Melt-pressure decompression

Devolatilization can be used to remove up to 5 percent of volatile impurities from the plastic melt. The first melt conveying (metering) zone builds up melt pressure. Then channel depth is increased abruptly in the vent zone, the melt is decompressed, and volatiles escape through the vent. After this the melt enters a second metering zone, which builds up melt pressure again, and feeds it to the die. 

The fractures in the lithospheric mantle reduce the pressure which causes the rocks of the plume head and the adjacent lithosphere to melt by decompression. 

Air trapped in melt] screw decompression/back pressure too low. 

Inadequate cooling time Extend cooling time. Use HR decompression to reduce residual melt pressure in HR. 

With dies designed to produce very rapid decompression at their outlets (e.g., by using wide entry angles and very short lands), transfer of dissolved gas from melt to expanding bubbles (as melt pressure rapidly drops) causes a corresponding... 

Allegre CJ, Dupre B, Lewin E (1986) Thorium/uranium ratio of the Earth. Chem Geol 56 217-227 Allegre CJ, Turcotte D (1986) Implications of a two-component marble-cake mantle. Nature 323 123-127 Asimow PD, Hirschmann MM, Ghiorso MS, O Hara MJ Stolper EM (1995) The effect of pressure-induced solid-solid phase transitions on decompression melting of the mantle. Geochim Cosmochim Acta 59 4489-4506... 

Oilfields in the North Sea provide some of the harshest environments for polymers, coupled with a requirement for reliability. Many environmental tests have therefore been performed to demonstrate the fitness-for-purpose of the materials and the products before they are put into service. Of recent examples [33-35], a complete test rig has been set up to test 250-300 mm diameter pipes, made of steel with a polypropylene jacket for thermal insulation and corrosion protection, with a design temperature of 140 °C, internal pressures of up to 50 MPa (500 bar) and a water depth of 350 m (external pressure 3.5 MPa or 35 bar). In the test rig the oil filled pipes are maintained at 140 °C in constantly renewed sea water at a pressure of 30 bar. Tests last for 3 years and after 2 years there have been no significant changes in melt flow index or mechanical properties. A separate programme was established for the selection of materials for the internal sheath of pipelines, whose purpose is to contain the oil and protect the main steel armour windings. Environmental ageing was performed first (immersion in oil, sea water and acid) and followed by mechanical tests as well as specialised tests (rapid gas decompression, methane permeability) related to the application. Creep was measured separately. 

Bubble Growth by Decompression 13.4.1. Thin shell of over-pressured viscous melt... 

In the case of flash degassing, the polymer solution is first heated under pressure to above the boiling point of the volatile components and decompressed directly into the ZSK. The polymer and solvent (monomer) spontaneously separate from each other inside the ZSK and the majority of the volatile components are released via the back venting system. Depending on the pressure and the temperature, up to 90% of the solvent can be removed in this way. Efficiency depends on the temperature of the polymer solution at the feed intake, the pressure drop in the back vent, and the material properties of the feeding system. The back vent is located upstream from the polymer or polymer solution feeding port (see Fig. 10.2). In this case, there is no melt in the screw channel so that the entire screw cross-section is available for the removal of gas or vapors. 

Figure 14 Normative spinel Iherzolite mineral abundances (wt.%) versus rock Mg for abyssal peridotite trends relative to calculated trends for polybaric, near-fractional melting. Three melt extraction models are shown over pressure ranges of 2.5-0.4 (short dashed line), 2.0-0.4 (long dashed line), and 1.5-0.4GPa (solid line). One percent melting occurs per 0.1 GPa of decompression, and 90% of the melt is extracted at each pressure. Starting mantle is fertile upper mantle (Table 1, 8).

Pressure exerts a hrst-order control on the solubility of volatile species (Equation (1)) because of the great increase in molar volume going from the melt to vapor phase. Therefore, decompression, which can accompany magma ascent, or failure of conhning rock, is a key process leading to saturation of volatiles and exsolution. Decompression also promotes undercooling, driving crystallization, and exsolution. 

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