Patent Grant
8225844
The present invention relates to a method for processing moulding sand.
Casting is probably the most important traditional moulding method. A melting charge of the material to be processed is cast into a mould in which it then solidifies to produce the casting.
Frequently, so-called lost moulds are used. Such moulds are produced from moulding sand, i.e. quartz sand and a bonding agent. Such moulds are usually formed by taking a mould from models. The liquid material is then cast into the mould. After the material has hardened, the moulding sand can be removed, i.e. the casting is unmoulded, whereupon the mould is destroyed. For this reason, this type of mould is known as a lost mould.
In order to optimize mould production the sand must, inter alia, be supplemented with suitable bonding agents. When producing moulds, it is thus essential to ensure that the properties of the moulding sand to be used are as best suited to the material as is possible. Thus, for example, the casting material used and the associated melting charge temperature and also the external and if appropriate the internal contour of the mould must be taken into account.
The quality of the moulding sand primarily depends on the clay content, the grain size and distribution, the shape and surface area of the quartz bodies, the type and quantity of the auxiliary materials, the moisture content and the degree of compression.
For economic and environmental reasons, the spent moulding sand is usually processed and reclaimed as completely as possible; the more so because 5 to 15 parts by weight of moulding sand is generally used per unit weight of casting. At least 90% of the spent moulding sand can be processed and fed back into the moulding circuit, since the bond coatings are usually still effective, so that only water and occasionally bonding agent has to be added. A portion of the old sand is discarded from the circuit and replaced with fresh replacement material.
Frequently, clay bonded moulding sands are used, which are usually fed back into the preparation method following the casting process, where appropriate quantities of water, bonding agent (for example bentonite), additives (for example coal dust) and fresh sand are again added to the old sand.
Processing is generally carried out in a mixer and usually under vacuum, in order to cool the moulding sand down at the same time. On processing, care has to be taken to ensure that the bonding agent coats the quartz sand grains in an optimal manner.
The intention of this processing is so that the processed sand leaving the mixer is of uniform quality. The quality of old foundry sand, however, varies due to the thermal load during the casting process which depends on the production program used, so that constantly, old sand with a variable moisture and clay content is fed back to the processing plant.
The perpetual aim of a properly functioning process is thus to detect variations in the old sand and to correct them by taking corrective measures in the preparation process, such as adapting the water addition or the bond content.
Many very different methods can be used for this purpose. As an example, the method disclosed in DE 32 20 662 usually employs a measuring device located downstream of or directly in the mixer to take a sample and directly determine the compressibility as well as other parameters, such as the compressive strength and/or shear strength. In addition, the moisture content in the old sand in the mixer is directly determined using a moisture sensor in order to correct the quantity of water to be added and the compressibility and moisture content data obtained are used to correct the quantity of water as well as the auxiliary material to be added in order to achieve a constant moulding sand quality for the processed moulding sand.
The disadvantage of all of these methods is that one or more additional, possibly expensive measuring devices are required to determine the moisture content as well as the sand parameters under consideration.
CH 517 541 discloses a method for adjusting the moisture content of mixing goods whereby the water is added to the mixture intermittently in two or more stages with varying addition and rest times as a function of several adjustable values for the output of the motor of an auxiliary tool, until a pre-set reference value is successively reached. Since the change in the motor output following addition of a certain quantity of water does not take place abruptly, but a certain mixing time is necessary until a stationary condition and thus a constant value for the measurement is reached, the mixing time changes substantially as a function of the required total water content. In order to achieve a uniform sand quality, in addition to as constant a moisture content as possible, a constant mixing time is also necessary for a sufficiently high water content which, however, is not achievable with the method cited above.
DE 2053936 describes a further development of CH 517 541 wherein in addition to the current drawn by the fast-running auxiliary tool, the current drawn by the rotating mixing container is considered for more precise determination of the moisture content of the mixture.
Here again, water addition is carried out successively in several steps into the moulding sand mixture to be processed at that time by controlling magnetic valves in water supply lines. In addition, to correct the quantity of water, a temperature signal is incorporated into the calculation. Even this improved solution results in unnecessarily long and above all variable length wet mix times, since after every addition of a part quantity of water, the mix needs a certain time before it builds up a constant higher resistance to the mixing tool.
DE 1 947 566 discloses a method with a mixing drum inclined to the horizontal fed continuously with a stream of moulding sand, whereby the motor output of the rotating mixing drum is used to regulate the added moisture. With varying added quantities or varying starting moisture contents for the added moulding sand, here again the amount of bulk in the drum varies and thus so does the mass of solid as well as the power consumption of the motor, so that long wavelength fluctuations in the properties of the old sand cannot be compensated for.
