Classifications

• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B22—CASTING; POWDER METALLURGY
  • B22D—CASTING OF METALS; CASTING OF OTHER SUBSTANCES BY THE SAME PROCESSES OR DEVICES
  • B22D18/00—Pressure casting; Vacuum casting
  • B22D18/06—Vacuum casting, i.e. making use of vacuum to fill the mould
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B22—CASTING; POWDER METALLURGY
  • B22C—FOUNDRY MOULDING
  • B22C7/00—Patterns; Manufacture thereof so far as not provided for in other classes
  • B22C7/02—Lost patterns
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B22—CASTING; POWDER METALLURGY
  • B22C—FOUNDRY MOULDING
  • B22C9/00—Moulds or cores; Moulding processes
  • B22C9/02—Sand moulds or like moulds for shaped castings
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B22—CASTING; POWDER METALLURGY
  • B22C—FOUNDRY MOULDING
  • B22C9/00—Moulds or cores; Moulding processes
  • B22C9/02—Sand moulds or like moulds for shaped castings
  • B22C9/04—Use of lost patterns
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B22—CASTING; POWDER METALLURGY
  • B22C—FOUNDRY MOULDING
  • B22C9/00—Moulds or cores; Moulding processes
  • B22C9/08—Features with respect to supply of molten metal, e.g. ingates, circular gates, skim gates
  • B22C9/082—Sprues, pouring cups

