Device for detecting material stresses in mouldings and castings
BACKGROUND AND SUMMARY OF THE INVENTION 
     In the processing of plastics, metals, alloys etc., internal stresses occur during solidification. They are largely retained in the finished moulding or casting, and are responsible for quality deterioration. These stresses are concentrated at certain places in a cast, injection-moulded or pressed part, and are caused mainly by unequal shrinkages during solidification. 
     Hitherto it has not been possible to detect such stresses non-destructively, neither during the production process nor after completion of the finished product. 
     At the present time, internal material stresses can be measured only with a complex drilling technique, and only to a limited extent. The suspected critical stress point is drilled out in stages. During this operation, the surroundings are tested for stress changes with strain gauges applied. It is thus possible today to assess internal stresses resulting from the production process only by destroying the casting or moulding. This is a lengthy and costly procedure. 
     The device according to the invention uses new approaches to measure, for the first time, the shrinkage or expansion process during the solidification of the casting or moulding in its mould. The solidification process (i.e. the transition from liquid or viscous to the solid state) is accompanied by crystal transformations and hence volume changes is known. In metal powder injection moulding (MIM), the shrinkage factor may amount to as much as 20%. 
     That the accompanying shrinkage or expansion (for example when water freezes into ice) can cause considerable stresses in the casting or moulding is also well known. Knowledge of these stress processes and means of monitoring them thus open up new possibilities for quality control. 
     New and in accordance with the invention therefore is the utilization of this exactly limited phenomenon in time during the production process to measure the changes in the stresses at predetermined points on the inner wall of the injection or casting mould. A measuring device has a measuring surface fitted into the injection or casting mould flush with the workpiece surface, so that this can move freely in all three orthogonal directions, i.e. it is separated from the mould wall by a minimal air gap. This air gap is so small that the material cannot pass through it in liquid or viscous state, even though it is under high pressure. The gap width depends on the viscosity of the fluid material, and can be determined by empirical tests. The measuring surface of the device is preferably the front face of a sensor screwed into the mould, which measures shear and compressive forces and is able to convert these into voltage changes with the appropriate electronics. However other measuring surfaces are conceivable and in accordance with the invention. 
     In the figures that follow, the invention is represented as a typical application in plastics injection moulding technology. However the invention is also applicable in metal pressure die-casting and injection moulding, and in a number of other processes where shrinkage phenomena have to be detected. 
     The process and device will be explained with reference to the figures. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 Section through a plastic injection mould with moulding and measuring device. 
     FIG. 2 A device in the form of a screw-in sensor for detecting one compressive and two shear stresses, δx and δy. 
     FIG. 3 A measuring diagram with one compressive stress curve pz and two shear stress curves δx, δy. 
     FIG. 4 A measuring diagram with one compressive stress curve pz and one shear stress curve δx for various materials M, N. 
     FIG. 5 A quality monitoring diagram with three assessment windows I, II and III for automatic process monitoring. 
     FIG. 6 A sensor according to FIG. 2 in section. 
     FIG. 7 A sensor similar to FIG. 6 with the measuring element accommodated in the actual sensor head 5. 
     FIG. 8 Plan section to FIG. 7 at VIII--VIII. 
     FIG. 9 Measuring rig for a single-component sensor δx. 
     FIG. 10 Measuring rig for a two-component sensor pz, δx. 
     FIG. 11 Alternative measuring rig to FIG. 10. 
     FIG. 12 Measuring rig and electrical circuitry for a two-component sensor pz, δx. 
    
    
     DETAILED DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows a section through a plastic injection mould, where 1 is the upper part of the mould, 2 the lower part, 3 the injection moulding or workpiece. The measuring device 4 is a cylindrical piece isolated from the wall of the mould, mounted with its face flush with the workpiece surface and sensitive in the three coordinate axes X, Y and Z. 
     FIG. 2 shows the front part of the measuring device 4 clearly. The sensor head 5 is positioned in the sensor sleeve 6 with the clearance S and supported on the sensor 7. By means of the screw thread 8, the sensor 7 is fitted in the lower mould part 2 for example. The shear stress δ reaches maximum values along the mould wall. By way of example, the measuring surface 9 is shown as a plane surface at right angles to the sensor axis, but it may also have more than one surface or be non-plane. 
     FIG. 3 shows stress curves recorded with a device in accordance with the invention. Two fundamental phases are conspicuous: 
     Phase A: Fluid phase, injection 
     Phase B: Solidification phase, shrinkage 
     While measurement of the compressive stress behaviour 10 is state of the art, 11 shows the shear stress curve δx, which yields previously unknown and surprising information about the solidification or shrinkage phase B. The invention exploits this information to open up entirely new possibilities. 
