What are the common faults of the bottle blowing system and their solutions?

2025-08-29 09:58:45

During long-term operation, the bottle blowing system is prone to malfunctions due to parameter deviations, component wear, raw material issues, etc., which can affect the quality of the bottle body and production efficiency. The following are the manifestations, causes and targeted solutions of six types of high-frequency faults, covering core links such as heating, blow molding and demolding:

I. Fault 1: Partial whitening and crystallization of the bottle body (appearance defect)

"Performance

White foggy areas appear on the side walls, bottom or near the bottle mouth, with a rough touch and reduced light transmittance (especially for PET bottles).

Core reason

Uneven heating of the preform: The heating tube corresponding to the whitening area has insufficient power or is damaged, or there are obstructions in the heating channel (such as dust, residual plastic).

Abnormal blow molding temperature: The heating temperature of the preform is lower than the "elastic stretch range" (PET bottles need 90-120℃), or the mold temperature during blow expansion is too low (< 15℃), causing the plastic molecules to crystallize too quickly.

Excessive moisture content in raw materials: When PET raw materials absorb moisture and are heated, the water vaporizes to produce bubbles. After the bubbles burst, local crystals form.

Solution

Check the heating system

Disassemble and inspect the heating channel, and clean the dust and residual plastic in the corresponding area. Use a multimeter to test the resistance of the heating tube and replace the damaged one.

Adjust the temperature of the heating zone: Increase the temperature of the heating group corresponding to the whitening area by 5-10℃, and at the same time ensure that the fan in the heating channel is operating normally (to avoid local overheating).

Optimize mold and blow molding parameters

Introduce 30-40℃ warm water (instead of cold water) into the mold cooling water passage to slow down the cooling rate.

Reduce the blow molding pressure by 5% to 10% appropriately (to avoid excessive stretching of the plastic leading to crystallization).

Control the humidity of raw materials

Check the dew point temperature of the dryer (≤-40℃) and replace the ineffective molecular sieve. The retention time of raw materials in the dryer should be ensured to be 4 to 6 hours (the standard drying time for PET raw materials) to prevent undried raw materials from entering the system.

Ii. Fault 2: Uneven wall thickness of the bottle body (size/strength defect

"Performance

One side of the bottle body is too thin (< 0.15mm, prone to deformation and cracking), while the other side is too thick (> 0.6mm, wasting raw materials), or there are "depressions/protrusions" at the bottom.

Core reason

Preform positioning deviation: When the preform is placed into the blow molding mold, the center is misaligned with the center of the mold cavity (such as loose preform grippers or worn mold positioning pins).

Improper blow molding pressure/time: The high-pressure blowing pressure is too low (< 2.5MPa), or the blowing time is too short, resulting in the plastic not fully adhering to the mold cavity.

Mold cavity wear: After long-term use, scratches and deformations appear on the inner walls of the mold cavity, resulting in uneven distribution of plastic.

Solution

Calibrate preform positioning

Check the tightness of the preform gripper and replace the worn gripper spring. Adjust the locating pins of the mold (replace them if worn) to ensure that the center of the preform is aligned with the center of the mold (error ≤0.05mm).

Manually test the clamping positions of 3 to 5 preforms to ensure there is no offset before mass production.

Adjust blow molding parameters

The blow molding pressure is increased to 3-4 mpa (adjusted according to the bottle type; 4-5 mpa is required for carbonated beverage bottles). Extend the blowing time by 0.2 to 0.5 seconds (to ensure that the plastic fully adheres to the cavity).

Repair/Replace the mold

Slightly worn cavities should be ground with diamond polishing paste of 1000# or above to remove scratches.

If the cavity is deformed (such as an ellipse), it needs to be returned to the factory for CNC correction or the mold cavity should be directly replaced.

Iii. Fault 3: Cracking/Depression at the bottom of the bottle (structural defect)

"Performance

After demolding, cracks appear at the bottom of the bottle body, or the center of the bottom is concave (with a depth greater than 2mm, affecting the standing stability).

Core reason

Insufficient bottom heating: The heating tubes at the bottom of the preform are not covered, or the heating temperature is too low, resulting in insufficient softening of the plastic at the bottom and causing it to crack under force when blown up.

Blow molding pressure fluctuation: Unstable pressure of the high-pressure air source (such as insufficient pressure from the air compressor, air pipe leakage), or blockage of the blow valve, resulting in insufficient expansion at the bottom.

Abnormal demolding mechanism: The position of the demolding ejector pin is offset, or the ejection speed is too fast, forcing the ejection and causing the bottom to crack.

Solution

Enhanced bottom heating

Add a dedicated heating tube at the bottom of the preform (if not equipped), or increase the temperature of the bottom heating group by 10-15℃.

Adjust the rotational speed of the preform in the heating channel (reduce by 5% to 10%) to ensure that the bottom is fully heated.

Stable blow molding air source

Check the pressure of the air compressor (which needs to be stabilized at 0.8-1.0MPa), and replace the leaking air pipes and joints.

Disassemble the air blowing valve, use compressed air to blow away impurities inside the valve, or replace the worn valve core sealing ring.

Optimize demolding parameters

Adjust the position of the top pin to ensure it is directly aligned with the center of the bottle bottom (deviation ≤0.1mm).

Reduce the demolding and ejection speed (from 50% to 30%-40%) to avoid impacting the bottle body.

Iv. Fault 4: Deformation and burrs at the bottle mouth (sealing defect)

"Performance

The bottle mouth thread is incomplete, with flash (plastic overflow), or the bottle mouth plane is tilted, resulting in an inability to seal the bottle cap (leakage during filling).

