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Comparison of "blow molding" and "casting" processes

Comparison of "blow molding" and "casting" processes

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  • Time of issue:2020-02-14 12:40
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(Summary description)In the field of film production, "blow molding" and "casting" are two commonly used processes. The following is a brief introduction and comparison of the most commonly used production processes in these two types of film production

Comparison of "blow molding" and "casting" processes

(Summary description)In the field of film production, "blow molding" and "casting" are two commonly used processes. The following is a brief introduction and comparison of the most commonly used production processes in these two types of film production

  • Categories:Industry news
  • Author:
  • Origin:
  • Time of issue:2020-02-14 12:40
  • Views:

In the field of film production, "blow molding" and "casting" are two commonly used processes. The following is a brief introduction and comparison of the most commonly used production processes in these two types of film production.

Multi-layer coextrusion blow molding

This production process is mainly divided into two forms: upper air cooling and lower air cooling.

Cold on top

It is mainly composed of multiple extruders, multi-layer stack die, multiple tuyere air ring, IBC internal cooling system, herringbone plywood, rotary traction system, lower traction system and winding system. At present, the imported units are mainly from Badenfeld, Germany, W & H, Germany, and Penton, Canada. Among the domestic units, Guangdong Jinming is the representative. Generally speaking, the core mechanical capacity is 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.

Blow water cooling

The basic components are the same as the top-blowing type. The difference is the cooling method. The bottom-blowing uses circulating cooling water as the main form of cooling. The design position of each component is different from the top-blowing. It is more common in PP film production. In the production of multi-layer co-extruded high-barrier films, the above blow-cooled units are still the mainstay. Regardless of which two types of units are used, the appearance of the unit is mainly characterized by a cylindrical film. In terms of vacuum bag making, the reduction of two heat sealing edges is better than the protection of the packaging contents. Cast film has very great advantages. Better in tensile strength than cast film.

Multi-layer co-extrusion casting method

This production process is mainly composed of multiple extruders, multiple runner distributors (commonly known as "collectors"), T-die, casting system, horizontal traction system, oscillator, and winding system.

The main characteristics of this production process are that the produced film products have good surface gloss, high flatness, small thickness tolerances, good mechanical elongation properties, good flexibility, and higher transparency than the blow molding method. At present, the imported units are represented by the former Badenfeld, Germany, W & H of Germany, Corinth of Italy, Lenzing of Austria, and former Mitsubishi Heavy Industries of Japan. The domestic units are represented by Foshan Shicheng. Similarly, the production method of domestic cast units is the same as that of blow molding. The main core components come from foreign suppliers. At present, this production process is mainly used in the domestic CPP industry, and there are also a small number of applications in the seven-layer co-extrusion high-barrier production.

Among the imported production lines are the seven- and eleven-story units of Xiangfu, Zhongshan, Guangdong, and the nine-story units of Changzhou Haiqi Plastic Industry.

These two production processes are the most common in China, 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, in-depth research and development have been carried out on the improvement of the inadequacy of the blow molding process by foreign enterprises, which are mainly reflected in (1) the improvement of the die design and the use of a unique parallel multi-flow channel design concept; The position and angle of the extruder; (3) the improvement of the Ling Que system (mainly the wind ring and internal cooling technology); (4) the four aspects of material formula and structure design. It has been proved through practice that the transparency, flatness, and thickness uniformity of each layer of material have reached or approached the level of the casting method. If domestic enterprises invest in high-barrier film equipment, after 2010, they will mainly use blown film.

Comparison of blow molding and casting

Blow molding and casting are the two main production process methods for polyolefin film production and processing. Blow molding was earlier used in the production and processing of films. Due to the limitations of its production process and the insurmountable defects of products, in the 1980s, a more advanced film production and processing process, the casting method, was gradually developed. In particular, the multi-layer co-extrusion technology is used to widely promote and apply the film production and processing technology of casting method, and gradually apply it to the high-end packaging of various articles 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.

I. Comparison of production processes

1. Casting method is more suitable for multi-layer co-extrusion film production and processing than blow molding method. Especially when a variety of materials are used for co-extrusion, the production process parameters of casting method are easier to control. The key part-the design of the die head can effectively ensure the uniformity of the distribution of various materials in the product. The technology of multi-layer coextrusion die of blow molding is more difficult, the structure is more complicated and the technology is not mature enough.