U.S. Pat. No. 3,838,847 discloses a further development of DE 1 947 566 wherein the liquid is admitted into a conical mixing drum inclined to the horizontal fed continuously with a stream of moulding sand as a function of the torque of a mixing tool operating in counter-current mode to the mixing container so that the torque on the mixing tool is constant.
The disadvantage of this solution is that the residence time in the mixer cannot be tailored and is dependent on the dosing power of the supply conveyor. Furthermore, the angle of repose of the inflowing moulding sand in the drum mixer is strongly dependent on the initial moisture content and thus the mixing tool is covered in variable amounts of moulding sand, which also has a large effect on the motor output. Since in addition, here again a certain time is required following the addition of the water to the moulding sand until the water can produce a noticeable change in the resistance and thus the power consumption, this method can easily result in overwetting of the moulding sand.
A similar situation occurs with the batch method described in DE 1 301 874, whereby water addition is continuous into the mix, after adding the old sand, until a certain power consumption is measured on the rotor. Due to the time-lapsed reaction of the mix in response to the water addition and the extreme dependency noted in the document of the consumption on very small changes in moisture content in the region of the desired final moisture content, this method can swiftly result in overwetting of the mixing material. Because of this problem, that same inventor developed the successive addition of portions of water as described in DE 2 053 936 and CH 517 541 with appropriate intervals between the individual addition steps.
JP 56053844 describes a method for correcting moulding sand quality by altering the weighed-in quantities of solids, which results from the time-programmed addition of old sand into a hopper, by measuring the output of a milling drive. In the method of the invention, the moisture content and the bentonite content of the old sand in a milling mixer are corrected on the basis of a difference in the motor output between a measurement following addition of old sand to the mixer and a second measurement following addition of a predefined quantity of water and bond as well as a fixed mixing period.
The compensation for the missing water and bond quantity is carried out following a second measurement into the same charge of moulding sand based on the experimentally determined relationship between the moisture content and the difference in motor output and the bond content and the difference in motor output. The simultaneous correction of two operational parameters, moisture content and bentonite content, which are also interdependent, based on only one measured parameter, motor output, as well as at a non-constant quantity of material in the mixer and also variable old sand compositions, necessarily results in larger rather than smaller variations in the quality of the moulding sand.
The aim of the present invention is to provide a method with a simple adjustment system to control the compressibility of a reclaimed moulding sand.
This aim is achieved by dint of a method for processing moulding sand, having the following steps:
Accordingly, initially a portion of the moulding sand to be processed is placed in a mixer and the force required to move the mixing tool is measured. The simplest way to measure this force is indirectly via the output of the mixer. It is not absolutely necessary to determine an exact value for the required force; rather, it is entirely sufficient to measure a magnitude which represents a measure of the required force since in this method it is not so much the force but rather the compressibility of the moulding sand which is of importance. There are many methods of measuring the compressibility of moulding sand. If, for example, the moulding sand is placed in a measuring cylinder and compressed with a predefined pressure, then the reduction in height of the moulding sand in the measuring cylinder, as a %, is termed the compressibility.
It is known from DE 3220662 that the compressibility of moulding sand is approximately linearly dependent on the degree of wetting or the moisture content for a constant clay content.
Experience has shown that this relationship is only valid for moisture contents of more than 2%. Below a moisture content of 2%, the relationship is distinctly non-linear, since insufficient binding of the grains of sand in the moulding sand is present. The compressibility increases with increasing clay content.
As an example, a constant quantity of old sand is charged into the mixer via a gravimetric solids weighing hopper. After all of the old sand has been added to the mixer, the power consumption of the drive motor MP1 is recorded and transformed into an actual moisture content F1 using the experimentally determined calibration curve between motor output and the moisture content. Using the known relationship between moisture content and compressibility for a given clay content SG, the necessary reference moisture content F reference is determined from the reference compressibility V reference and the resulting moisture content difference ΔF1 is compensated for by a single addition of water to the mixer.
After adding the quantity of water, the moulding sand is processed by mixing for a predetermined mixing period in the mixer and at the end of processing of this portion of moulding sand, shortly before emptying, a second measurement MP2 is taken of the output of the mixing tool. Using the known relationship between the output and the moisture content, an actual moisture content F2 or actual compressibility V2 can thus be determined for the moulding sand. Because of variations in the clay content of the old sand, this can now lead to divergences between the reference compressibility Vreference and the measured actual compressibility V2.
