Plastic Molding Design


The plastic molding design process is complex and generally, the design is only as good as it’s weakest link. Learning how to design runners is essential to a successful plastic molding design.

Sometimes, in the rush to get the plastic part designed and out on the shop floor, the runner system takes a back seat to the other seemingly more important components.

What is a runner?

A runner in a plastic molding design is almost always either a round or trapezoidal channel that connects the sprue to the gate,which feeds the molten plastic into the mold cavity.

Learning how to design runners is essential because if the runner is not properly sized and located the plastic part will suffer. Sink, warpage, short-shots and burn marks are just few of the imperfections that will result.

Runners must be polished by hand or with a rotary tool in order to remove cutter marks, or EDM craters in order to minimize friction and aid in the mold flow.

Round runners

The vast majority of runners are round, with half the runner machined in each half of the mold. This method requires a bit of expertise in order for the halves to match, forming a perfect circle. With today’s CNC milling machines and some careful surface grinding this is really not an issue anymore however.

Some companies strive to have highly polished runner in order to minimize friction and help with the flow of the molten plastic. Other companies insist on polished runners with the polish lines running in the direction of flow.

Trapezoidal runners

Trapezoidal runners are commonly used in three-plate molds and when the runner passes over a feature, such as a slide. Trap runners are easy to machine because they are only on one side of the mold and need not match anything.

Adding a radius to the bottom of the runner is recommended, as is hand polishing in order to remove cutter marks.

Runner sizing and location

A successful plastic molding design will ensure that the runner system is balanced. In the case of multiple cavities it is critical that the runners be balanced, otherwise one cavity will fill with quite different characteristics than the others.

Sometimes this process of balancing runners takes a bit of creativity, with cavities appearing to be upside down or backwards from one another. Often this can be accomplished by merely mirror-imaging the cavities in the CAD plastic molding design system.

The size of the runners should be as large as possible, yet as small as possible. This means that the runner should be large enough to quickly fill the part, yet small enough to reduce shot size and cycle time.

Large runners reduce warp, sink and stress on the part, but also take longer to cool, as well as consume more resin. Small runners use less resin and fill faster, thus the decision becomes somewhat of a balancing act. Most customer’s have some experience with this process and will often make good suggestions. Resin companies also have standards they have developed over time and have excellent recommendations.


As part of a successful plastic molding design, the type, size and location of the runners is critical. Time spent doing this correctly will help ensure a good Plastic mold and molding process.

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