Plastic Extrusions

Automotive manufacturers employ extruded plastics as engine components and as decorative and functional trim on auto exteriors. PVC extrusions, particularly PVC tubes, are used extensively in plumbing systems and in wastewater treatment operations. Chemical processing operations also make use of extruded plastic tubing, and because different plastics feature different qualities of reactivity when exposed to chemicals, plastic materials are chosen for their applications based on such qualities. Acrylic extrusions, for example, are characterized by their optical clarity and durability. They are, for these reasons, often well suited to use as tubing in chemical processing, because their contents can be easily visible when inspected by workers. Some unique applications call for an extrusion that features the qualities of more than one material. In these cases, coextrusion can be used to mix two plastics together, allowing for the qualities of two plastic materials to be present in one product.

The term polymer extrusion is sometimes used interchangeably with plastic extrusion. This is not incorrect, though it is not the most precise way to describe the process. Technically, a polymer is a large molecule made up of many similar monomers, which are small molecules. Biopolymers like proteins and nucleic acids are essential for life. These are not the kinds of polymers to which the words "polymer extrusion" refer. When the words "polymer" and "extrusion" are paired together, they refer only to plastics and elastomers (natural rubber is an example of an elastomer). In the case of plastic extrusion, a very long list of plastic materials is at the disposal of companies that provide plastic fabrication services. High density polyethylene (HDPE), low density polyethylene (LDPE), PETG, PVC, butyrate, vinyl, polypropylene, and polystyrene are just a few of the most commonly extruded thermoplastics. A thermoplastic is a plastic material that, when exposed to sufficient heat, becomes plasticized. A plasticized thermoplastic can be shaped into useful products and retain the shape once cooled. Thermoplastics are highly valued for this quality, and they are also valued because they can be recycled at the end of their lifespans and reshaped by the same process.

The plastic extrusion process is very standardized, and there are few deviations between extrusion operations in terms of the basic principles of extrusion. Operating temperatures and output speeds may vary depending on the properties of the plastic material, but plastic extrusion processes resemble each other very closely aside from these differences. The extrusion process begins with a collection of raw plastic materials in a hopper suspended above a conveyance channel. When a panel at the bottom of the hopper is removed or retracts, gravity directs the plastic into the channel. Inside the channel is a long shearing screw that forces the plastic down the channel as it turns. The friction caused by the turning of the screw causes the plastic to become molten. Some extruders feature supplementing electrical heating elements to aid in the melting of the plastic. By the time the plastic reaches the end of the channel, it is ready to be shaped. At the end of the channel is a die, which is a specially-designed tool used to form raw materials into usable products. In the case of plastic extrusion, a die is a metal plate cut with a hole through which the plastic is forced.

When the molten plastic reaches the die, it is forced through, taking its shape as a result. The plastic emerges on the other side of the die as newly extruded plastic material. The plastic is quickly cooled, which causes it to harden and keep its shape. The extrusion can then be cut and prepared for shipment or sent for additional processing like labeling, painting, anti-static treating or other surface treatment. The shape of an extrusion die determines the shape of the extrusion. Every die is different; they are specially designed to accommodate individual processes. The simplest possible die designs are nothing more than simple shapes. A circle-shaped die would produce a plastic rod, while a square-shaped die would produce a continuous plastic square. Extrusions like pipes require dies fitted with special pins that allow hollow places to form. Textured plastic sheets like floor mats can also sometimes be produced by extruders. Plastic sheet dies would be flat with grooves to create the texture. The coextrusion process, in which two or more plastic materials are combined in a single extrusion, features one die connected to a forked conveyance channel into which the different materials are directed and mixed together. The number of possible extruded plastic shapes is limited only by the imaginations of their designers.

Plastic Extrusions and Custom Plastic Extrusions Image Provided by Preferred Plastics, Inc.

Plastic Extrusion Types

• Acrylic extrusions are extruded plastic shapes made of acrylic materials. Acrylics are primarily characterized by their optical clarity.
• Coextrusions are materials that are a combination of one or more polymers combined in the extrusion process.
• Extruded plastic tubing products are tubes produced by a plastic extruder. 
• Extruded plastic netting is produced in a continuous process resulting in a stable net structure in which the hole size is held constant, unlike a woven or knitted product. Extruded plastic netting is typically utilized in vertical applications such as walls for pens, separation in water, filtration, predator control or barriers.
• Extruded plastic sheets are plastic powder or pellets that have been formed into sheet products with a range of thickness from less than 0.010 in. (film) up to and exceeding 2.0 inches, with widths as great as 30 ft. This sheet can be further processed into parts via thermoforming.
• Extruded plastics are made by pouring liquid plastic into dies.
• Plastic channels are strips of plastic that are shaped for a specific purpose and cut to length.
• Plastic extruders are machines that create plastic extrusions like rods, tubes and channels.
• Plastic profiles are extruded stock shapes of plastics including HDPE, polystyrene, butyrate, vinyl, PVC, LDPE, nylon, polyurethane, acrylics and fiberglass.
• Plastic rods are cylinder-shaped components made of plastic materials.
• Plastic shapes produced by extrusion include strips, trim, tubes, pipes and many other shapes.
• Plastic strips are extruded plastic shapes characterized by their length and narrowness.
• Plastic trim is a linear extruded profile that can be extruded to any length.
• Polymer extrusion is the shaping of raw plastic and elastometric materials into usable products. 
• PVC extrusions are extruded plastic shapes made of polyvinyl chloride. PVC is one of the most popular plastic extrusion materials worldwide.
• Vinyl extrusions are extruded products that are made of polyvinyl chloride (PVC), including pipes, siding and floor coverings

