You already understand the extrusion technique, but how do plastic extrusion machines work? Well, that’s the subject of this chapter. We will see what really happens in an extruder from when you start it to when the product exits the die. Additionally, we will be taking a look at the machine’s downstream equipment and how it helps complete the extrusion process.
How do plastic Extrusion Machines Work?
The plastic extruder machine working is pretty straightforward: Raw plastic materials enter the feed zone, are heated, compressed, and pushed through an opening in the machine’s end. The hot plastic then comes out already shaped. We will get in the details of the process in a short while but before that, the different materials and extrusion techniques that can be used with the machine.
Plastic Extrusion Materials
What plastic extrusion materials are can you use with an extruder? A majority of polymers can be used but mostly thermoplastics. Thermoplastics do not degrade in heat, which makes them best suited for extrusion. Here is a list of those plastics.
- HDPE (high-density polyethylene)
- ABS (Acrylonitrile Butadiene Styrene)
- PP (Polypropylene)
- PS (Polystyrene)
- PC (Polycarbonate)
- PETG (polyethylene terephthalate glycol)
- PVC (Polyvinyl chloride)
Plastic extrusion involves pushing heated and softened plastic through a die of the desired shape. Many different types of plastics can be extruded. Likewise, the extrusion process is done in several different ways as outlined below.
In the co extrusion process, molten material from a separate extrusion is added to a primary extrusion. This is used to combine different colors or create layered products, such as plastic siding for houses. In co-extrusion machine, two or more materials are fed into the screw channel and combined as they flow through the barrel and come out through the die.
Blown film extrusion uses air pressure to form a bubble of the molten plastic as the film is extruded. The bubble is then cooled by rollers before being made into various products. In a blown film extrusion machine, the die resembles that of tubing products. The blown film process is often used to create products such as shrinkwraps or carrier bags. These mostly use polyethylene.
Over Jacketing Extrusion
Over jacketing extrusion involves using molten plastic to coat a length of wire or cable. The wire is pulled through the middle of the extruder die. This introduces a polymer coating around it. Over jacketing helps protect bare wires from corrosion and other types of damage. It also helps to insulate the wire.
Tubing extrusion refers to the technique used to make hollow tubes and pipes. The heart of a tube extrusion process is the annular die. Other equipment is also used besides the extruder itself. This includes cutters, pullers, and vacuum sizing equipment.
The Extrusion Machine Process
The working of a plastic extruder can be divided into several different steps: feeding, heating, compression, and metering/conveying. These are also called the extruder barrel zones. Below is how plastic extrusion works (note that we chose the screw extruder since it’s the most common type of extrusion machine today)
- The extrusion process starts with material preparation. The extrusion material, mostly thermoplastic and in form of granules or pellets, is mixed with desired additives first (stabilizers, fillers, colorants, flame retardants, and UV inhibitors) or even other plastics (compounding).
- In some cases, the extrusion materials used may require drying to remove any moisture that may be present and prevent degradation. A conveyer belt (or any other method) then takes the plastic pellets to the extruder.
- Next, the material enters the extruder feeding system. This is typically a hopper that holds and directs the feedstock into the machine. The hopper often allows the plastic pellets to fall into the screw channel by gravity, but may also involve a vacuum or vibrating parts to enhance flow.
- In the extruder screw channel, the plastic pellets are subjected to frictional forces by the rotating motion. This causes them to mix and heat up. In most cases, a series of heaters located on the barrel also help to gradually bring the plastic to a melt. A cooling system on the barrel may also lower the temperatures if necessary to avoid material degradation.
- Meanwhile, the screw diameter begins to increase, compressing the molten plastic as it flows forward in the rotating motion. The purpose of this is to mix or homogenize the material as well as compress it. In a vented extruder, the screw diameter will also decrease to decompress the melt and vent out volatiles.
- The end of the extruder screw is a combination of a breaker plate and a screen or pack of screens. The extruder breaker plate helps “break” the material’s positive pressure. It also helps to discontinue the spiral motion of the melt and instead subject it to a straight flow. The screens, on the other hand, trap any solid parts from the liquid plastic and protect the extruder nozzle from clogging.
- Finally, the extruded form or product is pushed out of the die (nozzle) and immediately cooled using a water bath, spray, or air coolers and pulled away from the extruder. The extruder working may, most often, include a cutter. Typically, more processes are required before the extrudate can be called a product. More about that below.
Most of the time, the part or product coming out of the plastic extruder machine needs to undergo various stages and processes before it can be ready for use. The equipment to perform these processes are usually placed downstream with a few forming part of the extruder assembly.
The downstream extrusion equipment includes many different devices depending on the type of product or material. It also depends on the customization requirements. Examples of downstream extrusion machinery are coolers, vacuum systems, pullers, winders, and cutters.
Extrusion Cooling System
The plastic extrusion cooling system mostly consists of either air or water coolers. These help cool the part or profile coming out of the die. Air coolers are low-cost and simple to set up. However, they are not very efficient and, therefore, not popular. Water coolers are more effective. These can be water sprays surrounding the extrudate, or water baths that completely submerge it.
During the extrusion of pipes and other hollow structures, vacuum pressure is used to prevent the part from collapsing. The extrusion vacuum equipment is usually a part of the cooling system so it can hold the product form until it cools to a solid structure. This is usually a vacuum tank and hoses leading to the extruder assembly.
Extrusion pullers are devices that help pull the extruded product away from the die and other downstream equipment. This serves to protect the extrudate from bunching up until it’s completely cooled, especially when handling long extrusions. Plastic extrusion pullers come in many different types: belt, wheel, and cleat pullers. Belt pullers are suitable for low force pulls, while cleat pullers fit situations that require a lot of pulling force.
Extrusion cutters are devices that help size the extruded products. These are often customized to fit client requirements and include saws and knives. Different cutter versions use a variety of cutting methods and configurations. Timed cutters, for example, employ sensors and timing systems while continuous cutters use a pre-set cutting speed.
Other downstream extrusion equipment includes takeoff systems, winding/reel-off machines, and printers. There are also secondary extrusion processes such as printing, painting, flame treatment, hole-drilling, and label attachment before the product is inspected and packed to prep it for shipping.
Depending on the type of machine and product, the entire plastic extrusion process steps may be automated, from the hopper feed to the cutting and sizing stage. Some equipment may be or may not be present depending on the extruded product or type of machine.
To sum it up, the extrusion machine process is a very precise method that requires careful planning and management. Plastic extrusion machines normally come with the required controls for the various processes and manuals to help the operator work with the machine. The extruder working is also often customized to attain optimum production level and product quality, especially for uncommon extrusions. Next is a detailed look at the different types of the machine.