In the field of film production, "blowing method" and "casting method" are two more commonly used processes. The following is a brief introduction and comparison of the most commonly used production processes in the production of these two films.
Multi-layer co-extrusion blow molding
This production process is mainly divided into two forms: up-blowing cooling and down-blowing water cooling.
It's cold on the air
It is mainly composed of multiple extruders, multi-layer superimposed die heads, multi-vent air ring, IBC internal cooling system, chevrons, rotary traction system, lower traction system and winding system. At present, the imported units are mainly from the original Badenfeld in Germany, W&H in Germany, and Brampton in Canada. Among the domestic units, Guangdong Jinming is the representative. In general analysis, the current core mechanical capabilities are still controlled by foreign companies. For example, most of the core components of Guangdong Jinming's units still need to be supplied by foreign manufacturers.
Down blowing water cooling
The basic components are the same as the up-blowing type. The difference is in the cooling method. The down-blowing uses circulating cooling water as the main form of cooling. The design position of each component is different from that of the up-blowing. This production process is used in my country's PE and It is more common in PP film production. In the production of multi-layer co-extruded high-barrier films, the above-mentioned blow-cooled units are still the main ones. Regardless of which two types of units are used, in terms of appearance, they are mainly characterized by cylindrical film. In terms of vacuum bag making, due to the reduction of two heat-sealing edges, it is more effective in protecting the contents of the package. Cast film has a very big advantage. It is better than cast film in terms of tensile strength.
Multi-layer co-extrusion casting method
This production process is mainly composed of multiple extruders, multi-channel distributors (commonly known as "collectors"), T-die heads, casting systems, horizontal traction systems, oscillators, and winding systems.
The main characteristics of this production process are that the produced film products have good surface gloss, high flatness, small thickness tolerances, good mechanical extension properties, good flexibility, and higher transparency than blow molding methods. At present, the imported units are represented by the former Badenfeld of Germany, W&H of Germany, Corinth of Italy, Lenzing of Austria, and the former Mitsubishi Heavy Industries of Japan, and the domestically produced units are represented by Foshan Shicheng. Similarly, the production method of domestic casting line is the same as that of blow molding, and the main core components come from foreign suppliers. At present, this kind of production process is mainly used in the domestic CPP industry, and there are also a small number of applications in the production of seven-layer co-extrusion with high barrier properties.
The imported production lines are mainly represented by the seven- and eleventh-floor units of Guangdong Zhongshan Xiangfu and the nine-floor units of Changzhou Haiqi Plastic Industry.
These two production processes are the most common in my country. Because the blow molding process cannot be compared with the cast film in terms of product flatness and transparency, but its high production efficiency, low equipment investment, and low material loss are the casting process. incomparable.
Therefore, foreign companies have carried out in-depth research and development on the improvement of the deficiencies of the blow molding process, which is mainly reflected in (1) the improvement of the die head design and the use of a unique parallel multi-runner design concept; (2) The position and angle of the extruder; (3) the improvement of the cooling system (mainly air ring and internal cooling technology) (4) the four aspects of material formulation and structural design. Practice has proved that the transparency, flatness of the film, and the uniformity of the thickness of each layer of material have reached or approached the level of the casting method. If domestic companies invest in high-barrier film equipment, blown film will be the main focus after 2010.
Comparison of blow molding method and casting method
Blow molding method and casting method are the two main production process methods for polyolefin film production and processing. Blow molding has been applied to the production and processing of films earlier. Due to the limitations of its production process and the insurmountable defects of the products, in the 1980s, a more advanced film production process—casting method was gradually developed. In particular, the use of multi-layer co-extrusion technology has enabled the film production and processing technology of the casting method to be widely promoted and applied, and gradually applied to the high-end packaging of various items with its high-quality product characteristics. Due to its inherent defects, the blow molding method can only be satisfied with the packaging of ordinary middle and low-grade products.
1. Comparison of production process
1. The casting method is more suitable for multi-layer co-extrusion film production and processing than the blow molding method. Especially when a variety of materials of different materials are used for co-extrusion, the production process parameters of the casting method are easier to control, and the processing equipment The key part-the design of the die head can effectively ensure the uniformity of the distribution of various materials in the product. However, the multi-layer co-extrusion die head of the blow molding method is more technically difficult, the structure is more complicated and the technology is not mature enough.
2. In the processing equipment of the casting method, the distance from the die to the cold roll (forming to setting) is generally 10-20mm, the melt film curtain is very short and it is very good with the help of vacuum suction hood, air knife and edge fixing device. Quick setting, product quality is good and stable. In the blow molding method, the melt film has a blowing process and relies on air or water to cool and set the shape. The setting time of the product is too long and the change before setting is large (generally 4 to 7 times of inflation). It is easy to be affected by external factors and its own factors in the melting film stage, and the product quality control is very difficult and the stability is poor.
