Patent Application
20140348693
The present invention is related to the removal of risers from castings using breaker inserts in general, and more specifically the removal of risers from ductile iron castings using breaker inserts. Even more specifically, the present invention is related to an improved riser breaker insert, an improved casting system and improved molded products formed thereby.
Currently, the manufacture of metal castings requires the use of feed risers that supply molten metal to the casting to compensate for the volume contraction of the metal as it solidifies. This physical rule applies to the manufacture of ductile iron castings, which typically require about 20 to 30% of the casting volume to be created as feed riser volume in order to supply the required molten ductile iron to the casting cavity to compensate for the volume contraction of the ductile iron as it solidifies.
Metal solidification rates are determined by the rate of heat extraction of the mold and the section modulus of the casting. To ensure that the riser is the last portion of the casting to solidify, the section modulus and/or the heat extraction rate is carefully calculated and controlled for each casting design. The modulus of the connection area between the riser and the casting is critical to the production of a defect free casting. A section modulus that is too small will result in premature solidification of the riser contact, creating a shrinkage defect due to insufficient feed metal, whereas a section modulus that is too large will result in the inability of the riser to be removed from the casting or a fracture plane that penetrates into the casting, resulting in a scrap casting.
Currently, foundries use modeling software combined with empirical foundry art to design the size of the riser and the riser connection. The modeling software can accurately predict the size and shape of the riser and riser connection to provide the required feed metal to the casting, but is severely limited in predicting the impact required to remove the riser from the casting. Foundry experience is required to determine which method will be used to remove the riser from the casting. Currently foundries are limited to direct impact or sawing for the removal of risers from castings. Historically, foundries have used sand cores, mold notches, dense ceramic breaker cores or woven refractory cloths to assist the removal of the riser from the casting. Foundries may utilize either insulating or exothermic riser sleeves to alter, typically reduce, the rate of heat extraction from the riser. This reduction in heat extraction allows for a smaller riser modulus and a smaller accompanying riser connection. The smaller riser connection allows for the direct impact method of riser removal.
The problem with the riser in ductile iron foundries is both economic and mechanical. The use of insulating or exothermic risers raises the cost of the mold significantly and the insulating or exothermic material creates contamination issues in the molding sand. In addition, the insulating or exothermic material tends to get intermixed into the green sand, creating contamination issues that the foundry must cope with in subsequent casting cycles.
Significant process issues accompany the use of mechanical aids to break the riser from the casting. Resin bonded sand breaker cores tend to break down upon exposure to heat. If the riser connection remains liquid after this breakdown, there is a distinct probability that sand particles can be entrained in the liquid metal often leading to inclusion defects within the casting. Heat exposure limits the use of resin bonded sand breaker cores and restricts their use in castings weighing more than 500 pounds. A dense ceramic core will withstand the thermal exposure, but tends to increase the thermal extraction rate in the riser connection area, requiring an increase in the riser connection area. Woven refractory cloths are not rigid, which makes them difficult to place in a sand mold and this lack of rigidity creates an opportunity for potential movement or dislodgement from the original position within the mold due to the pressure of the flowing metal.
A significant issue with the current state of the art is the removal of the risers from the casting after the casting has been removed from the molding sand. Currently, there are two methods utilized to remove risers, direct impact or cut-off saws. Both of these methods have significant problems. Most artisans prefer to use direct impact to remove risers. Direct impact can be accomplished using either kinetic energy with a hammer strike or pulsed energy from a hydraulic wedge. Both of these impact methods are effective and cost efficient. The problems with the impact method arises when the section modulus of the connection area is too large to remove the riser by impact without creating a fracture plane that penetrates into the casting body, resulting in mechanical damage, which makes the casting unsuitable for its intended use. In some cases, the required section size of the riser connection combined with the ductility of the cast iron make it impossible for the riser to be removed from the casting using impact without breaking the casting itself. An additional problem with mechanical break off of the risers in many foundries is the ergonomic and safety issues associated with the use of the large sledge hammers required to deliver the manual impact needed to break the risers free of the castings. Any feature added to the contact area between riser and casting that reduces the impact required to separate the riser from the casting will result in improved ergonomics and reduced safety risk for this manual task.
