Profile dies

As the screw turns, it conveys the pellets from the feed zone towards the melting zone. A combination of external heating and mechanical work melts the polymer as it is transported towards the metering zone. The metering zone pumps a uniform stream of molten polymer to a forming device, such as a profile die. Other types of extruders that employ two or more screws are commonly used for compounding polymer blends. The principles of twin screw extrusion will be discussed in Chapter 12. 

Sketch the opening of a profile die that would produce the following products ... 

Profiles are all extruded articles having a cross-sectional shape that differs from that of a circle, an annulus, or a very wide and thin rectangle (flat film or sheet). The cross-sectional shapes are usually complex, which, in terms of solving the flow problem in profile dies, means complex boundary conditions. Furthermore, profile dies are of nonuniform thickness, raising the possibility of transverse pressure drops and velocity components, and making the prediction of extrudate swelling for viscoelastic fluids very difficult. For these reasons, profile dies are built today on a trial-and-error basis, and final product shape is achieved with sizing devices that act on the extrudate after it leaves the profile die. 

Streamlining of the complex profile dies is as necessary as with any other die shape, but obviously more difficult. For this reason, plate dies (82) made up of thin plates inserted in a die housing one behind the other, are common. The channel cross-sections in the individual plates differ in such a way as to streamline the polymer melt into the final plate. This construction makes both die modifications and die machining easier. In such complex dies, even approximate design expressions have not been developed yet in practice, the repeated filing off of metal in the approach plates achieves the desired shape. 

The availability of powerful three-dimensional flow computer simulation packages and personal computers capable of handling them is gradually transforming profile die design from an empirical trial-and-error process to one where design optimization benefits from computational results. Sebastian and Rakos (83) were the first to utilize realistic computational fluid-mechanical results in the design of profile dies. 

Different extrusion profile processes can be used such as a robotic profile. A robot delivers the hot melt over or around a substrate. This robotic extrusion process can uses a flexible, heated, high-pressure hose that is connected to the extruder s die exit. The hot melt travels through the hose. At the end of the hose is a nozzle the tip of this nozzle is, in effect the actual profile die. A computer-regulated multiaxis robot controls the positioning of the profile nozzle die. The nozzle/die is guided by the robot to deposit the profile s hot melt on, as an example, a substrate that is on a multi-station rotating table supporting other substrates to be covered. 

The flat dies, or slot dies as they are sometimes called, are used to produce webs in a variety of processes. They all have an interior manifold for distributing the plastic and lips for adjusting the final profile of the web (extrudate). Some dies have movable restrictor bars for changing the manifold for proper melt distribution (Figure 17.10). All flat dies have flexible lips that can be adjusted by bolts to remove humps or bumps in the web s profile. Die lips can have their adjustment bolts push only, where internal plastic melt pressures are adequate to keep the lips positioned against the bolts, or can be push/pull for low pressure applications. Direct acting or differential thread designs (for minute adjustments) are available. Profile variations of at least 3% or less can be achieved with flat dies. 

Computer-controlled automatic profile dies with electrical controlled sensors in closed-loop control systems have developed greater efficiency and accuracy to extrusion coating, cast film, and sheet lines. A scanner measures the web thickness and signals the computer, which then converts the readings to act on thermally actuated die bolts. The individual adjusting bolts expand or contract as ordered by the computer to control the profile. The more sophisticated systems measure adjusting bolt temperature and provide faster response time with less scrap and quicker startups. The scanner is typically an infrared, nuclear, or caliper-type gauge. 

Solid profiles can be simple flat plate dies with finished land geometry and pre-land dimensions determined by experience and trial in conjunction with sizing plates. If hollow shapes are extruded, supports are necessary, and tubing applications can have inflation air holes. Most of the profile dies, particularly those used in long production runs, require precision dies to meet very close tolerance requirements. 

The production of rigid PVC profiles is described with reference to state-of-the-art twin screw profile extruders and downstream equipment from Battenfeld. Features of the machinery are described, and particular details are given of the profile dies, calibration block, calibration table, haul off and profile saw. 

Profile dies are commonly made with a series of plates that are stacked together to form a complex... 

A commercial polymer flow simulation program was used to simulate the three-dimensional die flow and heat transfer through the U-profile die, shown in Fig. Because the last two die plates have the... 

Fig. 11 Profile vacuum calibration and take-off. (A) Section view of calibration process (1) melt enters profile die (2) profile die stack (3) molten profile extrudate (4) calibrator (cools, shapes, and sizes extrudate) (5) solidified plastic (6) puller (7) orientation of profile. (B) Partially disassembled calibrator (8) profile passing through calibrator (9) upper calibrator stack (10) lower calibrator stack (11) upper vacuum channel (12) lower vacuum channel (13) core feature of lower calibrator (14) cooling line.

Fig. 13 Computational model for U-Profile die design (A) preland, die land, and free surface as computational domain (B) finite element mesh (symmetry exploited to reduce computational requirements) (C) boundary conditions for simulation of polymer fiow through die and extrudate free surface and (D) relevant profiles (1) preland inlet (2) die land (uniform along flow length) (3) final free surface (target extrudate profile) and (4) symmetry plane.

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