WO1999058268A1

WO1999058268A1 - An infiltrated article prepared from particles covered with water glass

Info

Publication number: WO1999058268A1
Application number: PCT/DK1999/000248
Authority: WO
Inventors: Ole Huusmann
Original assignee: Dti Industri
Priority date: 1998-05-11
Filing date: 1999-05-05
Publication date: 1999-11-18
Also published as: AU3595499A; DK173646B1; NO20005645D0; EP1089841A1; NO20005645L; DK63998A

Abstract

An article prepared from particles covered with a layer of water glass is disclosed, wherein the water glass has a weight module between 1.8 and 3.5 and the article is infiltrated. The infiltration of the article results in an enhanced strength. Furthermore, a process for the preparation of an article is disclosed, wherein a green article is infiltrated by allowing a liquid material to be absorbed and subsequently solidified.

Description

AN INFILTRATED ARTICLE PREPARED FROM PARTICLES COVERED WITH WATER GLASS

The present invention relates to an infiltrated article prepared from particles covered with a layer of water glass. Furthermore, the invention relates to a process for preparing such articles.

Articles containing particles adhered together by a binder are used within many technical fields of application. Especially when casting iron or plastic objects, such articles have been used as moulds or cores. Usually, particles of quartz sand and binders of water glass are used.

The use of water glass as a binder to adhere quartz sand was introduced in foundries about 1920. In said process, sand is mixed with an aqueous solution of water glass, whereupon the mixture is teemed into a mould and cured by water evaporation. Said process was substantially improved about 1950 when the C0₂ process was developed. By said process, C0„ gas is led through the wet mixture of sand and water glass with a view to cure the binder. Cores or moulds obtained by the CO- process are relatively porous and their strength is poor. In the late 1960's, self-curing binder systems were developed, wherein the curing takes place by adding an organic ester to the wet mixture of sand and water glass prior to the mould being teemed.

In the late 1970' s, a dry curing process was developed, in which energy from hot air, a hot mould or microwaves make the water glass binder cure.

In the early 1990' s, the present inventor developed a curing process using injection of hot air into a ventilated mould by a three-stage temperature cycle, cf . "Miljøprojekt nr. 189, Miljøstyrelsen" . However, all above-mentioned processes, in which water glass is used as a binder, have the disadvantage that the flowability of the mixture of sand, water glass and water is relatively poor when the mould is teemed. In consequence, the mixture will tend not to flow into all mould cavities. Accordingly, the prior art processes, in which water glass is used, are limited to application in connection with moulds not having a detailed surface. Realizing the shortcomings of the prior art processes, the present inventor invented a process for the preparation of particles covered with water glass having a weight module, i.e. a Si0₂/Na-0 ratio, of 1.8 to 3.5 as well as articles prepared from such water glass covered particles. Said invention has been disclosed in International Patent Application No. PCT/DK97/00575, which had not yet been published on the date of priority of the present application.

Processes for the preparation of particles covered with water glass have been suggested earlier. US patent No. 3074802 discloses a process for the preparation of particles covered with water glass that can be cured by contact with C0_? . According to said prior art it is preferred to use a SiO_^/Na^O ratio of 0.5 to 1.2. Especially preferred is water glass having a SiO^/Na^O of 2/3.

The Finnish laid-open publication No. 89565 relates to grains of sand covered with water glass comprising a stable amount of crystallization water. Preferably, a water glass having a Si0₂/ a₂0 ratio of 1 and comprising 5 or 9 molecules of crystallization water is used. An article is prepared by heating the particles to a point above the water glass melting point . The melting point of a water glass having a Si0₂/Na₂0 ratio of 1 and comprising 9 molecules of crystallization water is 49°C.

The types of water glass used in the prior art have a high water solubility and a low melting point and will thus disintegrate when contacted with aqueous solutions and hot liquids or objects.

The articles obtainable by PCT/DK97/00575 have sufficient strength for many purposes. For instance, the articles are excellent for moulds or cores for metal casting. However, for certain fields of applica- tion enhanced strength may be desired. In consequence, it is the object of the present invention to provide such articles of enhanced strength as well as a process for the preparation of articles having enhanced strength. This object of the invention is met in providing an article prepared from particles covered with a layer of water glass having a weight module between 1.8 and 3.5 and in that the article is infiltrated.

