Abstract:
A method for producing silica sand-based foundry shapes useful informing metal castings and for reducing veining defects in sand-based foundry shapes by providing a foundry sand, adding an anti-veining composition that comprises bentonite to the foundry sand to form a mineral composition, then adding a foundry resin to the mineral composition to form a sand-based foundry composition and shaping the sand-based foundry composition to form a desired pattern.

Description:
CROSS REFERENCE TO RELATED APPLICATION  
       [0001]    This application claims priority from U.S. Provisional Application Serial No. 60/332,679, filed Nov. 14, 2001. 
     
    
     
       BACKGROUND OF THE INVENTION  
         [0002]    The present invention relates to a method for producing foundry shapes and, more specifically, to a method of reducing veining defects in sand-based foundry shapes by adding an anti-veining compound comprising bentonite.  
           [0003]    Sand casting is a process used in the foundry industry to produce metal parts. In sand casting, disposable foundry shapes, such as cores and molds, are made by forming a sand-based foundry composition into the desired shape and curing the composition. One or more binders mixed with the silica sand are required to maintain the sand in a predetermined shape. Commonly employed binders include inorganic binders such as clay and foundry resins such as phenolic resin binders. There are two basis types of binder systems used in the foundry industry. Green sands are produced by binding silica sand with clay, coal dust, and water. Chemically bonded sands use a variety of organic and inorganic resin binders.  
           [0004]    Green sand molding is the production of molded metal objects from tempered molding sand and is widely used to cast ferrous as well as non-ferrous metal castings. Green sand molding is economical and permits both quality and quantity production, particularly for smaller castings. Green sand is defined as a water tempered molding sand mixture with plasticity. A green sand mold used for casting steel usually consists of silica sand, and a binding agent mulled together with tempered water. Other useful foundry sands include chromite, zircon and olivine sands.  
           [0005]    Chemically bonded sands refer to sand-based foundry compositions comprising sand and a binding amount of a polymerizable or curable binder. The binder permits the foundry composition to be molded or shaped into the desired form and thereafter cured to form a self-supporting structure. The polymerizable or curable binder is caused to polymerize by the addition of catalyst and/or heat to convert the formed, uncured foundry sand composition into a hard, solid, cured state. Examples of curable resin compositions useful as binders in the foundry art include phenolic and furan resins. In a typical no-bake process, the sand, binder, and a liquid curing catalyst are mixed and compacted to produce a cured mold and/or core. A binder commonly used in the no-bake process is a polyurethane binder derived by curing a polyurethane-forming binder with a liquid tertiary amine catalyst.  
           [0006]    Silica sand grains expand upon heating. When the grains are too close, the molding sand moves and expands causing a variety of defects in the castings. One such defect is veining which refers to a discontinuity on the surface of the casting appearing as a raised, narrow ridge that forms upon cracking of the sand mold or core due to expansion of the sand during the filling of the mold with the molten metal.  
           [0007]    Iron oxides have been used for years in foundry applications to improve core properties and the quality of castings. Iron oxides have proven to be advantageous as an additive to foundry molding aggregates containing silica sand to improve the quality of castings by reducing the formation of thermal expansion defects, such as veining, scabs, buckles, and rat tails as well as gas defects, such as pinholes and metal penetration. There are several iron oxides which are currently used in foundries today. These include red iron oxide, also known as hematite (Fe 2 O 3 ), black iron oxide, also known as magnetite (Fe 3 O 4 ) and yellow ochre. Another iron oxide which is presently being used is Sierra Leone concentrate which is a hematite ore black in color. Red iron oxide and black iron oxide are the most popular iron oxides in use.  
           [0008]    One method of employing the above iron oxides is to add approximately 1-3% by weight to the sand mold aggregates during mixing. The exact mechanism by which iron oxides affect surface finish is not totally understood. However, it is generally believed that the iron oxides increase the hot plasticity of the sand mixture by the formation of a glassy layer between the sand grains which deforms and “gives,” without fracturing at metallurgical temperatures, to prevent fissures from opening up in the sand, which in turn reduces veining.  
           [0009]    Various other types of additives have also been employed in an attempt to improve core properties and the quality of sand castings. For example, other anti-veining compounds which have been utilized in sand aggregate mixtures include starch based products, dextrin, fine ground glass particles, red talc and wood flour, i.e. particles of wood coated with a resin. All of these additives have met with limited success in reducing veining.  
           [0010]    U.S. Pat. No. 5,911,269 to Brander et al. discloses the use of lithia-containing materials in silica sand molds and cores to reduce thermal expansion defects, such as veining. The addition of lithia-containing additives to the foundry sand composition can add significantly to the expense of the overall foundry operation.  
           [0011]    Although it is known to use bentonite clays as binders for foundry green sand molds or cores, bentonite clays have not been used as anti-veining additives in chemically bonded sand compositions. Quite surprisingly, it has been found that when bentonite clay is used as an anti-veining additive in conjunction with a chemically bonded-based foundry sand, the quality of the castings improves by reducing veining defects.  
