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Best Practices for Injection Molding Wall Thickness

September 20, 2022

From the 1950s to now, injection molding has dominated the consumer goods manufacturing industry, bringing us everything from action figures to denture containers. Although injection molding is incredibly versatile, it does have some design limitations.
The basic injection molding process is to heat and pressurize plastic particles until they flow into the mold cavity; Cooling the mold; Open the mold; Eject parts; Then close the mold. Repetitions and repetitions, usually one plastic manufacturing run 10000 times, and one million times during the life of the mold. It is not easy to produce hundreds of thousands of parts, but there are some changes in the design of plastic parts, the simplest of which is to pay attention to the wall thickness of the design.

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Wall thickness limit of injection molding
If you take apart any plastic appliances around the house, you will notice that the wall thickness of most parts is about 1mm to 4mm (the best thickness for molding), and the wall thickness of the whole part is uniform. Why? There are two reasons.
First of all, the thinner wall has a faster cooling speed, which shortens the cycle time of the mold and the time required to manufacture each part. If the plastic part can cool faster after the mold is filled, it can be safely pushed out faster without warping, and because the time cost on the injection molding machine is high, the production cost of the part is low.
The second reason is uniformity: in the cooling cycle, the outer surface of the plastic part cools first. Shrinkage due to cooling; If the part has uniform thickness, the whole part will shrink from the mold evenly when cooling, and the part will be taken out smoothly.
However, if the thick section and thin section of the part are adjacent, the melting center of the thicker area will continue to cool and shrink after the thinner area and surface have solidified. As this thick area continues to cool, it shrinks, and it can only pull material from the surface. The result is that there is a small dent on the surface of the part, which is called shrinkage mark.
Shrinkage marks only indicate that the engineering design of hidden areas is poor, but on decorative surfaces, they may require tens of thousands of yuan of reinstallation costs. How do you know whether these "thick wall" problems exist in the injection molding process of your parts?

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Solution of thick wall
Fortunately, thick walls have some simple solutions. The first thing to do is to pay attention to the problem area. In the following section, you can see two common problems: the thickness around the screw hole and the thickness in the part that requires strength.
For screw holes in injection molded parts, the solution is to use a "screw boss": a small cylinder of material directly surrounding the screw hole, connected to the rest of the shell with a stiffener or material flange. This allows for more uniform wall thickness and fewer shrinkage marks.

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When an area of a part needs to be particularly strong, but the wall is too thick, the solution is also simple: reinforcement. Instead of making the whole part thicker and difficult to cool, it is better to thin the outer surface into a shell, and then add vertical material ribs inside to improve strength and stiffness. In addition to being easier to shape, this also reduces the amount of materials required and costs.
Once you have made these changes, you can use the DFM tool again to check that the changes have resolved the problem. Of course, when everything is settled, before continuing to manufacture, prototype parts can be made in 3D printers to test them.