Pressure transducers for plastic substances
The present invention relates to pressure transducers for electrical measurement of pressure behavior during injection moulding of plastics and rubber. A large proportion of today&#39;s useful articles are made of rubber or plastics, produced by injection moulding under high pressure and temperature. The mould employed is usually of metal, consisting of more than one section, and is mounted in a moulding machine. In some cases, extrusion dies which are movable in relation to each other are also used. 
     The injection moulding and extrusion machines now in use are largely automated. Recently, such machines have been equipped with process computers, enabling a continuous operation under optimal conditions for the workpieces or articles to be made. Such computers have made it possible to determine the optimal filling quantity for each workpiece every time, allowing continuous operation without supervision by skilled personnel. 
     The process computer controls the machine on the basis of pressure, temperature and force measurements which are monitored and evaluated continuously. Particularly with exacting, intricate workpieces, it has been found that monitoring the pressure behavior during the injection moulding or extrusion operation is essential if workpieces with repeatedly accurate dimensions and characteristics are to be obtained. To achieve this, pressure transducers are mainly disposed at various points in the mould so that the front parts of the transducers are completely flush with the inner surface of the mould. It is, however, also possible to position the transducers in the nozzle of the moulding machine, although there is then no indication of the pressure distribution at critical points on the workpiece. 
     For these reasons, it is considered beneficial to fit pressure transducers directly into the moulds and dies. After completing a production run, the moulds with the transducers in position are put into storage until the same article is to be produced again. The process computer thus controls the machine according to a predetermined program, with the aid of the data measured at the temperature and pressure measuring points. In this way the required workpiece can be immediately produced again with the same tolerances and strength values, even after an interval of months, without any preliminary trials. Moreover, no highly paid skilled personnel is needed to get a second run started. By using injection moulds with permanently fitted monitoring elements at measuring points, which can be quickly connected to the control system of the machine, interruptions of work on expensive injection moulders are minimized, thereby bringing further savings on costs. 
     Pressure transducers for such demands must meet exacting requirements for continuous operation, repeatability, size, ambient temperature, etc. Owing to their size, the pressure transducers now available commercially for injection moulding can be fitted only at the nozzle of the machine. These commercially available transducers are too big for fitting into the moulds. Furthermore, their measuring systems cannot tolerate temperatures exceeding 80° C, because they are usually built on the operational basis of strain gauges. With these commercial pressure transducers, it is therefore necessary to separate a diaphragm part which is exposed to temperatures over 200° C from the measuring system, so that the necessary temperature difference can be obtained between the two parts. Examples of such transducers are found in the German Pat. Nos. 1,698,137, 1,936,528, 1,936,887 and 2,211,609. All these designs have resorted to complicated methods in order to eliminate as far as possible the influence of the necessary temperature differences on the pressure measurement. 
     Other methods are known, however, whereby force transmitting pins, similar to ejector pins, are fitted in the mould and connected to a force measuring cell. The ends of the pins are disposed flush with the surface of the mould. These pins vary in length and diameter according to the size of the mould, and are guided into fitted holes in the moulds. Owing to undefined lateral support, the clearance between hole and pin has to be very small. This means that the slightest distorsion of the mould exposed to high temperatures causes friction and sticking of pins, thereby leading to substantial and erratic errors in the transmitted force measurement. This method of measuring is therefore unsuited for reliable measurements. 
     By comparison the pressure transducers in accordance with this invention have a much simpler design. The measuring unit is in the form of a cylindrical element placed immediately behind the part where the pressure is to be measured. Such a unit can withstand temperatures above 200° C and is very rigid in construction. 
     The measuring system may be active or passive. Piezoelectric transducers are particularly suitable on account of their small susceptibility and sensitivity to temperature. However, there are also piezoresistive, inductive, capacitive and other measuring devices which would be suitable for pressure transducers in accordance with the invention. 
    
    
     The pressure transducer embodying the invention is shown without limitation in six illustrations, viz.: 
     FIG. 1: A cross section of a pressure transducer in accordance with the invention in the fitted state. 
     FIG. 2: A modified embodiment of FIG. 1. 
