FIELD OF THE INVENTION

The invention refers to a method of preparing a casting mould for precision casting, comprising the steps of preparing a pattern of destructible material on the basis of a master pattern, merging the pattern into fine sand of given density, drying the fine sand cover, repeating the merging and drying steps up to reaching a predetermined thickness of the cover, curing and firing the cover and destructing thereby the structure of the pattern. The method of the invention renders it possible to prepare a casting mould for making a mould of thin wall and desired surface complexity. The casting mould can be prepared also in more part form in order to make moulds of bigger dimensions by the means of the precision casting.

In the precision casting applying patterns of destructible material it is usual to define that a mould is of bigger dimensions if it is as long as at least 0,5 m and as heavy as about from 2 to 5 kg. BACKGROUND OF THE INVENTION

In the precision casting the application of a pattern made of wax or other destructible material is a general practice. The destructible material is generally a wax composition to be removed in molten state by the high temperature conditions of casting. This method is applicable in producing objects and parts of sophisticated, articulated surface. The most important steps of the known technologies are shown in the handbooks which are commercially available. Reference is made for example to the "Handbook of Casting" published (in Hungarian) by Mϋszaki Kόnyvkiadό, Budapest, 1985. This publication includes a detailed description of the precision casting and contains the warning that the known methods of precision casting require the use of expensive equipment and therefore they should be applied rather only in long series of products, when at least some of hundreds or more pieces should be manufactured. In this handbook it is stated also that the technology of the precision casting cannot be realised in practice or heavy difficulties are to be taken into account when the product is a hollow body of thin wall, wherein the outlet of the inner cavity is of smaller cross-section than the characteristic measure of the inner space. If the technology of the precision casting has to be applied to products of such kind the book proposes rather to modify the construction. The modification is, however, unacceptable at objects of individual character, as for example fine art or craft objects.

The technology of the precision casting as described in this book comprises the steps of preparing patterns from a destructible or meltable material by the means

of an appropriate tool, e.g. by extrusion, linking more pattems into one "tree" made of a destructible material and covering the tree of the patterns by a powdered refractory material, e.g. by merging it into a solution or by spraying. The refractory material is fired and in this way a casting mould is prepared. The destructible or meltable material is generally a wax mixture consisting of 50 wt.% paraffin and 50 wt.% stearin.

The precision casting applying wax pattems is also a general practice shown in many handbooks. Reference is made to the book of Dr. Bakό, K., Sandor, J., Dr. Szabό, Zs. and Szij, Z. "Technology of Casting Manufacture", ed. by Mϋszaki Kόnyvkiadό, Budapest, 1968. The essence of this method is that for preparing small mould products of articulate shape wax patterns are made from a specific wax, wax mixture or a mixture comprising an appropriate plastics. The first step is to prepare massive wax pattems which are the basis of producing massive moulds. Therefore the method can be applied in the practice only in the production of products of small mass, having when necessary holes. If products of greater volume are to be made other problems should be taken into account, because the process of cooling of metals of great volume causes heavy problems, especially with regard to the quality features and among them the mechanical strength.

In the method proposed the tree including the massive wax pattems is merged into a slurry made of fine sand or the fine sand is sprayed onto its surface and in this way a foamed cover is made which consists of a refractory binder, generally ethyl silicate or sodium silicate (water-glass) and a filler of fine graininess, as e.g. quartz sand or silimanite. Onto this cover a next quartz sand layer or other layers of compositions similar to that shown above are brought. Thereafter the covered tree is encased in sand and the material is dried and cured. The wax is expelled from the casting mould by melting. In this way big constructions can be prepared also for casting big pieces. The method is not capable of preparing casting moulds for making shell-type moulds comprising inner cavity because the casting mould cannot be prepared in a self-carrying structure.

The general literature and its items cited above disclose more methods for preparing moulds having inner cavity by the means of precision casting. At these methods cores made of an appropriate ceramic material or carbamide are applied but the cores of such kind require intensive work, specific equipment and tools of manufacturing, they are expensive with regard both to the material and equipment. This is the basis why the literature and the mentioned handbook proposes to prepare moulds having inner cavity by a method differing from the precision casting or when precision casting should be applied by altering the construction. This proposal refers also to the bigger moulds which are made in parts and the parts are united by welding

or other appropriate method and the surface is processed by abrasive treatment for reaching the required surface quality.

