Introduction: Injection molding is an efficient and widely used manufacturing process in the production of plastic products. However, in actual production, injection molding may encounter various defects that not only affect product quality but also lead to increased production costs and longer production cycles.

This article will explore common injection molding defects, their causes, and corresponding solutions, with specific case studies for analysis.

I. Short Shot

Definition:

The plastic melt fails to completely fill the mold cavity, resulting in an incomplete or partially missing part.

Causes:

1. Insufficient injection pressure: The pressure is insufficient to push the melt completely into the mold.

2. Poor mold venting: Air trapped inside the mold hinders melt flow.

3. Poor material flowability: The material viscosity is too high, making it difficult to flow.

4. Low mold temperature: Low mold temperature causes the material to cure prematurely.

Solutions:

1. Increase injection pressure: Ensure that the melt has sufficient pressure to fill the mold.

2. Improve mold venting: Add exhaust holes or exhaust grooves to release air.

3. Choose a material with better flowability: Adjust the material formula or choose a different grade.

4. Increase mold temperature: Delay material curing by heating the mold.

Case Study:

In the production of an electronic housing, short shots occurred. Analysis revealed insufficient venting. The problem was resolved by adding 0.03mm deep and 3mm wide venting grooves on the parting line of the mold.

Ⅱ. Flash

Definition:

Flash refers to the excess plastic material that appears around the mold parting line or inserts.

Causes:

1. Inadequate mold clamping: The mold is not completely closed, causing melt leakage.

2. Excessive injection pressure: Excessive pressure forces the melt out of the mold gap.

3. Excessive material flowability: The material viscosity is too low, making it prone to seeping into mold gaps.

4. Mold wear: Wear on the mold parting surface or inserts, resulting in increased gaps.

Solutions:

1. Increase mold clamping force: Ensure that the mold is completely closed.

2. Reduce injection pressure: Adjust process parameters to reduce pressure.

3. Choose a material with appropriate flowability: Adjust the material formulation or select a different grade.

4. Repair or replace the mold: Repair worn mold components.

Case Study:

In the production of household plastic products, excessive injection pressure led to severe flash problems. By adjusting the injection pressure, the flash problem was effectively controlled.

Ⅲ. Sink Marks

Definition:

Sink marks are depressions on the surface of a part, usually caused by uneven material shrinkage.

Causes:

1. Uneven cooling: Different parts of the component cool at different rates.

2. High material shrinkage: The material shrinks significantly during cooling.

3. Uneven wall thickness: Uneven wall thickness of parts leads to uneven shrinkage.

Solutions:

1. Optimize the cooling system: Ensure that parts are cooled evenly.

2. Choose a material with low shrinkage: Adjust the material formula or choose a different grade.

3. Adjust mold design: Make the wall thickness uniform and reduce shrinkage.

Case Study:

In the production of thick-walled plastic containers, uneven wall thickness led to severe sink marks. By optimizing the mold design to make the wall thickness uniform, the sink mark problem was improved.

Ⅳ. Bubbles

Definition:

Bubbles are cavities that appear inside or on the surface of a part, usually caused by air or volatiles trapped in the melt.

Causes:

1. High moisture content in the material: The moisture in the material turns into steam when heated.

2. Excessive injection speed: Excessive injection speed prevents air from being expelled in time.

3. Poor mold venting: Air cannot be expelled in a timely manner.

Solutions:

1. Poor mold venting: Air cannot be expelled in a timely manner.

2. Reduce injection speed: Allow air to escape.

3. Improve mold venting: Add exhaust holes or exhaust grooves.

Case Study:

In the production of transparent plastic products, high moisture content in the material led to severe bubble problems. By drying the material, the bubble problem was solved.

Ⅴ. Weld Lines

Definition:

Weld lines are lines that appear on the surface of a part, usually caused by two streams of melt meeting and not fusing completely.

Causes:

1.Low melt temperature: Insufficient melt temperature makes it difficult for the two melts to fuse.

2. Slow injection speed: If the injection speed is too slow, the melt will cool too early in the mold, affecting fusion.

3. Low mold temperature: Low mold temperature causes the melt to cool quickly, affecting fusion.

4. Improper gate design: Improper gate location or shape affects melt flow.

Solutions:

1. Increase melt temperature: Ensure that the melt temperature is high enough to promote fusion.

2. Adjust injection speed: Appropriately increase the injection speed to reduce the melt residence time.

3. Increase mold temperature: Increase the mold temperature to delay the cooling of the melt.

4. Optimize gate design: Improve gate location and shape to ensure smooth melt flow.

Case Study:

In the production of medical devices, weld lines appeared on the surface of the parts. By increasing the mold temperature and adjusting the injection speed, the weld line issue was significantly reduced.

Ⅵ. Silver Streaks

Definition:

Silver streaks refer to silver-white textures or spots that appear on the surface of injection-molded products along the direction of melt flow, usually appearing as needle-like stripes or flaky mica textures.

Causes:

1.High moisture content or excessive volatile substances in raw materials: Raw materials are not sufficiently dried, causing moisture or volatile substances to sublimate during heating, forming bubbles or steam that become trapped in the melt and solidify into silver streaks.

2.Poor mold venting: Moisture in the mold cavity or insufficient venting holes prevent gases from escaping during melt flow, resulting in silver streaks.