     FIG. 3: The frontal surface of the pressure transducer with the moulding plastics, on a very enlarged scale. 
     FIG. 4: The diaphragm part of the pressure transducer machined to match the inside shape of the moulding tool. 
     FIG. 5: Cross-section of a modification of the pressure transducer according to the invention. 
     FIG. 6: View of another measuring element fitted in the pressure transducer according to the invention. 
    
    
     As shown in FIG. 1, the pressure transducer consists of the housing 1 to which a screened or insulated connecting cable 2 may be joined with known connector means. The housing 1 is pressed onto the bearing surfaces 4 by means of the fitting screw 3. The measuring unit 5 is centered by the elastic clamping sleeve 6 and braced against the plane surface 8 of the housing 1 by the pressure piston 7 or diaphragm. 
     Owing to the narrowed portion 9, the clamping sleeve 6 is very elastic, consequently, the pressure p of the plastic mass is transmitted almost completely via the measuring unit 5 onto the transducer housing 1. The measuring system is susceptible to lateral forces and is enclosed by the sleeve 10, which is joined rigidly to the housing 1. The outside diameter of the sleeve 10 is dimensioned so that it can be inserted in the hole 11 virtually without clearance. The inside diameter D of the sleeve 10 and the diameter d of the pressure piston 7 are matched to each other so that a predetermined amount of uniform radial clearances 12 result. According to experience, such clearances should be less than 20 micrometers (μm). 
     The annular gap 12, which is formed by these clearances, is not sealed off with a metal diaphragm. Instead, the plastic mass 13 itself forms a membrane over this very fine gap, without penetrating into the gap, so that a sealing gap results between the pressure piston 7 and the sleeve 10. The exactly centered sleeve 10 needs to be cylindrical only on the inside diameter D, according to the invention, and its outer cross section might be □-shaped. The only essential condition is that the sleeve 10 must be rigid so that it is not deformed when mounted, otherwise the pressure piston 7 might touch the wall of the sleeve, which must be avoided. 
     To ensure high concentricity of the very fine sealing gap 12, the measuring device consisting of the pressure piston 7 and measuring unit 5 should be made as short as possible. With the clamping sleeve 6 in the form of a thin-walled, highly elastic tube, the measuring unit 5 is sealed in completely. 
     To prevent pressure building up in the space within the fine annular gap 12, which may possibly falsify the measurement, an air vent hole 14 may be provided although other means of connecting the gap space to atmosphere are also possible. 
     FIG. 2 shows a modification of FIG. 1. The pressure piston 7 has a reduced diameter from d to d 1  at the surface of the mould, and the sleeve 10 is correspondingly reinforced at this area by a flange portion. The critical annular gap 12 is maintained at this area of reduced diameter of the pressure piston 7. In this way, the force to be measured can be adapted largely to the capacity of the measuring cell. Often it is desirable that the frontal or facing area of the transducer to the plastic mass 13 should be as small as possible. In that case, the outside diameter of the sleeve 10 could also be reduced. 
     FIG. 3 shows the front part of a pressure transducer in accordance with FIG. 1 which is greatly enlarged. The annular gap 12 between pressure piston 7 and tubular sleeve 10 is bridged by the plastic mass 13. This mass penetrates to a depth f, which is less than the gap width S, preferably substantially less. This behavior is explained by the rapid cooling of the hot plastic mass against the water-cooled walls of the injection mould 15. When the cooled mass, i.e., the mould plastic mass 13, is expelled, the burr 16 is carried away with it. This burr 16 is found as a fine ring on the surface of the workpiece. 
     In special cases, it may occur that the pressure behavior is required to be measured at points on surfaces having no plane surface at right angles to the transducer axis. As shown in FIG. 4, the front part of the pressure transducer, i.e., the pressure piston 7, in accordance with the invention, may be modified by machining to a certain dimension X. The function of the transducer is not impaired by such shaping in any way. During machining, because of lateral elasticity, the pressure piston 7 may shift towards the sleeve 10 until it touches the wall, but it will move back into the central position again afterwards. It is advisable to finish the new surface 17 by grinding. The annular or sealing gap 12 may be covered over by a grinding burr, but the bypassing force resulting from this burr is negligible. The piston of the transducer may be machined on its front surface either separately or in the installed state. 
