Method for the manufacture of molds using casting sand or another mixture of raw material particles
FIELD OF THE INVENTION 
     The present invention relates to a method of manufacturing molds from casting sand or from an otherwise moldable mixture by the use of a pressure surge wave in a closed chamber. The other moldable mixture comprises particles of raw material, binder, water, and, if need be, additives. 
     BACKGROUND OF THE INVENTION 
     A number of processes are known whereby the manufacturing of molds is effected by means of a pressure surge wave. In such processes, a gas pressure is built up over a moldable mixture such as, for example, a casting sand, and then subsequently decreased. 
     The object of the invention is to select a range in pressure variation within which an optimal compaction of the moldable mixture is assured. 
     SUMMARY OF THE INVENTION 
     In accordance with the present invention, castings are made from casting sand or other moldable mixture by means of a pressure surge wave. The pressure surge wave is characterized by four parameters: these are the increasing pressure gradient, the maximum pressure, the duration of the maximum pressure, and the decreasing pressure gradient. The rise in pressure takes place with an increasing pressure gradient, dp/dt, of at least about 50 atmospheres (absolute)/second. A .[.maximum.]. pressure of at least about 2 atmospheres (absolute) is maintained for at least 0.01 seconds. The pressure drop takes place with a decreasing pressure gradient, -dp/dt, of up to 2.0 atmospheres (absolute)/sec. These values yield surprisingly good compacting of the casting sand and or other moldable mixture from which the castings are made. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The FIGURE shows four pressure surge waves as a function of time, in accordance with an illustrative embodiment of the invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The pressure range is characterized bythe fact that the moldable mixture is exposed to a gas pressure of at least 2 atmospheres (absolute) using an increasing pressure gradient, dp/dt, of at least 50 atmospheres (absolute)/second, maintaining at least this pressure for at least 0.01 seconds and then allowing the pressure to drop back to normal pressure within at least 0.2 seconds from the onset of the pressure increase. 
     Theoretically, no upper limit is set for the pressure-rise gradient. 
     Several tests have shown that an increasing pressure gradient of about 600 atmospheres (absolute)/second is a value which represents a suitable limit regarding both the hardness of the sand mold and the choice of the pressure medium. 
     The pressure surge wave should be applied to the surface of the sand as a mass flow through change of the amount of air per unit time. The pressure surge may be produced by compressing a gas or by an exothermic reaction of an explosive gas mixture. The pressure surge wave should be reduced to a low final pressure, that is, finally reduced to normal pressure. 
     A pressure maximum of 4 to 5.5 atmospheres (absolute) in a combustion chamber represents an optimization between the desired strength properties of the sand molds and the cost of sealing the combustion chamber. .[.The maximum.]. .Iadd.An elevated .Iaddend.pressure .Iadd.of at least 2 atmospheres (absolute) .Iaddend.should be maintained for at least 0.01 second and may be .[.mainted.]. .Iadd.maintained .Iaddend.for about 0.03-0.05 seconds. 
     With regard to the reduction of pressure, two oppositely directed phenomena must be weighed one against the other. On one hand, the pressure drop should not be too rapid, because cracks could form in the molds; whereas, on the other hand, too much time should not be taken for the pressure to drop, otherwise the cycle times for producing the molds would become too long. An upper limit would be in the range of about 2 atmospheres (absolute)/second, up to which limit, crack-free molds with a clean surface can be produced. The maximum pressure is reduced in a time ranging from 0.2 sec. to about 1.5 sec. 
     In the afore-described process use is made of four parameters to define a pressure surge. These parameters are the increasing pressure gradient, the maximum pressure, the duration of maximum pressure, and the decreasing pressure gradient. These parameters may be varied independently. 
     The FIGURE shows four pressure surges, all of which are characterized by increasing pressure gradients, maximum pressures, duration of maximum pressure, and decreasing pressure gradients in the aforementioned ranges in accordance with the present invention. Each of the surges starts at normal, i.e. atmospheric pressure and ends at normal, i.e. atmospheric pressure. 
     Additional influencing factors were investigated to determine the above-mentioned optimum values. Thus, a boundary surface concentration of the raw material particles of 10 -9  to 5·10 -9  mole/centimeter 2 , and especially, moreover, a concentration of 1.9 to 2.5·10 -9  mole/centimeter 2 , has been found to be advantageous. 
     In addition, with regard to making a choice of raw material particles, a determination was made relating to the velocity of propagation of elastic longitudinal waves in such particles, wherein a value of 4 to 7 kilometers/second, and preferably, a velocity of 5.8 to 6.0 kilometers/second, was found to be especially favorable. 
     For the tests, use was made of raw material particles which had a major constituent which was silicon dioxide. These particles had a compactability which lay between 27 and 60%, and preferably, 32 to 45%. 
     Compactibility relates to the decrease in volume of the casting mixture after the pressure surge is applied thereto. 
     Advantageously, the process described finds an application in the precise molding of a casting mold by using a pattern wherein the derived optimum values are so adjusted and combined that a pressure of 80 to 180 Newtons/centimeter 2  is produced on the surface of the pattern. 
     The limiting values thus determined for the pressure rise, for the maximum value and for the pressure reduction, yield data for the optimal operation of equipment for compacting sand.