If parts had completely uniform shrinkage, they wouldn’t warp, and they wouldn’t be injection molded. Non-uniform shrinkage exists in every part. The ugly ones warp, have sink or voids and the better ones meet specifications with some molded-in stress—the more uniform the shrinkage, the better the part.
Warpage is a symptom of non-uniform shrinkage, but it doesn’t tell you much about the root cause. Without analysis, you must solve these problems based on your experience and experiments at the molding machine. Wouldn’t it be better to look at the actual parameters throughout the part and the molding cycle to see how you can influence the results?
First, some definitions. Volumetric shrinkage is the change in volume of any discrete volume of material in the part from injection through ejection. Area shrinkage is the effect of volumetric shrinkage throughout the part broken down in the x, y and z directions. Mold shrinkage is a value used to scale-up the cavity so that the part meets nominal dimensional specifications.
Every molding parameter has some effect on volumetric shrinkage. Even though a single mold shrinkage value is typically used, shrinkage is never uniform, and, surprisingly, this works as well as it does. Non-uniform shrinkage is resisted by the inherent rigidity of the part based on its features and material. A very rigid part may not warp, but will still have molded-in stress.
Imagine if you squeeze the part around the outside and try to reduce the diameter. The middle will tend to dome upward. If the plastic shrinks more on the outside diameter, then that is precisely what happens. That volumetric shrinkage difference would be a typical result for a part packed with a single pressure until the part has reached the ejection temperature. The volumetric shrinkage would have larger values at the outside diameter and smaller values near the gate. The packing is not uniform.
With simulation by moldflow, the overall volume that makes up a part is broken down into smaller, tetrahedral volumes (finite element mesh) used to solve these problems. Since we can measure a myriad of molding parameters, including temperature, pressure and volumetric shrinkage throughout the cycle, it is equivalent to having thousands of sensors and probes in the tool.
With mold shrinkage, you are answering the question, “On average, how much bigger do you need to build the cavity for the molded part to meet dimensional requirements?” For volumetric shrinkage, you are answering the question, “How evenly and well packed is the part?” If you do a good job attending to volumetric shrinkage and pick a reasonable average material shrinkage value, then the part will meet dimensional requirements. One depends on a little luck, and the other depends on a bit of analysis.
The longer plastic cools until it reaches the ejection temperature, the more it shrinks. Thick areas of a part that have high values of volumetric shrinkage may result in dimensional issues and may have sink on the outside walls. With very high shrinkage, sink may effectively occur internally by forming vacuum voids. These are often mistaken for air traps that are due to melt-front advancement.
Analysis allows you to evaluate the root cause effects of all of these factors and the effect of modifying wall thicknesses, moving gates and improving cooling. This information will allow you to make informed, timely decisions whether you are designing a part, designing a mold or developing a process.