GB2061151A

GB2061151A - Expendable die casting sand core

Info

Publication number: GB2061151A
Application number: GB8031348A
Authority: GB
Original assignee: NL Industries Inc
Current assignee: NL Industries Inc
Priority date: 1979-10-01
Filing date: 1980-09-29
Publication date: 1981-05-13
Also published as: FR2466293B1; IT8024888A0; AU6254480A; CA1172825A; ES8105596A1; DE3036436A1; FR2466293A1; BR8006285A; JPS5650752A; AU539985B2; GB2061151B; IT1133622B; JPS649898B2; SE8006712L; DE3036436C2; ES495504A0

Abstract

Coated sand cores containing an acid curable resin binding agent are used in the production of die castings having undercut regions because of the favorable combination of shakeout properties, resistance to washout, and resistance to surface penetration. <IMAGE>

Description

SPECIFICATION Expendable die casting sand core The invention relates to the art of die casting such metals as aluminum, zinc, magnesium, copper and their alloys and to a solution to a long standing problem therein; i.e., the lack of a commercially feasible die casting technique to produce castings having undercut regions. Traditional pressure die casting requires molds or dies which are able to withstand the high temperatures and pressures to which they are subjected. Thus, ferrous materials are commonly used for die casting molds. Because these die materials are not easily collapsible, com plex undercuts and reliefs are not possible because the casting cannot be removed from the mold. Other common casting techniques, such as sand and semi-permanent mold casting, have utilized expend able or disposable cores to produce castings having undercuts.This has been possible because the pressure requirements for such techniques are usually underthe order of 30 psia as compared with at least about several thousand psia required during high pressure die casting.

Atypical core is composed of foundry sand mixed with a binder or resin. Through the use of heat, a catalyst, or chemical reaction, the sand grains are bonded together into a discrete shape, and can then be used in the casting process. The heat evolved during solidification and cooling of the actual cast parts evaporates moisture contained in the core or results in chemical breakdown of the binder. Relatively easy removal of the core from the casting is thereby facilitated.

Prior attempts to utilize sand cores for die casting have included the use of glass and soluble salt cores.

Such techniques are discussed in detail in British Patent Number 1,179,241. These systems are considered to be unsatisfactory from the standpoints of process control, economics, handling, and the corrosive characteristics of the salts.

Binding systems of the boronated aluminum phosphate type have been proposed for use in making expendable sand cores for die casting. Such systems are illustrated in United States Patent Number4,127,157 and in ourcopending British Patent Application No. 8003872. This application, however, relates to the use of a different binding system than that of this invention. In general, the system of the invention is considered to represent an improvement of the boronated aluminum phosphate system because, although both systems have excellent shakeout properties, the system of the invention produces cores having higher strength, especially immediately following coremaking.Moreover, coremaking procedures are simplified with use of the binding system of the invention bécause this system has higher blowabilitythan boronated aluminum phosphate binding systems and thus denser cores can be obtained.

A major problem in the development of a satisfactory expendable core for use in high pressure die casting has been the inability of a single core/binder system to simultaneously meet four primary core characteristics. They are good shakeout, good wash out resistance, freedom from surface penetration, and core strength. Good shakeout is necessary to facilitate core removal from the casting. Washout resistance is the ability of the core to withstand erosion from the high metal velocities that occur during die casting. Not only does washout adversely affect the tolerances on the finished part but the sand physically removed from the core becomes embedded in the casting.Surface penetration is caused by the combination of high heat and press ure that breaks down the core surface and permits the metal to penetrate between the sand grains thus causing a sand/metal mixture interface at the surface of the casting. This condition is extremely detrimental to subsequent maching and tool like. Moreover, should the sand become separated from the surface after component installation, damage to related parts, such as automobile parts, could result. High strength for cores is desirable in that the cores are more resistant to breakage during handling and are also more resistant to breakage during the rigors of the casting. This invention is considered to solve such problems with use of a core system that is capable of attaining the requisite balance of the four properties.

The Figure is a sectional view of a mold portion of a die casting machine and is useful for illustrating regions in the casting in which the three problems discussed above occur. Plunger 11 is used to inject molten metal 12 into the die casting mold formed by steel members 13 and 14 and sand core 15. Note that the final die casting shape includes an undercut region. Surface penetration of metal 12 into sand core 15 occurs along the dark shaded region identified as 16 in the Figure. Washout usually occurs at areas such as denoted by 17. Shakeout refers to the ability to remove core 15 upon solidification of the die casting, its removal from the die casting die, and subsequent cooling to ambient temperature.

It has been discovered that expendable sand cores can be used to produce die castings having undercut regions provided that a binding agent comprising an acid curable resin in an amount from about 0.25% to 5%, based upon weight of foundry sand, is used.

