Extruded strands

Figure 1 Optical micrographs in the flow direction of the extruded strands of the PP-LCP blends exhibiting viscosity ratios of (a) t7lcp i7pp = 0,6, and (b) 2.8 [44].

Figure 4 Optical micrographs from the skin region of the single-screw extruded strands processed at cylinder temperatures of (a) ISO C, (b) 200°C, (c) 230°C, (d) 250°C, and (e) 280°C.

To achieve a compound that meets these feeding requirements, compounders form their product into small pellets. To obtain these small pellets, they use specialized dies consisting of a metal plate with many small holes through which the molten compound is forced. This part of the process is very similar to how spaghetti is formed from pasta dough. To generate small pellets, the extruded strands are chopped into small pieces by rotating blades. 

One way to characterise thermoplastic melts is by using a Rheotens machine (31) which subjects an extruded strand of melt to tensile elongation at a fixed velocity while measuring the tensile force. The typical response (melt tensile force versus draw velocity) for branched PP extends to twice as high a draw velocity, with six times the force, than that for linear PP. Alternatively a... 

Mixing of the crystalline ingredients of oxidant, fuel elastomer binder (butadiene-2-methyl-5 vinyl-pyridine copolymer plasticized with the formal of diethylene glycol monobutyl ether) 26% by vol is done in a sigma blade mixer. The completed mix is blocked in a hydraulic press to form a chge suitable for extrusion. The extruded strand is cut into grain blanks which are cured at... 

It is manufd by using conventional solvent propellant processing equipment. The extruded strands are cut to fit lengthwise in the primer body. Eimite is a satisfactory igniter material for artillery-type primers and can be a suitahle substitute for BlkPdr. See also US Spec AEZ—54, Rev 1(16 Aug I960)... 

After leaving the die, the extruded strand shows an instantaneous increase in thickness. This is a consequence of the elastic elongation, in particular induced by the elongational flow in the die, which spontaneously recovers when the material leaves the die. This so-called die swell or extrudate swell has to be taken into account when designing the die in particular, when the cross-section of the extrudate is not circular, not only a diameter increase but also a distortion in shape occurs. The distortion can be prevented by adjusting the shape of the die opening. 

Zhang et al. studied the effect of conductive network formation in a polymer melt on the conductivity of MWNT/TPU composite systems (91). An extremely low percolation threshold of 0.13 wt% was achieved in hot-pressed composite film samples, whereas a much higher CNT concentration (3-4 wt%) is needed to form a conductive network in extruded composite strands. This was explained in terms of the dynamic percolation behavior of the CNT network in the polymer melt. The conductivity of extruded strand showed a hopping resistivity dominated behavior at low concentrations and a dynamic percolation induced network dominated behavior at higher concentrations. It was shown that a higher temperature can reduce the filler concentration required for the dynamic percolation to take effect. 

The dried components were melt mixed in a single-screw laboratory extruder. The extruded strands were regranulated and the extrusion cycle was repeated up to five times. After every cycle, samples were taken for the investigations. By the repeated extrusion, different degrees of dispersion were realized. 

With axial extruders, it is difficult to cut the extrudates into uniform lengths because the material extrudes faster on the outside of the die plate than nearer to the center (Fig. 8.28). This phenomenon is a result of the complex flow and pressure patterns that are developed in the extrusion zone. A rotating knife or an oscillating wire can be used to separate the extruded strands into pellets but, nevertheless, their length will usually vary considerably. 

Case III shows another aspect of the change of the material during extrusion. If certain volume throughputs, governed by the size and shape of the die, are exceeded, the extrusion pressure will gradually increase, then suddenly decrease and then increase again. The extruded strand becomes... 

S. Saikrasun, S. Bualek-Limcharoen, S. Kohjiya, and K. Urayama. Thermotropic liquid-crystalline copolyester/thermoplastic elastomer in situ composites. I. Rheology, morphology, and mechanical properties of extruded strands. J. Appl. Polym. ScL, 89 2676-2685, 2003. 

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