GB2308324A

GB2308324A - Producing shell moulds and cores

Info

Publication number: GB2308324A
Application number: GB9526327A
Authority: GB
Inventors: Claudio Luiz Mariotto
Original assignee: Individual
Current assignee: Individual
Priority date: 1995-12-22
Filing date: 1995-12-22
Publication date: 1997-06-25
Also published as: GB9526327D0; ITMI962403A0; IT1286381B1; ITMI962403A1

Abstract

Apparatus for producing shell moulds comprises a rigid structure 4 having vertical and horizontal arms. Pattern plates 1 are removably mounted in opposite parallel vertical faces of the structure. A coated sand reservoir 6 is capable of being raised and has a movable, permeable bottom 10 and an air inlet 11. To produce the shell moulds, the structure carrying the pattern plates is maintained at temperatures higher than 200{C. Air is blown through the sand reservoir fluidizing the sand contained therein. Simultaneously, the bottom of the reservoir is raised until the fluidized sand fills the internal space between the pattern plates. Then the bottom of the reservoir is lowered whilst fluidisation air flows.

Description

APPARATUS AND METHOD FOR PRODUCING SHELL MOULDS AND CORES The present invention relates to apparatus and a method for producing shell moulds, said shell moulds or cores being made, for example, of sand bonded with synthetic resin, and which may be used to produce metal cast components.

Throughout the present specification, the expressions "shell mould" and "shell core" are used to designate such moulds and cores made of sand bonded with thermosetting resin, whose walls thicknesses are essentially uniform, of the order of magnitude of one centimetre.

Conventionally, shell moulds are produced from mixtures of sand and thermosetting resin, usually of the novolac type, which is sold at room temperature. These sand-resin mixtures are, therefore, free-flowing aggregates. The most commonly used mixtures are prepared in such a way that the resin is distributed as a thin layer or on each individual sand grain surface, resulting in what is known as precoated sand or simply coated sand. The thermosetting property of a resin is achieved through the addition of a curing agent, hexamine being the most widely employed one.

Known apparatus for producing shell moulds comprises a dump box used in conjunction with pattern plates. However, changing hot pattern plates in a shell moulding machine would be a very arduous operation, and so pattern plates leaving production are allowed to cool before they are taken out of the machine, and the new pattern plate is fastened to the machine while at room temperature.

Therefore, any pattern plate change requires an interruption of production which takes longer than just the changing time: one must take into account also the times needed for cooling one pattern plate and for heating another one.

The changing of the pattern plates in a state-of-theart shell moulding apparatus is arduous due to the high temperatures at which the pattern plates operate; for preventing accidents (burns) it is necessary to wait for the cooling of the plates before removing them from the machine, while the substitute plates shall be heated to operating temperature only after they have been installed in the equipment.

In general, the manufacture of shell moulds with present apparatus is efficient only in cases of serial manufacture, where frequent changes of pattern plates are not necessary.

It is an object of the present invention to provide a method and apparatus for producing shell moulds able to reduce the disadvantages of known apparatus.

According to a first aspect of the present invention there is provided a method for producing shell moulds comprising the steps of arranging at least one hot pattern plate to extend substantially vertically, causing sand to be moved over the surface of the pattern plate, and compacting the sand on said face.

The invention also extends to apparatus for producing shell moulds comprising at least one pattern plate, and a supporting structure to support said pattern plate to extend substantially vertically, and means for moving sand over the surface of the pattern plate.

According to the invention, the equipment comprises; a parallelepipedic rigid metallic structure including vertical and horizontal arms; said pattern plates being removably mounted in opposite parallel faces of said structure, the faces carrying the patterns being directed to the inner part of the equipment and the ejection pins being directed outer of the equipment and each respective ejection plate is perpendicular to the same, the distance between the horizontal faces of the structure being larger than the height of the pattern plates and the distances between opposite vertical faces same; basic frame composed by metallic profiles presenting intermediate arms supporting at least a pair of inferior guides on which horizontally slides the structure carrying the pair of pattern plates; device for laterally closing the span between the two pattern plates, the coated sand reservoir being capable to be raised and having movable, permeable bottom and having interior air inlet, the widths of the pattern plates and the distance between the same when fixed to the structure being chosen so that the polygonal contour defined by the horizontal inferior edges of the pattern plates and by the imaginary straight tie lines that join their respective ends, be coincident in both contour and dimensions with the superior opening of the sand reservoir.

