GB2162456A - Process for the fabrication of cores or moulds for use in metal casting - Google Patents

Abstract

Refractory particles are mixed with a first inorganic binder and a second binder consisting of starch, with the optional addition of water. The mixture is introduced into a core box (or a mould box about a former) designed for effective exposure of the charged mixture to microwave energy, and the mixture is cured by exposing the core box (or the mould box) containing the mixture to microwave energy.

Description

SPECIFICATION Process for the fabrication of cores or of moulds for use in metal casting Description This invention relates to a process for the fabrication of cores or of moulds for use in metal casting.

In foundry work there is a continuing need for the fabrication of coherently bonded masses of sand or like reflectory grains or particles for use as disintegratable cores in combination with complementary moulds in casting applications, the core being used for forming a passage or opening in the casting.

Sand moulds and cores for metal casting applications must meet two contradictory requirements: they must be of high strength before and during the moulding of molten metal but should readily disintegrate and allow easy shakeout following the solidification of the metal. A variety of mould and core compositions, as well as methods of making moulds or cores, have been suggested in an attempt to fulfill the above and other requirements.

A typical conventional method dictates the bonding of refractory particles such as silica sand with an organic binder such as phenolic resin, furan resin, urethane resin and urea resin, and thermally curing the bonded refractory particles. Shell moulding is one well known curing method, wherein a mixture of sand and thermosetting phenolic resin is poured over a heated metal pattern, with the result that a thin shell of the mixture sticks to the hot pattern surface.

Microwave heating is another curing method, as disclosed for example in Cole U.S. Patent 4,331,197. The use of organic binders for holding refractory particles together is objectionable, however, as the moulds made therefrom, whether they have been cured by the hot pattern or microwave method, do not necessarily contract upon solidification of the cast metal to an extent sufficient to prevent the hot cracking of the casting. Moreover, being synthesized from petroleum, the above enumerated organic binders are generally expensive and subject to excessive cost fluctuations.

In view of the limitations and restrictions of organic binders, recent research and development efforts in the mould forming industry have again been centred on inorganic binders notably including water glass (sodium silicate). However, moulds or cores as so far fabricated with use of inorganic binders have had a problem with regard to their disintegration after the setting of the cast metal. The quality of the castings made with such moulds or cores has also been inferior in some instances to that of the castings fabricated with moulds or cores prepared with use of organic binders.

Take, for instance, the conventional carbon dioxide method wherein a body of sand bonded with water glass is hardened by a carbon dioxide gas. As much as four to six percent water glass has normally been added to the sand to ensure mould strength. The high proportion of water glass has sometimes resulted in sintering of the moulds or cores at the time of casting.

The sintered moulds or cores do not, of course, readily distingrate. They make shakeout difficult, and may partly stick fast to the castings. We have found moulds or cores so prepared to have relatively poor surface stability. We have also found pollution or contamination problems in reconditioning, for reuse, sand reclaimed from used moulds or cores.

In accordance with a first aspect of the present invention, we provide a process for making a core for use in metal casting, the process comprising the steps of: providing a mixture comprising refractory particles, a first binder consisting of an inorganic substance and a second binder consisting of starch, and, optionally, water; charging the said mixture into a core box having a cavity of predetermined configuration and designed for effective exposure of the charged mixture to microwave energy, and curing the said mixture by exposing the core box containing the mixture to microwave energy.

In a second and alternative aspect of the present invention, we provide a process for making a mould for use in metal casting, the process comprising the steps of: providing a mixture comprising refractory particles, a first binder consisting of an inorganic substance and a second binder consisting of starch, and, optionally, water; charging the said mixture into a mould box having a former or core of predetermined configuration and designed for effective exposure of the charged mixture to microwave energy; and curing the mixture by exposing the mould box containing the mixture to microwave energy.

As we shall explain in more detail below with reference to a number of worked Examples, processes in accordance with the present invention can provide a coherently bonded mass of refractory particles suitable for use as a mould or as a moulding core, which is free from the hot cracking problem conventionally encountered where moulds or cores are made with the use of organic binders, and which is superior in disintegration and surface stability characteristics to moulds or cores made by the use of inorganic binders.

In effect, the present invention enables the advantages of each of the conventional procedures to be obtained whilst avoiding the attendant disadvantages.

Preferably the mixture, apart from incidental impurities, consists of refractory particles, from 0.5 to 3.0 percent by weight of the first binder, from 0.5 to 2.0 percent by weight of the second binder, and from 0 to 5.0 percent by weight of water, the percentages all being relative to the weight of the refractory particles.