3.Excessive use of lubricants or release agents: Especially release agents containing silicon may prevent the melt from fully fusing, resulting in silver streaks.

4.Uneven wall thickness: Thicker sections of the product cool and contract, causing uneven melt flow and forming silver streaks.

5.Low mold temperature, low injection pressure, or slow injection speed: These factors slow down melt filling, accelerate cooling, and increase the likelihood of silver streaks forming.

Solutions:

1. Drying raw materials: Thoroughly dry plastic raw materials to remove moisture.

2. Improving mold venting: Add vent holes or vent slots to ensure that gas is discharged in a timely manner.

3. Reasonable use of lubricants and release agents: Reduce the amount of lubricants and release agents used, and avoid excessive use of silicone-containing release agents.

4. Optimizing process parameters: Increase mold temperature and appropriately increase injection pressure and speed to reduce melt cooling time.

Case Study:

When producing transparent plastic products, numerous silver streaks appeared on the surface of the finished product. Upon inspection, it was found that the raw materials contained excessive moisture and the mold had poor ventilation. After drying the raw materials and adding ventilation holes to the mold, the silver streak problem was effectively resolved.

Ⅶ. Jetting

Definition:

Spray patterns refer to snake-like patterns or stripes formed when molten material is sprayed at high speed onto the mold surface during the injection process.

Causes:

1. Injection speed too fast:When the melt is injected into the mold at too high a speed, it sprays onto the mold wall, forming serpentine patterns.

2. Improper gate design: Gates that are too small or improperly shaped can obstruct melt flow, resulting in spray marks.

3. Low mold temperature: When the mold temperature is low, the melt solidifies rapidly upon contact with the cold mold surface, forming snake marks.

Solutions:

1. Reduce injection speed: Adjust injection parameters to reduce injection speed.

2. Optimize gate design: Increase gate size or improve gate shape to ensure smooth melt flow.

3. Increase mold temperature: Appropriately increase mold temperature to delay melt solidification.

Case Study:

In the mobile phone case production line, spray marks frequently appear on the surface of the finished product. After analysis, it was found that the main causes were excessive injection speed and unreasonable gate design. By reducing the injection speed and improving the gate design, the spray mark problem was significantly reduced.

Ⅷ. Warpage

Definition:

Warpage refers to the bending and deformation of injection molded products after cooling, which affects their appearance and dimensional accuracy.

Causes:

1. Uneven cooling: The mold cooling system is poorly designed, resulting in inconsistent cooling speeds in different areas.

2. Uneven wall thickness: The transition between thick and thin walls in the product design is unreasonable, resulting in uneven cooling shrinkage.

3. High material shrinkage rate: The use of materials with a high shrinkage rate results in uneven shrinkage during cooling, causing warping.

4. Insufficient mold clamping force: Insufficient mold clamping force causes the melt to shift during cooling.

Solutions:

1. Optimize the cooling system: Ensure uniform mold cooling by adding cooling channels or improving channel design.

2. Adjust the mold design: Optimize the product structure design to ensure uniform thickness and avoid sudden changes.

3. Select low-shrinkage materials: Use plastic materials with low shrinkage rates to reduce cooling shrinkage.

4. Increase clamping force: Increase the mold clamping force to prevent melt displacement.

Case Study:

During the production process of automotive interior parts, warping issues arose in the finished products. After analysis, it was determined that the mold cooling system was poorly designed, resulting in uneven cooling. By optimizing the cooling channel design, the warping issue was effectively resolved.

Ⅸ. Black Specks

Definition:

Black spots refer to black spot-like defects that appear on the surface or inside of injection molded products, which may be caused by carbonization, impurities, or decomposition products.

Causes:

1.Barrel or mold contamination: Impurities or decomposition products in the barrel or mold enter the melt, forming black spots.

2. Material decomposition: Excessively high melt temperature or prolonged residence time causes material decomposition, resulting in the formation of carbonized particles.

3. Poor venting: Inadequate mold venting leads to the presence of gases or volatile substances in the melt, which condense and form black spots upon solidification.

4. Foreign objects in raw materials: Raw materials containing foreign objects or incompletely melted recycled material result in the formation of black spots.

Solutions:

1. Clean the hopper and mold: Regularly clean the hopper and mold to remove impurities and decomposition products.

2. Control melt temperature: Adjust the material temperature to avoid decomposition caused by excessive heat.

3. Improve mold venting: Add vent holes or vents to ensure timely gas removal.

4. Check raw material quality: Ensure raw materials are pure and free of foreign matter or excessive recycled material.

Case Study:

In the production line for home appliance casings, many black spots appeared on the surface of the finished products. After inspection, it was found that impurities had accumulated in the feed hopper. After cleaning the feed hopper and replacing the filter screen, the black spot problem was resolved.

Conclusion

Common defects in injection molding include short shots, flash, sink marks, bubbles, and weld lines. These defects are caused by multiple factors, including material properties, mold design, process parameters, and equipment conditions. By systematically analyzing the root causes and applying case-based solutions, manufacturers can effectively improve product quality and consistency.

In actual production, it is recommended to establish a complete injection molding condition record sheet and integrate CAE flow analysis technology to enable defect prediction and rapid response, thereby improving production efficiency and product quality.

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