Effect of shear stress and velocity profile development on hot runner flow bore wall slip

A recent trend towards higher flowing materials has caused significant problems in hot runner color change applications. These extreme high flow materials have a tendency to stagnate at the wall requiring an excessive number of parts to be molded before the color change is complete. Design criteria previously employed for color change applications, such as pressure loss and turnover, are no longer adequate when designing a hot runner system for a color change application when an extremely high flow material is to be used.; This thesis investigates the effect of shear stress at the wall on wall slip when molding polypropylene. It was found that at shear stresses below 66.9 KPa the material at the wall did not slip. Above shear stresses of 86.5 KPa the material at the wall did slip. This data correlated to data collected about past successes and failures in hot runner color change applications. The effect of velocity profile development was also investigated, in which a single shear stress was used with a varying number of shots molded. The results indicated that velocity profile development has no significant effect material slip at the hot runner wall.