There are many methods of plastic film molding, among which casting, extrusion, and blow molding are more common:
 
Casting method: A non-stretched, non-directional flat extruded film is produced by melt saliva quenching process. The films produced by this process do not have a distinct orientation structure.
 
Extrusion method: The common ones are unidirectional stretching and bidirectional stretching. The basic principle of biaxial stretching: the polymer raw material is first heated and molten and extruded into thick sheets by the extruder, and then stretched by a certain multiple along the longitudinal and transverse directions through the stretching machine within the appropriate temperature range above the glass transition temperature and below the melting point, so that the molecular chains are arranged in an oriented and orderly manner on the plane parallel to the film, and then heat set in the tension state to fix the oriented macromolecular structure, and finally form a film after cooling and subsequent treatment.
 
Blow molding: It is usually manufactured by extrusion blow molding, and the material is plasticized and extruded to form a tube blank blown into shape, and then cooled, pulled and coiled. Blown film is oriented. The blow molding process is simple and is the most commonly used film forming process.
 
Eleven film properties
 
HDPE
 
HDPE membranes are known as geomembranes or impermeable membranes. Its melting point is about 110 °C-130 °C, and the relative density is 0.918-0.965kg/cm3. It is a high-crystallinity, non-polar thermoplastic resin, and the appearance of the original HDPE is milky white, and it is translucent to a certain extent in a thin cross-section. It has good resistance to high and low temperatures and impacts, even at low temperatures of -40F. Its chemical stability, rigidity, toughness, mechanical strength, tear resistance properties are excellent, and with the increase of density, mechanical properties, barrier properties, tensile strength and heat resistance will be improved accordingly, and it can resist acid, alkali, organic solvents and other corrosion. Identification method: most of them are opaque, feel like wax, and there is a rustling sound when the plastic bag is rubbed or rubbed.
 
LDPE
 
LDPE film has low density, softness, low temperature resistance, impact resistance, good chemical stability, acid resistance (except for strong oxidizing acids), alkali and salt corrosion, and good electrical insulation. LDPE is mostly used in plastic bags, and the material symbol is - No. 4, and its products are mostly used in civil engineering and agricultural fields, such as geomembranes and agricultural films (shed films, plastic films, green storage films, etc.). Its recycled materials and leftovers can be recycled into elastic plastic recycled products such as garbage cans, tool boxes, bicycle mats, garbage trucks, etc. Identification method: The plastic bag made of LDPE is softer, and it will not make a rustling sound when rubbing, and the soft and easy-to-tear plastic film is LDPE, and the more brittle and hard is PVC or PP film.
 
PP
 
Ordinary PP film is blown molding, which has a simple process and low cost, but its optical performance is slightly lower than that of CPP and BOPP. The biggest feature of PP is that it is resistant to high temperature (about -20 °C ~ 120 °C) and has a melting point of up to 167 °C, which is suitable for filling soybean milk, rice milk and other products that need to be sterilized by steam. Its hardness is higher than that of PE, and it is mostly used to manufacture container caps, and the material symbol is No. 5. Identification: Generally speaking, PP has a higher hardness and a shiny surface, and does not produce a pungent odor when burning, while PE has a heavier candle smell.
 
CPP
 
Cast polypropylene film (CPP) is a non-stretched, non-oriented flat-extruded film produced by melt cast quenching. It is characterized by fast production speed, high output, good film transparency, gloss, barrier, softness, thickness uniformity, high temperature cooking (cooking temperature above 120 °C) and low temperature heat sealing (heat sealing temperature less than 125 °C), excellent balance of various properties. Follow-up work such as printing, composite convenient, widely used in textiles, food, daily necessities packaging, as the inner substrate of composite packaging, can extend the shelf life of food, increase aesthetics.
 
BOPP
 
Biaxially oriented polypropylene film (BOPP) is a transparent flexible packaging material developed in the 60s of the 20th century, which is a special production line to mix polypropylene raw materials and functional additives, melt and mix, make sheets, and then stretch into films. This kind of film not only has the advantages of low density, corrosion resistance and good heat resistance of PP resin, but also has good optical properties, high mechanical strength, rich raw material sources, excellent printing performance, can be combined with paper, PET and other substrates, and has high clarity and gloss, excellent ink absorption and coating adhesion, high tensile strength, excellent grease barrier properties, low static characteristics, etc. It is widely used in the field of printing and composite, and is also used as a packaging material in tobacco and other fields.
 
