Melt thickness

The previous chapter described the consequences of a nuclear reactor accident. Chemical process accidents are more varied and do not usually have the energy to melt thick pressure vessels and concrete basemats. The consequences of a chemical process accident that releases a toxic plume, like Bhopal did, are calculated similarly to calculating the dose from inhalation from a radioactive plume but usually calculating chemical process accidents differ from nuclear accidents for which explosions do not occur. 

Figure 6.18 Melt thicknesses for Zones C, D, and E around a solid bed for the melting simulation using a shear viscosity of 220 Pa-s. Melting started at the entry to the transition section at 6 diameters from the start of the screws and was completed at 13.7 diameters...

Maximum and Minimum Bounds on One-Dimensional Melt Thickness... 

For freezing, A would be replaced by — A. Equivalently, A can be redefined as a dimensionless heat absorbed per unit mass during the phase change. Equation (235) is an exact expression for the melt thickness directly in terms of an integral of the melt temperature profile and the boundary conditions at the plate. It has the usual advantage of integral formula-... 

This is an expression of the fact that at constant heat flux the melt thickness initially grows linearly with time. For small times an exact solution of this problem has been given by Evans et al. (E3), who expanded Ai (tji) in a Maclaurin series and found the first five coefficients by direct substitution ... 

Some numerical examples are given. For a semi-infinite copper melt initially at the fusion temperature, losing heat with an over-all heat transfer coefficient of 0.5 B.t.u./(hr.)(ft.2)(°F.) to the surroundings at ambient temperature, after 4 hr. 771 = 0.98, and the estimated thickness of solidified copper is 44 in. with a 12% error. A second example is a steel sheet subjected to a slowly flowing stream of very hot gas, such that a uniform heat flux of 105 B.t.u./(hr,)(ft.2)(°F.) is imposed at the surface with negligible motion of the melt. After 200 sec., 771 = 0.68, and the melt thickness is estimated to be 1.26 in., with a possible error of 8.6%. 

Figure 3 Schematic diagram showing how original lithospheric thickness and mantle potential temperature affect the amount of melt produced (melt thickness) and how these factors relate to continental flood basalts (CFB), volcanic rifted margins (VRM), off-ridge and ridge-centered oceanic plateaus (OP), and midocean ridges (MOR).

Mantle potential temperature Melt thickness is also controlled by melting temperature. This is because hot mantle will intersect the mantle solidus at a greater depth than cooler mantle, and as already established, melt thickness is a function of the depth of the melt column. However, since mantle temperatures increase with depth there needs to be... 

Melt thickness therefore can be expressed as a function of the mantle potential temperature. A potential temperature of 1,280°C equates to a melt thickness of about 7 km (normal ocean floor), whereas a mantle potential temperature of 1,480°C equates to a melt thickness of about 27 km (see Fig. 3.14). Clearly, these principles are important when considering melting processes in the early Earth, since many geoscientists believe that mantle temperatures were hotter in the Archaean (Section 3.2.3). 

If, as many suppose, the Archaean mantle had a higher potential temperature than the modern mantle, it is important to examine the implications of this for melt production during the early history of the Earth. The relationship between mantle potential temperature and melt thickness during adiabatic melting was outlined in Section 3.1.4.3 and may be briefly summarized by stating that as mantle potential temperature increases so will the melt production, as expressed in the depth of the melt column and the melt thickness. This is illustrated in Fig. 3.26, which shows how deeper, higher-temperature melting should lead to the formation of a thicker oceanic crust. 

Welding by Pressure - A form of heated tool welding in which flow of the molten plastic after heating is regulated by application of specific pressures to the hot tool and parts. Accurate pressure control is necessary, and final part dimensions may vary due to variations in melt thickness and melt viscosity. See also Welding by Distance, Heated Tool Welding. 

Matrix volume fraction, stabilizer and fat content Viscosity when melting Thickness, mouth coating, gumminess... 

This result governs melting, both in an extruder and in hot-plate welding. It only applies while is much less than the sheet thickness. It shows that conduction alone is too slow to melt thick layers of plastic it would take 100 s to melt a 3 mm surface layer. 

The melt bubble is stretched vertically and circumferentially by a factor of 2 or more, so that an initial melt thickness of about 1 mm is reduced to between 250 and 100 pm. In the biaxial tensile flow, the melt stress in the hoop (H) direction can be calculated from the pressure p inside the bubble, the current bubble radius r and thickness t, using Eq. (C.22) of Section C.3. 

The flow path length varies from 40 to 800 mm the wall thickness ranges from 0.5 to 3 mm with the possible flow path increasing as the wall thickness increases (for 0.5 mm thick walls the flow path ranges from 35 to 130 mm). The required specific cavity pressure varies from 18 MPa for low viscosity melts, thick walls to 200 MPa for high viscosity, thin walls. 

Micrometer measurements of thickness were made on the solidified PE samples. Errors due to polymer contraction on solidification were small, as the process of solidification generally results in a net volume change of the solid in the absence of constraints. As the polymer samples were not constrained in any dimension, contraction occurred along the length and width of the specimen as well as the thickness. That portion of the contraction resulting in a decrease in sample thickness was observed to be non-uniform across the face of the sample micrometer measurements on this face were taken as true melt thickness. 

Shims designed to allow an ontflow of excess molten polyethylene would facilitate thickness measurements as melt thickness would correspond to shim thickness. 

Figure 14.13 Pressure vs. time curve showing the four phases of heated tool welding. Parts to be welded are pressed (P,) against the hot tool in phase I, and heat is transferred to the parts by conduction. Melting begins when the melt temperature of the plastic is reached. In phase II, pressure (P ) is reduced in order to increase melt thickness. In phase III, the hot tool is removed, and in phase IV, the parts are brought together under pressure (P ) to cool and solidify.

Welding by pressure requires equipment in which the applied pressure can be accurately controlled. A drawback of this technique is that the final part dimensions cannot be controlled directly variations in the melt thickness and sensitivity of the melt viscosities of thermoplastics to small temperature changes can result in unacceptable variations in part dimensions. 

Step 3 includes phases I and II of Fig. 14.13. Molten material melts and flows out of the joint interface, decreasing part length until melt stops meet tooling stops. Melt thickness then increases until the heating platen is removed in step... 

Thickness and properties of the melt layer are important determinant of joint strength. Melt thickness is dependent on the amount of radiation reaching the weld interface and is influenced by heating time, heating temperature, heat source, geometry of the welding assembly, and characteristics of the polymers being welded. 

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