Blowing (or blown film) and casting are the two main production processes for producing and processing stretch films. The blow molding method was earlier used in the production and processing of stretch films. As a result, it is a widely used film processing method. However, due to the limitations of the production process and the insurmountable defects of special materials, in the 1980s, a new film production, and processing method – the casting method has gradually developed, especially the use of multi-layer co-extrusion technology, so that the stretch film production and processing technology of the casting method can be widely promoted and applied, and gradually applied to various film packaging. As a result, the new material is blown out with new particles, the color is uniform and clean, and the bag is stretched well.
In the early days, LLDPE stretch films were mostly blown films, from single-layer to two-layer and three-layer; LLDPE stretch films were mainly produced by casting method because the casting line production has the advantages of uniform thickness and high transparency, which can be It is suitable for the requirement of high ratio pre-stretching. Since single-layer casting cannot achieve single-sided adhesion, the application field is limited. The material selection of single-layer and double-layer casting is not as wide as that of three-layer casting. The formula cost is also high, so the structure of three-layer co-extrusion is still ideal. High-quality stretch film should have the characteristics of high transparency, high longitudinal elongation, high yield point, high transverse tear strength, and good puncture performance.
Blown Stretch Film
The process of blow molding is to melt and plasticize the resin into a thin-walled tube with an extruder, and then under the action of the traction device, the polymer has a better flow state at this time, and it is blown into the required state by compressed air. Thickness, after cooling and shaping, becomes a thin film. The raw materials used for the blown film are PE, PVC, PDVC, PP, PS, PA, EVA, PVA, etc. In addition to single-layer films, multi-layer composite films have also been developed. Most PEs are used for blown film processing. General low-density PE (LDPE) or linear low-density PE (LLDPE) are available. HDPE film grades are generally used where superior stretchability and excellent barrier properties are required. For example, HDPE films are commonly used in merchandise bags, grocery bags, and food packaging. PE film processing generally uses ordinary blown film processing or flat extrusion processing.
To meet the needs of some special functions of plastic film packaging materials in the production process, various characteristic materials (such as breathable, waterproof, thermal insulation, toughness, etc.) are blown and co-extruded together to form a multi-functional plastic film—layer co-extrusion blown film machine. The multi-layer co-extrusion blown film machine aims to play the advantages of multiple materials and avoid some shortcomings of a single material or a single-layer film blown machine.
For example, the co-extrusion blown film machine of PP and PE takes advantage of the combination of the high hardness of PP material and the high brightness of PE material to produce a film with a combination of the two characteristics.
Multi-layer co-extrusion blown film machine is divided into two types: two-layer co-extrusion blown film machine, three-layer co-extrusion blown film machine, five-layer co-extrusion blown film machine, seven-layer co-extrusion blown film machine, etc.
Cast Stretch Film
Casting is a non-stretched, non-oriented cast film produced by melt casting quenching. There are two ways single-layer drooling and multi-layer co-extrusion drooling. Because it is the subsequent process of flat extrusion films, such as printing and lamination, it is very convenient. Therefore, it is widely used in packaging food, medical supplies, textiles, flowers, and daily necessities. Unfortunately, my country’s production of salivary films started late (in the 1980s), and the foundation is relatively weak. Still, the development speed is relatively fast after nearly 20 years of continuous development and growth.
Casting stretch film has excellent heat-sealing performance and transparency and is one of the main packaging composite substrates. It is used to produce high-temperature cooking film, vacuum aluminized film, etc., and the market is extremely optimistic. Moreover, with the development of cast film production equipment in China, some technical indicators have reached the advanced international level. Therefore, the threshold for entering cast film production is getting lower and lower. Because of this, some companies compete to import cast film production lines to achieve one step.
At present, the imported units are represented by the former German Badenfeld, Germany W&H, Italy Corinth, Austria Lenzing, and the former Japan Mitsubishi Heavy Industries. Foshan Shicheng represents my country’s domestically produced units. Similarly, the production method of domestic casting units is the same as blow molding, and the main core is not from foreign suppliers. This production process is mainly used in the domestic CPP industry and has several applications in producing seven-layer co-extrusion with high barrier properties.
These two production processes are the most common in my country. Because the blowing process cannot compare with the cast film in terms of product flatness and transparency, its high production efficiency, low equipment investment, and low material loss are the main advantages of the casting process.
Foreign companies have conducted in-depth research and exploration on improving the deficiencies of the blow molding process, which are mainly reflected in these ways.
- The design improvement of the die head adopts a unique parallel multi-channel design concept.
- Improve the position and angle of the extruder control.
- The improvement of the cooling system mainly focuses on air circulation and internal cooling.