The invention also provides a method for producing shell moulds including the step of (a) providing a structure carrying pattern plates with ejection pins and maintained at temperatures higher than 2000C.

According to the invention, there are the following steps: (b) providing a reservoir containing sand and with movable bottom in an inferior side defined by the structure; (c) sealing the structure relative to the sand reservoir through sealing means; (d) blowing air through the inferior part of the reservoir and fluidizing the sand contained therein and simultaneously raising the bottom of the reservoir until the fluidized sand fulfils the internal space between the pattern plates; (e) interrupting, during predetermined interval of time, the fluidization, promoting the densification of the sand; (f) driving once again the fluidization air and simultaneously lowering the bottom of the reservoir; retracting the sealing means to the rest position; (g) laterally dislocating the structure towards the oven; and, after a predetermined time, extracting the formed shell.

Embodiments of the present invention will hereinafter be described, by way of example, with reference to the accompanying drawings, in which: Figure 1 shows schematically a substantially conventional method for the protection of shell moulds, Figure 2 shows a perspective view from the front of a structure carrying pattern plates of the invention, Figure 3 shows a perspective view of an alternative embodiment of a structure carrying pattern plates, Figure 4 is a front view of apparatus of the invention for producing shell moulds in an initial position, Figure 5 is a schematic two-part view showing, at the right, a reservoir raised towards a structure carrying pattern plates and, at the left, the reservoir at its highest position, within apparatus of the invention, Figure 6 is alternative front view of apparatus of the invention showing the relative position of ejection pins and actuating means therefore, and Figure 7 show a floor plan of a plant using several carrying structures and ovens.

Figure 1 shows schematically the use of substantially conventional apparatus for producing shell moulds. The apparatus comprises metallic pattern plates which are heated by way of gas or by way of electric resistances and which carry ejection pins which are used to remove the shell moulds. As can be seen in Figure 1, a dump box containing coated sand is articulated around its horizontal axis to provide the shell mould.

At stage A, the opening of the dump box is arranged upwardly and the sand contained therein rests on its bottom. A pattern plate, which is hot and horizontally disposed, has its face containing the patterns also positioned upwardly. One of the edges of the pattern plate is articulated against-one of the edges of the dump box opening.

At stages B and C the dump box is pivoted about its horizontal axis, with the pattern plate across its opening, so that at the end of stages B and C the dump box is positioned with its opening facing downwardly and with the sand contained therein resting upon the pattern plates. At stage D the dump box is returned to its original upward arrangements. The pattern plate, which closes the opening, stays momentarily in that position such that the recently formed shell hangs from it.

In stages E and F, the pattern plate is pivoted about its articulation axis with the dump box until its face containing the shell is positioned upwardly, which attitude will be maintained, except for its eventual transfer to the curing oven, until the final removal of the cured shell, after which a new cycle will begin.

During some of these described stages, some effects have been observed which adversely affect the quality of the formed shell, as explained below: During stages B and C, the coated sand, which should fall onto the pattern plate, actually glides onto it like an avalanche, sweeping from one of its edges to the opposite one. As a consequence, in the parts of the shell formed on pattern faces which are downstream of the sand flow, there is the possibility that the sand will not be well compacted. Furthermore, insufficiently dense regions may occur. As a consequence, variations in the thickness in the shells may be caused. These are known as the shade effect.

