Classifications

• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B22—CASTING; POWDER METALLURGY
  • B22C—FOUNDRY MOULDING
  • B22C1/00—Compositions of refractory mould or core materials; Grain structures thereof; Chemical or physical features in the formation or manufacture of moulds
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B22—CASTING; POWDER METALLURGY
  • B22C—FOUNDRY MOULDING
  • B22C1/00—Compositions of refractory mould or core materials; Grain structures thereof; Chemical or physical features in the formation or manufacture of moulds
  • B22C1/02—Compositions of refractory mould or core materials; Grain structures thereof; Chemical or physical features in the formation or manufacture of moulds characterised by additives for special purposes, e.g. indicators, breakdown additives

Definitions

• the present invention relates to a granulated material comprising a refractory framework coated with a material fit for providing "lustrous carbon” and suitable for being used as additive to foundry molding sand in order to improve its properties.
• the invention also relates to a method for the production of said granulated material as well as to the compositions of the molding sands containing such material.
• Molding sand actually used in foundry practice for the production of molds for cast- iron castings consists of sand and clay as binder. Bentonite is especially used with advantage as binding clay.
• a coal dust is usually added with the main purpose of preventing formation of defects on the surface of castings. Said defects, as it is known, are prevalently due to the thermal expansion of the sand and to metal-mold material reactions such as the formation of ferrous oxide on the surface of castings which, by reaction with silica, produces low-melting point silicates responsible for surface roughness in the castings.
• the coal dust absorbs the thermal expansion of the sand, since during casting, when the surface temperature of the mold rises to about 1000°C, it is caused to distill releasing volatile matter and increasing the void around the expanding sand grains. Furthermore said volatile matter generates an inert atmosphere and produces "lustrous carbon" which coats the sand grains with a refractory film, not wettable by molten metal, thus avoiding the metal from reacting with the mold material.
• “Lustrous carbon” is defined as an allotropic, optically isotropic state of the carbon having a microcrystalline, essentially bidimensional structure, intermediate between amorphous carbon and graphite. Lustrous carbon is obtained on wide specific surfaces by pirolysis of heavy hydrocarbons which are released by carbon-based materials during contact with the molten metal. Properties, characteristics and methods of analysis concerning lustrous carbon are extensively described by V. I. Bindermagel, A. Kolors, K. Orthus: Schnelltechnisch Kunststoff Anlagen aus Formstoffzusatzen (method of analysis for mold materials additives), Giesserei, Vol. 51, 12 Nov., page 729-730 (1964).
• coal dust improves green and dry strength of the sand and reduces gas permeability.
• Coal dusts which have been extensively used in the past are natural substances such as milled low-grade anthracite or pearl-pitch, characterized by a low content of ash and noxious products such as sulphur, with a lustrous carbon yield not greater than 12% of its own weight and normally lower than 10%. More recently advanced techniques for the preparation of coal dusts have introduced the use of synthetic substances yielding more than 70% of lustrous carbon, thus considerably reducing (up to 7 times) the amount of coal dust used.
• molding sands which contain 80-85% by weight of a refractory material, clay substance as binders and the above mentioned carbon-based materials
• synthetic substances with a high yield of lustrous carbon and natural substances such as low-grade anthracite powders or mixtures thereof.
• the molding sands after their use for molding, are continuously regenerated to produce new molds and are added with refractory material, clay binder and carbon-based additives at each cycle.
• silica sand The most common refractory material used for the preparation of molding sands is silica sand because of its low cost; other types of sand, such as olivine, chamotte, chromite, zircon, sillimanite, mullite and the like can be used for particular purposes. Nevertheless it is well known that with respect to other kinds of sands, a serious drawback of silica sand consists in that it has a higher thermal expansion coefficient, which moreover increases suddenly at about 500°C in correspondence to the quartz allotropic transformation from the alpha to beta state.
• silica sand is generally used, as well as compositions based on bentonite or binding clays as binders, low-grade anthracite powders or synthetic materials or mixtures thereof as carbon-based additives.
• Another object of the invention is to produce quartz- and dust-free material to be used for the . preparation of molding sands, thereby reducing both pollution of the working environment and material handling problems.
• a granulated material for adding to foundry molding sands for improving its properties is defined in claim 1.
• the refractory framework may be made with a material having a null or very low thermal expansion coefficient, such as zircon, chamotte, chromite, olivine, sillimanite or mullite sands and preferably consisting of a silicate with a content of free crystalline silica not greater than 3%.
