How Does an Injection Mold Work?

How Does an Injection Mold Work?

A (plastics) injection mold is a durable tool, that is, it will survive for long periods of time (useful life) if it is properly designed, constructed, and maintained. It is thus distinguished from one-time-use molds, such as those used in metal foundries. Injectable plastics can be made into various shapes with a mold.

Every mold must be capable of releasing the product after molding (such as in sand-casting), so that it can be removed.

China plastic mold structure

A cast iron part with an intricate inside shape could have been molded with conventional (permanent, “open and close”) molds in the past.
In addition to a molded metal composite being injected before injection, a molded metal composite will also be ejected after injection along with the molded product. Metal inserts will be removed by heat, but destroyed by the heat.

Molds consist of two halves, each with a cavity and a core that matches that cavity. There is a line that separates the two halves (parting plane). If the injected plastic (now shaped like the desired product) is sufficiently cooled and rigid, the mold opens, and either manually or automatically the product can be removed.

Injection-molding machines typically have an ejection mechanism on one side of the platen since the injection occurs on its stationary side. Stack molds always require ejection from the injection side, even though single level molds often do as well; any necessary mechanism must be added to the mold, or sometimes even to the machine; both of these calculations will increase complexity and cost. No external ejection mechanism is required for molds designed exclusively to use air ejection.

All products are normally ejected from the core (removed). In many hollow and cored molds, special provisions are required so that the products can be removed. The rule applies to products with severe undercuts, including screw threads, holes, ribs, or openings in their sides, or molding used for inserts.

Moving side cores may be required for some of these design features. These are inserts or sections of the mold cavity that are moved at a 90° angle to its “natural opening path.” The core or the cavity side of other models may require special unscrewing mechanisms.

It is sometimes necessary to use split cavities (or ‘splits’) in molds; that is, cavities are composed of two or more segments that can be manipulated mechanically or hydraulically during injection and then removed as one unit. Molds are sometimes required to introduce retractable inserts or to employ collapsible cores, which are both fairly complex processes (and much more expensive, too).

If the above special features are included in the mold, it would cost much more than an up and down mold, with machine ejectors that can easily remove the products during or after opening, not requiring these complicated features.

Using (simple) “up and down” molding, which comes from the early vertical molding machines, is described in the article. An injection molding machine usually works horizontally, and molds are opened and closed on a horizontal plane.