Injection Molding Design Solutions
Injection molding takes place when a thermoplastic or thermoset plastic material is fed into a heated barrel, mixed, and forced into the metal mold cavity where it cools and hardens before being removed.
Mold and die are used interchangeably to describe the tooling applied to produce plastic parts. They are typically constructed from pre-hardened steel, hardened steel, aluminum, and/or beryllium-copper alloy. Of these materials, hardened steel molds are the most expensive to make, but offer the user a long lifespan, which offsets the cost per part by spreading it over a larger quantity.
For low volumes or large components, pre-hardened steel molds provide a less wear-resistant and less expensive option. The most economical molds are produced out of aluminum. When designed and built using CNC machines or Electrical Discharge Machining processes, these molds can economically produce tens of thousands to hundreds of thousands of parts. Note that beryllium copper is often used in areas of the mold that require fast heat removal or places that see the most shear heat generated.
The injection molding process uses a granular plastic that is gravity fed from a hopper. A screw-type plunger forces the material into a heated chamber, called a barrel, where it is melted. The plunger continues to advance, pushing the polymer through a nozzle at the end of the barrel that is pressed against the mold. The plastic enters the mold cavity through a gate and runner system. After the cavity is filled, a holding pressure is maintained to compensate for material shrinkage as it cools.
At this same time, the screw turns so that the next shot is moved into a ready position, and the barrel retracts as the next shot is heated. Because the mold is kept cold, the plastic solidifies soon after the mold is filled. Once the part inside the mold cools completely, the mold opens, and the part is ejected. The next injection molding cycle starts the moment the mold closes and the polymer is injected into the mold cavity.
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