Classifications

• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B22—CASTING; POWDER METALLURGY
  • B22D—CASTING OF METALS; CASTING OF OTHER SUBSTANCES BY THE SAME PROCESSES OR DEVICES
  • B22D29/00—Removing castings from moulds, not restricted to casting processes covered by a single main group; Removing cores; Handling ingots
  • B22D29/001—Removing cores
  • B22D29/003—Removing cores using heat
• • 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
  • B22C9/04—Use of lost patterns

Definitions

• This invention relates to rapidly fabricating a run of functional durable parts having hollow complex interiors, and more particularly to making such parts using easily formed and easily removable resin bonded sand or salt cores.
• Non-durable plastic prototypes of complex parts have become commonplace; techniques such as stereolithography or cubital prototyping have been used where a non-functional plastic part is fashioned but limited in what can be done with such plastic part.
• Such plastic parts can be used to evaluate aesthetic aspects or fit and assembly of the design, but cannot be subjected to evaluation tests that require harsh operating conditions or high temperatures such as that needed to evaluate an internal combustion engine head or block.
• EP-A-92 690 discloses using a sand mould comprising a core which is arcuately ecompassed by a further core for casting a cylinder block.
• the present invention provides rapid fabrication of complex hollow metal castings, such as engine heads, blocks, manifolds and transmission cases.
• This invention has an advantage that it provides a method for rapidly making up to 100,000 castings of a complex hollow part using the same aluminium die sets, the produced parts being fully durable and functional parts with the method combining the rapid core making and core removal capabilities of resin bonded sand or salt cores and the rapid pattern making capabilities of the lost wax investment casting technique.
• This method allows for extreme ease and economy for modifying the interior flow characteristics of an engine head without need for changing the exterior shape of such head, leading to nimble manufacturing of engine families.
• the process of this invention does not require expensive repetitive machining of several slabs or laminations each time a single pattern is to be made, as in stratiform rapid prototyping (best suited cost-wise to making three or less prototypes) and does not require machining eight or more sets of steel dies to create moulds for each exterior or interior sand core characteristic (best suited to making hundreds of thousands of castings to justify the cost).
• the process herein described allows for faster and easier economic fabrication of 4-40,000 (even up to 100,000) castings by requiring machining of fewer die sets from aluminium metal or equivalent material; the die sets are limited to four: three for making non-fused resin bonded core elements, and one for making the wax pattern. This process combines the faster core making capabilities of particulate bonded cores and the repetitively faster pattern making of the lost wax investment technique.
• an assembly or family 10 of particulate bonded cores is formed to define passages within an internal combustion engine head; the assembly here consists of three intake cores 11, three exhaust cores 12 and an annular water jacket core 13.
• the assembly is unique because it does not comprise fused ceramic cores normally required to withstand the hydrostatic forces of fluid wax, but rather bonded particulate salt or sand cores capable of withstanding the controlled injected wax of this invention as well as molten metal.
• the intake passage core 11 is shown as having an enlarged common intake end 14 and a bifurcated end 15 to define two intake ports; such core, as well as the exhaust cores, are formed by blowing particles mixed with resin binder into a two-way two-piece die set that defines the core shape.
• the formed cores when cured, will have a modulus of rupture of 17.24-22.1 MPa (2500-3200 psi), a bulk density of about 1.92 g/cc, and a porosity of about 26%.
• a die set 18 is made to define a pattern or casting cavity 34 around the core assembly 10 (see Figures 2-4 and 8-13).
• Die set 18, as well as the die sets for the cores, are made of aluminium or other material that is easily machined.
• the exhaust passage core 12 is a curved single body having an outlet end 16 designed to be keyed into the die side part 20 of die set 18 for supporting such end and has another core end 17 designed to be supported on the base die part 19 on a wall 25 that defines the shape of the combustion chamber.
