Why is there white fog in your PET preform? Injection molding machine temperature control secrets revealed

During the PET preform production process, "white haze" (also known as "fogging" or "crystallization whitening") is a common defect that plagues many manufacturers. It not only affects the product's appearance but also reduces the mechanical strength and transparency of the preform. Understanding the root cause of white haze can help you better resolve this problem.

 

1. The Nature of the White Haze Problem: Crystallization Induced by Temperature Runaway

PET (polyethylene terephthalate) is a semi-crystalline material. The essence of white haze is localized light scattering caused by non-uniform crystallization. Improper temperature control during the injection molding process can lead to two typical problems:

 

Overcrystallization (too high a temperature): When the melt temperature is >285°C, the molecular chains become excessively oriented. This results in the formation of crystalline regions >50 nm, creating a noticeable white haze.

 

Laboratory data: For every 5% increase in crystallinity, light transmittance decreases by 30%.

 

Cold crystallization (too low a temperature): When the mold temperature is <90°C, the melt cools too rapidly, forming microcrystalline regions (20-100 nm), which appear as cloudy white spots.

 

2. Temperature control of PET-preform injection molding machine

Key temperature control points and parameter optimization

Barrel temperature partition control

The rear section (feeding area): 160-180℃ to avoid premature melting and causing material retention and degradation

Middle section (compression zone): 240-260℃. ±1℃ accuracy must be maintained, and PID control is used.

Front section (injection area): 265-285℃ maximum no more than 290℃ to prevent thermal degradation

Nozzle temperature: 265-275℃ Independent closed-loop control, dual thermocouple monitoring is recommended

 

Mold temperature control

Hot runner system: 285±2℃ It is recommended to use a high-response servo valve to control thermal balance

Mold cavity temperature: 8-12℃ (controlled by chiller) The temperature difference must be ≤1℃, and dynamic temperature change technology is recommended

Special measures to prevent white fog

Melt temperature stability control

Install melt pressure sensor (position ≤50mm from nozzle)

Control the actual melt temperature in the range of 280±2℃

Adopt resonant temperature control system (response time <0.5s)

 

Crystallinity management

Modular temperature gradient control during pressure holding stage: keep 12℃ at 0-2s, rise to 15℃ at 2-5s, and drop to 10℃ after 5s

Use pulse cooling technology (periodic 5Hz cooling)

Auxiliary system requirements

Drying system

Dehumidification dryer dew point ≤-40℃

Continuous insulation of the hopper is 120℃ (double tower drying is recommended)

 

Hydraulic system

Oil temperature is maintained at 45±2℃

Injection pressure fluctuations need <0.5%

Process window verification method

Perform DSC test (differential scanning calorimetry):

The melting peak should appear at 250-255℃

The crystallinity is controlled between 32-38%

 

Actual production verification:

Optimize parameters using orthogonal test method

Whiteness Index should be <15 (measured with a chromameter)

Exception handling process

Emergency adjustments when white fog appears:

Reduce nozzle temperature by 3°C every 2 minutes (minimum as 260°C)

Simultaneously increase injection speed by 15-20%

Check whether the mold exhaust is blocked (the exhaust groove should be kept at a depth of 0.02-0.03mm)

3. PET-preform injection molding machine FAQs and solutions

Appearance defect problems

• White fog/white hair

reason:

Melt temperature is too high (>290℃) or mold temperature is too low (<5℃)

Inadequate drying of materials (dew point >-30℃)

Excessive injection speed leads to overheating of shear

Solution:

Reduce the nozzle temperature below 265℃, and adjust the mold temperature to 8-12℃

Extend the drying time (≥4h@120℃), check the drying air volume

Use graded injection: low-speed filling 90% and then turn high-speed

 

• Black spot/yellow change

reason:

Residual degradation of the barrel (dwell time > 5 minutes)

Hot runner dead zone carbonization

Solution:

Clean the barrel with PP material when shut down

The hot runner temperature drops below 270℃, disassemble and clean every week

Dimensional accuracy issues

• Weight fluctuation > ±3g

reason:

Unstable back pressure (suggested value: 5-8MPa)

Screw wear (gap > 0.2mm)

Solution:

Install melt gear pump to stabilize output

Check the screw compression ratio (PET special screws must be ≥2.8:1)

 

• Neck ovality exceeds standard

reason:

Uneven cooling of the mold (temperature difference > 3℃)

Inadequate pressure holding pressure (＜40MPa)

Solution:

Adding point-cooling structure of mold cavity

Three-stage pressure holding: 60MPa→40MPa→20MPa(2s per section)

Performance defects

• The acetaldehyde (AA) content exceeds the standard

reason:

Melt temperature fluctuations (>±3℃)

Unreasonable design of screw compression section

Solution:

Use PID temperature control module instead (accuracy ±0.5℃)

Use barrier screws with mixing head

 

• Stress cracking

reason:

Cooling rate is too fast (>50℃/s)

Insufficient mold release slope (＜1°)

Solution:

Use slow cooling technology: the mold temperature is first 15℃ and then lowered to 5℃

Polish the mold cavity to below Ra0.2μm

Equipment failure category

• Hot runner leakage

reason:

Heating coil power mismatch (need > 30W/cm²)

Wear of valve needle sealing surface

Solution:

Replace the ceramic heating ring (lifetime increased by 3 times)

