WO1997029873A1

WO1997029873A1 - Method of making moulds and cores using a binding agent capable of reversible phase conversion between the solid and the liquid state

Info

Publication number: WO1997029873A1
Application number: PCT/DK1997/000057
Authority: WO
Inventors: Preben Nordgård HANSEN
Original assignee: Georg Fischer Disa A/S
Priority date: 1996-02-15
Filing date: 1997-02-10
Publication date: 1997-08-21
Also published as: DK16396A; AU1718697A; DK171631B1

Abstract

In a method of making moulds and/or cores from e.g. mould sand and a binding agent capable of melting and solidifying when being heated or cooled, respectively, at a conversion temperature, the main novel feature is that said conversion temperature lies above the highest expected ambient temperature, preferably above 30 °C. By proceeding in this manner it is possible to achieve the same advantages as are known from the use of frozen moulds or cores (i.e. in which the binding agent is water) without having to alter existing equipment for making moulds and/or cores or providing same with cooling facilities. Further, by choosing a suitable binding agent with a conversion temperature (melting point) as mentioned, it is possible to make moulds and/or cores with high mechanical strength, especially advantageous when operating with automated foundry equipment.

Description

METHOD OF MAKING MOULDS AND CORES USING A BINDING AGENT CAPABLE OF REVERSIBLE PHASE CONVERSION BETWEEN THE SOLID AND THE LIQUID STATE

TECHNICAL FIELD

The present invention relates to a method of making moulds and/or cores in the manner set forth in the preamble of claim 1. The invention is intended for use when making disposable moulds and/or cores for use in making castings in metals and alloys, such as steel, iron, light metals, heavy metals etc.

BACKGROUND ART

Disposable moulds and cores of the kind referred to initially are conventionally made by using granular materials, such as quartz, zircon, olivine, magnetite etc., as well as mixtures of these materials (in the following generally designated "mould sand"), bound together by a binding agent, such as bentonite/water, furan, "hot-box", "cold-box", salt/water etc. It is also known to enhance the binding effect by freezing the water in a bentonite/water binding agent (EFFSET).

Further, it is known to use casting-mould parts made from mould sand mixed with a given quantity of water, the latter being frozen after the shaping of the mould. Because the water as such does not possess adhesive properties, it is necessary to freeze the surface regions of the mould parts, at least to a certain depth, so as to make it possible to remove the mould parts from the moulding equipment. If necessary, it is possible to accelerate the freezing process by cooling the elements to be mixed and the moulding equipment to or below freezing point, such as is known e.g. from US-A-4,453,586, or by introducing a freezing agent in a mould under vacuum as known from US-A-4,576,215.

This method is very beneficial to the environment, as the binding agent is water. During the casting process itself practically no smoke or fumes are produced, since the binding agent gives off nothing but steam. When the casting process is completed, the mould and/or the core can relatively easily be disintegrated by shaking or rinsing with hot water, and the sand can be re-used substantially immediately together with possible residue of the binding agent in the form of water. Thus, this method is generally free from noxious by-products in the form of gases etc., and may allow re-use of the mould parts to a high degree.

There is, however, the disadvantage with this known method that it is necessary that the mould parts be cooled down to or below the freezing point of water and made to solidify for the binding agent to achieve its binding effect, thus requiring special equipment and considerations, and it can take some time to achieve a sufficient binding effect by freezing.

Further, it is necessary to keep the moulds cooled below freezing point, partly to allow them to solidify throughout their volume, partly to allow storage.

In addition to this, the properties of the binding agent during the phase transition from water to ice, during which the volume of the water increases, give rise to problems with dimensional changes when the mould is being made and when it is being poured. In the latter case, this can cause breakdown of the mould when the ice melts during the pouring process, its volume decreasing during the phase transition to water.

