December 2004 Design Tip

Consistent wall thickness can be a core issue

We've previously recommended that you maintain a consistent wall thickness throughout your part. One of the more useful techniques to do this is known as "coring out," which involves cutting away blocks of material that don't contribute substantially to the strength or function of the part. Of course we always try to hit it the first time, but if the second rev needs strengthening, the mould can be modified relatively easily by removing metal. In this way you can add plastic to the weak areas, leaving the rest of the part with consistent wall thickness.

The illustration to the left is an example of a simple part that was originally turned out of bar stock. When it came time to manufacture the part using injection moulding, the designer cored out all the thick sections to a consistent wall thickness.

Another example can be found on the Protomold sample part. It contains an example of the shrinkage problems that can occur with thick features and illustrates how coring out the feature can resolve the problem.

Resin Recommended Wall
Thickness (inches)
ABS 0.045 - 0.140
Acetal 0.030 - 0.120
Acrylic 0.025 - 0.150
Liquid crystal polymer 0.030 - 0.120
Long-fiber reinforced plastics   0.075 - 1.000
Nylon 0.030 - 0.115
Polycarbonate 0.040 - 0.150
Polyester 0.025 - 0.125
Polyethylene 0.030 - 0.200
Polyphenylene sulfide 0.020 - 0.180
Polypropylene 0.025 - 0.150
Polystyrene 0.035 - 0.150
Polyurethane 0.080 - 0.750

The table* shows wall thicknesses Protomold recommends according to resin. Please note that thin walls only work on small parts and thicker walls are required where the resin has a long way to flow (Protomold makes parts with dimensions of about 0.25" to 15" or more).

* Adapted from

Free Sample Part and Key

In our October Design Tip we offered you a free : Rapid Injection Moulding Sample Part and Key. Well, we've already shipped about 1500 of them! If you'd like us to mail one to you just send an email to and we'll get right on it.

As always, you can visit the Protomold Design Guide for helpful Rapid Injection Moulding design information.