If impact is not feasible or practical for a specific casting, then the foundry is required to use the more expensive cut-off option to remove the risers from the castings. In most cases, this is very difficult since these castings are usually very heavy, which will require mechanical assistance to manipulate the casting. Once the casting is manipulated into position, the next problem is access of the saw blade to the riser connection area. Often the casting design requires the riser to be placed in an area where there is no plane of access that will allow the saw blade to cut the riser without creating damage to the casting itself. These design restrictions generally result in a compromised feed design for the placement of the risers, which add significant expense to the casting process.
It is an object of the invention to provide an improved matrix breaker insert which provides a preferential sever point between a casting and the waste material in a riser.
A particular feature of the invention is the ability to provide preferential breakage due to a decrease in metal within a region without detrimental loss of thermal energy.
A particular feature of the invention is the ability to refine the art of casting in a manner which was previously unavailable.
These and other advantages, as will be realized, are provided in a casting system for molding metal. The casting system has a mold comprising a cavity in a predefined shape. A riser in flow communication with the mold wherein the riser provides molten metal to said cavity as the molten metal freezes. A riser breaker insert is between the mold and the riser wherein the riser breaker insert comprises voids. A molten metal supply is provided which is capable of filling the mold and the voids of said riser breaker insert and at least partially filling the riser.
Yet another embodiment is provided in a ceramic riser breaker insert. The ceramic breaker insert comprises a matrix of solid ceramic filaments forming a gross shape with an inner boundary, an outer boundary and a bulk density of at least 15 vol % to no more than 50 vol % between the inner boundary and the outer boundary.
Yet another embodiment is provided in a method for casting metal comprising:
Yet another embodiment is provided in a method of forming a ceramic riser breaker insert comprising:
The present invention is directed to an improved riser breaker insert and an improved method for casting metal. More specifically, the present invention is related to an improved riser breaker insert which allows metal to flow there through with minimal loss of thermal energy wherein the riser breaker insert forms a plane which is more easily severed, thereby separating the cast from the waste.
The invention will be described with reference to the figures forming an integral, non-limiting, component of the disclosure. The figures are intended to facilitate an understanding of the invention and are not intended to limit the invention in any way. Throughout the figures, various elements will be numbered accordingly.
An embodiment of the invention will be described with reference to
An embodiment of the invention is illustrated in cross-sectional schematic view in
An embodiment of the invention is illustrated in
A riser breaker insert is illustrated in top view in
The example illustrated comprises multiple structural rings, 38, of solid filament with solid filament forming an upper serpentine, 48, supported by the structural rings on one side and solid filament forming a bottom serpentine, 50, on the opposite side without limit thereto.
The solid filament is preferably formed as an extruded material wherein the extruded material comprises a ceramic precursor in a matrix. The extruded material has liquid properties suitable for allowing extrusion through an orifice, yet the extruded material has solid properties after extrusion sufficient for the extruded filament to be self-sustaining, without sag, over the span between support lattice. The ceramic precursor is selected for compatibility with the material to be filtered. Fired clay, mullite, alumina, zirconia-toughened alumina, zirconia-toughened mullite, silicon carbide, silica-bonded mullite, and silica-bonded silicon carbide are particularly suitable for demonstration of the invention for use with iron and particularly ductile iron. The precursor used to form the breaker insert can include rheology modifiers such as solvents, acids, bases, polymers, and the like where the modifiers are chosen to provide the viscoelasticity sufficient for extrusion and self-sustaining shape. Pore formers may be included to decrease the ceramic density. The ceramic material for the breaker insert may be chosen such that it has a significantly different coefficient of thermal expansion from that of the alloy being cast. Creating such a difference would tend to induce cracking and separation between the ceramic and metal as it cools, further weakening the zone surrounding the breaker insert, making it easier to remove the riser from the casting.
