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Components of an Extruder
1 The Extruder Screw
The heart of the extruder is the extruder screw. This is a long cylinder with a heli- cal flight wrapped around it, see Figure 1.6. The screw is very important because conveying, heating, melting, and mixing of the plastic are mostly determined by the screw. The stability of the process and the quality of the extruded product are very much dependent on the design of the screw. The screw rotates in a cylinder that fits closely around it.
Figure 1.6 A single flighted extruder screw
2. The Extruder Barrel
The cylinder is called the extruder barrel. The barrel is a straight cylinder usually equipped with a bimetallic liner; this liner is a hard, integral layer with high wear resistance. In most cases, the wear resistance of the barrel should be better than that of the screw. The reason is that the screw is much easier to rebuild and replace than the barrel. Bimetallic barrels usually cannot be rebuilt.
The barrel may have a vent opening through which volatiles can be removed from the plastic, see Figure 1.7, a process called devolatilization. An example is the re- moval of moisture from a hygroscopic plastic. An extruder with a vent port should use a special screw geometry to keep the plastic melt from coming out of the vent port; such a screw is called a “two-stage screw,” see Figure 1.7.
Figure 1.7 A vented extruder barrel with a two-stage screw
3. The Feed Throat
The feed throat is connected to the barrel; it contains the feed opening through which the plastic material is introduced to the extruder. The feed throat usually has water-cooling capability because we have to be able to keep the feed throat tempera- ture low enough to keep the plastic particles from sticking to the wall. To improve the intake capability of the feed throat, the feed opening can be offset as shown in Figure 1.8 and have an elongated shape. The length of the feed opening should be about 1.5 times the diameter of the barrel and the width about 0.7 times the dia- meter.
Some extruders do not have a separate feed throat, but the feed opening is ma- chined right into the extruder barrel. There are both advantages and disadvantages to such a setup. The advantages are lower cost, fewer parts, and no problems with alignment of the barrel to the feed throat. Disadvantages are that it is more difficult to create a thermal barrier between the hot barrel and the cold feed throat region and good cooling of the feed throat region is more difficult.
Figure 1.8 Preferred geometry for feed opening in the feed throat
4.The Feed Hopper
The feed throat is connected to the feed hopper and the extruder barrel. The feed hopper holds the plastic pellets or powder and discharges the material into the feed throat. The hopper should be designed to allow a steady flow of material. Steady flow is best achieved with a circular hopper with a gradual transition in the conical section of the hopper, see Figure 1.9.
Figure 1.9 Good hopper design (left) and bad hopper design (right)
For difficult bulk materials, special devices can be used to promote steady flow through the hopper; such as vibrating pads, stirrers, wipers, and even crammer screws to force the material to the discharge, see Figure 1.10.
Figure 1.10 : Example of crammer feeder
5. Barrel Heating and Cooling
The extruder barrel has both heating and cooling capability. Heating is usually done with electrical band heaters located along the length of the extruder. The heaters can be mica insulated heaters, ceramic heaters, or cast-in heaters. In cast-in heaters, the heating elements are cast in a semi-circular block of aluminum or bronze; these heaters provide good heat transfer. Aluminum cast heaters can heat up to 400 °C, while bronze cast heaters have a maximum operating temperature of about 550 °C. Other types of heating can be used, such as induction heating,
Various heating methods
radiation heating, and fluid heating. Induction and radiation heating are not com- monly used; fluid heating is used in rubber extrusion and on some older plastic extruders.
Most extruders have at least three temperature zones along the length of the barrel. Long extruders may have eight temperature zones or more. Each zone has its own heating and cooling capability and at least one temperature sensor to measure the zone temperature. The temperature is usually measured in the barrel. The die may have one or several temperature zones, depending on its complexity. Some dies have more than ten temperature zones. Dies have heating capability, but usually do not have cooling.
The barrel has to be cooled if the internal heat generation in the plastic raises the barrel temperatures above the setpoint. This is likely to occur when extruding high viscosity plastics and when running at high screw speeds. Cooling on single screw extruders is usually done with air. Blowers are placed under the extruder barrel and temperature zones are partitioned, so that one blower cools only one tempera- ture zone, see Figure 1.11.
Figure 1.11 Extruder with barrel heaters and blowers for cooling
Barrel vs. plastic temperature
Water cooling can be used as well, particularly if large amounts of heat must be re- moved. The extrusion process normally runs best when the screw supplies most of the energy needed in the process, so that little additional heating or cooling needs to be done through the barrel. As a result, air cooling is sufficient for most extru- sion operations using single screw extruders. Since water cooling removes heat more quickly, it can be more difficult to maintain good temperature control with water cooling. Oil cooling can be used as well; in fact, oil can be used both for heat- ing and cooling.
With barrel cooling, it is important to realize that even if the temperature is at its setpoint, the actual melt temperature is above the setpoint. With barrel cooling on, the heat flows from the plastic through the barrel to the outside. In this situation, the highest temperature occurs in the plastic. Even if the barrel temperature is at setpoint, the plastic temperature can be substantially higher. Therefore, barrel cool- ing should be minimized if possible.
