Screw core

The helix angle and the channel widths at the screw core or root are designated with a subscript c, and they are calculated as follows ... 

The analysis developed here is based on screw rotation physics [13], and thus several other definitions are developed here. The velocities at the screw core, indicated by the subscript c, in the x and z directions are as follows ... 

A mechanical clearance between the top of the screw flight and the barrel wall helix angle at the barrel 6c helix angle at the screw core 6 r) helix angle at radial position r... 

To visualize the difference between screw rotation and barrel rotation, a simple cardboard paper towel roller can be used to model the screw core and a wood block to model the solid plug. For barrel rotation, the roller is held constant and the block is moved downstream at a velocity of V. . Here the sliding velocity between the... 

Wc channel width perpendicular to flight at the screw core Wgr width of the grooves in a grooved barrel... 

Figure 6.20 Schematic of the K, velocity in the Film D between the screw core and the solid bed. The red and blue velocity lines are for pressure gradients that are zero and negative, respectively. The dotted vertical line is for K, = 0... 

The locally high pressure underneath the solid bed and the positive dP/dx in Film D causes some flow of resin from Film D to the melt pool. Thus, for a local Az increment for Film D, there is material entering the element from the melting process and from the drag motion of the screw core, and there is material leaving the increment from the motion of the screw core and from the flow of material into the melt pool due to a positive dP/dx. These complex flows are consistent with observations from Maddock solidification experiments. This flow is shown in detail for the Maddock experiment shown in Fig. 6.35. 

Cf compression ratio that should be used for a lower density feedstock resin Cpeiiets compression ratio used for a pellet feedstock specific heat of the solid resin diameter at the barrel wall local diameter at the screw core... 

Figure 7.2 Transforming the Cartesian reference frame a) cylindrical cross section of the screw and barrel with flow out of the surface of the page, and b) the unwound rectangular channel with a stationary barrel and the Cartesian coordinate frame positioned on the screw, is the velocity of the screw core in the z direction and it is negative...

A version of this free helix device was also constructed that had a recycle system such that there was essentially no pressure change from the exit to the entrance of the extruder, as shown in Fig. 7.4. That device had a barrel diameter of 58 mm and it was used to measure the effect of the different parts of the screw on the extruder performance where pressure gradients could he imposed [5], The devices shown in Figs. 7.3 and 7.4 could be operated as a conventional extruder if hoth core and helix elements were driven at the same angular velocity with the barrel stationary. In another mode, only the helix was rotated in the same direction as the screw had been while the core and barrel were stationary. And finally the screw core was rotated in the same or different direction as the screw and the helix and barrel were stationary. 

Due to the complicated helical screw geometry and the assumption that the down-channel drag flow was a result of matching the screw core velocity to the modeled barrel velocity, the literature assumption that the flow occurs in a rectangular channel is reasonable only if the ratio of channel depth to width is small, that is, a channel with a small aspect ratio (H/W). A schematic of the channel depth to... 

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