To explain what an extruder is, we will define some of the related terms. First of all, to extrude is to push out. When a material is extruded it is forced through an opening; the opening is called the die. For instance, when we squeeze toothpaste from a tube, we extrude tooth paste. As the material flows through the die it acquires the shape of the die flow channel. A machine that is used to extrude a material is called an extruder. Many different materials can be extruded, for instance clays, ceramics, food, metals, and of course plastics.
The main function of an extruder is to develop sufficient pressure in the material to force the material through the die. The pressure necessary to force a material through the die depends on the geometry of the die, the flow properties of the material, and the flow rate. So, basically, an extruder is a machine capable of developing pressure. In other words, an extruder is a pump. A plastics extruder is a pump for plastics materials. This is not to be confused with a plasticating extruder; this is a machine that not only extrudes but also plasticates or melts the material. A plasticating extruder is fed with solid plastics particles and delivers a completely molten plastics to the die.
Extruders are the most common machines in the plastics processing industry. Extruders are not only used in extrusion operations, most moulding operations also use an extruder, for instance injection moulding and blow moulding. Essentially every plastics part will have gone through an extruder at one point or another; in many cases, more than once!
1.2 DIFFERENT TYPES OF EXTRUDERS
There are many different types of extruder. In the plastics industry, there are three main types: the screw extruder, which is the most common, the ram extruder, and the drum or disk extruder, which is the least common.
In a screw extruder a screw rotates in a cylinder; the rotation of the screw creates a pumping action. A screw extruder can have one screw or more than one screw.
An extruder with one screw is called a single screw extruder; it is the most common machine in the plastics processing industry. An extruder with more than one screw is called a multi-screw extruder. The most common multi-screw extruder is the twin-screw extruder; it has two screws.
There are several types of twin screw extruder. In most twin screw extruders the screws are located side by side. If both screws rotate in the same direction, the extruder is called a co-rotating twin screw extruder.
If the screws rotate in opposite direction, we call it a counter-rotating twin screw extruder. Twin screw extruders can run at high speed or at low speed, depending on the application. High speed extruders run at around 200 to 500 rpm and even higher; they are primarily used in compounding. Low speed extruders run at about 10 to 40 rpm and used mostly in profile extrusion applications.
Most twin screw extruders for profile extrusion are counter-rotating extruders. This is because counter-rotating extruders tend to have better conveying characteristics than co-rotating extruders.
Most twin screw extruders have parallel screws, but some extruders have conical screws where the screws are not parallel.
Another distinguishing feature of twin screw extruders is the extent of intermeshing of the screws. The screws can be fully intermeshing, partially. Most twin screw extruders are intermeshing. The advantage of non-intermeshing twin screw extruders is that they can be made with very long length without concern of metal-to-metal contact between the screws. The L/D ratio can be as high as 100:1 and higher. The disadvantage of non-intermeshing twin screws is that they have poor dispersive mixing capability.
Ram extruders use a reciprocating piston to force the material forward and through the die. Ram extruders have very good conveying characteristics and can develop very high pressures.
The drawback of ram extruders is that they have low melting capacity. Therefore, they are not used very often for normal plastics. There are some unusual plastics, however, that are often processed on a ram extruder. These are the so called “intractable” plastics that cannot be processed on normal extruders. Examples of such plastics are PTFE (poly-tetra-fluoro-ethylene) and ultra high molecular weight poly-ethylene. These plastics do not melt like normal plastics and are formed by sintering. Continuous products can be made on a ram extruder; the line speed is quite low though, in the range of 25 to 75 cm per hour (10 to 30 inch per hour).