           [0012]    U.S. Pat. No. 4,216,133 to Johnson et al. discloses a shell process foundry resin composition containing novolak resins incorporating from about 0.5 to about 10% based on weight of the resin of a finely divided, siliceous material, such as bentonite. According to Johnson et al., the finely divided siliceous material incorporated in the foundry resin composition provides peel back resistance and increased stripping strength. Furthermore, Johnson et al. emphasized that the siliceous material is added to the resin material and is not merely added to the sand mixture in the muller. The incorporation of the siliceous material is thought to control viscosity during cure. The amount of siliceous material in the composition based on sand is only 0.05 to 0.8%. It should be noted that there is no indication or suggestion of using bentonite as an anti-veining composition in the &#39;133 patent. Veining is not typically considered a problem in a shell molding process.  
         SUMMARY OF THE INVENTION  
         [0013]    The present invention relates to a method for producing chemically bonded foundry shapes by incorporating an anti-veining composition comprising bentonite into a silica sand aggregate. The anti-veining composition is mixed with foundry sand used in the production of foundry cores and molds to improve the quality of castings by reducing thermal expansion defects, such as veining, in iron, steel and non-ferrous castings.  
           [0014]    In accordance with one embodiment of the present invention, a method of producing a silica sand-based foundry shape is disclosed. The process comprises the steps of providing a foundry sand, adding an anti-veining composition to the sand, adding a foundry resin to form a sand-based foundry composition, and shaping the sand-based foundry composition into a desired pattern, wherein the anti-veining composition comprises bentonite.  
           [0015]    The foundry molding and core silica sand mixture used to produce cores and molds in accordance with the present invention typically comprises about 85% to about 98.5% of commonly used molding and core silica sand together with about 5 to about 0.5% of foundry resin appropriate for sand cores and molds, and from about 1 to about 10% of bentonite as an anti-veining additive. The type of bentonite is not particularly limited and can be a water-soluble sodium bentonite clay or a low-soluble calcium bentonite clay. The composition may also include other clay minerals such as hectorite, illite, mixtures of illite and the family of smectites, shale, and other families of clay materials. In accordance with particular aspects of the present invention, the bentonite clay may have an average particle size of from about 74μ to about 3.5 mm.  
           [0016]    The addition of bentonite to foundry molding and core compositions significantly reduces the casting defects associated with the thermal expansion of silica and dramatically improves the surface finish of such castings. One of the major causes of veining occurs when silica sand is rapidly heated causing the silica to undergo a rapid expansion and form fissures that the hot metal penetrates. The addition of bentonite improves the resulting casting quality. Although not wishing to be bound, applicants believe that the reduction in veining defects relates to the crystalline structure of bentonite which can decompose and collapse thereby providing room for the expansion of the silica sand during heating. In addition, it is believed that the loss of crystalline water from the mineral reduces gas defects.  
           [0017]    The incorporation of bentonite into the silica sand foundry composition substantially improves the surface appearance of the casting and can eliminate or reduce the need for extensive surface grinding to remove any projecting veins from the casting. Accordingly, eliminating veining can significantly reduce the cost of the casting. Furthermore, bentonite is considerably less expensive than other anti-veining additives like lithia containing materials, thereby further reducing the cost of the casting.  
           [0018]    A method for producing foundry shapes by incorporating an anti-veining composition comprising bentonite in the silica sand-based foundry composition to reduce veining is disclosed. The process comprises the steps of providing a foundry sand, adding an anti-veining composition to the sand, and adding a foundry resin to form a sand-based foundry composition, and shaping the sand-based aggregate to form a desired pattern, wherein the anti-veining composition comprises bentonite.  
           [0019]    The bentonite can be selected from the group consisting of sodium bentonite, calcium bentonite and combinations thereof. The composition may include other clay minerals such as hectorite, illite, mixtures of illite and the family of smectites, shale, and other families of clay materials. The sand-based foundry composition typically comprises about 90% to about 99.5% of commonly used molding and core sand in combination with about 5 to about 0.5 of a polymerizable or curable foundry resin appropriate for sand cores and molds, and from about 1 to about 10% of bentonite.  
           [0020]    The foundry resin typically used is selected from the group consisting of phenolic hot box, phenolic urethane, furan, sodium silicate including ester and carbon dioxide systems, polyester binders, acrylic binders, alkaline binders, epoxy binders and furan warm box systems.  
           [0021]    The bentonite may be utilized in a granular form having an average particle size of from about 74μ to about 3.4 mm. Furthermore, the bentonite will typically have a moisture level from about 0.1% to about 12%, more particularly from about 3 to 5% with a target moisture level of about 4%.  
