Cavity pressure variation

In order to judge performance capabilities that exist within the controlled variabilities, there must be a reference to measure performance against. As an example, the injection mold cavity pressure profile is a parameter that is easily influenced by variations in the materials. Related to this parameter are four groups of variables that when put together influences the profile (1) melt viscosity and fill rate, (2) boost time, (3) pack and hold pressures, and (4) recovery of plastica-tor. TTius material variations may be directly related to the cavity pressure variation. Details on EQUIPMENT/PROCESSING VARIABLE are in Chapter 8. 

Thus material variations may be directly related to the cavity pressure variation (Chapter 4). 

The cavity pressure will vary across the disc and it is necessary to make some assumption about this variation. Experimental studies have suggested that an empirical relation of the form... 

Fig. 5.27 Variation of Cavity Pressure with Flow Ratio...

Fig. 5.28 Variation of Cavity Pressure Loss with Injection Rate...

Hydrodynamic cavitations in flowing liquids result from pressure variations in the CAV-OX system. Vapor-fiUed cavities form when the pressure is reduced to a critical value without a change in ambient temperature. 

Cavitation is the formation of gaseous cavities in a medium upon ultrasound exposure. The primary cause of cavitation is ultrasound-induced pressure variation in the medium. Cavitation involves either the rapid growth and collapse of a bubble (inertial cavitation) or the slow oscillatory motion of a bubble in an ultrasound field (stable cavitation). Collapse of cavitation bubbles releases a shock wave that can cause structural alteration in the surrounding tissue [13]. Tissues contain air pockets trapped in the fibrous structures that act as nuclei for cavitation upon ultrasound exposure. The cavitational effects vary inversely with ultrasound frequency and directly with ultrasound intensity. Cavitation might be important when low-frequency ultrasound is used, when gassy fluids are exposed, or when small gas-filled spaces are exposed. 

Dielectric hydration models serve as primitive theories against which more detailed molecular descriptions can be considered. Of particular interest are temperature and pressure variations of the hydration free energies, and this is specifically true also of hydrated polymer electrolyte membranes. The temperature and pressure variations of the free energies implied by dielectric models have been less well tested than the free energies close to standard conditions. Those temperature and pressure derivatives would give critical tests of this model (Pratt and Rempe, 1999 Tawa and Pratt, 1994). But we don t pursue those tests here because the straightforward evaluation of temperature and pressure derivatives should involve temperature and pressure variation of the assumed cavity radii about which we have little direct information (Pratt and Rempe, 1999 Tawa and Pratt, 1994). 

Another fundamental state variable that can be regulated during the cycle is cavity pressure. Closed loop control of cavity pressure could automatically compensate for variations in melt viscosity and injection pressure to achieve a consistent process and consistency of molded products. Adaptive control methods have been developed to track the cavity pressure profile at one location in the mold. In these earlier works, cavity pressure control was handicapped by the absence of actuators for distributed pressure control, as conventional molding machines are equipped with only one actuator (the screw), which prevents the simultaneous cavity pressure control at multiple points in the mold. This problem has been solved with the development of dynamic melt flow regulators that allow control of the flow and pressure of the polymer melt at multiple points in the mold.[ °l... 

Sink marks Melt temperature too high Insufiticient material injected Insufiticient dwell time Premature gate Ireezing Sharp variations in wall thickness Wrong gate location Part ejected too hot Cavity pressure too low... 

How large a cavity can exist at depth in an intact sandstone or in a dilated and yielded sand This question is a coupled mechanics-flow issue involving seepage forces acting at the local scale (cm), pore pressure variations acting at a larger scale (m), and stress redistribution at all scales. 

The bounding interface between the external and middle ear is the tympanic membrane. Pressure variations across the membrane move three ossicles, the malleus (hammer) connected to the membrane, the incus (anvil), and the stapes (stirrup) whose footplate is a piston-Hke structure fitting into the oval window, an opening to the fluid-filled cavities of the inner ear. Ligaments and muscles suspend the middle-ear ossicles so that they move freely. If sound reaches fluids of the inner ear directly, 99.9% of the energy is reflected [Wever and Lawrence, 1954], a 30-dB loss due to the mismatch in acoustic impedance between air and inner-ear fluids. Properties of the external meatus, middle-ear cavity, tympanic membrane, and middle-ear ossicles shape the responsiveness of a species to different frequencies. 

Variations in the cavity pressure p during solidification lead to variations in the reference lengths L/ of layers in the moulding. The latter are defined as... 

Birefringence variation with distance z from the mid-plane of a CD in the rz plane (d is the half thickness), (a) An z for a standard disc at various radial distances (b) Aurz for a short shot when the mould cavity pressure <40 bar (c) Aurs for a standard disc. The integral of this distribution gives a path difference of 10 nm (from R. Wimberger-FriedI, Polym. Eng. ScL, 30,813, 1990). 

Unbalanced cavity pressure buildup, mold distortion, dimensional variation/poor shrinkage control, stresses, flash, etc. 

Fig. 25.9 Measured pressure variation (solid line) inside the droplet generator cavity when a 4.61 ms pulse was used to trigger the solenoid valve. The applied pulse is also shown (dashed line). In this case, a single droplet is emerged (Reprinted from [40], With permission. Copyright 2003 of Elsevier)...

A change in resin viscosity is reflected as a change in melt pressure and can be detected by measuring mold or cavity pressure with respect to time. Other variations that similarly display themselves and can be detected include melt temperature, hydraulic pressure, oil temperature, and so on. 

In a conventional linear analysis of the dynamic characteristics of a journal bearing, the cavity is assumed to be fixed to the static state. However, the variation of the cavity pressure and the movement of the boundary affect the dynamic oil film coefficients, even if the displacement of the shaft and/or its velocity are infinitely small. 

Fig. 7 (a) and (b) illustrates the ratio of pressure variation to vertical shaft displacement velocity for constant pressure model (CP) and constant volume model (CV) respectively, (b) shows that the ratio is positive in the cavity because the cavity is compressed by the surrounding oil film. 

The movement of the cavity boundary affects the pressure variation just Inside the oil film boundary, and the dynamic coefficients of the oil film. A higher critical speed Is. predicted by taking the movement of the cavity boundary Into account. 

If the gas In the cavity Is conserved under dynamic load, the dynamic coefficients are affected by the variation of the cavity pressure. Constant cavity volume assumption leads to lower critical speed than constant cavity pressure assumption. 

Figure 6.9 Pressure variations in channels of an eight-cavity mould before and after...

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