WO2018104979A1

WO2018104979A1 - Integrated system for the automatic correction of the viscosity of a ceramic mixture for a lost wax casting process

Info

Publication number: WO2018104979A1
Application number: PCT/IT2017/050003
Authority: WO
Inventors: Giovanni Marina CARLOMAGNO; Corrado NATALE; Massimo QUATRALE
Original assignee: Europea Microfusioni Aerospaziali S.P.A.
Priority date: 2016-12-07
Filing date: 2017-12-07
Publication date: 2018-06-14
Also published as: IT201600124073A1

Abstract

The present invention concerns an integrated system for the automatic correction of the viscosity of a ceramic mixture for a lost wax casting process, comprising a reservoir of said mixture, means for the blending of said mixture, means of continuous measurement of the viscosity of said mixture, means of feeding of a diluent in the vicinity of the bottom of said reservoir, said means of feeding of a diluent being controlled by control means connected to said means of continuous measurement of the viscosity of said mixture.

Description

INTEGRATED SYSTEM FOR THE AUTOMATIC CORRECTION OF THE VISCOSITY OF A CERAMIC MIXTURE FOR A LOST WAX CASTING PROCESS

The present invention concerns an integrated system for the automatic correction of the viscosity of a ceramic mixture for a lost wax casting process.

The invention refers to the field of lost wax casting, i. e. micro- casting or, with English terminology, investment casting, and concerns in particular an integrated system for making continuous measurements to control the viscosity of a ceramic mixture (slurry) for investment casting and, whenever necessary, to bring it back to a prefixed value by automatically adding the diluent liquid to the slurry (correction).

It is well known that the process of lost wax casting (investment casting or microcasting) is used to make almost finished metal castings, starting from a model, generally made of wax, which is then discarded. After casting, to meet the final dimensional requirements of the product, castings require relatively limited mechanical machining, even if the resulting casting shape is geometrically very complex.

According to this method, each casting is made from a wax model obtained by injection into a mould. Several wax models are then welded onto supports (casting channels), also made of wax, forming a so-called cluster. The term "investment casting" derives from the use of ceramic liquid mixture (slurry), in which the cluster is repeatedly immersed (invested) to form layer by layer a shell up to the desired thickness of the shell to be built. The wax model is then removed by liquefaction, usually in an autoclave that leaves the shell empty. The shell is then heated to a high temperature and filled with molten metal (casting) which then solidifies inside. After casting, the metal is left to cool down before removing the ceramic shell by mechanical or chemical methods to obtain the casting part, which is finally subjected to the latest mechanical finishing operations.

As already mentioned, the shell is made by repeatedly bathing the cluster in the slurry and removing, each time, by dripping the excess material (draining), in order to obtain a uniform surface coating. The first coating (primary) represents the most important layer in the shell construction phase, because it is the part of the ceramic mould that comes into direct contact with the molten metal. It is composed of finer refractory materials than subsequent (secondary) slurries to ensure a good surface finish of the product. Both primary and secondary slurry are contained in tanks (tanks) of suitable capacity to receive the cluster, placed in continuous rotation and that have fixed blades to avoid sedimentation of solid particles present in the same slurry. Multiple grouped tanks, usually served by a robot, form a cell.

As a matter of fact, the slurry is a suspension of fine solid particles in a diluent liquid, which is prepared by mixing the raw materials that compose it, having specific proportions. The main raw materials used for the slurry are: refractories (thin solid particles), binders, wetting agents and anti-foaming agents. The proportions of these materials play an important role in determining the final characteristics of the shell, and it is necessary that they remain constant during the manufacturing process. Moreover, the liquid part of the suspension tends both to evaporate (from the free surface of the slurry in the tank and from the rivulets associated with the drainage) and to be absorbed by the cluster that is plunged dry. As a result, the viscosity of the mixture tends to increase over time, resulting in a different adhesion of the slurry on the cluster immersed in it. It is therefore necessary to periodically check the viscosity of the mixture and, if necessary, add a certain amount of diluent to the mixture itself.