A method of making shell-type cores for facilitating the process of casting is shown in the US-A 3,675,708. According to this method a pattern made of destructible material is merged into a slurry for covering its outer and inner surfaces and after reaching the required thickness of the cover brought on the cover is dried and partly removed from the outer surface. In this way ribs prepared previously on the pattems can be accessed and the outer cover divided into two separate parts. After expelling the destructible material the remaining inner part is fired out and thereby a casting mould is prepared which can be used for shell-type products. The method cannot be, however, applied in the case of products of articulate surface shape and is intended to facilitate the production of moulds at which there are no concave surface elements.

Another disadvantage is that in this way the bigger moulds cannot be prepared at all.

Hence, it can be stated that the technology of precision casting requires further improvements and especially with regard to the casting mould. Therefore the aim of the present invention is to improve the technology of the precision casting by creating a novel method of preparing the casting mould.

SUMMARY OF THE INVENTION

The invention is based on the recognition that during preparing the casting mould the cover should be prepared from two kinds of grainy material, especially corundum and if a mould of bigger dimensions should be prepared, the pattern has to be made from more parts. It is also the recognition that for covering the pattern fluidised bed should be used.

The present invention is intended to elaborate a method, whereby a casting mould can be prepared for manufacturing hollow cast products by precision casting, the costs of preparing the casting mould can be kept low, the method is simple and thereby allows to apply the precision casting to individual products of bigger dimensions, too. A further intention of the invention is to realise the method in that way that the precision casting becomes the basis of manufacturing shell-type products which have in given case big inner hollow space and which can be either individual or mass products.

Hence, the object of the invention is a method of preparing a casting mould for precision casting, comprising the steps of preparing a pattern of destructible material on the basis of a master pattern, merging the pattern into fine sand of given density, drying the fine sand cover, repeating the merging and drying steps up to reaching a predetermined thickness of the cover, curing and firing the cover and destructing

thereby the structure of the pattem, wherein the novelty lies in preparing the pattem of destructible material as a wax pattem of thin wall, making in the wall of the wax pattem more holes connecting to each hole a funnel type raising block for determining an inlet passageway, merging the wax pattem bearing the fine sand before drying into a fluidised bed established with a mixture comprising grains of corundum of first graininess, repeating the merging step up to reaching the predetermined thickness, arranging in the holes respective locks for connecting layers covering the inner and outer surfaces of the wax pattem, merging at least once the wax pattem into fine sand of higher than given density, and spraying it with a mixture comprising grains of corundum of second, exceeding the first graininess, then melting wax or wax mixture of the wax pattern and firing the body consisted of an outer pattem and inner core maintained together by the locks.

It is especially preferred when the proposed method is carried out by preparing a wax pattem having an inner cavity, arranging a hook in one of the inlet passageways opposite to the inner cavity, introducing a flexible tube, into the inner cavity and connecting the flexible tube with a blower of air stream, drying the mixture limiting the inner cavity by an air stream flowing from the outlet of the flexible tube through the inner cavity and at least one hole.

Another advantageous embodiment of the present invention proposes to realise the wax pattem of the master pattem in more parts corresponding to parts of the master pattem defined by at least one separation plane and preparing in respective two planes parallel to the at least one separation plane protrusions bearing matching figures outside of the master pattern, cleaning the fired surface of the matching figures, arranging the parts with the matching figures forming protrusions oblong the at least one separation plane, fixing the protrusions to one another and preferably closing the openings remaining in the inner surface of the parts with backing sand.

If the master pattem is of bigger dimensions it is advantageous to carry out the method of the inventions by fixing the protrusions to one another by applying flexible fixing clamps and pressing the fixing clamps by lashing wire to one another and preferably preparing at the closed surfaces oblong the at least one separation plane cone shaped protrusions crossing the dividing plane.

The mechanical strength of the mould prepared can be improved by applying corundum of average graininess in the range from about 0,15 mm to 0,20 mm or a mixture comprising corundum of such measures in the fluidised bed and in the range from about 1,0 mm to 1,7 mm or a mixture comprising corundum of such measures for spraying.

It is also preferred to prepare two layers by merging the mould into the fluidised bed and to spray at least two layers on the outer surface and at least one layer on the inner surface, further to apply an inner core of refractory material of destructible structure surrounded by the outer layer prepared by merging and spraying, the inner core being arranged by sliding into the inner space defined by the outer layers.