Core reason

Wear of the mold bottle mouth cavity: After long-term use, the threaded neck ring shows scratches or deformation, or the fit clearance between the cavity and the preform is too large.

Insufficient clamping force: When closing the mold, the clamping force is lower than the set value (for example, if 300kN is set but only 250kN is actually used), resulting in material leakage on the parting surface of the mold.

Insufficient blow molding holding time: During the holding stage, pressure was not continuously applied to the bottle mouth, and the plastic did not fully fill the threaded cavity.

Solution

Repair/Replace the bottle mouth cavity

Remove the threaded neck ring and clean the residual plastic in the cavity with a special tool. For minor wear, sand the threads with 800# sandpaper. For severe wear, replace the neck ring with a new one (made of S136 die steel).

Calibrate the clamping force

The force on the four pull rods is detected through the "clamping force testing function" of the equipment, and the pull rod nuts are adjusted to make the force uniform (deviation ≤5%).

If the clamping cylinder leaks, replace the cylinder seal to ensure the stability of the clamping force.

Extend the pressure-holding time

The holding pressure time is extended by 0.3 to 0.5 seconds from the original set value (for example, from 1.5 seconds to 2.0 seconds), while ensuring the pressure remains stable during the holding pressure stage (consistent with the blow molding pressure).

V. Fault 5: The bottle body cannot be demolded (production interruption)

"Performance

After blow molding is completed, the bottle body is closely attached to the mold cavity. When the ejector pin is pushed, the bottle body deforms or cannot be detached, and the mold needs to be manually disassembled to take out the bottle.

Core reason

There is no draft Angle in the mold cavity: or the draft Angle is too small (< 1°), and the plastic adheres too tightly to the inner wall of the cavity after cooling.

There is no mold release agent on the mold surface/the mold release agent has failed: The inner wall of the cavity has not been sprayed with food-grade mold release agent, or the mold release agent has been covered with plastic residues after long-term use.

Excessive cooling time: The bottle body cools down too much (such as cooling time > 15 seconds), and after shrinking, it "locks up" with the cavity.

Solution

Correct the draft Angle of the mold release

Return to the factory for CNC processing of the mold cavity, and adjust the draft Angle to 1°-2° (the higher the bottle height, the slightly larger the Angle should be).

If it is not possible to return to the factory in the short term, the inner wall of the cavity can be slightly ground (using 2000# sandpaper) to increase the surface smoothness.

Re-spray the mold release agent

Disassemble the mold, clean the inner wall of the cavity with a neutral cleaner, and remove the residual plastic and expired mold release agent.

Spray food-grade silicon-based mold release agent (a thin layer is sufficient to avoid excessive use that may cause the bottle to slip), and respray once every 1,000 to 2,000 bottles produced.

Shorten the cooling time

Reduce the cooling time by 0.5 to 1 second (for example, from 12 seconds to 11 seconds) to ensure that the bottle cools down to a state where it can be shaped but does not seize up (as long as the outer side of the bottle does not feel hot when touched).

Vi. Fault 6: The bottle blowing system alarms "No high-pressure gas" (equipment stops)

"Performance

When the equipment starts the blow molding process, the screen displays an alarm of "Insufficient high-pressure gas pressure", and the blow molding action cannot be carried out.

Core reason

Air source system failure: The air compressor has not started, the pressure switch is damaged, or the air compressor pressure has not reached the set value (0.8-1.0MPa is required).

High-pressure air pipe blockage/leakage: Impurities (such as plastic debris) blocking the air pipe, or wear of the air pipe joint or valve sealing ring causing leakage.

High-pressure valve failure: The valve core of the blow-molded high-pressure valve is stuck (such as being jammed by impurities), or the solenoid valve coil is burned out, preventing normal ventilation.

Solution

Check the source of the gas source

Confirm that the air compressor has started. Observe the pressure gauge of the air compressor. If the pressure is insufficient (< 0.8MPa), adjust the pressure switch of the air compressor to 1.0MPa.

If the air compressor does not start, check the power cord and contactor, and replace the damaged electrical components.

Inspect the air pipe and the joint

Turn off the air source, disassemble the high-pressure air pipe (from the air compressor to the bottle blowing valve), and use compressed air to blow back the air pipe to remove internal impurities.

Inspect the joints and valve sealing rings, replace the worn sealing rings (it is recommended to have spare sealing rings of the same model), and re-tighten the joints (ensure there is no air leakage).

Repair the high-pressure valve

Disassemble the high-pressure valve, clean the valve core with alcohol and remove impurities. If the valve core is worn, replace it with a new one.

Use a multimeter to test the resistance of the solenoid valve coil. If the coil burns out (with infinite resistance), replace the solenoid valve coil.

Summary: The key to fault prevention

Regular maintenance: Clean the dust in the heating channel and inspect the cooling water circuit of the mold every week. Calibrate the clamping force and replace the high-pressure valve sealing ring every month. Inspect the heating tubes and the molecular sieve of the dryer every quarter.

Parameter recording: Archive the production parameters (heating temperature, blow molding pressure, cooling time) of qualified bottle bodies. In case of faults, compare and restore the parameters to reduce debugging time.

Raw material control: Strictly control the moisture content of PET raw materials (moisture content after drying ≤0.02%), and avoid using damp or caked raw materials.

By the above methods, the high-frequency faults of the bottle blowing system can be quickly resolved, the scrap rate and downtime can be reduced, and the stability of production can be ensured.