In the industrial practice of lost wax casting, the viscosity of ceramic mixtures is generally indirectly controlled by the plate weight method or by a viscosimetric cup. The plate weight (PW) method consists of temporarily placing a plate of a certain size in the slurry and weighing, after a predetermined time, the mixture which remains adhering to it. The viscosimetric cup provides, during the flow of slurry from a cup (having a hole in the bottom and size depending on the type of mixture), the measurement of the time elapsed between the moment in which the cup begins to empty and the moment in which the blade of mixture flowing from the hole breaks near the outlet hole of the cup itself. In both cases, therefore, the industrial methods used to measure the viscosity of ceramic mixtures make it possible to obtain relative measurements.

At present, the viscosity of ceramic mixtures for a lost wax casting process can be controlled on the basis of plate weight measurements. In most cases, these are carried out on average twice per work shift (i. e. every four hours). The time for checking an entire standard cell is approximately 30 minutes net of corrections. Ultimately, a working cell can be unavailable on average 2 hours a day, which results in the interruption of dipping operations. Similar times can be observed when measuring with the viscosimetric cup.

As far as corrections are concerned, current procedures require the addition of a diluent liquid, when necessary, downstream of the viscosity measurement. Usually, this addition is made directly on the free surface of the mixture and a waiting time is therefore provided after the addition (often about half an hour) to allow the mixing of the diluent throughout the mixture, before being able to use it again for the immersion of the clusters. Very often, the waiting time resulting from correction operations results in a 10 to 20 hours per week of interruption of immersion.

In this context, the solution is inserted according to the present invention, which aims to eliminate the downtime of each cell for measuring viscosity and for any possible corrections, as well as to reduce the variability of the measurement related to the operator who carries out the test.

These and other results are obtained according to the present invention by proposing an alternative integrated system, placed online, to monitor the viscosity of the mixture for a lost wax casting process and manage the corrections.

The integrated system for the automatic correction of the viscosity of a ceramic mixture for a lost wax casting process, according to the present invention, allows continuous measurements to be carried out to monitor the viscosity of ceramic mixtures for investment casting and, whenever necessary, automatically make additions of diluent liquid (corrections) to bring it back around a predetermined nominal value. In particular, the integrated system for the automatic correction of the viscosity of a ceramic mixture for a lost wax casting process according to the present invention contemplates the use of a vertical rod, largely immersed in the mixture of which to control viscosity, on the end of which near the bottom of the reservoir containing it is placed a body and at the other end of which, opposite to that at the bottom of the reservoir, a transducer is connected (which may be a strain gauge, a load cell, a torque transducer, or equivalent devices), whose measured value, due to the fluiddynamic resistance of the body, is put in correlation to the viscosity of the mixture. Correction is achieved by using a dosing pump, which gradually injects the diluent liquid and automatically activates according to the measurement from the transducer.

The aim of the present invention is therefore to provide an integrated system for the automatic correction of the viscosity of a ceramic mixture for a lost wax casting process which makes it possible to exceed the limits of systems and methods according to the prior art and to obtain the technical results described above.

A further aim of the invention is that this integrated system can be implemented at substantially low cost, both in terms of production costs and operating costs.

Last but not least, the aim of the invention is to propose an integrated system for the automatic correction of the viscosity of a ceramic mixture for a lost wax casting process which is simple, safe and reliable.

It therefore forms a specific object of the present invention an integrated system for the automatic correction of the viscosity of a ceramic mixture for a lost wax casting process, comprising a reservoir of said mixture, means for the blending of said mixture, means of continuous measurement of the viscosity of said mixture, means of feeding of a diluent in the vicinity of the bottom of said reservoir, said means of feeding of a diluent being controlled by control means connected to said means of continuous measurement of the viscosity of said mixture.

Preferably, according to the present invention, said means of continuous measurement of the viscosity of said mixture comprise a body apt to be immersed in the mixture and to be placed in relative movement with respects to the mixture, said body being connected, by mechanical connecting means, to a transducer.

