Why is Wall Thickness Important in Injection Molding?

Proper wall thickness is one key design element that manufacturers need to hold in high standard for the effectiveness of the injection molding process. It influences the success or detriment of the end product, which is why it should be done appropriately. A host of problems such as sink marks, warping, and flow lines could result from poor wall thickness.

When engineering the wall thickness of a plastic part, engineers need to consider various variables such as weight, geometry and budget. For example, a thick wall molding offers more strength but is also more likely to warp during cooling.

What is the optimal wall thickness for injection molding?

If you have disassembled a plastic part before, you may have noticed that the wall thickness of these parts is uniform and ranges between 1mm-4mm. Most plastic parts are uniform and have a particular wall thickness. Here is why.

First, before a plastic part is ejected from the machine, it has to cool. The rate at which it cools depends on the material used and the thickness of the walls. Thinner walls cool faster, shortening the cycle time and facilitating high volume production. A shorter cooling period translates to less time on the injection mold machine, which means you incur fewer production costs.

Uniformity is the second aspect. During the cooling stage, the outer parts of the plastic harden first. Cooling causes contraction, and therefore the plastic part shrinks during this stage. If there’s uniform thickness, the shrinking occurs evenly, and the final part ejects out of the machine smoothly. However, if there is a mixture of thin and thick sections, the thicker areas contract and shrink after the outer parts have solidified.

As a result, the contraction pulls in the outer material, resulting in a tiny dimple called a sink mark. Sink marks often appear on thicker areas of a product as they require more cooling time. When they occur in hidden places, sink marks may show poor engineering. However, much money may be involved in retooling costs when sink marks form on cosmetic surfaces. The more the molten takes to cool, the more time it spends on the machine and the higher the production costs.

Other defects that may result from uneven wall thickness include warping. Warping is when twists and bends form on the shape of the final product. It is mainly due to the uneven cooling of plastic parts. Flow lines may also appear on the mold cavity. They usually appear as waves or lines and are generally of a different color and texture from the rest of the product. When molten plastic fails to reach certain parts of the mold, a part of the product may miss a defect called short shots. Short shots usually occur when molten from a thin section flows to a larger area and cools down prematurely.

Achieving uniform wall thickness

First, you want to look at the final product’s attributes including strength, color, and function. The type of material you use significantly determines the thickness of a plastic design. For this reason, you want to work with materials that allow different wall thicknesses. Suitable materials can also be molded into the required dimensions and geometry. Although some materials will enable you to achieve a thin and uniform wall thickness, they may have undesirable properties. For example, nylon is suitable for thin-walled parts, but it has average strength and is not heat resistant.

It is best to work with a specialist to choose the suitable material for your products. For example, an engineer may add glass fiber to the resin, making nylon more robust and heat resistant. Sometimes achieving uniform wall thickness may not be possible, but in such cases, specialists use gradual thickness variations to maintain design stability.

After choosing the suitable material for your plastic parts, it is essential to test the design before producing the final products. You can work with Design for Manufacturing (DFM) analysts to identify defects and make other design modifications to reduce wall thickness.

Of all the different design aspects, wall thickness and uniformity significantly impact a part’s cost and final quality. For this reason, it is best to work with experts to avoid defects such as warping and sink parts on your plastic parts.