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Injection Molding Defects: Causes and How You Can Prevent Them

An injection molding procedure incorporates several variables, and these have a profound effect on the quality of the injection molding product. Sometimes, these may incur minor errors. On other occasions, they may turn significant and impact the aesthetics, strength, and functionality of the final product. What are these defects, how are they formed, and is there a way to prevent them? Let’s uncover these queries through this article. 

Types of Injection Molding Defects 

Burn Marks

Extreme close-up of a white plastic component with a visible burn mark defect from the injection molding process

A blackened edge or discoloration seen sometimes on the final molded cavities is the burn mark. It happens due to air getting trapped in the cavities. As the injected plastic compresses the cavities, it leads to overheating of the mold and, hence, burning. This phenomenon leaves carbon residue behind on the burned edge of the molded part. 

Burn marks may also occur when the design of the injection mold and the plastic part trap air as the cavities are filled. The trapped air gets compressed, finally reaching the combustion temperature, thus burning the plastic and leaving behind black marks or burn marks. 

Causes, Symptoms, and Prevention 

The machines: Molding conditions may cause burn marks. For instance, if there is extremely hot molten resin running quickly into the mold or has high back pressure. Sometimes, the nozzle size and gate size may also cause burn marks. 

To prevent it, look for symptoms.

  • Black color towards the end. It displays two things: either the pressure or the molten speed is high. Due to this, the fill rate gets ignored. 
  • Black streaks. It shows that the gate size is small or at the wrong position. This poses a hindrance to the flow of the molten resin and, hence, burns.
  • Black specs. It means that the barrel or the screw requires cleansing or is damaged partially or completely.  

The mold: Inspect the mold to see if the machine seems to function fine. Remember, when the mold lacks proper manufacturing or favorable conditions, it can lead to hindrances in venting and the resin flow. 

You can prevent them by evaluating the signs such as:

  • Burn marks directly on the mold: This means that the air is trapped in the cavities. Improper construction or design may also cause this issue. Once the root cause is known, fix it by redesigning or adding more vents. 
  • Burn marks close to the end: Incorrect venting may result in this condition. Decrease the speed of the resin flow, as this will enable more venting. 

The material: If both the machine and the mold have no issues, then the problem may lie with the material. For instance, if there is a high moisture content in the resin or has water content, then the chances of it burning are high. 

Other than this, some materials, by virtue of their nature, are also prone to burn. These include acetate and PVC. So, cross-check the material you might have picked. 

Additionally, the signs that show that the burn marks are due to the material include resin chars. As the burnt resin mixes with the molten stream, it transforms into the finished product. So check the material for moisture or water in it. 

An external gas-assisted injection molding (EGAIM) is also used to reduce burn marks. This is a relatively new and significant achievement. 


Black plastic frame showing warping defect, isolated on a white background to highlight the uneven structure and distortions

When two parts of an injection molded product contract disproportionately, it is called as warping. This can make the part twist or fold in a way that may affect the functioning of the product. It may also affect the visual appeal of the product.   

Causes and Prevention

Machine errors are primarily the reason your injection molding material faces internal stress. 

  • Insufficient injection pressure: An inadequate amount of pressure will make the material solidify at an inconsistent rate. An easy way out is increasing the mold injection time or pressure. 
  • Insufficient residence time: Residence time is the total time spent by the resin in the barrel to get heated. If the time spent is less than required, then the material will not receive uniform heat. The molecules that did not receive enough heat will harden up sooner than the others, thus leading to warping. So, increase the residence time. 
  • Low Barrel temperature: The low temperature in this part causes the resin to fail to attain the required flow temperature. Eventually, the molecules solidify unevenly, leading to warping. Maintaining the optimum temperature in the barrel so that the molecules receive heat homogeneously is suggested. 
  • Low Nozzle temperature: If the temperature of the nozzle is low, it will slow down the resin’s travel time. This will prevent the packing of the molecules and, hence, uneven stacking. Therefore, the nozzle temperature should be increased by at least 10 degrees Fahrenheit. 

Vacuum Voids

Overhead view of a clear plastic part with vacuum voids, showing bubbles and gaps caused by air trapped during the injection molding process

When there is a shortage of plastic material in the mold, vacuum voids can arise. This takes place as the plastic is prone to contracting on cooling and will leave behind hollow areas. 

