What is Plastic Extrusion?
Plastic extrusion, sometimes referred to as plasticating extrusion, is a manufacturing technique that’s used to produce large quantities of continuous products with a uniform cross-section. The process involves taking a thermoplastic material — such as powder, pellets, or granules — melting it uniformly, then pushing the molten plastic through a die under pressure.
In screw extrusion, this pressure is generated by the rotation of a screw against the walls of a barrel. As the molten plastic flows through the die, it takes on the shape of the die’s openings and exits the extruder. The resulting product, known as extrudate, is then cooled, often through water baths or cooling rolls, until it solidifies into its final form.
Plastic extrusion can be used to produce pipes, tubes, sheets, films, profiles, and other custom shapes. The equipment that’s used is called an extruder, and this machine is part of a larger system that supports the high-volume production of plastic components.
Plastic Extruder Components
The key components of a plastic extruder are the hopper, feed throat, breaker plate, barrel, feed pipe/adapter, die, and cooling system (see below). These components work together to facilitate the efficient melting, shaping, and solidification of the plastic material.
The functions of the different components within a plastic extruder are as follows:
1.Hopper: The first stage of the extruder, where plastic granules are stored and prepared for processing.
2.Feed Throat: Transfers the plastic from the hopper to the barrel.
3.Breaker Plate: Acts as a filter and maintains pressure within the barrel.
4.Barrel: Heats and softens the plastic while a rotating screw conveys the molten material toward the feed pipe.
5.Feed Pipe/Adapter: Conveys the molten plastic toward the die.
6.Die: A metal tool that determines the shape of the extruded profile.
7.Cooling System: Rapidly cools and solidifies the extruded profile.
The Plastic Extrusion Process
During the plastic extrusion process, the thermoplastic raw material is fed into the extruder through a top-mounted hopper. The material, in the form of small beads or resin, can be mixed with additives like colorants and UV inhibitors before entering the hopper.
The following steps provide an overview of the entire plastic extrusion process.
1.Material Feeding: Raw material enters the extruder through the feed throat, which is located near the rear side of the barrel. The molten thermoplastic then contacts a rotating screw inside the barrel.
1.Melting and Heating: The screw rotates at a controlled speed, typically up to 120 rpm, pushing the plastic beads forward in the barrel. The barrel is heated to the required melt temperature, which ranges from 200–275°C, depending on the type of polymer that’s being extruded. Typically, the barrel has multiple PID-controlled heater zones that gradually increase the temperature from the rear to the front.
2.Melt Formation: As the plastic beads are forced through the heated barrel, they gradually melt. Temperature controls prevent overheating and polymer degradation. Friction and pressure in the barrel also contribute to the melting process. In some cases, and if the extrusion speed is fast enough, the heaters can be turned off so that the melt temperature is maintained through friction and pressure alone. Cooling fans or cast-in heater jackets are used to regulate temperature and prevent overheating.
3.Filtration: Molten plastic leaves the screw and passes through a screen pack to remove contaminants. A breaker plate reinforces the screens, as pressures can exceed 34 Mpa. The screen pack and breaker plate assembly also create back pressure in the barrel for proper mixing and uniform melting of the polymer.
4.Shaping: After passing through the breaker plate, the molten plastic passes through the die, which imparts a specific shape to the final product. The die ensures a uniform flow of molten plastic, transforming it from a standard cylinder to the desired shape of the final product.
5.Cooling: The product is cooled by pulling it through a water bath or using cooling rolls, depending on the type of extrusion. Plastics have poor thermal conductivity, so controlled cooling is essential. For pipes or tubes, a vacuum is applied to the water bath to prevent their collapse. Plastic sheeting is cooled by passing through a set of cooling rolls.
Four Types of Plastic Extrusion Processes
Plastic extrusion has different processes for different applications. The four major processes are tubing, blow-film, sheet film, and over-jacketing.
1.Tubing Extrusion: This process uses a mandrel or pin inside the die to create internal cavities. Tubing extrusion is ideal for producing tubes, pipes, and hollow items.
2.Blow-Film Extrusion: This is commonly used for creating items like shopping bags. This process uses an air ring to cool and expand the extruded plastic into a bubble, then nip rollers flatten the bubble into a double-layered film extrusion.
3.Sheet Film Extrusion: Similar to blow-film extrusion, this process creates flat sheets of plastic. The desired shape and surface texture are achieved through a pulling and rolling process.
4.Over-jacketing Extrusion: Specifically used for coating wires, this type of plastic extrusion uses either pressure or jacketing to cover wire with molten plastic, depending on the required level of adhesion.
Two Types of Plastic Extruders
Plastic extruders use two different types of equipment. Both are capable of performing the four processes described above, but this equipment differs in terms of production speed, mixing capability, and operational flexibility.
Here’s a look at each extruder type.
Single-Screw Extruder
A single-screw extruder consists of a barrel with a single rotating screw. These extruders are well-suited for applications that require simple profiles, such as pipes and sheets. Although single-screw machines perform adequately, they tend to operate at slower speeds, have limited mixing capabilities, and may not be the most suitable choice for complex shapes.
Twin-Screw Extruder
Twin-screw extruders feature two parallel screws within the barrel. Depending on the specific application, the twin screws can rotate in the same direction or in opposite directions. Due to their design, twin-screw extruders offer enhanced stability, superior performance in exhausting air and volatile fumes, increased production speed, and improved mixing capabilities.
Material Selection for Plastic Extrusion
The plastic extrusion process supports the use of a wide range of materials, each with different properties. Some examples of commonly extruded plastic materials include:
1.ABS (Acrylonitrile Butadiene Styrene)
2.Acrylic
3.PVC (Polyvinyl Chloride)
4.Polyethylene
5.Polycarbonate
6.Styrene
7.TPA (Thermoplastic Alloy)
8.Polypropylene
9.CPVC (Chlorinated Polyvinyl Chloride)
10.TPV (Thermoplastic Vulcanizate)
