Processing, thermoplastics channels

Recendy, storm doors have been constmcted of advanced thermoplastic composites. Stampable, glass-mat reinforced polypropylene sheet is used to create a high strength outer skin. These compression molded skins are welded together using a friction or ultrasonic process then injection molded with a polyurethane foam core to produce an insulated structure. New technology for window frames incorporate the pultrusion of frame channels to produce a thermoset composite channel that can be filled with fiber glass for further insulation enhancement (12,31,33,34,48,49,54—56,60—67). 

Fig. 6.12 Down-channel velocity distribution for pure drag flow from Eq. 6.3.17 for various HjW ratios. [Reprinted by permission from E. C. Bernhardt, Ed., Processing of Thermoplastic Materials, Reinhold, New York, p. 290 (1959).]...

The laminar character of the flow is violated in thermoplast and thermoplast-based compositions flow throught the channels at high volume consumption rates. In extrusion formation processes this leads to undesired distortion of the extrudate... 

Figure 1 Typical stages in thermoplastics processing. (1) Solids flow (2) screw filling and solids conveying (3) melting (4) polymer melt shearing/pumping (5) shear flow in channels (6) die swell (7) elongational flow (8) volume change under pressure (9) thermal conduction and (10) shrinkage.

Flows can be classified into streamline, when particles in the fluid follow paths (streamlines) that remain constant with time, and turbulent, when vortices cause unpredictable changes in the flow pattern with time. The changeover occurs at a critical value of the Reynolds number, which is defined as the melt velocity, divided by the viscosity times the channel diameter. The high viscosity of thermoplastic melts causes velocities to be low. Hence, the Reynolds number is very low and the flows are streamline. We will consider steady flows, and ignore the start and end of injection and blow-moulding flows, when the melt accelerates and decelerates, respectively. However, in the RIM process (Section 5.6.5), turbulent flow of the low viscosity constituents in the mixing head achieves intimate mixing. 

If the filaments are not wound onto a mandrel but simply gathered into relatively large bundles and led slowly into a heated consolidation zone, emerging from a die partly or completely cured, the primitive essentials of pultrusion are present (Fig. 2.3(b)). This process began as the thermosetting resin equivalent of extrusion, that is, it was a continuous operation for manufacturing profiles, such as rod and channels. The method has been extended to fibre reinforced thermoplastics. 

It is important to note that the back flow increases in proportion to the third power of the depth of the thread. For this reason, screws with deep cut channels are not the best choice for thermoplastics (but are okay with thermosets). If the speed of single screw extruders is increased, especially in the processing of high molecular weight viscous melts, the extrudate obtained may be rough, unattractive in appearance, and unsalable. Such results also can occur with slower-running machines using relatively deep cut screws in conjunction with extrusion dies of low resistance to flow. 

An alternative method to avoid the problems associated with high molecular weight materials has been described by Li et al. (2004). They used an approach where the periodicity was introduced using monodisperse silica microspheres (in this example with a diameter of 700 nm) these spheres were sintered at 950 °C. This process introduces connections between the beads and this provides channels for subsequent removal of the silica by etching. The array thus produced was used as a template and a solution of a thermoplastic elastomer in... 

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