Injection Molding Quality Issues

image of plastic injection mold makingWhen you develop a plastic injection parts, you must meet few defects that happens to your project. Some of defects might be minor and would not affect your product property, but some are not, they could have very bad impact to your customer feeling. So here below we would like to list the main defects existing in injection molding process. And you can watch out when you review your project and avoid them to happen in advance.

  1. Flash
  2. Deformation
  3. Sink Marks
  4. Weld Lines
  5. Flow marks

Generally those defects could be result by material, mold tooling, and injection molding process. Let’s look it into one by one


Flash is an excess material that overflow on the edge/corner of parts, instead of the original part structure. It will affect your part function, as well your component assembly. The flash is visible when you check the molded part. Most of injection molding factories will sort out when they found it by QC or operators. But to have a long-term troubleshooting solution, you should also always check the mold tooling where should be some small gap among mold parts. On the other hand, if you injection molding pressure is big, it could also result in flash to happen.


Deformation is also a common defect for injection molding parts. When you take the deformed parts, you can obviously find out the part is deviation in shape, especially it’s visual when you put the parts surface on a inspection platform. If you want to know exact data of how much it deformed, you can use a pin guage to check.

Deformation could be result by incorrect material, mold tooling, and improper injection parameter. When you encounter a deformation problem, you should check it seriously and comprehensively. Otherwise you might be hard to resolve the problem.

Sink Mark

Sink mark is a small recessed dot or area from a platform or surface of parts. It will affect cosmetic of part seriously. The causes of sink mark could be uneven wall thickness and bad cooling system that concentrate the plastic material to accumulate in a certain area.

To solve sink mark issue, you must adjust the wall thickness of part, and try to balance the material flow system. Secondly you should cool down the mold temperature. Please keep these two tips on mind. It’s very helpful for you.

Weld line

Weld line usually happens on the circle or arc features of a part. It’s a nature phenomenon of material flowing. You can minimize it, but you can never avoid it. You can improve it by raising the temperature of mold tooling or molten the raw material. And you can also adjust the injection molding speed

Flow marks

Flow marks is identified as uneven lines, wavy lines, or patterns. It is very common defect for plastic molding process. The main reason for this is that the Injection speed is set too slow, and plastic material becomes partially solid while still filling up the mold tooling that causing the wave pattern to appear.

To solve flow marks issue, you should adjust your injection speed of machine. And make the nozzle diameter larger, so the flow process can move smoothly.


Finally I think you should have some basic idea for main molding defect after my sharing above. If you still have other question or project to run, you are welcome to contact us for help. our team will be always for you here in Shenzhen China.