Extrusion barrel diameter

Instead of the extrusion barrel diameter, the same dimensions are frequently quoted when referring to the auger diameters, which are almost identical but are, in fact, approx. 1-10 mm smaller than the extruder barrel diameter. Table 3 gives a rough guideline as to how the extrusion barrel diameters can be allocated to the application range and type of products. 

Table 3 Range of application of extruders determined by the extrusion barrel diameter...

As plastics can have quite different viscosities, they will tend to behave differently during extrusion. Fig. 4.3 shows some typical outputs possible with different plastics in extruders with a variety of barrel diameters. This diagram is to provide a general idea of the ranking of materials - actual outputs may vary 25% from those shown, depending on temperatures, screw speeds, etc. 

The difference between a screen and a die plate extruder is quite substantial. While die plates are 2-30 mm thick, screens usually feature the same thickness as the hole diameter screens are rarely thicker than 1 mm. For extruders with the same barrel diameter, a radial discharge with a screen will have more than 6 times the open extrusion area than an axial discharge with a die plate.This has consequences for screw design but will also generally translate into higher specific capacity and cost advantages for the radial discharge extruder if the low density and small product diameter can be tolerated. 

Conical extrusion barrel. When using a conical extrusion barrel, the diameter of the auger is gradually reduced in the direction of feed, whereby the augers have a linear pitch. The pressure build-up is in this case generated by the conicity of the extruder barrel. An extrusion auger of pure conical design is however rarely employed nowadays. 

Enlarged extrusion barrel. With this design the auger diameter increases towards the front end this version is pre-eminent in the extrusion of slugs. The purpose of using a conical extrusion barrel is to facilitate backward release of air still entrapped in the material. 

Increasing the barrel diameter will increase the shear rate—other factors being constant. This will increase viscous dissipation and melt temperatures as discussed earlier. Another problem with larger barrel diameters is that the heat transfer surface area increases with the diameter squared, while the channel volume increases with the diameter cubed. As a result, the heat transfer becomes less effective with larger diameter extruders. It is well known in the extrusion industry that the ability to influence melt temperature by changes in barrel temperature is very limited for large extruders. 

The main design criteria of most TPE dies are to ensure that changes in flow channel diameter from the extruder barrel bore to the die exit are equal. Most of the viscoelastic materials exhibit a die swell on exit from a die. TPEs tend to show die swell significantly lower than that of typical thermoplastics. This swell must be taken into consideration in designing dies and adjusting extrusion condition to achieve a perfect profile. The die swell normally increases with increasing hardness and shear rate and decreasing temperature. 

Compression of the rubber compound as it travels up the barrel is developed in the extruder by either decreasing the thread pitch but maintaining a constant root diameter, or alternatively by increasing root diameter whilst maintaining constant thread pitch. Each of these situations increases the pressure as the rubber compound travels up the barrel. The last portion of the screw prior to the die entry, however, is maintained at a constant pitch or root diameter to enable stock to stabilise in characteristics just prior to entering the die head, to ensure uniformity for extrusion through the die. Conventional extruder screws achieve a compression ratio of 2.5 1. 

A three-dimensional simulation method was used to simulate this extrusion process and others presented in this book. For this method, an FDM technique was used to solve the momentum equations Eqs. 7.43 to 7.45. The channel geometry used for this method was essentially identical to that of the unwound channel. That is, the width of the channel at the screw root was smaller than that at the barrel wall as forced by geometric constraints provided by Fig. 7.1. The Lagrangian reference frame transformation was used for all calculations, and thermal effects were included. The thermal effects were based on screw rotation. This three-dimensional simulation method was previously proven to predict accurately the simulation of pressures, temperatures, and rates for extruders of different diameters, screw designs, and resin types. 

An extrusion trial was performed at the processor s plant using a 38.1 mm diameter production extruder, a proprietary screw design, and resin that had previously exhibited flow surging and reduced rate. The extruder was equipped with three barrel zone heaters with control thermocouples (labeled Tl, T2, and T3) and two pressure sensors. One pressure sensor was located in the midsection (zone 2) of the barrel (P2) and the other at the end of the barrel near the tip of the screw (P3). Both transducers were positioned over the top of the screw such that a pressure variation due to screw rotation would be observed. 

Figure 12.12 Barrel, discharge, and pump inlet pressures and motor current for stable and unstable extrusion for a large-diameter extruder running HIPS resin...

Fig. 9.20 Cross sections obtained from cooling experiments of a 2.5-in-diameter 26.5 length-to-diameter ratio screw extruder. Material rigid PVC. Operating conditions are listed in the figure Tb is the barrel temperature, N the screw speed, P the pressure at the die, and G the mass flow rate. Numbers denote turns from the beginning (hopper side) of the screw. The screw was of a metering type with a 12.5 turn feed section 0.37 in deep, a 9.5 turn transition section, and a 4.5 turn metering section 0.127 in deep. [Reprinted by permission from Z. Tadmor and I. Klein, Engineering Principles of Plasticaling Extrusion, Van Nostrand Reinhold, New York, 1970. The experiments were carried out at the Western Electric Engineering Research Center, Princeton, NJ.]...

Fig. 9.37 Simulated and measured pressure profiles for an LDPE extruded in a 2.5-in-diameter, 26.5 length-to-diameter ratio extruder, with a metering-type screw having 12.5 feed section with channel depth of 0.37 in and 4.5 turns of metering section of depth of 0.1275. The flow rate is 136 lb/h, the screw speed 60 rpm, and the barrel temperature was set at 300°F. The SBP is shown in Fig. 9.24. The screw geometry is shown at the top of the figure. Simulation was carried out by the first computer simulation package for plasticating extrusion developed by the Western Electric Princeton Engineering Research Center team (17). [Reprinted by permission from Z. Tadmor and I. Klein, Engineering Principles of Plasticating Extrusion, Van Nostrand Reinhold, New York, 1970.]...

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