PVC
 
The advantages of PVC film and PET are similar, and they are transparent, impermeable, and acid-alkali resistant. However, PVC is not suitable for filling food-grade substances due to the incomplete polymerization of some monomers due to the incomplete polymerization of some monomers in the manufacturing process, so PET bottles have been mostly used with the material symbol No. 3. Recycled materials can be remade: water pipe elbows, artificial leather, wire coverings, fireproof building materials, etc.
 
PS
 
PS film has low water absorption, but its dimensional stability is good, and it can be processed by injection molding, compression molding, extrusion, and thermoforming. Generally, it is divided into two categories: foamed and unfoamed according to whether it has undergone foaming process. Unfoamed PS is mainly used in building materials, toys, stationery, etc., and can also be commonly made into containers filled with fermented milk products (such as Yakult, bodybuilding, Toto and other dairy products or lactic acid products), etc., and in recent years, it has also been widely used to make disposable tableware, and the material symbol is No. 6. Recycled materials can be recycled: that is, camera shells, hangers and other hard, fragile, inelastic plastic recycles.
 
PET
 
Polyester film (PET) is a thermoplastic engineering plastic. It is a film material made into thick sheets by extrusion and then biaxially stretched. Polyester film is characterized by excellent mechanical properties, high rigidity, hardness and toughness, puncture resistance, friction resistance, high and low temperature resistance, chemical resistance, oil resistance, air tightness and fragrance retention. The thickness of the film is generally 0.12mm, and it is often used as the outer material of food packaging, which has good printability. In plastic products, the material symbol is marked as No. 1. Its recycled material can be remade: monofilament→ wigs, zipper serrations; Flakes → packaging boxes (egg cartons, stationery L clips); Polyester cotton→ padded cotton, non-woven, polyester fabric.
 
PA
 
At present, there are many varieties of nylon plastic film (polyamide PA) industrial production, among which the main varieties used to produce film are nylon 6, nylon 12, nylon 66, etc. Nylon film is a very tough film with good transparency and good luster. Tensile strength, tensile strength, high and low temperature resistance, oil resistance, organic solvent resistance, abrasion resistance and puncture resistance are all very good, and the film is relatively soft, excellent oxygen resistance, but the barrier to water vapor is poor, moisture absorption, moisture permeability is large, and heat sealing is poor. It is suitable for packaging hard items, such as greasy food, fried food, vacuum-packed food, retorted food, etc.
 
PVA
 
Polyvinyl alcohol (PVA) high barrier composite film is a film with extremely high barrier properties formed by coating a modified polyvinyl alcohol water-soluble liquid on the substrate with polyethylene plastic as the substrate. Because the polyvinyl alcohol high barrier composite film has good barrier performance and meets the requirements of environmental protection, the market prospect of this kind of packaging material is very bright, and there is a broad market space in the food industry.
 
 Aluminized
 
Aluminized film has the properties of both plastic film and metal. The function of aluminized plating on the surface of the film is to shade and prevent ultraviolet radiation, which not only prolongs the shelf life of the contents, but also improves the brightness of the film, replaces the aluminum foil to a certain extent, and also has low price, beauty and good barrier performance. Therefore, the aluminized film is widely used in composite packaging, mainly used in dry and puffed food packaging such as biscuits, and the outer packaging of medicine and cosmetics.
 
PBAT
 
PBAT material can be decomposed into carbon dioxide, water and organic fertilizer by microorganisms within 180 days under composting conditions, and the degradation process will not produce toxic gases, which is the main raw material of biodegradable plastic film, which can not only be biodegradable but also compostable, and can effectively fight against white pollution. Biodegradable polyester raw materials are produced by one-step polymerization, and the products fully comply with the US and European food contact safety standards, as well as the EU EN13432 and US ASTM6400D biodegradable plastic standards, and the shelf life is 12 months under sealed packaging conditions, and can also be guaranteed to be stored for 12 months when reprocessed into products.
 
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