At stage E, when the dump box is returned to its original position, the coated sand that remained unaffected by the heat flows back on the shell, as a new avalanche but in a reversed direction as compared to the first one. The impact of that sand avalanche against parts of the shell which are not well-compacted and have low resistance properties, the shell being recently formed and not-cured, tends to reinforce the shade effect and provoke laminations of the shell.

Furthermore, at the end of stage D, inertia forces act, in addition to the weight of the shell which is hanging from the inverted pattern plate, these forces tending to detach the shell from the pattern plate. The resulting partial detachment of the shell is called peelback.

Similarly, between stages D and E, when the pivoting movement of the inverted pattern plate starts, inertia forces also occur and the possibility of peel-back detachment is augmented.

There is an additional negative aspect of the known apparatus. The dimensions of the pattern plates which can be used are limited by the size of the opening in the dump box. Therefore, when the production schedule requires that, during a certain production cycle, pattern plates of several sizes should be utilised, it is generally required to adapt a respective frame to the dump box opening, to use a specific production unit for each pattern plate size, or to adopt modular pattern plates, which can be juxtaposed so as to form external dimensions the same as those of the opening of the dump box.

Apparatus of the invention for producing shell moulds comprises, as shown in Figure 2, a parallelepipedic rigid metallic structure 4 constituted by substantially vertical and horizontal arms carrying pattern plates 1 which are removably mounted in opposite substantially parallel, substantially vertical faces of said structure 4.

The substantially vertical faces of the structure carrying the pattern plates 1 are positioned within the structure, and ejection pins 3 are placed over an outer part thereof.

The distance between the substantially horizontal faces of the structure 4 is larger than the height of the pattern plates 1, and the distance between respective opposed substantially vertical faces is larger than the width of a pattern plate 1 and the distance between respective plates.

Figures 4, 5 and 6 illustrate apparatus of the invention comprising a basic frame 17 comprising metallic profiles presenting intermediate arms 14 supporting at least a pair of inferior guides 5 on which the structure 4 carrying the pattern plates 1 slides horizontally.

Therefore, the structure 4 can be placed just above a coated sand reservoir 6. The widths of the pattern plates 1 and the distance between them when fixed to the structure 4 are chosen so that the rectangle (or square) defined by the horizontal inferior edges of the pattern plates 1 and by the imaginary straight tie lines that join their respective ends, coincide in both contour and dimensions with the superior opening of the sand reservoir 6.

Apparatus of the invention for producing shells (M) is shown in Figure 5 in two stages. At the right, is shown an initial position and, at the left, is shown a working position. The apparatus provides a closing device 7 in the form of walls able to laterally close the span between two pattern plates 1. As soon as the lateral closing device 7 is driven, two other actions occur. The coated sand reservoir 6 rises by mechanical or hydraulic actuating means. From the lower part of the apparatus. In the example, the actuating means is pneumatic. The elevation continues until the lower edges of the assembly composed by the two pattern plates 1 and by the walls of the lateral closing device 7 are reached. In addition an upper closing device defined by a permeable dome 8 descends from the upper part of the apparatus.The permeability of the dome 8 may be achieved by providing it with windows fitted with wire or cloth mesh having openings smaller than the dimensions of the sand grains.

More specifically, a permeable bottom is constituted, for instance, by a chamber which is hermetic except for a lower air inlet 11 and the superior face, which is composed by a porous plate and is responsible for the homogeneous distribution of air injected in the lower part of the reservoir 6 containing coated sand. The upward flow of air causes fluidization of the coated sand. The porous plate may be constructed, for example, of polyurethane rigid foam of metallic plate having multiple perforations.

In an alternative arrangement, a movable bottom 10 comprises an impermeable plate crossed by the air inlet 11 and whose upper end is connected to a metallic tube (not shown) having multiple perforations and folded to form a spiral disposed in plan substantially parallel to that of the plate.

The perforations should be directed downwards, to avoid their clogging with sand grains.

