Barrel rotation

Barrel plating of parts in copper cyanide solutions utilizes various formulations, some weaker, some stronger than the high speed baths. When plating parts that tend to stick together or nest during the barrel rotation, the free cyanide may need to be increased. This may require 35—40 g/L free potassium cyanide or more with an equal copper content. 

The optimum quantities and grades of powder, impact media, water and promoter, and plating conditions such as barrel-rotation speed, are best decided by trial runs. 

A7.3.1 Viscous Energy Dissipation for Barrel Rotation Generalized... 

As discussed previously in Section 5.2.4, screw rotation physics need to be used in order to calculate the sliding velocity of the solid bed relative to the barrel and screw surfaces. For barrel rotation physics, the sliding velocities at the barrel and screw surfaces are considerably different than that for screw rotation. At the barrel wall, the z component of motion must be corrected for the moving velocity of the barrel wall, as provided in Eq. 5.38. For the example above, V(,s= 12.5 cm/s. Because the screw is stationary for barrel rotation physics, = 0, and the sliding velocity at screw surface using Eq. 5.39 sets = 4.4 cm/s. At a pressure of 0.7 MPa... 

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... 

Figure 6.11 Schematics of the solid bed just prior to complete melting (a) the solid bed is pushed to the trailing flight with the Tadmor melting model and barrel rotation physics, and (b) the solid bed is a thin plate and positioned as in the diagram (screw rotation and observation). The cream color represents molten resin...

Model Parameter New Analysis (Screw Rotation) Historical Analysis [8] (Barrel Rotation)... 

The new melting model presented in this section qualitatively fits the experimental data observed by many previous researchers. Like the Tadmor and Klein model [8], this model is based on simplistic assumptions and linear mathematics for the melt films. The new model, however, does not require the reorganization of the solid bed like the Tadmor and Klein model. Furthermore, the new model allows viscous dissipation and melting in all four melt films, and does not restrict all melting to the Zone C film. Melting in the Zone D melt film becomes highly important when the boundary conditions are switched from barrel rotation to the actual conditions of screw rotation. 

The model developed here uses a fitting parameter to obtain melting lengths that are consistent with those observed experimentally. This fitted model is utilized to adjust for some of the non-linearities in the model. The model is not meant as a screw design tool. An improved model could be written based on the original Lindt and Elbirli model [27], The improved model would set the boundary conditions for screw rotation rather than barrel rotation as used by Lindt and Elbirli. 

The degradation ribbon at the merger of the flows occurs because of the crosschannel flow of material from the region between the solid bed and the screw root to the melt pool. As shown by Fig. 6.35, this flow is relatively large. As previously stated, the flow occurs because of pressure-induced flow and the dragging of fresh material under the solid bed by the backwards motion of the screw root. This process is consistent with the physics presented for screw rotation. The flow fields developed for a barrel rotation system would not create the low-flow region such as shown in Fig. 6.37. 

Because it is more complicated to solve the moving boundary problem for the rotation of the screw, the barrel rotation models described above have been extensively adopted and investigated. In practice the screw is rotated and not the barrel. The barrel rotation theory has several limitations when describing the real extrusion process, so correct interpretation of the calculated results based on barrel rotation becomes necessary. Most screw design practitioners, with substantial previous design experience, make major adjustments in design specifications to obtain effective correiations. 

Equation 7.21 is the literature expression for motion in the x direction for barrel rotation physics. The boundary conditions here are = 0 aty = 0 (screw root) and Erf = Kx aty = // (flight tip). Cross-channel velocity in the laboratory (Eulerian)... 

Figure 7.13 Comparison of literature drag and screw rotation for deep channels [45], The experimental data for screw rotation and barrel rotation and the theory lines were for screws with a 7° helix angle...

Two different calculation methods were used for the simulations (1) the generalized Newtonian method as developed above, and (2) the three-dimensional numerical method presented in Section 7.5.1. The generalized Newtonian method used a shear viscosity value that was based on the average barrel rotation shear rate and temperature in the channel. The average shear rate based on barrel rotation (7ft) is provided by Eq. 7.52. Barrel rotation shear rate and the generalized Newtonian method are used by many commercial codes, and that is why it was used for this study. 

Equations 7.57 and 7.58 that are developed above use the as the velocity component as shown for screw rotation physics. As previously discussed, the classic drag flow model [8] assumed that the equivalent flow rate will be obtained if the barrel is rotated in the direction opposite to that of the screw as long as the linear velocity of the unwound barrel Is numerically equal to the linear velocity of the screw core. For this classic barrel rotation model, is used as the velocity component instead of V(,z-Since is less than the drag flow rate would be reduced. It Is Interesting to note here that the classical model using reduced the drag flow rate such that It provided a better estimate of the actual rotational flow rate, but for the wrong reason. 

There is not an analytical velocity function for the y-direction velocity at the flights, so the wide channel approximation is used for demonstration purposes with a pressure gradient of zero. Using the equation developed previously for screw rotation for a very wide shallow channel, the transformed Lagrangian form of is the same as the laboratory form for barrel rotation and is as follows ... 

It follows that the dissipation rate should be different for screw rotation In the laboratory frame from the expected dissipation In the Lagrangian frame for screw rotation and the laboratory frame for the barrel rotation for the same pumping rate of a particular screw. 

In order to show the difference between screw and barrel rotation, a simple energy dissipation example is provided here. A complete analysis of energy dissipation will be presented in Section 7.7. For this example, a 50.8 mm diameter screw is rotated at a speed of 50 rpm and with a pressure gradient of zero, producing a rate of 6.8 cmys. The geometric data and physical property data are provided in Table 7.4. 

Evaluating the process aty = 0.5//, one obtains for the Lagrangian frame or barrel rotation the dissipation rate ... 

This analysis indicates that screw and barrel rotation should have different temperature increases even though they have the same rate predicted for the same rotation rate. In the following section the predictions will be checked experimentally. 

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