Omar Khalid
Plastic wrap is a form of food packaging consisting of a thin film of flexible, transparent polymer. The process of making plastic wrap involves heating plastic granules until they melt and then forcing the liquid plastic through a die to form a tube of warm, stretchable plastic. Compressed air is then blown into the side of the moving tube at regular intervals to form large bubbles, stretching the plastic to the desired thinness. The thin plastic then cools rapidly and is collapsed between metal rollers to form a film. This film is then wound around a large metal roller to form a roll, which may hold several kilometres of plastic wrap.
| Characteristics | Values |
|---|---|
| Plastic film production methods | Extrusion, casting, coextrusion, lamination, solution deposition, skiving, blown or tubular process, calender rolls, extrusion coating, roll slitting |
| Plastic film types | Polyethylene, PVC, PVDC, Bakelite, cellophane |
| Plastic film properties | Flexible, transparent, thin, glossy, printable, barrier against gases and moisture |
| Plastic film thickness | Controlled by speed of extrusion and amount of air inserted in the bubble |
| Plastic film width | Controlled by the amount of air inserted in the bubble |
| Plastic film uses | Food packaging, self-adhesive labels, flexible packaging, product design |
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Plastic extrusion
The molten plastic is then forced through a die, which shapes the plastic and helps it harden during cooling. Different types of dies, such as annular, spider, and spiral, are used depending on the desired product. For example, in the production of plastic film for food packaging, a blown film line is used, where air is forced into an extruded ring of plastic to expand and stretch it into a thin film.
After the extrusion process, the plastic must be cooled. The cooling method depends on the product being manufactured. For thin plastic films, air cooling can be effective as an initial stage, followed by cooling rolls to complete the process. The cooled plastic film is then wound around a large metal roller, forming a roll that can hold several kilometres of plastic. This roll is then cut and rerolled onto small cardboard tubes for distribution.
Plastic extruders can also be used to process recycled plastic waste or other raw materials. This process involves cleaning, sorting, and blending the recycled plastic before extruding it into filaments that can be further processed. Plastic extrusion is a versatile and efficient process that allows for the continuous production of a wide range of plastic products, from films and sheets to pipes and tubing.
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Casting and quenching
Casting is particularly useful for producing biodegradable polymer materials. This method, also known as the "solvent process", involves drying a film-forming solution after its dispersion in a solvent. The casting process results in a web of thin film that is extruded onto a chilled, highly polished, turning roll. The speed of the roll determines the thickness of the film.
The casting process is followed by quenching, which is a rapid cooling step. In this stage, the molten polymer is quenched on a series of highly polished chrome rollers that are cooled with circulating water. This rapid cooling improves the mechanical properties and clarity of the thin plastic film.
After quenching, the film thickness is measured, and it may undergo a corona treatment to increase surface tension and facilitate printing. The film then passes through a set of rolls for slitting and is wound onto a roller for storage.
Overall, casting and quenching are crucial steps in the production of thin plastic films, particularly for biodegradable polymers, and they offer advantages in terms of simplicity, cost, and mechanical properties.
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Stretching and thinning
Plastic film is produced through a process called extrusion, where molten plastic is pushed through a die—a device that shapes the plastic. The plastic is formed into a continuous tube, known as a "bubble", which is inflated with air and drawn up a tower to cool. This process is used to create the desired diameter of the tube. Once cooled, the tube is flattened, and the speed at which this is done determines the thickness of the film.
The plastic film can then be stretched and thinned further through a process called "blowing". Compressed air is blown into the side of the moving tube at regular intervals to form large bubbles, which stretch the plastic. This process is repeated until the desired thinness is achieved. This method of stretching and thinning is also known as a blown or tubular process.
Flat tenter frames are another method used to stretch and thin plastic film. These frames physically stretch the extruded film before annealing, which is a heat treatment process used to alter the properties of the plastic.
The final method for stretching and thinning plastic film is through the use of calender rolls. These rolls form the film from hot polymers, which are then cooled. This process can be used to create extremely thin films, as the layers of plastic can be very thin when using this method.
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Coextrusion
The coextrusion process begins by heating granules of plastic until they melt, typically at 212°F (100°C) for polyethylene and 392°F (200°C) for PVC and PVDC. Multiple extruders are then used to form the molten plastic into layered or encapsulated parts, with each extruder delivering the precise amount of material needed for the operation. The output from each extruder can be split to create two or more layers in the final material if desired. The layers are combined in a way that allows each resin layer to retain its individual identity and characteristics.
The molten plastic then passes through a single die to build up layers of melted resin on the inside and outside of the film. This die can be a multilayer die assembly, allowing compatible thermoplastic materials to be fed from multiple extruders into the same feed block. The internal flow channel system within the die is designed to combine the melt and glue, after which the film is shaped by a cooling wheel. The film is then wound up on a roll, stretched, thinned, or oriented in one or two directions.
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Lamination
In the automotive industry, PVC/acrylonitrile butadiene-styrene (ABS) mixtures are often applied to interiors by laminating onto a polyurethane foam to create a soft-touch surface. Laminated plastic film is also used to improve safety in vehicle windshields, where a tough plastic film is laminated between two layers of glass to prevent shards of glass from detaching in the event of a break.
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Frequently asked questions
Plastic film is a thin, flexible, transparent polymer that is used for packaging. It can be made through various processes, including extrusion, coextrusion, and lamination.
In the extrusion process, plastic granules are heated until they melt and are then forced through a die to form a tube of warm, stretchable plastic. Compressed air is blown into the side of the moving tube at regular intervals to form large bubbles, stretching the plastic to the desired thinness.
Coextrusion is a process that produces a plastic film containing two or more distinct plastic layers without requiring any intermediate steps. In contrast, lamination adheres two or more plastic films together, with each layer needing to be thick enough to be produced and handled on its own.
Plastic film is commonly used for packaging, particularly for food. It helps keep food fresh by protecting it from air and moisture. It also has applications in labelling, where it can provide a "no-label" look on transparent containers.