By means of an elevation device 12, herein exemplified by a hydraulic piston, the movable bottom 10 of the reservoir 6 is raised until the coated fluidized sand entirely fills the volume defined by the pattern plates 1, by the two lateral closing walls 7 and the upper closing dome. It will be appreciated that the pattern plate will be hot, whilst the closing walls 7 and the upper dome 8 are cold. The structure 4, which supports the pattern plates 1, is also hot but due to the construction, remains out of the space that will be filled by the coated sand and, therefore, there is no contact with the same. This situation is represented in the left side of Figure 5.

The elevation force of the movable bottom 10 of the coated sand reservoir 6 is maintained even after the coated sand completely fills the aforementioned volume. However, during the period for forming the shell, the fluidization air flow is interrupted so that the sand bed remains dense.

Once this time period has ended, air for fluidization is again turned on, while the movable bottom 10 of the reservoir 6 is lowered, so that the coated sand that does not form the shell (M) returns to the reservoir 6, leaving above the same two pattern plates 1, each having an adherent sand-resin shell.

The lateral closing walls 7, the upper closing dome 8 and the reservoir 6 retract to their original positions, releasing the structure 4 and the two pattern plates 1, each one with a recently formed and non-cured shell.

On the guide means 5, for instance composed by rails extending to the interior of an oven 13 provided for curing the shells (M), the structure 4 can slide on upper and/or lower wheels from the position over the reservoir 6 to said curing oven 13. By means of these rails the structure 4 carrying the pattern plates 1 and respective non-cured shells is directed to the curing oven 13. This transfer can be done manually or through an adequate automated system.

In another solution represented in Figure 3, pattern plates or core boxes 1 are mounted in the structure 4 in such a way that they can move in a direction substantially perpendicular to their respective faces, being allowed to move closer to or further from each other as required. If the tooling is a core box, the closing device 7 is not necessary. In the case of pattern plates, each of the closing devices 7 is divided into two halves, each half being fastened to one pattern plate of the pair. In the first case, by the end of each cycle, one or several shell cores are obtained. In the second case, each cycle produces a mould whose half-cavities are located opposite to each other, that is to say, in opposing faces of the same piece of mould. By stacking several of these pieces of mould, a sequence of complete cavities which can be filled with liquid metal can be obtained.

As in conventional shell moulding methods, the time needed to cure a shell mould is determined by the characteristics of the resins employed and by the temperatures chosen to run the process. For the resins commonly used and the temperature ranges typical of the state-of-the-art, the cure time is typically 5 to 6 times longer than the corresponding investment cycle time. The mode of mounting the tooling frees the rest of the apparatus while the oven is being used to cure a pair of shells. In one example, represented diagrammatically in Figure 6, one assembly 17 can work together with 4 to 6 curing ovens 13, so that, after delivering one pair of pattern plates with their recently formed, uncured shells to one of the ovens, the assembly 17 can be moved sidewise so as to be indexed to any of the other ovens where shells produced by other pairs of pattern plates are being cured.

With this layout, one single assembly 17 can work with four to six curing ovens with least idle time.

Each structure 4 with its respective pair of pattern plates can be moved freely on the rails or guides 5. As a consequence, the removal of a hot pair of pattern plates from production and the substitution of another already hot pair for those which come out presents no difficulty. This implies that, if an additional oven is available to preheat pattern plates scheduled to enter production; mounted on their respective structures 4; the influence of pattern plate change on production rhythm will be negligible. An important consequence of this is, that even though the speeds of shell build-up and cure remain in this method similar to those of the conventional method, the effective productivity per pattern plate tends to be much improved due to the absence of interruptions in production for changing of pattern plates.This confers on the present invention a flexibility which is impossible to achieve by the use of the conventional methods of shell mould making.

This means that the present invention allows shell moulding to be a competitive production process even if employed in foundries producing components in short series.

Also, as compared to the conventional method, the present invention allows for a productivity, expressed as shell moulds produced per hour and per unit floor area occupied by the moulding installation, which is, at least, twice as large, mainly because in the same floor area that by the conventional method one single pattern plate is operated, at least two vertical pattern plates are operated by the present invention.