More specifically, always according to the invention, said body apt to be immersed in the mixture is a fixed body and said relative movement of said body with respect to the mixture is generated by the rotation of said reservoir.

Additionally, according to the invention, said mechanical connecting means comprise a rod, apt to be immersed in said mixture, with a first end coupled with said body and a second end apt to be placed externally to said mixture and connected to said transducer.

In particular, according to the present invention, said transducer can be chosen amongst: a strain gauge, a torque transducer, a load cell, or equivalent means.

Alternatively, according to the present invention, said means of continuous measurement of the viscosity of said mixture comprise a viscometer, apt to be placed inside the mixture of which the viscosity has to be measured, by means of the provision of a shielding.

Lastly, according to the present invention, said means of feeding of a diluent at the bottom of said reservoir comprise a dosing pump.

It is evident the effectiveness of the integrated system for the automatic correction of the viscosity of a ceramic mixture for a lost wax casting process, according to the present invention, which makes it possible to carry out on-line checks and corrections without having to stop production, i. e. eliminating downtime and, at the same time, gaining in accuracy of the measurement and effectiveness of the corrections, which take place in a distributed and not concentrated way, as often happens according to the prior art.

The present invention will now be described, by way of illustration, but not as a limitation, according to a preferred embodiment, with particular reference to the figures of the enclosed drawings, in which:

- Figure 1 shows a view of an integrated system for the automatic correction of the viscosity of a ceramic mixture for a lost wax casting process according to the present invention,

- Figure 2 shows a first detail of the integrated system in Figure 1 ,

- Figure 3 shows a second detail of the integrated system of Figure 1 ,

- Figure 4 shows an illustrative diagram showing the correlation between the output of the transducer in millivolt (mV) and plate weight (PW), and

- Figures 5A and 5B show an example of the functioning of the integrated system for the automatic correction of the viscosity of a ceramic mixture for a lost wax casting process according to the present invention, in particular Figure 5A shows an illustrative diagram of the mV signal generated by the transducer as a function of time and Figure 5B shows the operating status of the dosing pump at the same time.

Referring to the figures, the integrated system for the automatic correction of the viscosity of a ceramic mixture in a lost wax casting plant, according to the present invention, involves the use of a fixed body 15, placed close to the bottom of the rotating reservoir 1 1 , at the end of vertical rod 18, largely immersed in the slurry. The other end of the rod 18 is connected to a transducer 16 (strain gauge, torque transducer, load cell, etc.) that measures the torque present there. This moment is due to the fluiddynamic resistance force of the body, and to a lesser extent of the rod, in the slurry moving due to the reservoir rotation. Since slurry viscosity is very high, the fluid-dynamic resistance of the body (next to Stokes regimen) can be inherently linked to the viscosity of the mixture. Therefore, the transducer output allows direct monitoring of the slurry viscosity. The measurement can of course also be performed with a more expensive conventional viscometer suitably shielded to avoid the influence of slurry motion. Figure 1 also shows fixed blades 24 that stir the slurry during its rotation.

The advantages of using a measuring system with a transducer compared to conventional measurements, using the plate weight method, or with a viscosimetric cup, are: - elimination of the cell downtime to perform control tests;

- continuous measures over time, which ensure more accurate control of the state of the mixture;

- elimination of measurement uncertainty due to the operator carrying out the measurement.

To make in-line corrections without the need to stop the immersion operations, the system uses a dosing pump 20 that gradually feeds the mixture diluent through a tube 25 the outlet of which is placed near the bottom of the reservoir containing the slurry, in particular, near the fixed body 15. This position near the bottom of the reservoir is due to the fact that the diluent liquid has a relative density close to the unit, while the slurry density is higher (generally between 1 .5 and 3.5, depending on its composition), preventing the diluent from tending to stratify, floating on the mixture, with a consequent increase in mixing time.