Some of the advantageous properties found in frozen moulds are also found in other types of moulds and cores, e.g. thermosetting moulds and cores in which salt binding agents are used. Thus, WO 89/05204 and US-A-5, 248, 552 disclose examples of methods in which mould sand is mixed with a salt binding agent dissolved in water, after which the mould parts are formed. The water is removed by evaporation, the desired binding effect being created by the crystallization of the salt from the aqueous solution. The salt being used must have a higher melting point than the mass to be poured into the mould later. Thus, the salt binding agent will typically attain its greatest binding strength after a heat treatment for expelling the solution water and/or during the pouring of the casting mass (metal). This process does not, however, produce mould parts with quite the same good properties as frozen moulds and cores, because the strength of the binding agent is not attained until the water has evaporated, for which reason the greatest strength of the mould parts will occur after the pouring process, and at this stage, they are most difficult to remove. Generally, moulds and cores having been produced with this method will require the use of large quantities of energy before the mould parts can be used, either to remove the water by heating the complete core or mould up above 100°C, or by storing the core or moulds for a longer period under specially dry conditions or under vacuum in order to expel the water.

DISCLOSURE OF THE INVENTION

It is the object of the present invention to provide a method of the kind referred to initially, with which the bonding of the material for the moulds or cores can be achieved with the same advantageous properties as are known from frozen moulds with water binding agents, at the same time making it possible to use ordinary sand-moulding equipment without the necessity of using special cooling equipment, and with which it is possible to store finished mould parts without the necessity of special cooling spaces.

Further, the method should be highly suitable for automated casting processes, whereas the moulds and the cores should attain and possess high mechanical strength prior to pouring, thus being very robust in handling, without the necessity of special heating or cooling.

This object is achieved by, according to the present invention, proceeding in the manner set forth in the characterizing clause of claim 1.

Thus, the present invention is based on the principle known from making moulds, in which a phase conversion of water to ice occurs for the binding agent, constituted by water.

In contradistinction to the known frozen moulds and cores, however, the binding agent being used is not water, but e.g. hydrates of salts having melting points above 30°C, so that they are in the solid state at the ambient or operating temperature in the foundry, at which temperature the binding agent possesses high strength.

When using this binding agent, moulds and/or cores are produced by heating the mould sand to a temperature above the melting point of the binding agent, and the latter is mixed with the mould sand to make the total mixture reach a temperature above the melting point of the binding agent, after which the mould or the core is shaped and cooled. In this process, the binding agent may be mixed with the mould sand prior to heating or after the process of heating the mould sand.

Alternatively, the binding agent can be heated to a temperature substantially above its melting point, and then mixed with mould sand having a temperature below this melting point, so that a quick solidification is achieved.

Thus, when shaping the mould sand into moulds or cores, it is possible to use dry mould sand, because the binding agent "lubricates" the sand, but if wet mould sand and e.g. salt hydrate are used, it is possible to use salt hydrate in which the water of crystallization has been expelled to a degree corresponding to the water content in the mould sand, so that the salt hydrate will bind substantially all the water in the finished mould or core. Thus, when using a binding agent possessing these properties it is possible to achieve the advantages known from frozen moulds without the need for using cooling equipment. Further, the consumption of large amounts of energy used for evaporating water from water-dissolved binding agents, and the long time required for the evaporation, are avoided. Further, the cores or moulds can be made without chemical reaction products arising as a consequence of the use of hardeners or solvents that can produce insoluble or environmentally harmful residues.

A binding agent consisting of a salt hydrate, especially fixing salt (sodium thiosulphate), is an example of such a binding agent having these properties.

Using fixing salt it is possible to make a core or a mould by heating the mould sand up to 65°C, after which a quantity of fixing salt of approximately 10% by weight is admixed to it. Then, the mixture is allowed to rest for approximately 5 minutes, after which it is again agitated and left to rest for another 5 minutes before use. The mixture is now ready for use, the fixing salt having melted and wets the grains of sand. In this process, the core can be moulded by hand or using a "core shooter" or using a further known method, e.g. based on the use of sand throwing, vibration and/or compression, or some other known method. After the shaping process, the core is cooled to room temperature, the fixing salt solidifying and providing a strong binding effect.

After this, the cores can be placed in wet-sand moulds produced according to the DISAMATIC® principle using e.g. quartz sand, after which a series of castings can be made.

