Common Defects in Plastic Injection Molding and How to Avoid Them

Cold Die Casting: A Guide to the Process and Its Benefits

Plastic injection molding is a widely used manufacturing process, however, it sometimes leads to some common defects that affect the quality of the final plastic part. To get the best results in injection molding, it’s crucial to understand these defects and how to mitigate or prevent them.

First, a common defect is air bubbles. During the injection molding process, air bubbles can occur if the mold is not completely closed or if there is gas trapped in the plastic. These bubbles can cause uneven surfaces on parts, affecting their appearance and strength. In order to reduce or prevent the generation of bubbles, the following measures can be taken: ensuring good sealing of the mold, using high-quality raw materials, controlling injection speed and pressure, and performing appropriate preheating and cooling.

Second, another common defect is warping. As the plastic cools and solidifies in the mold, the part can warp due to differences in temperature and shrinkage. This can cause the part to not fit or function properly. In order to reduce or prevent warpage, the following measures can be taken: designing a reasonable mold structure, ensuring uniform cooling of parts, controlling injection speed and pressure, and using appropriate cooling media and temperatures.

In addition, defects such as shrinkage, weld lines, and burrs may occur. Shrinkage refers to the plastic shrinking in volume during cooling, resulting in unstable part dimensions. A weld line is a distinct line formed at the junction of two or more materials that affects appearance and strength. Burr is excess plastic that appears on the edge of a part and requires additional processing to remove. In order to mitigate or prevent these defects, the following measures can be taken: optimizing mold design and manufacturing, controlling injection speed and pressure, using appropriate cooling media and temperatures, and performing appropriate post-processing and trimming.

In conclusion, understanding the common defects that can occur during plastic injection molding and how to mitigate or prevent them is crucial to obtaining high-quality plastic parts. By taking appropriate measures, such as ensuring the sealing of the mold, controlling injection parameters, optimizing mold design and manufacturing, etc., the success rate and product quality of injection molding can be improved.


Warping is a common defect that can occur during the plastic injection molding process. It refers to the deformation of a part, where it takes on an unintended shape or form. Warping happens when there is uneven cooling between different areas of the molded part, causing some parts to shrink and others to expand more than they should. This leads to stresses in the material which cause it to bend or twist out of shape.


There are several reasons why warping can occur during plastic injection molding. One primary cause is improper cooling time and temperature control. If a mold cools too quickly in one area, while another area remains hot for longer periods, this will lead to uneven shrinkage rates across the part leading to warpage.

Another contributing factor could be inadequate packing pressure or filling speed during injection molding which causes gaps within the material that acts as stress concentration zones; these zones are susceptible spots for creating warped sections within a product.

Furthermore, if there is not enough draft angle incorporated into either side of your mold design before production begins – due largely because designers rarely incorporate this feature thinking its unimportant – then problems related specifically with warping may arise from internal tension being created by locked-in stresses generated by insufficiently drafted walls.


To prevent warping from occurring during plastic injection molding processes, there are several things manufacturers must consider:

Firstly, proper cooling time and temperature controls must be implemented throughout all stages of manufacturing – initial heating up through final extraction phases – so that each section cools uniformly avoiding any difference in shrinkage rates among other factors like humidity levels which may affect how long materials continue their expansion phase once removed from heat sources such as molds used for shaping objects via injections thereof onto them at high speeds under intense pressure settings;

Secondly ensuring adequate packing pressures/filling speeds when injecting melt into molds helps reduce chances gaps forming inside said melts after solidification has occurred thus reducing occurrences relating directly towards warping defects;

Lastly, designers should take adequate draft angles into account during the design phase of their products to ensure that they are capable of withstanding internal stresses generated by locked-in tension. This will help in avoiding any unnecessary deformation or warpage issues.

Sink Marks

One of the most common defects that occur during plastic injection molding is sink marks. Sink marks are depressions or dents on the surface of a molded part caused by uneven shrinkage in thicker sections of the part. Even though these depressions do not affect the functionality of the product, they can greatly impact its aesthetic appeal and reduce its value.

The primary cause of sink marks is thermal contraction which occurs when molten plastic cools down and solidifies at different rates in different parts of the mold. As a result, areas with more material take longer to cool than thinner sections causing them to contract more than other areas leading to visible dents or depressions on the surface.

Another factor that contributes to sink mark formation is high packing pressure during injection molding process which causes displacement and deformation on thick-walled regions resulting in voids underneath those surfaces as it cools down.