• Figure 2 illustrates how one of the intake cores 11 is supported on the die set 18 (the die set 18 being comprised, as shown in Figure 3, of a base part 19 left side part 20, a top part 21a, another top part 21b, and end parts 22, 23).
• the other end 14 of such core is keyed at 28 into a groove formed into both side and base die parts.
• the annular water jacket core 13 has a primary longitudinally extending wall 26 which extends between the rows of intake and exhaust cores; annular webs or walls 27, 28 extend from wall 26 and wrap around such respective cores while spaced therefrom.
• Core walls 27 are much thicker than walls 28 because they define water channels adjacent the exhaust port which demands greater heat extraction.
• the ends of water jacket wall 26 have core extensions 29, 30 to define passages that connect the water jacket to a fluid circulating system.
• Core 13 thus provides annular walls around each intake and exhaust core, but in spaced relation. The space therebetween can be quite thin - as little as 3-4 mm.
• cores 11 and 12 have one end within and arcuately encompassed by the annular webs 27 or 28 and have another end that curvingly projects around and to the outside of such web walls. This creates a spaced relationship that defines hidden spaces or gaps 31 and 32 therebetween. Such thin spaces promote increased heat transfer to the water jacket when replaced by metal, such as aluminium, in the final casting. Such thin spaces have heretofore presented a difficult problem to accurately form with pattern material.
• Inlets 33 for injecting hot fluid wax into casting cavity 34 is shown in Figure 4 as step three of the process.
• the cavity 34 also includes spaces 39 opposite the inlets to allow for complete filling by the wax.
• the interior cavity does not need to have any draft angles or relief tapers incorporated.
• Hot wax is injected under a controlled injection pressure between 2.07-3.45 MPa (300-500 psi) with the wax at a temperature in the range of 54.5-60°C (130-140°F).
• the wax is preferably a mineral base pattern wax or any investment casting was.
• An injection apparatus 35 is utilised to force the wax into the ingate 36 and through the plurality of inlets 33, which are sized to a diameter of about 12.7 mm (one half inch).
• the injection of hot wax is sustained at such pressure for a time period of about 120-240 seconds until all of the wax fills the voids in the cavity 34 without entrapment of any gases.
• the wax typically will become solidified in a period of 120-300 seconds because of the heat sink provided by the aluminium die assembly.
• the injection apparatus 35 after appropriate formation of a skin on the wax pattern 37, is removed and the ingate 36 is separated.
• the parts 19-23 of the die set 18 are separated from the wax pattern 37; the wax pattern 37 still retains the bonded particulate cores 11, 12 and 13, which can be seen at the pattern surface where the cores intersect the exterior of the wax pattern.
• a shell mould 40 is formed about the wax pattern 37 containing the core assembly 10 as step four of the process.
• the mould 40 is created by multiple dipping of the pattern into a ceramic slurry 38, draining the excess slurry, applying a refractory stucco, and drying or gelling the coating. This is repeated until a shell of about 7.65 mm (.3 inches) or greater is achieved.
• the slurry preferably consists of a ceramic flour in colloidal silica which forms a layer 41 that is then sprinkled with a fine sand.
• the stuccoed silica/ceramic layer mould is then dipped into another ceramic slurry and then into a fluidised bed containing granular molochite to thereby stucco the surface again to form other layers 42.
• the slurry may be composed of refractory binders and refractory fillers or solids.
• the refractory binders can be silica sols, ethyl silicate, sodium and potassium silicate and gypsum type plasters.
• Common refractory fillers that can be used in the process are silica, fused silica, zircon, and aluminium silicate.
• the stucco in many cases, is the same type of refractory as the dip coat, but it has a much larger grain size.
• the stucco is applied to the wet surface of the slurry to provide a mechanical key for the next coating and to minimise the drying stresses in the slurry coating, thus preventing cracking of the coating.
• the slurries are kept in suspension by use of a continually rotating drum with paddle mixture arrangement or by use of pneumatic prop mixers.
• the primary coat is most important to ensure that good surface finish and details are obtained; subsequent coats are used to build the shell thickness and strength in order to withstand de-waxing and metal pressures.
