MELT PRESSURE

Even at the lowest temperatures, a substantial pressure is required to soHdify helium, and then the soHd formed is one of the softest, most compressible known. The fluid—soHd phase diagrams for both helium-3 and helium-4 are shown in Eigure 1 (53). Both isotopes have three allotropic soHd forms an fee stmeture at high pressures, an hep stmeture at medium and low pressures, and a bcc stmeture over a narrow, low pressure range for helium-4 and over a somewhat larger range for helium-3. The melting pressure of helium-4 has been measured up to 24°C, where it is 11.5 GPa (115 kbar) (54). 

The width of molecular weight distribution (MWD) is usually represented by the ratio of the weight—average and the number—average molecular weights, MJM. In iadustry, MWD is often represented by the value of the melt flow ratio (MER), which is calculated as a ratio of two melt indexes measured at two melt pressures that differ by a factor of 10. Most commodity-grade LLDPE resias have a narrow MWD, with the MJM ratios of 2.5—4.5 and MER values in the 20—35 range. However, LLDPE resias produced with chromium oxide-based catalysts have a broad MWD, with M.Jof 10—35 and MER of 80-200. 

A web of molten plastic is pulled from the die into the nip between the top and middle roUs. At the nip, there is a very small rolling bank of melt. Pressure between the roUs is adjusted to produce sheet of the proper thickness and surface appearance. The necessary amount of pressure depends on the viscosity. For a given width, thickness depends on the balance between extmder output rate and the take-off rate of the pull roUs. A change in either the extmder screw speed or the puU-roU speed affects thickness. A constant thickness across the sheet requires a constant thickness of melt from the die. The die is equipped with bolts for adjusting the die-gap opening and with an adjustable choker bar or dam located inside the die a few centimeters behind the die opening. The choker bar restricts flow in the center of the die, helping to maintain a uniform flow rate across the entire die width. 

Purification using carbon is accompHshed by addition to the melt or to the soHd charge before vacuum melting. Pressure rises as a result of the reaction of the carbon with dissolved oxygen. Completion of the deoxidation process is marked by a rapid pressure drop indicating when the evolution of CO is complete. 

Problems of corrosion and staining can occur under conditions of high melt pressure. Modified grades are now available that largely overcome these difficulties. 

There are several types of nozzle. The simplest is an open nozzle as shown in Fig. 4.34(a). This is used whenever possible because pressure drops can be minimised and there are no hold up points where the melt can stagnate and decompose. However, if the melt viscosity is low then leakage will occur from this type of nozzle particularly if the barrel/nozzle assembly retracts from the mould each cycle. The solution is to use a shut-off nozzle of which there are many types. Fig. 4.34(b) shows a nozzle which is shut off by external means. Fig. 4.34(c) shows a nozzle with a spring loaded needle valve which opens when the melt pressure exceeds a certain value or alternatively when the nozzle is pressed up against the mould. Most of the shut-off nozzles have the disadvantage that they restrict the flow of the material and provide undersirable stagnation sites. For this reason they should not be used with heat sensitive materials such as PVC. 

A circular plate of diameter 0.5 m is to be moulded using a sprue gate in its centre. If the melt pressure is 50 MN/m and the pressure loss coefficient is 0.6 estimate the clamping force required. 

Narrow molecular weight distribution, which is characteristic of metallocene-based polyethylene (Fig. 7), causes processing difficulty in certain applications due to increased melt pressure, reduced melt strength, and melt fracture [14,15]. This problem can be overcome by blending the metallocene polymer with other prod-... 

The product designer should caution the tool designer to keep the gate area away from load-bearing surfaces and to make the gate size such that it will improve the quality of the product. It so happens that the product wall in the gate area develops the minimum tolerance due to the high melt pressure in that area. 

Blind hole In regard to molding products that include holes, it is important to ensure that sufficient material surrounds the holes and melt flows property. A core pin forming blind holes is subjected to the bending forces that exist in the cavity due to the high melt pressures. Calculations can be made for each case by establishing the core pin diameter, its length, and the anticipated pressure conditions in the cavity (3). 

