Extrusion correction factors

The 500 mm diameter extrusion problem presented in Section 7.5.1 was simulated using the generalized Newtonian method with and without the iy correction factor. The simulations and the experimentally determined pressure at a position 5.6 diameters downstream are shown in Fig. 7.29. Recall that the generalized Newtonian method over-predicts the pressure by a factor of 1.7 (a 70% over-prediction) at 10.8 MPa. The method with the F, correction factor predicts a pressure of 5.9 MPa, a pressure that is slightly lower than the experimentally determined pressure of... 

Fig. 9.8 Curvature correction factor for drag flow for a Power Law model fluid. /v,. fl 0 denotes the ratio of drag flow between parallel plates to drag (Couette) flow between concentric cylinders at equal gaps and moving surface velocities. The subscript 6 — 0 indicates that in screw extrusion this correction factor is rigorously valid only in the limit of a zero helix angle. [Reprinted by permission from Z. Tadmor and I. Klein, Engineering Principles of Plasticating Extrusion, Van Nostrand Reinhold, New York, 1970.]...

Basically these correction factors can be used not only for single-screw extruders but for all types of extrusion processes. 

The differences of the intrusion and extrusion mechanisms are the main factors, leading to the different pathways (hysteresis) of the branches in Fig. 1.16A. Furthermore, this effect causes the pore size distribution obtained from the intrusion curve to be incorrectly shifted towards smaller pore sizes. Unlike some inorganic materials of very regular pore structure (e.g. zeolites), permanently porous organic polymers consist of a very complex network of pores of different sizes connected to each other. Correction of these falsifications in the results described above is virtually impossible, since it implies a detailed understanding of the network. 

It would appear from equations (12.7) and (12.8) that there are two different values for W, the work of extrusion. This is not the case. When equation (12.7) is used to calculate the work of extrusion, A is determined from Fig. 12.1 where the intrusion contact angle 6- was used to obtain the extrusion curve. In fact, A is a smaller area than that of the pores actually emptied. When equation (12.8) is employed A represents the correct pore area and is larger than A by the factor cos 0 /cos 0. Thus, regardless of whether equation (12.7) or (12.8) is used, the calculated value of will be the same. 

The major processing methods that process well over 80wt% of all plastics are extrusion, injection molding, and blow molding. These processes as well as a few others use a plasticator to melt plastics. It is a very important component in a melting process with its usual barrel and screw. If factors such as the proper screw design and/or barrel heat profile are not used correctly fabricated products may not meet or maximize their performance and very important not provide for low cost process. 

We continue with the extrusion-spheronization project, discussed in chapter 2, section III. Here the effects of a large number of factors on the yield of pellets of the correct size were examined. Amongst those found to have quite large, and statistically significant, effects were the speed of the spheronizer and the spheronization time. 

The effect of five parameters in the extrusion spheronization process was studied by Chariot et al. (8) for a placebo formulation. In an initial study they estimated the main effects of five variables on the percentage yield of pellets of the correct size. This can be done most efficiently using a reduced factorial design of 8 experiments. The factors studied and their levels are shown in table 3.17. 

It is usually relatively simple to find the optimum conditions for a single response that does not depend on more than 4 factors, once the coefficients of the model equations have been estimated - provided, of course, that the model is correct. Real problems are usually more complex. In the case of extrusion-spheronization it is not only the yield of pellets that is important, but their shape (how near to spherical), friability, smoothness, and their ease of production. The optimum is a combination of all of these. 

The practical need to recover waste from plastics production is self-evident to all who have witnessed the operation of any extruder, and since extrusion is, in various guises, involved in a number of production techniques it can logically, and correctly, be assumed that the ability to reprocess is an important factor in profitable plastics conversion. Injection moulding, blow moulding and film production are all extensions of the extrusion process and the only known method of checking their correct operation is by processing material, usually to waste. 

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