When a melt pump is the method selected for die pressurization and metering, the extruder and all upstream equipment will be contributing to the polymer melt properties.
When an extrusion system’s output is not completely driven by the extruder, the functions of the polymer feeder and extruder are not constrained as they are when the extruder determines the rate and quality of output. Melt conditioning becomes crucial, including preheating of the resin, controlled feed of the material to the extruder, melt homogeneity of the polymer in the extruder, and delivery at the required pressure and rate to the melt pump to avoid cavitation. All these system elements are discussed below.
Heating and drying the resin.
The incoming plastic material, in the form of pellets or powder, needs to be heated to a consistent temperature while in the feed hopper. In the case of hydroscopic materials such as acrylic and ABS, the equipment includes a hopper dryer unit that heats the material and reduces the moisture by a recirculated airstream with very low humidity, produced by either a desiccant bed or a refrigeration unit. To improve the output rate in the most effective way, the material should be at the highest temperature that does not cause bridging in the feed throat or other operating problems. When working with non-hydroscopic materials such as polyolefins and rigid PVC, drying is not required. The heat input will, however, contribute substantially to the output capability and stability of the process.
Probably one of the simplest ways to handle the preheating is to use a hopper dryer unit without the dehumidifying device. Some such units can be operated with the dehumidifier off.
The energy load on the extruder is substantially reduced by using the hopper preheating. For example, rigid PVC can be brought to a temperature of 175F from a room temperature of 75F. With a specific heat of 0.35, this represents a heat input of 35 Btu/lb, which is 20 percent or more of the total needed to melt the material. For materials with higher melting points, especially crystalline ones that soften close to the melt temperature, the heat input is a larger fraction of the total needed. Since the remainder of the melting heat is generated by the mechanical working of the plastic, the result is lower horsepower consumption or much higher melting rates for the extruder.
Powders represent a somewhat different problem. To preheat powders, a fluidized-bed heating unit is usually needed. Another situation exists with oxygen-sensitive materials that may be degraded by the heated air. These can be dried with an inert gas such as nitrogen. Since the heated gas is recirculated, little is needed, and it does not represent a significant production cost.
The beneficial effect of the hopper preheating is to make the Inlet-resin feed temperature uniform and to reduce the energy load on the extruder. The energy-load reduction occurs in single-screw extruders, twin-screw extruders, and various other melting extruder devices.
In normal operation of single-screw extruders, it is uncommon to use a weigh feeder or volumetric feeder to supply the resin, because the machine is often operated in a starve mode and the normal melt-bed configuration is disturbed, resulting in erratic output. The feed may be controlled, but surging and other output inconsistencies frequently occur. However, when the extruder’s function is to deliver plasticated material to the melt pump this condition does not apply. It is possible to use feeders, both of the weigh and volumetric type, to regulate the throughput of the extrusion device, in twin-screw as well as other types of extrusion machines.
The extruder is the most important element in the melt-delivery system. Practically speaking, the vast majority of machines in use are single-screw extruders.
To plasticate material in most extrusion equipment, using typical materials, heat generation is done largely by means of the shear work done on the material by the screw in addition to heat transferred through the barrel wall. When used to feed a melt pump, the operating mode can be quite different. For example, the use of a preheated feed will materially reduce the amount of shaft power needed for the shear to plasticate the material. Also, the equipment can be set up so that a larger percentage of heat is transferred into the plastic through both the barrel wall and the screws.
One approach to supplying additional heat is to place a cartridge heating element into the core hole of the extruder screw. Barrel heaters are then used to increase the amount of transferred heat. Operating in this mode, an extruder is able to deliver much greater than the normal output, because the only function of the extruder is to supply the melt pump at a pressure needed to prevent cavitation.
The same sort of changes can be done to intermeshing co-rotating and counter-rotating twin-screw machines. The machines can be operated with high barrel temperatures and with internal heat supplied to the screws. Adding the screws as heat-transfer surfaces approximately doubles the available heat-transfer area. Be aware that overheating the screw will cause a non-conveyance situation. The feedscrew pumps because the polymer pellets are sticking to the barrel and slipping on the screw. If the screw is overheated, the pellets will stick to the screw and not be conveyed, stalling the process.
Note that there is one very important consideration in operating the extruder in this mode. Since shaft power is reduced, the amount of shear imparted to the material is substantially reduced. A significant degree of shear mixing is essential to achieve effective melting and mixing of most plastics. Mixing elements in the machine must supply the shear. In twin-screw machines kneading-type elements are typically incorporated into the screw to supply the shear. Straight and spiral Maddox-type mixers or other mixers such as the Egan and Rapra can be incorporated in the pin mixers of single-screw machines to intensify the shear levels for proper plastication.
The screw design must be considered for its effect on plastication, although the design is not as critical in the operating mode for the machines introducing the melt into a melt pump. Special screw designs are available for use specifically with the melt pumps. Head pressures must be low for screw operation. To prevent cavitation in the entry port of the melt pumps, the typical pressure required is in the range of 500 to 1000 psi. It is possible to retrofit the melt pump and conditioning units on machines that have worn barrels and screws. In most cases, loss of head pressure and variations in output are the first Indicators of screw and barrel wear. The melt pump can operate with low inlet pressure. Low inlet pressure will smooth out variations in output from the extruder. If the machine suffers from excessive wear, it will not plasticate the material, but machines that would normally need barrel and screw replacement can be operated for a long time past this point when used in conjunction with the melt pump.
The control interface between the melt pump and the extruder unit is important. The pressure to the inlet side of the melt pump must be controlled to ensure high enough levels to prevent cavitation and low enough levels to prevent excessive pressure buildup, which can damage the equipment. The pressure is sensed with a melt-pressure transducer that feeds back a signal through an appropriate control system to the extruder delivery system.
This can be done in two ways. The drive units of most extruders are SCR controlled. With the pressure signal properly set and conditioned, the screw speed can be changed in response to the pressure readings. This will have an effect on the melt-bed pattern, which would be a problem in normal operation; however, in this case the melt pump controls the output.
For equipment having a weight or volumetric feeder, the pressure signal can also be used to change the rate of delivery of material to the extruder. This is a slow-response control loop, but it can be used in conjunction with the screw speed control for long-term stability of head pressure. Commercially available electronics packages can do the feedback control.
In summary—to operate a melt-pump/melt-conditioning system, a plasticating device must be used. It can be a single-screw extruder, a twin-screw extruder, or one of the less commonly used continuous plasticating machines. The unit is equipped with hopper preheating (either with or without drying elements) to introduce the resin into the extruder with as much heat content as is practical. The extruder can be equipped with weight or volume feeder control useful in the overall control. The extrusion device is operated as an efficient plasticating device without regard to normal output-stability requirements, since the output is completely controlled by the melt pump.
Since this arrangement involves much lower shear than normal, the extruder will have additional shear elements Incorporated. Feedback control is used to maintain proper head pressures by screw and/or feed control. The reduced output-stability requirements permit the system to be used on equipment with somewhat worn screws and barrels.