Replace valve needle seal assembly every 50,000 molds

 

• Screw slide

reason:

The temperature of the rear section of the barrel is too low (＜160℃)

PET viscosity decreases (intrinsic viscosity IV < 0.72)

Solution:

Increase the temperature in the rear section to 170-180℃

Check raw material IV value (qualification range: 0.78-0.85)

Systematic preventive measures

Daily maintenance:

Clean the mold exhaust tank (0.02mm depth × 5mm width)

Verify thermocouple error (±1℃ allowed)

 

Weekly maintenance:

Ultrasonic cleaning of hot runners

Check the water content of hydraulic oil (needed to be <500ppm)

 

Process monitoring focus:

Melt pressure fluctuation <±2MPa

Mode locking force deviation <±3%

 

PET-preform injection molding machine FAQ and processing plan table

Question Type	Possible Causes	Solution
Incomplete filling of the bottle preform (short injection)	1. Residual air in the melt (insufficient plasticization) 2. Insufficient screw back pressure 3. The temperature of the feed section of the barrel is too high or the bridge is built	1. Increase the back pressure of the screw 2. Adjust the back buffer position of the screw 3. Check and clean the bridge of the discharge port to reduce the temperature of the feed section
Hydraulic system leak	1. The sealing ring is aging or damaged 2. The oil pipe is worn or the joints are loose	1. Replace the sealing ring 2. Tighten the threaded connection or replace the oil pipe
Hydraulic oil temperature is too high	1. Cooling system failure 2. Oil contamination or insufficient oil 3. Pump wear	1. Clean the cooler (soft water is recommended) 2. Replace the hydraulic oil (every 5000 hours or once a year) 3. Check the pump and lower the load
Whitening/atomizing	1. Inadequate drying of raw materials 2. Poor plasticization or too low nozzle temperature	1. Increase the drying temperature or extend the drying time 2. Adjust the screw speed/back pressure 3. Check the nozzle temperature and cooling time
Bottle thread missing	1. The mold lip exhaust groove is blocked 2. There is residual material in the thread	1. Clean the exhaust tank with acetone 2. Clean the thread residue with copper needle
Injection molding machine has no action	1. General pressure valve failure 2. Electrical control signal interruption (such as sensor or solenoid valve failure)	1. Check the general pressure valve and amplifier plate 2. Check the sensor, solenoid valve and line connection
Poor transparency of bottle preforms	1. The heating temperature is too high 2. The compressed air contains moisture 3. The raw material quality is poor	1. Reduce the heating temperature or shorten the time 2. Use a dryer to remove water 3. Replace high-purity PET raw materials

 

Preventive maintenance recommendations

• Regular cleaning and lubrication: clean the material pipes, mold residues, and lubricate mechanical components (such as bent elbow mechanisms).
• Hydraulic system maintenance: Replace hydraulic oil and filter element regularly to keep the oil temperature at 40-50°
• Electrical inspection: fasten the wire connectors, test the sensors and heating elements.
• Raw material management: Ensure that the PET raw materials are fully dry (intrinsic viscosity control) to avoid mixing metal impurities.

4. FAQ for PET-preform injection molding machine

Q1: How long is the maintenance cycle of screws and barrels?

A:

Daily: Clean with PP material every shift (prevent carbonization).

regular:

Check the screw wear every 3 months (gap > 0.2mm needs to be repaired).

Replace the thermocouple of the barrel every year (to prevent temperature distortion).

 

Q2: What should I do if the oil temperature of the hydraulic system is too high?

A:

Causes: The cooler is blocked, the oil pump efficiency is reduced, and the oil quality is aging.

deal with:

Clean the oil cooler (water-cooled type requires descaling).

Check the viscosity of hydraulic oil (46-68cSt is preferred at 40℃).

The oil temperature should be controlled at 45±2℃ (the machine must be shut down after exceeding 50℃).

 

Q3: How to set the temperature parameters of PET preform injection molding machine?

A:

The barrel temperature: 160-180℃ in the back section (anti-degradation), 240-260℃ in the middle section, 265-285℃ in the front section (≤290℃).

Nozzle temperature: 265-275℃ (independent temperature control, avoid casting).

Mold temperature: 8-12℃ (controlled by chiller, temperature difference ≤1℃).

Key point: The actual melt temperature should be controlled at 280±2℃ (required with a melt thermometer).

 

Q4: How does the injection speed affect the quality of the bottle preform?

A:

Too fast: causes shear heating (white mist) and molecular chain breakage (decreasing intensity).

Too slow: insufficient filling and premature cooling.

Recommendation: Use graded injection (slow-fast-slow), the first stage speed is ≤30% maximum firing rate.

 

Q5: What to do if there is stress crack in the bottle preform?

A:

Process adjustment:

Reduce the cooling rate (model temperature slows down from 15°C to 5°C).

Increase the holding time (extend 1-2 seconds).

Mold improvements:

Polishing mold cavity (Ra≤0.2μm).

Increase the demolding slope (≥1°).

 

Q6: How to deal with emergency treatment for hot runner leakage?

A:

Stop the machine immediately and turn off the heating system.

Check after cleaning up leaks:

Whether the heating coil is damaged (resistance value deviation > 10% needs to be replaced).

Is the sealing surface of the valve needle worn (roughness > Ra0.8μm required to be ground).

Preventive measures: Check monthly for hot runner torque screws to be loose.