Causes and Prevention

  • Moisture: A void may be present in the material if the resin in use is not sufficiently dry. To prevent it, use resin dryers. Second, if the molten plastic has water, that may transform into vapor. There is also a possibility of water lying in the mold’s plastic resin; as it evaporates, it causes bubbles to form. So, ensure that the materials are dry before proceeding with the procedure.
  • Poor venting: It is important to have proper air circulation in the mold. Venting issues, such as insufficient ventilation or poorly designed mold, result in air becoming trapped. As a result, insufficient venting or airflow will lead to the retention of air within the mold instead of the complete replacement of air with the molten material in the mold. Increasing the injection pressure will inject the air pockets out. 
  • Short shot: When the mold does not have enough plastic in the molten form, it can cause some empty spaces, known as vacuum voids. This issue arises from a “short shot,” meaning the mold was not filled properly with the plastic as needed. Speed up the flow of molten plastic material. 
  • Additives: When the plastic is not hard enough or does not have sufficient fiber or additives mixed in, it shrinks unevenly and creates vacuum voids. Add a mold release or lubricant master batch to fix the issue. You can also try a spray-on mold release.

Sink Marks

Red plastic part with a highlighted area showing sink marks, visible as depressions on the surface due to improper cooling during molding

This type of defect appears as depressions on the surface of the injection molding product. It is completely harmless and does not affect the function or the strength of the mold. What really causes this is the sudden shrinking of the inner material, drawing material from the outside towards the inside. 

Causes and Prevention

  • Low melting temperature: The low temperature of the resin material can make sink marks. To prevent it, change the temperature range of the resin material prior to injecting the material. 
  • Incorrect mold temperature: An ideal mold temperature should be 80-120℃ / 176-248℉. However, if it is above this range, then it can stop the gate from closing completely. An easy way to prevent this is to apply the optimum temperature. 
  • Low pack and hold time: If the holding pressure is not distributed properly, then it may deter the part gate from properly preventing the mold resin from moving in and out of the gate. Increase the holding pressure.  
  • Flaws in mold design: When a part of the mold design is not proper, then sink marks are formed. These depend on elements such as bosses, ribs, consistent wall thickness, mold wall thickness, variations in wall thickness, etc. 

To prevent it, make sure there is a balance between the wall thickness and the rib. Ribs with a gradual 7-degree slope at the base can also help prevent sink marks. The hoop should receive negligible stress and the wall should be uniformly thick. 


White plastic object with a visible jetting defect, characterized by a wavy pattern on the surface caused by turbulent flow of material

When molten plastic is pushed through the gate or nozzle quickly, and it starts turning hard before the cavity is filled, then it leads to jetting. It can be identified by its appearance as a squiggly line of the surface of the molded product. Jetting makes the product weak.

Causes and Prevention

Mold design

The location of the gate, along with the size of the gate, nozzle, and runner, can all contribute to jetting. 

  • Small gate size: Changing the size of the gate can change the flow of the material. Depending on the shape and size of the product, you can select a wider and shorter gate. 
  • Small nozzle size: Pick a nozzle size that matches what you need to make the mold. It should be at least 7/32″ in diameter. Also, consider the thickness of the material. 
  • Incorrect gate location: Make sure the gate is in the right place. Move it so that the resin can touch a wall or surface inside the mold quickly.
  • Small runner diameter: Make sure the diameter is enough to enable the free flow of resin. Do not use small runners.

Note: You can evaluate the size of the gate and the runner using a computer program. 

Mold process

  • Low holding pressure: When the holding pressure is not sufficient, it will not allow pressing of the material as needed. So, to remedy this, increase the holding pressure.
  • High injection speed: High speed can trap air or cause disturbance. Add more vents to the mold, as this allows the air to be let out when the molten material goes in.

Mold material

  • High viscosity of the material: It is a known fact that all materials that possess high viscosity are unable to flow freely. Increase mold temperature and resin temperature and use high-fluidity materials to lower the resin viscosity. 


Black plastic component with a significant delamination defect, highlighted by a peeled layer exposing the rough texture beneath

If the layers of a molded product start coming off easily, we call this phenomenon delamination or, lamination or even layering. 

Causes and Preventio

The material

  • Contamination: When foreign material enters the molten plastic, it may lead to defects. Make sure you hire only trained professionals to handle the materials.
  • Incompatibility: Two materials that are not compatible with each other will fail to combine properly. Use only suitable and compatible materials and additives.
  • Moisture: The fillers may contain moisture-soaking materials. So, use only dry materials, and these should conform to the manufacturer’s recommendations. 

