Effect of screw speed on devolatilization for a vented barrel injection molding machine

Vented barrel machines have long been used to process hygroscopic materials. A great deal of research has been concentrated on screw design, but obtaining literature on optimizing the process parameters leaves much to be desired. Prior to devolatilization, moisture co-exists with the hot melt in the screw and could potentially initiate hydrolytic degradation for materials like PC. Attaching a starve feeder gives greater flexibility in screw speed.; This study was done to investigate the optimum screw speed that should be used to obtain maximum devolatilization and least reduction in molecular weight. Both wet and dry resins were used to compare the effect of moisture on molecular weight degradation. A 2{dollar}sp5{dollar} factorial design was also conducted to determine the factor which influenced the physical properties the most. The processing parameters studied were: cycle time, feed rate, screw speed, melt temperature and moisture condition. The properties used for comparison were % elongation, toughness and impact strength. Actual values of molecular weight and MWD were determined using a GPC instrument.; It was found that amongst the different factors studied, the moisture condition of the resin was most significant. The % elongation and toughness values for wet resin peaked at a certain rpm value and then decreased. An opposing trend was observed for the dry resin. The rpm variation did not produce a consistent trend for either the wet or dry resin. A higher molecular weight degradation and broader molecular weight degradation was observed for wet resin. Increase in the screw speed and feed rate decreased the MWD. Increasing the melt temperature decreased the MWD for dry resin and increased the MWD for a wet resin.