polypropylene film films are widely used in packaging, industry and agriculture because of their excellent physical properties, chemical stability and processing properties. In food packaging, it can effectively block air and moisture, extending the shelf life of food; in industry, it can be used to make a variety of packaging materials, insulation materials, etc. In view of the importance of polypropylene (PP) films, it is important to explore the key steps in the production of PP films in order to improve the quality of the films and meet the needs of different fields.
Key steps of raw material processing prior to PP film production
(I)Raw material selection.
Different types of polypropylene resin have unique characteristics and are suitable for different applications. Homopolymer PP has high tensile strength, elastic modulus and melting point, so the films made of it have good stiffness. However, it is not suitable for frozen food packaging due to its high heat sealing temperature and low temperature toughness. Random copolymer PP, although relatively high in price, has advantages of transparency and flexibility in some properties.
Choosing the appropriate raw material according to the specific application of the film is crucial. For example, for food packaging requiring high temperature distillation, polypropylene resin resistant to high temperature distillation must be selected to ensure that the film does not deform or decompose under high temperature conditions, thus guaranteeing food safety and quality. Metallized films has special requirements for raw material additives and special additives are needed to improve the adhesion between metallized film and aluminum layer.
(II) purification of raw materials and preparation of PP raw materials purchased may contain impurities and introduce appearance defects such as crystal points and fisheyes into the films, affecting the quality and properties of the films. Therefore, the purification of raw materials is necessary.
PP plastic granules are usually mixed in proportion to additives such as EVA, matte masterbatch and white in the production of films for different purposes. For example, the addition of matte masterbatch can reduce the gloss of the film, giving it abrasive and suitable for packaging that requires a low-profile appearance. The mixed raw materials are then transported by pneumatic or other means to the extruder in preparation for subsequent molding processes.
(III) Catalyst preparation (if Polymerization Reaction is involved) Catalyst is required if the production of certain polypropylene films begins with a polymerization reaction. Ziegler-Natta catalysts are commonly used and consist of titanium compounds and organometallic compounds. They must be prepared under strictly anhydrous and oxygen-free conditions, as water and oxygen inactivate catalysts and affect the polymerization reaction and the quality of polypropylene resin.
Key steps in PP PP Film Molding
(I) Extrusion
1. Method of casting
T-joint die is the key equipment in casting film production. The die design must ensure that the material flows evenly across the width of the die lip and has an even temperature distribution. In industrial production, the extruder has certain parameters such as diameter, output, melt temperature, crack width and backpressure. For example, too high a melt temperature can cause film to melt and crack, while too low a temperature can lead to poor material flowability and affect the quality of the film.
the extruder melts melts and plasticizes the raw material and then extrudes it through a T-shaped seam mold. The molten film preform is immediately pulled onto the casting roller to form a film. The diameter, surface treatment and cooling mode of casting roller have important influence on the quality of casting 1. **Coating Method:** Cooling water circulates in the casting roller, and the diaphragm bishell structure is employed to realize the spiral cooling water flow, so that the film can be cooled and molded quickly. At the same time, the temperature of the die-casting roller must be strictly controlled and generally maintained within the appropriate range to ensure the smoothness and performance of the film.
2. **Blow Molding Method:** blown film forming process involves extruding polymers from a tubular preform, blowing them to a specified thickness with high pressure air, and then cooling them to form a film.
Both flat extrusion blowing film and flat extrusion blowing film have their own characteristics. Polypropylene is a crystalline polymer with low melting strength, high crystallinity and fast crystallization speed. The production line is reasonably distributed, using water ring spray for cooling. It has advantages of convenient film drawing, fast production speed, high cooling efficiency efficiency, low melt viscosity and high product transparency.
3. **Coating Method:** Coated film is the process of extruding polypropylene molten film onto base mold (other polymer films, metal foil or paper) to form a composite film material. Multilayer composite films can be used for single-sided or double-sided extrusion coating.
The coating process is suitable for the preparation of polymers non-oriented multilayer composite films with different material properties and processing conditions. For example, applying PP film to a paper base can improve the paper's water resistance and strength of the paper while maintaining its printability. Coating can also improve the corrosion resistance, surface gloss and abrasion resistance of composite film products.