In the present description and claims, the term "water glass" is to be understood as lithium, sodium or potassium silicates. The lithium, sodium or potassium component (M_pO, M = Li, Na , or K) may be present in variable amounts in proportion to the silicate component (Si0₂) . The ratio SiC /lVLO is designated the weight module. A water glass having a low weight module is easily soluble in water and strongly basic due to a high content of the basic component M₂0. Correspondingly, a water glass having a high weight module is less basic and less water soluble. In the present invention water glass having a weight module between

1.8 and 3.5 is used for the covering of the particles. If water glass having a lower weight module is used, the article will disintegrate totally or partially during the infiltration. Furthermore, it is preferred that M = Na. Unless otherwise specified, in the follow- ing, the term "water glass" refers to water glass in which M = Na.

The particles to be covered with a layer of water glass may be any material to which water glass can adhere .

Examples of such materials are metallic materials and ceramics. Examples of usable metallic materials include aluminium, copper, iron, wolfram, chrome, vanadium, molybdenum and manganese. Examples of ceramics are Si0₂ (quartz sand), TiC>₂ , Zr0₂ , Al₂0₃, CaO and MgO.

The size and size distribution of the particles is not essential when preparing the covered particles. For mould and core boxes, particles having a size between 0.05 mm and 2.0 mm, especially between 0.10 and 0.60 mm, may be used. For castings requiring a high degree of details and a reproduction of the design as precise as possible, particles having a diameter of 10 to 100 μm may be used. The selected particles may have a broad or narrow particle size distribution, or the particle mass may consist of so-called double-sieved particles (double-sieved sand) showing two peaks on the grain size curve.

The particles of the article may be bonded by the layer of water glass, or the particles may be sintered together so that a direct contact between adjacent particles exist.

The article according to the invention is preferably infiltrated by water glass and/or a metal or a metal alloy. Optionally, the article may be infiltrated with a binder, for instance of the epoxy type.

In case the article is infiltrated with water glass, the water glass is chosen with a view to ensure the desired solubility. Water- glass having a maximum weight module of appr. 3 may be dissolved directly by water contact at ambient temperature within a foresee- able time frame, whereas water glass having a weight module above appr. 3.0, e.g. a weight module between 3.0 and 3.5, is sparingly soluble in water. Usually, a durable article that does not dissolve or disintegrate at high air humidity is desired, possibly combined with a temperature above normal ambient temperature. Therefore, a water glass having a weight module between 3 and 4, especially between 3.0 and 3.5, is often chosen. For certain purposes, especially cores to be removed after completed casting, it is advantageous to use easily soluble water glass for the infiltration since such a core can be removed by hot water cleansing. Relatively easily water soluble articles may for instance be infiltrated with a water glass having a weight module between 1.8 and 3.0.

When infiltrating with water glass, in addition to an enhanced article strength, a complete or partial filling of the pores is obtained so that a smooth surface of the article appears. In case the article according to the invention is infiltrated with metal or metal alloy, said metal or metal alloy may be randomly_, chosen from among known metals or metal alloys. It is preferred to use copper or a cupriferous alloy for the infiltration of the article according to the invention. Especially, it is preferred to use a bronze, e.g. containing appr. 90% by weight copper and appr. 10% by weight tin. Examples of applicable metals to be used either alone or combined as alloys are aluminium, antimony, bismuth, cadmium, calcium, cobalt, copper, gold, iron, lead, magnesium, manganese, mercury, nickel, silver, thallium, tin and zinc.

The article according to the invention may have any outer shape. Since the particles covered with a layer of water glass are very easily flowing when dry and easy to pound into all cavities of a mould when a little humid, articles may be prepared as casts of original models having a very detailed surface. For instance, the article according to the invention may be a positive or a negative cast of a model. A negative cast may be used as a mould for preparing a copy of the original model, whereas the positive cast per se represents a complete or partial cast of the original model . When used for iron or metal casting, or for die- casting of plastics, the article according to the invention may constitute a mould and/or a core. The article according to the invention is applicable for other purposes as well, such as catalysts or carriers thereof, spark electrodes, electrodes for electrolysis etc .

The article according to the invention may be prepared by a process for the preparation of an article, wherein particles covered with a layer of water glass are provided in a mould, the particle mass is cured into a green article, and the green article is infiltrated by allowing a . liquid material to be absorbed and subsequently solidified.