The quality of the casting mould can be improved also by preparing the wax pattem by the steps of encasing partly the master pattem in a curable carrying material composition for preparing a part of the mould and leaving its surface partly free, forming at least one matching element on the surface of the carrying material composition adjacent to the master pattem, providing a silicone polymer based covering mixture and making therefrom at least two interconnected layers on the free surface of the master pattem, arranging a mesh like flexible sheet on the surface of one of the interconnected layers before forming the next layer for preparing a first composite mould part, curing the interconnected layers receiving the mesh like flexible sheet, pouring a curable substance onto the first composite mould part and curing the curable substance for preparing a first carrier, separating the first carrier and the first composite mould part from the carrying material composition, yielding the master pattem, clearing its free surface and arranging it back in the first composite mould part, leaving free a further part of its surface, preparing at least one further composite mould part by repeating the steps of preparing the first composite mould part by providing a silicone polymer based covering mixture, forming therefrom at least two interconnected layers on the free surface of the master pattem and on the accessible surface of the composite mould part, arranging a mesh like flexible sheet on the surface of one of the interconnected layers before forming the next layer for preparing at least one further composite mould part, curing the interconnected layers receiving the mesh like flexible sheet, pouring a curable substance onto the at least one further composite mould part and curing the curable substance for preparing at least one further carrier, separating the at least one further carrier and the at least one composite mould part from the carrying material composition for preparing a rigid carrier construction, opening the rigid carrier construction and taking out the master pattem from the inside of the at least two composite mould parts, uniting the at least two carriers into the rigid construction and fixing therein the composite mould parts forming the mould, providing the wax mixture in the form of a melt comprising approximately equal amounts of a stearin wax and a paraffin wax and from approximately 1 wt.% to about 4 wt.% pine resin and/or polyethylene in the range from about 3 wt.% to about 5 wt.%, filling up partly the inner space of the mould with the

molten wax mixture, vibrating the mould and forming thereby a wax layer on the inner surface thereof and leaving the wax layer to cool to a temperature exceeding ambient temperature, before lowering the temperature to ambient value at least once filling up partly the inner space of the mould with the molten wax mixture and preparing thereby interconnected wax layers on the inner surface. If necessary, the mould is prepared with loose pieces. According to the general practice a passageway can be made in the curable carrying material composition for ensuring the inlet opening and the covering mixture is applied to the passageway during preparing the composite mould parts. It is especially advantageous when a clay composition is applied for the curable carrying material composition. In the curable carrying composition, which is advantageously made of plaster of Paris it is proposed to build in cooling elements or cooling pipes and to apply them for ensuring the correct temperature of the wax mixture.

The use of the cooling means can be avoided in the curable carrying material composition, when the wax mixture is applied in charges of decreasing temperature for preparing the wax layers, an excess of the molten wax mixture is poured into the mould and after preparing the wax layer of desired thickness the rests of the wax mixture are removed from the mould.

The efficiency of preparing the wax layer is improved when in the method of the invention advantageously the inner space of the mould is filled with the molten wax mixture at most to the three quarter of its height, and after it the wax mixture is exposed to irregular movement in order to expel therefrom the air bubbles.

Material savings can be ensured during casting the product according to the wax pattem if in the method of the invention a wax pattem is made the wall thickness of which is in average approximately equal to that of the product.

The arrangement of the invention can be reliably kept together if supporting elements are built in to the curable carrying material composition, especially plaster of Paris applied.

If products of sophisticated shapes should be made, it is especially preferred to realise the invention by arranging in the curable carrying material composition cooling means, especially cooling construction parts and/or cooling pipe and during preparing the wax layers cooling the carrying material composition and the connected elements. In this aspect it is also preferred to prepare the mould by dividing surface of the master pattem into more than two parts and to apply the covering mixture separately to the parts for making the composite mould parts.

In the practice very good results can be obtained by applying a covering mixture including a polysiloxane of relative molecular mass in the range from about

30000 to about 600000 and another polysiloxane of relative molecular mass in the range from about 100000 to about 200000 in mass ratio from about 1 : 1 to about 2 : 1 , from about 1 wt.% to about 7 wt.% catalyst of known composition and accelerant, and from about 0,5 wt.% to about 35 wt.% filling material. This filling material or a part of can be a flaked silicate which represents from about 0,5 wt.% to about 3 wt.% of the mass of the covering mixture.

