The Most Dangerous Plastic Injection Molding Defects

 Norah Amboko
  Nov 14, 2017

Plastic injection molding is a complex process that required great attention to details. Often, mistakes or omissions can result in millions of dollars in losses. Avoiding these mistakes is all about competency and commitment to delivering extra value to clients.

The moment you decide to look for a plastic injection molding company, it is important to take a fore-look to establish its ability to deliver flawless results. You can achieve this in three ways;

  1. Working with the company's product design team at all stages.
  2. Reviewing the company’s success in the plastic injection molding orders.
  3. Understanding the defects related to injection molding and ensuring they are avoided at all costs.

Knowing the defects that arise in plastic injection molding is an eye-opener. It helps you better understand the process and ask the hard questions. An enlightened client can put the design (he thought was perfect) on hold to take a second look at discoveries.

Remember that once the design is completed, defects such as flash could easily give your competitors a competitive edge. Do not hold back on anything that can guarantee better products, a higher value for money, and enhancing customer satisfaction. Therefore, what are these defects? How can they be minimized or eliminated?

Sink Marks

These are small depressions that form on the thick sections of an injection molding prototype or final part during cooling. Note that unlike the sinkholes that result from erosion, sink marks arise from shrinkage.

The main cause of sink marks is inadequate cooling that does not allow total curing when the part is in the mold. In some cases, sink marks arise from low pressure in the mold. To prevent this defect, a great plastic injection molding manufacturer can use the following;

  • Lowering the mold temperature and raising the holding temperatures.
  • Ample time should be allowed before opening the mold.
  • Making the thickest sections of the mold thinner. This will allow faster cooling before the opening of the mold.

Flow marks

This is one of the process-caused defects that manifest itself as lined parts on the surface. The marks are caused by the differing speed of molten plastic especially when the direction in the mold changes. You are also likely to experience the problem if plastic flows via sections that have varying wall thickness.

To address the problem, experts in plastic injection molding companies use these methods;

  • Optimizing the injection pressure to allow for better filling of all cavities in the mold.
  • Raising the molten plastic temperatures. This ensures the plastic reaches all the sections before cooling starts.
  • Rounding the corners to make molten plastic flow easily to all sections and cavities.
  • Positioning the plastic injection molding gate at the section with thinner walls.


These are wafer-like protrusions on the surface of the finished plastic part. The problem arises when molten plastic escapes from the mold cavity via a parting line. The escaping molten plastic cools down but remains attached to the final part. Often, the issue arises from poor clamping that leaves some open sections. The remedies for addressing flaw marks include;

  • Increasing clamp pressure to seal any parting line.
  • Proper maintenance and cleaning of the mold before the process starts.
  • Making sure that the plastic injection molding process takes place at the optimal condition such as injection speed, pressure, temperature, and gas venting.

The vacuum voids

These are trapped pockets of air that are close to the surface of the finished product or injection molded prototype. The main cause of the problem is uneven solidification of the mold’s outer and inner parts. In some cases, the problem can arise from the wrong alignment of the mold cavity and mold core. To prevent or rectify the defects, the following tactics are used.

  • Positioning the gate on the thickest section of the mold.
  • Using a less viscous plastic.
  • Increasing the holding time and pressure so that the part cools adequately.
  • Perfect alignment of the mold cavity and mold core.


This defect occurs from resin or its additives at the early stages of the injection molding process. It is a serious problem that makes the final part has an additional loose/peeling layer on the surface. The additional layer compromises the product’s strength as well as its attractiveness.

The main cause of this problem is contamination of the resin by moisture or other impurities. The contaminants prevent the resin from bonding well and causing a loose layer on the surface.

The problem can be addressed by pre-drying the resin pellets well before starting the production cycle. Depending on the design of the mold, raising the temperatures could also address the problem.

Weld Line

This is a plane type of line on the section where the molten plastic meet when flowing inside the mold. The main cause of the defect is the weak bonding of the flow fonts because of partial solidification. The problem can be rectified through the following;

  • Elevating the mold temperature.
  • Raising the molten plastic temperature.
  • Adjusting the flow pattern to ensure it only flows from one direction
  • Using less viscous plastic.

The final take

Understanding these defects and methods of addressing them is very crucial for any plastic injection molding process. Most of the defects can be prevented by identifying the right injection molding company. A good company will put extra effort in the design process and applying the best tooling. This is not all. A top plastic injection company helps to ensure the gates are perfectly located, the temperatures and pressure are optimized, and timing perfectly calculated. Some will even present simulations on how such defects have been avoided in the past.

Well, if you pick the plastic injection molding manufacturer with extra careful teams, the chances are that it will have carried multiple simulations to identify the problems and methods of avoiding it. They use experts and guarantee users of the best. Remember to absolve yourself of any blame in the case of defects that arise from the process.

The Most Dangerous Plastic Injection Molding Defects

Norah Amboko

Norah Amboko is a versatile writer who loves the limitless creative opportunities that writing presents. She loves blog writing, Article writing and has an uncanny ability to present copy in a number of niches. When not molesting the keyboard, she loves reading, going out for a swim or catching on the latest TV shows. Writing to her is more of a hobby rather than a boring duty. I bet we are done here. Thanks and have a lovely day!

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