WO2000027562A1

WO2000027562A1 - Casting mold assembly

Info

Publication number: WO2000027562A1
Application number: PCT/US1999/026420
Authority: WO
Inventors: Ralph E. Showman; Ronald C. Aufderheide; Helena Twardowska
Original assignee: Ashland Inc.
Priority date: 1998-11-10
Filing date: 1999-11-09
Publication date: 2000-05-18
Also published as: AU2146600A

Abstract

This invention relates to a casting mold assembly (1, 6) comprising at least one sleeve (3, 4, 7) wherein said sleeve (3, 4, 7) is at least partially in contact with an insulating layer of gas (2, 5, 9), preferably air. It also relates to preparation and use of such sleeves (3, 4, 7) in casting metal.

Description

CASTING MOLD ASSEMBLY

FIELD OF THE INVENTION

5 This invention relates to a casting mold assembly comprising at least one sleeve wherein said sleeve is at least partially in contact with an insulating layer of gas, preferably air. It also relates to preparation and use of such sleeves in casting metal.

BACKGROUND OF THE INVENTION 0 A casting mold assembly consists of a pouring cup, a gating system (including downsprues, choke, runners, and gates), risers, sleeves, molds, cores, and other components. To produce a metal casting, metal is poured into the pouring cup of the casting mold assembly and passes through the gating system to the mold cavity and/or core assembly where it cools and solidifies. The metal part is then removed by 5 separating it from the core and/or mold assembly.

During casting, most metals and alloys undergo a volume reduction or shrinkage during cooling and particularly during solidification. When producing castings, this volume reduction can lead to "shrinkage" holes, cavities, or dimensional inaccuracies in the casting. Because of this, risers (also known as feeders) are often incorporated into the casting mold to o provide a reservoir of liquid metal to "feed" the casting as it solidifies and to offset the shrinkage.

Sleeves are sometimes used to surround portions of the riser in order to keep the molten metal in the riser hot and maintain it in the liquid state while the casting cools and solidifies. Sleeves are made of exothermic and/or insulating materials which retain the 5 heat in the riser and thus retard its solidification. Thus the metal from the riser is allowed to remain in a liquid state for a longer period of time, thereby providing more metal to the casting as it cools and solidifies. The temperature of the molten metal and the amount of time that the metal in the riser remains molten is a function of the sleeve composition, thickness, and the geometric design, among other factors. o Typically, sleeves are made of a single layer composition and uniform properties.

Sleeves are generally made from a mixture of materials such as aluminum metal, oxidizing agents, fibers, fillers, refractory materials such as sand, alumina and aluminosilicate, and aluminosilicate in the form of hollow aluminosilicate spheres. The materials used to make sleeves increase the cost of making of the mold assembly, but the overall cost is reduced because using the sleeve improves the yield and reduces cleaning costs. In order to serve their function, sleeves typically have exothermic and/or insulating properties. The exothermic and insulating thermal properties of the sleeve are different in kind and/or degree than the thermal properties of the casting mold assembly into which they are incorporated. Predominately exothermic sleeves operate by liberating heat which satisfies some or all of the specific heat requirements of the riser and limits the temperature loss of the molten metal in the riser, thereby keeping the metal hotter and in liquid phase longer. Insulating sleeves, on the other hand, do not add heat to the riser, but help to insulate it from the surrounding mold assembly and reduce heat losses. A riser sleeve may have excellent insulating characteristics, but not contain any exothermic material. Alternately it may be highly exothermic, but have poorer insulating characteristics. Traditionally, there were three basic processes used to produce sleeves:

"ramming", "vacuuming", and "blowing or shooting" as described in PCT publication WO 94/23865, which is hereby incorporated by reference into this disclosure. More recently it was shown that sleeves with improved dimensional accuracy can be produced by chemically curing the sleeves by the no-bake (liquid curing catalyst) or cold-box process (vaporous curing catalyst). See PCT publication WO 97/35677 which is hereby incorporated by reference into this disclosure.

DESCRIPTION OF THE FIGURES

Figure 1 shows a cross sectional view of an insertable exothermic sleeve 1 where an air gap 2 is incorporated between the sleeve 1 and the mold wall 3.

