Defects and Causes of Injection Molding Products Made of Thermoplastics
Introduction
Thermoplastics are polymers materials with the ability to be repeatedly softened on heating and hardened on cooling, which makes them suitable for injection molding processes. With injection molding, thermoplastic polymers can be produced into many shapes and sizes such as personal accessories, medical devices, and car parts. The injection molding process is an automated process that involves injecting molten plastic into a mold (also called cavity) to represent desired shapes and parts. Although beneficial, the injection molding process can suffer from some defects due to the complexities and complexities in controlling the process variables, such as temperature, pressure, and moisture. This article will examine some of the common defects seen in injection molded thermoplastic products and their potential causes.
Weld lines
Weld lines, also known as knit lines, occur when two separate streams of molten plastic meet at a welded surface. Typically, weld lines are weak points in the plastic and can lead to deformation of the part or even stress cracking. Weld lines can appear for different reasons, but the most common is when there is a low injection pressure. Low injection pressure is due to highbackpressure or in other words, an injection speed that is too slow or too fast. A slow injection speed may not be able to keep the molten plastic streaming at a fast enough rate, while a fast injection speed may lead to pressure.
Warpage
Warpage is a type of defect in injection molded thermoplastic products that can be classified as nonuniform dimensional distortion. Warpage happens when the surface of the part does not remain level as it cools down after the injection molding process. This may be due to differences in the cooling rate of different regions of the part, or due to improper clamping forces. To prevent warpage, it is important to match the part design and material choice to the clamping force and ensure that the mold’s cooling channels are evenly distributed.
Flash
Flash is another common defect seen in injection molded thermoplastic products, and is caused by molten plastic that is forced out of the mold during the injection molding process. This is mainly due to an improper clamping force that allows molten plastic to escape through any gaps between the mold and the workpiece. Flash defects can be prevented by controlling the clamping force and by reducing the amount of plastic that is injected into the mold.
Conclusion
Injection molding thermoplastics is an effective method of producing many shapes and sizes of products. Although beneficial, the injection molding process can suffer from some common defects due to difficulties in controlling process variables such as injection pressure, temperature, and cooling rate. These defects include weld lines, warpage, and flash. In order to minimize these defects and improve the overall quality of the products, it is important to match the part design and material choice to the clamping force, ensure that the injection pressure and speed are controlled, and reduce the amount of plastic being injected into the mold.
