Melt-fed extruders

This chapter will present the three most common uses for melt-fed extruders (1) pumping of resin from a compounding operation to a pelletizer, (2) large-diameter extruders used for finishing sections in resin production plants, and (3) secondary cooling extruders for foam sheet lines. 

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Large diameter, melt-fed extruders are commonly used for the final devolatilization and pelletization of LDPE and PE copolymers in resin manufacturing plants. A full description of this type of extruder and process is provided in Section 15.3. Simulation of these processes is complicated by the multiple flights used in the design and the high H/W aspect ratios of the channels. The processes can be simulated from the feed hopper to discharge, however, since they are not required to convey solids and melt resin. This section will show the requirements and difficulties for simulating these processes. 

The melt-fed extruder is typically designed with a relatively short barrel length of 6 to 1L/D. Since the resin is molten when it enters the extruder, the motor size is relatively small compared to a plasticating extruder with the same barrel diameter. The extruders are typically starve-fed. That is, the extruder is operating at a screw speed that is higher than required, and thus the rate is not controlled by the screw speed, instead, the rate of the overall process is controlled by the feedstock addition rate to the compounding machine. 

Figure 15.2 Schematic showing the channel depths for a 304.8 mm diameter melt-fed extruder designed to pump an ABS resin...

Figure 15.3 Simulated axial pressure and temperature for a melt-fed extruder running a compounded ABS resin. The process is operating at a rate of 5500 kg/h and a screw speed of 70 rpm...

Common Problems for Melt-Fed Extruders on Compounding Lines... 

Table 15.1 Typical Extruder Specifications for Melt-Fed Extruders Used in the Manufacturing of LDPE Resins (Some of the Data Courtesy of Peter Hunziker of Berstorff USA). The Data Are for Resins with an Ml of 2 dg/min (190 °C, 2.13 kg)...

Early design and simulation of large-diameter, melt-fed extruders were described by Fenner [17]. A numerical simulation of the axial pressure and temperature fora screw similar to that shown in Fig. 15.8 is shown in Fig. 15.10. This simulation was performed using a three-dimensional method using a finite difference approach. The process starts with an LDPE resin (2 dg/min, 2.16 kg, 190 °C) in the low-pressure separator at a pressure of 0.04 MPa (gauge) and a temperature of 230 °C. 

Figure 15.12 Schematic of a visco-seal on the shank of a melt-fed extruder screw...

Large-diameter, melt-fed extruders are almost always flood-fed as described above. As such they will typically operate at a specific rate that is very predictable based on numerical simulations. The extruder will operate at these rates in a very stable manner. That is, the motor current and discharge pressure are essentially constant with time. An improperly designed extruder or a malfunctioning extruder can, however, operate at much lower specific rates and in a very unstable manner. This mode of operation is often referred to as slipping because the feed section of the... 

Elastic melt extruders Screwless extruder Screw or disk type melt extruder Melt fed extruder... 

Rearward devolatilization is used on melt fed extruders. In rearward devolatilization, the volatiles are extracted upstream of the feed opening of the extruder. This is shown schematically in Fig. 8.44. 

The main function of an extruder is to develop sufficient pressure in the material so as to force the material through the die. The pressure necessary to force a material through the die depends on the geometry of the die, the flow properties of the material, and the flow rate. Basically, an extruder is a machine capable of developing pressure. In other words, an extruder is a pump. Aplastics extruder is a pump for plastic materials. However, this is not to be confused with aplasticating extruder, which is a machine that not only extrudes but also plasticates, or melts, the material. A plasticating extruder is fed with solid plastic particles and delivers a completely molten plastic to the die. On the other hand, a machine that extrudes molten plastic without melting it is called a melt-fed extruder. 

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