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The extruder drive
The screw drive is usually an electric motor that supplies the power to turn the screw. The nominal speed of the motor is typically around 1800 rpm, while a typical screw speed is about 100 rpm. As a result, a speed reducer is needed between the motor and the screw. Various motors can be used for extruders. DC motors were most com- mon in the past, although AC motors are now starting to be used more frequently. Older extruders often use a DC brush motor with tachometer feedback. This yields a speed regulation of about 1% of full speed. One percent regulation may be acceptable for some products, but it may not be good enough for high precision operations, such as medical tubing, particularly when the extruder operates at low speed.
For example, if an extruder has a maximum screw speed of 100 rpm and a DC brush motor with tachometer feedback, the screw speed can vary 1 rpm, see Table 1.2.
Table 1.2 Screw Speed Variations at Different Screw Speeds
However, if the screw speed is reduced to 10 rpm, the screw speed can still vary 1 rpm. This 1 rpm variation now represents 10% ! Obviously, this would be too much for a high precision extruded product. Fortunately, nowadays we have drives with better speed control. For example, there are brushless DC drives with a speed regu- lation of about 0.01% of full speed. Drives with similar speed control are digital DC brush drives and variable frequency AC vector drives. Servo drives can achieve a speed regulation that is considerably better than 0.001% of full speed.
When we have a direct coupling between the motor and the gear reducer, we call this a direct drive, see Figure 1.19.
Figure 1.19 Example of extruder with direct drive
Some extruders have a belt transmission between the motor and the gear reducer;
these are called indirect drives, see Figure 1.20. Advantages of direct drives are:
no chance of slippage
The disadvantage of a direct drive is that it may be more difficult to change the re- duction ratio.
Advantages of indirect drives are:
ease of changing the reduction ratio
more freedom to position the motor
Disadvantages of indirect drives are:
chance of slippage
energy loss in the belts
more parts that can wear out and fail
Even though direct drives appear to be more attractive, many extruders are made with indirect drives. An example of an extruder with an indirect drive is shown in Figure 1.20.
Figure 1.20 Example of an extruder with indirect drive
2.The Speed Reducer
Issues to be considered with gear reducers
Fast and easy change of gear
A speed reducer is necessary because the speed of the motor is much higher than the speed of the screw. Typical reduction ratios range from 15 : 1 to 20 : 1. The most common reducers implemented with extruders are gear reducers using spur gears. A popular spur gear is the herringbone gear. The V-shaped tooth design practically eliminates axial loads on the gears.
Important issues with gear reducers are the reduction ratio, energy efficiency, power transmission capability, cost, and backlash of the gearbox. Backlash is basi- cally the slop in the gears. If the screw speed is maintained at a constant value, backlash is not a big issue. However, if the screw speed is changed quickly, as is done in some advanced control schemes, then the backlash should be minimal to maintain good speed control and to avoid rapid wear of the gears.
Gearboxes can be made with a quick-change gear provision. This allows a quick and easy change of the gear ratio. A quick-change gear provision can improve the flexi- bility and versatility of an extruder. Changing the gear ratio on a regular gearbox is a precision job and is quite time-consuming.
2.1 Thrust Bearing Assembly
Thrust bearings are necessary because the extruder screw has to develop substan- tial pressure to overcome the flow resistance of the die. Typical diehead pressures range from 7 MPa to 28 MPa (about 1,000 to 4,000 psi). The melt pressure at the end of the screw causes a thrust force on the screw, pushing it in the direction of the drive. The thrust bearings are necessary to take up the thrust load acting on the screw. A 150 mm (6 inch) extruder running at a head pressure of 35 MPa (about 5,000 psi) experiences an axial thrust of about 620 kN (about 140,000 lbf). The thrust bearing assembly may be connected to the gear reducer or it may also be part of the gearbox itself. An example of a thrust bearing assembly is shown in Figure 1.21.
Figure 1.21 Example of thrust bearing assembly
The ability of the thrust bearings to handle the thrust load on the screw is reflected in the B-10 life of the thrust bearings. The B-10 life is the number of hours that 9 out of 10 identical bearings last at a certain load and speed. The B-10 life for the extruder thrust bearings is usually given at a head pressure of 35 MPa (5,000 psi) and a screw speed of 100 rpm. If an extruder is operated 24 hours a day, the B-10 life should be at least 100,000 hours to get a useful life of more than 10 years out of the thrust bearing.
3. The Gear Pump
Extruders have some limitations, including the fact that high output stability is difficult to achieve. This is important in high-precision extrusion operations such as fiber spinning or medical tubing extrusion. The best output variation that can be obtained in a regular extruder is about one percent. To improve the output stability, a gear pump can be added to the extruder; it is placed between the extruder and the die. The gear pump consists of two intermeshing, counter-rotating gears, see Figure 1.22.
Expected useful life under load
Improving output stability
Figure 1.22 The gear pump
Problems with gear pumps
Material entering the gear pump is trapped in the space between two teeth and moves forward in a circular path with the gears. At the point where the teeth of the gears start to intermesh, the plastic melt is forced out of the gears and to the dis- charge of the pump. The action of a gear pump is similar to that of a bucket brigade. The “buckets” are filled at one end and emptied at the other end. If each bucket has the same volume and is filled completely, precise control of the flow rate can be achieved.
The conveying of plastic melt in the gear pump is achieved by forced conveying rather than by drag as is the case in the extruder. As a result, good output stability is easier to achieve with a gear pump than without one. Another advantage of the gear pump over the extruder is that it generates pressure more efficiently. As a re- sult, gear pumps make sense in the following applications:
High-precision extrusion where the output variability must remain less than 1%
Operations where the extruder does not have sufficient pressure generating capability; for instance, a vented extruder that has to operate at high barrel pressure
There are some operations where the use of gear pumps can create problems. One is when a plastic containing abrasive fillers or other abrasive ingredients is ex- truded, causing wear of the gear pump and reducing its pumping accuracy. Another potential problem can occur when a gear pump is used with a plastic that is suscep- tible to degradation. In many gear pumps, the plastic melt is used to lubricate the gears. This means that a small fraction of the plastic spends a long time in the gear pump, sometimes as long as 15 minutes or longer. The long residence time in the gear pump combined with the high temperatures can cause degradation.