What are the five common problems with injection molding of plastic parts?

Injection molding is one of the most common manufacturing processes because it is fast, economical, and produces high-quality products. However, many factors can affect the quality of a product that is injection molded.

There’s a lot of room for expensive errors, ranging from minor surface defects to serious issues that can affect the performance and overall function of the product. These problems can be caused by a myriad of factors, including inappropriate molding process, low-quality materials, incorrect tooling design, or a combination of all three.

But as with any quality problem, knowing how molding effects arise is half the battle. Manufacturers should understand the root cause of problems to avoid them to happen in the future.

Here, we have put together the five common issues that often occur when carrying out injection molding of plastic parts. Check them out and find out how you can fix them without putting much hassle.

A Brief Introduction to the Injection Molding

Injection molding is a manufacturing process for making plastic parts by forcing molten plastic under high pressure into the shapes of the parts. The process is used to create objects like toys, car parts, and appliances.

This approach is ideal to meet the bulk requirements of the client and lets you manufacture tons of identical products within the least possible time. It is an efficient production method and requires a high degree of technical expertise to master.

But with plenty of variables in play, small and seemingly minor mistakes in the early phases of product development can lead to major problems and even compromise product integrity down the line.

These defects can further reduce the speed and cost-efficiency of the entire product development process while shortening product life spans if left unchecked. Further, we will learn about the 5 most common problems with Injection Molding and how product teams can avoid them.

A Guide to the 5 Common Injection Molding Defects & how to fix them

Every manufacturing process is prone to glitches and Injection Molding is no different. These defects and issues can be caused by a host of reasons, including quality control failures, production process mistakes, and poor design.

It’s important to take a rightful approach to cut down the risk throughout the product development process. It finally decreases the chances of potential injection molding problems and helps you carry out the process effectively. Here, we have outlined the 5 major and common problems manufacturers generally face when using the Injection molding approach.

1.Flow Lines

Referred to as the major Injection Molding defect, Flow Lines seem like a wavy pattern of a slightly different color than the surrounding area. It generally appears on narrower sections of the molded component.

In addition, Flow Lines may also appear as ring-shaped bands on a product’s surface near the entry points of the mold or gates, where the molten material flows through.

While these flow lines won’t typically impact the integrity of the component, they are unacceptable if found in certain consumer products, like toys or sunglasses. Various reasons might trigger these streaks or unusual patterns. Here are mentioned some of them.

Common Reasons For Flow Lines

  • Flow Lines occur due to the varying speed at which the molten plastic flows when it changes direction through contours and bends within the mold tool.
  • These streaks are also caused when the plastic flows through sections with varying wall thickness.
  • Another reason for this defect to occur lies in its injection speed. It might be caused when the injection speed is too low, making the plastic solidify at different speeds. In that case, the plastic injected into the mold becomes half gummy and half solid when filling the mold, which further created a flowy, wavy pattern.

How to Fix the Flow Lines in Injection Molding?

Follow these common remedies to fix the Flow Lines when carrying out the Injection Molding process.

  • Keep the injection speed, pressure, and material temperature high and ensure that the material fills the mold before solidifying.
  • Always smoother the edges of the mold to promote a consistent flow rate.
  • Elevates the nozzle diameter to increase the flow speed and restricts early cooling.
  • Create more distance between the mold gates and mold coolant.

Burn Marks

Burn marks are yet another common problem in injection molding of plastic parts. They typically appear as black or rusty discoloration on an edge or surface of the molded plastic component.

Burn marks don’t impact the integrity of the product unless the plastic is burned to the extent of degradation. There are several causes of burn marks in injection molding, and each requires a different approach to remedying them.

Causes of Burn Marks

  • The primary cause for burn marks in the injection-molded component is none other than trapped air, or the resin itself. It overheats the mold cavity during injection, thus leading to such a problem.
  • Yet another common reason that triggers burn marks in Injection Molding is excessive injection speed or high material temperature that often leads to overheating that causes burns.

How to Fix Burn Marks in Injection Molding?

Here’re mentioned some common solutions to keep these annoying burn marks at the bay.

  • Reducing the melt and mold temperature might help in dealing with this issue. It prevents overheating and protects the component from burning.
  • Increase the size of gas vents and gates to allow trapped air to escape.
  • The next best solution that can help fix the problem is shortening the mold cycle time so that any trapped air and resin don’t have a chance to overheat.
  • Decrease the injection speed to mitigate the risk of trapping air inside the mold.

Sink Marks

Sink marks are depressions that can be seen on the surface of the part where the molten plastic has pooled and solidified. They can be caused by several factors, including poor injection techniques, flawed parts, and improperly designed molds.

Sinks marks often occur near areas of high stress or heat, such as junctions between parts or areas where two pieces of metal intersect.

Causes of Sink Marks

  • These small depressions or sink marks appear when the part inside a molded component shrinks. 
  • The material near the exterior of the part cools too slowly, resulting in shrinkage and sink marks. As with voids, these depressions are more likely to occur in thicket parts of a plastic component.

How to Fix Sink Marks in Injection Molding?

Follow the remedies mentioned below to avoid the occurrence of sink marks in the future.

  • Always create a mold with thin walls to attain faster cooling near the surface.
  • Enhance the cooling time to mitigate shrinkage and sink marks.
  • Increasing the holding time and pressure may also help as it allows the material near the component’s surface to cool.

Vacuum Voids

Also known as air pockets, vacuum voids look like air bubbles trapped inside a finished molded component. These are similar to sink marks but they are reversed in cause and effect.

While these air pockets are often termed minor defects, larger or numerous voids can directly impact the molded part and weaken it.

Common Causes of Vaccum Voids

  • Inappropriate molding pressure is the basic reason to trigger vacuum voids in the plastic component.
  • The problem is also caused when the material nearest to the mold wall solidifies too quickly, causing it to harden and creating an abscess.
  • Vacuum Voids also occur if the density changes significantly from the molten to a hardened state.
  • These are quite popular in molded parts that are thicker than 6mm.

How to Fix Vaccum Voids in Injection Molding?

The common ways to limit vacuum voids in the molded part are mentioned below.

  • Increase the injection pressure to force trapped air pockets out.
  • Select a material with lower viscosity to reduce the risk of the formation of air bubbles.
  • Inset gates close to the thickest parts of the mold.


Warping is a distortion of the plastic part caused by heat and pressure. The most common type of warping is called “crease-back” warping. Crease-back warping happens when the plastic cools and contracts in the middle of the part. Other types of warping can also occur, depending on the type of injection molding process used.

There are several causes of warping in injection molding and some of them are mentioned here.

Causes of Warping

  • One of the major causes of warping is that solidification happens too quickly. And what makes the problem worse is the excessive temperature and low thermal conductivity of the molten component.
  • Mold design can also contribute to warping, especially when mold walls are not of uniform thickness.

How to Fix Warping in Injection Molding?

Use these simple methods to resolve the warping problem in injection-molded products.

  • Make sure that the solidification process is long enough to prevent uneven shrinkage.
  • Keep in mind the wall thickness and redesign the mold to promote part symmetry.
  • Reduce the mold temperature to avoid warping.


These are the five common Injection Molding defects. We hope you have learned about major causes and remedies for the same to avoid them in the future. Injection Molding is one of the leading approaches to producing identical products; however, it typically requires significant upfront investment in tooling. 

Make sure to design the molds appropriately and then initiate the process to avoid minor and serious defects later on.

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