• the stucco is applied either by raining or by using a fluidised bed.
• Drying of the shell 40 is important; it begins with applying at constant velocity, temperature and humidity to remove the surface binder liquid (a constant rate drying). This is followed by a falling rate drying period which results in capillary transfer of the binder liquid from inside of the shell to the surface.
• Control of humidity and temperature is important. Temperature control affects pattern expansion and contraction which can cause the shell to crack.
• Humidity is preferably controlled to 50% and air velocity is controlled to 18.3-365 m/min (60-1200 ft/min).
• the wax pattern 37 is removed from the layered shell mould 40 by shock firing, steam autoclaving, or other heating technique, which drains the wax through suitable drain openings 50 in the shell. Once the wax is removed, leaving the part cavity 34 vacant, molten metal, such as aluminium, is poured into the cavity 34, as part of step four, through a sprue 51, to produce the required cast object, such as the finished cylinder head 43 shown in figure 7.
• Removal (melting) of the pattern 37 is done during the mould firing cycle.
• the strength of the solid mould must be adequate to withstand the expansion stresses of the wax pattern.
• the wax pattern material then burns off in the firing furnace.
• Firing is carried out in an oxidised environment so that no carbon is left on the mould surface and may be accomplished by several techniques including autoclaving, flash firing and microwave de-waxing.
• Shell firing is then carried out after the moulds are de-waxed to increase the mould strength, along with heating and removing of residual pattern material prior to the final operation of pouring the metal.
• the moulds are heated in an oxidised atmospheric condition to a temperature of 873-1098°C (1600-2000°F) depending upon specific foundry requirements.
• Firing and preheat temperatures depend on the shell material and the type of material being poured. Aluminium castings typically are poured at a heated shell temperature of 221-316°C (400-600°F) and steel at 873-1098°C (1600-2000°F). The metal is poured slowly into the hot shell without causing turbulence. While the shell is heated to a temperature of 221°C (400°F), as described earlier, metal (632°C (1200°F) for aluminium 356) is poured through the gating system to fill up the cavities and the risers. The metal is then left to cool in atmospheric conditions. In the case of a cylinder head, a chill may be used to draw heat away from the relatively large sections of the cylinder head.
• the final casting 43 still retains the particulate bonded cores therein which can now easily be removed as step five by subjecting the casting to a heat treatment cycle; the resin or other bonding agent, holding the particulates together, is reversed so that the sand or salt becomes freeflowing and easily pours from the openings, such as 46-49 of the casting, by gravity (as shown in Figure 7).
• Such heat treatment may also be employed to concurrently treat the aluminium metal to enhance its metallurgical characteristics. It is important to keep in mind that cleaning the casting of core material is not carried out by use of hammers or salt baths, but rather by simple use of inquiescent water at 17°C (62°F) that completely dissociates the bonded cores in less than 12 minutes. Hotter or pressurised water will further reduce dissolution time.
• the described process also promotes economical nimble manufacturing of a family of engine heads or blocks. Only new sets of cores need be made to change the flow characteristics of a head or block, the exterior configuration defined by the pattern die set 18 can remain the same and continue to be used to make up to at least 100,000 wax patterns for different members of an engine family. This is a significant economic breakthrough.

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• Engineering & Computer Science (AREA)
• Mechanical Engineering (AREA)
• Molds, Cores, And Manufacturing Methods Thereof (AREA)

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Publications (2)

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• 1998
  • 1998-11-25 CA CA002254505A patent/CA2254505A1/en not_active Abandoned
  • 1998-12-09 JP JP10350257A patent/JPH11244994A/ja active Pending
  • 1998-12-22 EP EP98310605A patent/EP0924008B1/en not_active Expired - Fee Related
  • 1998-12-22 DE DE69817989T patent/DE69817989T2/de not_active Expired - Fee Related

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