Mold Design

The mold-making process typically starts with a plug (a representation of the final part) and the initial mold (the mirror image that is “pulled off” the plug to make production parts). It’s important to start projects off right with a solid design. By investing time, energy and money in the design stage, you can avoid or minimize production issues later. Your return on capital – realized only after production starts – depends on how quickly a high-quality mold can make high-quality parts that require minimal additional input to produce a finished, shippable part. If you build a poorly-designed mold, that return on capital may never happen.


After hand-working the plug surface, an easy-to-smooth coating is applied and sanded to size and surface. Photo Credit: RTM North


This high-gloss coated, polished, buffed plug is ready for mold manufacturing. Photo Credit: RTM North

There are lots of factors that need to be considered during mold design, many of which have already been listed. One primary concern is shrinkage from plug to mold. With older mold-making materials, a rule of thumb was to expect 1/32-inch of shrinkage per inch of mold. Part shrinkage related to materials must also be considered in the design stage. With newer controlled-shrink mold-making materials, shrinkage can be minimized, but parts made with shrinking materials still need to be compensated for.

When designing plugs and molds, manufacturers also need to consider things like ease of gel coating, fiber placement, core placement, movement and storage at the plant and their impact on labor costs. Poorly planned designs will lead to build issues for manufacturing. Staff who oversee mold loading, mold care, mold prep, mold movement, demolding and production controls should be consulted during the design phase if production issues and mold durability/life expectancy are of any concern.

There are many software packages available to design plugs via computer aided design (CAD) files and then machine them, including SOLIDWORKS®, Autodesk, NX for Design and ESAComp.

Plug Production

The plug can be designed using various materials, including plaster, foam, wood, tooling board, plastics and modified fiberglass parts. In addition to CAD design, machining and 3-D printing, plugs are often still made by hand, using wood products like medium density fiberboard (MDF) with fairing materials and coatings that are then sanded to the required surface finish. This is a common plug-making method for simple and smaller shapes.