Nozzle pressure control

The resulting motion of the beam is detected by the pneumatic nozzle amphfier, which, by proper sizing of the nozzle and fixed orifice diameters, causes the pressure internal to the nozzle to rise and fall with vertical beam motion. The internal nozzle pressure is routed to the pneumatic relay. The relay, which is constructed like the booster relay described in the Valve Control Devices subsection, has a direct hnear input-to-output pressure characteristic. The output of the relay is the controller s output and is piped away to the final control element. 

The Majac jet pulverizer (Ho.sokawa Micron Powder Sy.stems Div.) is an opposed-jet type with a mechanical classifier (Fig. 20-55). Fineness is controlled primarily by the classifier speed and the amount of fan air dehvered to the classifier, but other effects can be achieved by variation of nozzle pressure, distance between the muzzles of the gun barrels, and position of the classifier disk. These pulverizers are available in 30 sizes, operated on quantities of compressed air ranging from approximately 0.6 to 13.0 mVmin (20 to 4500 ftV min). In most apphcations, the economics of the use of this type of jet pulverizer becomes attractive in the range of 98 percent through 200 mesh or finer. 

A detailed study of mold filling was done by Krueger and Tadmor (6). The various stages of the injection process was followed by pressure gauges in the mold, as shown in Fig. 13.5. In this experiment polystyrene (PS) was injected into a shallow rectangular cavity with inserts as shown in the figure. Pressure transducers were placed in the nozzle, runner, and mold cavity their output was scanned every 0.02 s and retrieved by a computer. The nozzle pressure was set by machine controls to inject at a constant 10,000 psi and, upon mold filling, to hold the pressure at 5500 psi. 

Developments in micro-processing have made it much easier to monitor and to control conditions in the course of manufacture, and to prepare and maintain written records. Injection moulding may be supervised with the help of melt thermocouples fitted in the nozzle, pressure transducers in the hydraulic line and in the mould, and a transducer to monitor linear movement of the screw. Information from these sensors may be displayed and depicted in relation to the control limits (upper and lower) and the operator thus made aware immediately of variations that could be significant. Conditions can be corrected more rapidly and if need be any mouldings that might not be satisfactory diverted for examination. Should the number of suspect products exceed a limit set in advance, a line can be shut down automatically. 

Chemical barriers are similar to deflagration suppression systems. Typically, optical sensors are installed in upstream locations of the pipeline in which the flame is to be stopped. Upon a detection of flame, the sensor sends a signal to the control unit, which amplifies the signal and triggers the detonator-controlled valve in a suppressant bottle. The extinguishing agent is injected into the pipeline through a suitable nozzle. Pressure... 

Although pressure control is the most common way of adjusting the volume output of boom sprayers fitted with conventional nozzles, two other main approaches have also been used, namely the use of twin-fluid nozzles and arrangements based on the use of solenoid valves. It should also be noted that the use of other types of spray generation system, such as spinning discs or air shear nozzles give other options for control of applied volume rate, but such arrangements are not commonly used on boom sprayers. 

Flow through a given nozzle is controlled by fixing the reservoir and receiver pressures. For a given flow through a specific nozzle, a unique pressure exists at each point along the axis of the nozzle. The relation is conveniently shown as a plot of p/po versus L, where pg is the reservoir pressure and p the pressure at point... 

As the ball reseats, it shoots a droplet of fluid out of the end of the nozzle. Adjusting the nozzle orifice, air pressure and fluid pressure controls the size of droplets. Precise heat control at the nozzle tip maintains fluid temperature at an optimum viscosity for jetting and reduces variation in production. 

The Megaject Mark II injection system, from Plastech IT, offers facilities such as pre-setting the amount of catalyzed resin to be injected (pre-determining counter), pre-setting safe nozzle pressure (mold pressure guard), catalyst flow sensor and auto-stop (Catal), and automatic valve and controller (autosprue). 

Temperature control of the hydraulic fluid is used to provide consistent stripping times. Proper washing of the cathode is provided by a sequence of wash solenoids and water pressure control to spray nozzles. 

Time, temperature, and pressure controls indicate whether performance requirements of a molded part are met. Time factors include rate of injection, duration of ram pressure, time of cooling, time of plastication, and screw RPM (Fig. 2-2). Pressure factors are injection high and low pressure, back pressure on the extruder screw, and pressure loss before the plastic enters the cavity, which can be caused by a variety of restrictions in the mold (Fig. 2-4). Temperature factors are mold (cavity and core), barrel, and nozzle temperatures, as well as the melt temperature due to back pressure, screw speed, frictional heat, and so on (see Fig. 2-5 and references... 

K control must be exercised from anywhere, apart from screw penetration, then most molders will go for line pressure control as this is easier, simpler and the system is less prone to damage it is also virtually maintenance free. The use of mold parting, which occurs on injection, is now a commercial option. This is a very easy system to fit as, no machining is required as the sensor location is on the outside of the mold. However, it is probable that with the development of suitable nozzle pressure transducers that, control will be exercised in future from this part of the machine. 

Switching from the first stage or velocity (V) control to the second stage or pressure (P) control is known as first to second stage transfer, or VP transfer. This switchover occurs as the screw moves forward and reaches a set point or position that provides the right amount of material to fill the mold 95-99.9% full. Other possible switchover conditions can be based on time, hydraulic pressure, or cavity pressure. Switchover by cavity pressure is more repeatable than by position, which is more repeatable than time. Switchover by hydraulic pressure is not recommended, as it limits the machine s ability to control velocity. To properly control the development of cavity pressure in the mold requires sharp machine response from the first to the second stage and accurate pressure control (see Figure 17, nozzle pressure curve). 

In the PGSS operations, the physical mixtures of the drug and the polymer are first exposed to SCF. In the presence of SCF and elevated pressure, the mixture starts to plasticize and melt. Following initial melt, introduction of the SCCO2 continues to dissolve the mixtures and the viscosity is decreased further. This nonviscous solution is then sprayed into a receiver using a nozzle and a pressure controlling valve. As... 

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