The cross-sectional shape of the solid filament is not particularly limited herein with triangular, rectangular, trapezoidal and higher polygonal shapes suitable. Round, oval or obround are particularly preferred due to the ease of extruding cross-sectional shapes without edges and the decreased probability of breakage at an edge during use.
There are two densities of importance for the instant invention. One density is referred to herein as the ceramic density which is a function of the composition and the volume of voids within the ceramic. The ceramic density is reported relative to theoretical density, wherein the theoretical density of a material is defined as the mass per unit volume of the material in its pure, pore-free state. Theoretical density of all of the ceramic parts is not easily achieved in practice and pore formers are often incorporated in the ceramic precursor to purposely add microporosity thereby decreasing the ceramic density. Adding microporosity to the ceramic matrix could reduce the strength of the breaker insert thereby weakening the zone surrounding the breaker insert and making it easier to remove the riser from the casting. It is preferable that the ceramic density of the solid filaments be at least 30% of theoretical density, and can approach 100% of theoretical density. The second density is bulk density, which is the total percent volume of ceramic, including its intergranular porosity volume, as a function of total volume of the breaker insert. The bulk density of ceramic as more specifically set forth in
The bulk density of the breaker insert is preferably at least 10 vol % to no more than 50 vol %. More preferably, the bulk density is at least 20 vol % to no more than 40 vol %. Below about 10 vol %, the strength of the breaker insert is insufficient and the breaker insert is susceptible to breakage during the pour. Above about 50 vol %, the flow of molten metal through the breaker insert may be insufficient to allow the riser to function properly.
The cross-sectional size of the solid filament, or strut, is selected to provide sufficient ceramic area to provide a clean break in the metal, yet small enough to allow adequate flow of molten metal though the voids in the riser breaker insert. The cross-sectional size the solid filament must be sufficiently large to be self-supporting during formation and firing of the riser breaker insert which depends, in part, on the material used. A solid filter equivalent diameter of 0.5 mm to 5.0 mm is preferable. Below about 0.5 mm the ceramic may not be sufficiently self-sustaining. Above about 5.0 mm equivalent diameter the number and arrangement of voids becomes limited. More preferably the solid filament has an equivalent diameter of 0.75-mm to 2.5-mm and most preferably 1-mm to 2-mm. Equivalent diameter is the diameter of a circle having the same cross-sectional surface area as the solid filament.
The gross shape is selected based on the orientation of the mold and riser and is not limited by the ability to design the riser breaker insert. The instant invention allows for great flexibility in gross shape with the current limit being installed equipment and the typical desire to match the disposable riser breaker insert with existing equipment and processes. A particular advantage of the instant invention is the ability to design the gross shape in concert with design of the mold thereby providing optimizations which were previously not considered. Based on current molds and risers available in the art, a gross shape of round is preferable since this matches the design criteria established by most existing equipment. With optimization, the gross shape may become trigonal, rectangular or may be the shape of a higher level polygon wherein flow dynamics can be altered as desired by the design.
The riser breaker insert can be manufactured in multiple geometric shapes, including but not limited to circles, ovals, squares, rectangles and trapezoids. In addition, the cross sectional design of the matrix riser breaker insert can be engineered to provide the foundry with the required reductions in fracture resistance and concentration of mechanical force. An example of a wedge shaped, circular matrix riser breaker insert is shown in
The cross-sectional shape provides a level of control for the flow profile of the molten metal through the riser breaker insert. Previously, this was not readily adjusted. The profile can be adjusted by the combination of the cross-sectional shape and the size of the major and minor voids to allow a more free flow or a more restricted flow in certain regions. For the purposes of illustration, flow through the center of the major void is relatively free with no limits thereon except those associated with flowing metal, such as viscosity. Closer to the wall of the riser, the flow is less free due to friction with the riser walls which may create a form of a slip stream. The riser breaker insert can be thicker at the outer boundary and thinner at the inner boundary to minimize the impact of flow restriction inherent in the design of the riser. Alternatively, the cross-sectional shape can be used to increase flow restrictions in certain regions of the flowing metal if so desired based on flow dynamics of the metal and shape of the riser and related components.
