Common Issues, Causes, and Solutions in PET Injection Molding

PET (polyethylene terephthalate) is a crystalline plastic widely used in preforms, food packaging, electrical components, transparent parts, and other applications. However, it is highly hygroscopic, temperature-sensitive, prone to crystallization and whitening, and susceptible to the formation of silver streaks and bubbles; consequently, it is one of the most problematic materials encountered in injection molding.

1.The moisture content of the raw materials is excessively high.

Symptoms: Silver streaks, bubbles, voids, product brittleness, hazy surface finish.
Causes:

• PET has a high moisture absorption rate and readily absorbs moisture from the air.
• Moisture undergoes hydrolysis at high temperatures, leading to a reduction in molecular weight and the generation of gas.

Solutions:

• Drying *must* be performed using a dehumidifying dryer; standard hot-air drying is ineffective.
• Drying temperature: 120–160°C.
• Drying time: 4–6 hours.
• The hopper must be insulated to prevent moisture re-absorption.
• Moisture content must be controlled to below 0.02%.

2. Excessive proportion of recycled material.
Symptoms: Black spots, yellowing, poor mechanical strength, unstable melt flow.
Solutions:

• The ratio of virgin material to recycled material is recommended to be ≤ 20%.
• Recycled material must be re-dried.
• Avoid subjecting the material to repeated heating cycles at high temperatures.

1. Silver Streaks, Flow Marks, Flow Lines
Causes:

• Raw material not sufficiently dried
• Material temperature too high, leading to decomposition
• Injection speed too fast, entraining air
• Poor mold venting

Solutions:

• Intensify drying process
• Appropriately lower barrel temperature
• Implement segmented injection speed control: Slow → Fast → Slow
• Improve mold vent design

2. Surface Haziness/Whitening, Opacity, Crystalline Whitening
Causes:

• Mold temperature too low; rapid cooling leads to uneven crystallization
• Material temperature too low
• Injection speed too slow

Solutions:

• For transparent PET parts, control mold temperature between 80–120°C
• Increase the temperature of the front section of the barrel
• Appropriately increase injection speed
• Use a mold temperature controller if necessary

3. Black Spots, Yellowing, Scorching
Causes:

• Excessive temperature causing thermal decomposition of PET
• Material residence time in the screw is too long
• Carbon accumulation in barrel dead spots
• Worn check ring (non-return valve) causing backflow and overheating

Solutions:

• Control barrel temperature within 260–290°C; strictly prohibit exceeding 300°C
• Purge the barrel before shutdown; clean using PE material
• Shorten the cooling cycle to avoid prolonged material residence
• Inspect and replace the check ring

1. Short Shots / Incomplete Filling
Causes：

• Poor material flowability
• Material/mold temperature too low
• Slow injection speed; insufficient pressure
• Runner or gate size too small

Solutions：

• Increase material and mold temperatures
• Appropriately increase injection pressure and speed
• Enlarge gates; optimize runner design
• Check the screw non-return ring for leakage

2. Sink Marks / Depressions / Shrinkage
Causes：

• Insufficient holding pressure
• Holding time too short
• Uneven wall thickness
• Gate freezes prematurely

Solutions：

• Increase holding pressure
• Extend holding time until the gate solidifies
• Increase mold temperature to reduce internal stress
• Optimize product wall thickness design

3. Warpage / Deformation
Causes：

• Uneven cooling
• Uneven crystallization
• Excessive internal stress
• Unbalanced ejection

Solutions：

• Ensure uniform layout of cooling channels
• Stabilize mold temperature
• Reduce injection speed and pressure to minimize internal stress
• Adjust ejection balance

4. Flash / Burrs
Causes：

• Material temperature too high
• Injection pressure too high
• Insufficient clamping force
• Poor mold mating/fit

Solutions：

• Reduce temperature and injection speed
• Appropriately reduce injection pressure
• Verify clamping force
• Repair/adjust the mold parting line

1. Internal Bubbles and Voids
Causes:

• Moisture content in raw materials
• Excessively rapid cooling, causing the outer skin to harden prematurely while the interior shrinks to form voids
• Insufficient holding pressure

Solutions:

• Thoroughly dry the material
• Extend the holding time
• Increase mold temperature to ensure uniform shrinkage
• For thick-walled parts, reduce the injection speed

2. Cracking, Brittleness, and Fracture
Causes:

• Hydrolysis leading to a reduction in molecular weight
• Excessive internal stress
• Material degradation due to excessively high melt temperature
• Excessive use of recycled material

Solutions:

• Strictly control the drying process
• Reduce injection speed and pressure
• Reduce screw rotation speed
• Decrease the proportion of recycled material

1. Screw Slippage / Unstable Melt
Causes:

• Insufficient raw material drying
• Back pressure is too low
• Melt temperature is too high
• Screw wear

Solutions:

• Ensure adequate drying
• Appropriately increase back pressure
• Lower barrel temperature
• Inspect the clearance between the screw and the barrel

2. Unstable Product Transparency
Causes:

• Mold temperature fluctuations
• Melt temperature fluctuations
• Inconsistent drying
• Fluctuating injection speed (inconsistent flow rate)

Solutions:

• Use a mold temperature controller
• Ensure stable temperature control
• Maintain continuous production; avoid prolonged shutdowns
• Stabilize process parameters

• Drying Temperature: 120–160°C
• Drying Time: 4–6 h
• Barrel Temperature: 260–290°C
• Mold Temperature (Transparent Parts): 80–120°C
• Injection Speed: Medium to High (Segmented Control)
• Holding Pressure: High Pressure, Extended Duration
• Screw Speed: Medium to Low (To Avoid Shear Overheating)