The difference in compressibility ΔV2 arising from the divergence is now transformed into a moisture content corrective value Fcorr using a predetermined correction function which is taken into account for the subsequent moulding sand charge to be processed when determining the required quantity of water to be added.
Freference, i=F1,i+ΔF1−Fcorr,i+Fevap(Ti) (1)
where Fcorr,i=Fcorr,i−1+Fcorr(ΔV2,i−1)
where i is the charge number, i.e. i=1 for the first moulding sand portion, i=2 for the second portion of moulding sand, etc.
Processing of the subsequent moulding sand portions is thus influenced by the corrective measurement which was made for the immediately preceding step for processing the moulding sand portion following its processing. By means of this corrective intervention in the processing of the subsequent moulding sand portion, on the one hand the mixing time in the mixer can be kept constant and on the other hand, long wavelength variations in the old sand composition can be compensated for. This results in auto-adaptation of the corrective water quantities to gradual changes in sand composition. In other words, the compressibility at the end of processing is monitored and—if a divergence from the reference value is observed—the processing of subsequent moulding sand portions is correspondingly adapted. The corrective value is no longer applied to the moulding sand portion for which the divergence was established, but only to subsequent moulding sand portions to be processed.
In case the moulding sand to be processed has a raised temperature with respect to the surroundings, following the addition of water, a portion of the water which is to be added is evaporated in the parts of the plant which are downstream of the mixer, for example the discharging belt. In order to compensate for this loss of moisture, in a preferred implementation, the expected loss of moisture through evaporation is calculated from the temperature of the old sand using an energy balance and this additional moisture Fevap(T) is also added to the moulding sand.
In a further embodiment, the mixer is evacuated during processing. This results in a reduction of the boiling point of the water contained in the moulding sand, so that at least a portion of the water evaporates and the evaporation energy required means that the remaining moulding sand is effectively cooled. Since the reclaimed moulding sand is primarily obtained from the destroyed mould, it is in any event too hot for further processing and must be cooled down. Processing under vacuum not only shortens the preparation method, but also results in better quality of the moulding sand to be processed.
In order to retain the moisture content of the moulding sand, in this variation for processing the moulding sand, in addition to being supplemented with a quantity of water for evaporation, which in this case is given by the final temperature of the moulding sand to be processed which corresponds to the applied final pressure, a supplement prior to processing of exactly the quantity of water Fcool which is necessary to cool the moulding sand from its actual temperature to the reference temperature is made. To this end, a measurement of the temperature of the unprocessed moulding sand can be used, whereupon the temperature measurement can be carried out in the old sand supply line.
The temperature of the old sand which, for example, is conveyed via old sand belts to the weighing hopper, is then captured on the way to the weighing hopper and used for the subsequent water correction to compensate for the evaporation water or for processing under vacuum to determine the water content which is used for evaporation cooling.
Thus, the temperature-dependent water loss Fevap(T) by evaporation is calculated by the previously measured old sand temperature in the old sand or from the boiling point calculated from the final pressure of the vacuum processing using the steam pressure curve in known manner via an energy balance, and is then added to the mixture in addition.
In a particularly preferred implementation, the corrective function for the moisture content correction as a function of the determined moisture content difference between the actual compressibility and the reference compressibility at the end of the preparation is divided into 3 sections. In a first section, the corrective function follows an nth order polynomial with n>1, so that small divergences result in only small changes in the moisture addition and large divergences have a greater effect. In a second section, which lies directly next to the first section, the moisture content correction follows a linear relationship, and in a third section, which lies directly next to the second section, it is limited by the set maximum value.
In a further implementation of the invention, the correction of the compressibility difference is carried out alternatively to or in combination with the addition to the mix of new sand or a mixture of finely divided materials such as bentonite, coal dust and filtered dust. After addition of a predefined and gravimetrically checked quantity of solid into the mixer is complete, the power consumption of the drive motor is recorded and transformed into an actual moisture content using the calibration curve between the motor output and the moisture content. The difference between a previously defined final moisture content taking into account the evaporation water based on the temperature of the old sand is compensated for by adding water to the mix.
Freference,i=F1,i+ΔF1+Fevap(Ti) (2)
After adding the entire quantity of water, the moulding sand is processed in the mixer for a predetermined mixing period and when processing of this moulding sand portion is complete, a second measurement of the output of the mixing tool is taken shortly before emptying. The known relationship between the output and the moisture content or the compressibility for a given clay content is used to determine the difference between the actual and reference compressibility.
