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5 Injection Molding Defects to Prevent and How to Fix Them

It’s always ideal to understand the common injection molding defects to avoid or fix. Learn from this helpful guide below.

5 Injection Molding Defects to Prevent and How to Fix Them - KB Delta

Injection molding defects happen, even to the most knowledgeable and experienced operators today. Molding defects are not purely symptomatic of inattention or inexperience as several factors can contribute significantly to them. But they usually occur due to lack of experience or use of the right tools.

Of course, an incredibly high level of attention to detail and technical expertise is necessary to prevent small errors. Small mistakes usually cost organizations a lot of money with regard to the mass production of new and original parts.

This is why ingenuity and creative solutions should abound in staff or employees with the right experience and an ideal combination of software and hardware.

Prevention is always a much better option and has to do with highly competent design. This post will discuss a few molding defects that may occur during injection molding, as well as several ways you can fix them.

Here are some of the common injection molding defects:


  • Weld lines
  • Flow lines
  • Surface delamination
  • Jetting
  • Flash


Let’s discuss each injection molding defect and how to fix them.


1. Weld Lines

Weld lines usually appear in parts where molten plastics flowing from 2 different mold parts meet each other.


  • Causes of Weld Lines

The insufficient bonding of 2 – or more – flow fronts due to the partial solidification of the molten plastics causes weld lines.


  • How to Fix Weld Lines

You can do any of the following to fix the problem:


  1. Boost the injection speed.
  2. Increase the temperature of the molten plastic or mold.
  3. Substitute the current plastic with one that has a lower melting pressure or lower viscosity.
  4. Alter the design of flow patterns until it becomes a single source flow.


2. Flow Lines

Flow lines are usually off-tone colored patterns, streaks, or even lines. They appear on prototype parts as the aftermath of the cooling profile and physical path the molten plastic follows right into the injection mold tooling cavity.

The journey of injection-molded plastic starts its journey by flowing through the part tool by way of a ‘gate,’ i.e., an entry section. The plastic continues its journey via the tool cavity, starts cooling, and finally hardens into a solid.


  • Causes of Flow Lines

The varying speed of the flow of molten plastic, especially when its direction changes via the bends and contours within the mold tool, causes off-toned flow lines. This defect can also occur when the molten plastic travels through sections where the wall thickness varies.

When the injection speed is somewhat low and causes the solidification of the molten plastic at different speeds, flow lines occur.


  • How to Fix Flow Lines

The following can be done to remedy flow lines:


  1. Ensure the gate or entry section is located at an excellent spot – with thin walls – within the tool cavity.
  2. Ensure every location and corner is perfectly rounded, especially in places where the wall thickness varies. This prevents sudden changes in flow rate and direction, which causes flow lines.
  3. Significantly increase pressure and injection speeds until they reach optimal levels. This ensures the proper filling of all cavities while preventing the molten plastic from cooling unceremoniously.


3. Flash

Flash refers to a molding defect that generally occurs when some molten plastic runoff from the mold cavity via ejector pin or parting line locations. The extrusion eventually cools, solidifies, and attaches itself to the finished prototype.


  • Causes of Flash

Flash may occur due to the improper clamping of the mold with an adequate force that is potent enough to readily withstand the opposing forces that the molten plastic generates as it flows through the mold.

Moreover, flash can also occur due to the continued use of worn-out molds. Excessive injection mold pressure can also force out the molten plastic through any route that offers little resistance.


  • How to Fix Flash


  1. Proper maintenance and regular cleaning of the mold ensure flawless operation. Any mold that has reached its lifespan must be replaced immediately.
  2. Increase the clamp pressure adequately. This ensures that the mold parts are always shut during shots.
  3. Opt for optimal molding conditions such as injection pressure, injection speed, adequate gas venting, and mold temperature.


4. Surface Delamination

Surface delamination refers to a condition in which thin surface layers appear on the prototype part due to contaminants. Such layers usually appear like coatings and can be peeled off – or ‘delaminated’ – by hand in most cases.


  • Causes of Surface Delamination

At times, foreign materials somehow drift into molten plastic before it becomes a finished product. The contaminants cannot bond with the plastic, affecting the appearance of the part, including its strength.

Since the contaminant is trapped within the prototype, it becomes a localized fault. Moreover, depending too much on mold release agents can also bring about delamination.


  • How to Fix Surface Delamination

The following can be done to fix surface delamination:


  1. Intensify the mold temperature.
  2. Proper pre-drying of the plastic before molding should be carried out.
  3. Do away with or minimize over-dependence on mold release agents by focusing more on the ejection mechanism.
  4. Smoothen every sharp turn and corner within the mold design in order to prevent unexpected changes in melt flow.


5. Jetting

Jetting is the situation in which molten plastic somehow fails to adhere to the mold surface as a result of the speed of injection. Since molten plastic is a fluid, it becomes a solid and shows every wavy fold of the jet stream that appears on the surface of the injection molded part.


  • Causes of Jetting

Jetting occurs, for the most part, due to the high viscosity of the molten plastic. It also occurs when the melt temperature is somewhat too low. These conditions end up boosting the overall resistance of the molten plastic’s flow through the injection mold.

As soon as the molten plastic comes into contact with the walls of the injection mold, its viscosity increases even as it cools down rapidly. However, there is the material that flows right behind the viscous plastic, which pushes it further.

The result of this ‘push’ leaves scrape marks on the surface of the finished prototype.


  • How to Fix Jetting


  1. Increase the melt and mold temperatures.
  2. Efficiently optimize gate design such that it ensures enough contact between the mold and the molten plastic.
  3. Enhance the gate size to such an extent that it slows down the injection speed.


Preventing Injection Molding Defects

As you can see, most of the injection mold defects highlighted in this article can easily be prevented during the design process. This is possible when the proper tooling design is incorporated into the iterative process.

Attention to detail and a very high level of tech expertise also play crucial roles in preventing injection molding defects from occurring.



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