Compression molding squeezing flow

Example 6.14 Squeezing Flow between Two Parallel Disks This flow characterizes compression molding it is used in certain hydrodynamic lubricating systems and in rheological testing of asphalt, rubber, and other very viscous liquids.14 We solve the flow problem for a Power Law model fluid as suggested by Scott (48) and presented by Leider and Bird (49). We assume a quasi-steady-state slow flow15 and invoke the lubrication approximation. We use a cylindrical coordinate system placed at the center and midway between the plates as shown in Fig. E6.14a. 

Figure 13.44 represents the various stages of the compression molding cycle from the point of view of the plunger force needed to close the mold at a constant rate. In the first region, t < the force increases rapidly as the preform is squeezed and heated. At tf, the polymer is presumably in the molten state and, as such, is forced to flow into the cavity and fill it. Filling terminates at tc, when compression of the polymer melt takes place, to compensate for the volume contraction that results from the polymerization reaction. The bulk of the chemical reaction occurs after tc. We now comment on each of the steps of the compression molding process. 

Squeeze flow between parallel plates was analyzed in Section 6.3 as an elementary model of compression molding. In that treatment we were able to obtain an analytical solution to the creeping flow equations for isothermal Newtonian fluids by making the kinematical assumption that the axial velocity is independent of radial position (or, equivalently, that material surfaces that are initially parallel to the plates remain parallel). In this section we show a finite element solution for non-isothermal squeeze flow of a Newtonian hquid. The geometry is shown schematically in Figure 8.16. We retain the inertial terms in the Navier-Stokes equations, thus including the velocity transient, and we solve the full transient equation for the temperature, including the viscous dissipation terms. The computational details. 

Squeezing flow is interesting to fluid mechanicians because it simulates such polymer processes as compression molding and stamping. In addition, it is a simple model for the action of a lubricant film under a bearing. Because polymers are typically added to lubricant oil, a number of studies have used squeezing flow to determine whether viscoelasticity will improve load capacity of bearing. 

Figure 9.39 Trace of normal stress for a compression-molded 73/27 HBA/HNA copolyester specimen (a) during sample loading at 320 °C, (b) during squeezing and cooling to 290 °C, (c) during temperature equilibration at 290 °C for 4 h, (d) transient and steady-state shear flows at y = 0.5 for 200 s, and (e) during relaxation after cessation of shear flow. An unrelaxed normal stress of 195 Pa was present in the specimen before being subjected to shear flow. (Reprinted from Han et al., Molecular Crystals and Liquid Crystals 254 335. Copyright 1994, with permission from Taylor Francis Group.)...

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