Extruder specification drive

An enthalpy diagram also shows that the temperature increase in the example shown is directly linked to the specific energy input. Figure 6.6 gives the specific enthalpies for different products and converts them from kj/kg (left) to kWh/kg (right). In this example, the drive power of the extruder is converted into the dissipative product heat increase . 

The specific heating and cooling power is an often-underestimated source of error. It is physically possible for a large production extruder to require less heating and/or cooling power than a small laboratory extruder in terms of drive power (Fig. 11.15). Therefore, laboratory extruders are often operated adiabatically or only moderately heated in order to ensure comparability with a large extruder. 

There are extruders that are utilized for just one process or a specific task. In this case, it suffices to define exactly one speed for their drive. A fixed-speed motor is used and operated directly from the mains. In such cases, a manual switching stage is provided on the gear box so that it can run at a second constant speed. The screw speed typically ranges between 200 and 300 rpm. 

Medium Pressure Axial Screw Extruders Axial screw extruders normalty operate with low (see Section 8.4.1) or high (see Section 8.4.3) pressure. The basic principle of an axial screw extruder was shown in Fig. 6.4b.1 (Chapter 6) and Fig. 8.24 (Section 8.4.1). The pressure that is developed by the screw(s) depends on the power of the drive and the frictional resistance in the extrusion channel or other discharge device. Axial screw extruders that rely solely on the pressure developed by the rotating screw(s) employ hydrostatic pressure as the driving mechanism for extrusion. Such machines generally use high pressure. However, under certain conditions and for specific applications, some can be classified as medium-pressure agglomerators. 

A certain amount of energy has to be provided to heat and melt the materials above their transition temperature in the extruder. The energy required for melting can be approximately calculated from the typical specific enthalpy of each polymer. The energy is supplied either by heat transfer through the heated barrel walls or by dissipation, frictional heat, and mechanical energy supplied by the extruder drive. 

In this formula, E is the power provided by the drive unit, Q is the heat flow through the wall into the extruder. Hr is the (exothermal) reaction enthalpy per unit mass, Q AP is the energy needed to give a volume flow Q a pressure rise AP, AT is the temperature rise of the material, and is the conversion enthalpy (heat required for melting 1 kg of material), p and Cp are the density and the specific heat, respectively. With this simple balance it is possible to evaluate some overall effects without the need to solve the complete flow field and the energy balances. The temperature rise can be written as... 

A second consideration is the screw design. Most of the commercial lines have extruder screws that have been optimized to run olefin resins at the maximum output for the diameter of the screw. To do this, they need to design for the specific melt flow properties (rheology) of the resin. PLA has a significantly different rheological behavior from olefin materials. It does not shear thin as readily and therefore typically requires additional power input from the drive. This additional power input manifests itself as an excessive increase in melt temperature. Quite often, if the drive has enough power to process PLA, it will be screw speed limited (and hence rate limited) by excessive melt temperature. 

Table 25 shows typically worst-case torque requirements for four extruder sizes. The range of screw speed represents plastomers at the low end and LDPE at the high end. The specific torque value can be simply mul-tiphed by the maximum screw speed to determine the recommended drive size. The transition from LDPE to plastomers is the most severe. When running LDPE and plastomers on the same machine, field weakening must be used to satisfy the increased screw speed and lower torque requirements of LDPE. The processing performance of narrow MWD, branched, unimodal and bimodal metallocene polymers is illustrated in Fig. 56. 

Standard commercially available single-screw extruders can be used to process SPS with some attention to specific machine design parameters. Specifically, due to the higher melt point (270 °C) and semicrystalline nature of SPS attention must be paid to the extruders heating capabilities, screw design and drive capability. 

Extruder Drives The higher specific heat requirements of SPS and lack of preheating through drying necessitates extruder drive with greater capability. On a theoretical basis, extrusion of SPS requires 2.5-2.75 times the drive horsepower typically needed to process a preheated polyethylene terephthalate (PET). For preheated PET extrusion, extruder drives are designed for approximately 12.01b/h/hp room temperature SPS would require approximately lOlb/h/hp [13]. 

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. 

The vast m ority of adjustable-frequency AC controls applied to date have been on low-performance applications such as pumps, fans, and mixers. Only recently have significant numbers been applied to applications such as extruders, winders, and coordinated web processes that meet the criteria above. As improvements in control technology make these applications commonplace there is a need for definite purpose motors designed specifically to optimize the performance of the drive. 

The power consumption of an extruder was measured with a standard wattmeter connected to the extruder drive unit. These results were converted into an index of specific mechanical energy (SME) consumption according to an equation formulated by Levin [2, 3] ... 

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