Melt film thickness

The melt film thickness 6 for Zone C is determined using Eq. 6.13. Note that V x is essentially numerically equal to the traditional specified by Tadmor and Klein because of the small value of the film thickness. 

Figure 6.14 Qualitative shape of A and Kbed dimensions and melt film thickness for melting in a conventional transition section a) top view, and b) side view. The cream color represents molten resin...

Figure 6.17 Melt film thickness at Zone C (6J for the simulations using resins with shear viscosities of 220 and 880 Pa-s...

Two constraints are placed on the solids-conveying channel. That is, the sum of the widths and thicknesses for the components must equal the width and depth of the solids-conveying channel. Here, the melt film thicknesses 6t, and 6 for Zones B and D were found by difference ... 

When examining eqn. (6.276) it is clear that the only two unknowns are the speed at which the solid moves, Usy, and the melt film thickness, 6. To solve for melt film thickness we can perform a energy balance by setting the heat conduction through the thickness of the film equal to the heat of fusion of the melting material and the energy required to raise its temperature from T0 to Tm... 

In the compression or barrier zone of single screw, the melting occurs approximately 85—90 percent with drive power requirement. The same mechanism occurs in metering zone too. At the metering zone, the melt film thickness equals the channel depth due to complete melting at the point [6]. 

The penetration depth is about 0.1 mm when VD t = 25-10 [m]. If the diffusion coefficient is assumed to be D - 10 mys, then the corresponding time is 0.0625 s. If the diffusion coefficient is assumed to be D = 10 mys, then the corresponding time is 6.2510 s. Since these are typical values of the diffusion coefficient, the assumption of infinite melt film thickness may not be valid in the analysis of devolatilization in extrusion equipment. A typical exposure time of the melt film in an extruder is of the order of one second a typical melt film thickness is 0.1 mm ( 0.004 in). 

The heat supplied by the barrel heaters has to be conducted through the entire thickness of the barrel and through the entire thickness of the melt film before it can reach the solid bed. Problems with this energy transport are considerable heat losses by conduction, convection, and radiation. Another, probably more severe, problem is the low thermal conductivity of the polymer. The heat has to be transferred across the entire melt film thickness. Therefore, the conductive heat flux will be small, particularly when the melt film thickness is large. Increasing the barrel temperature can accelerate the heating process however, this temperature is limited by the possibility of degradation of the polymer. 

The prime with the mass flow indicates mass flow per unit length. The prime with the x-coordinate indicates the direction determined by the vectorial difference between the barrel and solid bed see Fig. 7.36. This increase in mass flow will require an increase in melt film thickness. The corresponding increase in drag flow... 

It can be assumed that the changes in melt film thickness occur only in direction x of solid bed relative motion. In that case, by using Eqs. 7.101 through 7.103, a differential equation describing the melt film thickness can be formulated ... 

If the melt film thickness at x = 0 is taken as the local radial clearance 8 between flight and barrel, then the melt film thickness can be expressed as ... 

It should be noted that Eqs. 7.102 through 7.109 are slightly different from the equations developed by Tadmor [62] see also reference 5 of Chapter 1. The reason is that Tadmor assumed a constant melt film thickness. However, it is clear that the melt film... 

For a temperature-independent power law fluid, factor Bj becomes and the melt film thickness becomes ... 

Factor AH is the enthalpy difference between T , and T,. The melt film thickness can be determined from a mass balance in direction x. This leads to the following expression ... 

The extraction number is a measure of the overall devolatilization efficiency. The backmixing term disappears when the melt film thickness is zero. This is to be expected because the backmixing is caused by the transport in the melt film. If the film thickness is zero, there can be no transport in the melt film and consequently no backmixing. If the film stage efficiency X, = 1, the backmixing term will disappear as well. This is also easy to understand because In this case the concentration C (z) at which the melt film reenters the melt pool is known and will equal Cg. Thus, the Taylor s series expansion is no longer necessary and the first-order differential equation can be determined directly from the mass balance, Eq. 7.463. 

As the number of flights is increased, the width of the individual solid beds becomes smaller. As a result, the average melt film thickness reduces, because the melt film thickness increases with the solid bed width. The thinner melt film will improve heat conduction through the melt film and increase the viscous heat generation in the melt fllm, causing improved melting performance. 

Average shear rate (assuming a melt film thickness of 0.01 cm) is... 

Masson, J.L., Green, P.F. Viscosity of entangled polystyrene thin film melts film thickness... 

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