           [0022]    The present invention is also directed to a method of making a metal casting from sand-based foundry compositions comprising the steps of preparing a sand-based foundry composition of silica sand, a foundry resin appropriate for sand molds, and an anti-veining material, wherein the anti-veining material is present at a concentration of from about 1 to about 10% based on sand, the anti-veining material comprising bentonite; shaping said sand-based foundry composition to form a sand mold or core having a desired pattern therein; and pouring molten metal into the pattern formed in the sand-based foundry composition to produce a metal casting having little or no veining.  
           [0023]    A foundry core comprising a matrix of sand and foundry resin having bentonite particles uniformly dispersed through such that the formation of veins is reduced is also within the scope of the invention.  
         DETAILED DESCRIPTION OF THE INVENTION  
         [0024]    The present invention relates to a method of making a silica sand-based foundry shape wherein an anti-veining composition comprising bentonite is incorporated in the silica sand-based composition to reduce veining. The anti-veining additive produces a sand-based foundry mold and core composition which resists the formation of some of the defects commonly associated with the production of castings produced by silica, sand-based molding and core foundry compositions. In particular, the anti-veining additive improves the quality of the castings by reducing thermal expansion defects, such as veining, in iron, steel and non-ferrous castings.  
           [0025]    The anti-veining additive of the present invention may be utilized with conventional foundry silica sand molding and core compositions used in the manufacture of sand-based shapes. Such foundry compositions are typically made from silica sand, with the sand grains being bound together with a mechanical or chemical means. An example of a commercially available foundry sand is Wedron 520 available from Fairmount Minerals. Typically, the mold or core mixture may comprise between about 85% to about 98.5% of silica sand, and about 5% to about 0.5% of a foundry resin. The resin used may be of any of numerous conventional core and mold foundry resin systems such as phenolic hot box, phenolic urethane, furan, sodium silicate including ester and carbon dioxide system, polyester binders, acrylic binders, alkaline binders, epoxy binders, and furan warm box systems. A particularly useful binder is a no-bake resin binder system available from Ashland. This resin binder system comprises a three part phenolic urethane system which includes a series of binders and a liquid catalyst. Each of the above binder systems is well known in the art and therefore a detailed description thereof is unnecessary.  
           [0026]    The order of additive of bentonite is important to its function. The sand is preferably mixed with the bentonite first and then the foundry resin is added so that the resin coats the surface of the sand particles and provides a foundry sand composition with bentonite particles dispersed throughout. It is believed that in this manner the bentonite prevents the formation of fissures in the sand.  
           [0027]    The anti-veining composition of the present invention comprises bentonite. In accordance with one aspect of the present invention, bentonite will be added to the sand-based aggregate in an amount of from about 1 to 10% based on sand. More particularly, the bentonite may be present in the aggregate in an amount from about 1% to 7% based on sand. Bentonite is a type of clay composed primarily of montmorillonite minerals. The bentonite used in accordance with the present invention can be a sodium bentonite, a calcium bentonite, or mixture thereof. The composition may also contain other materials including clay minerals such as hectorite, illite, mixtures of illite and the family of smectites, shale, and other families of clay materials.  
           [0028]    In accordance with a particular aspect of the present invention, the bentonite is utilized in a granular form having an average particle size of from about 74μ to about 3.4 mm. More particularly, the granular bentonite clay may range in size from about 105μ to about 2.0 mm. The use of bentonite having a particle size smaller than 74μ has been found to give rise to no or only very little improvement in casting quality. In particular embodiments of the present invention, the particle size of the bentonite is from about 1.0 to 2.0 mm.  
           [0029]    Particles having an average size of about 74μ or greater are those which are generally retained on the surface of a U.S. standard No. 200 mesh sieve screen. Particles having an average size of less than about 3.4 mm are those which generally pass through a U.S. standard No. 6 mesh sieve screen. Particles having an average particle size of 105μ or greater are those which are generally retained on a surface of a U.S. standard No. 140 mesh sieve screen. Particles having a nominal size of less than about 2.0 mm are those which generally pass through a U.S. standard No. 10 mesh sieve screen.  
           [0030]    The moisture level of the bentonite clay can also affect the quality of the casting. If the moisture level is too high, the product can potentially fail, and, therefore, it is believed that veining decreases with decreasing moisture levels. However, from a practical standpoint, the bentonite will typically have a moisture level of from 0.1% to about 12%, more particularly from about 3 to 5% with a target moisture level of about 4%.  
           [0031]    It is within the scope of the present invention to incorporate other anti-veining compounds in the silica sand-based foundry composition. Examples of specific anti-veining compounds include, but are not limited to, dextrin, starch-based products, fine ground glass particles, red talc, wood flour, and lithia-containing materials. In accordance with a particular embodiment of the present invention, the anti-veining composition comprises a mixture of lithia-containing material described in U.S. Pat. No. 5,911,269 and bentonite. The other anti-veining compounds and more particularly the lithia-containing material can be used with the bentonite in a ratio from about 3 to 1 to 1 to 3 lithia-containing material to bentonite. 
       
