Plastic bag film blowing is a highly technical production process with significant influence of process parameters. In order to ensure stable film quality, uniform thickness, good transparency and excellent physical properties, special attention should be paid to the following aspects during the film blowing process:
1. Raw materials
Raw material drying: Some materials (such as nylon and PLA) have strong hygroscopicity and must be fully dried, otherwise bubbles, crystal points and uneven thickness are easily generated. LDPE and LLDPE are generally not sensitive to moisture, but they should also be properly dried when the ambient humidity is high.
Raw materials are pure and mixed evenly: avoid impurities affecting the transparency and strength of the film. When multiple raw materials are blended (such as co-extrusion), the proportions must be precise and the mixing must be uniform to avoid color difference or unstable performance.
2. Equipment and process control
Temperature control: The heating temperature of each section (extruder barrel temperature, die head temperature) must be accurately controlled to prevent material decomposition or insufficient fluidity. Different materials have different melting temperatures, such as about 160-190℃ for LDPE and about 180-220℃ for HDPE.
Wind ring and cooling: The wind ring (cooling wind) should be balanced and of moderate strength, otherwise the film bubble swing will cause uneven thickness and film wrinkles. The cooling rate affects transparency and strength. Water-cooled films have higher transparency and air-cooled films have more stable structures.
Take-up ratio and blow-up ratio:
Take-up ratio: affects longitudinal stretching and thickness;
Blow-up ratio: affects transverse stretching and width;
Improper ratio adjustment will lead to unstable film bubbles, large size changes, and poor physical properties.
Die adjustment: The opening uniformity, cleanliness, and temperature consistency of the die head directly affect the consistency of film thickness.
Film bubble stability: Use auxiliary devices such as stabilizers and shaping rings to keep the film bubble vertical and not swing excessively to avoid film deviation or poor winding.
3. Product quality control
Film thickness control: Use an automatic thickness detection system or an artificial online thickness measuring instrument to ensure uniform thickness and avoid thickness fluctuations.
Transparency and haze: Too fast cooling, mixing problems, and poor raw material quality will reduce transparency. The temperature and cooling speed can be adjusted appropriately to improve visual quality.
Crystal points and impurities: Black spots and crystal points will be caused if the material is not filtered and the die head is not cleaned.
Physical performance testing: Test indicators such as tensile strength, heat sealing, drop performance, and puncture resistance to ensure the reliability of the actual application of the film bag.
4. Safety and maintenance
Equipment regular maintenance: such as air rings, screws, die heads, cooling systems, etc. need to be cleaned and inspected regularly to prevent carbon deposition and blockage.
Production environment control: dustproof, anti-static, and anti-foreign matter, especially the production of food packaging film must be completed in a clean workshop.
Safety operating specifications: The film blowing equipment has high temperature and high pressure, and the operating specifications must be strictly followed to avoid dangers such as burns and explosions.
5. Additional precautions for multi-layer co-extrusion blown film
The melt index (MI) of each layer of raw materials should be matched to prevent delamination and peeling.
The design of the bonding layer (Tie layer) should be considered between different materials.
The layer thickness ratio needs to be strictly controlled to prevent the weak layer from affecting the overall strength.
The synchronization, temperature control and extrusion rate of multi-layer equipment are more important.