This compressibility difference is now transformed via a sectionally defined corrective function into a corrective value to correct the clay content in the recipe which is taken into account in the subsequent preparation of another moulding sand portion to determine the required additional quantities to be added.
In the case of a positive difference between the actual and reference compressibility, the clay content in the mixture is too low and must be increased by adding fines, for example in the form of a mixture of bentonite, coal dust and filtered dust, while a negative difference between the actual and reference compressibility means that the clay content in the mixture is too high and coarse new sand must be added to reduce it.
The corrective function for the additional substances as a function of the compressibility difference determined at the end of processing between the actual compressibility and the reference compressibility can be divided into 3 sections. In a first section, the corrective function follows an nth order polynomial with n>1, so that small divergences result in only very small changes in the amount of additional substances to be added. In a second section, which lies directly next to the first section, the additional substance correction follows a linear relationship, and in a third section, which lies directly next to the second section, it is limited by the set maximum value.
In a further advantageous implementation of the invention, in order to shorten the total preparation period while maintaining a constant wet mixing period, which determines the quality, a portion, preferably 80-90%, of the required quantity of water can be metered into the mixer, the quantity being based on the quantity of water determined for the previously processed moulding sand portion, simultaneously with the addition of the old sand or new sand and the additives to the mixer.
In this manner, on the one hand it can be ensured that the moisture content of the sand at the beginning of the first measurement of the output is definitely above the required minimum moisture content of 2%, and on the other hand the required wet mixing time for high moisture contents can be kept to significantly shortened moulding sand portion processing times. The minimum moisture content of 2% is necessary in this case since only here is the relationship between the compressibility and moisture content linear.
The missing quantity of moisture necessary to achieve the predetermined reference compressibility can be determined based on the first output measurement after adding and mixing in the water. After determining and adding the remaining quantity of water under equation (1), which in this case only compensates for the missing 10% to 20%, shortly before emptying the second output measurement is recorded for an overall constant wet mixing time so that the actual moisture content or the actual compressibility can be determined therefrom and is available for correction of the quantity of water to be added in the subsequent moulding sand portion.
Further preferred implementations are defined in the dependent claims.
Further advantages, features and embodiments of the present invention will become apparent from the following description made with reference to the accompanying drawings, which show:
The individual weights for the solid weighing hoppers are also gravimetrically metered via the control device in order to be able to provide a constant total weight of solids to the mixer 1.
The lower part of the diagram shown in
The utility of this region for the purposes of regulating compressibility is only limited, and so preferably a starting moisture content of more than 2% is selected.
As an example, this can be ensured by adding water in an amount representing 80-90% of the quantity of water which was added to the previous moulding sand portion (also termed the preceding charge) at the same time as the solids are added to the mixer.
A negative divergence between the compressibility at the end of processing in the mixer, V2, and the reference compressibility means that the clay content in the mixture is too high, so that this can be compensated for by adding coarse material in the form of new sand.
Both the corrective functions for the sand addition and the fines addition of bentonite, for example, are also preferably divided into three different sections. In a first section, the corrective function follows an nth order polynomial with n>1, so that small divergences from the reference are only slightly corrected if at all, while for larger divergences an over-proportionally large correction is carried out. In order that the correction for large divergences is not too large, the first section leads into a second section which preferably exhibits linear behaviour, wherein the divergences between compressibility and moisture content are directly proportional. In order to prevent the control loop from oscillating, for very large divergences, which as a rule are due to isolated events rather than to long wavelength variations, are limited by an upper corrective value.
| Number | Date | Country | Kind |
|---|---|---|---|
| 10 2007 027 298 | Jun 2007 | DE | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/EP2008/054769 | 4/18/2008 | WO | 00 | 12/10/2009 |
| Publishing Document | Publishing Date | Country | Kind |
|---|---|---|---|
| WO2008/151874 | 12/18/2008 | WO | A |
| Number | Name | Date | Kind |
|---|---|---|---|
| 3838847 | Tegelhutter | Oct 1974 | A |
| 4569025 | Eirich et al. | Feb 1986 | A |
| 5816312 | Suginaka et al. | Oct 1998 | A |
| Number | Date | Country |
|---|---|---|
| 517 541 | Jan 1972 | CH |
| 13 01 874 | Aug 1969 | DE |
| 19 47 566 | Aug 1970 | DE |
| 20 53 936 | May 1971 | DE |
| 32 20 662 | Aug 1983 | DE |
| 195 36 803 | Apr 1996 | DE |
| 56 053844 | May 1981 | JP |
| Number | Date | Country | |
|---|---|---|---|
| 20100181042 A1 | Jul 2010 | US |
