Plastic films have become a ubiquitous part of our daily lives. They are used in packaging, agriculture, construction, and even healthcare industries. Two of the most common methods for producing plastic film are cast extrusion and blown film processes using polyethylene (PE) as the base material. In this article, we will delve into each process to understand their differences and how they affect the properties of the final product.
Characteristic of Plastic Film
There are many methods of plastic film molding, of which three processes are more common: calendering, extrusion and blow molding:
Casting method: A non-stretching, non-oriented, flat extruded film is produced through a melt salivation quenching process. The film produced by this process does not have an obvious orientation structure.
Extrusion: Commonly available are one-way stretch and two-way stretch methods. The basic principle of bi-directional stretching: polymer raw materials are first heated and extruded into thick sheets by the extruder, and then stretched by the stretching machine in the appropriate temperature range above the glass transition temperature and below the melting point by a certain number of times along the longitudinal and transverse directions, so that the molecular chains are oriented and arranged in an orderly manner parallel to the plane of the film, and then heat-set under tension, so that the oriented macromolecular structure is fixed. Finally, the film is formed by cooling and subsequent processing.
Blow molding method: Usually extrusion blow molding method, the material is plasticized and extruded to form the billet blow molding, then cooled, traction, and rolled. The blown film is oriented. The blown film forming process is simple and is the most commonly used film forming process.
Multiple Film Characteristics
1.HDPE
HDPE membrane is known as geomembrane or impermeable membrane. Its melting point is about 110℃-130℃ and relative density is 0.918-0.965kg/cm3. It is a kind of high crystallinity, non-polar thermoplastic resin, and the appearance of HDPE in its original state is milky white, with a certain degree of translucency in the thin section. It has good high and low temperature resistance and impact resistance, even at a low temperature of -40F. Its chemical stability, rigidity, toughness, mechanical strength, tear strength properties are excellent, and with the rise in density, mechanical properties, barrier properties, tensile strength and heat resistance will increase accordingly, can be resistant to acids, alkalis, organic solvents and other corrosion.
Identification method: mostly opaque, feel like wax, plastic bags rubbed or rubbed with a rustling sound.
2.LDPE
LDPE film density is low, soft, low-temperature resistance, impact resistance chemical stability is good, in general, acid resistance (in addition to strong oxidizing acid), alkali, salt corrosion, has good electrical insulation. lDPE is mostly used in plastic bags, marked with a material symbol for – 4, its products are mostly used in civil engineering and agricultural fields The products are mostly used in civil engineering and agricultural fields, such as geomembranes, agricultural films (shed films, ground films, bitter cover films, green storage films, etc.). Its recycled materials, corners, etc. can be made into garbage cans, tool boxes, bicycle pads, garbage carts and other flexible plastic products.
Identification method: LDPE plastic bags made of soft, rubbing more will not make a rustling sound, the outer plastic film soft and easy to tear is LDPE, more brittle and hard is PVC or PP film.
3.PP
Common PP film using blow molding, simple process, low cost, but optical properties slightly lower than CPP and BOPP. PP is the most characteristic of high temperature resistance (about -20 ℃ ~ 120 ℃), and melting point of 167 ℃, suitable for filling soybean milk, rice pulp and other products that require steam sterilization. Its hardness is higher than that of PE, and it is mostly used for manufacturing containers and caps, and is marked with the material symbol No. Identification method: generally speaking, PP hardness is higher, and the surface is more shiny, does not produce irritating odor when burning, while PE has a heavy candle smell.
4.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 polyethylene plastic as the substrate. Since polyvinyl alcohol high barrier composite film has good barrier properties and meets environmental requirements, the market for this packaging material is very bright and has a broad market space in the food industry.
5.CPP
Cast polypropylene film (CPP) is a non-stretch, non-directional, flat extruded film produced by melt flow quenching. It is characterized by fast production speed, high yield, good film transparency, gloss, barrier, flexibility, thickness uniformity, high temperature cooking (cooking temperature above 120℃) and low temperature heat sealing (heat sealing temperature less than 125℃), and excellent balance of properties. Follow-up work such as printing, lamination is convenient, widely used in textiles, flowers, food, daily necessities packaging, do composite packaging of the inner layer of the substrate, can extend the shelf life of food, increase the beauty.
6.BOPP
Biaxially oriented polypropylene film (BOPP) is a transparent flexible packaging material developed in the 1960s, which is made by mixing polypropylene raw materials and functional additives in a special production line, melting and mixing, making sheets, and then stretching them into film. This film not only has the original advantages of low density, corrosion resistance and heat resistance of PP resin, but also has good optical properties, high mechanical strength, abundant raw material sources, excellent printing performance, and can be laminated with paper, PET and other substrates with high clarity and gloss, excellent ink absorption and coating adhesion, high tensile strength, excellent grease barrier properties, and low electrostatic properties. It is widely used in the field of printing lamination, and also as packaging material in tobacco and other areas.
