what is a mold?

A mold used to produce plastic parts is a metal form consisting of a core and a cavity for each part. The core forms the inner shape of the part, while the cavity forms the outer shape of the part. The space between the core and the cavity is where the melted material flows (in injection molding) or the heated, softened material is pressed (as in pressure forming or thermo-forming) to form the plastic part. Molds range in size from single cavity to upwards of 200 cavities, depending on the size of the component and the number of parts required. A car bumper mold, for example, is generally only one cavity. The type of mold and runner system selected depends on the design and complexity of the part to be produced.



A mold can be a complex piece of equipment that makes very complex components, and can even allow other functions to be performed inside the mold cavity such as decorating the part, assembling the part, and other functions to save time and money on secondary operations. A mold can also be a simple piece of equipment that molds simple parts. The cost of the mold is determined by the complexity, size, process type being used (i.e. blow molding, injection molding, thermoforming/pressure forming), and design requirements of the plastic component being molded.

Traditionally, molds have been expensive to manufacture. They were usually only used in mass production where thousands of parts were being produced. Molds are typically constructed from hardened steel, pre-hardened steel, aluminum, and/or beryllium-copper alloy.

Molds can be manufactured by standard machining or by using electrical discharge machining processes.

Standard Machining, in its conventional form, has historically been the method of building injection molds. With technological development, CNC machining became the predominant means of making more complex molds with more accurate mold details in less time than traditional methods.

The electrical discharge machining (EDM) or spark erosion process has become widely used in mold making. As well as allowing the formation of shapes which are difficult to machine, the process allows pre-hardened molds to be shaped so that no heat treatment is required. Changes to a hardened mold by conventional drilling and milling normally require annealing to soften the steel, followed by heat treatment to harden it again. EDM is a simple process in which a shaped electrode, usually made of copper or graphite, is very slowly lowered onto the mold surface (over a period of many hours), which is immersed in paraffin oil. A voltage applied between tool and mold causes spark erosion of the mold surface in the inverse shape of the electrode.