The required strength of the composite mould parts can be ensured by applying a flexible net made of plastics or metal or a dressing gauze between two layers of the composite part as a mesh like flexible sheet. The long series production of the wax pattems and thereby that of the casting products on the basis of the master pattem can be facilitated when in the method of the invention the wax pattem made from the composite mould parts for preparing a metal body bearing the negative pattem of the wax pattem are applied and further wax pattems are made from this body. It is well known in the casting practice and advantageous also when realising the invention if the surface of the master pattem is degreased before encasing it in the curable carrying material and is covered with a composition facilitating the separation of the composite mould part.

BRIEF DESCRIPTION OF THE DRAWINGS The invention will be further discussed in more detail on the basis of advantageous embodiments shown by way of examples only. In this discussion reference will be made to the drawings wherein

Figure 1: is a top view of an arrangement carrying one half of a master pattem, Figure 2: illustrates the cross-section ll-ll of the arrangement of Fig. 1., Figure 3: shows the cross-section and arrangement of a wax pattem made according to the invention, Figure 4: a cross-section I -IV of the arrangement shown in Fig. 3, Figure 5: is the cross-section of a wax pattem having surface open from one side and holes, Figure 6: is the cross-section of a wax pattem closed from each side and having holes, Figure 7: is the side-view of a lock applied for supporting a core in the casting mould of the invention, Figure 8: shows a preferred embodiment of a casting mould with inner cavity and its inlet system,

Figure 9: is the part IX of Figure 8 in enlarged view,

Figure 10: is the basic structure of an apparatus for ventilating the inside of a casting mould having inner cavity, Figure 11 : shows the cross-section of a casting mould having inner cavity connected with a ventilation unit, in drying position, Figure 12: illustrates a possible division of a master pattem of bigger dimensions, Figure 13: is a cross-section of a wax pattem corresponding to the lower part of the master pattem shown in Fig. 12 and the top view of a mould made of silicone polymer receiving the master pattem, Figure 14: illustrates the side view of a fixing clamp applied for maintaining elements of a casting mould forming its outer shell and

Figure 15: is the cross-section of a casting mould having inner cavity after applying the clamps for maintaining the parts of the mould together, the mould including also locks.

MODES OF CARRYING OUT THE INVENTION During the realisation of the method of the invention the first stage is the preparation of a more layer composite mould by the application of an appropriate, e.g. silicone polymer based composition. The first step is to define one or more dividing planes in a master pattem 4 according to the known principles of casting. If the master pattem 4 has a sophisticated, articulate surface to selected parts thereof loose pieces are connected. The dividing planes are regularly parallel to the longitudinal axis of the master pattem 4. The next step is to clear, degrease the surface of the master pattem 4 and if necessary, to cover it with an aqueous solution of yellow soap or other agent facilitating the later separation.

If the master pattem 4 is of bigger dimensions, i.e. it is a piece having length at least about 0,5 m and mass about 2 kg to 5 kg or over the master pattem 4 is divided also into parts by separation planes 31 as shown in Fig. 12. The separation planes 31 are in the length shorter than 50 cm and they should be selected on those places of the master pattem 4 which are easily accessible during later processing. The master pattem 4 shown in Fig. 12 may be divided into two halves, i.e. a lower part 36 and an upper part 37. In the separation planes 31 in positive and negative pattems matching elements 33 (protrusions, grooves, etc.) are to be prepared on all surfaces taking part in the preparing of the wax pattem and the casting mould.

After these introductionary measures a more layer casting mould should be prepared from the master pattem 4 or the parts, i.e. in the given case from the lower part 36 and the upper part 37 by applying a silicone mixture. For realising this step the master pattem 4 or its part defined by the separation plane 31 is encased up to the

level determined by a dividing plane in a curable carrying material composition 7, e.g. in a moulding clay composition or other similar material (Figs 1 and 2) wherein elements defining raising blocks 5 and ventilation passageways 6 are arranged or made out and an element is also arranged which ensures a higher level for pouring a molten wax mixture. This element is e.g. a truncated cone 9 made of plaster of Paris. By applying the raising blocks 5 a cone shaped place is ensured in which an inlet opening of relatively big measures of a casting mould can be determined. The inlet opening is selected to lie in the case of a cast product having an opened inner space in the plane of the opening and if the cast product has a closed inner space, in a surface area which is not exposed and/or in given case can be left open. On the surface of the curable carrying material composition 7 in the dividing plane matching elements 3', i.e. protrusions or troughs are shaped for example by pressing an element of individual shape into the composition. Thereafter the curable carrying material composition 7 and the master pattem 4 therein are surrounded by a frame 1 which protrudes from the plane of the curable carrying material composition 7 (Figs 1 and 2).