The use of the pump 20, which gradually doses the diluent, offers a twofold advantage:

- eliminating the waiting time to obtain a complete mixing between the diluent and the mixture;

- making a liquid addition in a distributed over time and not concentrated way.

The final objective of the system object of the invention is to integrate the two techniques described above so as to have a fully automated system that can make corrections, when necessary, on the basis of the transducer's measurements, without stopping the operations of immersion of the clusters.

In order to verify the functioning of what is proposed, the results of an integrated measurement system (with strain gage transducers) and viscosity control of a mixture for a lost wax casting process are given as an example only, which system, with reference to Figure 1 , has been realized in such a way as to include:

- a transducer 16 consisting of two semiconductor strain gages;

- a stabilized power supply with fixed voltage to supply the two strain gages; - a data logger 19 which, in addition to measuring the output of the strain gages 16 and feeding them, also makes it possible to control the automatic operation of a pump 20, depending on the value of their output;

- a measuring rod 18, having at its bottom a fixed body 15 immersed in the slurry of which the viscosity has to be checked and at its top the transducer 16, mounted on top of a container 12 external to the rotating reservoir 1 1 ;

- a peristaltic pump 20 with a constant flow rate of 400 imL/h and a pressure head of 1 .5 bar, connected on the one hand to a reservoir 22 of diluent and on the other hand to a tube 25 which, running inside the rod 18, has its outlet close to the bottom of the reservoir 1 1 ;

- a reservoir 22 containing a correction liquid (demineralised water in this case).

In particular, the data logger 19, which also includes the stabilized power supply and the transducer signal meter, activates the pump 20 (mode 1 ) if the output of the transducer 16 rises above a set value (mVmax, high slurry viscosity) and stops it (mode 0) when the signal drops below a lower value (mVmin, low viscosity), which is also prefixed.

Figure 2 shows a detail of the area surrounding the body 15 immersed in the mixture, in which are visible: the body 15 itself, the lower end of the rod 18 supporting it, the exit of tube 25 for the injection of the diluent (whose outlet is also placed close to the bottom of reservoir 1 1 which, in this case, runs inside the rod 18.

Figure 3 shows a detail of the area around the transducer 16, in which are visible: the transducer 16 itself, attached to the external fixed container 12, the upper end of the rod 18 connected to it, the cable 31 connecting the transducer 16 to the data logger 19, the tube 25 for the injection of the diluent that enters the rod 18 and then runs inside it and goes out to its outlet, part of the rotating reservoir 1 1 , part of the outer container 12 and the free surface of the mixture 38.

Figure 4 shows the regression curve between the output of the transducer 16 (strain gages), expressed in millivolt, and the plate weight (PW) measurement, expressed in grams. It is fairly linear and shows a strong dependence of the fluiddynamic resistance of the body immersed in the slurry on the plate weight value.

Figure 5 shows an example of how the integrated transducer/pump system works in a rotating reservoir 1 1 containing a equiasy primary in the absence of cluster immersion. In particular, the diagram above shows the mV signal generated by the transducer 16 as a function of time; the diagram below shows the operating status of the dosing pump 20. During the period under consideration, pump 20 operated automatically (mode 0 - off; mode 1 - on).

From the comparison of the output data from the transducer 16 with the plate weight measured at the same time, a range in millivolts has been chosen to adjust the pump 20 start/stop. In particular, to lower the viscosity of the mixture, the injection of diluent liquid starts when the transducer 16 signal exceeds the maximum value of the range indicated with mVmax (31 .20 mV, in this case), and stops when it falls below the minimum value indicated with mVmin (30.65 mV), when the nominal viscosity is restored.

Despite the unavoidable measurement noise, the measured strain gauge measurements remain confined in the chosen range and follow a "regular" pattern, with an almost constant slope between two peaks; the peaks at the top are always corresponding to the start up 20 of the pump. The pump 20 works on average for 15 minutes after switching on, therefore, since it has a constant flow rate of 400 imL/h, it means that each time it performs a correction of about 100 mL distributed over time. The reference parameter against which to set the pump intervention interval has always been the PW, which, in this case, must remain around its nominal value of 1 1 grams for the mixture considered.

