Injection molded thermoplastics are a common form of manufacturing due to their versatility and relative low cost when compared to other methods of fabrication. They are used in a wide variety of industries, from automotive to home appliances. Despite the advantages of injection molded thermoplastics, it can still be susceptible to a variety of defects. Understanding the types of defects that can occur and the causes can help manufacturers rectify them and produce parts with the best-possible quality.
The most common defects with injection molded thermoplastics are burn marks, flow marks, voids, sink marks, warpage, die lines and weld lines. Burn marks occur when local temperatures reach high levels on the surface of the part due to wear of the barrel. Flow marks are wavy lines on the surface of a part caused by inadequate flow of the plastic melt. Voids are cavities in the part resulting from non-uniform flow of the plastic. Sink marks are depressions in the part caused by non-uniform wall thickness or insufficient cooling. Warpage is the distortion of a part caused by non-uniform shrinkage of the plastic due to inadequate cooling or high temperature. Die lines are lines or grooves on the part caused by differential pressure between the dies. Weld lines are caused by different flows of plastic from two different injection points meeting in the mould.
There are several common causes for the above defects in injection molded thermoplastics. Poor material selection and incorrect residence time of the plastic melt in the barrel are two of the main causes. Poor design of the part often results in insufficient cooling, which can also lead to defects. Wear and tear on the barrel, as well as settings on the barrel that are incorrect, can result in a build-up of heat in the barrel and ultimately cause defects on the surface of the part. Poor maintenance of the mould can also lead to defects, such as an inadequate release of the part from the mould.
In order to avoid these defects, manufacturers must employ proper selection of materials, correct residence time in the barrel, ensure adequate cooling of the mould, and ensure proper maintenance of the barrel and mould. By taking these steps and understanding the common defects that can occur in injection molded thermoplastics, manufacturers can produce injection moulded parts with the best-possible quality.
