Burn marks on injection molding products can result from numerous factors. Issues can arise from hurried production processes or the lack of experience among operators, leading to potential quality issues at any stage.
Below are some of the primary causes of burn marks in injection molding.
Rapid Injection Speed
When molten material is injected into the mold at excessive speeds, it can become trapped with air and gases, preventing their escape. This creates a high-pressure situation inside the mold. The compressed air heats up significantly due to the fast-moving molten plastic, and turbulence may develop, further hindering the escape of trapped gases.
This heated, compressed air can scorch the sensitive plastic material, resulting in burn marks. Moreover, high injection speeds increase the shear stress on the material, which can lead to its degradation and make it more vulnerable to burns.
High Mold and Material Temperatures
It is vital to maintain appropriate temperatures during the injection molding process. If either the mold or the plastic material becomes too hot, it may lead to a chemical breakdown of the resin.
An overly hot mold can reduce the material’s viscosity, causing it to flow too swiftly through the channels. This situation may exacerbate issues related to high injection speeds, as the rapid flow does not allow enough time for trapped gases to escape, increasing the likelihood of burn marks. This is especially critical with temperature-sensitive materials like PVC, which can break down quickly and are particularly prone to burn marks.
Degradation of Resin
Resins are affected by various factors, such as heat, shear forces, and exposure to certain chemicals.
High temperatures in the barrel or keeping the resin in the barrel for an extended period can lead to its breakdown. Similarly, excessive shear forces from worn components, like the screw and barrel, can damage the resin. When the resin degrades, it loses its original thermal and flow properties. A compromised resin does not process like a fresh one, making it significantly more prone to burn marks during injection.