Classifications

• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B22—CASTING; POWDER METALLURGY
  • B22C—FOUNDRY MOULDING
  • B22C5/00—Machines or devices specially designed for dressing or handling the mould material so far as specially adapted for that purpose
  • B22C5/04—Machines or devices specially designed for dressing or handling the mould material so far as specially adapted for that purpose by grinding, blending, mixing, kneading, or stirring
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B22—CASTING; POWDER METALLURGY
  • B22C—FOUNDRY MOULDING
  • B22C5/00—Machines or devices specially designed for dressing or handling the mould material so far as specially adapted for that purpose
  • B22C5/08—Machines or devices specially designed for dressing or handling the mould material so far as specially adapted for that purpose by sprinkling, cooling, or drying
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B22—CASTING; POWDER METALLURGY
  • B22C—FOUNDRY MOULDING
  • B22C9/00—Moulds or cores; Moulding processes
  • B22C9/12—Treating moulds or cores, e.g. drying, hardening

Definitions

• This invention relates to water dispersible moulds which are used in the manufacture of foundry castings. More particularly it relates to a method for making such moulds.
• mould as used in this specification includes both a mould for producing castings with or without cavities and a core for producing a cavity in a cavity containing casting and combinations of such moulds and cores.
• Cores and moulds are made from sand or other refractory particulate materials and it is customary to add binders in order to give the necessary properties of flowability (to enable the core/mould to be formed), stripping strength (to enable the core/mould to be handled soon after forming) and the ultimate strength to withstand the conditions occurring during casting.
• the refractory particulate material and binder are formed into a core or a mould by various processes which include ramming, pressing, blowing and extruding the mix into a suitable forming means such as a core box, a moulding flask, or a moulding or mould box.
• a suitable forming means such as a core box, a moulding flask, or a moulding or mould box.
• Water is present in the mix of refractory particulate material and binder, and must be removed after the core or mould has been formed in the forming means, and before it can be used in the manufacture of foundry castings.
• the core box is heated to a temperature in the range of 50-90 ⁇ C, and then purged with compressed air at a pressure of 80 pounds per square inch for a period of time in excess of 30 seconds depending on the core size and binder composition.
• the core is then transferred to an oven where final removal of water is accomplished by heating the core to a temperature in the range 120-150°C.
• steps (b) to (d) are carried out in a saturated atmosphere of steam.
• the binder may be any composition which after being mixed with the water- insoluble particulate material in the presence of water is capable of binding together the particles of the water-insoluble particulate material upon removal of water (e.g., by volatilisation) from the mixture.
• the method of the invention may utilise silicate binders, for instance alkali metal silicate, and other inorganic binding materials.
• the binder includes polyphosphate chains and/or borate ions, and even more preferably these are respectively derived from at least one water soluble phosphate and/or borate glass.
• the binder may be added to the particulate material as an aqueous solution.
• the binder and the particulate material may be mixed together and then water may be added to the mixture.
• the binder has been mixed with the particulate material in the form of an aqueous solution of the at least one water soluble glass.
• the binder has been mixed with the particulate material in the form of particles of the at least one water soluble glass and the polyphosphate chains and/or borate ions have been formed by mixing water with the mixture of particulate material and glass particles.
• the glass particles may be wholly or partially dissolved into the water thereby to form the polyphosphate chains and/or borate ions.
• the water-soluble glass may be wholly vitreous or partially devitrified, in the latter case the water-soluble glass having been heated and cooled thereby to form crystalline regions in an amorphous or glassy phase.
• the polyphosphate chains are formed following the dissolution of the respective water soluble glasses into aqueous solution. These chains form an interlinking matrix throughout the mould, which is enhanced by hydrogen bonding of the chains by chemically bonded water molecules. After removal of excess water, the resulting dried mould retains the polyphosphate matrix which firmly binds together the water-insoluble particulate material. If excess water were not removed, the resulting wet mixture could be structurally weakened by the presence of water and would generally not be usable as a mould or core. In addition, the excess water would generate steam during the casting process which, as is well known in the art, would degrade the quality of the resultant casting.