The particles covered with a layer of water glass are preferably prepared by

(a) providing a mixture of particles to be covered, water and 0.1-5% by weight water glass, calculated on the basis of the weight of the particles, wherein the water glass is present in dissolved form,

(b) stirring the mixture mechanically, optionally simultaneously supplying heat from an external heat source, and allowing the water to evaporate until at least so much water has evaporated from the mixture that it is no longer sticky. 7

The amount of water present in the mixture in step (a) depends on several factors, such as the specific surface, the porosity and the electrostatic nature of the particles, but, preferably, calculated on the basis of the weight of the particles to be covered, it is at least 0.1% by weight. Typically, an amount of 1-3% by weight is chosen to ensure adequate humidity of the particles. Preferably, the amount of water does not exceed 5% by weight since a higher water supply does not contribute further to the humidity of the surface of the particles to be covered.

The amount of water glass in the mixture comprising particles to be covered, water and water glass depends on factors such as the desired layer thickness and the specific surface of the particles to be covered. On the basis of the weight of the particles, 0.1-5% by weight water glass may be used. Generally, it is preferred to use 1-3% by weight.

The mixture of step (a) is preferably obtained by (al) mixing water and particles to be covered,

(a2) stirring the mixture so as to have the water evenly distributed in the particle mass, (a3) adding 0.1-5% by weight water glass to the mixture , and (a4) continuing stirring until the water glass is evenly distributed and dissolved.

In step (a3), the water glass is mainly added to the mixture in a solid form as it is preferred to use particulate solid water glass prepared by spray drying. It has proven expedient to keep stirring during the entire preparation of the particles covered with a layer of water glass. Said stirring is made mechanically, mainly using rotating blades. The stirring speed is adjusted so as to ensure that no cured lumps of par- tides are formed, which would have to be subsequently 8 crushed.

Since the stickiness of the mixture increases significantly during the water evaporation, it has proven practical to use a container of plastic for the mixture as water glass hardly sticks to plastic. The stirring is mainly performed with such an intensity that the mixture is heated and the water thus evaporated. If desired, the evaporation rate may be increased by supplying heat from an external source and/or evaporation may take place in vacuum. If desired, the particles used may be preheated before being mixed with water and water glass. Said procedure is particularly advantageous if the water glass is sparingly soluble, i.e. has a weight module between 3.0 and 4.0, in which case the particles may preferably be heated to a temperature not exceeding 100°C, preferably 80-90°C, before being mixed with water and water glass.

It must be ensured that the mixing leaves no lumps of either sand or water glass before essential evapor- ation of the water is allowed. During the evaporation of the water from the mixture, the viscosity of same increases, and after a while, the mixture turns sticky so that the particles tend to stick both to each other and to the apparatus used. The stirring must be adapted to said stickiness tendency so as to prevent a too strong mutual binding of the particles. When the water content of the water glass layer has dropped below the lowest level allowing mutual binding of the particles, the viscosity decreases again. The stirring may con- tinue until all water has evaporated and the dry particles covered with a layer of water glass have been obtained, however, the humid and nonsticky covered particles may be removed at an earlier stage if desired. Unexpectedly, it has turned out that the flowability of such particles covered with water glass is excellent, both when they are dry and slightly humid, but not when they are sticky.

Presumerably, said flowability is obtained both as a result of the layer of water glass being smooth and hard and because the particles covered interact during the drying so that the covered particles obtained are rounder than the uncovered particles .

A green article may be prepared by

(c) providing the particles covered with water glass in a mould,

(d) ensuring the presence of water for the activation of water glass in the particle mass, and

(e) curing the particles covered with water glass in the mould into a green article by supplying energy from a source thereof.

The particles covered with water glass may be provided in the mould in an arbitrary way. In a first preferred embodiment, loose particles are teemed into a mould, and the mould is afterwards being slightly vibrated to ensure teeming of all mould cavities in order to obtain a tight and homogenous stuffing. Such a vibration will make the small particles move towards the surface of the particle mass and thus increase the density of the surface of the green article. After the vibration of the loosely stuffed particles, under the influence of the particles an after-vibration at a suitable pressure may be carried out, e.g. using a plumb, to further a tighter particle stuffing.