7.PET
Polyester film (PET) is a thermoplastic engineering plastic. It is made into a thick sheet by extrusion and then made into a film material by biaxial stretching. 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 good fragrance retention, is one of the commonly used permeability barrier composite film substrates, but poor corona resistance, higher prices. Film thickness is generally 0.12mm, commonly used as the outer layer of food packaging materials, printing sex is better. In the plastic products marked material symbol for No. 1. Its recycled material can be made again: monofilament → wig, zipper serrated; sheet → packaging boxes (egg carton, stationery L clip); polyester cotton → filling cotton, non-woven, polyester fabric.
8.PA
Nylon plastic film (polyamide PA) is currently industrially produced in many varieties, of 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 a good gloss. Tensile strength, tensile strength, high and low temperature resistance, oil resistance, organic solvent resistance, abrasion resistance and puncture resistance are very good, and the film is relatively soft, excellent oxygen barrier, 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 goods, such as greasy food, meat products, fried food, vacuum-packed food, steamed food, etc.
9.PVC
PVC film has similar advantages to PET, with the same transparent, impermeable, acid and alkaline resistant characteristics. However, PVC is not suitable for filling food-grade substances because the manufacturing process may release carcinogens due to incomplete polymerization of some monomers, and has now been replaced by PET bottles, which are marked with the material symbol No. 3. Recycled materials can be made into: water pipe elbow, artificial leather, wire cover, fireproof building materials, etc.
10.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 distinguished into foamed and non-foamed according to whether it has undergone the foaming process. Unfoamed PS is mainly used in building materials, toys, stationery, etc. It can also be commonly made into containers filled with fermented dairy products (such as Yakult, Kenmore, Dodo and other dairy products or lactic acid products), etc. In recent years, it is also used in large quantities to make non-washable tableware, marked with the material symbol 6. Recycled material can be made into: i.e. camera shells, hangers and other hard and fragile, non-elastic plastic re-products.
11.Aluminized
Aluminized film has the characteristics of both plastic film and metal. The role of film surface aluminizing is to shade and prevent ultraviolet radiation, which not only extends the shelf life of the contents, but also improves the brightness of the film, replacing aluminum foil to a certain extent, and also has inexpensive, beautiful and better barrier properties. Therefore, aluminized film is widely used in composite packaging, mainly in cookies and other dry, puffed food packaging, as well as some pharmaceutical and cosmetic outer packaging.
Plastic Films Preservation Conditions
1. Foils should be stored in a well-ventilated, cool and dry warehouse (indoor temperature below 30 degrees centigrade, relative humidity below 80 degrees centigrade is appropriate) on a shelf 200mm above the ground.
2. Do not come into direct contact with strong acids, strong alkalis and other chemicals.
3. Do not approach the source of fire, contact with high temperature.
Types of PE Films
Ⅰ. PE Cast Extrusion Film
Cast extrusion is one of the oldest methods used for making plastic films. It involves melting PE resin pellets in a hopper that feeds it to a screw compressor where it is melted thoroughly before being pushed through a narrow die that shapes it into flat sheets or films.
The molten polymer emerges from the die onto two cooling rolls which control both its thickness and width. The distance between these rolls determines how fast or slow the film cools down; faster speed leads to thinner but stronger sheets while slower speeds create thicker but weaker ones.
Because cast extruded films cool more quickly than blown ones, they tend to be stiffer with less elasticity and limited ability to stretch without breaking. As such, cast films tend to work better when rigidity is required like in bags or pouches for dry products such as pet food or cement additives.
Another advantage of cast extruded films over blown type is uniformity; since they come out evenly on both sides due to being pulled out by rollers rather than inflated like balloons during blowing process gives them superior dimensional stability which makes them ideal for printing labels/graphics on high-end products like shampoo bottles.
Ⅱ. PE Blown Film
Blown film technology was invented in 1930s; however modern versions were developed post-World War II era when polyethylene became available commercially at affordable prices opening doors for large-scale production techniques capable meeting consumer demand efficiently while reducing cost per unit produced significantly compared to traditional foil materials previously used predominantly until then.
Blown film differs from cast extrusion in that the polymer is melted and then fed into a circular die. Compressed air is introduced as it exits to inflate the molten tube which becomes thinner and longer as it rises up from the floor; creating a bubble of plastic film.
At the top, two collapsing frames compress both sides of the inflated plastic film against each other to form thin, flat sheets with uniform thickness throughout its width.
Blown films tend to have more elasticity compared to cast films due stretching process they undergo during production; making them ideal for applications where flexibility is required such as bags or wraps for foods like cheeses or meats being vacuum packed since their ability to stretch without ripping/breaking ensure food remains fresh while also preventing freezer burn by sealing out moisture completely.
Another advantage of blown films over cast ones is their optical clarity thanks to lack of surface defects resulting from not having contact with rollers like in cast process. This makes them excellent choices for packaging items that need high levels of transparency such as cosmetics, toys, or electronics.
Conclusion
Both PE cast extrusion and blown film processes are widely used today because they offer unique advantages depending on what kind of product you’re looking for. Cast extruded plastics tend to be stiffer than blown types but offer superior dimensional stability while blown films possess greater elasticity combined with optimal clarity and strength. As technology continues evolving new innovations will arise allowing us create even better products using these techniques ultimately benefiting consumers worldwide through reduced costs higher quality goods available at lower prices than ever before!
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