FIELD OF THE INVENTION

The invention refers to a method of preparing a wax pattern for precision casting, comprising the steps of preparing a mould consisting of two or more parts bearing the negative pattern of the surface of a master pattern, the mould having an inlet opening, matching the parts of the mould to one another, providing and melting a wax mixture including stearin and paraffin, pouring the wax mixture in molten state into the inner space of the mould, allowing the wax mixture to solidify and yielding a wax pattern. The method of the invention renders it possible to prepare a wax pattern on the basis of which a casting mould for making a mould of thin wall and desired surface can be obtained. The casting mould can be applied also for manufacturing massive products.

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 composition which transforms into molten state in 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 patterns 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 patterns 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 patterns 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.

The advantages of the precision casting induced the efforts for applying this method for manufacturing small and/or hollow products in short series. The problem is with the quality of the wax pattern and some solutions have become known to overcome this problem. According to the HU-B 199,085 a method is proposed, whereby for preparing a wax pattern a master pattem made of a material which cannot be vulcanised, e.g. of metal is selected, the master pattem is arranged in raw rubber or in one component heat vulcanisable silicone rubber and this rubber is pressed against the master pattern. The rubber surrounding the master, pattern is vulcanised and the product thus received is cut on two parts and the master pattem is taken out. The two parts are united: the inner space thereof is limited by a surface which is the negative pattern of the master pattern. From this negative pattern the wax pattern can be prepared. The proposed method is applied generally only in the jewellery and in the dentistry and it shows the disadvantage that the cast product should be small, rather massive and have convex surface. A method of preparing a tool for making a meltable pattern for precision casting is shown in the specification of the Hungarian patent application HU-A 197,238. According to the method disclosed at a part of a master pattem a cover is made from a silicone polymer of clearly defined composition. The cover consists of more interconnected layers which are brought one another on the surface of the previous one. The cover is cured and poured with plaster of Paris or an appropriate plastic resin. The remaining surface of the master pattern is covered in the similar way with a silicone polymer and poured. The tool realised can be used for preparing the

wax pattern. If necessary the curing process of the silicone polymer cover can be prolonged. In this way theoretically a tool of high quality can be produced but the problem is that the silicone polymer is not applicable for following sophisticated and articulated surface shapes because it shows low viscosity and slides down from the surface element. This causes inaccuracy in the shape of the wax pattem and thereafter of the mould and can lead to high material wastes during casting.

The aim of the present invention is to improve the technology of the precision casting in this aspect that the methods of preparing the wax pattern necessary in making the casting products are improved. SUMMARY OF THE INVENTION

The invention is based on the recognition that during preparing the wax pattem the first step should be the making of an appropriate silicone or metal mould and in the inner space of this mould a wax mixture of specific composition should be applied for preparing the pattem in more steps. The present invention is intended to elaborate a method, whereby a wax pattem can be prepared for manufacturing hollow cast products by precision casting, the costs of preparing the wax pattem can be kept low, the method is simple and thereby allows to apply the precision casting to individual products, 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 wax pattern for precision casting, comprising the steps of preparing a mould consisting of two or more parts bearing the negative pattern of the surface of a master pattem, the mould having an inlet opening, matching the parts of the mould to one another, providing and melting a wax mixture including stearin and paraffin, pouring the wax mixture in molten state into the inner space of the mould, allowing the wax mixture to solidify and yielding a wax pattern, wherein the improvement lies in the steps of encasing partly the master pattern in a curable carrying material composition for preparing a part of the mould and leaving its surface partly free, preparing 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 forming therefrom at least two interconnected layers on the free surface of the master pattern, 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 the surface of the master pattern, 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 pattern 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, and 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 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 successive wax layers, the molten wax mixture is poured into the

mould in excess 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 pattern 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 pattern 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 patterns and thereby that of the casting products on the basis of the master pattern can be facilitated when in the method of the invention the wax pattern made from the composite mould parts for preparing a metal body bearing the negative pattern of the wax pattem are applied and further wax patterns 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 pattern 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 pattern made according to the invention, and

Figure 4: a cross-section IV-IV of the arrangement shown in Fig. 3.

MODES OF CARRYING OUT THE INVENTION

During the realisation of the method of the invention the first stage is the preparation a more layer composite mould by the application of an appropriate, e.g. silicone based composition. The first step is to define one or more dividing planes in a master pattern 4 according to the known principles of casting. If the master pattern 4 has a sophisticated, articulate surface to selected parts thereof loose pieces are prepared and applied. 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 which facilitates the later separation.

After these beginning measures for realising the invention the master pattern 4 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 open inner space in the plane of the entry 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

pattern 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 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 pattern 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 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 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 mixture makes on the matching elements 3' protruding from the dividing plane protrusions 3 and these protrusions 3 are reflected by a negative pattern 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 pattern 4 is taken

out from the composite mould part 2. The surfaces of the master pattem 4 are cleaned and the master pattern 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 pattern 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 pattern 4. This cavity is connected with one raising block 5.

The two or more composite parts and the raising block 5 forming the composite mould can be prepared also from metal, 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 pattern of the invention. In this case the raising block 5 constitutes a cone shaped inlet (funnel) as it is shown in Figs 3 and 4 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 as many times as necessary 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 interconnection of the layers is weak. 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.

INDUSTRIAL APPLICABILITY The method of the invention offers the possibility of preparing a wax pattern which is the basis of a casting mould applicable in precision casting of shell-type products. The method is simply realisable, and the wax pattem can be manufactured in long series if necessary. The wax pattern received is self-consistent, it is characterised by relatively high strength depending on the composition of the applied wax mixture.