With PBAs the compressed gases often used are nitrogen or carbon dioxide. These gases are injected into a plastic melt in the screw barrel under pressure (higher than the melt pressure) and form a cellular structure when the melt is released to atmospheric pressure or low pressure. The volatile liquids are usually aliphatic hydrocarbons, which may be halogenated, and include materials such as carbon dioxide, pentane, hexane, methyl chloride, etc. Polychlorofluoro-carbons were formerly used but they have now been phased out due to environment problems. 

Change in the physical state freezing (freeze-drying), crystallizing, condensation, melting, pressure-pelletizing, vaporization etc. 

The ortho to para conversion of hydrogen [18,19] is exothermic the heat release is 1.06 kJ/mole starting from an ortho concentration of 75% and 1.42 kJ/mole for 100%. Such conversion can be very slow, for example, 1.9%/h for solid H2 at the melting pressure [20]. 

Afterwards, measurements of the specific heat have been carried out in a wider pressure range [29,37-39]. It was thus evident that there is a discontinuity at the transition temperature which, at the melting pressure, gave a AC/C 2, (well above the 1.43 value expected from BCS theory) which decreased as the pressure diminished. 

Such transition is called phase A transition (T = 2.505 mK at the melting pressure) this is a second-order transition, with a specific heat discontinuity more similar to that of normal superconductors than to the 4He A-transition (Fig. 2.11). 

When temperature decreases, a second transition to a distinct superfluid phase B is observed. The latter is first-order transition at T — 1.948 mK at the melting pressure. 

For example the p(T) relation in a 3He melting pressure thermometer (see Section 9.3) can be used to deduce T. 

PLTS-2000 was formally adopted by the Comite International des Poids et Mesures in October The PLTS-2000 covers die range of temperature from 1K down to 0.9 mK and is defined by an equation for the melting pressure of 3He... 

The choice of a primary thermometer such as the 3He melting pressure thermometer to define the PLTS 2000 witnesses the great difficulties encountered in the measurement of very low temperatures. For example, at the beginning of 1980s, it was realized that differences up to 40% existed in the data of 3He specific heat obtained... 

Fig. 8.2. Melting pressure pm of 3He (full line) and absolute value of the derivative dpm/dr (dashed line) as a function of temperature. rN, TB, TA are the temperatures of the three phase transitions in solid and liquid 3He.

The PLTS 2000 is defined by the following relation between the melting pressure p and the temperature T2000 (see Table 8.6) ... 

The 3He melting pressure thermometer has been chosen to extend the ITS 90 for several reasons, such as the good sensitivity over three temperature decades, except around the pressure minimum at 315.24 mK. On the other hand, such a minimum is a reference point in the calibration of the pressure transducer in fact, the pressure must be measured in situ since, below 315.24 mK, the entrance of the measurement cell is blocked by solid 3He. 

Even if nowadays, the MCT may be considered a primary thermometer only on a narrow temperature range, it is considered the best dissemination standard in the millikelvin range [56-59], In fact, the 3He melting pressure is a good thermometric property because of its sensitivity over three decades of temperature with a resolution A T/T up to 10 5 [56], The good repeatability, the insensitivity to magnetic fields up to 0.5 T [60] and the presence of temperature-fixed points allow for the control of possible shifts in the calibration curve of the pressure transducer. The usefulness of these fixed points is evident, considering that the ITS-90 is based just on the definition of fixed points. 

Gas bubble nucleation in a melt is often cast in terms of a pressure decrease. If the actual process is due to a temperature increase or concentration increase, it is equivalent to a pressure decrease as long as the saturation pressure (that is, the equilibrium pressure) corresponding the concentration and temperature of the melt can be found. Let the equilibrium pressure be Pg, the ambient melt pressure be P, and the gas pressure in the bubble be Pg. The gas pressure in a bubble of radius r and the ambient melt pressure are related as... 

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