GB2205516A

GB2205516A - Compacting sand casting moulds, by compressed air

Info

Publication number: GB2205516A
Application number: GB08810649A
Authority: GB
Inventors: Johannes Iburg; Karl-Heinz Weikert
Original assignee: Kuenkel Wagner GmbH and Co KG
Current assignee: Kuenkel Wagner GmbH and Co KG
Priority date: 1987-05-25
Filing date: 1988-05-05
Publication date: 1988-12-14
Anticipated expiration: 2008-05-05
Also published as: DE3717558A1; GB8810649D0; DE3717558C3; DK282288A; DK282288D0; CH678405A5; GB2205516B; DE3717558C2

Description

A Process for the Production of Sand CastifQg Moulds The invention relates

to a process f or the production of sand casting moulds of patterns, in particular of patterns with considerable differences in heights and depths with the features of the preamble of claim 1.

Numerous different processes are known for the production of sand casting moulds of patterns. The jolt squeeze or multi-plunger pressing devices are especially effective and simple, in which the sand mass is compacted by being mechanically acted upon. However, such machines work at a high noise level; and it is difficult to reduce it to the level required nowadays.

Therefore a number of other moulding processes have been developed, in which the sand mass is compacted in a pulse-like or abrupt fashion, e.g. by shooting the sand into the mould chamber by means of excess pressure or partial vacuum.

All these known processes require a lot of energy and lines of great diameter to achieve the necessary pulse-like acceleration of the amount of sand and pulse-like changes in pressure and to be able to cope with the resultant considerable amounts of air.

In processes in which the sand mass filled in must be compacted in abrupt fashion within seconds, a large numbers of nozzles distributed over the mould chamber limits is necessary. Complex measures are moreover necessary to avoid in the sides not facing the side from which the impact is applied a precompacting of the sand mass, which might lead to air inclusions and to the subsequent breaking of these points.

It is the object of the invention to indicate a processs of the type indicated in detail at the beginning and having the features of the preamble of claim 1 which can be carried out substantially more easily and which requires substantially less energy and can be carried out with devices, in which lines of substantially smaller diameter can be used.

This object is fulfilled by the measures of claim 1.

In this process the amount of sand is present in the mould chamber in bulk, preferably in sieved condition.

The increase in pressure in the mould chamber which is effected after the sealing of the mould chamber penetrates the entire amount of sand and reaches all, even the smallest air inclusions. Neither the filling process nor the process of the increase in pressure involve a special deadline pressure so that narrow cross-sections may be used for the pressure leads. The aeration carried out after the increase in pressure is carried out specifically from the side of the pattern and the side of the pattern supporting plate by providing aeration nozzles in this area in the supporting plate, preferably also in the pattern. The aeration nozzles in the pattern itself are preferably provided in the aeras of great pattern depth and thus of a relatively small distance from the pattern supporting plate. The aeration of the mould chamber from the pattern side must be effected abruptly.

Whereas the aeration of the air head space still being under pressure directly towards the external atmosphere takes place relatively slowly as compared to the pulse-like processes. Due to the aeration of the mould chamber from the pattern side a drop of pressure occurs in the amount of sand until the pressure compensation with the external atmosphere. Because the bulk of sand is loose, the sand grains can shift relatively to each other following this drop in pressiire. The result is a relatively high compacting of the sand layer near the pattern surface and the pattern supporting plate. The remaining layers of the amount of sand remain in a condition of relatively low density as compared with this. To obtain a sufficient strength of these layers, as well, the mechnical recompacting is effected with the customary mechanical pressing means. A multi-plunger press with freely movable or individually contolled plungers is especially suitable for this.

Since in the new process the filling of the mould chamber with compressed air requires a relatively long period of time as compared with the known pulse and impact moulding processes, the compacting of the amount of sand and the removal of the sand agglomeration is achieved with relatively little energy expenditure. The fact that due to the long. time required by the processes it is also advantageous for the process, but also for the simplicity of the device necessary for it that only relatively small flow rates occur and that flow cross-sections -of only relatively small size are required. Nevertheless the strength of the sand casting mould is very good even in complex patterns with great differences in heights and depths, the decreasing density f rom the side near to the pattern to the rear side of the sand agglomeration being favourable for practice.