In another preferred embodiment, the particles covered with water glass are provided in the mould by being blown into same by means of an airflow. The supporting airflow escapes though mould valves, and the particles will be stuffed in the mould under the influence of the pressure of the airflow. In yet another preferred embodiment, the particles 10 covered with water glass may be provided in the mould by being extruded into same by a process designated "impact moulding". In said procedure, the particles are pounded into the mould under the influence of a high pressure, e.g. obtained by suddenly released pressure air.

In yet another preferred embodiment, slightly humid particles covered with a layer of water glass are provided in a mould, whereby it is ensured that the humid covered articles fill out all cavities of the mould, e.g. by pounding the particle mass or vibrating the mould containing the particle mass as described above. Said process entails the advantage that the amount of water necessary for the activation is already present in the particle mass.

The presence of water in the particle mass and the supply of energy from a source thereof will activate the water glass so that a coherent green article is formed. Therefore, the presence of water for the activation of the water glass in the particle mass must be ensured. Said water may for instance be present in the form of crystallization water, it may be added as water vapour, or the particles used may be humidified by a small quantity of water prior to the teeming of the mould, e.g. 0.1 to 0.7% by weight water, preferably appr. 0.3% by weight. The energy source for the curing may for instance be a source of microwaves or high- frequency waves, hot air, convection heat or steam.

In a first preferred embodiment vapour is conveyed through the dry particles covered with water glass provided in a mould to activate the water glass layer. Subsequently, pressure air at a temperature of 160- 200°C is added to cause a further heating of the covered particles and an initial evaporation of the water. Then the temperature is lowered to 80-160°C to 11 remove the water from the green article prepared. Optionally, a pressure air temperature of 0-80°C may be used at last to cool the green article and the mould. During the humidification of the particles covered with water glass in the mould, it is important to make sure that an essentially homogenous humidification of all areas of the mould takes place, without the water glass being flushed off the particles.

In a variant of said embodiment, particles covered with water glass humidified with water, e.g. up to 0.7% by weight water is used instead of vapour for the humidification of the covered particles. The pressure of the pressure air and the duration of the various temperature periods vary, dependent on the amount of water used for humidification, the size of the green article, the amount of used water glass etc., and said time periods may be determined by the person skilled in the art by routine tests. When preparing a 10 kg article, in which the particles are quartz sand having a medium particle size of 0.30 mm covered with a water glass amount of 0.8% by weight (module 2.0) and humidified by a water amount of, 1% by weight at an air pressure of 700 kPa, the typical duration periods of the various temperatures are: 10 sec of pressure air at a temperature of 160-200°C, 30 sec at a temperature of 80-160°C and 20 sec at ambient temperature. When changing the temperature, it has turned out to be advantageous to maintain the same pressure to prevent the green article from breaking. In another preferred embodiment, the particles covered with water glass are cured by means of microwaves or high-frequency waves. The water necessary for the activation of the water glass may be present as crystallization water in the water glass layer, it may be added by using humid particles having a water 12 content of for instance 0.1-0.7% by weight or be provided by supply of water vapour. As far as the latter option is concerned, it has turned out to be possible to obtain curing of an article by placing a mould containing particles covered with water glass in a microwave oven, whereby the mould inlet is facing a moist blotting paper.

A third preferred embodiment relates to the use of moulds receiving heat by convection, e.g. by placing the moulds in an oven, placing the moulds on a heating plate, or a mould with a heating jacket may be used. If the mould containing a moist particle mass is heat- treated in an oven, an oven having air circulation is preferably used in order to ensure a more homogenous distribution of the heat and thus a more homogeneous curing of the green article. The oven is mainly heated to a temperature of 130-200°C. The duration of the heat-treatment in the oven depends on the size and wall thickness of the green article. No matter which of the three above-mentioned embodiments is used for the curing a green article, an article comprising a large amount of particles adhered together by a layer of water glass is obtained. Surprisingly, it has turned out to be possible to produce green articles that do not contract essentially during the curing so that an essentially precise cast of the mould is obtained. Said property is particularly advantageous when casting a model with a view to produce an article to be used as a mould for preparing essentially identical copies of models.

Furthermore, it is possible to produce a green article as a cast of a model, making the size of the cast larger or smaller than the surface of the model by means of adjusting the temperature of the model. Said property is particularly advantageous if the model to 13 be copied is a spare part from which an oversize cast is desired.