To prevent sink marks from occurring, there are several tips that manufacturers should consider. First, designers must pay attention to wall thickness variations across different parts of their products since thicker walls increase chances for thermal contraction. They need to make sure that all thicknesses are uniform throughout each section so as not to create any unnecessary stresses within certain areas.

Secondly, selecting appropriate resin materials can help prevent sink marks from forming. Resins with higher melt flow indexes (MFI) allow for better flowability during injection thereby minimizing packing pressure required while providing enough material volume needed for a specific application.

Lastly, adjusting processing parameters such as temperature settings and cooling time durations can significantly minimize incidences of sink mark formation in molded parts. Manufacturers may also consider using Gas-assisted Injection Molding (GAIM) technique where nitrogen gas is injected into molds under high pressure forcing melted plastic onto cavity walls uniformly reducing density variation hence preventing sinking issues.


Flash is a common defect that occurs in plastic injection molding. It refers to the excess material that protrudes from the mold cavity and solidifies during the cooling process. This can be caused by several factors, including excessive injection pressure, high temperatures, poor clamping force, or an incorrect amount of material.

One way to prevent flash is by optimizing the machine settings. Injection pressures should be set at appropriate levels and monitored closely throughout production runs. Temperatures should also be controlled carefully to avoid overheating the mold and causing flash.

Another important factor in preventing flash is proper maintenance of molds and machinery. Regular cleaning and inspection can help identify potential issues before they become major problems. It’s also important to ensure that molds are properly aligned with machine platens so that there is no movement during production runs.

In addition, using high-quality materials can help reduce the risk of flash occurring in plastic injection molding processes. Choosing resins with good flow properties and consistent melt viscosity can help ensure smooth filling without creating excess pressure or temperature changes.

Finally, it’s essential to have skilled operators who understand how to set up machines correctly and monitor production for signs of defects like flash. By investing in training programs for employees, manufacturers can improve their overall efficiency while reducing scrap rates caused by defects like flash.

Overall, preventing flash requires a combination of careful machine settings, regular maintenance practices, high-quality materials selection ,and skilled personnel who understand how these variables interact during production runs . With these measures in place ,manufacturers will be ableto achieve higher yield rates while producing parts with fewer defects overall .

Short Shots

Short shots are a common defect in plastic injection molding that can occur when the mold cavity isn’t completely filled with molten plastic. Incomplete filling of the mold results in portions of the final product missing, leading to issues such as weakness or incompleteness. This is one of the most commonly occurring defects in injection molding.

There may be several causes for short shots, but some of them are more prominent than others. One reason could be insufficient resin being injected into the mold due to incorrect settings on machines and equipment. Another cause could be inadequate pressure within the machine, which prevents enough molten plastic from flowing into all parts of the mold cavity.

A third possible cause for short shots could be improper temperature regulation during production. When temperatures aren’t controlled properly during molding, it can result in solidification before all sections have been filled with liquid plastic.

To prevent this imperfection from happening again, there are a few tips that manufacturers should follow:

Tips on how to Prevent Short Shots

Firstly, ensure proper resin selection and machine setup by carefully reviewing instructions provided by material suppliers and adhering strictly to manufacturer’s guidelines regarding machinery usage and maintenance.

Secondly, use high-quality molds that can withstand high pressures required for successful completion of injection molding processes without deformation or failure under stress conditions.

Finally yet importantly, closely monitor temperature fluctuations throughout production cycles using thermocouples so that any deviations from normal operating ranges can quickly identify potential problems early on before they become major challenges down-the-line.

By following these best practices consistently over time while also testing new approaches when necessary (such as implementing process improvements like automation), you’ll find yourself well-equipped not only avoid future instances where your products may have flaws due simple errors like incomplete filling up molds but also improve overall quality control measures along way!

Burn Marks

Burn marks are a common defect in plastic injection molding that can greatly affect the quality of the final product. They appear as dark or discolored areas on the surface of the molded part, caused by overheating during the molding process. Burn marks can be caused by various factors, including high temperatures, prolonged residence time in the barrel or mold, and excessive shear rates.

One of the main causes of burn marks is high temperatures. When resin is exposed to high temperatures for too long, it begins to break down and release gases that create carbon deposits on the surface of the mold cavity. These carbon deposits then transfer onto the surface of the part being formed, causing burn marks.

Another possible cause is prolonged residence time in either barrel or mold. If plastic material remains in contact with hot surfaces for too long without proper cooling, it may start to degrade and produce unwanted particles which will negatively impact its properties such as color and strength.