Onto the master pattem 4 and the connected surfaces of the curable carrying material composition 7 in the dividing plane and if applicable in the separation plane 31 a mixture is brought which is based on a silicone polymer as shown e.g. in the HU-A 197,238. According to this specification the basic mixture is made from a blend of two polysiloxanes in the range from about 1 : 1 to about 2 : 1 , the first of them having relative molecular mass in the range from about 30000 to about 600000 and the second in the range from about 100000 to about 200000, from about 1 wt.% to about 7 wt.% catalyst and an ^' accelerant whereto a filling material is given in the amount from about 0,5 wt.% to about 35 wt.%. According to the invention the basic mixture comprises as the filling material or a part thereof about 0,5 wt.% to 3,5 wt.% flaked silicate, e.g. the product Aerosil 200 or Aerosil 2000. After bringing up the first layer the partly solidification of this layer is awaited and on the surface of the partly solidified mixture a second layer is brought up. On the last layer a flexible mesh like sheet is arranged, e.g. a net and the sheet or net is covered with at least one silicone layer. For the sake of simplicity the drawings do not show the layered structure obtain. The flaked silicate is applied for increasing the viscosity of the mixture which cannot now flow down from the highly sloped or even vertical surfaces of the master pattem 4 and its flexibility is important because it ensures that the prepared layer silicone structure has good consistency and advantageous strength. In the dividing plane and in the separation plane 31 if determined the whole surface of the curable carrying material composition 7 or a part thereof including the matching elements 3' and in

given case the surface of the areas defining the raising blocks 5 are covered also by the mentioned silicone polymer mixture. The same procedure is followed at the loose pieces the surface of which are covered also with a silicone layered structure. By curing the silicone polymer mixture results in a composite mould part 2. Thereafter the master pattem 4, the loose pieces and the surface of the cured polymer mixture (composite mould part 2) are poured within the frame 1 up to the dividing plane and if applied up to the separation plane 31 with plaster of Paris or similar curable material and if necessary in the space filled up with this plaster of Paris or curable material strengthening elements, e.g. metal wires, rods and other are arranged. The silicone polymer based mixture results in protrusions 3 or other kinds of matching elements 33 on the matching elements 3' protruding from the dividing plane and/or the separation plane 31 and the protrusions 3 or the matching elements 33 are reflected by a negative pattem in the plaster of Paris or other curable material.

When the plaster of Paris or the curable material within the frame 1 is cured the carrier obtained with the structure including the curable carrying material composition 7, the composite mould part 2 and the master pattem 4 is reversed, the curable carrying material composition 7 is separated and the master pattem 4 is taken out from the composite mould part 2. The surfaces of the master pattem 4 are cleaned and the master pattem 4 is arranged back in the composite mould part 2. The structure thus received is arranged in the frame 1 back. The accessible surfaces of the composite mould part 2 and the free surface of the master pattem 4 are covered thereafter by a separating composition, e.g. an aqueous solution of yellow soap and onto the covered surfaces the silicone polymer mixture defined above is brought. Repeating the steps previously described more layers are prepared and on the second (or further) layer a mesh like flexible sheet is also arranged. Having access to these surfaces the silicone polymer cover of required thickness is prepared also on the loose pieces and the free surfaces. After curing the mixture the free space within the frame 1 is poured with plaster of Paris or other curable material and in the free space if necessary, strengthening elements, e.g. metal wires are applied. In this way a further composite mould part 2 is prepared and the composite mould parts 2 define respective parts of a mould. The protrusions 3 and the matching elements 3' match in one of the composite mould part 2 troughs in the matching composite mould part 2. The composite mould parts 2 can be united in a way that practically no opening between them remains. After curing the plaster of Paris or the curable material the carrier obtained comprising in its structure the composite mould parts 2 is opened and the master pattem 4 can be taken out from the inside of the composite silicone mould. The inner surface of the remaining cavity follows with high accuracy even the smallest

details of the surface of the master pattem 4. This cavity is connected with one raising block 5. The two or more parts of the composite mould are prepared for the whole surface of master pattern 4, i.e. if necessary such composite mould parts 2 are manufactured for the parts defined by the separation plane(s) 31 , e.g. for the lower part 36 and for the upper part 37.