(II) Forming
Casting method: Under the tug and cooling action of casting roller, the melt is lengthways stretched and cooled to form a thin film. Then precut, thickness measurement, sway adjustment, surface corona treatment and edge trim before winding. The corona treatment can improve the surface performance of the film and enhance the adhesion of the film to other materials.
2. Blow Molding Method: A tubular preform is tightened by a traction device. A small amount of low pressure air is blown into the premolded body to prevent adhesion and the formation of enclosed bubbles. High-pressure air is then blown in, stretching and expanding the preform into thin-walled tubular films. Melted tubular films are cooled and solidified by means of air conditioning or spray bath. Thickness and uniformity of tubular films can be regulated by controlling die clearance, airflow, cooling conditions and traction speed. Finally, the PP blown film is achieved through herringbone plate collection, clamping rollers folding and winding.
3. Thermal forming (if applicable): Polypropylene sheet as the main raw material, vacuum forming forming, air compression forming and other thermoforming processes to produce a variety of polypropylene plastic products. Specific steps are as follows:
- Plate feeding and positioning: Accurately place polypropylene sheets in the specified position of the thermoforming equipment.
- Heating and plasticizing: control heating temperature and residence time to achieve a suitable plasticizing state of the plate. High heating temperature will lead to plate decomposition, while low temperature will hinder adequate plasticizing.
- Molding: Apply molding force after closing the die. Vacuum forming uses atmospheric pressure to press a softened sheet material to the surface of the die cavity; pneumatic forming uses compressed air above the sheet material to force it into the mold cavity. Vacuum pressure can also be combined with air pressure. Maintain sufficient vacuum or pneumatic pressure and retention time to ensure molding quality.
- Cooling and setting: Cooling is achieved through an air cooling system in a cooling water line or die, and the cooling time is controlled.
- Demolding: The plastic part is removed by vacuum backflushing, compressed air blowing or ejecting mechanism.
INTRODUCTION Key post-processing steps in PP film production
(I) Surface treatment
Grafting polymerization graft polymerization discharge corona discharge irradiation laser irradiation used to improve the wettability and adhesion of polypropylene film surfaces. Among them, corona treatment technology has been widely concerned for its environmental protection. It can improve the concentration of reactive oxygen species (ROS) on polymer surface. It has advantages of wide processing range, short treatment time, fast processing speed, on-line processing, simple operation, easy control, minimal impact on mechanical properties and no waste liquid discharge. It improves the wetness and adhesion of plastic surfaces and is commonly used for surface treatment before film printing, coating and laminating.
Corona treatment is based on a process in which a high voltage electric field causes an electron flow to hit the surface of the film, roughening the surface and producing a large number of plasma particles. Ozone interacts with molecules on the surface of plastics, causing polymer bonds to break, creating free radicals and unsaturated centers. These free radicals and unsaturated centers then cross with water in the air to form polar groups that activate the surface. The increase of surface tension is mainly caused by polar components.
(II) Slitting and Rewinding
The film master rolls is transferred to a cutting machine and is cut according to specific size requirements. The slit film is then rewound using a coiler for subsequent storage, transport and use.
Quality Inspection of the appearance, dimensions, uniformity of wall wall thickness uniformity specific performance requirements of the The finished film. In terms of appearance, the film must be free from material defects, bubbles, dents, black spots, silver streaks, deformation, etc. In terms of dimensions, key dimensions must be consistent with drawings. The uniformity of wall thickness directly affects the properties and performance of the films.
Inspection methods include first inspection, process inspection and finished product inspection. Establish Sampling plans in accordance with relevant standards and norms to ensure accuracy and reliability of inspection results.
(IV) Aging Treatment (where applicable) Some films, such as BOPP film, require to be aged after winding after production (stored at room temperature for at least 72 hours). Aging treatment can make the film completely free of shrinkage, release internal stress and improve the stability of dimensions. At the same time, it allows additives to migrate from the core layer to the surface, improving the film's performance.
Conclusion Each stage of polypropylene film production process contains key steps such as selection of raw materials, purification and preparation, catalyst preparation, extrusion and molding, post-production surface treatment, slitting and coiling, quality inspection and aging treatment. Every link is interconnected and interactive. Strict control of parameters of each step is essential to ensure the quality of the film and to meet the needs of different applications. Only by continuously improving production process can we produce higher quality andperformance polypropylene films to meet the changing market demand.
