Screw compression ratio

Fabrication. Acetal resins are most commonly fabricated by injection mol ding. A homogeneous melt is essential for optimum appearance and for performance of injection molded parts. A screw compression ratio of no less than 3 1 is advised and the size of the injection molded shot should be 50 to 75% of the rated capacity (based on polystyrene) of the barrel. 

The copolymers are used in the manufacture of filaments. These may be extruded from steam-heated extruders with a screw compression ratio of 5 1 and a length/diameter of 10 1. The filaments are extruded downwards (about 40 at a time) into a quench bath and then round drawing rollers which cause a three- to four-fold extension of the filaments and an increase in strength from about 10000 to 36 000 Ibf/in (70-250 MPa). The filaments are used for deck chair fabrics, car... 

Cyro produces a family of acrylic-based resins Acrylite , Cyrolite and XT). All types of injection molding machines can be used to mold these blends. If reciprocating screw machines are used, the screw compression ratio should not exceed 3.5 (to avoid material breakdown). Injection temperature should be the lowest at which the part will fill. Since the material has sparkling clarity, great care should be taken to avoid contamination. 

Kurian et al. [1993] examined gamma-irradiation of the HDPE/LLDPE blends, at a dose rate of 10 kGy/h (Table 11.9). The blends of HDPE and LLDPE were prepared by melt mixing in an extruder attached to an internal mixer (L/D ratio, 20 screw compression ratio, 3 1 screw speed, 30 rpm extruder heater zones, 140°C die temperature, 150°C). Eor irradiation, the samples were compression-molded at 150°C in a steam-heated laboratory hydraulic press for 5 min. Eor the dicumyl peroxide (DCP) crosslinkable samples, 1 % DCP was added during compound-... 

The thermoplastic starch beads are foamed on a Demag D60 injection molding machine equipped with a standard PE screw (compression ratio 1 2). Beads with the desired moisture content (10-15% b.w.) are fed into the injection imit, melted and injected in the same fashion as in a regular injection molding process. The exception, however, is that the melt is injected into free air instead of a mould. The temperature used for foaming ranges from 150 to 200°C. 

As explained in the IM chapter, for TSs the processor uses a screw compression ratio of one, the barrel heat is kept relatively low (below the curing/ hardening heat), and the mold is at a higher heat to permit final cure. 

Extrusion of TPUs requires extruders with higher torque or hors power drives compared with other thermoplastics. Recommended screw compression ratio is 2.5 1 and minimum L/D ratio of 24 1 for most polyurethanes. 

Table 9.1 Selected results of mechanical tests (screw compression ratio 2.0).

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