We have found that the use of the two binders, namely an inorganic binder on the one hand and starch on the other hand, together with the use of microwave curing, makes it possible to use a proportion of inorganic binder substantially less than in tve conventional practice. We find that this results in a core which does not sinter from the heat of cast metal and readily disintegrates following the setting of the metal. By controlling the amount of the inorganic binder, a desired degree of hot strength can be imparted to the resuiting core or mould to prevent the production of scabs that might take place if starch only were used as the binder.We find that in the practice of the present invention, shorter periods of time are required for the production of cores lor moulds than in comparable conventional processes employing heat transfer for curing of sand mixtures.

We have also found for best results that water is preferably present in the range of 0.5 to 3.0 percent by weight of the refractory particles. In this particular range we find the greatest improvement in characteristics of the core, notably in surface stability and strength at room temperature.

In the preferred procedure for preparing the mixture preparatory to microwave curing, the starch and water are fixed mixed together into paste form in the required proportions. This paste is added together with the inorganic binder to the refractory particles. The ingredients are then intimately inter-minged to provide the desired mixture for charging into a core box or mould box for curing. This procedure results in a more uniform dispersion of the starch with consequent improvement in surface stability and other characteristics of the core or mould produced.

The starch employed is preferably an alpha starch prepared by processing a natural starch.

The use of an alpha starch has also been found by us to improve the surface stability and strength, both at room and elevated temperatures, of the core or mould.

Preferred examples of the inorganic binder for use in the practise of our invention are silicates such as, typically, water glas and potassium silicate. Potassium silicate in particular is effective to avoid the pollution problem in reconditioning the refractory particles recovered from the core or mould after use.

The principal utility for the present invention is in the production of disintegratable cores, and the invention is described hereinbelow with particular reference to the fabrication of such cores.

It is to be understood, however, that the invention is equally applicable to the manufacture of moulds formed in a similar fashion from refractory particles.

The invention is accordingly hereinafter more particularly described by way of example only with reference to the accompanying drawings in which: Figure 1 is a perspective view of an embodiment of core box suitable for use with the present invention; Figure 2 is a vertical sectional view through the core box of Fig. 1; Figure 3 is a graph plotting the strength of cores fabricated by examples of methods in accordance with the present invention in comparison with the strength of cores manufactured by certain prior methods, at various temperatures; and Figure 4 is a graph plotting the surface stability of cores fabricated by examples of methods in accordance with the present invention and of cores fabricated by certain prior methods.

In order to put the invention into practise, a mixture of refractory particles and of the two binders must first be prepared.

In the preferred procedure, a mixture consisting of 0.5 to 2.0 percent by weight of starch and up to 5 percent (most preferably from 0.5 to 3.0 percent) by weight of water, in each case relative to the weight of the refractory particles to be employed in the process, are inter-mingled to form a paste. The starch may be chosen from among natural starches (for example those derived from wheat, rye, rice, corn or maize, potato and tapioca), processed starches, cellulosic starches (for example carboxymethyl cellulose), or water-soluble synthetic high polymer starches (for example those composed principally of polyvinyl alcohol). We have found, however, that the most preferable starches are alpha starches (for example alpha corn starch) obtained by processing natural starches. We have found these to be effective to enhance the strength of the cores or moulds produced.

The starch/water paste and an inorganic binder are then added to the refractory particles.

Suitable inorganic binders, preferably present in an amount of 0.5 to 3.0 percent by weight relative to the weight of the refractory particles, are silicates such as water glass and potassium silicate. Water glass is recommended in applications where low manufacturing costs are essential, being readily available commercially at a small cost. Potassium silicate, on the other hand, lends itself to use in applications where pollution or contamination problems in the recovery and reconditioning of used cores or moulds must be avoided. The refractory particles suitably comprise silica sand, zircon sand, alumina sand, or mullite. Such sands may be either fresh or recovered. The recovered sands may or may not be reconditioned. Recovered sands, whether reconditioned or not, which contain dielectric substances are also acceptable.

The various ingredients are well intermingled and the prepared mixture is charged into a core box having the desired cavity configuration, either by hand, by blowing, or by any other method. The core box should be one which has been especially designed for use with microwave curing of charged mixtures, being easily permeable to microwave energy. The core box described hereinbelow with reference to Figs. 1 and 2 is an example of a core box which we have found suitable in the practise of this invention.

The core box containing the required mixture is introduced into a microwave oven, and the mixture is irradiated with microwave energy through the core box. Dielectric materials, such as the water which may be present in the mixture, convert microwave energy into heat with the result that the mixture hardens in the shape of the core box cavity. The core box itself is designed so as not to emit much heat, as further explained below.

