Piston Forces

RATING AND DESIGN RULES FOR ROD AND TUBE EXTRUSION PRESSES 

Calculation of the extrusion force, i. e. of the necessary ram force, at close approximation is only possible in few, particularly simple cases, because it depends on a variety of factors that can only be evaluated in an imperfect manner. These factors include the resistance to deformation of the alloy to be extruded, in dependence of the temperature and speed at which the work is performed, the resistances caused by the varying flow phenomena in the billet, the frictional resistance on the tools, the extrusion ratio, the shape of the section, etc. The latter factor alone may essentially increase the required force, if the section is of intrical shape as compared with round rods of identical cross-sectional area (see Fig. 107). 

These calculations require first of all a clear understanding of the internal flow phenomena in the billet. An important point to be noted here is that - in direct extrusion - it is not the complete billet, as it is pushed forward by the ram, that flows towards the die, but that certain portions of the billet even flow back against the ram. 

The flow of metals may easily be ascertained from experiments ) run on a 1,500-ton capacity rod extrusion press, in which - as illustrated in Fig. 108 - a 100 mm rod was extruded from a 163 mm container. The billet was made up with disks of 50 mm thickness and 150 mm diameter (sketch a) alternatively of copper and brass, and first of all compressed to a diameter of 163 mm and a total billet length of 592 mm (sketch b). Sketches c-e show clearly how the speed of flow is slowed down by the friction of the billet on the container wall and how the various disks bulge out towards the die aperture. From the movement of the various disks it is seen that disk no. for example, is dislodged for a distance of 205 mm from the die after a ram stroke of 92 mm (see sketch c). In sketch b the distance between this very edge and the die - in the com- 

Also it has been ascertained that only little movement occurs at the container wall and that it becomes more intense in the shell zone. Consequently, the resistance to friction inside the material is lower than that at the interface of billet and container wall. 

---

Capillary rheometers are used extensively to measure viscosity in the intermediate to high shear rate range. The rheometer has for all practical purposes a lower limit in viscosity measurement because of the plunger seals. These seals are shown on the bottom of the plunger in Fig. 3.16, and they induce a frictional resistance when they are pushed through the rheometer barrel. The piston force can be evaluated without polymer in the barrel, but it is always a source of error at low viscosities because of experimental variability. Moreover, barrel friction is one of the critical corrections that must be made when evaluating viscosity measurements... 

Fig. 13.25 Experimental setup on an apparatus for displacing a multilayered, two-color melt by the driving piston force (Fp - Fg). [Reprinted by permission from E. Vos, H. E. H. Meijer, and G. W. M. Peters, Multilayer Injection Molding, Int. Polym. Process., 6, 42-50 (1991).]...

Equations (1) to (7) are rather general and apply to conical or pyramidal pistons. For example, if a = 45° and we let (h — ho) be 6Ro, the maximum chamber pressure is about 45 and approximately 14% of the piston force is due to chamber pressure 86% is ideally taken by gasket load. 

A sample of chlorine gas is used as a pool disinfectant. The sample occupies a volume of 500 mL in a cylinder with a moveable piston. The piston forces the gas out into the water when needed. The gas is stored at 25.0 atm at standard temperature. If the temperature remains unchanged and the piston compresses the gas to 220 mL, what is the pressure inside the cylinder ... 

High-duty pumps are equipped with steam cylinders in which the steam is used expansively. As the load is constant throughout the stroke and the steam piston force diminishes during the period of expansion, some compensating device is necessary to equalize the forces. This may be a fly wheel, an auxiliary... 

The important parameters are temperature, pressure (or piston force) and time. The process is usually described in terms of pressure cycle. Figure... 

The particular ESM extruder was modified in such a manner that it can be used with a measuring barrel serving as a large capillary rheometer, whereby an auger in lieu of a pressure piston forces the material into the nozzle. This means that the pressure drop is measured analogously to the capillary rheometer. The results of this test produce the yield point and the apparent flow curve of the body, amongst other readings. 

A capillary rheometer (Fig. 6.8) is similar to a melt indexer. The main difference is that instead of by gravity, the piston is driven by a variable speed motor. Also, a load cell in-line with the piston measures ram force in real time. This configuration allows tests to be run at a controlled shear rate, and even to vary the rate over a large range during a single test run. The piston force and the orifice geometry provide the data necessary... 

Figure 8.14 Opposed-piston compressor balances piston forces...

These pumps are often used for processing highly viscous adhesives. The design of a twin piston pump is schematically shown in Fig. 38.4. The special feature of a twin piston pump is that it transports materials in both movement directions. This is achieved due to the fact that the two chambers, which are separated by the piston, are kept in phase with each other by a nonreturn valve, and the cross section of the lower chamber is twice as large as the cross section of the upper chamber. When the pump is at the bottom dead center, both nonreturn valves close (= piston valve and foot valve). If the piston now moves upward, the foot valve opens and the lower chamber is filled. Simultaneously, the adhesive in the upper chamber is transported via the outlet into the connected pipe system. After reaching the top dead center and reversal of movement of the piston, the piston valve opens and the foot valve closes. The upward movement of the piston forces the volume of adhesive from the lower chamber... 

In process 2-3, the displacer is moved toward the piston, forcing the compressed gas through the regenerator where it is cooled. The energy removed is stored in the regenerator. 

The apparatus used was a non-commercial porosimeter which enabled us to make accurate measurements between 0.4 and 2000 bar corresponding to a pore radius between 2 10 and 3.6 nm. The porosimeter consists of a steel cylinder and a piston forced into the cylinder by a ram (ref. 2). The pressure and the volume change were measured accurately by a strain gauge and by the displacement of the piston respectively. Up to a pressure of 5 bar the mercury was forced into the porosimeter by an air pump and the amount of mercury was determined accurately by a balance (ref. 3). Fig. 1 shows schematically the... 

---