Extruders, extrusion melt flow

EPDM-ZnO-stearic acid systems could not be extruded even at 190°C. This is not unexpected since the material, in the absence of zinc stearate, shows no transition from the rubbery state to the viscous flow state (Fig. 1). In the presence of 10 phr of zinc stearate, the m-EPDM-ZnO-stearic acid system could be extruded but melt fracture occurred at a lower temperature (150°C) at all shear rates. At 160°C and 170°C, however, the extrudates showed melt fracture only at high shear conditions. At 20 phr loading of zinc stearate, melt fracture of the extrudate occurred at high shear conditions at 150°C, but at higher temperatures no melt fracture occurred and the extrusion was smooth under all shear conditions. At 30 and 40 phr loadings of zinc stearate, the extrudates were smooth under all shear conditions at all temperatures. 

Figure 5.5 shows a schematic diagram of a melt indexer (which is also sometimes referred to as an extrusion plastometer). To determine the melt flow rate of a polymer resin, we place a suitable mass of it into the barrel, which is pre-heated to a standard temperature appropriate to the polymer. We then place a weighted piston on top of the sample. After allowing the polymer to reach the temperature of the barrel we allow it to extrude from the capillary orifice. The melt flow rate is the mass of polymer in grams that extrudes in ten minutes. 

Polypropylene (PP) films were first produced by extrusion casting. Polymer is extruded through a slit or tubular die and quenched by cooling on chill rolls or in a water bath. Cast films can be sealed over a wide range of temperatures and do not shrink in a steam autoclave, Polymers with melt flow rates below 5 dg/min are usually used to maintain the stability of the extra date. Higher clarity films are produced using random copolymers. 

The melt flow indexer. The melt flow indexer is often used in industry to characterize a polymer melt and as a simple and quick means of quality control. It takes a single point measurement using standard testing conditions specific to each polymer class on a ram type extruder or extrusion plastometer as shown in Fig. 2.45. 

Repeated Extrusions. 0.25% Tris(nonylphenyl) phosphite and 0.25% polymeric phenolic phosphite were added to unstabilized polypropylene, and the resins were run through the laboratory extruder four times. The barrel temperature was 450°F., and the stock temperature of the extrudate was approximately 500°F. Melt flow rate of samples taken after each pass was measured according to ASTM 1238-62T. 

Extrusion Stability (20). Processing stabilities were also determined by six succesive extrusions of the resins through a 1-inch extruder and measuring the change in melt flow (ASTM D 1238-65T, Condition L) after each extrusion. The data shown in Figure 7 indicate that Resins C, E, and I are least stable and G, A, and D are the most stable. 

Because of its high melt viscosity it has no useful melt flow index. Conventional screw plasticizing extrusion and injection molding can noy process them. The processing methods used are compression molding, ram extrusion, ram injection, and warm forming of extruded slugs from powdered plastic. In turn many components are machined from semifinished products. 

Purpose of the screens is primarily twofold (1) to change the melt s spiraling motion, caused by the screw rotation and (2) to filter contaminants out of the melt. Most plastics contain contaminants and these particles can be conveniently removed by means of a screen placed after the extruder barrel and before the melt flow reaches the extrusion die. The simplest means for filtering plastic melts are woven wire mesh disks of about the same diameter as that of the extruder barrels. Several... 

Against the forces exerted by the melt flow, the screen packs are backed by a thick, densely perforated steel disk called a breaker plate. The outer rims of the breaker plate and of the screen pack fit into a round recess in the end of the extruder barrel and are clamped in place by the adapter flange of the adjoining piece of equipment, usually that of the extrusion die. To change a clogged screen pack, the die adapter flange has to be removed, the old pack taken out and replaced with a new one, and the equipment reassembled. 

Weld lines (also known as knit lines) are a potential source of weakness in molded and extruded plastic products. These occur when separate polymer melt flows meet and weld more or less into each other. Knit lines arise from flows around barriers, as in double or multigating and use of inserts in injection molding. The primary source of weld lines in extrusion is flow around spiders (multiarmed devices that hold the extrusion die). The melt temperature and melt elasticity (which is mentioned in the next section of this chapter) have major influences on the mechanical properties of weld lines. The tensile and impact strength of plastics that fail without appreciable yielding may be reduced considerably by in doublegated moldings, compared to that of samples without weld lines. Polystryrene and SAN copolymers are typical of such materials. The effects of weld lines is relatively minor with ductile amorphous plastics like ABS and polycarbonate and with semicrystalline polymers such as polyoxymethylene. Tliis is because these materials can reduce stress concentrations by yielding [22]. 

Table 3.12 shows that the melt flow index of HDPE in the presence of 50% wood flour decreases rather significantly, from 1.6 g/10 min for salt cedar to 0.2 g/10 min in the presence of pine. Fnrthermore, if we consider the initial MFI value for the neat plastic, a drop in MFI is qnite significant. For example, a polypropylene with MFI 23 g/10 min (MFI was 29 g/10 min after processing in the extruder at 100 or 300 rpm) was loaded with 20% bleached sulfite cellulose fibers, and after extrusion at 100 rpm MFI dropped to 2 g/10 min. With 30% loading with the fiber, MFI further decreased to 0.5 g/10 min [131]. 

Spider lines are essential elements in extruded composite hollow deck boards. Spiders in an extrusion die interrupt the melt flow, causing it to separate and then rejoin (see Figs. 15.6 and 15.7). This welding of the profiles creates the so-called spider lines along hollow composite deck board profiles. 

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