In order to keep up with advances in mold cooling and the faster-cycling parallel movements of electric and hybrid machines, injection presses need ability to melt crystalline resins faster. Inadequate melting in conventional three-zone screws can affect part quality and cause increased wear on screw, barrel, and nonreturn valve. To address this, Engel Machinery, York, Pa., has come out with a double-flighted screw said to enable cycle-time reductions averaging around 30%.
Its flat-cut feed zone provides high input of shearing energy, so that a high degree of melting occurs before the compression zone. This prevents formation of a “solid wedge” that can cause screw deflection and wear. To avoid feeding problems, the feed zone starts with a single flight. Then a second flight is introduced in the center of the feed zone, directly opposite the first flight for a symmetrical pressure profile.
The new screw is available in 20:1 L/D with diameters of 35 to 80 mm. One application for it is hard-to-melt PP. It is a more economical alternative to barrier screws that are normally employed far below their maximum capabilities, Engel says.
Another key use is molding nylon, whose hard granules and high melting temperature are not suitable for barrier screws, according to Engel. The new screw can allow much smaller screw/barrel diameters than would otherwise be necessary for fast cycles.
The same is said of molding acetal (POM), which starts to degrade just above its processing temperature. High throughput rates with standard screws tend to push solids into the compression zone, resulting in high wear.