Figure 2 shows a cross sectional view of a sleeve 4 where the air gap 5 is incorporated into the design of the sleeve 4. Figure 3 shows a cross section of a mold assembly 6 with a sleeve 7 having protrusions 8 and recesses 9 which create an air gap.

Figure 4 shows a cross section of a test casting 10, riser 11, and the safety margin 12.

SUMMARY OF THE INVENTION

This invention relates to a casting mold assembly comprising at least one sleeve, preferably a riser sleeve, wherein said sleeve is at least partially, or totally, in contact with an insulating layer of gas, preferably air. It also relates to preparation and use of such sleeves in casting feπous and non ferrous metals, e.g. iron, ductile iron, steel, aluminum, grey iron, and brass. The cold-box process is particularly useful for preparing sleeves containing an air gap because dimensionally accurate sleeves can be designed to seal the air gap and prevent metal from filling it.

The sleeve, preferably a riser sleeve, may be an exothermic, insulating, or both. The sleeve may also consist of multiple layers where the layers are exothermic, insulating, or both. The invention is particularly useful for sleeves made by the cold-box process. The dimensional accuracy of such sleeves allows the sleeve to be inserted into a sleeve print cavity in the mold without special fixturing or equipment.

The efficiency of riser is improved by the presence of the gas because the heat of the riser is more effectively contained (i.e. the bulk thermal conductivity is reduced) than when no gas is present and the sleeve is in direct contact with the mold material. Thus the metal of the riser can be maintained at a higher temperature, and the heat loss of the metal in the riser is reduced. As a result, it is possible to use smaller risers resulting in proportionally higher casting yields. Alternately, the use of the presence of the gas can reduce the volume and/or cost of material needed in the sleeve to produce the same results.

Another advantage of using an air gap is that less sleeve material can be used without sacrificing the effectiveness of the sleeve. By using an air gap some of the more expensive sleeve mix can be decreased, thus lowering the total cost of the sleeve. An exothermic sleeve liberates heat once it ignites, but much of the heat escapes to the surrounding surfaces and is not effective in keeping the liquid metal of the riser - hot. Replacing some exothermic material of the sleeve with an insulating air gap keeps heat from the riser from escaping. This "saved" heat maintains the temperature of the molten metal of the riser for a longer time.

DEFINITIONS

The following definitions will be used for terms in the disclosure and claims:

Casting mold assembly - assembly of casting components such as pouring cup, downsprue, runners, ingates, molds, cores, risers, sleeves, etc. which are used to make a metal casting by pouring molten metal into the casting mold assembly where it flows to the casting cavity and cools to form a metal part.

Downsprue - main feed channel of the casting assembly, which generally connects the pouring system to a runner system, through which the molten metal is poured.

Exothermic sleeve - a sleeve which has exothermic properties (i.e. produces net heat) compared to the mold/core assembly into which it is inserted. The exothermic properties of the sleeve are typically generated by an oxidizable material (typically aluminum metal) and an oxygen source which can react to generate heat.

Gas or air gap - a cavity separating the sleeve from the mold material or within the body of the sleeve that is filled with air or gas. Gating system system through which metal is transported from the pouring cup to the mold and/or core assembly. Components of the gating system include the downsprue, runners, choke, gates etc.

Insulating sleeve a sleeve having better insulating properties (i.e. lower thermal conductivity and/or heat capacity than the mold/core assembly into which it is inserted). An insulating sleeve typically contains low density refractory materials such as fibers and/or hollow microspheres.

Insulating/exothermic sleeve - a sleeve which has both exothermic and insulating properties.

Mold assembly - an assembly (typically made of sand and binder) that has an internal cavity shaped so that when molten metal is poured into it and is allowed to cool, the shape of the metal part formed is essentially the same shape as the shape of the mold assembly.

Multiple sleeve - a sleeve having more than one discrete layer. The layers may be discrete because of composition, physical properties, density, etc.

Riser - a cavity connected to a mold or casting cavity of the casting mold assembly which , when filled with liquid metal, acts as a reservoir for excess molten metal to prevent cavities in the casting as the casting contracts on solidification. Risers may be open or blind.