When the master pattem 4 is a big piece, the next step is to prepare closures (Fig. 13), which are not reflected in the casting mould and which leave the surface corresponding to the separation planes 31 free. The closures are prepared at the composite mould parts 2 and at the surface of the plaster of Paris bearing the matching elements 33 and this is repeated at all bodies obtained from the master pattem 4 which are defined by the dividing planes and the separation planes 31. At each closure in each composite mould part 2 oppositely to the surface bearing the matching elements 33 and to the closing surface of the composite mould part 2 lying in the separation plane 31 a depression is made which leaves the separation plane 31 free and this depression is also covered by a silicone mixture. In this way a cavity lined with a silicone mixture layer is prepared which defines a space for precise connecting the parts of the master pattem 4 with one another and if the parts are nested into one another, the matching elements 33 ensure the accurate position of the parts. The two or more composite parts and the raising block 5 forming the composite mould can be prepared also from metal (Fig. 13), e.g. by casting or metal processing technologies, advantageously by template milling. The metal parts are rather expensive products and therefore this solution is applicable in the industry when the casting process should be automated for manufacturing long series of products. It is preferred to prepare the metal mould parts with the application of a pattern cast into the silicone based mould made of the composite mould parts 2 and the surface of the last can be advantageously projected in the metal. The use of metal parts is preferred at products of relatively simple shapes and when long series should be made. By the application of the mould made of the composite mould parts 2 comprising also the raising block 5 and other carrying elements the next step is to prepare the wax pattem of the invention. In this case the raising block 5 constitutes a cone shaped inlet (funnel) as it is shown in Figs 3, 4 and 15 and the inlet gives the passageway for a wax mixture into the inner space of the mould made of the composite mould parts 2. According to the invention a wax mixture is applied which differs in composition from those applied according to the art. This mixture comprises over the approximately equal amounts of a paraffin wax and a stearin wax from about

1 wt.% to about 4 wt.% pine resin and/or from about 3 wt.% to about 5 wt.% polyethylene. This mixture of novel composition is a very important element of the method of the invention and it is the basis why the wax pattem for precision casting can be prepared. The exact composition of the mixture depends on the circumstances in the place of production and it is intended to ensure relatively quick directed solidifying of the mixture after pouring it into the inner space of the mould. In the practice the next step of the method of the invention is to melt the wax mixture and keep it in molten state in a temperature higher than the melting point and pouring the melt into the inner space of the mould consisted of the composite mould parts 2. The molten mixture is left within the space for a time during which a wax layer 8 comes into being oblong the inner surface. In the meantime it is advantageous to knock the unit consisting of the mould and the hard carrier elements made of plaster of Paris, to vibrate it with low amplitude movement in order to expel the air bubbles from the mixture. It is also preferred to prepare only a rather thin wax layer 8 on the inner surface of the mould. After preparing the first wax layer 8 which lasts a time determined by the experience the rests of the wax mixture are expelled from the mould. After a short period of waiting a next portion of the same or similar molten wax mixture is poured into the mould but this portion should be a bit cooler than the previous one. It is advantageous to fill up the inner space of the mould at most up to the three quarters of its height. For preparing the next wax layer 8 the mould should be vibrated, knocked, rotated, etc. for making the wax move therein. In this way the thickness of the wax layer 8 is increased. The steps previously described are repeated in the necessary times for depositing wax in a layer of desired thickness. In the repeated steps it is advantageous to apply wax mixtures of stepwise decreased temperatures. With each wax mixture the wax layer 8 is made by vibrating, knocking and otherwise moving the mould and other parts. In this way it is preferred to prepare a wax layer 8 having thickness approximately equal to that of the product desired to be manufactured.

Between bringing up the layers from the wax mixture it is advantageous to leave the deposited substance to cool but it is proposed to avoid cooling it up to ambient temperature because according to the experience in this case the layers are interconnected weakly. If the conditions or the structure of the product to be manufactured make it necessary or eventually the workplace is too hot for relatively quick lowering the temperature of the wax layer 8, the speed of cooling can be increased if the elements made of plaster of Paris are previously cooled. Another possibility is to apply cooling rods or pipes within the carrier of the mould. The cooling pipe can be fed with cooling water.

When the master pattem 4 is a big piece, the wax pattems 11 created according to the lower part 36 and upper part 37 are bodies which either bear an open surface 13 or surround an inner cavity 19 (Figures 5 and 6). At the latter a closing element 14 is applied for covering the opening which remained in the place of the raising block 5. According to the invention the wax pattem 11 is the basis of preparing the casting mould from which the casting process can be realised.

