high viscosity, in order for plastic injection molding to be successful melted polymer must be injected into a hollow mold with a large force.

Preparing the Mold
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Feeding the polymer resin (pellets) down to the auger (screw) is a large open-bottomed container. An electric (or hydraulic) motor is responsible for turning the auger inside a heated cylinder which feeds the pellets up through the grooves of the auger. A gate before the injection mold cavity restricts the flow of the melt into the mold and limits backflow. The pressure created by pushing the forward through the grooves up to the gate also produces heat on the inside of the cylinder which helps to melt the polymer and prepare it for injection into the mold.

Injection of Polymer Melt Into the Mold

As the auger moves forward it injects polymer melt into the mold at high pressure (typically 10,000 – 30,000 psi), holds it, and adds more melt to ensure the contraction due to cooling and solidification does not leave gaps in the final product. Eventually the gate solidifies and isolates the mold from the injection cylinder.

Cooling the Mold

Molds are typically air or water cooled. Sometimes small holes are bored into the mold that allow a cooling liquid (such as water) to be circulated. Injection mold cooling consumes about 85% of the cycle time for the entire process.

Unloading / Demolding

After solidification, the clamp holding the two halves of the mold together closed is opened allowing the part to be removed. The injection molding process can then be repeated.

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Edited by Leafly Mould Provides Injection Mold, Die Casting Mold, Stamping Mold

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