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Different Types of Injection Molding Defects and Solutions

Injection Molding Defects Caused by Molding Process and Solutions

The injection molding processes are not the same for all parts. When you use any wrong method, it might cause an injection molding defect. However, you can prevent such a possibility with some simple adjustments. You need to adjust the flow rate, temperature, and some other parameters to avoid defects. In the following, we will cover some defects, causes, and their solutions.

injection molding defects

Flow Lines

These lines look a bit different than the surrounding area. Yes, they will have a slightly different color. You can recognize them from their ring-shaped bands on the product surface. Also, you will find flow lines close to the entry point of molds. Flow lines might not affect the integrity of products. But your customers will not appreciate this injection molding defect.

What Causes Flow Lines

The material temperature and low mold temperature might lead to this defect. Besides, slow injection speed and pressure can cause this condition. Also, the small size of the flow channel and gate can create this issue. Another factor is variations in the cooling speed while flowing in diverse directions.


You need to boost the speed, pressure, and temperature of the material to avoid this defect. Besides, you can add distance between the mold coolants and mold gates. You can increase the diameter of the nozzle and minimize the first channel size.

Sink Marks

You will notice small recesses and depression in this defect. You can find them on the small surface of molded parts. You can have this defect when there is a shrinking in the inner part of the molded component. The shrinking will pull the material inward. That will result in sink marks.

What Causes Sink Marks

You can have this defect with a low injection or holding pressure. Some other causes are too short holding or cooling pressure and time. Also, very high molding or melting temperature will lead to this condition. Improper structural design is another factor for such defects.


You need to reduce the wall size of the material. Also, you will have to control the thickness of the adjacent wall. Besides, you can boost the holding pressure and time. You can increase the cooling time as well.

Weld Lines

You can also have this defect in molded parts. You will see molten material on the part’s surface. Initially, the material will split off into more areas. You can experience this defect when the material bonding is weak. The poor fix will impact the strength of the part.

What Causes Weld Lines

Too high or too low temperature can lead to molten materials. Other causes are uneven thickness on the wall and holes on the surface.


You need to improve the flexibility of your molten material. Consider using a venting slot and adjusting the gate position. Apart from that, you can boost the melting temperature and minimize the release agent’s amount.

Burn Marks

The burn marks appear as black discoloration. Yes, you can find them on molded part’s surface. They might appear rust-colored as well. Even if these marks do not affect your parts’ integrity, they might cause fast degradation.

What Causes Burn Marks

The molded parts can have burn marks when air removal takes time. Also, too much melting temperature can cause this condition. Excessive injection speeds, improper design, and high speed in the runner’s system are other factors.


You can avoid this mistake by minimizing the molding and melting temperature. You can also slow down the injection speed. Besides, you can consider adding an exhaust system.


The molded parts can have this defect when they cool and solidify unevenly during plastic processing. This condition will lead to twists and bends in the end product.

What Causes Warping

The cause is premature cooling of the raw material. Also, it can happen due to uneven walls and overheating.


A gradual cooling process will prevent this condition. Also, you will have to adjust the temperature of the mold during production. Besides, you can consider some materials that will not shrink fast and excessively during the cooling process. Focus on the uniform thickness as well.

Air Pockets

You can experience this bubble defect in injection molding. It might not be a noticeable defect. But, it can impact the strength of the final product.

What Causes Air Pockets

The most common cause is insufficient mold pressure during low-volume manufacturing. You will find the air bubbles trapped in the mold. They will not cool and solidify during the molding process.


You will have to increase the injection pressure to force all the trapped bubbles to come out of the mold. You can also use a material that comes with low viscosity. As a result, the air bubbles cannot form and cause such defects. Also, you will have to make sure that the gates are close to the thickest parts. If you do so, the material will take time to cool.


Unlike many other defects, jetting can impact the integrity of the end product.

What Causes Jetting

This defect happens due to a sudden and fast jet of molten materials. The parts can have this problem while experiencing high injection pressure. The jet stream will start to cool fast compared to other areas of the cavity. That will cause wavy formation in the part’s surface.


You can minimize the injection pressure to avoid jetting defects. Also, you can increase the temperature of the material and mold to ensure that the jet stream will not solidify fast compared to other materials.

Injection Molding Defects Related to Storage or Materials Used and Solutions

The molding process can cause most of the defects in the end part. However, the material itself can cause some issues. The improper handling by the manufacturer can lead to some problems. However, most defects will be minor and impact the look. But, some flaws can affect the strength and cause safety issues as well. Here are some material and storage defects.


Discoloration and color streaking can happen when the parts have a different color than the intended appearance. However, it mostly comes in localized areas. This defect will not impact the strength and durability. However, it will affect the appearance.

What Causes Discoloration

Some residual resins or leftover pellets in the nozzle or hopper can cause such defects. Besides, the mold from the previous production can result in discoloration. Some other factors are inadequate masterbatch mixing and inappropriate colorant thermal stability.


You will have to clean the nozzle, mold, and hopper between your production processes. Also, you will have to ensure that the masterbatch mixing is even. Besides, you can make sure that the colorant thermal stability is adequate.


Delamination defects can cause surface layer flaking. However, it is a different type of defect. It can impact the strength and durability of the molded component.

What Causes Delamination

The molded parts can have this defect due to the mixing of the mold with any incompatible polymers. Also, if manufacturers use release agents excessively for the molding, they can have this defect. Apart from that, excessive moisture can lead to such defects.


Make sure that you have compatible materials during the production process. Also, you need to remove all the possible impurities from the mold.

Injection Molding Defects Caused by Maintenance or Poor Mold Design and Solutions

In addition to all these, poor mold design and improper maintenance can lead to some defects. These defects are not minor. If you notice these flaws at a later stage, you will have to spend more to correct them. Some of these mistakes are in the following.


A flash defect in injection molding can happen due to the excess molding material. The molded part can have this defect when some materials flow out of their intended channels. These materials will flow from the surface and enter the space between tooling plates. Some even flow to the injection pin.

What Causes Flash

Any part can have this defect when there is an issue with the molding material. Also, the insufficient clamping force can lead to this problem. Some other causes are improper design, inadequate exhaust system, and poor management.


You can increase the plate’s clamping force. Also, you can design the mold the way it can close whenever needed. You will have to ensure the proper flow of the material as well.

Short Shot

This defect will occur when molten materials do not flow to fill the cavity of the mold. It is not ignorable and can change the appearance of the part. It can impact the functionally as well.

What Causes Short Shot

High viscosity material, trapped air pockets, and inadequate injection pressure are a few causes of such defects.


You can use a thinner base material. You can increase the injection speed and pressure. Also, you can boost the mold temperature and use more air vents.


You can experience all these injection molding defects. However, you can avoid many of them by following some simple measures. You will have to take care of the temperature, material, and speed during the manufacturing process to reduce the defects significantly.

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