The diagrams in Figure 5 show, additionally, that the time interval during which the strain gauge signal decreases as a result of correction (decrease in viscosity) is practically the same as the period during which the pump operates. This indicates that the diluent injection method adopted is very effective, resulting in an almost immediate mixing of the diluent in the mixture.

It can also be seen that the slope of the curve between peaks is almost constant; this is another proof in favour of the correlation between strain gauge and viscosity measurements. In fact, the almost constant slope indicates a practically constant evaporation of liquid from the free surface, which leads to a constant increase in the viscosity of the mixture. Measurement with strain gauges is certainly more sensitive, objective and reliable than plate weight or viscosity cup, both because it is continuous over time and because it is not subject to accuracy errors linked to the operator performing the check.

With the system described above, a standard field trial was also carried out, in particular, on a rotating tank of equiaxic secondary, in which measurements of slurry level and cluster immersion were also performed, to carry out a series of tests including:

- variation of the strain gauge signal during plate weight measurement,

- possible influence on the measurement of the opening/closing of the lid of the reservoir,

- influence of measurement by the laser mounted on the robot of the bath level,

- influence of cluster immersion on the signal, and

- influence of the corrections.

The variation in strain gage signal during plate weight measurement was relatively small.

The opening/closing of the lid of the reservoir did not give rise to appreciable variations in the strain gauge signal, partly because the cable connecting the transducer 16 to the data logger 19 was shielded.

The measurement of the bath level by the robot showed a sharp decrease in the signal. This is because, in the cell used, the measurement of the bath level by the laser is made with a simultaneous temporary decrease of the reservoir rotation speed (to dampen the surface waves). This decrease inevitably results in lower body resistance 15 and therefore in a lower strain gauge signal.

The influence of the immersion of the rotating cluster on the signal was also quite consistent, of the same order of magnitude as the measurement of the bath level, but with a longer duration. This, in addition to the variations induced on the slurry's motion field due to the presence of the cluster, is also partly due to the rotation movement of the cluster.

These temporary decreases, as recorded by the data logger 19, would result in an interruption of pump 20 operation if it were running. To reactivate, pump 20 would then have to receive from the transducer 16 a signal higher than mVmax and, therefore, its operation would be interrupted by these decreases. It was therefore decided that the interruption of pump 20 operation should only take place when the value of the strain gage signal lower than mVmin (to change from mode 1 to mode 0) has a consecutive duration equal to a certain time interval At, in this case At = 80 seconds (immersion time plus substantial conclusion of the induced transient motions).

The influence of corrections on the strain gage signal was similar to that found on the reservoir containing the primary mixture.

In conclusion, according to a preferred embodiment of the integrated system for automatic correction of the viscosity of ceramic mixes, in particular refractory mixes for investment casting, according to the present invention, it is provided for the presence of:

- a vertical rod 18 partially immersed in the reservoir 1 1 containing mixture;

- a body 15, located at the bottom end of the rod 18 and immersed in the mixture;

- a transducer 16, external to the mixture and connected on one side to the other end of the rod 18 and on the other to container 12, whose output, resulting from the fluiddynamic resistance of body 15 in the mixture, indicates the viscosity of the mixture itself;

- a dosing pump 20 that feeds a diluent liquid into the mixture by means of a tube 25, the outlet of which is immersed in the mixture, the operation of which is automatically adjusted according to the measurement from transducer 16 (in terms of voltage or other electrical quantity);

- a data logger 19 which, in addition to measuring the output of the transducer 16 and powering it, automatically controls the operation of the pump 20, depending on the value measured by the transducer 16;

- a reservoir 22 containing the correction liquid which feeds the dosing pump 20.