It has turned out that a green article produced from metal particles covered with water glass is not conductive, which is an indication of the completeness of the layers around the particles.

If desired, in a subsequent step, the green article may be heated to a temperature corresponding to or above the temperature when sintering the particles so that a direct contact is obtained between the particles. The direct contact between the particles lead to an increase of the strength of the green article and enhanced resistance to water or steam. The temperature chosen for the sintering depends on the material of the core. Typically, a temperature of 10-

40 °C, especially 15-30 °C above the minimum temperature when sintering particles without the presence of water glass, will be chosen. Preferably, the sintering period should be adjusted not to destabilize the green article.

Before the green article is infiltrated, if desired, it may be worked so ,as to achieve the desired shape .

The green article according to the invention may be infiltrated by a process comprising the steps of contacting the green article with a solution of water glass so that part of the solution is absorbed therein, heat-treating the green article, wherein a sol- ution of water glass has been absorbed so that the water glass is solidified, and, optionally, repeating the preceding steps in order to obtain an infiltrated article.

The solution of water glass is usually aqueous, which makes it less detrimental to health, but other 14 organic or inorganic solvents, in which water glass is soluble, are applicable. Since the solvent of the solution is to be removed in the subsequent step, a solution having a relatively high content of water glass is preferred. A solution having water glass and water in the weight ratio 1:2 has turned out to be suitable. The water glass may have a high or a low weight module, depending on whether a low or high water solubility is requested for the infiltrated article. If a low water solubility of the article is to be obtained, water glass having a weight module of 3.0 to 4.0, particularly 3.0 to 3.5, has turned out to be appropriate .

The green article may be contacted with a solution of water glass in any suitable manner. For instance, the solution may be applied by spraying or brushing, or the article may be submerged into the solution.

The green article may be infiltrated in part or totally with the solution of water glass. By a partial infiltration, the solution of water glass will penetrate into the outer layer of the green article so that a reinforced shell is obtained around the article. By a total infiltration all air is displaced from the article and replaced by the solution of water glass so that an article is obtained in which the entire article is reinforced. By partial infiltration the penetration depth may be controlled, e.g. by adjusting the submersion time of the green article in the solution of water glass. If a partial infiltration is desired, the article may be submerged in the solution of water glass for 1-20 sec, preferably 2-10 sec. In case a longer submersion time of the article in the solution of water glass is desired, it is advisable that the water glass layer around the particles consists of water glass that is sparingly soluble in the solvent so that disintegra- 15 tion is prevented.

The green article, in which a solution of water glass has been absorbed, may be heat-treated in any suitable manner ensuring the solidification of the water glass in the green article. By the heat-treatment, the solvent evaporates and the water glass remains in the article and supports the article structure. By the heat treatment possibly present crystallization water may be released as well. The heat treatment is usually carried out at a temperature of 100 to 250°C, preferably at a temperature of 120 to 180°C. The temperature chosen for the heat treatment, inter alia, depends on the water glass used since water glass having various weight modules may comprise crystallization water that is being released at various temperatures. It may be desirable to carry out the heat treatment at a higher temperature than mentioned above in order to release possibly present crystallization water, especially if the infiltrated article is foreseen to be used at higher temperatures .

When infiltrating with water glass an infiltrated article having enhanced strength is obtained. Primarily, because the water glass in the infiltration liquid deposits within the porous structure of the green article. By the evaporation of the solvent during the heat treatment a porous infiltrated article according to the invention is obtained that may be used for many purposes, e.g. iron and metal casting. A further strength enhancement may be obtained by mixing the solution of water glass with a metal powder having a diameter smaller than the pore diameter of the green article. When contacting the green article with the solution of water glass with suspended metal particles, metal particles and water glass will infil- 16 trate the green article and solidify therein at the subsequent heat treatment .

It may be desirable to obtain an article having a non-porous surface. This may be obtained by repeating the above-mentioned steps, i.e. contacting the infiltrated article with a solution of water glass and subsequently carrying out a heat treatment . The surface of the article that has been infiltrated once or several times is smooth and non-porous and may thus advantageously be used as a mould, e.g. for injection moulding.