     FIG. 5 shows an arrangement wherein the exactly centered sealing gap, according to FIG. 1, can be achieved and maintained. The protecting and centering sleeve 10 can, for example, be drawn onto a fixed stop 20 of the body 1 with the threaded part 4. An axial preload can be achieved by other means also, however, as shown in FIG. 1 by the fitting screw 3. 
     The sleeve 10 is centered by the fitting or mating portion 16 of the measuring device 30. The measuring device 30 is joined securely to the body 1 with known means, and has three carefully machined cylindrical portions which ensure exact radial distribution of the minimum annular gap 12 of only a few μm, so that the portion of the pressure piston 7, which can be integrally formed with the unit 30, does not touch the wall at any point. These portions are the fitting portion 16, the measuring portion 17 in which the measuring element 30 is placed under elastic preload, and the sealing portion 18 of the pressure piston 7 with the exactly toleranced sealing gap 12, bounded by the cylindrical surface of the centering sleeve 10 having a diameter D. 
     The arrangement described might also be employed in succession, in accordance with the invention, by using a measuring unit 30 preloaded with a central means, such as a screw body 1, and the element 30. In all cases, however, the sealing zone 18 and measuring zone 17 must be made short, in order to bring the subsequent fitting zone 16 with the stop 20 as close to the sealing zone 18 as possible. It has been found that preferably the ratio of the length L of the measuring device 30 to the diameter D of the centering sleeve 10 should be less than 3 : 1. The invention is not limited to this dimension however. 
     FIG. 6 shows a typical arrangement assuring a high-precision sealing cap 12, according to the invention. The body 1 with the fully assembled measuring device 30 is mounted on a machine tool, preferably a circular grinding machine, after which the fitting zone 16 with stop 20 and (after adjusting the rest by the specified gap width 12) the sealing zone 18 are machined in one chucking. In this way, only the difference, and not an absolute dimension, has to be adjusted on the machine tool, and this can be done with great accuracy. 
     Calibration of the pressure transducer during operation is no longer necessry. If it should nevertheless be required, the transducer can be mounted on a hydraulic testing device with an adapter. The plane front portion should first be sealed with adhesive film to provide the diaphragm function. Alternatively, a simple calibration is possible by introducing force onto the pressure piston 7 with a force measurement testing device, but this calls for great care. 
     The invention makes it possible for the first time to measure the pressure behavior directly on the workpiece during the injection moulding operation. In this way, many new possibilities have been opened up for the automation of plastics and rubber injection moulding and extrusion machines. The geometry of the transducer, which may for example conform to that of a standard ejector pin, as used in the plastics industry, with the measuring element fitted directly into the front part and protected by a strong centering sleeve, together with the described arrangements for obtaining an exactly toleranced and centered sealing gap, results in a miniaturized, high-precision measuring tool. This tool can be constructed to be fitted very simply in standard holes of a size 4 mm or less. It thus represents a considerable advance over the present state of the art. 
     The known errors, resulting from thermal shock, which occur when the hot plastic mass comes into contact with the usual metallic diaphragms, is not detectable with the arrangement of the invention. Finally, the ability to machine the front portion of the transducer to which this invention relates facilitates installation, and enables the transducer to be disposed in locations where previously ruled out. In this way, the techniques in accordance with the invention enable pressure transducers to be produced which for the first time can be disposed directly into even small injection moulds. By dispensing with complicated diaphragms, sticking pressure plugs, complicated temperature compensation systems etc. of the prior art, these transducers permit accurate, hysteresis-free and repeatable measurements of pressure behavior during injection moulding of plastic substances in a continuous operation, thereby opening up new technical possiblities. 
     While we have shown and described several embodiments in accordance with the present invention, it is understood that the same is not limited thereto but is susceptible of numerous changes and modifications as known to a person skilled in the art, and we therefore do not wish to be limited to the details shown and described herein but intend to cover all such changes and modifications as are obvious to one of ordinary skill in the art.