The major void and minor void sizes and positions are chosen to allow optimum flow through the riser breaker insert, yet with the desire to minimize the columns of metal passing there through. As would be realized, the size and shape of the major void is preferably large enough to minimize flow restriction since this represents the maximum flow rate, yet it must be sufficiently small to minimize the cross-sectional area of the pillar formed therein upon freezing of the metal contained in the major void. If the major void is too large, the pillar formed therein is large which makes severing the waste from the cast more difficult, thereby limiting the value provided by the riser breaker insert. If the major void becomes too small, the flow is restricted which may be a disadvantage. With prior art inserts this was a conundrum, yet with the instantly claimed invention the minor voids may be large, particularly closest to the inner boundary, thereby providing sufficient flow while minimizing the size of the central pillar formed in the major void. In one embodiment, the major void is the same size as a minor void, thereby eliminating the large pillar completely. Graduated void sizes, preferably with the largest voids towards the center, are particularly preferred in some embodiments.
Various techniques are known in the art for extruding solid filaments including the use of biphasic ink as the extruded material wherein the biphasic ink comprises a gel phase with a plurality of flocculated and non-flocculated particles in a carrier liquid as set forth in U.S. Pat. No. 8,187,500, which is incorporated herein by reference. The biphasic ink employs different dispersants such that particles can attain flocculated or non-flocculated states within a carrier liquid. The non-flocculated particles remain repulsive through the use of a particularly preferred comb polymer dispersant with ionizable and nonionizable side-chains.
Slurries may also be employed as the extruded material wherein the slurry has a sufficiently high concentration of particles to be pseudoplastic and very little, if any, organic binder.
Sol-gel inks can be employed as the extruded material particularly for forming metal oxide structures. The sol-gel ink preferably has a metal oxide precursor with at least one member selected from the group consisting of Ti, Sn, Zr, and In. A stress reliever is typically included with polyvinylpyrrolidone, poly(N,N-dimethylacrylamide), poly(2-methyl-oxazoline), poly(ethylene glycol), poly(propylene glycol) or poly(vinyl alcohol) being suitable stress relievers for demonstration of the invention. A solvent is typically included and optionally a polymerization inhibitor.
Chemically reactive suspensions can be used to form the extruded material. Chemically reactive suspensions employ two fluids in separate containers which are fed into a cylindrical mixing chamber. The two liquids are mixed and deposited from the mixing chamber onto a platform that moves relative to the mixing chamber to form the riser breaker insert. The mixture gels shortly after being deposited. Alternatively, the platform may be stationary and the mixing chamber can move to form the pattern.
The slurry can be extruded into a bath containing solution, solvent or oil with high concentration of acid, base or salt that induces flocculation of the particles within the slurry. The rate at which the slurry flocculates can be controlled by the concentration of acid, base or salt relative to the slurry and the materials used.
The extruded material may eventually be dried, if necessary, to remove any volatile components which is referred to herein as a green riser breaker insert. The green riser breaker insert is then heated to a temperature necessary to sinter the ceramic precursor thereby forming a ceramic riser breaker insert.
While not limited to any theory, it is thought that a porous ceramic riser breaker insert will maintain the desired section modulus of the contact area between the riser and the casting, while creating an accompanying fracture plane and a smaller solid metal interface at the riser connection. These new features will allow easy removal of the riser with appropriate impact and will create a fracture plane that will protect the casting from mechanical damage created during riser removal.