Risers are also known as feeders or heads.

Safety margin - distance from the top of the casting surface to the shrinkage cavity within the riser. A positive value indicates that all shrinkage was confined to the riser and the casting was sound. A negative value indicates that shrinkage extended into the casting. The safety margin can be measured in inches or as a percentage of the total height of the original riser. Generally, more positive values indicate better performance.

Sleeve - any formed shape having exothermic and/or insulating properties which covers, in whole or part, the riser, or is used as part of the casting mold assembly, and separates the metal from the mold material. The sleeve can be inserted into the mold, rammed up, or mold-in-place. Sleeves can have a variety of shapes, e.g. cylinders, domes, cups, boards, cylindrical, neckdown, spherical, neckdown dome, or insertable sleeves.

BEST MODE AND OTHER MODES FOR PRACTICING THE INVENTION

The type and shape of sleeve used not critical. The sleeve may be a cylinder, dome, cup, board, core, neckdown, spherical, neckdown dome, or insertable. The sleeve may also be a multiple layer sleeve. The materials used to prepare the sleeves are any of the exothermic and insulating materials known in the art. The sleeves can be prepared with any methods known in the art. Usually, sleeves are not totally exothermic or insulating in their properties, but have a hybrid of properties.

For purposes of this invention, an exothermic sleeve defined as a sleeve that produces heat when exposed to the heat of molten metal during pouring. An exothermic sleeve is formed from (a) an oxidizable material, (b) an oxygen source capable of generating an exothermic reaction at the temperature where the metal is poured, and (c) a refractory or filler material such as fibers, sand, alumina, aluminosilicate refractories, and/or hollow aluminosilicate microspheres. The oxidizable material typically is aluminum, although magnesium and similar metals and certain metal salts can also be used. When aluminum metal is used as the oxidizable metal for the exothermic sleeve, it is typically used in the form of aluminum powder and/or aluminum granules. The amount of aluminum metal in the sleeve composition typically ranges from 5 weight percent to 50 weight percent based upon the weight of the sleeve composition. The oxidizing agent used for the exothermic sleeve includes iron oxide, manganese oxide, nitrate, potassium permanganate, etc. In addition, the sleeve composition may contain fillers and additives, such as cryolite (Na₃AlF₆), potassium aluminum tetrafluoride, potassium aluminum hexafluoride, sand and wood flour.

For purposes of this invention, an insulating sleeve is defined as a sleeve having better insulating properties, i.e. lower thermal conductivity and/or heat capacity, than the mold material. Insulating sleeve are formed from any insulating material such as mineral fibers, paniculate refractory materials, and preferably hollow aluminosilicate microspheres. The novel feature of this invention is that the sleeve is in contact with, or incorporates, a gap containing a gas, preferably air. The gas is preferably trapped air in the casting mold assembly and partially or totally surrounds the sleeve in the casting mold assembly. This air provides acts as an insulating layer sleeve. The basic methods of creating an air gap are:

1. The sleeve may be manufactured with a different taper, draft, diameter, geometry, or height than the mold cavity into which it is inserted to create an air space between the mold and sleeve. The insertable sleeve is intentionally sized so that an air gap is formed between the insertable sleeve and mold cavity. The air gap acts an insulating layer for the insertable sleeve.

2. The sleeve itself may be designed so that an air gap will result when the sleeve is inserted into the casting mold assembly, e.g. with recesses and/or protrusions. This design lends itself to both insertable and rammed-up applications.

3. The air gap can be molded in an insertable multiple sleeve having more than one layer. The sleeve preferably is designed to prevent metal from flowing into the gap. A breaker core can also act as a seal at the bottom of the sleeve. A seal that reduces the permeability of the sleeve and/or mold adjacent to the air gap will increase the effectiveness of the air gap. The size of the sleeve layer and the gap of gas suπounding the sleeve depends upon the application. The thickness of the air gap is at least 1/16", preferably at least 3/16". Generally, it can be said that the ratio of the volume of the exothermic sleeve to the volume of gas is from 1.0 to 1.0 to 5.0 to 1.0.

