Molding Defects and Troubleshooting
Injection molding is a manufacturing process for producing parts from thermoplastic and thermosetting plastic materials. It is one of the most commonly used techniques for producing plastic parts. It is a very complex process and involves the injection of molten material into a precision-machined mold that had been designed from an engineering perspective to produce parts from the injected plastic material. The quality of the end product is greatly affected by the molding process and any defects or imperfections in the finished part can be costly and time-consuming to fix. In this article, we will discuss some of the common injection molding defects and how to troubleshoot them.
One of the most common injection molding defects is sink marks, which occur when the molding process causes the plastic material to contract or shrink during cooling. This typically happens in thinner sections of the part and can cause the appearance of “sinking” in the plastic material. The cause of this defect can usually be traced back to poor injection mold design, such as an incorrect gate location or poor part thickness. To prevent sink marks from occurring, a proper gate location, proper part thickness, and a balanced runner section should all be considered. Reducing the mold temperature can also help to reduce the likelihood of sink marks.
Another common injection molding defect is flash, which is a thin film of plastic around the edges of the part caused by plastic leaking from the parting line of the mold. Flashes occur when the mold is not designed properly and the plastic material can escape the confines of the mold. To prevent flash from occurring, the mold should be designed with a proper draft angle to ensure that the plastic has a way to escape. Additionally, ensuring that the injection speed is properly set and the material being used is properly filtered can also help reduce the occurrence of flash.
A third common molding defect is warpage, which is caused by an imbalance of forces within the mold or a variation in the cooling pattern inside the mold. Warpage is typically caused by an inadequate number of cooling lines in the mold or an unbalanced cooling rate. To prevent warpage, an even cooling rate should be maintained throughout the mold. This can be accomplished by using multiple cooling lines or by adding a cooling fan to the mold to ensure that the plastic cools evenly. Additionally, a properly designed mold should also be considered to reduce the amount of stress on the plastic during injection and cooling.
Finally, there are often voids, which are pockets of air or other gases in the plastic material. These voids can cause pockets of weak spots which can reduce the strength of the part. To prevent voids, the injection starting pressure must be adjusted correctly and material flow should be balanced. Additionally, the mold itself should be properly vented to allow excess gas to escape.
In summary, injection molding defects can cause costly and time-consuming setbacks to the production of a part. Fortunately, many of these defects can be prevented and or corrected with proper understanding of the injection molding process and proper design of the injection mold. By following the troubleshooting tips discussed above, most injection molding defects can be avoided or corrected.
