5 Injection Molding Defects Explained

Injection molding defects
Injection molding defects
Resource: https://youtu.be/dgadvQmCrSE

Injection molding defects can make parts less appealing, weak, or unable to properly function in their specific applications. This guide discusses the five most common of these injection molding problems and how to avoid them. Also included are troubleshooting tips to help fix the defects.

About Injection Molding

Before we can discuss injection molding defects, let’s review the process itself. Injection molding is a technique to manufacture parts where melted plastic (or powered metal and glass material) is injected into a pre-made cavity in a metal die.

Injection molding is quite efficient. It allows engineers to control all aspects of the manufacturing process, from ensuring precise temperature levels to using specified pressures.

However, just like it would happen in other manufacturing processes, this technique is not exempt from defects. The most common injection molding issues are explained below.

Injection molding sink marks
Injection molding sink marks
Resource: https://youtu.be/6WEh3ZysU6A

Injection Molding Defects

The five most common injection molding defects we’ll be discussing are: sink marks, weld lines, warping, injection molding short shot, and flash. Later, we will see how these problems can be avoided and how to troubleshoot them.

1. Injection Molding Sink Marks

Plastic injection molding sink marks, also known as shrinkage marks, are caused by the material shrinking in the back of the part being molded. This is usually due to the melt cooling too slowly. Sink marks show up as depressions, or marks that are visible on the top surface of the part.

As one might expect, this defect is typically more common with thick-walled parts (or sections where the part is thick) than thin-walls. To avoid injection molding sink marks, engineers should use a plastic material with the right properties.

2. Injection Molding Weld Lines

Injection molding weld lines occur when two or more melted plastic streams from different injection points meet together in the mold and don’t bond properly.

These are visible as lines on the surface of the part, which is why they are also called flow or weld lines. The strength of a weld line can be weaker than the rest of the part, making it a weak point.

3. Injection Molding Warping

Warping is one of the major injection molding defects, where the part ends up with a bent or distorted shape due to uneven shrinking stress different section of the molded part.

This commonly happens when the plastic in one area of the cavity cools down faster than other areas, causing it to shrink. The results of warping in injection molding are unevenly shaped parts that may not function correctly.

Causes include poor mold design, molding parts with thick walls which are also not uniform. Warps will also occur if the material used does not adequately conduct heat.

4. Injection Molding Short Shot

Injection molding short shot occurs when the molten plastic is unable to fill the entire cavity. This leads to a part that is only partially formed and can have missing features or incorrect dimensions.

These types of injection molding defects are usually caused by poor material flow or insufficient injection pressure. Other causes include narrow or blocked injection molding gates restricting the melt from optimally flowing into the mold.

This problem may also result from trapped air hindering the flow of material. As mentioned, an injection molding short shot may appear as incomplete features, missing details, or even voids.

5. Injection Molding Flash

Flash in injection molding refers to the excess material that leaks out of the mold cavity and sticks to the part. This defect is caused by one or more of the following: improper gate size, incorrect plastic pressure, too high injection rates, or not enough venting.

The result is a part with protrusions (or lips) on the edges, which is why these defects are also called burrs or spews. In some cases, these injection molding defects do not present serious functional problems; however, they can affect the aesthetics of the part and must be trimmed off.

Injection molding troubleshooting
Injection molding troubleshooting
Resource: https://youtu.be/uWBObqn6eJI

Injection Molding Troubleshooting Chart

Having looked at the common problems with injection molding, let’s take a look at a troubleshooting chart that can help you identify and solve these problems quickly.

Injection Mold Defect

Possible Solutions

Sink Marks

Increase part holding pressure and duration

Increase cooling time

Optimize mold temperature

Use thinner part walls

Weld lines

Increase injection speed

Increase hold pressure

Inspect the vents

Increase melt and mold temperatures


Ensure gradual mold cooling

Use materials offer good thermal conduction

Use lower mold ad melt temperatures

Short shot

Adjust injection speed and pressure

Increase barrel and mold temperatures

Optimize venting

Increase gate, runner, and, sprue diameters


Optimize gate size and position

Reduce mold temperature

Ensure proper venting

Reduce material temperature

As you can see, injection molding issues are easily solved with a combination of engineering principles and process controls. With that in mind, it’s important to consider the injection molding troubleshooting chart given above when dealing with any type of issue.

Injection molding best practices to reduce flash
Injection molding best practices to reduce flash
Resource: https://youtu.be/vsJzM_NjU_k

Injection Molding Best Practices

Injection molding defects can be prevented, usually by following proper molding rules and mold design strategies, right from the design stages of the manufacturing process to production and quality control. To give you an idea of what that means, here is a list of injection molding best practices:

Injection Molding Undercuts

Injection molding undercuts make it difficult to extract a part without destroying it. The rule of the thumb is to avoid them altogether if possible. If you must, the solution is to use side-action mechanisms or sliders, which slide out of the way. Other design tricks include using shut-offs that obstruct the molding material and bump-offs, or collapsible cores.

Injection Molding Wall Thickness

In injection molding, wall thickness helps control part strength and shrinkage. Parts should be designed with uniform wall thickness to ensure even cooling, which helps prevent defects. The ideal wall thickness will depend on the design. Generally, 1 to 4mm is recommended, and the use or ribs and bosses advised to keep uniformity.

Injection Molding Rib Design

Ribs are used to reduce the wall thickness in areas where a uniform wall is not a possibility. The biggest challenge with injection molding rib design is to avoid sink marks and warping. To do this, ribs must be designed properly and optimized for the specific material being used.

The ribs must be 50 to 60% thick and less than 3 times high than the nominal wall thickness. They should also be designed with a 1° draft angle, and spaced not bigger than two times the nominal thickness.

Injection Molding Draft Angle

An injection molding draft angle helps make it easier to remove the parts from the mold, minimizing potential damage and deformation of the part. The ideal angle will depend on the material and design, but in general, a draft between 1°-3° is recommended for injection molded parts. Check out this video for more.

Inspection of Injection Molded Parts

It cannot be stressed enough how important it is to regularly inspect your injection molded parts. This helps detect potential defects early and can save a lot of time in the long run. With the right inspection of injection molded parts, manufacturers can quickly identify and address any issues that arise.

By following these best practices, you can avoid many of the common injection molding issues. Additionally, it’s important to use high-quality materials and select the right process parameters for each part design.


Injection molding defects present a variety of challenges for manufacturers, but fortunately, they can be prevented by following best practices and troubleshooting correctly. By understanding the causes of these issues and taking a systematic approach to solving them, you can ensure your parts are manufactured correctly and quickly.

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