Troubleshooting for Injection Molding Process

Troubleshooting for Injection Molding Process

Injection molding is a widely used manufacturing process for producing plastic parts. However, it can also pose many challenges and problems that affect the quality and efficiency of the final products. In this blog post, we will discuss some common injection molding defects and how to troubleshoot them.

First, let’s understand what injection molding is. Injection molding is a manufacturing process in which molten plastic material is injected into a mold, then cooled and solidified to form the desired shape. This method has the advantages of high efficiency, precision and economy, so it is widely used in manufacturing various plastic products. Injection molding is a process of producing plastic parts by injecting molten material into a mold. Injection molding can produce complex shapes with high precision and low cost. However, injection molding also has some common problems that can affect the quality and performance of the molded parts. Some of these problems are:

  • Short shot: This occurs when the molten material does not fill the entire cavity of the mold, resulting in incomplete or defective parts. Possible causes of short shot are low injection pressure, low injection speed, insufficient material, cold mold temperature, or blocked vents.
  • Flash: This occurs when the molten material leaks out of the mold cavity and forms thin projections or excess material on the part. Possible causes of flash are high injection pressure, high injection speed, worn or damaged mold, or improper clamping force.
  • Sink marks: These are depressions or dimples on the surface of the part caused by shrinkage of the material as it cools. Possible causes of sink marks are thick or uneven wall thickness, low cooling rate, low packing pressure, or insufficient cooling time.
  • Warping: This occurs when the part is distorted or twisted due to uneven shrinkage or internal stresses. Possible causes of warping are non-uniform wall thickness, non-uniform cooling rate, excessive injection speed, high mold temperature, or improper ejection.
  • Burn marks: These are black or brown spots or streaks on the surface of the part caused by overheating or degradation of the material. Possible causes of burn marks are high injection temperature, high injection speed, excessive residence time, insufficient venting, or contaminated material.

To troubleshoot these problems, injection molders need to identify the root cause and adjust the process parameters accordingly. Some general guidelines for troubleshooting are:

  • Check the material specifications and properties and ensure that they are suitable for the application and the mold design.
  • Check the mold condition and maintenance and ensure that it is clean, aligned, lubricated, and free of defects.
  • Check the machine settings and operation and ensure that they are consistent with the material and mold requirements.
  • Check the quality and appearance of the molded parts and measure their dimensions and properties.
  • Compare the actual results with the expected results and identify any deviations or defects.
  • Analyze the possible causes of the problem and test different solutions until the problem is resolved.

Table of Content

  • Introduction
  • Common Injection Molding Defects
  • Short Shot
  • Flash
  • Sink Marks
  • Warping
  • Bubbles
  • Burn Marks
  • Troubleshooting Tips
  • Optimize the Process Parameters
  • Check the Mold Design and Condition
  • Use Quality Material and Equipment
  • Perform Regular Maintenance and Inspection


Injection molding is a process that involves injecting molten plastic into a mold cavity under high pressure and temperature. The plastic then cools and solidifies into the shape of the mold. Injection molding can produce complex and precise parts with high efficiency and low cost.

However, injection molding is not without its drawbacks. There are many factors that can affect the quality and performance of the injection molded parts, such as the material properties, the mold design, the process parameters, and the environmental conditions. These factors can cause various defects and problems that compromise the functionality and appearance of the parts.

Common Injection Molding Defects

Some of the most common injection molding defects are:

Short Shot

A short shot occurs when the molten plastic does not fill the mold cavity completely, resulting in a part that is incomplete or missing features. This defect can be caused by insufficient material, low injection pressure or speed, high viscosity of the material, or cold spots in the mold.


Flash is a thin layer of excess plastic that extends beyond the edge of the mold cavity, creating unwanted projections or burrs on the part. Flash can be caused by excessive injection pressure or temperature, worn or damaged mold components, improper clamping force, or poor mold alignment.

Sink Marks

Sink marks are depressions or dimples on the surface of the part that occur when the inner part shrinks more than the outer part during cooling. Sink marks can be caused by thick or uneven wall sections, low cooling rate, high injection pressure or temperature, or insufficient packing pressure.


Warping is a distortion or deformation of the part that occurs when different sections of the part cool at different rates, creating internal stresses. Warping can be caused by non-uniform wall thickness, high injection pressure or temperature, low cooling rate, poor mold design or ventilation, or excessive ejector force.


Bubbles are air pockets or voids inside the part that occur when air is trapped in the molten plastic during injection. Bubbles can be caused by low injection speed or pressure, high back pressure, high moisture content in the material, or inadequate venting of the mold.

Burn Marks

Burn marks are black or brown spots on the surface of the part that occur when the material is overheated or degraded by excessive friction or compression. Burn marks can be caused by high injection speed or pressure, high melt temperature, long residence time in the barrel, insufficient cooling, or poor venting of the mold.

Troubleshooting Tips

Injection molding is a complex process that involves many variables, such as temperature, pressure, speed, material properties, mold design and condition, and equipment quality. Sometimes, defects or problems may occur during the injection molding process, affecting the quality and functionality of the final product. To troubleshoot these issues, here are some tips that can help you improve your injection molding process and avoid common pitfalls.

  • Optimize the process parameters. The process parameters are the key factors that determine the quality of the injection molded part. They include injection pressure, injection speed, melt temperature, mold temperature, cooling time, holding pressure, and holding time. By adjusting these parameters according to the material characteristics and the part geometry, you can optimize the filling, packing, cooling, and shrinkage of the part and reduce defects such as warping, sink marks, flash, short shots, or bubbles.
  • Check the mold design and condition. The mold design and condition also affect the quality of the injection molded part. The mold design should match the material properties and the part geometry, ensuring proper venting, gating, runner system, ejector system, and cooling system. The mold condition should be checked regularly for wear and tear, corrosion, dirt, or damage that may cause defects or problems during the injection molding process. If necessary, repair or replace the mold components to maintain optimal performance.
  • Use quality material and equipment. The quality of the material and equipment used in the injection molding process also influences the quality of the final product. The material should be suitable for the injection molding process and have consistent properties and specifications. The material should be stored properly and dried before use to avoid moisture or contamination. The equipment should be reliable and well-maintained to ensure accurate and stable operation. The equipment should also be calibrated and cleaned regularly to prevent errors or malfunctions.
  • Perform regular maintenance and inspection. Regular maintenance and inspection are essential for ensuring the efficiency and quality of the injection molding process. Maintenance and inspection should be performed on the material, equipment, mold, and product at different stages of the injection molding process. By doing so, you can detect and correct any issues or deviations that may affect the quality of the final product. You can also prevent potential problems or failures that may cause downtime or waste.

By following these troubleshooting tips, you can improve your injection molding process and achieve better results. However, troubleshooting is not a one-time activity but a continuous improvement process that requires constant monitoring and evaluation. Therefore, you should always keep track of your process data and performance indicators and use them to identify areas for improvement and optimization.