For example, EP 0,599, 507 discloses implementation of a casting process by using a so-called disappearing foam plastic pattern, e. g. in the production of metal pieces containing low carbon steel. In this process, there is first provided a moulding box to be filled with sand, which contains a plastic pattern in a shape consistent with a cast article to be produced. This is followed by covering a top of the moulding box and by applying therein a vacuum, e. g. within the range of 67-98 kPa. Then, a molten metal is poured into the plastic pattern for displacing the same, as well as for producing a cast article in a shape consistent therewith. The above-described method is generally used for producing sizable cast articles, which have a relatively high dimensional accuracy. On the other hand, quite often does this method result in a surface quality not good enough, which is why bearing surfaces, and other such surfaces with precise tolerances, must be prepared by machining the cast billet.

In addition to a so-called full mould process, as described above, it is also current knowledge to employ a precision casting or shell mould technique, or else e. g. more recent Replicast CS or RP processes.

A traditional precision casting process is commenced by first making a disposable wax pattern, which is generally performed by pressing the molten wax in a wax pattern mould, manufactured e. g. in aluminium. The wax pattern is mounted on a separate block of wood for patterns, which can be fitted with up to several dozen of such patterns, depending on cast articles to be produced. When the pattern is completed, regarding a system of casting conduits usually connected therewith at a later stage, it is immersed in a ceramic slurry for coating the same with e. g. various types of zirconic sands. A binder in the slurry used in this process is generally e. g. colloidal silica, sodium silicate, or ethylene silicate. The actual sanding is effected by means of a sprinkler or a fluidized bed.

The coating process is continued until the shell has a thickness of about 6-8 mm. This is followed by removing the wax either by the application of heat or, if necessary, with additional pressure as well, and then the mould is sintered for its final strength.

First of all, a problem with the above method is that the manufacture of a wax pattern requires an expensive wax pattern mould, which is generally made from aluminium or steel by machining, or else by using a vulcanized silicone mould. The more complex the cast article, the more expensive naturally the required wax pattern mould. The use of a wax pattern in itself causes numerous extra operations and costs, especially because of equipment and accessories needed in the working of wax. The accuracy attainable with the method is at the highest level obtainable today, although, for the time being, precision casting can only be used in the manufacture of very small cast

articles. In Finland, for example, the articles have. average weights in the order of a few dozen grams. In addition, difficult contours, such as counter- clearance sections, and a large size render the application of such traditional technology unreasonably expensive also in terms of its costs, which on the other hand requires primarily large-scale series production.

It is an object of a method of the present invention to provide a decisive improvement regarding the foregoing problems and limitations of current technology, and thus to raise essentially the available prior art. In order to achieve this objective, the inventive method is principally characterized in that the casting pattern is assembled from the plastic pattern, and from one or more mould shells, attachable thereto for casting and manufactured in a refractory material, such as a ceramic material, for providing the resulting cast article with surfaces particularly exacting in terms of the surface quality and/or dimensional accuracy thereof.

As one of the most important benefits gained by the inventive method, it should be noted that the mould shell used therein, manufactured e. g. in a ceramic material, enables first of all the casting of geometrically complex and relatively sizable cast articles, which include portions highly exacting in terms of the surface quality, surface contours, and/or dimensional tolerances thereof. On the other hand, by virtue of a basically traditional full mould process, which is utilized in this method, the partially plastic casting pattern is positioned in a mould during a casting process, which makes it possible that no division surface or mould joint be needed in the mould. By virtue of this, the casting pattern can also

be designed exactly as desired, nor does the employed casting pattern require any drafts; in principle, there is no other casting method that would match a full mould process in terms of freedom of design for a cast article to be produced. By virtue of the invention, the cast article becomes also lighter and metal is saved. Since the real situation is generally such that cast articles have just a few regions that require high-quality surface and/or high dimensional or contour-related accuracy, it is possible, by virtue of the invention, that particularly secondary portions, in which the surface quality and dimensional requirements are not crucial, can be cast in a plastic pattern, and exacting primary portions, such as e. g. apertures, sliding surfaces, bearing housings, etc., can be cast with refractory mould shells. Thus, the master patterns for refractory mould shells will be simpler and more compact than by having the entire casting mould made of a ceramic shell. Consequently, in this respect, the invention brings substantial cost savings in the production of manufacturing tools for a casting pattern and mould. In addition to that, the inventive method enables production in small series.

One of the basic concepts in a method of the invention is also that it enables the finishing of a cast article for a final product without subsequent machining and grinding operations, which also contributes to considerable cost savings and reduces manufacturing time for products.

Preferred embodiments for a method of the invention are disclosed in the non-independent claims directed thereto.

