Extruder hydraulic drives

Some early DC extruder drives used fixed-speed AC motors to drive DC generators that produced the variable voltage for the DC motor. Nowadays, the DC motor drives usually operate from a solid-state power supply, since this power supply is generally more cost-effective than the motor generator set. The DC motor drive can be simpler and cheaper than the variable frequency drive, even when the higher cost of the DC motor is included. The smaller number of solid-state devices tends to give the DC drive a better reliability than the variable frequency drive. Brushes and commutator maintenance is the principal drawback to the use of DC motors. If the drive has to be expiosion-proof, the additional expense associated with this option may be quite iarge for a DC drive, more so than with a variable frequency AC drive or a hydraulic drive. A schematic of the DC drive is shown in Fig. 3.4. 

Despite the attractive features of hydrostatic drives, they are rarely used on regular (non-reciprocating) extruders. The reason for this situation is not obvious since the hydrostatic drive is in many respects competitive with, for instance, the SCR DC drive and in some respects better e.g., there is no need for a gearbox. A possible reason is that the hydrostatic drive is still regarded with suspicion by many people. This is because early hydraulic drives were not very reliable and accurate. However, this situation has changed dramatically over the years, but the hydrostatic drive still seems to suffer from its early unfavorable reputation. Very few U.S. companies supply hydraulic drives for extruders Feed Screws Division of New Castle Industries and Wiimington Plastics Machinery. It is claimed [5] that hydraulic drives are less expensive than DC drives on smaller extruders, up to about 90 mm. 

Fluid film thrust bearings have been applied to extruders on a few occasions. Their load carrying capability at low speed is generally poor and a loss of fluid film would have disastrous results. If a hydraulic drive is used to turn the screw, application of hydraulic thrust bearings may deserve some consideration. Reference 28 describes an extruder with hydraulic drive that incorporates a patented thrust bearing assembly with hydraulic axial screw adjustment. By measuring the pressure of the hydrostatic chamber of the thrust bearing, the pressure in the polymer melt at the end of the screw can be determined. 

Extrusion at a Point away from the Head The adhesive may be extruded from a point away from the applicator head and feeding the material to an on/off valve at a given rate. A system of this type is now available it is based on a hydraulic drive unit which drives the extrusion cylinder piston. The size of the cylinder can be readily tailored to suit the specific applications without imposing excessive loads on the robot wrist. 

Most ram extruder designs allow for interchangeable dies. The hydraulic unit used to drive the ram should be sized to handle the pressure requirements of the largest or most intricate profile contemplated. The axial die pressures during compaction are high, approximately 44 MPa. The compaction force (F required to advance the compacted plug can be expressed by Equation 7 (19) ... 

The three basic types of drives are alternating current (ac), direct current (dc), and hydraulic. While a number of drives have been used in extruders, the most common are dc silicon control rectified (SCR) and ac adjustable frequency drives. A dc SCR drive is a sohd-state dc rectifier connected to a dc motor. The base speed is about 1 percent, but reduces to 0.1 percent when a tachometer is added to the drive. These drives are very reliable, can handle high starting torques, can maintain a constant torque through a speed range of 20 1, and are relatively easy to maintain (that is, replace brushes). However, since the drives have brushes, they are limited to noncorrosive polymers. 

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