    
    
     EXAMPLE 1  
       [0032]    Different silica sand-based foundry compositions were prepared for the purpose of evaluating various anti-veining additives for effectiveness in preventing veining and for tensile properties. Accordingly, identical silica sand-based aggregate mixes were prepared utilizing various anti-veining additives. Test samples were prepared by blending the silica sand and the anti-veining material in a mixer for 30 seconds. The addition of the 3 part Ashland binder system was completed according to the manufacturer&#39;s recommendations. The testing specimens were prepared for evaluation.  
         [0033]    Tables 1 and 2 summarize the effectiveness of various anti-veining additives. Table 1 is directed to sand cores coated with EZ Kote Graphite Coating while Table 2 is directed to uncoated cores.  
                                                                                                                         TABLE 1                           Comparison Of Anti-Veining Additives       Coated Sand Cores                Formula (wt) (g)                        Example 2                   Material   Control   Example 1   (Comparative)   Example 3   Example 4   Example 5                    Ashland (Part 1)   10   10   10   10   10   10       Pepset XI 1000       Ashland (Part2)   8   8   8   8   8   8       Pepset XII 2000       Ashland Catalyst 3502   0.5   0.5   0.5   0.5   0.5   0.5       Sand   2000   1900   1900   1900   1940   1860       Bentonite (#40)   —   100   —   100   60   140       Veinseal   —   —   100   —   —   —                       —            Veining (# observed)            Horizontal   1   None   None   None   1   None       Vertical   2   None   None   None   2   None                  
 
         [0034]    Examples 1 and 2 illustrate the effectiveness of bentonite as an anti-veining additive as compared to a commercially available lithium-containing anti-veining additive. Examples 3-5 illustrate the effect of bentonite concentration on anti-veining.  
                                                                                         TABLE 2                           Comparison Of Anti-Veining Additives       Uncoated Sand Cores                Formula (wt) (g)                    Material   Control   Example 6                            Ashland (Part 1) Pepset XI 1000   10   10           Ashland (Part 2) Pepset XII 2000   8   8           Ashland Catalyst 3502   0.5   0.5           Sand   2000   1900           Bentonite   —   100            Veining (# observed)                Horizontal   2   None           Vertical   4   1 (minor)                      
 
