Dies for extruders

Pipe and Tubing. A typical die for extruding tubular products is shown in Figure 4. It is an in-line design, ie, the center of the extmded pipe is concentric with the extmder barrel. The extmdate is formed into a tube by the male and female die parts. The male die part is supported in the center by a spider mandrel. Melt flows around legs of the mandrel and meets on the downstream side. The position of the female die part can be adjusted with bolts adjustment is required to obtain a tube with a uniform wall thickness. 

Die for Extruding a Square Extrudate (a) Explain why a die with a square cross section cannot produce an extruded product with a square cross section, (b) What cross section should the die have to produce a square-shaped product ... 

Plate die n. An inexpensive and easily modified die for extruding a plastic profile, into which a orifice of the desired shape has been machined typically by electrical-discharge machining. The plate is bolted to the front of a universal die body. 

Pipe can be coextruded with more than one layer, which results in a composite sandwich wall. Figure 10.4 shows a multimanifold die for extruding PVC or ABS pipe with three layers where the intermediate layer is a foam core. This results in a saving of weight the foam core pipe is 75-80% of the weight of solid pipe. Such pipe is used in city sewer applications. The ASTM standards for foam core pipe are F-268 for ABS and F891 for PVC. 

Figure 3-35 provides design features in extrusion dies that influence product performance. The guiding principle should be to keep it simple whenever possible, (a) The method of balancing flow to produce this shape requires having a short land where the thin leg is extruded. This design provides the same rate of flow for the thin section as for the heavy one. (b) This die for making square... 

The die-swell (extrudate swell) effect describes the significant expansion of the diameter of the fluid column after exiting from a small pipe (Figure 4.3.8(b)). Some polymer fluids can have a swelling of up to two or three times the exit diameter. A simple proposition for the mechanism of the die-swell phenomenon is that while the fluid is inside the exit pipe, it is subject to a velocity shear, similar to the pipe flow with a maximum shear stress at the wall [18]. This velocity shear stretches... 

Production of extruder dies for complex products, such as profiles, requires a method for accurately calculating the required shape of the die and the flow inlets. 

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. 

Figure 16.13 Extruding equipment. The extruder is opened in the middle of this picture to show the die for making over 1000 simultaneous strands. (Courtesy of BP Chemicals, Alvin, Texas)...

We now follow the modeling approach outlined for the melt extrusion process. We assume steady state conditions and a given mass flow rate then, starting from the hopper, where initial conditions are known, calculations are made in finite steps, ending up at the die, with extrudate pressure, mean temperature, and solids content. If the flow rate at these conditions does not match that of the die, or if the calculations break down for some reason (e.g., insufficient solids conveying), calculations are repeated at a new mass flow rate. 

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