     Similar, though on much reduced scale, is the solidification shear stress δy orthogonal to δx. From these two values, after the familiar vector addition the magnitude of the maximum shear stress vector δmax and the direction angle α can be ascertained. 
     From the geometry of the workpiece, the expert can find the critical measuring point or points where the sensor should be placed. At the same time, he recognizes the direction in which the most dangerous solidification stresses act. In most cases, especially for production monitoring, he will therefore verify only one shear stress axis such as δx, to enable the influence of various process parameters to be assessed. 
     FIG. 4 shows a typical investigation of this kind with two different materials M and N. The compressive stress curves for different materials pz M and pz N are denoted by 15 and are almost identical. Very surprising, however, are the differences in the shear stress curves 16 during the solidification phase B, so that the two materials reveal considerable residual stress differences Δδx. From this it is clear that knowledge of the shear stresses opens up a new dimension with regard to quality demands. Interesting is the drop of the shear stress curve Δx M at K. 
     In FIG. 5 the three assessment windows I, II and III of a process-controlled injection moulding machine are explained. For this a two-component sensor pz, Δx according to the invention is mounted at the critical place. In window I the injection pressure limit curves 18 are monitored, in window II the shear stress limits 19 during the fluid phase, and in window III the limits of the shear stresses 20 during the solidification phase. On this new basis according to the invention a previously unattained quality level can be reached. 
     FIG. 6 shows a sensor according to FIG. 2 in section. The sensor may be fitted into the mould part 2 directly, or with the sleeve 6 which is part of the sensor 7. The clearance S may be provided in certain cases with a seal ring 22. A transmission pin 25 leads the measuring forces sustained by the sensor head 5 onto a crystal pair 23, which has like polarity, i.e. both crystal pieces 23 yield a like, i.e. negative charge on the inside when loaded in the Z direction. In accordance with the state of the art, electrodes 24 pick up the electrical charges from the metallized crystal surfaces and conduct them to connecting cable 36 (FIG. 12). 
     FIG. 7 shows a variant to FIG. 6, with the crystal array 23 accommodated in the actual sensor head 5 itself. 
     FIG. 8 shows the plan to FIG. 7. The crystal pair 23 is separated by the filler 26, and the electrodes 24 are fitted in the filler 26. 
     FIG. 9 shows the measuring rig and circuitry where the sensor is to measure only one component--the shear stress δx--which is the simplest arrangement. The electrodes 24 can be interconnected and their signals led to a charge amplifier 30, which delivers an output voltage U directly. This simplification necessitates a crystal pair of unlike polarity 23 (+) (-). 
     FIG. 10 shows the same configuration with two two pairs of crystals 35 of like polarity for two components pz and δx. 
     FIG. 11 shows the configuration with two pair of crystals 35 of unlike polarity, again for two components pz and δx. 
     The electronic outlay on charge amplifiers 30, summing units 31 and difference unit 32 is the same in both cases. 
     FIG. 12 shows more clearly the electronic circuitry of the arrangement according to the invention. The simpler two-component case is illustrated, fitted with the crystal pair 23 of like polarity, e.g. in the sensor according to FIGS. 6 and 7. The two-wire, externally shielded connecting cable 36 leads to the amplifier box 33, which accommodates the two charge amplifiers 30, the summing unit 31 and the differentiator 32. 
     The output consists of the two signals pz and δx and the &#34;operate&#34; connection for switching-on the units. Thanks to modern hybrid technique, all four subunits have small dimensions and are reliable and inexpensive due to solid-state technology. 
     The process and the device according to the invention for detecting material stresses will open up new possibilities above all in injection moulding technology with plastics, metals and ceramics. 
     Yet there are a number of other production processes in which the transition from a viscous to a solid phase causes material stresses to be set up, which likewise have defied detection till now. Here also the invention opens up entirely new avenues. With present technology piezoelectric crystal sensors can be engineered to withstand peak temperatures up to +500° C. 
     The examples shown can be embodied most simply with piezoelectric crystal elements, as has been illustrated, in particular with quartz elements. 
     However similar individual elements are possible based on piezoresistance with diffused silicon elements. Also measuring elements with thin films or strain gauges may be used. Nevertheless piezoelectric configurations are best suited on account of the simple signal pickup, and they are also the most reliable.