6. Screw Heating and Cooling
The screw of an extruder is usually neither heated nor cooled; such a screw is called a “neutral screw.” However, it is possible to either heat or cool the screw by coring the screw (making it hollow) and circulating a heat transfer fluid through the hol- low section, see Figure 1.12.
The screw can also be heated by a cartridge heater. The heater’s electrical power has to be supplied through a slip ring assembly at the drive end of the screw. If the cartridge heater is equipped with a temperature sensor, the power to the heater can be controlled to maintain a constant temperature.
Temperature control of the screw
Figure 1.12 Circulating a heat transfer fluid through the screw
7. The Breaker Plate
The breaker plate is located at the end of the barrel. It is a thick metal disk with closely spaced holes as shown in Figure 1.13.
Figure 1.13 Example of a breaker plate
The main purpose of the breaker plate is to support a number of screens, located just ahead of the breaker plate. The screens are used to trap contaminants so they do not end up in the extruded product. Usually, several screens are stacked together starting with a coarse screen followed by increasingly finer screens and then a coarse screen again right up against the breaker plate. The plastic melt thus flows
Trap for contaminants
through screens with increasingly smaller openings. The last coarse screen acts merely as a support for the finer screens. The collection of screens is called the screen pack.
8. The Screen Pack
The screen pack is not only used to trap contaminants; in some cases the restriction of the screen pack is increased to increase mixing in the extruder. This works to some degree; however, the mixing can be improved more efficiently by adding mix- ing sections to the extruder screw, as discussed in Chapter 5. The most common filters are wire mesh screens. The mesh number of the screen represents the num- ber of wires per inch (25 mm). The higher the mesh, the more wires per inch and the smaller the openings of the screen.
Figure 1.14 shows the relationship between the mesh value of the screen and the micron rating. The micron rating indicates what size particles the screen is able to trap. The higher the mesh, the lower the micron rating and, therefore, the finer the screen. A typical screen pack can consist of a 20-mesh screen, followed by a 40-,60-, and 80-mesh screen with a final 20-mesh screen for support against the breaker plate.
Figure 1.14 The micron rating vs. the mesh value for wire mesh screens
There are a number of different filter materials. Wire screens are the most common. Several types of wire screens are available, such as the square mesh with plain weave and the square mesh with Dutch twill. There are also depth filtration media, such as sintered metal powder and random metal fibers. Advantages and disadvan- tages of different filter materials are shown in Table 1.1.
Other filter materials used in some cases are plates with small holes and filters made of sand. Plates with small holes are useful in filtering out coarse contaminants and, for that reason, are used for pre-filtration. Sand is inexpensive and be- cause of the large volume, sand filters can operate for long periods before they have to be changed. A drawback of sand is that the filtration action is not uniform. Also, the run-in period for sand filters can be long and the installation is complicated.
Table 1.1 Comparison of Different Filter Media
Wire mesh square weave Wire mesh
Dutch twill Sintered
metal powder Random metal fibers
Gel capture Poor Fair Good Very good
Contaminant capacity Fair Good Fair Very good
Permeability Very good Poor Fair Good
Price Low Fair High High
8.1 Screen Changers
In some cases, screens have to be replaced at short intervals, for instance, every two hours. This may happen when the plastic contains a substantial level of con- taminants. In such a situation, it can be advantageous to use a screen changer, which is a device that allows a quick change of the screens. Some screen changers allow the extruder to keep running while the screens are changed; these are called continuous screen changers. Screen changers are useful when pressure increases rapidly at the screens. Screen changers can be manual or automatic. In many auto- matic screen changers, the screens are changed when the pressure drop across the screens reaches a preset value.
There are various types of screen changers, including manual screen changers, hydraulic screen changers, semi-continuous screen changers, screen changers with a continuous moving screen, and rotary type screen changers. Manual screen changers are used on smaller extruders, up to about 90 mm or 3.5". They use a slide plate design with two circular screen blocks; one is in the melt flow at all times. When the pressure builds up to a certain level, the operator uses a hand lever to insert a new screen block.
Hydraulic screen changers use a hydraulic ram to push the block containing a new screen into the melt stream, see Figure 1.15. They can be used with larger extrud- ers. Semi-continuous screen changers allow the screen to be changed without affecting the melt flow. In most units, trapped air is prevented from going into the melt stream by a bleed valve. Even if the entrapped air can be eliminated com- pletely, there may still may a pressure spike when a new screen is moved into position.
Fast screen exchange
Types of screen changers
Figure 1.15 Typical slide plate screen changer
Continuously moving screen
Dies for various extrusion products
Die flow channel
In screen changers with a continuously moving screen, a continuous band of screen material passes across the melt flow at a speed determined by the pressure differ- ence across the screen. The rotary screen changer uses a moving wheel with 10 to 16 kidney-shaped cavities containing the screens. Each cavity moves slowly through the melt stream. This is similar to the continuously moving screen, except that in the rotary screen, the screen itself is not continuous because of the webs between the cavities. The webs, however, are relatively thin, so that at any given time at least 90% of the channel is occupied by the screen. Rotary screen changers use separate screen inserts that are changed by the operator. The advantage of the screen changer with the continuously moving screen and the rotary screen is that pres- sure disturbances during screen changes are minimized.