The core box with the now cured mixture forming a core therein is withdrawn from the microwave oven and the core extracted from the core box. We have found that cores so formed have sufficient strength immediately after microwave curing to allow themselves to be used straight away in metal casting.

We have found that cores so formed have extremely high surface stability with the result that the casting defect known as sand holes (that is the entrapment in the casting of sand eroded from the core) is virtually eliminated. Destruction of the core after a casting operation is also straightforward. The use of the combination of the two binders allows adjustment in the hot strength of the cores so that there is a substantial reduction in the production of scabs on castings as compared with procedures where starch alone is used as a binder. The use of a microwave curing technique with attendant rapid heating of the body of the mixture from within, enables the concentration of binders to be reduced to approximately 20 to 50 percent of binder concentrations that would have been necessary in prior art methods relying on heat transmission for cure.

Turning now to the core box 10 illustrated in Figs. 1 and 2, it should be noted that the core box illustrated in these figures is not itself novel and has been described in Japanese Patent Application 37215/81 and in European Patent Application 82301379.2 (Publication No: 0060731) which claims priority from the said Japanese Patent Application. We shall therefore describe the core box 10 in the present specification only to the extent necessary for an understanding of the present invention.

The core box 10 has a rectangular frame 1 2 of rigid material, preferably metal, which is open on both sides. The metal frame 1 2 is apertured at 14 for pouring the bonded refractory mixture to be moulded and cured. Within the metal frame 1 2 there is provided a relatively thin facing layer 1 6 defining a cavity 1 8 of the desired shape into which the mixture is to be formed.The aperture 1 7 in the metal frame 1 2 is open directly to the cavity 1 8. The facing layer 1 6 is enclosed in a backing layer 20 having a greater thickness than the facing layer and serving to reinforce the facing layer and to mechanically join the same to the metal frame 1 2. The backing layer 20 need not be of excessively large thickness as the metal frame 1 2 also functions to reinforce and protect the facing layer 16. The facing layer 1 6 and backing layer 20 should both be moulded of materials pervious to microwave energy.A preferred example of material for the facing layer 1 6 is heat-resistant silicone rubber, and that for the backing layer 20 is a suitably proportioned mixture of nonpolar epoxy resin and dry silica sand, cured in situ by heating.

In the core box 10 constructed as in the foregoing, the backing layer 20 enveloping the facing layer 1 6 is mostly exposed through the pair of opposite side openings 22 of the metal frame 1 2. Thus, irradiated through these openings 22 without being substantially affected by the metal frame 12, microwave energy will easily penetrate the backing layer 20 and then the facing layer 1 6 and will be effectively converted into heat energy in the refractory mixture that has been charged into the cavity 1 8 through the aperture 14 in the metal frame 1 2. The core box itself will not generate much heat.

At 24 is shown a parting line along which the core box 10 may be split into a pair of halves for the withdrawal of the cured mixture or core. The core halves may be held against each other following the withdrawal of one completed core, and th.e next batch of bonded refractory mixture may be charged into the cavity 1 8 for the fabrication of the next core. A large number of cores of the same shape and size can thus be manufactured by the same core box. For further details the reader is directed to our aforesaid European Patent Specification.

The invention will be further illustrated by reference to a number of worked Examples in accordance therewith.

Example I Silica sand was admixed with 1.0% by weight of water glass as a first binder, 1.0% by weight of alpha corn starch as a second binder, and 2.0% by weight of water. After well intermingling the listed ingredients, the water glass and starch bonded sand mixture was blown into a core box which was constructed in accordance with the teachings of Figs. 1 and 2 but which had a cylindrical cavity with a diameter of 50 millimeters (mm) and a length of 50 mm.

The core box containing the mixture was introduced into a microwave oven, in which the mixture was cured by exposure to microwave energy at 30 kilowatts (kw) for three minutes and 18 seconds.

The cores fabricated as above were then used for steel casting. After the usual vibratory shakeout process the castings were inspected for the attachment of sand and were found to be nearly free from sand. The sand that had been attached to the castings could be thoroughly removed by the application of shakeout shots.

Fig. 3 is a graphic illustration of the compressive strengths, at various temperatures, of the cores fabricated as above and, by way of comparison, of similar cores prepared by certain prior methods. It will be seen from this graph that the strength at room temperature of the cores constructed as in the present Example is adequately high, being 22 kilograms per square centimeter (kg/cm2). The strength of the inventive cores rapidly decreases with temperatures, and the residual strength at 300"C is zero. This attests to the favourable disintegration of the cores after use for metal casting.