Molding process

  • Low mold temperature: When looking for precision in mold temperature, use the correct mold temperature. Ideally, it should be 120 – 180 degrees F. The supplier is usually the right person to know about this. So, do inquire about it.
  • Low barrel temperature: Different materials need different temperatures to shape them. Therefore, it is important that the machine’s heat matches the material and the design of what is being molded. Again, ask the material supplier about this. 
  • Low hold time: Till the gate freezes, you should make sure that the holding pressure is maintained. Once that happens, there would be no impact of pressure on the material.
  • Insufficient injection speed: Boost injection speed till there is no danger of delamination anymore.

Molding machine

  • Size of the gate and the runner: Put rounded corners at every bend and gate entry in the runner system. It stops materials from splitting during injection.
  • High mold release: Keep the mold clean so that there is no need to apply for external mold releases. 
  • Inadequate cushioning: The cushion should be at least 1/8″. Smaller cushions can lead to under-packing. 

Weld Lines

Close-up of weld lines on a gray plastic vent, highlighting the slight imperfections where molten plastic has met and solidified

When an injected plastic comes together inside a mold, it can result in weld line defects. These are also known as knit lines. 

Causes and Prevention

  • Temperature: simultaneous welding of the inner and outer surface of the material can lead to weld lines. Increase the temperature of the barrel and the nozzle to avert this condition.
  • Pressure: Insufficient pressure to rejoin the material can form a weld line. It may occur due to a fault in the machine or the settings. Mold design is also responsible sometimes. Just increase the pressure. 
  • Low speed: As the material flows slowly, the fronts cool irregularly. In other words, if the molten materials cool before touching each other, wed lines are created. Again, enhancing the parameter is the only solution. 
  • Presence of impurities: Resin laden with impurities in any form will not allow the molten material to flow as smoothly as it needs to. So, get your resin from a trustworthy supplier. 

Flow lines 

Close-up of flow lines on a black plastic surface, showing subtle streaks and patterns due to variations in the flow of molten plastic

When the waves on the surface appear to be in a different color, then this is referred to as flow lines. It is usually caused due to the uneven cooling of the molten plastic. 

Understand it like this: while the molten materials start stiffening as soon as they approach the cavity wall, the remaining is still in a moving state on the inner side. Therefore, a ripple effect is visible when two forms of the same material (movable and immovable) come in contact with each other. 

Causes and Prevention

The machine

  • Inadequate holding pressure: Both holding pressure and injection pressure are found to be insufficient in compressing the material. This leaves behind flow lines. Enhancing the pressure prevents the formation, but keep doing so till the product is formed. 
  • Inappropriate cycle time: A short cycle time does not allow the material to melt properly. The temperature of the melt is also found to be low. Though the cavity is not fully filled, the pressure cannot be applied fully, resulting in a flow line. Increase the cycle time till the material melts completely. 
  • Low barrel temperature: low barrel temperature means low melt temperature and also low holding pressure. This is the reason the material cannot be pressed enough, making flow lines instead. So, raise the barrel temperature. 

Note: the temperature of each zone of the barrel (center, front, rear, and nozzle) should be increased one by one; 6 degrees for each zone, to be precise. 

The Mold 

  • Inadequate venting: When the venting is not enough, it will block the melt filling. This will stop the melt front from pressing the material. Begin releasing air at the end of each runner, as this will let most of the air out before it fills the cavity.
  • Size of the sprue: A tiny sprue can increase the flow resistance. Low injection pressure will slow down the melt and cool the plastic. At the same time, without enough pressure, the solid part will not press against the mold, thus leaving flow lines. To avert the situation, evaluate the filling state on a computer before proceeding.

The material

  • Bad fluidity: Whenever the material is not fluid enough, it will make the flow slow, and cooling will occur. This will, in turn, not allow sufficient pressing leaving lines. Ask the material suppliers, and they will help you find the right material for your injection molding. 

Parting Thoughts 

Those looking to use injection molding to manufacture products must understand that it should be done right at the very first instant. If not, it may lead to high production costs due to redesigning and remolding. Therefore, choose HiTop Industrial mold designing for your injection molding needs. Their professionals can help you with precise injection molding techniques with minimum risk of defects. 

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