• the principal component in a mould is a water insoluble particulate material which may be a refractory such as foundry sand e.g. silica,olivine ,chromite or zircon sand or another water insoluble particulate refractory material such as alumina, an alumino silicate or fused silica.
• the silica sands used for foundry work usually contain 9&% by weight SiO_.
• the mould may also contain minor amounts of other additives designed to improve the performance of the mould.
• the binder comprises at least ⁇ ).2 ⁇ ⁇ % by weight, and the particulate material comprises up to 99-75X by weight, of the total weight of the mixture of particulate material and the binder. More preferably the binder comprises from 0.5 to 50J. by weight and the particulate material comprises from 99-5 to 50% by weight, of the total weight of the particulate material and the binder.
• the water soluble phosphate glass comprises from
• the water soluble phosphate glass comprises from 58 to 72 vt.% P-,0,., from 2 to 28 wtj. Na-0 and from 0 to 16 wtj. CaO.
• Such glasses include glasses of the following compositions in weight %'.
• soluble glass it is preferred to use a glass which has a solution or solubility rate of 0.1-1000 mg/cm 2 /hr at 25 * C.
• the glass preferably has a saturation solubility at 25°C of at least 200 g/1, more preferably 800 g/1 or greater, for phosphate glasses, and of at least 50 g/1 for borate glasses.
• the commonly available phosphate glasses are those from the binary system Na_0.P_0,_.
• the selection of glasses containing K_0 or mixed alkali metal oxides can be made on the same basis but glasses containing K_0 and/or mixtures of alkali metal oxides are less likely to be satisfactory as they are more prone to devitrification, and are also likely to be more costly.
• a preferred glass is a phosphate glass from the binary system
• a mould made with a glass with a chain length of about 30 requires about 10 minutes soaking in water and 30 seconds flushing with water for removal, compared to less than 1 minutes soaking in water and 30 seconds flushing for a glass with a chain length of about 4. Thus where quick removal is required the shorter chain length glass is preferred.
• the temperature of the casting process can affect the binder in the core having consequential implications for the dispersibility of the core.
• the centre of a core may be subjected to temperatures of around 4 ⁇ O°C but the skin of the core may reach temperatures as high as 500°C.
• the dispersibility of cores generally decreases with increasing temperature to which the cores have been subjected.
• the variation of dispersiblity with composition may vary at different temperatures.
• indispersibility of the core after the casting process may be related to the removal of all combined water in the core which was previously bound with the sodium polyphosphate binder.
• thermogravimetric analysis provides a relationship between weight loss and temperature. Thermogravimetric analyses were carried out on a number of sodium polyphosphate glasses and it was found that in some cases after a particular temperature had been reached there was substantially no further weight loss which appeared to suggest that at that temperature all combined water had been lost from the glass. We have found that if this temperature is lower than the temperature to which the core is to be subjected to during a casting process, this indicates that the core may have poor post-casting dispersibility resulting from excessive water removal from the core during the casting process.
• a suitable core binder also required a number of other features in order to be able to produce a satisfactory core, such as dimensional stability, absence of distortion during the casting process, low gas evolution and low surface erosion in a molten metal flow.
• dimensional stability e.g., dimensional stability
• absence of distortion during the casting process e.g., low gas evolution
• low surface erosion e.g., low metal flow
• an inorganic binding material such as a polyphosphate
• a polyphosphate can be subjected to the temperatures involved in a casting process and still remain readily soluble so as to enable a sand core which is held together by a binder of the polyphosphate material rapidly to be dispersed in water after the high temperature casting process.
• the mixture of particulate material and binder are heated to a temperature in excess of 100°C prior to being formed into the desired shape.
• the particulate material e.g., foundry sand
• the particulate material is heated prior to being mixed with the binder. This enables a supply of hot sand to be maintained in reserve so that the period for which the mixture of sand, binder and water needs to be heated together may be reduced. If the sand is prior heated to a temperature of 100°C or greater then when this is mixed with the binder the resultant mixture will have a temperature close to or even greater than 100°C.
• the binder may also be heated prior to mixing although care should be exercised to ensure that water is not excessively volatilised off from the binder composition.