More specifically, in the integrated system for the automatic correction of the viscosity of refractory mixtures for a lost wax casting process according to the present invention, the transducer 16 power supply unit and/or the transducer 16 output meter and/or the data logger 19 constitute separate units.

Additionally, in the integrated system for automatic correction of the viscosity of refractory mixtures for a lost wax casting process, according to the present invention, the body 15 placed at the lower end of the rod 18 may have any shape, tapered or otherwise, in particular that of a blade.

Alternatively, according to the invention, in the integrated system for automatic correction of the viscosity of refractory mixtures for a lost wax casting process, the transducer can consist of one or more strain gages, or a load cell, or a torque transducer.

Alternatively, additionally, in the integrated system for automatic correction of the viscosity of refractory mixtures for a lost wax casting process according to the present invention, the rod 18, the body 15 and the transducer 16 can be replaced by a conventional viscometer, suitably shielded to avoid the influence of slurry motion.

In particular, in the integrated system for automatic correction of the viscosity of refractory mixtures for a lost-wax casting process of the present invention, the diluent liquid dosing pump 20 may consist of a diaphragm or piston or peristaltic, gear or centrifugal pump.

Alternatively, in the integrated system for automatic correction of the viscosity of refractory mixtures for a lost wax casting process of the present invention, tube 25 that feeds the diluent liquid into the mixture can be connected to the rod 18, i.e. it can run inside or be separated from the rod 18.

Lastly, according to the present invention, in the integrated system for automatic correction of the viscosity of refractory mixtures for a lost wax casting process, the interruption of the dosing pump that feeds the diluent liquid into the reservoir of the mixture occurs only when the transducer signal value is lower than the preset mVmin for a predetermined period of time.

The present invention has been described by way of illustration, but not as a limitation, according to its preferred embodiments, but it is to be understood that variations and/or modifications may be made by the experts in the field without departing from the relevant field of protection, as defined by the attached claims.

Claims

1 ) Integrated system for the automatic correction of the viscosity of a ceramic mixture for a lost wax casting process, comprising a reservoir of said mixture, means for the blending of said mixture, means of continuous measurement of the viscosity of said mixture, means of feeding of a diluent in the vicinity of the bottom of said reservoir, said means of feeding of a diluent being controlled by control means connected to said means of continuous measurement of the viscosity of said mixture.

2) Integrated system for the automatic correction of the viscosity of a ceramic mixture for a lost wax casting process according to claim 1 , characterised in that said means of continuous measurement of the viscosity of said mixture comprising a body (15) apt to be immersed in the mixture and to be placed in relative movement with respects to the mixture, said body (15) being connected, by mechanical connecting means, to a transducer.

3) Integrated system for the automatic correction of the viscosity of a ceramic mixture for a lost wax casting process according to claim 2, characterised in that said body (15) apt to be immersed in the mixture is a fixed body and said relative movement of said body (15) with respect to the mixture is generated by the rotation of said reservoir (1 1 ).

4) Integrated system for the automatic correction of the viscosity of a ceramic mixture for a lost wax casting process according to claim 2, characterised in that said mechanical connecting means comprise a rod (18), apt to be immersed in said mixture, with a first end coupled with said body (15) and a second end apt to be placed externally to said mixture and connected to said transducer.

5) Integrated system for the automatic correction of the viscosity of a ceramic mixture for a lost wax casting process according to claim 2, characterised in that said transducer is chosen amongst: strain gauge, torque transducer, load cell having a voltage output signal or other electrical quantity.

6) Integrated system for the automatic correction of the viscosity of a ceramic mixture for a lost wax casting process according to claim 1 , characterised in that said means of continuous measurement of the viscosity of said mixture comprise a viscometer, apt to be placed inside the mixture of which the viscosity has to be measured, by means of the provision of a shielding.

7) Integrated system for the automatic correction of the viscosity of a ceramic mixture for a lost wax casting process according to claim 1 , characterised in that said means of feeding of a diluent in the vicinity of the bottom of said reservoir comprise a dosing pump (20).