Injection molding is a complex process with a number of “moving parts” — both figuratively and literally — that should be monitored during production. The process is versatile enough to be the manufacturing method of choice for a broad range of products, but unfortunately, it is rarely as simple as just building a mold and running it through an injection molding machine. Several factors inherent to the process can affect the quality of your end product, and should be monitored and measured as closely as possible in order to maintain the safety and integrity of your parts.
Residual stress is one of these factors. As mentioned above, residual stress is an inherent part of injection molding, meaning that it cannot be avoided in the production of your parts. Instead, it can be managed, monitored and even built into the design of your part. This way, it can be predicted and planned for, keeping your part quality at a high level and your pieces safe and operational.
So, what is residual stress? It is defined as the byproduct of the manufacturing process, which is a part of the internal makeup of a part. A corresponding term and concept is the idea of “applied stress” — stress that a part undergoes as a result of an external force or load. While both types of stress are natural and expected components of a piece’s creation and life cycle, they can also create major problems if not handled properly: part rejection off the line, part failure in use, and damage or injury to other components as well as to users of the part.
Different types of residual stress can be caused by different factors, and can lead to different types of effects. First, let’s look at what can lead to unplanned residual stress:
Flow rate issues. Flow rate is a critical part of successful injection molding, and is defined as the rate at which the heated, liquefied injection molding substrate is pushed into the mold cavity. Different materials have different flow rates, so it is important for you and your injection molding service provider to understand the properties of the material that you choose to manufacture your product. Non-uniform or unexpected flow rate problems can create unpredicted residual stress, which in turn can affect the required performance of the finished part.
Cooling time problems. Flow rate can affect cooling time for a part, but there are other factors that may lead to problems in this area as well. The biggest issue with cooling time is differing cooling times and rates for different areas of a part. If this situation occurs, the piece will set (or cool down and harden) unpredictably, and its residual stress will in turn be affected. Flow rate, material choice, part design and adequate cooling time are all ways to avoid problems here.
Improper part design. As mentioned above, the design of your product is very important in managing residual stress. Always keep injection molding best practices in mind when designing your part: rounded corners, simplicity in design and draft, just to name a few. Avoiding other design problems in this way serves the double benefit of helping to keep residual stress managed.
Temperature fluctuations. While injection molding can seem simple in concept — fill a mold cavity with a heated, liquefied plastic or rubber substrate and wait for it to cool and harden — the reality is that there are many complicating factors at play, especially when it comes to material choice. Different types of polymers have different material properties, and can be affected by post-production heating or cooling, whether as part of a finishing process or during shipping and transportation. The flaking, softening, warping and other problems that may occur as a result of these temperature fluctuations will often have the secondary effect of complicating the residual stress of a part.
Now that you know some of the causes of residual stress, let’s examine the kinds of problems that it can lead to when left unmanaged:
Part failure. One of the worst effects of residual stress is part failure in the field, in the course of use. Residual stress can weaken a part or cause it to be more susceptible to bending, warping or breaking. Aside from the corresponding maintenance costs for both the piece and the product that it may be a component of, part failure can of course lead to injury or worse for the people who are relying on it to operate successfully. Such an incident can be potentially devastating for your business.
Line rejection. Residual stress will often be detected as a result of improper functioning of a part during QA or other testing. As you know, every piece that has been manufactured but remains unused is a blow to your bottom line and your profit margins. By understanding residual stress and factoring it into your part designs, you will be able to attenuate and even potentially avoid these types of issues.
Cosmetic issues. If the cosmetic appearance of your part is critical to its sale or operation, residual stress can lead to problems in this area. It can often lead to flaking, cracking, crazing (microcracking), and optical issues for items like PVC windows or lenses.
Finally, let’s examine some ways of detecting and managing residual stress. Since the safe and successful operation of your product is always a paramount concern, testing should always be done to identify any pieces that may have potential problems in use. An instrument called a “polariscope,” which generates polarized light is one of the simplest and most effective ways of testing for residual stress. When shone through an injection molded piece, polarized light will reveal areas of residual stress due to the interference generated in the polarized light waves as they pass through the product.
As mentioned throughout this piece, management of residual stress is the most crucial part of the process — not its avoidance, which is not possible. For this reason, it is important to understand and plan for where in your part residual stress is acceptable — for instance, in the thickest or strongest areas of it, not any extremities or isolated parts. You can also determine how much residual stress is acceptable to the operation of your part — especially on areas where applied force will also be present — and ensure that your finished pieces stay within this range through smart product design.