The first step of preparing the casting mould of the invention is to make a hole or opening of preferred diameter in the range to 8 mm to 10 mm in a place corresponding to the closure of the wax pattem 11 , in the line of an inner opening of the piece connected with it. This should be done independently on the character of the wax pattem 11, i.e. whether it is closed or opened. The wax pattem 11 should be connected with a support which is similar to the raising block 5 and is made of wax 28. The support defines an inlet system which is necessary for casting. Before solidifying in the wax 28 a hook 25 is arranged (the wax pattem 11 completed with this is shown in Fig. 9 but as a product which is covered). Independently on whether the wax pattem 11 is opened or made with an inner cavity 19 (Figs. 5 and 6) it is necessary to prepare more holes 12 in it. The number of the holes 12 and their arrangement depends on the dimensions and the simplicity or complexity of the shape of the casting mould. In the next step of the production the wax pattem 11 is merged into fine sand.

The composition of fine sand can be the usual, i.e. it comprises a solid and a liquid component, the last consisted in about 95 wt.% of a mixture of ethyl silicate and industrial (denatured) alcohol in ratio 2 to 1, of about 0,3 wt.% to about 0,7 wt.% hydrochloric acid and of water. The viscosity of fine sand can be adjusted by dosing quartz sand of average graininess from about 20 μm to about 80 μm and it is determined by the means of a metering orifice of 4 mm diameter from which a 1 dm ³ charge of the fine sand should flow out in about 50 to 60 sec. It is preferred to maintain the temperature at a stable value in the range from about 18 °C to about 24 °C during preparing the wax pattem 11 and merging it into the fine sand. The wax pattem 11 has humid surface when it is taken out from fine sand. The surface is now covered with a mixture comprising corundum and this is made according to the invention in a fluidised bed. The wax pattem 11 is suspended on the hook 25 before introducing into the fluidised bed, where - this is not shown in the drawings - corundum of average graininess in the range from about 0,15 mm to about 0,20 mm is kept in stream. The wax pattem 11 is merged regularly two times into the fluidised bed and after preparing the first layer it is left for a short time to dry or if the

environment is humid, it is dried. The layer prepared by the means of the fluidised bed should be dried out before further processing.

Into the holes 12 of the wax pattem 11 covered with the layer made in the fluidised bed locks 15 shown in Fig. 7 or of similar construction should be arranged and the locks are made so that they press at the periphery of the holes 12 to the material of the layers prepared on the outer surface of the wax pattem 11 and on the surface of the inner cavity 19. The locks 15 serve as core supports in the mould cast to be prepared. In the next step the wax pattem 11 is merged against into a new charge of fine sand which is characterised by higher density than the fine sand applied previously for preparing the first two or more layers. The surface wetted by fine sand is covered by spraying with a further mixture comprising corundum. This further mixture is made with corundum of average graininess in the range from about 1,0 mm to about 1,7 mm, the value about 1,6 mm especially preferred. From this mixture of corundum of relatively big graininess further two or three layers can be made also and on the inner surface it is sufficient to prepare only two further layers. When preparing a fourth or further layer it is preferred to merge the wax pattem 11 again into fine sand having density greater than the previous mixture and to dry the layers brought on. The layers can be dried out completely or partly.

In the consequence of the steps shown an intermediate product is obtained (Fig. 8) which includes a shell-type structure covering the wax pattem 11 and consists of an outer shell 17 and an inner core 18 defining a casting cavity 20. The casting cavity 20 between the outer shell 17 and the inner core 18 communicates with an inlet passageway 16 filled out now with wax 28. The inner surface of the outer shell 17 follows the outer surface of the master pattem 4 and the outer surface of the inner core 18 has to determine the thickness of the cast product prepared. It follows that in given case the inner core 18 can be made also by other means than by merging and spraying referred to above, if it is expected that it can be destructed or its mechanical strength is low, it can be replaced by an appropriate elements made of destructible material which will slowly be destructed during casting. By these elements the appropriate parts of the inner space determined by the outer shell 17 are filled out. They are introduced into the inner space defined by the outer shell 17. For maintaining the metal elements forming the inner core 18 it is preferred to prepare protruding matching elements on the outer surface of the wax pattem 11 which will be transformed into groove-type elements on the inner surface of the inner core 18 and can be applied for fixing the inner core 18.

