Apart from the air ring, factors causing uneven film thickness
It is divided into circumferential uneven thickness (one side is thicker and the other is thinner in a circle) and longitudinal thickness fluctuation (the thickness varies during the film feeding process). Small machines have low tolerance for errors, and many minor factors can amplify the thickness deviation.
1. Uneven thickness in the circumferential direction (persistently thicker or thinner at a fixed position)
1. The temperature distribution of the die head is uneven
The heating ring is loose and aged, the temperature measurement of the thermocouple is inaccurate, and there is a large temperature difference at various positions of the die head. The local temperature is high, the melt fluidity is large, and the film is relatively thick. The temperature is low, the fluidity is poor, and the film is relatively thin. The mold head of a small-sized machine is small in size. Even a slight displacement of the heating coil can cause a significant temperature difference.
2. The die gap is uneven
The tightness of the die bolts is inconsistent, and the size of the gap around the die lip is not the same. As a result, the material discharged is thicker on one side and thinner on the other. This situation is most likely to occur after disassembling, assembling and cleaning the die head.
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For small die molds, the bolts cannot be tightened significantly; they can only be adjusted slightly.
3. Carbon deposits and impurities adhering inside the die head
Plastic coke or impurities getting stuck at a certain gap of the die lip prevent the melt from flowing out, causing local continuous thin spots or thick strips, and may also be accompanied by pinholes.
4. Partial blockage of the filter screen
The filter screen is partially blocked by impurities, causing the melt flow direction to shift and the die head discharge to be unbalanced in the circumferential direction.
5. The die head and the wind ring are not concentric
The centers of the die head and the air ring are misaligned. Even if the air ring itself has uniform air output, the cooling conditions around the film bubble are still inconsistent, which is manifested as circumferential thickness deviation.
Ii. Longitudinal thickness Fluctuation (Along the direction of film movement, the thickness fluctuates)
1. The extrusion discharge is unstable
- Screw speed fluctuation: Inverter failure, speed setting too high; The screw of the small-sized machine is short, and it is prone to unstable discharge at high speeds.
- Unstable melt pressure: The filter screen gradually gets clogged, the pressure fluctuates, and the output volume is sometimes large and sometimes small.
- Barrel temperature fluctuation: Temperature control drift, material plasticization is sometimes good and sometimes bad.
2. Issues with the raw materials themselves
- Moisture in raw materials: PLA and modified materials contain water, the melt is mixed with bubbles, and there is thickness fluctuation. PE generally has a relatively small impact.
Excessive addition of recycled material and mixed raw material particles: unstable fluidity and fluctuating extrusion discharge.
The raw material grade has been changed, and the fluidity index does not match.
3. Abnormality in the traction system
The traction roller slips, the rubber roller wears out, and the traction speed fluctuates: when the traction is fast, the film becomes thinner; when the traction is slow, the film becomes thicker.
The pressure on both sides of the traction roller is inconsistent, and the film is unstable under tensile force.
4. Fluctuations in winding tension
The winding tension fluctuates greatly, causing the film to be repeatedly stretched, and the finished roll shows thickness fluctuations.
Iii. External Environment and Assembly Factors
1. Workshop draft
The external airflow directly blows the film bubbles, forcing local cooling and causing thickness differences. The small film bubbles are significantly affected. It's not a fault of the wind ring itself, but it's superimposed with the effect of the wind ring.
2. Insufficient or excessive plasticization of the screw
The overall temperature is relatively low, the plasticization is incomplete, and the melt fluidity is poor and unstable. When the temperature is too high, the material degrades, the melt performance deteriorates, and the thickness fluctuation increases.
3. The die head was not adequately insulated during heating and temperature rise
The machine was started right after the temperature was raised. The temperature at all parts of the die head has not yet reached thermal equilibrium, and the thickness remains unstable. It is necessary to keep it warm and wait for thermal equilibrium.