- The improvement of material formula and structural design.
- It has been proved by practice that the transparency and flatness of the film and the thickness uniformity of each layer of materials have reached or approached the level of leaving a message.
Therefore, if domestic enterprises invest in high-barrier film equipment, they will focus on blown film after 2010.
Mechanical properties of stretch films
In film inflation, the blow molding method causes the film to be stretched to varying degrees in the longitudinal and transverse directions, so it has a certain tear resistance in the longitudinal and transverse directions. In processing, there is a certain amount of stretching in the
longitudinal direction, but there is no stretching in the transverse direction. Therefore, the blown stretch film is more advantageous than the casting method for packaging films with certain mechanical properties in the longitudinal and transverse directions.
Stretch film thickness control
The casting method adopts a flat die head, and the die head is provided with a special retention groove, which can ensure the uniformity of the 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. Thick films can be controlled below ±3%.
The annular die head and multi-layer superimposed die core used in the blow molding method has very high requirements for processing accuracy and assembly accuracy, and it is difficult to control the thickness. Therefore, the thickness of the common blown film is controlled at about ±5%. Although the latest technology shows that the blow molding method can currently control the film thickness below ±3%, from the perspective of the overall global technical level, the casting method has more advantages in film thickness control than the blow molding method.
Production process
In the processing equipment of the casting method, the distance from molding to setting is generally 10-20 mm, and the melt film curtain is very short. Therefore, it can be quickly shaped with the help of various devices, and the product quality is good and stable. In the blow molding method, because the molten film has a blowing process and relies on air or water to cool and shape, the product takes a long time to shape, and it is easy to be affected by external and own factors in the melting film stage. In plastic molding, it is easy to accumulate residues at the die, and it is easy to be entrained in the film if it is not removed in time, resulting in excessive impurities in the content or obvious scratches on the surface of the film.
The casting method can clean the precipitates without stopping the machine, which is relatively easy to clean, but there are many wastes; the residues in the film bubble of the blow molding method are generally cleaned by shutting down the die, which is relatively complicated.
Stretch film temperature resistance
Although the temperature resistance of the film is mainly derived from the characteristics of the raw material resin, the blow molding method and the casting method also have a certain influence on the temperature resistance of the film. The blow molding method causes the vertical and horizontal molecules of the product to be stretched to different degrees due to an inflation process. As a result, the product’s temperature resistance and heat-sealing performance will be affected to a certain extent. In addition, when the product needs to be retorted and sterilized (such as steam at 121°C), sometimes there will be shrinkage after molding, resulting in deformation of the film, and will reduce the strength of the heat-sealed joint, leading to leaking bags.
The film is produced by the casting method, the molecules are arranged in an orderly manner, and the cooling speed of the product is fast, which is beneficial to improve the transparency and gloss of the product. In addition, the product has a soft texture, and its heat resistance and low-temperature adaptability are better than the blow molding method.
Productivity
The casting method has high production efficiency and good product quality stability. As a result, there is less waste in the production process, and it is easier to realize the online recycling of edge and waste. As a result, the utilization rate of materials is high.
The blow molding method has a low single-machine output and has obvious advantages for scheduling and cost control of small batch orders and customized orders. In contrast, the casting method has a high single-machine output and is suitable for scheduling large-volume orders.
Material selection
In selecting raw materials, the blow molding and casting methods are also very different. Therefore, it is necessary to choose materials with similar fluidity as possible to match each other. The casting method selects a material with a higher melting index (generally 3~4). However, many low-melting index materials are on the market, and material replacement is easier. Therefore, the blow molding method is superior to the casting method in selecting material technology.
Although there have also been successful cases in the market with blow-molded formulations processed on casting equipment, the overall output will be reduced accordingly.
The down-blowing method and water-cooling setting are required for materials with low melt strength. The casting method uses steel casting rolls (cooling water inside) to cool and shape the film, and the water does not directly contact the film.
For films that need to be retorted (121°C steam sterilization), retort-resistant raw materials must be selected for either casting or blow molding. Cooking non-cooking grade materials can cause the film to harden, become brittle and whiten. It not only affects the film’s appearance but also leads to a serious decrease in the bag’s impact resistance and pressure resistance, which cannot meet the testing standards and usage requirements.
Investment cost
The blow molding method has less one-time investment, a small plant area, cheap equipment, small investment, and quick results (short equipment processing cycle, short installation and commissioning period, less auxiliary equipment and facilities, and low requirements).
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 current application of blow molding is more than that of casting.
You can choose the appropriate manufacturing process for the stretch film according to your needs. If you have other questions can contact us, we provide a variety of packaging solutions free of charge.