3. There is also a big difference between the blow molding method and the casting method in the selection of raw materials. In the multi-layer co-extrusion, the blow molding method requires the use of materials with similar fluidity as much as possible to match each other. When the fluidity of the materials differs greatly, the corresponding The measures are adjusted so that when one of the materials is selected, there is not much room for choice with the other materials co-extruded, which greatly restricts the embodiment of the advantages of the multi-layer co-extrusion processing technology in the product quality. The processing equipment of the casting method can easily solve the almost unsolvable problems in the blow molding method by adjusting the runners.
2. Comparison of product features
1. In the blow molding method, the vertical and horizontal molecules of the product are stretched to different degrees due to an inflation process. The temperature resistance and heat sealing performance of the product will be greatly affected. When the product needs to be retorted and sterilized (such as 121°C steam), the stretched molecular chain in the molding process will shrink significantly after being heated, which will cause the product to be severely deformed. , And will greatly reduce the strength of the heat-sealed joints and lead to bag leakage.
2. When the film is produced by the casting method (also known as unstretched film), the molecules are arranged in an orderly manner, and the product cooling speed is fast, which is beneficial to improve the transparency, gloss and thickness uniformity of the product, and the product texture is soft and tough; Impact resistance, heat resistance and low temperature adaptability are better than blow molding.
3. The production efficiency of the casting method is extremely high, and the product quality is stable. There is less waste in the production process, and it is easier to realize online recycling of edges and waste, and the utilization rate of materials is high.
4. The casting method adopts a flat die head with a special retention groove in the die head, which can ensure the uniformity of the material flow, and the uniformity of the die lip gap can be automatically controlled by the automatic adjustment device of the die head, and the control accuracy is very high. , The thicker film can be controlled below ±3%. However, the annular die head and the multi-layer superimposed die core used in the blow molding method require very high processing accuracy and assembly accuracy and are particularly difficult to control. If there is a small error in the processing and matching accuracy, it cannot be compensated by adjustment. The thickness tolerance is basically around ±10%. For a 200μm thick film, it means that the thinnest part is 180μm and the thickest part may be 220μm. When the bag is welded and sealed, the gap of the mold and the heat sealing temperature are adjusted anyway. It is difficult to ensure that the thickest and thinnest parts can achieve the necessary sealing strength. The phenomenon of bag leakage is inevitable, and the yield is naturally very low.
5. For materials with lower melt strength, down-blowing and water cooling are required. Incomplete moisture drying or improper selection of corona layer materials will result in poor corona treatment effect or serious corona attenuation. Affect the fastness of the printed ink and cause discoloration. The casting method uses a steel casting roll (with cooling water inside) to cool and shape the film, and the water does not directly contact the film.
6. For the film that needs to be retorted (121℃ steam sterilization), both the casting method and the blow molding method must use retort-resistant raw materials. Non-cooking grade materials will cause the film to become hard, brittle and whiten after cooking. It not only affects the appearance of the film, but also results in a serious drop in the impact resistance and pressure resistance of the bag, which cannot meet the testing standards and use requirements.
7. In the process of plastic molding, it is easy to accumulate residues at the die due to thermal decomposition or low-molecular-weight products. The casting method can be easily removed, while the residue in the blow molding method can only be removed by shutting down. If it is not removed in time, it is easy to be entrained in the film, causing the content of impurities in the content to exceed the standard or causing obvious scratches on the surface of the film. Due to the low output of the blow molding method and the large waste of driving, the unit production cost of such as frequent driving and parking has risen sharply.
3. Comparison of investment costs
1. Due to the low one-time investment of the blow molding method, the plant occupies a small area, the equipment is cheap, and the investment is small and effective (the equipment processing cycle is short, the installation and debugging period is short, the auxiliary equipment and facilities are small, and the requirements are low). The one-time investment in equipment, workshops and facilities of the casting method is very large, which is 5 to 10 times that of the blow molding method. Therefore, the application of blow molding method is more than that of casting method.
2. Because the blow molding method is restricted by the process conditions, the product quality is poor, the output is small, and the unit production cost is high. Although it has been continuously improved and perfected, it is still difficult to replace the casting method. This is the most fundamental reason why the output of cast film in food packaging substrate film is much higher than that of blown film.
Article link: China Plastics Online http://info.21cp.com/industry/News/201612/1226270.htm