Excessive shear rate can also lead to burn marks during plastic injection molding. High shear rates occur when there is an abrupt change in flow direction within a narrow channel inside a tooling system like an injection moulding machine – this creates frictional heat leading ultimately leads to burnt look on molded parts

To prevent burn marks from occurring during plastic injection molding process several steps must be taken into consideration:

First off ensure you check material specifications before proceeding with production runs

Secondly make sure you have good control over temperature settings throughout all stages (heating /cooling)

Proper maintenance practices should be followed regularly so that machinery performance remains consistent.

Use appropriate fillers if necessary; some materials require additives such as anti-static agents which help minimize static charge build-up while others need lubricants/slip agents added at specific levels.This helps reduce friction between components thereby preventing overheating issues

Ensure adequate ventilation systems are installed especially around areas where excessive amounts of dust/debris accumulate easily causing blockages and other issues.

By following these tips, you can greatly reduce the occurrence of burn marks in your plastic injection molding process. This will help to ensure that your final products are of high quality with no defects or blemishes.


In conclusion, plastic injection molding is a highly efficient and effective manufacturing process. However, it is important to be aware of the common defects that can arise during the process in order to prevent them from occurring. By understanding and addressing these issues early on in the design and production stages, manufacturers can save time, money, and resources while improving product quality. It is crucial for manufacturing professionals and engineers to stay up-to-date with industry standards and best practices to ensure successful plastic injection molding projects. Overall, preventing defects in plastic injection molding not only benefits manufacturers but also leads to satisfied customers who receive high-quality products that meet their expectations.

What are the common defects in plastic injection molding?

Several common defects can occur in plastic injection molding, including:

  1. Sink Marks: These are depressions or dimples that appear on the surface of the molded part due to uneven cooling or insufficient packing of the plastic material.
  2. Warpage: Warpage refers to the deformation or distortion of the molded part caused by uneven cooling or inadequate mold design.
  3. Flash: Flash occurs when excess plastic material escapes from the mold and creates thin, unwanted protrusions around the edges of the part.
  4. Voids: Voids are air pockets or bubbles trapped within the molded part due to inadequate venting or trapped gases during the injection molding process.
  5. Short Shot: A short shot happens when the molten plastic fails to completely fill the mold cavity, resulting in incomplete parts with missing sections or features.
  6. Weld Lines: Weld lines are lines or marks formed when two or more flowing molten plastic streams meet during injection molding, resulting in weak cosmetic or structural areas.

How do you fix injection molding problems?

Fixing injection molding problems often requires identifying the root cause and implementing appropriate measures. Potential solutions include:

  1. Adjusting Mold Temperature: Modifying the mold temperature can help alleviate sink marks, warpage, and reduce the likelihood of short shots, depending on the material used.
  2. Optimizing Injection Speed and Pressure: Adjusting the injection speed and pressure can help address flash, voids, and weld lines, ensuring better material flow and minimizing trapped air.
  3. Enhancing Cooling Systems: Improving cooling efficiency through mold design modifications, such as adding cooling channels or employing conformal cooling, can help mitigate warpage and improve overall part quality.
  4. Fine-tuning Process Parameters: Adjusting process parameters, such as melt temperature, injection time, and holding pressure, can assist in resolving various defects by optimizing material flow and packing.
  5. Proper Venting and Gate Design: Ensuring adequate venting in the mold and optimizing gate design can help minimize voids and eliminate trapped gases, enhancing part quality.
  6. Mold Maintenance: Regular maintenance of the injection mold can prevent defects caused by wear, damage, or buildup that may compromise the molding process.

How do you prevent voids in injection molding?

To prevent voids in injection molding, several measures can be taken:

  1. Adequate Venting: Ensure proper venting in the mold design to allow trapped air or gases to escape during the filling phase, preventing voids from forming.
  2. Decrease Melt Temperature: Lowering the melt temperature can minimize the potential for trapped gases in the molten plastic, reducing the likelihood of voids.
  3. Balanced Fill: Ensure uniform and balanced filling of the mold cavity to minimize void formation. This can be achieved through appropriate gate design, gate size, and proper runner system to maintain consistent flow rates.
  4. Optimize Venting Placement: Place vents strategically at locations prone to trapped air, such as corners, thin sections, or areas with complex geometries, to ensure effective air evacuation.
  5. Purge and Pre-dry Material: Properly purging the injection molding machine and pre-drying the plastic material can help eliminate moisture, which can lead to void formation.
  6. Mold Temperature Control: Maintaining consistent mold temperature throughout the injection molding process can prevent temperature differentials that contribute to voids.

By understanding these common defects and implementing effective prevention or mitigation strategies, manufacturers can achieve higher-quality plastic parts and optimize their injection molding processes.