9 Common Errors in Injection Molding and Effective Solutions

9 Common Injection Molding Defects and Effective Solutions
In the injection molding process, defects are inevitable, especially when producing in large quantities or requiring high precision. These defects not only affect the product's appearance but also increase costs, time, and the scrap rate.
This article will summarize common injection molding defects, their causes, and solutions, helping businesses optimize their production process and ensure product quality.
Short Shot
- Phenomenon
The product is not completely filled, lacking corners or small details.
- Causes
 Insufficient injection pressure 
 Low plastic or mold temperature 
 Suboptimal flow design 
 Trapped air (air trap) 
- Solutions
Increase injection pressure for better mold filling 
Increase plastic and mold temperature for easier plastic flow 
Enlarge or optimize gates and runners to prevent plastic flow obstruction 
Add vents to allow air in the mold to escape
Flash
- Phenomenon
Plastic overflows the mold parting line, creating a thin edge.
- Causes
 Excessive injection pressure 
 Mold not sealed or worn 
 Insufficient clamping force 
- Solutions
Reduce injection pressure to prevent plastic overflow 
Increase mold clamping force for a tighter mold closure 
Inspect and repair worn or open mold surfaces
Sink Mark
- Phenomenon
Slight depressions appear on the product surface, especially in thick plastic areas.
- Causes
 Uneven material shrinkage 
 Insufficient holding time 
 Overly thick product design 
- Solutions
Increase holding time and pressure to compensate for shrinkage 
Reduce thickness in overly thick areas (redesign product if necessary) 
Improve the cooling system for more uniform product cooling
Warpage
- Phenomenon
The product deforms and warps after cooling.
- Causes
 Uneven shrinkage between areas 
 Non-uniform mold temperature 
 Unbalanced flow design 
- Solutions
Adjust mold temperature uniformly across areas
Optimize gate position for balanced plastic flow 
Choose materials with more stable shrinkage
Burn Mark
- Phenomenon
Black or brown burn marks appear on the product.
- Causes
 Compressed air not escaping 
 Excessively high injection speed 
 Excessively high temperature 
- Solutions
Add vents to prevent air trapping 
Reduce injection speed to avoid rapid air compression 
Reduce plastic temperature if it is too high
Weld Line
- Phenomenon
A visible line appears where two plastic flow fronts meet.
- Causes
 Low temperature 
 Long and complex flow path 
 Suboptimal mold design 
- Solutions
Increase plastic and mold temperature for better fusion of the two plastic flows 
Increase injection speed to prevent plastic from cooling before meeting 
Adjust gate position to avoid weld lines in critical areas
Flow Mark
- Phenomenon
The surface has wavy patterns or uneven streaks.
- Causes
 Unstable injection speed 
 Low temperature 
 Unsuitable gate design 
- Solutions
Increase injection speed for more uniform plastic flow 
Increase plastic temperature to reduce premature solidification 
Adjust gate design for stable flow
Bubbles / Voids
- Phenomenon
Air pockets appear inside or on the surface of the product.
- Causes
 Material contains moisture 
 Air trapped in the mold 
 Uneven cooling 
- Solutions
Dry the material before molding to remove moisture from the plastic 
Create or improve mold vents to allow air to escape 
Adjust cooling time to prevent uneven shrinkage
Silver Streak
- Phenomenon
White or silver streaks appear on the surface.
- Causes
 Moist material 
 Plastic degradation 
 Excessively high temperature 
- Solutions
Dry material at the correct temperature and time according to standards 
Reduce processing temperature if plastic degradation occurs 
Control plastic residence time in the machine to avoid overheating
Core Principles for Reducing Injection Molding Defects
In actual production, injection molding defects often stem from 4 main factors:
Material
Machine
Mold
Method
Analyzing using the 4M model helps businesses identify the root cause and avoid incorrect solutions.
Understanding common injection molding defects and their solutions not only helps reduce the scrap rate but also optimizes production costs and enhances quality stability.
For businesses manufacturing high-tech components (automotive, electronics, IoT, etc.), controlling defects right from the mold design stage and process setup is a key factor to ensure long-term effectiveness.
CNCTech Thang Long is a comprehensive injection molding solution provider, from mold design and manufacturing to complete assembly. The factory is equipped with over 120 automatic injection molding machines with capacities ranging from 75–700 tons, flexibly meeting the needs for high-precision and complex-shaped parts. The company is capable of processing various engineering plastics such as ABS, PC, POM, PA, etc., along with a strict quality control system according to ISO 9001 and IATF 16949 standards. Additionally, CNCTech also provides post-molding services such as insert welding, printing, painting, and assembly, helping to optimize the production process for customers.