It will be appreciated that variations in and modifications to the method and apparatus described above, and illustrated, may be made within the scope of the appended claims.

Claims

1. A method for producing shell moulds comprising the steps of arranging at least one hot pattern plate to extend substantially vertically, causing sand to be moved over the surface of the pattern plate, and compacting the sand on said face.

2. A method as claimed in Claim 1, wherein at least two substantially parallel pattern plates are provided and the sand is moved into the internal space between the two plates.

3. A method as claimed in Claim 1 or Claim 2, wherein the sand is fluidised to move it, and subsequently fluidisation of the sand is stopped to cause compaction of the sand.

4. Apparatus for producing shell moulds arranged to perform a method as claimed in any preceding claim.

5. Apparatus for producing shell moulds comprising at least one pattern plate, and a supporting structure to support said pattern plate to extend substantially vertically, and means for moving sand over the surface of the pattern plate.

6. Apparatus as claimed in Claim 5, wherein said means for moving sand comprises a sand reservoir arranged to be raised and lowered relative to said pattern plate.

7. Apparatus as claimed in Claim 5 or Claim 6, wherein said means for moving sand comprises fluidisation means for fluidising sand and directing it over the surface of the pattern plate.

8. Apparatus for producing shell moulds comprising pattern plates carrying ejection pins, a curing oven, ejection plates, and a sand reservoir, characterised in that said apparatus further comprises a structure having vertical and horizontal arms, the pattern plates being removably mounted in opposite parallel substantially vertical faces of said structure, a frame presenting intermediate arms supporting at least a pair of inferior guides on which the structure carrying the pair of pattern plates horizontally slides, a device for laterally closing the span between two pattern plates defined by lateral closing walls, a permeable superior closing dome for the pattern plates wherein the coated sand reservoir is arranged to be raised and has a movable, permeable bottom and an having inferior air inlet.

9. Apparatus for producing shell moulds as claimed in Claim 8, wherein the permeable bottom is constituted by a chamber which is hermetic except for the inferior air inlet and the superior face, which is constituted by a porous air-distribution plate.

10. Apparatus as claimed in Claim 8, wherein the movable bottom is crossed by the air inlet, which is connected to a tube with multiple perforations directed downward and folded to form a spiral and disposed in a plan parallel to that of the bottom plate.

11. Apparatus as claimed in any of Claims 8 to 10, wherein the dome has windows fitted with wire or cloth mesh having openings smaller than the dimensions of the sand grains.

12. Apparatus as claimed in any of Claims 8 to 11, wherein the elevation of the bottom of the reservoir and the driving of the ejection plates are achieved by means of pistons.

13. Apparatus as claimed in any of Claims 8 to 12, further comprising upper guides for directing and laterally sliding the structure carrying the pattern plates towards the oven.

14. Apparatus as claimed in any of Claims 8 to 13, wherein said guide means comprise rails wherein slide wheels place in profiles.

15. Apparatus as claimed in any of Claims 8 to 14, wherein elevation of the sand reservoir is achieved by springs.

16. A method for producing shell moulds comprising the steps of providing a structure carrying pattern plates and maintained at temperatures higher than 2000C, providing a reservoir containing sand with movable bottom in an inferior side defined by the structure; sealing the structure relative to the sand reservoir through sealing means; blowing air through the inferior part of the reservoir and fluidizing the sand contained therein and simultaneously raising the bottom of the reservoir until the fluidized sand fills the internal space between the pattern plates; interrupting, during a predetermined interval of time, the fluidization, promoting the densification of the sand; driving once again the fluidization air and simultaneously lowering the bottom of the reservoir; retracting the sealing means to the rest position; laterally dislocating the structure towards the oven; and, after a predetermined time, extracting the formed shell.

17. Apparatus for producing shell moulds substantially as hereinbefore described with reference to the accompanying drawings.

18. A method for producing shell moulds substantially as hereinbefore described with reference to the accompanying drawings.