The Importance of Uniform Wall Thickness in Plastics

Keeping the walls of your injection molded plastic part the same thickness is important for maintaining good cosmetics and structural integrity. In this article, we’ll explain in depth why uniform wall thickness is so important.

Preventing Sink Marks

The number one reason to maintain uniform wall thickness is to prevent sink marks (also called “shrink marks”). Sink marks usually manifest as small depressions on a part, in areas which should be uniformly flat. They are often caused by non-uniform wall thickness creating non-uniform cooling of the center of the wall (since heat from the center takes longer to escape through thicker sections of plastic). This non-uniform cooling in turn creates non-uniform shrinkage, because not all areas of the part solidify at the same time. When the still liquid plastic in the center shrinks and solidifies, it pulls on the neighboring areas which have already solidified, causing them to cave in.

The process is similar to how sinkholes growing underground can cause a depression at the surface. Just like sinkholes, sink marks may or may not be a problem depending on where they form. A sinkhole under your house is a disaster, but one in an uninhabited field isn’t. Likewise, shrink holes on non-cosmetic surfaces, especially if they are too small to undermine the structural strength of a part, can often be accepted. This same process, however, can lead to warping, twisting and cracking in extreme cases.

It’s important to keep in mind that uniform wall thickness won’t always prevent shrink marks. If your there are areas of your part which are too thick, shrink marks will appear, even if that area is uniformly thick. “Too thick”, just like so many other things when it comes to injection molding, is dependent on the specific plastic you are using. In addition to asking your contract manufacturer, there are many tables online which list recommended wall thicknesses for each plastic.

Exceptions

There are few special cases when it’s advisable to deviate from this general rule. One of these is in cases where  two walls (ribs included) meet in a “T”. It’s a good rule of thumb to make one of these walls around 60% of the wall thickness of the other wall. This reduces the volume of plastic at the junction, which will act like a small thick section of plastic, and thus exhibit the non-uniform shrinking process described above. This ratio may need to be fine-tuned for your particular plastic, mold and mechanical requirements.

Another exception to the uniform wall thickness rule is when the change in thickness along the wall is gradual, not abrupt.

Finding the Optimal Wall Thickness During Design

Determine the best thickness for each all in your part is a balancing act between mechanical strength (the part must support the intended load and any additional loads from other parts in the assembly) and other considerations, like strength-reducing bubbles inside the wall (which can occur if the part is too thick) and sink marks.

Other Tools in Your Arsenal

It’s worth discussing here that uniform wall thickness isn’t the only tool at your disposal for preventing shrink marks and warping. It is, however, usually the easiest fix to implement because you catch this via design rule early in design, or later during your design for manufacturing (DFM) review (you are doing those, right?).

Other ways to prevent shrink marks include modifying the placement of the gates, as well as increasing their size. Coring out excessively thick sections also works, plus they have the added benefit of reducing part weight and price per part (due to decreased cycle time and decrease amount of plastic used).

Shrinkage can also be reduced by increasing the hold time and injection pressure. Increasing hold time of course increases total cycle time per part, which in turn increases the price per part. Increasing the injection pressure may require moving the tool to an injection molding machine that’s the next size up, which can also slightly increase cost per part. In practice, it’s usually best to go this route only after the other methods have been exhausted.

Then there are gussets. Think of gussets as the flying buttresses of injection molding. Like their medieval counterpart, these are support structures which stick out from the sections they are supporting, a bit like external ribs. Gussets can prevent warping by adding rigidity to long, thin unsupported sections of the part.

Maintaining uniform wall thickness is an effective way to ensure your finished part has both the cosmetic appearance and mechanical strength necessary for your application when it pops out of the mold. Achieving uniform wall thickness can be an engineering challenge in its own right, as you confront the tradeoffs between the optical and mechanical properties of your chosen material and your design requirements. This is why you need a contract manufacturer who can bring more injection molding skills and knowledge to bear on getting your design right the first time. Providence Enterprise is that contract manufacturer.

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