Plastic Extrusion Terms

Adiabatic Plastic Extrusions - Plastic extrusions whose only source of heat is the conversion of the drive energy through the viscous resistance of the plastics mass in the plastic extruders.
 
Back Pressure - The resistivity of molten plastic material to forward flow.
 
Barrel - The part of the plastic extruders encasing the screw or plunger.
 
Barrel Liner - The sleeve forming the inner surface of the plastic extrusions barrel.
 
Calendering - The plastic extrusions process of pressing or smoothing material between rollers.
 
Cladding - Sometimes referred to as sidings it is extruded PVC-U boards that are used as outdoor weather-resistant fade panels.
 
Compound - Any plastic material prepared for subsequent manufacturing processes, specifically plastic extrusions, molding or calendering.
 
Compression Section - The transition section of a screw channel in which a reduction in the screw channel volume occurs.
 
Cooling Tank - A tank typically containing water through which plastic extrusions are constantly passed for cooling.
 
Cure - The technique of cross-linking a plastics material.
 
Decompression Section - The section of two-stage plastic extruders in which an increase in screw channel volume occurs.
 
Die - The component on plastics extruders affixed to the plastic extruder's head through which the melt is pushed to form the desired profile.
 
Die Plate - In moulds, the main support for the punch or mould cavity.
 
Dry Blend - A free flowing blend of compound or resin and other ingredients as prepared for an additional manufacturing operation specifically for plastic extrusions or molding.
 
Extrudate - The product or result of the plastic extrusions process. An extrudate is a product or material forced through a shaping orifice as a continuous body.
 
Extruder Size - The minimal inner diameter of the plastic extruders barrel
 
Extrusion Coating - A coating technique in which molten plastic feeds directly from plastic extruder dies into a nip-roll assembly combined with the substrate.
 
Haul-off - Also called a caterpillar, it is an apparatus used for the continous removal of extrudate from the die.
 
Heat Aging - The unique process of aging a thermoplastic or thermoset product and examining the percentage of retained physical and chemical properties after exposure to heat for a prolonged period of time.
 
Melt - Any plastic extrusions material heated to a plastic condition.
 
Melt Strength - A term that refers to the strength of molten plastic.
 
Outer Die Ring - The element of tubing tie that shapes the outer surface of a tube.
 
Pellets - Resins or mixtures of resins with compounding additives in the shape of similar-sized tablets and granules that have been extruded or chopped into short segments to prepare them for molding or extrusion operations.
 
Ram Extruder - A barrel with a temperature control, wherein a plunger pushes material in a melted state to the die.
 
Resin - Any of several physically similar polymerized synthetics or chemically altered natural resins, such as thermoplastic materials (polyvinyl, polystyrene, polyethylene) or thermosetting materials (epoxies, polyesters, silicones used with fillers, stabilizers, pigments).
 
Screw - A helically grooved rotating element inside the barrel of screw plastic extruders. The main purpose of a screw is to melt and feed raw material from the feeder to the die, but it also homogenizes, compresses and pressurizes the material.
 
Screw Extruder - A machine comprised of a barrel with a temperature control. It houses one or more rotating screws, which pass plastic materials from the feed aperture and move them in the form of melt under pressure through a die.
 
Take-up - An apparatus for reeling extruded plastics material.
 
Thermoset - A term that refers to the family of materials that can be melted only once during the original processing and cannot be reprocessed after the original part is made.
 
Thermoplastic - Any material, such as polyethylene, Santoprene and ABS, that can be remelted and reprocessed without considerable loss of properties or scrap loss.
 
Torpedo - An apparatus at the discharge stage of the screw for finishing homogenizing and blending of the melt.
 
Trunking - An extruded PVC-U channel used to contain and protect pipes or cables.
 
Vacuum Sizing - A procedure utilizing a sizing die with a vacuum applied to the outer surface of the extrudate.
 
Vinyl - A generic term for PVC, it is one of various compounds of ethylene that are polymerized to form resins and plastics (e.g. polyvinyl or polyethylene plastics).