The presence of a non-metallic material in a cast metal creates a boundary plane that is inherently weaker than the crystalline structure of the cast metal. Engineering the geometry of the non-metallic structure will have a predictable influence on the fracture resistance of the cast metal. This effect on the impact resistance of the cast metal has a dramatic effect on cast iron specifically. Cast iron is a natural composite material consisting of a controlled mixture of graphite particles intermingled with iron crystals. The graphite particles generally crystallize out of the liquid metal simultaneously with the formation of iron crystals during the solidification of the casting. The shape of these graphite crystals can be flakes/planes, tubes or spheroids depending upon how the molten metal is chemically modified by the foundry. For ductile iron, the molten metal is chemically modified to produce a spheroidal graphite shape which increases the elongation and tensile strength of the metal. While the formation of the spheroidal graphite is highly desirable in the casting, it makes the removal of the feed riser significantly more difficult to remove from the casting, particularly using direct impact. It is documented in the literature that a fracture propagates along the boundary between the iron matrix and the graphite crystals. In grey iron, the graphite crystals are flakes/planes and both the impact resistance and tensile strength are governed by the flake length. Longer flakes result in lower impact resistance and tensile strength. In ductile iron, the impact resistance and tensile strength are significantly increased through the modification of the graphite shape from interconnected flakes and planes to discontinuous spheroids of graphite. To reduce the impact resistance of the riser connection area, a weak boundary needs to be created to allow the iron to fracture readily with reduced impact loads. Ceramic materials used to filter ductile iron are known to have limited wettability with molten iron. This limited wettability between ceramic filament and molten iron creates the weak bond required to reduce the impact resistance at the riser connection area. The filament length of the ceramic insert will be orders of magnitude greater than the length of the boundary between the graphite spheroids and the iron matrix. This significant increase in the length of the weak boundary area in the iron will significantly reduce the impact resistance of the metal in the contact area between the casting and the riser. A design of the matrix breaker insert can be created for each casting application to further reduce the impact resistance in the iron by concentrating the impact force in a smaller area. This tailoring of the geometrical shape of the breaker insert will provide the foundry with the reduction in cast metal fracture resistance required to allow for removal of the riser with available impact force.
The open design of the matrix breaker insert allows the full penetration of the molten metal into the matrix breaker insert's cross sectional area. The complete filling of the voids in the matrix combined with the low mass of the matrix structure will have a minimal impact on the section modulus of the riser connection area. This increase in modulus will allow the riser connection to remain liquid longer, thus providing the desired feed path from the riser to the casting. Since the riser contact will remain open longer, smaller risers can be employed in the casting application design resulting in increased efficiency and lower cost.
Creating a molten metal feed path that has lower fracture resistance, but maintains a higher section modulus, allows those of skill in the art to achieve several desirable benefits in the casting application design. The artisan can use smaller risers with larger contact areas to feed molten metal into the casting without creating the risk of casting damage during riser removal. The artisan can more readily use the direct impact method for riser removal, which is preferable. The artisan is also now provided with the tools to re-evaluate the long stagnant art of molding and particularly the use of insulated/exothermic riser sleeves to maintain the liquid path between the riser and the casting. In summary, incorporation of the riser breaker insert technology into the foundry process will provide opportunities to reduce cost by reducing riser size, utilizing the direct impact riser removal method and eliminating the use of costly insulating or exothermic riser sleeves.
Throughout the description shapes: such as polygonal, round, obround, oblong, etc. refer to the general shape with the understanding that deviations from the shape may occur during the treatment process due to flow and the like. A round extrusion, for example, may be partially altered at one face, such as flattened, due to the material contouring to a surface upon which the round filament is placed and is therefore still considered to be a round filament even though there are perturbations to the extruded shape.
The invention has been described with particular reference to preferred embodiments without limit thereto. One of skill in the art would realize additional embodiments and improvements which are not specifically enumerated but which are within the scope of the invention as specifically set forth in the claims appended hereto.
The present invention claims priority to pending U.S. Provisional Patent Application No. 61/827,065 filed May 24, 2013 which is incorporated herein by reference.
| Number | Date | Country | |
|---|---|---|---|
| 61827065 | May 2013 | US |
