If you have ever looked into injection molding for your plastic parts, your first question was likely why are injection molds so expensive? An injection mold is easily the most expensive part of the project, and can dramatically increase costs for small part runs. The main factors that influence the cost of an injection mold are the size and intricacy of the part, the material used, and the number of parts being produced.
Injection molding is a process in which plastic pellets are heated to their melting point then injected into a cavity (the mold). As the plastic cools, it hardens into the shape of the cavity to form the part.
Injection molds are subject to thousands of pounds of pressure every cycle. As a result, the molds must be made from very strong materials that can withstand repeated use without deforming. The most common choice for injection molds is steel, with the grade of steel determined by the number of parts to be produced and the material to be injected into the mold. (For example, a fiberglass filled material causes a lot of wear when injected, so the mold would need to be made from a higher grade of steel.)
Another factor that determines injection mold costs is the complexity of the part to be produced. The more complex your part, the more complex and costly the mold. And once made, molds are difficult and many times impossible to modify, removing your ability to modify the design.
While injection molding has a high upfront cost for the molds, at a certain volume injected parts do become cheaper than machined parts. This crossover can occur from as little as 100 parts up to about 5000, depending on the part. But there are still several scenarios in which CNC machining is a better choice than injection molding.
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