The exothermic sleeves are prepared according to well know techniques. When using fibers, the sleeves are typically prepared by "ramming", "vacuuming", and "blowing or shooting" as described in PCT publication WO 94/23865. When using refractory materials and hollow aluminosilicate microspheres, it is useful to chemically cure the shaped sleeve mix as described in PCT publication WO 97/35677.

Any no-bake or cold-box process can be used to chemically cure the sleeves. Curing the sleeve by the no-bake process takes place by mixing a liquid curing catalyst with the sleeve mix and binder, shaping the sleeve mix containing the catalyst, and allowing the sleeve shape to cure, typically at ambient temperature without the addition of heat. The cold-box process is particularly useful for making sleeves because dimensionally accurate sleeve layers can be molded in succession with custom sized tooling.

With respect to the cold-box process, a sleeve mix and binder is shaped by blowing or ramming it into a sleeve pattern box, and curing it by contacting it with vaporous or gaseous curing catalyst. Such catalysts include tertiary amines, sulfur dioxide and an oxidizing agent, carbon dioxide (see U.S. Patent 4,985,489 which is hereby incorporated into this disclosure by reference), or methyl esters (used with alkaline phenolic resole resins as describe in U.S. Patent 4,750,716 which is hereby incorporated into this disclosure by reference). Carbon dioxide is also used with binders based on silicates (see U.S. Patent 4,391,642 which is hereby incorporated into this disclosure by reference). Those skilled in the art will know which gaseous curing agent is appropriate for the binder used. Preferably used as the cold-box process is used with an ISOSET® binder (based upon epoxy-acrylic binders cured with sulfur dioxide in the presence of an oxidizing agent as described in U.S. Patent 4,526,219, which is hereby incorporated by reference), or an ISOCURE® binder (based on phenolic urethane binder cured by passing a tertiary amine gas, such a triethylamine, in the manner as described in U.S. Patent 3,409,579, which is hereby incorporated by reference) .

EXAMPLE 1

The sleeve mixes used to prepare the exothermic sleeves comprised 66% aluminum silicate microspheres, 25.7% aluminum powder, 5.3% magnetite and 3% cryolite. The sleeve mixes used to prepare the insulating sleeves consisted entirely of aluminum silicate microspheres.

The sleeve mix and 8.8% parts of binder, based upon the weight of the sleeve mix were mixed in a Hobart N-50 mixer for about 2-4 minutes. The exothermic sleeves were produced by the cold-box process in much the same way a core is produced. The cold-box binder used in the examples is an ISOCURE® binder sold by Ashland Chemical Company, a division of Ashland Inc. This binder is a two part polyurethane-forming cold-box binder where the Part I is a phenolic resin similar to that described in U.S. Patent 3,485,797. The resin is dissolved in a blend of aromatic, ester, and aliphatic solvents, and a silane. Part II of the binder is the polyisocyanate component and comprises a polymethylene polyphenyl isocyanate, a solvent blend consisting primarily of aromatic solvents and a minor amount of aliphatic solvents, and a benchlife extender. The weight ratio of Part I to Part π is about 55:45. The sleeves used in the experiments were prepared by mixing the sleeve mix and curing by the cold-box process. The dimensional accuracy of the sleeve produced by the cold-box process allows the sleeve to be inserted into a pre-molded cavity in the mold without special fixturing or equipment. The sleeve will fit into the sleeve cavity tightly with no gap at the bottom of the sleeve that would allow liquid metal to flow up and into the air gap around the sleeve. By using the correct dimensions on the sleeve and the tooling to produce the sleeve cavity, it is possible to insert the sleeve into the core print so that it is tight at the bottom (top of the casting), but so a cavity or air gap is created around the upper sides and top of the sleeve. In some instances it may be recommended to mold certain surfaces in the sleeve cavity, or the sleeve itsel-ζ to serve as locators for the sleeve to provide extra support and dimensional accuracy in sleeve placement. Trials were conducted using a "shrink cube" casting with the sleeves (see Figure 3).

The "shrink cube" is a 3 1/2" cube that was cast in steel and used to evaluate the relative performance of sleeve materials. Molds were produced using 2 1/2" x 3 3/4" riser sleeves.

