Patent Publication Number: US-7900685-B2

Title: Investment casting

Description:
BACKGROUND OF THE INVENTION 
     This invention relates to improvements in investment casting and more particularly to improvements in an investment casting procedure where the heat utilised to melt the wax-type pattern and to sinter the ceramic mould is provided by microwave energy. 
     In investment casting first a model of the article to be moulded, usually from molten metal, is wax injected into a reverse engineered mould, or fabricated from a wax type pattern material. The pattern material may be natural or synthetic wax, polystyrene, or blends of various waxes, thermoplastic materials usually, but not exclusively, including fillers such as adipic acid and plasticizers. As used herein and in the appended claims the expression “wax type pattern material” is intended to include all such heat fusible pattern materials suitable for use in a “lost wax” moulding procedure. Typically a number of similar models are attached to a “sprue” to form a “tree” of the pattern material and the whole is coated several times typically, but not exclusively, with ceramic slurry and sand type material. The ceramic coating is then dried to provide a hard mould around the “wax type pattern material”. The pattern material is melted out and the ceramic “shell” is sintered and molten metal is then poured into the “shell” void. When the metal has hardened the ceramic shell can be removed. 
     Investment casting using conventional sources of heat is a very lengthy and expensive procedure. It has been proposed e.g. in British Patent No. 1 457 046 to use microwave energy, thereby shortening the procedure and making it more economical. However the principal problems encountered in investment casting arise from differential expansion and contraction of the different materials involved when being heated up and cooled down. A particular danger is that if the pattern material cannot escape fast enough from the ceramic shell when being melted it may crack the shell due to its expansion. British Patent No. 1 457 046 offers as a solution to this problem the inclusion in the ceramic slurry of a so called “lossy material” which will induce a rapid melting of the pattern material adjacent to the shell. The solution however is imperfect especially when moulding articles of such a shape that the pattern material can only escape from the ceramic shell through a restricted bottle-neck, sprue or pour cup. If the material of the sprue is not melted first, or is imperfectly melted, the escape path for the rapidly expanding material within the shell is blocked with the result that the shell may be cracked. 
     It has been proposed in Japanese patent publication JP56117857 to use a resin type mould that can be melted out of the shell without deformation or cracking. This solution however is imperfect as it relies on placing the resin mould into a container of water allowing the water to penetrate through the honeycomb sections of the mould by capillary action. By this technique the volume of water will be generally constant throughout the mould where exposed above the water surface, i.e. there will be no gradient of susceptor content throughout different areas of the mould. Moreover this type of resin moulding cannot be used on high specification finishes of the cast components (such as aero engine blades) without a further polishing process, due to the manufacturing type of process of resin moulds, which do not produce a smooth finish to the casting. 
     A principle object of the present invention is to resolve these problems by providing a differential melting characteristic for wax pattern material in different parts of the mould, such that material in a sprue or other restricted opening will melt before material in other areas of the mould upstream of the opening. Thus when the latter material in turn becomes molten its escape route is not blocked and it can exit the mould while expanding without endangering the mould shell. The current virgin wax patterns, which must be used in the production of engine blades, can be used in accordance with this invention. 
     SUMMARY OF THE INVENTION 
     In accordance with the present invention there is provided an investment casting procedure using microwave energy as a heat source, characterised in that models of virgin wax are attached to a sprue of a wax-type pattern material incorporating a susceptor having a relatively greater heat absorption characteristic than the virgin wax and the sprue is attached to a pour cup of a wax-type pattern material incorporating a greater percentage of said susceptor than is incorporated in the material of the sprue. 
     The susceptor may be confined to regions of the sprue and the pour cup which will be restricted openings of the mould when the wax-type pattern material is melted. 
     The susceptor may be water, carbon, graphite or any combination thereof. 
     A tree on which multiple virgin wax models are mounted may incorporate said susceptor and may have a pour cup which incorporates a greater percentage of said susceptor than the remainder of the tree. 
     The susceptor content of the tree may be in the region of 12% and the susceptor content of the pour cup may be in the region of 15%. 
     A preferred embodiment of the invention will now be described by way of non-limitative example with reference to the accompanying drawings, in which: 
    
    
     
       BRIEF DESCRIPTION OF SEVERAL VIEWS OF THE DRAWINGS 
         FIG. 1  is a front elevation of a sprue with a pour cup; 
         FIGS. 2A and 2B  respectively illustrate the sprue of  FIG. 1  in front and side elevation with multiple models attached, so that it is now called a tree, and 
         FIG. 3  illustrates the tree of  FIGS. 2A and 2B  in side elevation showing that the whole has been coated with a ceramic material. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The drawings illustrate a sprue  10  having a pour cup  14  filled with wax-type material  11 . Models  12  of articles to be moulded are attached to the sprue by wax, glue or hot knife attachment. As is known per se all of the models  12 , the sprue  10  and the pour cup  11  are fabricated from a wax-type pattern material. In accordance with the present invention, however, the sprue  10  has a higher percentage of susceptor content than the virgin wax models  12  and the pour cup  11  has a higher susceptor content than the sprue  10 . The models  12  are virgin wax and the sprue  10  and the pour cup  11  are made up from reclaimed wax emulsions with known fixed percentages of susceptor in the emulsions. 
     The prime sand coat has a percentage of susceptor, likely to be carbon, graphite or any other suitably susceptible material or any combination thereof 
     The entire assembly, the tree,  10 ,  11 ,  12  and  14  is prime coated with a ceramic slurry. While still wet the prime coat  15  is covered with the susceptible prime sand coat and then dried. Any number of additional coats of ceramic slurry  13  and sand are then applied to the prime coat to build up a ceramic shell of the desired thickness. The tree is then stood on the pour cup  14  over an opening in a microwave oven (not shown) and microwave energy is used to melt the wax-type material, which is now encased in a dried ceramic shell  13 . Because of its higher susceptor material content the pour cup  1  will melt first and run out of the oven where it may be collected for reclamation. The material of the sprue  10  will melt next and run out through the pour cup thus unblocking the exits  16  from the models  12  enabling the virgin wax to run out when melted. 
     The doped prime coat will heat up, thus melting the pattern material adjacent to it. Due to the exits  16  from the pattern material being unblocked by prior melting of the sprue and pour cup the resulting melting of the virgin wax, by thermal transfer, will not endanger the shell  13 . 
     Microwave energy is continuously applied to sinter the ceramic material and until the shell reaches an elevated temperature, e.g. 1000 degrees centigrade, whereupon it is cooled to pouring temperature, and metal, at a similar temperature, is poured into it through the pour cup  14 . Alternatively the ceramic shells can be cooled to ambient temperature and supported mechanically, usually by sand, while being filled with molten metal. After the casting has cooled and the metal hardened the shell  13  can be removed conventionally and the individual articles can be removed from the sprue and finished in the conventional way. 
     It will be apparent that the procedure of the present invention is not limited to the use of a tree such as  10  and to the simultaneous casting of multiple moulds. In any investment casting procedure using microwave energy as the heat source the wax-type pattern material in the region of a restricted opening of a cast ceramic shell may be given a higher susceptor content than the remainder of the pattern material, thus ensuring that the pattern material can run out of the shell before its expansion endangers the shell during the start of the sintering process.