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Different Types of Extruders
What is an Extruder?
To describe what an extruder is, we first need to define some related terms. First, to extrude is to push out. When a material is extruded, it is forced through an opening called the die. For instance, when we squeeze toothpaste from a tube, we extrude toothpaste. 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, including 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 ma- terial, and the flow rate. 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 plastic materials. However, this is not to be confused with a plasticating extruder, which is a machine that not only extrudes but also plasticates, or melts, the mate- rial. A plasticating extruder is fed with solid plastic particles and delivers a com- pletely molten plastic to the die. On the other hand, a machine that extrudes molten plastic without melting it is called a melt-fed extruder.
Extruders are the most common machines in the plastics processing industry. Extruders are not only used in extrusion operations; most molding operations also use an extruder, for instance, injection molding and blow molding. Essentially every plastic part has gone through an extruder at one point or another; in many cases, more than once.
Different Types of Extruders
In this section we will discuss:
single screw extruders
twin screw extruders
In the plastics industry, there are three main extruder types: the screw extruder, which is the most common one, 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, see Figure 1.1.
Figure 1.1 : A single screw extruder
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 plas- tics 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 with its two screws.
126.96.36.199Co-Rotating Twin Screw Extruders
There are several types of twin screw extruders. 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, see Figure 1.2.
Figure 1.2 : A co-rotating twin screw extruder
If the screws of a twin screw extruder rotate in opposite directions, it is called a counter-rotating twin screw extruder. Twin screw extruders can run at high or low speed, depending on the application. High speed extruders run at around 200 to
500 rpm, sometimes even higher; they are primarily used in compounding. Low speed extruders run at about 10 to 40 rpm and are 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, see Figure 1.3.
Figure 1.3 Conical twin screw extruder
Another distinguishing feature of twin screw extruders is the extent to which the screws intermesh. The screws can be fully intermeshing, see Figure 1.4a, partially intermeshing, see Figure 1.4b, and non-intermeshing, see Figure 1.4c.
Figure 1.4 (a) Fully intermeshing counter-rotating screws; (b) partially intermeshing counter- rotating screws; (c) non-intermeshing (tangential) counter-rotating screws
Degree of intermeshing
High pressure, low melting capacity
Most twin screw extruders are intermeshing. The advantage of non-intermeshing twin screw extruders is that they can be very long yet avoiding problems with metal-to-metal contact between the screws. The length-to-diameter (L/D) ratio can be 100 : 1 and higher. The L/D of intermeshing twin screw extruders is generally limited to values less than 50 : 1. A disadvantage of current non-intermeshing twin screws is that they have limited dispersive mixing capability; however, new disper- sive mixing technology described in Chapter 5 may negate this limitation.
In a ram extruder, a reciprocating piston forces the material forward and through the die, as shown in Figure 1.5.
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, such as 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 (UHMW) polyethylene. These plastics do not melt like normal plastics and are formed by sintering. Continuous products can be made on a ram extruder, although the line speeds are quite low − typically in the range of 25 to 75 cm per hour (10 to 30 inches per hour).