Plastic extrusion, a method through which plastic shapes profiles are produced, is an important process for many industries. In this process, shapes are produced when molten plastic is forced through a die. Basic profiles include strips, channels and trim.
While output speeds and operating temperatures may vary depending on properties of the material, the basic principles of plastic extrusion remain standardized. Extruded plastics begin their journey as a collection of raw plastic materials, suspended above a conveyance channel in a hopper. Then, after the bottom of the hopper retracts or is removed, they fall directly into the channel, where they become molten, thanks to friction caused by a long shearing screw that pushes it and turns. By the time the now molten plastic reaches the end of the channel, it’s ready to be shaped by the die. A plastic extrusion die is a metal plate with a custom-shaped hole in it, through which plastic is forced and formed into a usable product. When the plastic comes out on the other side of the die, it can officially be called extruded plastic material. After the extruded plastic is cooled quickly, so that it will harden and keep its shape, it can be cut and prepared for shipment or sent on for additional processing like painting, labeling, anti-static treatment or other surface treatment. Read More…
As noted earlier, plastic extrusion is a process that plays an important role in many industries, such as automotive, chemical processing, plumbing, industrial water treatment and many commercial industries. Extruded plastics have applications as engine components and functional and decorative auto exterior trim.
Plumbing systems, wastewater treatment systems and chemical processing operations all use extruded PVC and plastic tubing. Plastic extrusions are chosen for their various qualities, like how they react or do not react to certain chemicals or their level of transparency. Sometimes, an application requires qualities of more than one material, in which case, engineers may mix two plastics together using coextrusion.
The list of plastics available for extrusion services is quite long. It includes, among others, low density polyethylene (LDPE), high density polyethylene (HDPE), vinyl, polypropylene, polystyrene, PVC, butyrate and PETG. Many of these plastics can be recycled and reshaped and the end of their lifespans. With such allowances, the possibilities of plastic extrusion are virtually endless.
How Plastic Extrusions are Made
Extruded plastics are similar to extruded metals, however, the difference, as the name implies, is in the material used. To make extruded plastics, plastic in the form of granules is molded into an array of profiles, PVC channels, plastic strips, and plastic tubing. Extruded plastic parts have a vital role in present day manufacturing and fabrication. Extruded plastics have applications in essentially every commodity, whether it is consumer products or automotive fabrication.
Plastic Extrusion Process
Unlike metal extrusion, extruded plastic can be manufactured, easily and in high volumes, since the manufacturing process is continuous. Moreover, the plastic extrusion process, if observed closely, is more of a misnomer, since the process is more similar to plastic injection molding than extrusion. Plastic extrusion is a similar to the pultrusion process, where plastic is pulled through a die to make profiles and other shapes, but heating is not involved.
Even though the extrusion process is continuous, it can be divided into number of steps, for the sake of understanding.
The first step involves feeding the extruder with the raw materials, which are pre-processed plastic grains or resin specifically for extruding. The granules, in the shape of beads, are simply fed into to extruder’s barrel under the effect of gravity through a hopper, without any additional mechanisms. Sometimes, to equip specific properties like UV resistance and color to the product, chemical supplements are mixed before the granules reach the hopper.
As the raw material reaches the barrel, a screw, which rotates at certain RPM, pushes the beads forward in to the heated chamber. There are two ways the plastic is melted; firstly by passing the raw material through the heating zones and secondly by the friction and pressure that the raw material encounters. With the combination of the two heating actions, the material steadily reaches the melting point. For some applications, there is no need for heating zones when heat produced by friction is enough to melt the plastic. Regulating heat is the most important factor in the manufacturing process, overheating can degrade the polymer chemical structure. Therefore, heat regulating equipment is used, ranging from fan to special heating regulating jackets.
When the molten plastic reaches the barrel end, the release is regulated by a break plate, which performs three functions, removing impurities by a screen, creating back pressure and straightening the plastic.
At the end of the process, the molten plastic encounters a die, which based on the tooling, gives shape to the profile. Any imaginable profile can be created with this process, as long as the profile is continuous.
The last step involves cooling the product under a controlled environment, so that stress points do not disorient the shape. The most common way include, bathing the product in water. In some more sophisticated methods, a vacuum is created during the bath, and for some requirements, cooling rolls are used.
The versatility of this process makes it most sought after manufacturing process, maybe after injection molding.