• the mixture is formed into the desired shape. This may be achieved by blowing the heated mixture into a suitable core box using a core blower. Preferably, the temperature of the mixture is maintained during transfer to the core box.
• water is then removed from the formed mixture with air at a temperature in excess of 100°C.
• the core mould is provided with heating means, such as an electrical heater, so that the temperature of the formed mixture can be controlled.
• Air is passed into the core box to effect removal of water from the mixture.
• the air may be heated up to a temperature of about 250°C. Typically, however, the air temperature will be maintained at about 150°C.
• the particle size of the particulate material is relatively small, a relatively large amount of binder will be required in order to ensure that the binder matrix binds together the larger number of particles which provide a correspondingly large surface area.
• the amount of binder is relatively small as compared to the quantity of sand or other particulate material, it is preferable to introduce the water and glass in the form of a solution of the glass in water.
• a coarse foundry sand i.e., AFS 50
• AFS 100 equivalent glass:water ratio for a fine foundry sand
• AFS 100 equivalent glass:water ratio for a fine foundry sand
• the glass in a powdered form is simply added to water and mixed with a high shear mixer to achieve full solution. A portion of the solution is then added to the refractory particulate material and mixed thoroughly before e.g., blowing the mixture into a core box preheated to 80°C with compressed air at a pressure of about 80 pounds per square inch, and then purging with compressed air at ambient temperature for about 50 seconds. Cores with good handling strengths are obtained in this manner. Moulds can also be formed.
• Figure 1 is a schematic layout for apparatus arranged to carry out the method of the present invention.
• a hopper 1 receives binder and feeds the binder to a heated continuous mixer 5.
• the binder may be heated, for instance by a ribbon heater 4.
• Sand is fed from a hopper 2 to a heater 3.
• the heated sand is then fed to the continuous mixer 5 where it is mixed with the binder.
• the mixer 5 is heated and insulated to maintain the temperature of the binder/sand mix above 100°C.
• the sand binder mix is fed through a buffer hopper 7 to a heated screw feeder 8. From the screw feeder discrete amounts of mix are fed via a slide valve 9 to a shooter head 10. From the shooter head the mix is blown into a core box 11 to form a core. It should be noted that in addition to the continuous mixer, hopper 7, screw feeder 8 and the shooter head 10 are all heated and insulated to ensure that the binder sand mix is maintained at a temperature in excess of 100°C.
• the water content of the binder/sand mix remains above a particulate level prior to being blown into the core box to prevent the mix from hardening as it passes through the machinery. This is achieved by keeping the mix in a saturated atmosphere of steam from the mixer 5 through to the shooter head. A vent 6 in the mixer prevents the atmosphere being pressurized and each individual apparatus from the mixer 5 to the shooter head 10 is sealed to prevent water losses.
• the core box 11 is heated and purged with hot air typically at a temperature of 150 C C to remove the water held within the mix. After a relatively short time period substantially all the water has been removed from the core, and the core is now suitable for use in. making foundry castings.
• the polyphosphate glass was dissolved in water to give a 50% solution (w/w).
• a hot strength additive comprising sodium aluminium silicate in an amount of 10% by weight based on the weight of the solid binder was mixed in to give a slurry.
• the binder slurry was added to Chelford AFS 60 (4.4% w/w) and intimately blended using a Ridsdale mixer.
• Portions of the sand/binder mixture (1.0kg) were heated to temperatures in the range 50°C to 110°C in a microwave oven. A sealed container was used to prevent evaporative losses.
• the heated mixture was hand rammed into the preheated cylinder mould and final compression achieved using a pneumatic piston.
• a purge head was clamped onto the core mould and heated air passed through the mixture for predetermined periods of time.
• Rapid hardening was achieved when a significant proportion of the energy to meet latent heat of evaporation requirements has been supplied to the mixture prior to its addition to the mould.

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• Engineering & Computer Science (AREA)
• Mechanical Engineering (AREA)
• Mold Materials And Core Materials (AREA)

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• 1993
  • 1993-03-29 AU AU38947/93A patent/AU3894793A/en not_active Abandoned
  • 1993-03-29 WO PCT/GB1993/000636 patent/WO1993019870A1/en not_active Application Discontinuation
  • 1993-03-29 MX MX9301739A patent/MX9301739A/es unknown
  • 1993-03-29 EP EP93907933A patent/EP0632753A1/de not_active Withdrawn

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