In many cases it cannot be avoided that in the sealingly closed mould chamber f illed with the amount of sand there remains an air head space over the amount of same. It is kept as small as possible in the new process. If there is such an air head space it is essential that the same is aerated separately during the aeration of the mould chamber, i.e. directly to the external atmosphere. It has proved to be essential that the aeration of the head space begins only after the beginning of the aeration of the mould chamber, however clearly before the pressure compensation between external atmosphere and mould chamber. A too early aeration of the head space prevents the formation of a sufficient pressure gradient above the bulk height of the amount of sand in the mould chamber and impairs therefore the density of the sand layers near the pattern surface. A too late aeration of the air head space leads, on the other hand, to a fraying of already compacted layers near the pattern surfaces in the area of the aeration nozzles.

The aeration of the mould chamber can be effected continuously and at predetermined flow rate, which is determined as a function of the other conditions by the outlet cross-section of the aeration nozzles and the associated lines. However, it also proved to be advantageous if the aeration process is discontinuous, i.e. if the mould chamber is aerated in stuttering or oscillatingly varyig fashion. This' can be easily achieved by a frequency -controlled closing or throttle valve in the aeration line, which intermittently changes the outlet cross-section of the line during the aeeration process. The change can include the instantaneous closing of the line. However the change can also be effected only in the range of a throttling of the flow.

The recompacting of the residual amount of sand over the layers of sand near the pattern surface, which are strongly compacted can be effected with the customary compacting means such as press plates, pressure cushions or multi-plunger presses. The mechanical recompacting process can be triggered directly after the occurrence of the pressure compensation between external atmosphere and mould chamber. It may also be advantageous in some cases if the mechanical recompacting process is triggered clearly after the beginning of the aeration of the mould chamber, however still prior to the achieving of the complete pressure compensation to let the mechanical compacting take effect as long as the sand grains in the residual amount of sand are still moving due to aeration. After the beginning of the aeration of the mould chamber it may also be suitable to subject the pattern supporting means to the oscillation of a high-frequency exciting means to support the movement of the sand grains along the produced drop in pressure. The high-frequency oscillation of the pattern supporting means can also be used in combination with a vibratory control of the aeration flow..

The aeration nozzles disposed at the bottom can advantageously also be used for facilitating the separation of the sand casting mould from the pattern, by subjecting the surface of the sand casting mould facing the pattern to a slight air pressure of apporx.

- (.0 - 0.5 bar. If required, the pressure of a gas other than air may be used for this purpose.

Details of the process measures are illustrated by the following example which is explained in more detail by means of the enclosed drawings.

Fig. 1 shows a kind of circuit diagram of an arrangement suited to carry out the new process.

Fig. 2 shows schematically the time relationship of the d-'Lfferent phases of the new process and Fig. 3 shows schematically the effect of the new process.

In the schematic arrangement shown in Fig. 1 for the carrying out of the new proce,ss a mould device 1 is shown, which consists e.g. of a pattern supporting plate 2, a mould flask, a sand-filling frame and an upper closing element which is not designated in more detail and which may be designed and disposed together in such fashion that they can limit an externally sealed mould chamber 5. The pattern 3 is disposed on the pattern plate 2 as this is customary. The pattern plate has at 11 and the pattern 3 has at 12 in each case air passage nozzles in the surface which are connected to a line 21 via a collective line 20. The air passage nozzles may have one of the known nozzle shapes. Their distribution and arrangement over the pattern supporting plate 2 on the one hand and over the pattern surface on the other hand depends on the si.ze and shaping of the pattern.

