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 uniformly be flat. They are often caused by non-uniform wall thicknesses creating irregular cooling in the center of the wall (since heat from the center takes longer to escape through thicker sections of plastic). This uneven cooling in turn creates unintentional shrinkage, because not all areas of the part solidify at the same time. When the plastic in the center of the walls shrinks and solidifies, it pulls on the neighboring areas which if solidified already, cause 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 marks 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 doesn’t always prevent shrink marks. If there are areas of your part that 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 chosen. In addition to asking your contract manufacturer, there are many tables online which list recommended wall thicknesses for each resin.
There are few special cases when it’s advisable to deviate from this general rule. One such case is 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. 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. Keep in mind, this ratio may need to be fine-tuned for your particular material, mold and/or mechanical requirements.
Another exception to the uniform wall thickness rule occurs when the change in thickness along the length of a wall is gradual, not abrupt.
Finding the Optimal Wall Thickness During Design
Determining the best wall thickness for each part is a balancing act. It must have great mechanical strength so that the part can support the intended load of any additional parts in an assembly. A good design must also prevent oddities like strength-reducing bubbles that could occur inside the wall if the part is too thick and a material that doesn’t have the proper flow properties is chosen. Any experienced design engineer with a background in plastics should be able to assist a customer with part design to avoid costly mistakes later in manufacturing.
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 if you catch it early in the design phase 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 and has the added benefit of reducing part weight and piece price (due to decreased cycle time and decreased amount of plastic used).
Shrinkage can also be reduced by increasing the hold time and injection pressure. However, 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 a larger tonnage injection molding machine, 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. Achieving this 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 design requirements. This is why you need a contract manufacturer who can bring more injection molding skills and experience to the table to get your design right the first time.