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This post was last edited by *aojiaoya0546 on 2016-11-27 at 22:08. Stretch film is generally produced using two processes: blow molding and cast molding. Cast polyethylene stretch film has a uniform thickness, but it is rarely used at present due to its high cost. Blown polyethylene stretch film is made by blowing blown-grade PE particles using a blow molding machine; it has a low cost, which is why it is the most widely used. So, how many types of raw materials are there for stretch films? 1. Polyester: It boasts excellent transparency, heat resistance, and wear resistance. The film is available in 12 microns and 25 microns thickness; printing is done on polyester, and it can be used as a covering material for retort bags or disc-shaped containers. 2. Nylon: It is essential as the base material for packaging materials used in soft canned foods. There are two types: biaxially stretched and unstretched. Biaxially stretched nylon 6 film possesses excellent heat and cold resistance, as well as good tear strength and puncture resistance; therefore, it can be used in combination with other materials. Additionally, to prevent curling and improve strength, nylon 6-6 films are also used, but their price is higher than that of nylon 6. 3. Polyvinylidene chloride: It can be used in soft-packed foods as a single-layer film, and serves as packaging material for ham and sausages. It boasts excellent sealing properties, bundling capabilities, heat-shrinkability, and gas barrier properties. Polyvinylidene chloride can also be used as a coating for films such as nylon and polyester. In addition, there are composite substrates used as steaming bags, as well as high-barrier polyvinyl chloride films that are excellent for sterilization under high temperature and pressure. 4. High-density polyethylene: A special high-density polyethylene used for soft cans is already available as a commercial product in Japan. Although this film has poor gas barrier properties, it can be used as a sealing layer due to its excellent tensile strength, elongation, tear strength, and good sealing adaptability. However, high-density polyethylene has certain characteristics in terms of heat resistance, with a maximum heat resistance temperature of 120°C. 5. Polypropylene: It has good transparency and oil resistance, and its heat resistance is better than that of special polyethylenes; therefore, it can be used as a sealing layer for retorting bags and retorting containers. The sealing layer of soft canned food packaging is mostly made of unstretched polypropylene film or sheets. 6. Aluminum foil: Aluminum foil is made by rolling electrolytic aluminum with a purity of over 99%. When used as a packaging material, it is the only metal-based material among packaging materials. The aluminum foil used for cooking bags has a thickness of 7–15 microns, while the aluminum foil used for cooking containers has a thickness of 50–130 microns. Since aluminum foil is non-toxic and tasteless, possesses excellent light-blocking properties, as well as high levels of moisture resistance, gas barrier properties, and confidentiality, it can provide the most effective protection for the items being packaged; therefore, it is the most suitable material for packaging fatty foods. However, since there is a risk of pinholes forming when the aluminum foil is bent, it is necessary to use a plastic film as an outer layer and combine it with it.