The inner core 18 and the outer shell 17 defining the casting cavity 20 are kept together by the corundum mixture which undergoes crosslinking during drying. If the

inner core 18 is of bigger dimensions, it is supported also by the locks 15 or similar elements shown in Fig. 7. The locks 15 are surrounded on both surfaces and connected by the corundum containing mixture. A fragment of this arrangement is shown better in Fig. 9., from which it follows, that the outer shell 17 made with the fine sand and the mixture comprising corundum fully surrounds the locks 15 and the same is the situation if a shell-type inner core 18 is applied. At each part defined oblong the separation plane(s) 31 between this closure and the part is a cavity filled out with the wax 28 and on one of the inner surfaces of this cavity there are the matching elements 33 made of the silicone polymer mixture or metal. During preparing the layers forming the outer shell 17 and the inner core 18 a very important step of the method of the invention, i.e. the drying step is carried out. This step is always required when the product has a closed inner cavity 19 and it should last as long as necessary for expelling the solving medium, up to crosslinking the ethyl silicate. This is made preferably by an arrangement which is schematically shown in Fig. 10 and in that way that the intermediate product shown in Fig. 11 forming a body 26 with the inner cavity 19 is suspended by the hook 25 fixed in the wax 28. Into an inner cavity 19 a flexible tube 24 is introduced which is connected with an inlet of a ventilator 21 over a tube 23 connected with a pressure regulator 22. If the ventilator 21 works the air flows in the direction of the arrow E and reaches the inner cavity 19 through the tube 23 and the flexible tube 24 for reaching the surface of the inner core 18. Arrows A, B, C and D represent the way of forwarding the air stream which leaves the inner cavity 19 by the holes 12 and the opening where the flexible tube 24 was introduced. The air flows oblong the surface of the inner core 18 and takes away the components which evaporate from the cover layer. From the outside the outer shell 17 regulariy does not require drying, the natural circulation of the air in the place of production is sufficient. If not, ventilator 21 can be applied for keeping the air in stream along the outer surface of the outer shell 17.

The structure consisting of the outer shell 17, the inner core 18 and the wax pattem, further of the locks 15 arranged in the holes 12 is thereby dried out and the appropriate components of the fine sand crosslinked. The next step is to merge the structure into hot liquid, e.g. water for removing wax 28 from the casting cavity 20 and from the raising element 5 forming the inner passageway 16. Wax 28 can be used again for preparing a next wax pattem 11. By the above steps an intermediate product of self-carrying structure resistant in some extent to mechanical claims is obtained which should be fired out - the normal temperature of this operation is about 900 °C - whereby the different parts of the desired casting mould can be made.

If a casting mould of bigger dimensions is prepared, the parts are connected with respective head parts including cavities. Within the cavities the matching elements 33 are arranged in the dividing planes 31, and especially it is advantageous when in the middle region of this cavity there is a protruding cone shaped body 38 crossing the dividing plane 31. The received parts should be fixed to one another and one of them is connected with an inlet passageway 16 through which the metal of the cast products can be poured into the casting cavity 20.

During the mounting operation the wall limiting the cavity is cut up, in order to have access to the matching elements and the protruding cone shaped body 38. The last is also cut up and the opening thus received is closed by backing sand. The same is made at those surfaces of the dividing planes 31 at which the danger exists that the molten metal can flow away through them.

The parts of the bigger casting mould are matched to one another by connecting the respective surfaces of the lateral protrusions or extensions bearing the matching elements 33, they are connected to one another and fixed by the means of clamps 34 shown in Fig. 14 or similar elements and if necessary lashing wire 35 is applied for securing the relative position of the parts connected. In this way a body is obtained which is similar in construction to that represented in Fig. 15.

If the shape of the product to be obtained is of relatively simply shape the inner core 18 can be prepared in the form of a massive insert which is supported by the locks 15 or other way. The massive insert should be rather a cylindrical body arranged in the central part of the inner cavity 19. In long series this solution simplifies the process of manufacturing the inner core 18, the danger of making cast products of unacceptable quality is restricted. The casting mould prepared according to Fig. 8 or Fig. 15 is encased in warm sand and the sand is vibrated in order to pack it as dense as possible.

INDUSTRIAL APPLICABILITY

The method of the invention offers the possibility of preparing a casting mould which is the basis of a cast product of very high quality, the surface of which follows with very good accuracy the surface of the master pattem. The method is simply realisable, the casting mould and the wax pattem can be manufactured in long series if necessary. The casting mould received is self-consistent, it is characterised by relatively high strength depending on the composition of the applied components. The product obtained from the casting mould requires some completing surface processing operations, if any.

The method of the invention results in shell-type products of high quality.