It is assumed in Fig. 1 that the mould chamber 5 is already closed. Before this, an amount of moulding sand 4 was f illed over the pattern into the mould chamber, namely up to the filling level which is represented in Fig. 1 in dotted fashion at 6. The filling of the moulding sand is effected in such fashion that the amount of sand lies loosely in the mould chamber. The f illing of the sand into the mould chamber through a sieve proves to be especially advantageous.

An air head space 7 remains above the f illing level 6 of the amount of sand filled into the mould chamber 5, which should however be kept as small as possible. The size of the air head space 7 depends on various factors, - in particular also on the fact whether a mechanical compacting means is disposed above the amount of sand. Such a compacting means is outlined in dotted fashion as a multi-plunger press at 9 in the head space 7 in Fig. 1. A line 10 is also connected to the air head space 7, which connects the air head space to a compressed air source 13 of predetermined pressure P via several valves. A pressure switch 15 is disposed in the line 101 which can be reversed from the represented closed position to the open position at the beginning of a compacting cycle via a corresponding time switch programme. There is a control means 14 before the pressure switch, by means of which the maximum pressure level and/or the building up time of the pressure can be adjusted. There is morover a change-over and aeration valve 16 between the pressure switch 15 and the air head space 7, which connects the air head space 7 directly with the external atmosphete in the represented position via an adjustable throttle means 17. With the reversing of the pressure switch the change-over valve 16 can be changed over from the represented position into the passage position. There is a change-over valve 18 in the line 21, which can connect the collective line 20 directly to the external atmosphere. In exceptional cases the line 21 can also be connected to a vacuum receptacle or the like via the valve 19 instead of with the external atmosphere.

However, the connection of the line 21 directly-to the external atmosphere via the valve 18 is preferred. A control means 23 can be associated to the valve 18, which reverses the valve 18 during the active phase of the valve 18 in discontinuous fashion e.g. optionally in stuttering or cyclically varying between the locking position and the passage position. For this purpose a separate flow throttling means may be provided in the line 21 which is controlled via the means 23. As outlined in dotted fashion a high-frequency generator 24 may be associated to the pattern supporting plate 2, wh-'Lch subjects the pattern supporting plate 2 to a high-frequency oscillation with the beginning of the aeration of the mould chamber 5.

The arrangement represented schematically in Fig. 1 is described as follows as regards the carrying out of the new process.

After the loose filling in of the amount of sand 4 over the pattern in the mould chamber 5, the mould chamber is closed correspondingly sealingly. Thereupon the pressure switch 15 is switched on and the change-over switch 16 is changed over to connect the line 10 with the compressed air source 13. This is the time which is illustrated with the start line in the time table according to Fig. 2. In accordance with the adjustment of the device 14 compressed air flows into the head space 7 and into the amount of sand 4 filled in over a certain period of time. Due to the resultant f low a relatively small directed force is applied to the sand grains according to Fig. 3a so that in this phase of the filling of the mould chamber with compressed air a slight precompacting takes place, during which the sand grains of the amount of sand loosely filled in are slightly shifted against each other. At the end of the filling duration tO the pressure switch 15 is switched off, while the valve 16 remains in its chanje-over position so that a predetermined air pressure is maintained in the mould chamber 5 for a predetermined period of time. The air pressure in the sand filling 4 is the same as in the air head space 7. The result of this is that the same pressure is exerted on the grains of the sand filling from all sides so that the sand grains are in each case in an equilibrium of forces as this is outlined in Fig. 3b. As f rom the time of the closing of the pressure switch 15 the aeration valve 13 is opened af ter the expiration of a time interval t2 (Fig. 29, which deaerates the mould chamber via the line 21, the collective line 20 and the aeration nozzles 11 and 12 in the pattern plate 2 and in the pattern 2. It is decisive that the deaeration of the mould chamber takes place from the side of the pattern and the pattern supporting plate, i.e. from below in the represented case. The duration of dearation of the mould chamber 5 from below is represented by the time interval t3 in Fig. 2. The air expanding during dearation escapes through the deaeration nozzles 11 and 12, the air escaping at first very quickly from the sand layers being closest to the pattern surface and the surface of the pattern supporting plate. At the time of the opening of the deaeration valve 18 an especially great drop in pressure is formed in these areas.

