Fill time nozzle

Fill times can now be estimated for the nozzle pressure-viscosity data. The results follow, together with observed fill times (all tf are in seconds) ... 

Differently filled cavities are the result of viscosity-related filling time differences in the individual cavities. Temperature sensors close to the end of the flow path recognize, however, exactly when the melt reaches this position. An intelligent control system evaluates the information and controls the individual nozzle temperatures until the individual filling times and thus the viscosities are identical [9]. 

Rapid production rates can be achieved with little limitation on size and shape. Cycle times may be as low as 10 seconds for small components, being dependent on the time required to fill the mould and the cooling time. Filling time is proportional to the pressure drop across the nozzle raised to a power dependent on the type of plastics being injected, and cooling time is roughly proportional to the square of the wall thickness of the moulding. 

The pressure required to pack the cavities in the mold mainly depends on the temperature of the mass at the nozzle and on the temperature increase due to viscous dissipation. This dissipation is proportional to the viscosity and quadrati-cally proportional to the injection speed. Increased temperatures and the resulting decreases in viscosity improve the flow into the cavities, thus decreasing filling time, but they increase the cooling time needed before the object has solidified sufficiently to be ejected. A balanced choice of melt temperature and mold temperature is important for optimal production. Determination of these parameters is generally done experimentally. 

Figure 7.8 Rapid-mixing rapid-freezing apparatus.Two syringes (2) are filled with a solution of enzyme and substrate, respectively. By applying a large force (I) on the syringes, the solutions are driven into the mixer (3) whereafter the reaction starts.The reaction mixture flows through a reaction hose (4) and is then sprayed via a thin nozzle (5) in a funnel filled with cold isopentane (—I40°C). This causes a rapid (Sms) quenching of the reaction.The frozen powder is subsequently collected in an EPR tube attached to the funnel (6) and then is ready for EPR measurements.The funnel and EPR tube are held in a dewar with isopentane (—l40°C).The reaction time can be varied by changing the length and diameter of the reaction hose.

When the mixture of monomers is cast in an open mold, the air bubbles formed at the jet nozzle in the mold usually have enough time to leave the material due to the low viscosity of the reactive liquid. When more viscous oligomers and prepolymers are used, particularly in the case of low-lifetime reactive mixtures, it may be necessary to use some simple procedures, for example, filling through a pipe immersed into the mold, to prevent the formation of air bubbles in the product. 

Categorized by work duties, mixers (those handling concentrate) received the highest internal dose of 2,4,5-T, followed in order by backpack sprayers, mist blower drivers, helicopter pilots, supervisors, and flagmen for the helicopter operation. One helicopter pilot excreted considerably less 2,4,5-T in his urine than did the other pilot. This difference appeared to be related to the fact that the second pilot routinely checked and unplugged nozzles at each fill-up time. In... 

The two become equal upon cessation of flow when the mold is full. The difference I — P2 indicates the pressure drop over the sprue and runner system. The pressure drop across the gate is given approximated by P2 — Pj,. We note that just downstream of the gate, the pressure P3 increases with time throughout the filling process (from about 0.4 s to 1.3 s). As Example 13.1 pointed out, such a pressure trace approaches conditions of constant filling rate. This is supported by ram position measurements, which were also retrieved at 0.02-s intervals. We further note that, upon mold filling, when P5 sharply increases, there is also a steep increase in all the pressures except the nozzle pressure, which is then reduced to 5500 psi. 

At the end of this injection process (80-90% filled), the two pistons will hydraulically separate the mold from the nozzle. The missing amount of polymer is now added from the small cylinders, countering the phases in the mold and the different molten Section 5 of the plasticized plastic is moved back and forth two to three times over the theoretical weld lines. After completion of this mixing process the cylinders are synchronized (phase B) (Fig. 14.18). 

---