However, it is preferred to employ between about 1% to 2%. The choice of a specific binder level will be dependent upon core shape, core thickness, complexity, the manner in which the core is secured within the casting die, and casting conditions. The binder, mixed with foundry sand and an appropriate amount of oxidizing agent forms the core. The core can also be coated to provide improved resistance to penetration and washout.

The acid curable resin binding agent mentioned above is more fully described in United States Patent Number 3,879,339 which patent, in its entirety, is incorporated by reference herein for such purpose.

This agent has proven to be beneficial to shakeout properties in die casting applications when present in amounts from about 0.25% to 5% by weight of foundry sand. The lower limit is required to provide sufficient core strength to withstand handling while the upper limit should not be exceeded due to blowing problems during coremaking caused by a lack of uniform- density related to variations in sand flow and unacceptable decreases in shake-out efficiency. When using typical silica foundry sands of a fineness of AFS No. 65, it is preferred to employ from about 1.0% to 2.0%. Should heavier foundry sands such as zircon be used, less binder is required, i.e., preferably on the order of from about 0.5% to 1.0%.

The respective preferred upper and lower limits are chosen for the same reasons as for the more general range. Of course, the use of other commonly used foundry sands having different densities than the above mentioned sands is within the scope of the invention Such other sands would preferably require the use of binder amounts consistent with density.

Acid curable resin binding systems that are suitable for use in the invention include urea/formaldehyde, phenol/formaldehyde, furane, and copolymer resins. It is preferred to use furane because this system is more resistant to distortion immediately following coremaking than the other systems. it is also possible to use copolymers of these resins with epoxidised compounds or with unsaturated compounds.

The silianization of the resins of the invention through the addition of from about 1% to 10% by weight based on resin of a silane such as gammaamino-propyl-triethoxysilane is optional. Such additions function to strengthen the core.

An oxidizing agent such as methyl ethyl ketone peroxide, should be present in the system in quantities ranging from about 20% to 70%, based upon weight of the resin. Amounts from 30% to 50% are preferred to ensure complete curing. The oxidizing agent functions to react with gaseous sulfur dioxide to form sulfuric acid, which, in turn, cures the resin.

Other suitable oxidizing agents include but are not limited to peroxide, hydroperoxide, hydroxyhydro peroxide, chlorate, perchlorate, chlorite, hydro- chloride, perbenzoate, metal oxide, permanganate, monoperiphthalic acid, and hydrogen peroxide.

Such oxidizing agents are normally added as a liquid to the resin to facilitate mixing although the use of solid or gaseous agents is not outside the scope of this invention.

To typically prepare the expendable core of the invention, sand and an acid curable resin are mixed, then silane may be optionally added to the mixture, and then the oxidizing agent is added to the mixture.

The mixture may then be air blown or hand packed into a core box having the desired shape of the core.

A curing gas such as sulfur dioxide is passed through the core box to cure the resin. The core is removed from the box as a solid mass and utilized in the die easting process. Other techniques and curing gases are disclosed in United States Patent Number 3,879,339 and 3,639,654, which patents are incorporated by reference, in their entirety, for such purpose.

Following its preparation, the core may be coated to further improve performance with respect to washout and surface penetration. Core coatings generally comprise a suspending agent, a refractory material, a binding agent, and a solvent.

Core coatings for die castings are more critical than core coatings suitable for other casting methods. The core coating should possess the capability of being able to substantially seal the pores on the surface of the core. Because die casting places molten metal under pressure, any porosity at the surface of the cores will lead to penetration of the molten metal and thus trap sand on the surface of the as-cast part. An application of the proper core coating to the core will provide a die like finish with no resultant penetration of the molten metal into the sand core.

Suspending agents are usuallyclayorclayderiva- tives. These materials should be present in amounts sufficient to perform the function of maintaining the refractory material in suspension. Based upon total solids weight, such agents may be present in amounts ranging from about 4% to 30%.

Typical particulate refractory materials that are useful in the coating formulation include but are not limited to graphite, silica, aluminum oxide, magnesium oxide, zircon, and mica. These materials are present in amounts generally ranging from about 60% to 95% based upon total solids weight The mass of particles is bound together through use of binding agents such as thermoplastic resins.

Binding agents useful in the practice of the invention generally comprise from about 1% to 10% by total solids weight of the coating composition. The binding and suspending agents should be compatible with the particular solvent which may be an organic liquid. The solvent should be included in an amount which is effective to obtain the necessary viscosity to control coating thickness and uniformity.

Atypical suitable core coating comprises, based upon total solids weight, from 4% to 30% of an amine treated bentonite suspending agent, from 1% to 10% of a thermoplastic resin binding agent, and from 60% to 95% of a refractory such as silica or the like. The above constituents, in powder form, are mixed with a sufficient quantity of organic liquid vehicle-to attain the necessary viscosity to produce, upon drying, the desired coating thickness and serve to seal the pores on the surface of the core.