         [0035]    The tensile properties of various compositions were calculated based on the retained tensile strength in reference to the control material as indicated in Table 3 below. Tensile strength is important to maintain the desired form of the mold or core before and during casting.  
                                                                                                                                                 TABLE 3                           Comparison Of Anti-Veining Additives       Uncoated Sand Cores       Retained Tensile Properties of Anti-Veining Aggregates                Formulation of Prepared Mixtures (wt) (g)                    Example   Example   Example   Example   Example   Example   Example       Material   Control   7   8   9   10   11   12   13                    Ashland (Part 1)   10   10   10   10   10   10   10   10       Pepset XI 1000       Ashland (Part2)   8   8   8   8   8   8   8   8       Pepset XII 2000       Catalyst   0.5   0.5   0.5   0.5   0.5   0.5   0.5   0.5       Sand   2000   1970   1970   1970   1970   1970   1970   1900       Dextrin       30   20   10   —   —   —   —       Bentonite (#40)   —   —   0   0   0   0   0   100       Bentonite (#200)           10   20   7.5   15   22.5   0       Iron Oxide   —   —   —   —   22.5   15   7.5   —       Relative Tensile   100%   34%   7%   1%   39%   7%   4%   47%       Strength            Veining (# Observed)            Horizontal   3   2   2   2   1   1   1   1 Minor       Vertical   4   2   2   0   3   2   1   0                  
 
         [0036]    Tables 4 and 5 illustrate additional examples in accordance with some embodiments of the present invention.  
                                                                                                             TABLE 4                           Comparison Of Anti-Veining Additives       Uncoated Sand Cores                Formula (wt) (g)                        Example 15               Material   Control   Example 14   (Comparative)   Example 16   Example 17                    Ashland (Part 1)   10   10   10   10   10       Pepset XI 1000       Ashland (Part 2)   8   8   8   8   8       Pepset XII 2000       Ashland Catalyst 3502   0.5   0.5   0.5   0.5   0.5       Sand   2000   1900   1900   1900   1900       Bentonite (#40)   —   100   —   —   —       Veinseal   —   —   100   —   —       Calcium Bent. (Gran)               100       Shale                   100            Veining (# observed)            Horizontal   3   None   None   None   1       Vertical   4   None   None   None   2 minor                  
 
         [0037]    [0037]                                                                                                                         TABLE 5                           Comparison Of Anti-Veining Additives       Uncoated Sand Cores                Formula (wt) (g)                        Example 15                   Material   Control   Example 14   (Comparative)   Example 18   Example 19   Example 20                    Ashland (Part 1)   10   10   10   10   10   10       Pepset XI 1000       Ashland (Part 2)   8   8   8   8   8   8       Pepset XII 2000       Ashland Catalyst   0.5   0.5   0.5   0.5   0.5   0.5       3502       Sand   2000   1900   1900   1900   1900   1900       Bentonite (#40)   —   100   —   25   50   75       Veinseal   —   —   100   75   50   25            Veining (# observed)            Horizontal   3   None   None   None   1 minor   1       Vertical   4   None   None   None   1 minor   none                    
         [0038]    As indicated in the foregoing examples, the addition of bentonite to molding and core aggregates used in casting manufacture can significantly improve quality of the castings by reducing thermal expansion defects, such as veining. The addition of bentonite significantly reduces the casting defects associated with the use of foundry binder systems and molding sand aggregates and increases the strength the resulting bond aggregates. The use of bentonite as an anti-veining composition reduces the amount of surface grinding necessary to remove any imperfections at the surface of the casting. Furthermore, the cost of bentonite is less than other anti-veining additives thereby providing for lower cost mold and core production, while improving the resulting casting quality.  
         [0039]    Having described the invention in detail by reference to particular embodiments thereof, it will be apparent that modifications and variations are possible without departing from the scope of the invention.