In trials using binding agents on a fixing-salt basis, the water of crystallization in the fixing salt evaporates at the surface of the core when pouring-in the casting material, making this core surface hard as stone. In a trial with fixing salt in the form of sodium thiosulphate (Na₂S₂O₃ + 5 H₂O), melting point 48°C, it has been possible using this binding agent to produce cores with compressional strength above 13 kg/cm², with both quartz sand and magnetite sand and a proportion of sodium thiosulphate of approximately 8% by weight.

Further, these cores were easily removed, as they could simply be flushed out with hot water. Alternatively, it is also possible to melt them out, which in many cases can be achieved by letting the heat from the cast material propagate through the cores and/or the mould, after which the latter can easily be shaken out.

It is especially easy to re-use mould materials when using a binding agent that upon being heated firstly becomes liquid and, then possibly having its water of crystallization expelled, possibly finally to change into the gaseous state, without any of the remaining components of the binding agent in solid or liquid state having undergone irreversible chemical changes. In this case, the mould material may be re-used immediately, possibly adding extra binding agent to replace expelled binding agent (gas). Thus, with the method described above, it is possible to achieve:

- a low-cost procedure,

- simple re-use of the mould sand,

- a core with a water-soluble binding agent that is immune to humidity at room temperature, and that e.g. can be stored in the foundry for weeks without special measures having to be taken,

- uncomplicated storage without ventilation or other conditioning measures,

- simple removal of cores by dissolving in water or melting-out,

- that the maximum strength of the mould or core is attained prior to the pouring-in of the casting material,

- low energy consumption for making the moulds and/or cores, since it is not necessary to evaporate water to harden them, and

- easier coating, as it is not necessary to take account of the presence of water or to keep the mould parts frozen.

LIST OF SUBSTANCES THAT CAN BE USED AS BINDING AGENTS WHEN CARRYING OUT THE METHOD ACCORDING TO THE INVENTION

Salts

Hydroxides

Nitrates

Tartrates

Lactates

Oxalates

Chlorides

Silicates

Chromates

Phosphates

Asparagine

Iodates

Bathophenanthroline

Acetates

Citrates

Sulphates

Bromides

Molybdates

Acids

Claims

C L A I M S

1. Method of making moulds and/or cores from a granular base material and at least one binding agent, in which at least one binding agent is capable of undergoing a reversible phase transition between the solid and the liquid state at a certain conversion temperature, in which method

a) the granular base material and the binding agent or agents are mixed, at least one binding agent during the making of the moulds and/or cores possessing or achieving a temperature above said conversion temperature, and

b) subsequently, the moulds and/or cores are cooled to a temperature below the lowest conversion temperature for a phase-convertible binding agent in the mixture, c h a r a c t e r i z e d by

c) the use of such a binding agent or such binding agents, that the lowest conversion temperature for a phase-convertible binding agent in the mixture is above the highest expected normal ambient temperature, preferably above 30°C.

2. Method according to claim 1, c h a r a c t e ri z e d by the exclusive use of a binding agent or binding agents capable of undergoing a reversible phase conversion between the solid and the liquid state.

3. Method according to claim 1, c h a r a c t e ri z e d by the use of binding agents comprising at least one binding agent, e.g. a bentonite, that is not capable of undergoing a reversible phase conversion during the making of the moulds and/or cores.

4. Method according to claim 1, 2 or 3, c h a r a ct e r i z e d by the use of at least one phase-convertible binding agent chosen among hydrates of salts, such as sodium thiosulphate.

5. Method according to claim 1, 2, 3 or 4, c h a ra c t e r i z e d by the use of at least one phase-convertible binding agent chosen among waxes, paraffins and stearins.

6. Method according to any one or any of the claims 1-5, c h a r a c t e r i z e d by the use of a binding agent capable of being softened or melted-out by heating after the making of the mould and/or core.

7. Method according to any one or any of the claims 1-4, c h a r a c t e r i z e d by the use of a water-soluble binding agent.

8. Method according to any one or any of the claims 4-7, c h a r a c t e r i z e d by the use of at least one binding agent capable of receiving water of crystallization, when wet or moist base material, especially sand, is used for making the moulds and/or the cores.