The performance of screws of advanced design in injection molding has been investigated with respect to four different objectives: 1) improvement of distributive mixing; 2) improvement of dispersive mixing; 3) increase of plasticating capacity; and 4) reduction of inhomogeneity of melt temperature.The screws used are three zone screws with different compression ratios, screws with pineapple or MaddockIEgan mixing elements, with one or two channel barrier sections, with static mixers mounted in the valve or in the nozzle, or with combinations of these different elements. The best mixing quality is obtained with multi-channel Maddock sections. The highest plasticating capacity and, consequently, the shortest cycle times are achieved with the barrier screws. Temperature measurements show that these screws improve melt homogeneity considerably with a relatively small loss of plasticating time. In all cases, increasing the back pressure gives inferior results compared with improvement of the screw design.

Slip at boundaries is possible in viscous flows: for instance, in the extrusion of foodstuffs, water-containing materials, and some polymers. There are two phenomenological descriptions of slip: The first is based on the presence of a very thin, low viscosity boundary layer and has been derived for capillary flow and extrusion: the second is based on a Coulomb-friction mechanism and has been derived for capillary flow only. After a survey of these results, the friction model is derived for the extrusion process. All calculations are a s simple as possible: twodimensional, Newtonian, isothermal, and with constant boundary layer parameters or coefficients of friction. A strong dependence of pumping characteristics and efficiency on the slip boundary conditions, and also on the extruder length in the case of friction, was found, especially when slip is only allowed for a t the screw surface. Exercises like these may help in understanding abnormal extrusion behavior of slippery materials in practice.