2. In the processing equipment of casting method, the distance from the die to the cold roll (forming to setting) is generally 10-20mm. The molten film curtain is very short and with the help of vacuum suction hood, air knife and fixing device Quick setting, good product quality and stability. The blow molding method has an inflation process due to the melting film, and relies on air or water to cool and set. The setting time of the product is too long and the change before setting is large (usually 4 to 7 times inflation). It is easy to be affected by external factors and its own factors during the melting film stage. The quality control of the product is very difficult and the stability is poor.

3. There is also a big difference in the choice of raw materials between the blow molding method and the casting method. In the multilayer co-extrusion, the blow molding method requires that materials with close fluidity be used as much as possible to match each other. When the material fluidity is greatly different, there is no corresponding response. The measures are adjusted, so when one of the materials is selected, there is not much choice for the other materials co-extruded with it, which restricts the manifestation of the advantages of multi-layer co-extrusion processing technology in product quality to a great extent, and The processing equipment of the casting method can easily solve the almost intractable problems in the blow molding method by adjusting the flow path.

Comparison of product characteristics

1. Due to an inflation process, the longitudinal and transverse molecules of the product are stretched to different degrees. The product's temperature resistance and heat-sealing performance will be greatly affected. When the product needs to be sterilized (such as 121 ° C steam), the stretched molecular chain in the molding will obviously shrink after heating, resulting in serious deformation of the product. , And will greatly reduce the strength of the heat-sealed connection and cause bag leakage.

2. When the film is produced by casting (also known as unstretched film), the molecules are arranged in an orderly manner and the product is cooled quickly, which is beneficial to improving 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. Casting method has extremely high production efficiency and good product quality stability. There is less waste material in the production process, and it is easier to realize online recycling of edge and waste material, and the utilization rate of materials is high.

4. The casting method uses a flat die head, and a special retention groove is provided in the die head to ensure uniform and consistent material flow. 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. , Thicker films can be controlled below ± 3%. The ring die and multi-layer superimposed core used in the blow molding method have very high requirements for processing accuracy and assembly accuracy and are particularly difficult to control. If there are slight errors in processing and fitting accuracy, it cannot be compensated by adjustment. The thickness tolerance is basically about ± 10%. For a 200 μm thick film, it means that the thinnest part is 180 μm and the thickest part may be 220 μm. When welding and making bags, the mold gap and heat sealing temperature are adjusted no matter how It is difficult to ensure that the thickest and thinnest parts can achieve the necessary sealing strength. Leakage of bags is inevitable, and the yield is naturally low.

5. For materials with low melt strength, down-blowing and water-cooling are required. Incomplete drying of the moisture or improper selection of the corona layer will result in poor corona treatment or severe corona attenuation. It affects the fastness of printing ink and causes discoloration. The casting method uses a steel casting roll (internal cooling water) to cool and shape the film, and the moisture does not directly contact the film.

6. For films that need to be cooked (steam sterilization at 121 ° C), no matter whether the casting method or the blow molding method is used, the raw materials that are resistant to cooking can be used. Non-cooking grade materials will cause the film to harden, become brittle and whiten after cooking. It will not only affect the appearance of the film, but also cause the bag's impact resistance and pressure to be severely reduced, failing to meet the testing standards and use requirements.

7. In the process of plastic molding, due to thermal decomposition or low-molecular exudates, residues are easily accumulated at the die. The casting method can be easily removed and the residue in the blown film can only be removed by stopping. It is easy to be entrained in the film if it is not cleared in time, causing the content of impurities in the content to exceed the standard or causing obvious scratches on the film surface. Due to the low output of the blow molding method and the large waste of driving, such as frequent driving, the unit production cost of parking has increased significantly.

Comparison of investment costs

1. Due to less one-time investment in blow molding, small plant area, cheap equipment, small investment and quick results (short equipment processing cycle, short installation and commissioning period, few auxiliary equipment and facilities, low requirements). The one-time investment in equipment, plant and facilities of the casting method is very large, which is 5 to 10 times that of the blow molding method. Therefore, the current application of blow molding method is more than cast method.

2. As the blow molding method is limited by process conditions, the product quality is poor, the output is small, and the unit production cost is high. Although it is continuously improved and perfected, it is still difficult to replace the casting method. This is the most fundamental reason why the current 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

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