If, as represented, there is an air head space in the mould chamber 5, it is important that the deaeration of the air head space 7 is not ef f ected through the sand f illing 4 or only to a limited extent, if the good and f irm formation of the sand layers close to the pattern surface is not to be impaired. It is therefore important that a separate dearation path is provided f or the deaeration of the air head space 7. In the respresented example according to Fig. 1 it is provided above the pressure build-up line 10, namely upon the change-over of the change-over valve 16 into the normal position which is represented in Fig. 1. In this position the line 10 is directly connected to the external atmosphere via the change-over valve 16 and an adjustable throttle means 17. It can be seen from Fig.

2 that a change-over of the change-over valve into the normal position is effected at a point in time during the time interval t3, during which the deaeration valve 18 is opened.

The compacting of the mould sand is effected by the expansion of the previously compressed air in the mould chamber 5. The described arrangement and process result in a good compacting and hardening of the sand layers in the direct vicinity of the pattern surface and the surface of the pattern supporting plate 2. The sand areas behind them are also compacted, but not to that extent so that it may be suitable to compact these layers to the desired hardness by a mechanical compacting means as it is represented at 9 in Fig. 1.

With the beginning of aearation, i.e. after the sum of the process times tO and t2 the pattern supporting plate 2 may furthermore be subjected to the action of a high-frequency oscillation by the oscillation generator 24 to support the compacting of the sand f illing by means of the expansion of the compressed air.

The duration of the air expansion effective for compacting, i.e the effective expansion time is represented by the dif f erence of the times tl and t2, because with the change-over of the change-over valve 16 into the normal position the compacting ef f ect of expansion is stopped. Th effect of the expansion forces is represented in Fig. 3c. One recognizes that the equilibrium of forces still prevailing in Fig. 3b is cancelled one-sidedly, i.e. in the represented example towards below so that the sand grains can shift themselves abruptly and in directed fashion under the action of the air expansion.

The building up of pressure in the mould chamber 5 to a value of approx. 4 to 5 bar has proven to be advantageous in practice. The effective expansion time, i.e the difference between tl and t2 should be preferably between about 0.3 and 0.6, the absolute magnitude of the times tl and t2 being capable of being varied in limits, namely for tl between about 0.5 and 1.8 seconds and for t2 between 0.1 and 1.0 seconds. It must be ensured that in each case the end of the time interval tl is before the end of the time interval t3 so that the direct aeration of the air head space 7 via the change-over valve 16 begins in time. The duration of cycle is about 2 seconds.

It has been still discovered that the optimal time parameters must be ascertained in the individual cas.e, because the time parameters depend to a certain extent on the nature of the used moulding sand.

il- The time relationships dscribed above and absolute times and their effect on the result will be illustrated again by the following table: 1 q 1 Table of the adjusted parameters tl t2 t3 t6 texp. Sand Mould behaviour humidity (S) (S) (S) (S) (S) (%) 0.7 0.1 0.9 2 0.6 3.2 good mould, little.

porosity in the area of the nozzles 1.1 0.5 1.3 2 0.6 3.2 good mould, little porosity in the area of the nozzles 1.3 1.0 15 2 0.3 35 very good mould, little porosity 1.6 1.0 1.8 2.5 0.6 3.2 good mould, little porosity 2.0 1.0 2.2 3 i 3.5 great porosity 1.5 1.0 1,7 2.5 -0.5 3.5 good mould, little porosity 1.4 1.0 1.6 2 0.4 3.5 good mould, little porosity The opening time of the individual valves and their relationship to each other depend of course also on the flow cross-sections for the air flowing off, in particular on the nozzle cross-sections.