In another preferred embodiment according to the invention, the green article is infiltrated with a metal or a metal alloy. Preferably, the article is infiltrated by contacting the green article with a melted metal or a melted metal alloy so that melted metal or metal alloy is absorbed in the article, cooling the article comprising melted metal or metal alloy with a view to making the metal or the metal alloy solidify, and, optionally, repeating the preceding, steps in order to obtain an infiltrated article. Usually, the green article is infiltrated by being placed in an oven in physical contact with the metal in a solid form. It is preferred to use a briquette comprising compressed metal powder. The heat treatment in the oven usually consists of two steps, whereby an intermediate sintering step may be foreseen. In a first step, the oven is preheated to a temperature below the melting point of the metal for infiltration to ensure the core temperature of the article being sufficiently high. In a second step, heating continues until the melting point of the metal or the metal alloy has been reached or passed, whereby the metal will flow into the 17 pores of the green article. The duration of the first and the second step inter alia depends on the wall thickness of the green article.

The sintering step that may optionally be placed in between the two steps mentioned, may be carried out by heating to a temperature corresponding to or above such a temperature at which sintering between the particles surrounded by water glass will take place. The duration of the sintering step will be kept at a level at which the green article does not essentially disintegrate .

Preferably, the green article substantially has the same temperature as the melting temperature of the metal or the metal alloy, or, during the infiltration, a higher temperature. Accordingly, the metal or the metal alloy will infiltrate the green article completely so that the total strength of the porous structure of the green article is enhanced. If only penetration of the outer layers of the green article is desired, the core temperature of the green article may be kept lower than the melting temperature of the metal or the metal alloy for infiltration.

It may be necessary to support the green article during the infiltration of same since water glass softens when heated to the melting point of many metals or the metal alloys for infiltration. For instance, water glass has a softening point at 640°C in case of a weight module of 3. However, the actual melting point of water glass is above the melting point of most relevant metals or metal alloys. Therefore, to obtain a successful result it is usually a necessary prerequisite to choose a metal or a metal alloy having a melting point below the melting point of the water glass covering the particles. By complete infiltration of the green article with 18 a metal or a metal alloy, a surplus of infiltrating metal or metal alloy is preferably used to ensure the complete infiltration. Before the cooling of the infiltrated green article with a view to obtain a solidification of metal or metal alloy, excessive metal/metal alloy is usually removed from the article.

The green article may be infiltrated with metal or metal alloy by any prior art method known to the person skilled in the art, e.g. as described in Metals Hand- book, 9th Edition, Volume 7, pages 551-566.

In the following the invention will be illustrated by way of examples. The examples are not to be considered limiting to the scope of protection defined in the claims .

EXAMPLES Example 1

Preparation of particles of stainless steel covered with water glass 6.0 kg particles of stainless steel having particle sizes from 22 to 53 mμ were placed in a cylindrical plastic container having, a diameter of 200 mm and a height of 190 mm. The plastic container was supplied with a stirrer comprising a central axis from where four blades extended. The length of the blades was 95 mm. The stirrer was started and adjusted to a rotating speed of 450 rpm.

During the stirring, 180 ml water was slowly admixed to the particles followed by a mixing for appr. 1 min to distribute the water within the particles. Subsequently, 180 g solid water glass having a weight module of 2.0 was admixed to the mixture of particles and water. A practically instant solvation of the water glass was observed. The stirring continued for appr. 45 min consisting 19 of two periods, viz.

Period 1: 0-30 min after the admixture of water glass, the mixture was heated by the mechanical energy of the stirrer and water was allowed to freely evapor- ate;

Period 2: 30-45 min after the admixture of water glass, the water evaporation was so advanced that the water glass began turning sticky and a tendency towards formation of loosely coherent agglomerates was noticed. Towards the end of the time interval, the stickiness of the mixture decreased again as the amount of water in the mixture had been reduced to a point lower than the lowest adhering level and the agglomerates were broken by the stirring. At the end of the period, the water content of the mixture amounted to appr. 0.7% by weight .

The product obtained was examined in a microscope and showed an even and smooth layer of water glass, which was assumed to be the reason for the easy flow- ability.

Example 2

Preparation of an article comprising particles of stainless steel covered with water glass The aqueous water glass covered stainless steel particles obtained in Example 1 were teemed into a mould and pounded sufficiently to ensure the filling of all cavities. The silicone mould was a cast of an object from which a copy was desired. The mould was cylindrical having a diameter of 41.1 mm and a pattern at the bottom.

