A closer look at screws and pellets (part 1)

摘要

Screw design, pellet type/size, and material behaviour are all critical issues in delivering uniform melt to the mold, but they don"t always get the consideration they deserve. I'd like to help change that by examining them in greater detail, beginning with screw design. Screws have three zones: one for feeding; a second for transitioning solids into melt, also known as the compression or melting zone; and a third zone for metering. The function of the feed zone is to convey unmelted plastic to the transition zone. Usually the feed zone accounts for about half the total length of the screw, and the flights in this section are deep. The feed section of the screw is not designed to melt the plastic, but to auger the granules forward and prepare them for melting. The granules are "gravity fed" through the feed throat, and there is lots of air between the granules in this part of the screw. This air, as well as volatiles such as residual moisture and light fractions of the poly- mer (and perhaps of the additives), must be vented through the feed throat as the material begins to heat in the rear zone.