The invention will be described in more detail in the following specification with reference to the accompanying drawings, in which

figs. la-ld illustrate one preferred operating principle for a method of the invention in four successive working sequences, figs. 2a-2c illustrate one preferred preliminary preparation step for a method of the invention, regarding particularly the manufacture of a shell mould, in three separate working sequences, figs. 3a-3c illustrate further an assembly for a casting pattern shown in fig. 1 and used in a casting process for a cast article (figs. 3a and 3b), and a cast article produced therefrom in a perspective view (fig. 3c).

The invention relates to a method for casting, the casting pattern used therein comprising at least partially a pattern 1 manufactured in a plastics material, whereby, according to the principle shown e. g. in figs. la-ld, the casting process is performed by first providing a chamber 2, such as a moulding assembly or the like, which is filled with a refractory material, such as sand, and in which is placed the prefabricated plastic pattern 1, after which a top of the chamber 2 filled with the discussed material is covered according to the principle shown in figs. la and lb, by finally placing on top of it a pouring basin 5b. This is followed by delivering a molten metal into the plastic pattern 1 present in the optionally underpressurized chamber 2 for displacing the same and for producing a cast article 4 in a shape essentially consistent therewith according to the principle shown in figs. lc and ld. In this context,

a casting pattern is assembled, as shown e. g. in figs.

3a and 3b, from the plastic pattern 1, and from one or more shell moulds 1', attachable thereto for casting and manufactured in a refractory material, such as a ceramic material, for providing the resulting cast article 4 with surfaces particularly exacting in terms of the surface quality and/or dimensional accuracy thereof.

Referring particularly to figs. 2a-2c, the refractory shell mould 1'for use in connection with the plastic pattern 1 included in a casting pattern is manufactured e. g. by thermally spraying the surface of a master pattern M with one or more successive refractory material layers.

In another technique, which is an alternative to what is described above, the refractory mould shell 1'for use in connection with the plastic mould 1 included in a casting pattern can of course be also manufactured by scattering on top of the master pattern M one or more refractory material layers, which are cured by means of a chemical and/or thermal finishing treatment by bonding, sintering, and/or the like.

Referring particularly to the preferred embodiment shown in figs. 3a and 3b, the shell mould 1'made of a refractory material is attached to the plastic pattern 1 included in a casting mould chemically, such as by means of an adhesive L, and/or mechanically, such as by means of a fitting joint or the like. In the embodiment of figs. 3a and 3b, the mould shells 1' are bonded e. g. with the adhesive L to the plastic pattern 1 for providing bearing surfaces at the opposite ends of the cast article 4 and for producing holes in its mid-section.

In a further preferred embodiment for a method of the

invention, the plastic pattern 1 included in a casting mould is manufactured e. g. in expanded polystyrene EPS, expanded polypropene EPP, expanded polymethyl- methacrylate EPMMA, and/or a mixture of the above. In a further preferred embodiment, in this context, the plastic pattern 1 included in a casting mould is manufactured by machining from a blank, by expanding in a mould, or by casting, the plastic pattern 1 included in a casting mould having its components further coated, if necessary.

In a further preferred embodiment of the method, a casting system 5 included in a casting pattern, or an element thereof, such as a system of casting conduits 5a, a pouring basin 5b, and/or the like, is assembled from the plastic pattern 1 and/or the ceramic mould shell 1'. Thus, the element functioning as a plastic pattern can be just a component in a casting system, such as a head or the casting conduit 5a, which is attached to a casting mould made of ceramic shells. A system of plastic casting conduits can be used e. g. for integrating a plurality of ceramic moulds in a common casting system, wherein a single moulding vessel is capable of concurrently casting large numbers of ceramic moulds at a time. On the other hand, the plastic section of a casting pattern may constitute almost the entire casting pattern, with the exception of a few portions provided with a ceramic shell, such as e. g. the inside surface of a hole or the end of an axle.

It is obvious that the invention is not limited to the embodiments illustrated or described above, but it can be varied as necessary within the basic inventional concept, depending on each particular casting process.

Naturally, it is obvious that the inventive method can be implemented by using the most diverse of constructions, whereby e. g. the bottom and/or wall

frame of a moulding assembly can be built as best appropriate for each cast article, e. g. by varying dimensions or systems of vacuum conduits. In a further preferred embodiment, the distribution of a vacuum in a mould can be enhanced by using separate convertible systems inside or outside the mould. Furthermore, it is possible to use a bubbling through during or immediately after a casting process, wherein a selected gas or gas mixture is passed by way of an upper mould section through the mould and further into a vacuum system. This provides certain special features, such as reduced carbonization of casting steel, since generally, when using this type of method, the casting steel picks up carbon from the pattern material.