Two castings were poured with low carbon steel at 2950° F. The Control used a standard exothermic sleeve in the sand mold without an air gap. The castings of Example 1 used and air gap of 1/4" around the outside of the sleeve.

Following casting, the molds were broken open and examined. Where the sleeve was molded directly against the sand mold without an air gap (Control), the sleeve materials remained in contact with the metal riser on the inside and with the sand mold on the outside. Where an air gap was used around the sleeve (Example 1), there was a visible cavity surrounding the sleeve and separating it from the sand mold.

The sleeve performance was evaluated by measuring the distance from the top of the cube casting to the bottom of the shrinkage pipe in the riser. This is referred to as the "safety margin". The Control had a shrinkage pipe that extended close to the top of the casting for a safety margin of 0.32". The casting of Example 1, which had the air gap, had a safety margin of 1.43". Thus the performance of the sleeve was enhanced by the presence of the air gap.

The results of the experiments are shown in Table I.

TABLE I

SAFETY MARGIN OF SLEEVES WITH AN AIR GAP COMPARED

TO CONVENTIONAL SLEEVES WITHOUT AN AIR GAP FOR STEEL

2x3" sleeve 3/8" thickness, with 3.5"cube test casting

Table I shows that the safety margin of the sleeve is improved significantly when there is a 1/4" air gap surrounding the sleeve.

The results in the Table clearly show the advantages of using a an air gap with an exothermic sleeve. The greater safety margin indicates that sleeves in contact with an air gap improve the thermal properties and performance of the sleeve. The heat of the riser is evidently more effectively contained than when no air gap is present. Thus the metal of the riser can be maintained at a higher temperature longer.

Claims

CLAIMS We claim:

1. A sleeve shaped such that a layer of gas will result when the sleeve is incorporated into the casting mold assembly.

2. The sleeve of claim 1 wherein the layer of gas is air.

3. The sleeve of claim 2 which contains recesses or protrusions.

4. The sleeve of claim 2 wherein said sleeve is an insertable sleeve.

5. The sleeve of claim 2 wherein the sleeve is a mold-in-place sleeve.

6. The sleeve of claim 2 wherein the air gap is at least 1/16 inch.

7. The sleeve of claim 2 wherein the sleeve is a multiple layered sleeve.

8. A casting mold assembly comprising at least one sleeve wherein said sleeve is at least partially in contact with an insulating layer of gas.

9. The casting mold assembly of claim 8 where the layer of gas is air.

10. The casting mold assembly of claim 9 where the sleeve is selected from the group consisting of exothermic sleeves, insulating sleeves, and sleeves have both insulating and exothermic properties.

11. The casting mold assembly of claim 10 wherein the sleeve contains hollow aluminosilicate microspheres.

12. The casting mold assembly of claims 11 wherein the sleeves are prepared by a cold- box process.

13. The casting mold assembly of claim 12 wherein the gas layer is between the wall of the mold and the sleeve.

14. The casting mold assembly of claim 12 wherein the gas is within the walls of the sleeve.

15. A casting mold assembly of claim 12 where the air gap is created by the shape and geometry of the sleeve and the shape and geometry of the cavity into which the sleeve is inserted, such that the sleeve is not in contact with the mold wall at all points and air is trapped in the resultant cavity between the sleeve and the mold.

16. The casting mold assembly of claim 12 wherein the binder used to prepare the sleeve layers is selected from the group consisting of phenolic urethane binders and epoxy-acrylic binders.

17. The casting mold assembly of claim 16 wherein the chemical binder is a phenolic urethane binder and the curing catalyst is a vaporous tertiary amine.

18. The casting mold assembly of claim 16 wherein the chemical binder is an epoxy- acrylic binder and the curing catalyst is sulfur dioxide.

19. A process for casting a metal part which comprises:

(1) forming a casting mold assembly comprising a casting mold assembly which comprises a sleeve of claims 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, or 18, wherein the thermal conductivity of said mold assembly is higher than the thermoconductivity of said sleeve;

(2) pouring metal, while in the liquid state, into said casting mold assembly;

(3) allowing said metal to cool and solidify; and

(4) then separating the cast metal part from the casting mold assembly.

20. A metal part prepared in accordance with claim 19.