A special advantage of the new process resides in its extensive use possibility. While the former pulse moulding processes have an upper limit in the dimensions of the moulding flasks of 1.g. 1.5 m/m 2 moulding flask cross-section, the new process can be used in moulding flasks of optionally large cross-section, because due to the substantially static pressure build-up there is always an excellent pressure distrubtion also over large cross-sections so that substantially the same pressure gradients occur everywhere at the beginning of expansion.

claims.

1 A process for the production of a sand casting mould for a pattern, of the type in which a sealable defined mould chamber with the desired pattern configu'ration at its base is filled with a desired amount of sand, and compacted: in which (a) the sand is filled loosely into the chamber (b) the chamber is then sealed (c) the air pressure in the filled and sealed mould chamber is increased to a predetermined value and (d) the chamber is then vented from outlets in, or in the close vicinity of, the pattern configuration, thereby to produce a compacted layer of sand over, and reproducing the shape of, the pattern configuration.

2. A process for the production of sand casting moulds of patterns, in particular of patterns with a considerable difference of height and depth, in which a mould formed by a base pattern, a mould flask supporting means, a frame-shaped mould flask and a sand filling frame is filled with a predetermined amount of sand and the said amount of sand is compacted by a relative.

movement between the elements defining-the mould chamber and a pressing means covering it, characterized in that (a) the predetermined amounL of sand is loosely introduced into the mould chamber preferably by means of a sieve (b) after the sealing closing of the mould chamber the pressure in the mould chamber is increased to a predetermined value. and (c) the mould chamber is de-aerated from the regions near the sides of the pattern and the supporting means to the external atmosphere to cause the sand layer near the pattern and the pattern supporting means to become compacted and (d) any remaining uncompacted sand is mechanically compacted by relative movement between the said pressing means and supporting means.

3. A process according to claim 2, characterized in that de-aeration is effected by means of nozzles located at points of the pattern supporting means and points of the pattern surface.

4. A process according to claim 2 or 3, characterized in that the mechanical compacting is triggered directly after pressure compensation between mould chamber and external atmosphere has been effected.

5. A process according to claim 2 or 3, characterized in that the mechanical compacting is triggered when a predetermined residual excess pressure, with respect to the external atmosphere, has been reached in the mould chamber.

17 6. A process according to any one of the claims 2 to 5,characterized in that any air head space remaining in the sealingly closed mould chamber, over the amount of sand filled, is kept as small as possible.

7. A process according to claim 6, chararaterized in that the said air head space is itself directly de-aerated to the external atmosphere after the excess pressure has been built up in the mould chamber.

8. A process according to claim 7, characterized in that the deaeration of the air head space is triggered after a delay from starting the de-aeration of the mould chamber at the pattern and the supporting means, the triggering being effected before pressure compensation is reached between the mould chamber and external atmophere.

9. A process according to any one of claims 2 to 8, characterized in that the de-aeration of the mould chamber is effected continuously and at predetermined outflow speed.

10. A process according to any one of claims 2 to 8, characterized in that the de-aeration of the mould chamber is effected in a discontinuous fashion. such as a "stuttering" or oscillatingly varying fashion.

11. A process according to any one of claims 2 to 10, characterized in that the pattern and mould flask supporting means is subjected to a high-frequency oscillation at the beginning of the de-aeration of the mould chamber.

12. A process according to any one of claims 2 to 10, characterized in that to facilitate separation of the pattern and the sand agglomeration thereon, a positive relative air or gas pressure of approximately 0.5 is applied bar via the nozzles in the pattern and in the supporting means.

13. A process as claimed in claim 1 or 2 and substantially as herein defined.

14. Sand moulds made by the process as claimed in any one preceding claim.

Pubhshed 1988 at The Patent Office, State House. 66"71 High Holborn, Londor WCIR 4TP. Further copies maybe obtained from The Patent Office, Sales Branch, St Mary Cray, Orpington, Kent BR5 3RD. Printed by Multiplex techniques ltd, St Mary Cray, Kent. Con. 1187.