Cavity pressure profiles

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. 

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... 

Figure 10.12. Typical cavity pressure profiles during compression molding. Stages are 1 - polymer heating, 2 - flow, 3 - compression, 4- cooling. Full and dashed lines represent respectively thermoplastic and thermoset molding...

FIGURE 5.16 Cavity pressure profile during the injection phase (four pressure sensors in different positions)... 

FIGURE 5.18 Cavity pressure profile at different holding pressure times... 

Figure 11.5 Cavity pressure profiles of solid molded part near gate and near end of fill.

Also, in the cold jet case, pressure profiles were measured to assess possible thrust penalty associated with the flow-induced resonance. Near-held pressure prohles, which are plotted in Fig. 29.11 for typical forced and natural cases, again show the faster growth associated with the excitation. In the far held, the static pressure became identical to the ambient pressure. To obtain the thrust force, far-held total pressure prohles were integrated over the jet cross-sectional area. The measurement at 18 exit diameters downstream for the excited case showed that there was a force deheit of about 8% compared to the natural case. This appears to be the maximum amount of thrust penalty caused by periodic impingement of shear how on the cavity trailing edge. 

In the simplest case under consideration (Newtonian, isothermal, and inertialess motion in the absence of structuring) at h(t) 8 the flow may be assumed to be quasi-one-dimensional, and the distorsions of velocity and pressure profiles in the vicinity of a front and the gate into a forming cavity may be neglected. The basic equation will then have the form ... 

To compare theory and experiment, calculate pressure profiles at the points of pressure gauges (at the distance z. from the entrance into a forming cavity). By using Eq. (6.11a) in the dimensionless representation, we obtain universal expressions ... 

Figure 10.9. Topical pressure profiles during injection molding. and stand respectively for nozzle and mold cavity pressure. Stages are 1 - dead time, 2 - filling, 3 - packing, 4 - cooling, 5 - ejection.

The numerical results obtained from simultaneous integration of (1) and (7) are shown in figs 1—3 for a planar cavity cluster at Pm = 20-103 Pa with f = a /27r = 20 kHz when = 0.3 mm. It is apparent from fig. 1 that initially the tensile stress in the cluster drops exponentially with the distance from the cluster boundary [1], but after 4 fjs the pressure profile steepens near the boundary due to the significant growth of the cavities here. The tensile stress decays beyond the first few cavity "layers" where growth of the cavities primarily occurs. The profile of increase of cavity radius vs. position is found to be essentially exponential, fig. 2. The pressure assumed from (15) is set up if an incident acoustic wave with a pressure Api vs. time t as shown in fig. 3 reaches the cluster boundary. It is noticed that very quickly the reflection coefficient approaches —1, showing that the boundary becomes an essentially compliant interface. 

If the stress amplitude is reduced to Pm = 2 kPa with f = 2 kHz a cavity growth as above is obtained after about 70 ps, and then Ux=o = - 0.49 m/s with k = - 0.996. The pressure at the cluster boundary penetrates slightly deeper than above, but still only the outermost cavity "layers" are appreciably affected and the cavity growth profile is essentially the same in all cases, only the time of development changes. 

The development of the stresses can be illustrated at sequential time instants by means of numerical results. Figure 6.5 shows a cavity pressure evolution profile. Figure 6.6 shows the gap-wise in-plane stress profiles of at successive times. Five typical time instants are chosen to display the results. They are t — 0.56 s when the location has just been filled t2 — 0.86 s at the end of the filling stage 3 = 1.68 s when the pressure reaches the peak value — 10.0 s after the core is solidified (Complete solidification occurred at t = 8.7 s) — 35.9 s just before... 

It was found that toe switchover point set earlier than toe normal caused hesitation in the pressure profile as shown in Fig. 3. The hesitation in toe cavity pressure means that the melt front stalls or flows backward for a moment. It would cause surface defects so that it should be avoided in terms of part quality. 

After the optimization test finishes, the optimized molding condition should be maintained. Also, there is a need to have means assuring the part quality immediately in the shop floor for the quahty control purpose. Another contribution of the PMS to the injection molding process would be the immediate represaitation of part qu ity. In order to utilize the PMS for the purpose, the pressure or temperature profiles cannot be used as it is. Even well trained engineers have difficulty in telling the difference by reading the profiles only. Therefore, a proper index from the measurement of toe PMS should be made to represent a quality target. It can be called as the PMS index. Usually the cavity pressure peak has been widely... 

Transient cavitation is generally due to gaseous or vapor filled cavities, which are believed to be produced at ultrasonic intensity greater than 10 W/cm2. Transient cavitation involves larger variation in the bubble sizes (maximum size reached by the cavity is few hundred times the initial size) over a time scale of few acoustic cycles. The life time of transient bubble is too small for any mass to flow by diffusion of the gas into or out of the bubble however evaporation and condensation of liquid within the cavity can take place freely. Hence, as there is no gas to act as cushion, the collapse is violent. Bubble dynamics analysis can be easily used to understand whether transient cavitation can occur for a particular set of operating conditions. A typical bubble dynamics profile for the case of transient cavitation has been given in Fig. 2.2. By assuming adiabatic collapse of bubble, the maximum temperature and pressure reached after the collapse can be estimated as follows [2]. 

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