One of the methods of producing plastics is forming them using an injection mold. This involves heating the material to the appropriate temperature and then injecting it under pressure into the mold. This technology provides a wide range of possibilities for creating parts of various thicknesses and complex shapes. However, despite the universality of this process, we may encounter obstacles in the form of injection defects, which have a significant impact on the subsequent painting process. In this article we will try to present and briefly discuss them.

What are molding defects?

These are primarily imperfections on the surface of the formed part. There are many reasons for their appearance. The most common of them are:

• incorrectly set injection process, including inappropriate selection of parameters
• incorrectly made or damaged injection mold
• incorrectly selected material or using too much regranulate

Types of injection molding defects

There are many different defects that may appear in the part forming process. They are usually noticeable immediately after the part comes out of the molding, but there are also those that become visible only after painting. In our article we have listed the most common ones.

• Sink marks – visible as a depression on the surface of the part. They arise as a result of shrinkage of the volume of the material in the injection mold or its uneven pouring, which results in differences in the thickness of the walls of the part. This defect is usually eliminated by filling the cavities with the pressure of the piston pressing the material.
• Flash – this is the pouring of material through the slots of the injection mold, most often felt as a thickening on the edge of the part. It is usually caused by an inaccurate fit of the mold or too much pressure distorting it.
• Short shot – a defect in the material that has not been filled in the molding cavity. The cause may be a poorly set gating system or too little material delivered to the injection mold.
• Weld lines – this is the place on the surface of the part where the plastic streams connect with each other, most often after bypassing an “obstacle”, e.g. a hole. Joint lines may also be formed by too low a temperature of the material, which leads to its uneven spreading. Sometimes it is a thin line that can be covered during the varnishing process, but it also happens that even several layers of varnish cannot hide this unsightly defect.
• Bumping – usually appears at the stage of pushing the part out of the injection mold, sometimes during packaging, when it is still warm and susceptible to shape changes, or during incorrect mounting on the painting frame.
• Warping – characterized by an uneven shape of the part, most often in the place of the ejector pins, which leave marks when the part is placed too tightly in the mold.
• Burns – these are colorless streaks, most often visible only after varnishing. They are caused by improper venting of the injection mold.

How do mold defects affect painting?

If we plan to varnish a raw part, we must take into account possible defects of the product. We can hide some of them by selecting the appropriate paint or primer, structural finishing of the part (i.e. rough to the touch) or additional activities such as sanding. However, very visible defects cannot be eliminated by painting. Here, the solution is increased control of the injection process, changing parameters or taking a closer look at the injection mold. It is worth contacting a professional industrial paint shop which will help you choose the right solution and advise you what to do to make the defects as less visible as possible.

Summary

Molding defects are a common problem in the plastics industry. They result not only in the lack of aesthetics of the parts, but also in reducing their strength and functionality. It is important to regularly perform technical inspections of the injection machine and its components, and to follow the recommendations of manufacturers and technologists.