Other suitable core coatings include those shown in United States Patent Number 4,001,468 which patent, in its entirety, is incorporated herein for such purpose. The patent discloses coating compositions comprising an organic liquid solvent having a kauributanol value of at least 36, such as liquid 1,1,1trichloroethane; a suspending agent; a powdered refractory material such as graphite, coke, mica, silica, aluminum oxide, magnesium oxide, talc or zircon flour; and an organic polymer such as vinyl toluene/butadiene copolymer, styrene/butadiene copolymer, vinyl toluene/acrylate copolymer, styrene/acetylene copolymers, or acrylate homopolymers. The ratio by weight of organic polymer to organic liquid solvent is maintained between about 1:50 and about 1:200 and the ratio by weight of powdered refractory to organic liquid solvent is maintained between about 1:2.5 and 1:3.5.

An additional core coating that has proven to be satisfactory for use in combination with the binding system of the invention is that described in United States Patent Number 4,096,293 which patent, in its entirety, is incorporated by reference herein for purpose. The coating material has a viscosity sufficient to substantially seal surface porosity of the core and is suitable to obtain a coating thickness and uniformity that leads to good resistance to washout and penetration during die casting.The coating consists of from about 5% to 90% of an organic liquid solvent, from about 0.1% to 2% of a suspending agent, from about 5% to 80% of calcium aluminate particles having an average particle size of 20 to 25 microns and-no particles being larger than about 70 microns, and a hard resin which is the reaction product of fumaric acid, gum rosin, and pentaerythritol, the resin is within the ratio by weight between about 0.5 and 5 parts per 100 parts of composition, all percentages expressed by weight of composition. A wetting agent may optionally be added in amounts ranging from about 0.01% and 2%.

Following manufacture of the core in a core box and its removal, the core is sufficiently strong enough to be handled. A core coating is then applied by brushing, dipping, spraying or an equivalent method. Once the coating is dry, the core is placed into a die located on a casting machine. The steel portion of the die forms the surface shape of the metal part that is not formed by the core. The core is placed in this die and is located by pins, impressions or other methods commonly known to those skilled in the art. The die is then closed, thus trapping the core in a fixed location and molten metal is then injected into the die.

During the process of solidification in the die, heat is emitted from the casting. A portion of the heat flows into the core and increases its temperature.

This flow breaks down the binder and drives off moisture and gaseous materials. Once the molten metal has solidified in the die, the machine is opened and the resultant casting and expendable core are removed. Upon cooling to ambient temperature, the core may be shaken out mechanically.

The following example illustrates an embodiment of the invention: An aluminum alloy was die cast into the shape shown in the Figure with use of a core containing silica foundry sand (AFS Fineness No. 65). Furane in an amount of 1.5%, based upon weight of the foundry sand was used as the binding agent and 40%, based upon weight of the resin, of the oxidizing agent methyl ethyl ketone peroxide, and 3%, based upon weight of the resin, of silane. A core coating as set forth previously was applied to the core prior to die casting. Core strength was good. Good shakeout properties were noted following mechanical separation of the core from the casting upon cooling to ambient temperature. The casting exhibited good resistance to surface penetration and washout resistance was good.

Claims

1. A method of forming a die casting having an undercut region from molten metal, comprising: injecting molten metal into a die casting mould having a casting surface that includes at least one expendable sand corethatforms an undercut region on the die casting, the core consisting of foundry sand and from 0.25% to 5% by weight on the weight of the foundry sand of a binding agent consisting of an acid curable resin, from 20% to 70% based upon weight of the resin of an oxidizing agent, permitting the injected molten metal to solidfy along the casting surface to form a die casting; removing the die casting from the mould; and separating the die casting from the core.

2. A method as claimed in Claim 1, wherein the foundry sand comprises silica sand and the binder is present in an amount from about 1.0% to 2.0%.

3. A method as claimed in Claim 1, wherein the foundry sand comprises zircon sand and the binder is present in an amount from about 0.5% to 1.0%.

4. A method as claimed in any one of claims 1 to 3, wherein the core contains from 30% to 50% of the oxidizing agent.

5. A method as claimed in any one of claims 1 to 4, wherein the core contains from 1% to 10% based upon weight of the resin of at least one silane.

6. A method as claimed in any one of claims 1 to 5, wherein the oxidizing agent is methyl ethyl ketone peroxide.

7. A method as claimed in any one of claims 1 to 6, wherein the acid curable resin comprises furane.

8. A method as claimed in any one of claims 1 to 7, wherein the binding agent has been cured by the passage of S02 gas through the core.

