17 Jun 2015

Rex Plastics Product Development

Plastic Part Design Fundamentals

Injection Molding

Plastic design is the building block of plastics manufacturing, and can make or break not only the initial investment required to get a product launched, but the long term profitability of that product. In this series we will discuss the fundamentals of this very important component of product development.

The phrase “Design For Manufacturing (DFM)” describes the affect a product’s design has on its ability to be mass produced. If a product is too thick, has undercuts or insufficient draft, or poorly designed features, it will increase cost, slow production, and create other delays in getting your product to market.

If a plastic part is too thick or has an uneven wall thickness, excess shrink, sink, internal bubbles and voids, and poor cosmetics are possible. This is a result of the plastic cooling from a molten state to a solid, room temperature state.

Too Thick

Cored Out

“Coring out” a part will eliminate the issues listed above. The idea behind coring out is to simply remove material from a plastic part, leaving a distinct rib structure behind. Ribs not only provide structural integrity to the part but also provide mating surfaces for other parts in the assembly if needed. Other advantages of coring out a part are reduced weight and production time and therefore cost. How a part is cored out will affect the strength. Leaving ribs in the right location or direction, particularly in bending, will maintain strength throughout the part and improve cosmetics.

Undercuts can be thought of as any indentation, protrusion, or hole that prohibits the injection mold from opening and ejecting the part in a linear motion. Undercuts often add significant cost to the mold as well as increased lead times. Many times undercuts can be eliminated with clever plastic design. This will provide a more efficient mold design and save tooling costs.

Below are a couple examples of undercuts. If a product can be modified to eliminate these features, it can save significant time and money.

Plastic Design

Plastic Design

Internal Threads Requires Collapsible Core/Unscrewing Mold

As you can see, and as explained in our previous post The Value Of Plastic Part Design, having the proper plastic design is critical to product development, and having it optimized before tooling is made can save exponential costs later in the product cycle.

In our next article we will discuss draft, sink and warp, and surface finishes.


  1. Thanks for helping me learn more about plastic design. I actually didn’t know that coring out a part of the plastic can help prevent any internal bubbles and voids. I’m kind of interested to learn more about how this coring out process is done or how to accurately measure it to make sure you still have a functional design.

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