Modern Plastic injection mold Making
Plastic injection molds are now made using a highly advanced process, capable of producing molds accurate to a hundredth of a millimeter. This is remarkably precise, as a millimeter is only 0.254 of an inch, meaning plastic molds are now produced accurately within 0.00254 of an inch.
Molds are made from chromium steel, a high durability metal that can withstand repeated high-pressure injection of plastic. Several bars of chromium steel are assembled into a block, called a mold base, and are then mounted on a milling machine to shave the bars to the right dimension.
Strategically positioned holes are drilled in the bases for guide pins and bushings to hold together the two halves of which most molds consist. Following this step, a grinder is used to smooth all the surfaces, preparing the base for the high-precision machining operations necessary to shape it into a mold component.
A computer-guided machine, called a CNC, wears the base slowly to create the mold component’s shape. The CNC machine works at a painstakingly slow pace in order to insure precise accuracy, taking several hours, often more than twenty, to complete a single mold component.
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Most mold components, but especially ones with fine detailing, will then go on to a second tooling machine. The second machine, called the EDM, is outfitted with a copper electrode in the shape of the plastic part. The electrode is polished to ensure faultless casting and measured with a highly sophisticated measuring device.
On the EDM, strong electric currents are passed through the electrode to penetrate the mold formed by the CNC machine in the first part of the production process, leaving a cavity, or imprint, shaped like the electrode itself.
The surface of the cavity is polished smoothly to even out any roughness still left after the initial tooling is done.
Very specialized skills are needed to produce plastic molds and, although Plastic injection mold making still remains time-consuming and costly, when the expense is viewed in terms of what is achieved, impeccable part design combined with low-cost mass production of goods, the process is well worth the investment made.