The prior art methods tested by way of comparison in this Example were as follows: 1. The sand recovered from used green sand and reFonditioned by a calciner was mixed with 5.5 wt.% water glass, and the mixture was cured by carbon dioxide gas.

2. "Pep-set" (trademark for the product manufactured by Ashland Chemical Co., of the U.S.) organic cores prepared by a process analogous with the cold box method.

3. Fresh silica sand (Mikawa 5.6) was mixed with 2.5 wt.% water glass, and the mixture was microwave cured.

4. Fresh silica sand (Mikawa 5.6) was mixed with 3 wt.% potassium silicate, and the mixture was microwave cured.

Fig. 4 graphically represents the surface stability of cores fabricated through the procedure of Example I and of those by comparative prior methods. The surface stability was tested by rolling cylindrical test pieces, each having a diameter of 50 mm and a length of 50 mm, over an approximately 10-mesh sieve for 24 hours. Fig. 4 gives the ratios of the weights of the thus treated test pieces to their initial weights in percent. The surface stability of the cores produced by a method in accordance with the invention has proved to be adequately high (99.5%) in comparison with cores produced by prior methods.

Example II Silica sand (Yunotsu) was mixed with water glass, alpha starch, and water in various sets of proportions set forth in the Table below. In preparing these mixtures the silica sand was charged into a commercially available universal mixer and was agitated for 30 seconds for uniform dispersion of the various sized grains. Then a pasty mixture of starch and water, prepared separately, was charged into the mixer, and the whole intermingled for two minutes. Then water glass was introduced into the mixer, and the materials were intermingled for five minutes.

The water glass and starch bonded sand mixture prepared as above was then introduced into a core box of the same construction as that used in Example I. Then the core box containing the mixture was placed in a microwave oven, in which the mixture was subjected to microwave energy at 7 kw for two minutes.

The following Table lists the compressive strengths, at room temperature, of the cores of several different compositions fabricated as above: Compressive Strength (kg/cm2) Alpha Starch (parts) 0.5 1.0 2.0 3.0

0.5 .... 9.8 18.0 29.3 1.0 11.2 18.3 26.0 31.1 2.0 2.0 40.8 38.2 57.6 52.4 v, , < 3.0 46.6 58.1 64.4 4.0 90.9 85.1 av 3 5.0 134.5 93.2 ....

1.0 2.0 4.0 4.5 Water (parts) From the above tabulated results, as well as from the results of additional experimentation conducted by the applicant, the proportions of inorganic binder and starch are selected in the range of 0.5-3.0 wt.% and 0.5-2.0 wt.%, respectively, with respect to the amount of refractory particles, for good results. Our experiments also show that water should be limited to the range of 0-5.0 (and preferably 0.5-3.0) wt.% for good results.

Claims (9)

1. A process for making a core for use in metal casting, the process comprising the steps of: providing a mixture comprising refractory particles, a first binder consisting of an inorganic substance and a second binder consisting of starch, and, optionally, water; charging the said mixture into a core box having a cavity of predetermined configuration and designed for effective exposure of the charged mixture to microwave energy, and curing the said mixture by exposing the core box containing the mixture to microwave energy.

2. A process for making a mould for use in metal casting, the process comprising the steps of: providing a mixture comprising refractory particles, a first binder consisting of an inorganic substance and a second binder consisting of starch, and, optionally, water; charging the said mixture into a mould box having a former or core of predetermined configuration and designed for effective exposure of the charged mixture to microwave energy; and curing the mixture by exposing the mould box containing the mixture to microwave energy.

3. A process according to Claim 1 or Claim 2, wherein, apart from incidental impurities, the said mixture consists of refractory particles, from 0.5 to 3.0 percent by weight of said first binder, from 0.5 to 2.0 percent by weight of said second binder and from 0 to 5.0 percent by weight of water, the percentages being by reference to the weight of the refractory particles.

4. A process according to any preceding claim, wherein the proportion of water is in the range of 0.5-3.0 percent by weight of the refractory particles.

5. A process according to any preceding claim, wherein the mixture is prepared by: intermingling the second binder and water into paste-like form; adding the first binder and the paste-like mixture of the second binder and water to the refractory particles; and intermingling the refractory particles, the first binder, and the paste-like mixture of the second binder and water.

6. A process according to any preceding claim, wherein the first binder is a silicate.

7. A process according to any preceding claim, wherein the second binder is an alpha starch.

8. A process according to any preceding claim, wherein the refractory particles are silica sand.

9. Substantially as hereinbefore described with reference to the Examples and/or to the accompanying drawings, a process for making a core or a mould for use in metal casting.