• the addition of only one component to the foundry molding sands has the double effect of adding a coal dust and of regenerating the sand with a refractory material, thus avoiding the handling of powder-producing materials and contemporaneously adding a refractory material, during regenerative additions, which progressively improves the dilatometric properties of the refractory framework as a whole.
• the refractory frameworkmateriai is precoated with a substance fit for producing a considerable amount of lustrous carbon upon melting.
• lustrous carbon producing material organic polymers with softening temperature range of 70-180°C are generally used, such as plastics or resins of the type of polyethylene, polypropylene, coumaronic resins, polystyrene, glycerophtalic resins, petrolic resins, glysonite and pitchy substances, phenolic resins and more generally any other material fit for producing, during casting, lustrous carbon according to foundry methods in an amount ranging between 15 and 90% of its own weight.
• a method for producing the granulated material according to the invention may comprise melting the organic material and mixing it in a hot process with the granules of refractory material.
• Another possible method is based on the fact that said organic material is solvent-soluble and therefore can be directly added to the granules of refractory material when mixed in a drum with a solvent, in order to produce a proper resin coating on the granules by evaporating the solvent, which is recovered in a subsequent step.
• a person skilled in the art is well aware of how selection of the solvent, on the basis of the type of organic material used, is to be carried out.
• a particularly preferred solvent is white spirit.
• the material produced according to said methods is homogeneous, granular and dust-free, each granule being coated with a layer of carbon-based material yielding a high percentage of lustrous carbon.
• Such homogeneous granulated material is directly used for regenerating molding sands, in place of the two separate components of refractory framework and coal dust.
• a practical, particularly preferred application of this invention provides for the use of olivine sand, chromite sand or the like or their mixtures either with a granule size ranging between 0.1 and 0.4 mm or as a powder.
• the sand, or sand mixture is heated to 150°C and then mixed with coumaronic resin; the percentage of the resin is 10% calculated on the weight of the refractory framework. After a few minutes of mixing, during which liquefied resin produces a coating on the refractory granules, the mixture is cooled and sieved to break any possible lump, thus obtaining a dust-free granulated material.
• the mixture is cooled and discharged onto a vibrating screen to obtain a granulated material with a granule size ranging between 0.1 and 0.3 mm, each granule being coated with polystyrene.
• Said cromite sand precoated with polystyrene yields 7-8% of its own weight in lustrous carbon, the same amount being provided by a coal dust obtained from the grinding of a low-grade anthracite.
• a synthetic material can be added to a molding sand according to normal practice in the art for the addition of coal dust.
• olivine sand with granule size ranging between 0.06 and 0.4 mm are fed to a mixing drum with 100 kg of a solution containing 50% of white spirit and 50% of petrolic resin, with the following characteristics: after a few minutes of mixing, the solvent is sucked from the mixing drum to completely coat the sand granules with the resin. Any possible dot is broken on a vibrating screen thus obtaining a dust-free material yielding a percentage of lustrous carbon corresponding to 3-3.5% of its own weight.
• Said material is added to a molding sand in such an amount so as to have a constant lustrous carbon yield equal to 0.2-0.6%.
• the molding sand is also enriched with a non-expandable refractory material, thus minimizing surface defects on castings due to thermal expansion problems.
• the amount of preferably resin-based coating on the refractory material will be of 0.5 to 60% by weight and in particular more preferably 1.0 to 30% by weight.
• a refractory material with 25% of coating consisting of a synthetic substitute of a mineral coal dust, would be able to release upon melting lustrous carbon in the percentage of 15 to 16% by weight.
• the granulated material according to the invention further offers the advantage of improving the extracting properties of castings and obtaining better surfaces on same.

Landscapes

• Engineering & Computer Science (AREA)
• Chemical & Material Sciences (AREA)
• Materials Engineering (AREA)
• Mechanical Engineering (AREA)
• Mold Materials And Core Materials (AREA)
• Glanulating (AREA)

Priority Applications (1)

Applications Claiming Priority (2)

Publications (2)

Family

ID=11128442

Family Applications (1)

Country Status (4)

Cited By (1)

Families Citing this family (4)

Citations (7)

• 1980
  • 1980-01-21 IT IT8009323A patent/IT1209745B/it active
• 1981
  • 1981-01-21 EP EP81830010A patent/EP0032881B1/en not_active Expired
  • 1981-01-21 AT AT81830010T patent/ATE6835T1/de not_active IP Right Cessation
  • 1981-01-21 DE DE8181830010T patent/DE3162845D1/de not_active Expired

Patent Citations (7)

Also Published As

Similar Documents

Legal Events