Plastic extrusion is a high-volume manufacturing process in which raw plastic is melted and formed into a continuous profile
When finished, products made from plastic take many different forms, but they all start in the same place and of the same raw material – resin.
In plastic extrusion, resin beads are melted down, filtered for uniform consistency, pushed through a die cut for the final product shape, and then cooled.
Plastic extrusion is used to produce a wide range of products on the market, from building materials to consumer products to industrial parts. Pipes, window frames, electrical covers, fence, edging, and weather stripping are just a few of the common items made by plastic extrusion, along with thousands of custom profiles.
The process itself is highly technical. However, it can be summed up in just five steps.
Step 1: Raw plastic materials like granules, pellets, or powder are loaded into a hopper and then fed into a long heated cylindrical chamber called an extruder. It is moved through this chamber by a revolving screw. Some extruders can have either one or two revolving screws.
Step 2: The raw materials flow from the hopper down through the feed throat and onto a large spinning screw operating within a horizontal barrel.
Step 3: Not all materials have the same properties, so the raw material is processed by a heating temperature specific to the type of raw material being passed through. At the end of the heated chamber, the molten plastic is forced through a small opening called a die to form the shape of the finished product.
Step 4: As the plastic is extruded from the die, it is fed onto a conveyor belt for cooling. Cooling can happen by air or water. The process is similar to the injection moulding process except that the melted plastic is forced through a die instead of into a mould.
Step 5: The die for an individual plastic profile is designed so that the plastic flows smoothly and evenly from the cylindrical profile of the extruder into the final profile shape. Consistency in this flow is critical to achieving an end product with integrity.
We go by our everyday lives with little regard to how the things we use are done. We utilize a lot of plastic products in our homes, but little do we know that plastic extrusion plays a big role in the creation of those products.
Plastic extrusion involves forcing melted plastic through a die into a shape with a fixed cross section. It’s an efficient way to produce many shapes, and is essential in both industrial and domestic product applications. Since the plastic is melted from a solid form and then resolidified, only thermoplastics can undergo extrusion. The properties of this type of plastic make it good for recycling both scrap pieces and post consumer goods.
PVC water and sewer pipes are common forms of plastic extruded pipes. In industrial use, other plastics transport liquids and gasses that PVC may not be able to handle. Diameters of several feet are average for larger sewer pipes, which represent some of the largest plastic extrusions found. On the other end of the scale are micro-miniature medical tubes that can have an OD of less than .010”. Plastic tubing is essential for transporting oxygen in the ICU, fuel in small gasoline engines, and soda in a drinking straw.
These shapes—forms of plastic used as raw materials in machine shops—include round, square, and hexagonal bar stock, as well as rectangular pieces. All standard thermoplastics are available in these forms in sizes that match the need, and are available in any length.
Also included in this category are extruded decking, planks, and structural pieces that have become popular in recent years. Plastic decking and lumber are good venues for recycled plastics due to their large mass and non-critical nature. Outdoor furniture, fencing, and park benches are all made with extruded plastic pieces. These boards are weather proof, splinter proof, maintenance free, and generally outlast any exterior wooden member.
Polycarbonate sheeting appears in window glazing and machine guards, and is impact resistant. It is manufactured in thicknesses ranging from thin films up to several inches. Acrylic plastics are also extruded into thin sheets for glass replacements. Extruded clear plastic films are common in the short-term protection of foods and modern retail packaging, which makes use of blister packs–extruded hard films that are softened by heat and then formed in a mold to their correct shape.
Extrusion is the preferred in the production of long cross-section shapes. Windshield wiper and squeegee blades, plastic rain gutters, and automotive trim, are all extruded shapes. Vinyl siding is also made from this process, and is found in over half of all new home construction. Like other exterior grade plastic parts, vinyl siding stands up to the elements and is maintenance free.
Bottles and jars start as extrusions. After extending to a certain length, a two-piece mold pinches off and closes around the blank, now called a parison. The mold forms the threads in the parison, and while it is still hot, it is injected with compressed air that expands to the interior size of the mold. After cooling, the mold ejects the bottle and begins the cycle again.
The insulation on electrical wire is also created through extrusion. In this process, the wire is pulled through the die, where the insulation is then extruded around it. In order for this to work, the wire should have strong electrical insulation properties, suppleness, and abrasive resistance—the wire should also be impervious to liquids, gasses, and corrosives. Non-electrical cables are also coated by plastic extrusion for corrosion resistance, abrasion resistance, and to enable smooth handling.