What Factors Influence the Thickness Uniformity of PP Blown Films?

Polypropylene blower is widely used in food packaging, agricultural mulch films and industrial packaging due to its excellent mechanical properties, transparency and barrier. However, thickness uniformity, as the core index of film quality, directly affects the mechanical properties, thermalsealing effect and printing property of the film. Through industry practice and frontier research, the key factors affecting the uniformity of polypropylene blowing film thickness are analyzed systematically, which provides theoretical basis for production optimization.

1.1 Die structure and Gap Uniformity
Die is the key part to determine the initial distribution of melt. If there is a local difference in die clearance (e.g. gap error exceeding 0.1 mm), uneven extrusion volume distribution will be uneven and the thickness of film will fluctuate. For example, areas with larger gaps may have higher extrusion volumes, resulting in thicker films and, conversely, thinner films. In addition, the temperature distribution inside the mold is not uniform, which also affects the degree of resin plasticization. High-temperature regions increase melt fluidity, increased extrusion volume, and cause thickness differences.
Optimization Measures:

• Periodically measure die gaps with a laser distance meters and mechanically adjust to ensure that the error per point is less than 0.05 mm.
• The temperature fluctuation of the die + -1 ℃ or less is realized by using the partition heating control system and PID algorithms.
• Chromium plating is applied to the ageing mould to repair surface scratches and reduce local overheating caused by the retention of the melt.

1.2 Screw and Barrel wear
During long-term operation, the increase of the gap between the screw and the cylinder will cause melt to flow back, which will lead to the fluctuation of extrusion volume. For example, uneven clearance can lead to a melt flow difference of over 15%, directly resulting in transverse thickness deviations. In addition, poorly designed screws (e.g., insufficient compression ratio) can lead to uneven melt plasticization, further worsening thickness uniformity.
Optimization Measures:

• The gap between the screw and barrel is inspected every 5,000 hours and repair or replaced when the gap exceeds 0.3 mm.
• Two-stage screw structure is selected and shearing elements is added to improve melt uniformity.
• Install a stirring head at the end of the screw to enhance the dispersion and mixing of PP with functional materials such as EVOH.

2.1 Temperature Gradient Management
Polypropylene melting temperature range from 160 to 230°C and temperature control shall be determined according to the structure of the layer:

• Inner layer (LDPE/LLDPE): 160–180°C (low viscosity to ensure mobility).
• Mesosphere (PP/EVOH): 180-210°C (equilibrium plasticization and interlayer adhesion).
• Outer layer (PP): 210–230°C (high melt strength to prevent film breakage).

Excessive temperature difference leads to uneven shear stress between layers, resulting in stratification, while low temperature difference affects the adhesion strength of functional materials, such as EVOH.
Case study: One enterprise reduced the standard deviation of film thickness from 8 microns to 3 microns by reducing mold temperature fluctuation from + -3°C to + -1°C.
2.2 Matching to Draw Ratio
Coordination of blast ratio (bubble diameter/die diameter) and pull ratio (traction speed/extrusion speed) is essential:
Excessive bloating (>3.5) results in unstable bubble formation, resulting in a 20% difference in transverse tensile strength.
Too high a stretch ratio (>15) can cause excessive longitudinal stretching, thinning the film and making it easy to "ripple."
Optimization Model:

• To establish a mathematical relationship between BUR and (DR:
• BUR * DR =constant

Ensure balance of transverse and longitudinal stretching stresses. For example, when BUR = 3.0, DR should be controlled between 10 and 12.

2.3 Cooling System Design
The cooling speed directly affects the film's crystallinity and thickness uniformity:

• Excessively high air ring air speed (>10 m/s) can cause the films to become too cold and brittle, resulting in a 15% difference in local shrinkage.
• Insufficient air speed (less than 3m/s) can lead to overheating and adhesion, creating "orange peel" textures.

Innovation and Technology:
Double gas ring structure is adopted. The inner gas ring controls the cooling at the top of the bubble and the outer gas ring regulates the temperature in the middle of the bubble. The frost line height fluctuation less than ±5 mm with independent wind volume control.

Melt Flow Rate
The MFR of PP directly affects melt fluidity:

• An excessively low MFR (<1 g/10 min) causes extrusion pressure to fluctuate, resulting in uneven thickness.
• An excessively high MFR (>10 g/10 min) results in insufficient melt strength and film breakage.

Recommended Solution:

• Copolymer PP resin with MFR of 2-5 g/10 min was selected and LLDPE of 0.5-1% was added to improve processing performance.

3.2 Raw Material Drying Treatment
Too much moisture content (>0.02%) causes the following problems:
Melt expands to form bubbles, causing a sudden increase in local thickness.
Poor plasticization results in melt viscosity fluctuations and a 30% increase in standard deviation of thickness.
Drying Process:
Adopt a three-stage drying system:
Preheat (80°C for 2 hours) to remove surface moisture.
Dehumidification stage (120°C for 4 hours) reduces internal moisture content.
Keep warm (100°C for 1 hour) and maintain moisture levels low.

4.1 Workshop temperature and Humidity Control
Temperature fluctuations in the workshop environment (> 5°C) can cause:
Die temperature drift, resulting in periodic changes in die thickness.
Bubbles cool at different rates, forming "transverse stripes."
Solutions:
Install a constant temperature and humidity system to maintain workshop temperature at 25 + 2°C and humidity at 50% + 10%.
4.2 Operator Skill Training
Human factors can account for up to 20% of thickness uniformity issues:
Delayed parameter adjustments (e.g., a traction speed response delay >5 seconds) may cause thickness fluctuations.
Inadequate equipment cleaning (such as charred material left on moulds) can lead to a sudden increase in local thickness.
Training system:
Establishment of a three-tier skills certification system:
Basic level: master basic parameter settings (e.g. temperature, wind speed).
Intermediate: Possess fault diagnosis capabilities (e.g., analysis of the cause of thickness fluctuation).
Advanced: Capable of optimizing process formulations (e.g. adjusting interlayer ratios).

5.1 Online Thickness Detection System
Beta-ray or laser thickness are used to achieve:
Thickness of films was monitored in real time (accuracy ±1 μm).
Process parameters (such as wind volume, traction speed, etc.) are automatically adjusted by closed-loop control.
Case study: after one enterprise launched an online detection system, the product qualification rate increased from 85% to 98%.
5.2 Digital Twin Technology
The digital model of the blow-film machine is established and the optimization process is simulated.
Predicting thickness distributions under different process parameters.
Reduce test run and shorten process development cycle by over 50%.
Implementation path:
Collect equipment operation data (temperature, pressure, speed).
The computational fluid dynamics (CFD) model established.
Optimizing control strategies through machine learning.
Conclusion:
The thickness uniformity of PP blown films is influenced by equipment precision, process parameters, raw material characteristics, production environment and control technology. quality stability can be greatly improved by implementing the following strategies:
Establish preventive maintenance system to ensure the accuracy of key parts such as die and screw.
Process parameters were optimized by using gradient temperature control and blow-up rate-pull ratio matching model.
Strict control of raw material drying and MFR to reduce thickness fluctuations at source.
Online detection and digital twin technology are introduced to realize intelligent production control.
In the future, as IoT and AI technologies are deeply integrated, PP blowing film production will move toward higher automation and refinement, providing better material solutions for the packaging industry.