Definitions

• This invention relates to the casting of articles of molten metal and is particularly concerned with casting by the so-called investment or lost wax process.
• a very accurate model of the required product is produced in wax.
• a ceramic shell is then formed around the wax by applying successive coats of ceramic slurry and stucco.
• the wax is dipped into a tank of slurry; on removal, excess slurry adhering to the wax model is drained off and dry granular ceramic stucco is applied to the surface either by raining it over the model, or by immersing the model in a fluidised bed containing the stucco.
• the first coating must be dried before the next coating can be applied.
• the resultant shell is then cooled to room temperature, repaired and cleaned of any internal debris, after which it is heated for several hours in a pre -heat furnace before being taken to a casting furnace or casting station to be filled with metal while at high temperature.
• Pre-heating is necessary to avoid thermal shock and to ensure that the mould fills completely, especially in thin sections. In many foundries, the same furnace is used for all firing and pre-heating operations which reduces capital investment but leads to logistical and productivity constraints.
• Investment casting offers a manufacturing process capable of producing components of high definition, good dimensional accuracy and excellent surface finish. Its drawbacks, however, are that it is limited in the size of components which can be cast and it is expensive to operate. One problem is caused by the fact that wax expands on heating and this can cause the surrounding ceramic shell to crack. It is one object of this invention to address these drawbacks by providing a casting process capable of delivering the benefits of investment casting but which is applicable over a greater size range and is cheaper to operate.
• the invention provides a method of casting an article of molten metal, the method comprising casting the liquid metal into a supported thin ceramic shell which has not been pre-heated before ingress of the liquid metal.
• the method includes the preliminary step of supporting the shell in a bed of compacted granular material and applying a vacuum while the liquid metal is being cast into the shell whereby to draw all the metal into the shell.
• a mould which is cast hot must be strong enough to withstand being handled at high temperature and also to avoid breakout during pouring. This is achieved by building up a substantial thickness of shell consisting of as many as 15 dip coats.
• the mould is cold at the point when molten metal is poured into it and it is supported in a bed of loose sand compacted to a high bulk density. In this process it is normal to apply only 5 to 7 coats depending on the size and geometry of the casting being produced leading to very significant savings in moulding materials, mould manufacturing lead time, work in progress and waste disposal costs.
• VOC volatile organic compound
• moulds are cast cold, it is possible to cast several components together from a single large charge of metal. (Pre-heated, investment cast moulds are usually cast individually, each mould requiring a small, single billet of metal to be melted and poured, which is time consuming and costly.)
• the ceramic shell is supported in a bed of granular material, for example, sand, while the molten metal is being cast into the shell. It is also preferred that the bed of granular material be at or near ambient temperature.
• the sand may beneficially be compacted by vibration before pouring and it may be further compacted by applying a vacuum while the metal is being poured.
• the vibration is preferably of high frequency and low amplitude, typically 40 - 50 Hz and 0.045 mm RMS (root mean square) to optimise compaction of the backing material and to provide acceptable support for the mould.
• the thickness of the ceramic shell is typically around 3 mm, i.e. relatively thin compared to most investment casting shells and is made by applying a relatively small number of slurry and stucco coat; each coat being dried before the next is applied. For most castings only five coats need to be applied to provide the necessary shell thickness.
• the ceramic slurries used to form the coatings which make up the shell mould are preferably water-based. Suitable materials for use in these slurries include, but are not restricted to, zircon, silica, alumina and the alumino-silicate group of materials.
• the invention provides a method of casting an article of molten metal, the method comprising the steps of:
• the casting method is applicable to a wide range of alloys including iron, steel, aluminium, cobalt/ chrome and nickel based superalloys.
• a further adaptation of this process is its application to vacuum cast alloys, e.g. nickel-based or cobalt-based superalloys.
• the alloy to be cast is melted under vacuum in a vacuum melting furnace.
• a bed of granular material containing the mould is positioned inside the casting chamber of the furnace; the granular material in the bed having been vibrated in advance to compact it.
• An inert gas e.g. argon
• a pump is activated which draws the inert gas through the mould and the bed of granular material and then re-admits it into the casting chamber.
• the invention includes components cast by the method.
• FIG. 1 is a flow diagram representing the steps involved in one process of the invention.
• Figure 2 is a section through bed of granular material containing the mould immediately prior to filling the mould with molten metal.
• the second and subsequent coatings were of back-up material, comprising a slurry of fused silica filler and water-based colloidal silica binder and a stucco of MolochiteTM, an alumino- silicate material produced from china clay.
• five coatings were applied, including the primary coat. Drying times between coats ranged from 36 minutes between coats 1 and 2 to 54 minutes between coats 4 and 5. After coat 5 was applied, the mould was left for 20 hours in the drying room before removing the wax in a steam autoclave and firing at 1050°C in a gas fired kiln to remove all traces of wax. Drying conditions used throughout mould build and final dry were 23°C dry bulb temperature and 55% relative humidity.
• the cold shell S was placed in a mould box 3 measuring lm x lm x lm.
• Sub-angular silica sand 4 was then poured into the bed around the outside of the mould.
• the box was vibrated at a frequency of 40 - 50 Hz and displacement of 0.045 mm RMS (root mean square) for a period of 90 seconds. This compacted the sand ensuring a high bulk density and intimate contact of the sand with all areas of the mould.
• a vacuum of approximately 500 mm measured at the pump mercury gauge was drawn in the sand bed and steel was then poured into the cold shell via a plenum chamber 5 in the base of the box.
• the top of the box may be covered by a sheet and/ or protective gas may be supplied during casting.

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• Engineering & Computer Science (AREA)
• Mechanical Engineering (AREA)
• Molds, Cores, And Manufacturing Methods Thereof (AREA)
• Sorption Type Refrigeration Machines (AREA)
• Control Of Steam Boilers And Waste-Gas Boilers (AREA)
• Pyridine Compounds (AREA)
• Soil Working Implements (AREA)
• Dental Prosthetics (AREA)
• Mold Materials And Core Materials (AREA)

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Publications (2)

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• 1997
  • 1997-11-19 GB GBGB9724568.2A patent/GB9724568D0/en not_active Ceased
• 1998
  • 1998-11-18 DE DE69828523T patent/DE69828523T2/de not_active Expired - Fee Related
  • 1998-11-18 AT AT98954632T patent/ATE286443T1/de not_active IP Right Cessation
  • 1998-11-18 WO PCT/GB1998/003472 patent/WO1999025511A1/fr active IP Right Grant
  • 1998-11-18 EP EP98954632A patent/EP1027180B1/fr not_active Expired - Lifetime

Non-Patent Citations (1)

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