9. A method as claimed in any one of claims 1 to 8 wherein the core is coated with a coating material having a viscosity sufficient to substantially seal surface porosity on the core which coating material consists of from 4% to 30% of a suspending agent, from 60% to 95% of a particulate refractory material, from 1% to 10% of a binding agent, and an effective amount of a vehicle for interacting with the suspending agent and the binding agent to achieve a viscosity suitable to obtain a coating thickness and uniformity that leads to good resistance to washout and penetration during die casting.

10. A method as claimed in Claim 9, wherein the vehicle comprises water.

11. A method as claimed in any one of claims 1 to 8, wherein the core is coated with a coating material having a viscosity sufficient to substantially seal surface porosity on the core, which coating material consists of an organic liquid solvent having a kauri-butanol value of at least 36; a suspending agent; a powered refractory material which is graphite, coke, mica, silica, aluminum oxide, magnesium oxide, talc and zircon flour or a mixture thereof; and an organic polymer which is at least one of vinyl toluene/butadiene copolymer, styrene/ butadiene copolymer, vinyl toluene/acrylate, a homopolmer of one of such monomers or a mixture of such polymers; the ratio by weight of organic polymer to organic liquid solvent being from 1:50 to 1:200 and the ratio by weight of powdered refractory to organic liquid solvent being from 1:2.5 to 1 :3.5.

12. A method as claimed in any one of claims 1 to 8, wherein the core is coated with a coating material having a viscosity sufficient to substantially seal surface porosity on the core and suitable to obtain a coating thickness and uniformity that leads to good resistance to washout and penetration during die casting which coating material consists of from 5% to 90% by weight of an organic liquid solvent, from 0.1% to 2% by weight of a suspending agent, from 5% to 80% by weight of calcium aluminate particles having an average particle size of from 20 to 25 microns and having no particles larger than 70 microns, and from 0.5 to 5% by weight of a hard resin which is the reaction product of fumaric acid, gum rosin and pentaerythritol.

13. An expendable sand core adapted to define an undercut region in a die casting mould, the said sand core comprising foundry sand, and from 0.25% to 5% by weight on the weight of the sand of a binding agent consisting of an acid curable resin.

The sand core providing for a die casting operation a favourable combination of shakeout properties, resistance to washout, resistance to surface penetration by molten metal, and core strength.

14. A die casting mould having an undercut region comprising an expendable sand core as claimed in claim 13.

15. A mould as claimed in Claim 14 in which the sand core at least initially contained an oxidizing agent in the amount of about 20% to about 70% by weight of the resin.

16. A mould as claimed in claim 15 in which the oxidizing agent is methyl ethyl ketone peroxide.

17. A mould as claimed in any one of claims 14to 16 in which the foundry sand comprises zircon sand, and the binder is present in an amount of about 1% to about 2%.

18.- A mould as claimed in either any one of claims 14 to 16, in which the foundry sand comprises zircon sand, and the binder is present in an amount from about 0.5% to about 1.0%.

19. A mould as claimed in any one of claims 13 to 18 in which the core contains from 1% to 10% based on the weight of the resin of at least one silane.

20. A die casting mould as claimed in any one of claims 13 to 19 in which the acid curable resin comprises furane.

21. A mould as claimed in Claim 14 in which the core has a surface coating comprising in weight percent from 4% to 30% of a suspending agent, from 60% to 95% of a particulate refractory material, from 1% to 10% of a binding agent, and an effective amount of a vehicle for interacting with the suspending agent and the binding agent to achieve a viscosity suitable to obtain a coating thickness and uniformity adapted to provide resistance to washout and penetration during die casting.

22. A mould as claimed in Claim 14 in which the core has a surface coating comprising an organic liquid solvent having a kauri-butanol value of at least 36, a suspending agent, a powdered refractory material which is graphite, coke, rhicas, silica, aluminum oxide, magnesium oxide, talc, zircon flour, or a mixture thereof, and an organic polymer which is vinyl toluenelbutadiene copolymer, styreneíbuta- dien copolymer, vinyl toluene/butadiene copolymer, styrene/butadiene copolymer, vinyl tolueneiacrylate, and homopolymers thereof, the ratio by weight of organic polymer to organic liquid solvent being from 1:50 to 1:200, and the ratio by weight of powdered refractoryto organic liquid solvent being from 1:2.5 to 1:3.5.

23. A mould as claimed in Claim 14 in which the core has a surface coating comprising from 5% to 90% by weight of an organic liquid solvent from 0.1% to 2% by weight of a suspending agent, from 5% to 80% by weight of calcium aluminate particles having an average particle size of from 20 to 25 microns and, having no particles larger than 70 microns and from 0.5 to 5% by weight of a resin which is the reaction product offumaric acid, gum rosin and pen taeryth ritol.