synopsisA semiquantitative model is proposed to explain the complex molecular orientation distribution, observed in injection moldings of amorphous polymers. The model incorporates flow and heat transfer mechanisms coupled with molecular theories. The orientation in the surface skin is related to steady elongational flow in the advancing front, whereas the orientation in the core is related to the shear flow, behind the front, between two solidyfying layers. Coupled with the elongational and shear-induced orientations, a molecular relaxation process takes place which is determined by the rate of heat transfer. The bead-and-spring macromolecular theory was used to calculate root mean end-bend distances of macromolecules in the various flow fields, &s well as the relaxation process.

his article proposes a model which provides the equa-T tions that are required to calculate the length of the melting zone. This length depends on the physical properties of the polymer and operating conditions. The familiar, simplified picture of the metering zone is adopted (1, 2) in which the channel is unwound from the screw and located along a rectangular coordinate system. The z coordinate is in the down channel direction, x is in the cross channel direction, and y is in the channel depth direction. Figure 1 shows an idealized cross section of the unwound channel. The barrel surface is at the top, moving at constant velocity, Va, with velocity components Vbr and V,. in the cross channel and down channel directions, respectively. The screw surface is at the bottom, and the screw flights are at the two sides.Following an experimental investigation, Maddock (3) proposed a qualitative mechanism for the melting process. According to his observations, the solid particles in contact with the hot surface partially melt and smear a film of molten plastic over the barrel surface.This film, and probably some particles, are dragged by the barrel surface, and as they meet the leading edge of the advancing flight, they are mixed with previously molten material. The molten material collects at an area in the rear of the channel while the forward portion of the channel is filled with solid particles. The width of the solid bed, X, gradually decreases toward the exit end of the screw. The melting process comes to an end when the solid bed disappears. The proposed model is based on these observations. Description of the Model and Simplifying AssumptionsThe basic assumption of the model is the existence of a steady state of temperatures and velocities with time, which implies that at any cross section of the channel the solid-melt interface remains at the same position in time. Furthermore, it is assumed that the interface boundaries are sharp boundaries, which implies that the plastic has a sharp melting point rather than a softening range. The solid bed is considered to be a homogeneous and continuous one. Both the channel and the soIid bed cross sections are rectangular.The proposed idealized mechanism for melting is the following. Referring to Figures 1 and 2, heat is con-ducted from the barrel surface through the moving film to the solid-melt interface. Additional heat is generated by viscos dissipation in the f i l m . Heat transfer from the circulating melt at the rear of the, channel to the solid bed is neglected, since the solid bed height is much smaller than its width through most of the melting process.The melting of the solid particles takes place only at the interface. The steady state is maintained since the molten material is dragged by the barrel surface to the rear of the channel while the solid bed moves at a constant velocity, Vsy, into the interface.The heat conducted and convected in the down channel, z, direction is neglected. For the heat transfer calculation, the solid bed is assumed to have an infini...

The residence time distribution (RTD) functions were derived for screw extruders, based on the “parallel plate” and curved channel flow models. The results indicate a relatively narrow distribution, and they explain several characteristics of screw extruders. The strain distribution in the fluid across the channel was also derived. With the aid of these two functions an average strain of the fluid leaving the extruder was defined. The resulting weighted‐average total strain (WATS) provides a quantitative criterion to the “goodness of mixing” in extruders.

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