While the correct application of the science of plastic injection molding can take you a long way toward creating the perfect part, there are still a number of issues that can arise. That’s where experience, and the art of plastic injection molding come into play.
Three of the most common issues are:
Knowing what they are and how to avoid them can make all the difference between structurally-sound, properly-formed parts, and weak, deformed pieces that ultimately need to be scrapped.
Like a geologic sinkhole you might see on the news, when it consumes part of a roadway, sink marks are depressions, or craters, visible on the surface of parts. However, unlike sinkholes, which are formed by the erosion of material beneath the surface, sink marks are formed when shrinkage occurs within a piece.
They can result when the plastic is not fully cooled before a part is removed from the mold, either because the cooling mechanism is not working properly or because the cooling time is not sufficient. Insufficient pressure in the mold cavity or excessive heat at the gate can also cause the problem.
Prevention relies on reversing the causes: cooler mold temperature, increased hold time, and greater holding pressure. As sinks are more common in thicker sections of a piece, designing for more uniform thicknesses throughout a part can also help.
Warping results in pieces having twists, bends, or uneven shapes that are not part of the design. It is typically caused when different parts of a mold cool at different rates. This non-uniform cooling results in internal stresses that warp the part when it is released from the mold.
A longer, slower cooling process can help prevent warping. Designing parts with uniform wall thicknesses, and selecting types of plastic that are less prone to shrinkage and warping are also helpful.
Voids are air pockets or bubbles trapped within a part. They are the result of uneven hardening between the inner portions of a piece and its surface. Holding pressure that is insufficient to condense the viscous plastic and force air out can cause, or exacerbate, the problem as well. Voids can also result when the halves of a mold are not properly aligned.
Voids can be prevented by using a less viscous plastic, which allows air to escape more readily, increasing hold time and pressure, and ensuring the perfect alignment of mold parts.
The Injection Molding Balancing Act
Ultimately, the ability to produce defect-free parts requires a detailed understanding of all the forces at work in an injection molding process, from architecture to physics to chemistry. To ensure that you get the end product you’re looking for, be sure the company you work with can confidently and completely explain the challenges, and the right responses, to you.