The mould teemed with the water glass covered particles was placed in an oven at 150°C for 40 min. Subsequently, the mould was removed from the oven and cooled to ambient temperature. The green article 20 obtained showed the same dimensions as the original object from which the mould was a cast. The weight of the green article was 100 g.

Example 3

Preparation of an article infiltrated with water glass

The green article obtained in Example 2 was completely submerged into a solution consisting of one part of water glass (Crossfield P60) having a weight module of 3.2 and two parts of water for four seconds. Thus, 4% by weight of the solution of water glass was absorbed by the green article. The article containing the solution of water glass was subsequently placed in an oven at 150°C for 40 min.

The cooled treated article was once more completely submerged into the above-mentioned solution of water glass for four seconds. Thus, 2% by weight of the solution of water glass was absorbed. The article containing the solution of water glass was subsequently placed in an oven at 150°C for 40 min.

An infiltrated article , having a smooth surface suitable for use in connection with injection moulding was obtained. The weight of the infiltrated article amounted to 102 g. The compression strength of the infiltrated article was almost 10 times higher than the compression strength of the green article. When slitting the infiltrated article it was noticed that by the infiltration a shell of water glass had been formed which had penetrated the outer layer of the article. 21

Example 4

Preparation of an article infiltrated with water glass and metal particles

A solution containing 1 part of water glass having a weight module 3.2 and 2 parts of water was prepared. To said solution iron powder having a diameter of 1-40 μm was admixed so that a solidification occurred.

A green article as obtained in Example 2 but prepared from particles of iron having a diameter of 40-100 μM was brushed with the solidification. The article was subsequently placed in an oven at 130°C for

40 min.

After cooling of the article it was sprayed with a solution of water glass (module 3.2) and water in the ratio 1:2 by weight. The article was placed in an oven at 130°C for 40 min.

An article infiltrated with both water glass and iron powder was obtained. The surface of the article was smooth. The compression strength was higher than that of the article prepared in Example 3.

Example 5

Preparation of an article completely infiltrated with water glass The green article obtained by Example 2 was completely submerged into a solution of one part of water glass (Crossfield P60) having a weight module of 3.2 and two parts of water until all air had been displaced. The article containing the solution of water glass was subsequently placed in a cold oven with air circulation in order to cure the surface for 15 min.

In the course of 30 min the oven containing the infiltrated article was heated to 130°C. 22

Example 6

Preparing an article infiltrated with metal

A briquette of compressed bronze was placed in a ceramic crucible. The bronze contained 90% by weight Cu and 10% by weight tin. The green article was placed on top of the briquette and both were exposed to a preheating in an oven at a temperature of 700°C for 20 min. Subsequently, the temperature was increased, and a temperature of 1,120°C was maintained for 20 min, which made the bronze melt and penetrate into the green article .

After cooling the article proved to be completely infiltrated. The weight of the infiltrated article amounted to 165 g. The infiltrated article was resis- tant to compression strengths above 250 MPa.

Claims

23P A T E N T C L A I M S

1. An article prepared from particles covered with a layer of water glass, wherein the water glass has a weight module between 1.8 and 3.5 and the article has been infiltrated.

2. An article according to claim 1, wherein the particles are adhered together by the water glass layer.

3. An article according to claim 1, wherein the particles are sintered together so that a direct contact exists between adjacent particles.

4. An article according to any of the claims 1 to

3, wherein the article has been infiltrated with water glass . 5. An article according to any of the claims 1 to

4, wherein the article has been infiltrated with water glass having a weight module between 3.0 and 4.0, preferably between 3.0 and 3.

5.

6. An article according to any of the claims 1 to 5, wherein the article has been infiltrated with a metal or a metal alloy.

7. An article according to claim 6, wherein the article has been infiltrated with copper or a cupriferous alloy.

8. A process for the preparation of an article, wherein particles covered with a layer of water glass are provided in a mould, the particle mass is cured into a green article, and the green article is infiltrated by allowing a liquid material to be absorbed and subsequently solidified.

9. A process according to claim 8, wherein the particles covered with a layer of water glass are prepared by (a) providing a mixture of particles to be covered, water and 0.1-5% by weight water glass, calcu- 24 lated on the basis of the weight of the particles wherein the water glass is present in dissolved form,

(b) stirring the mixture mechanically, optionally 5 simultaneously supplying heat from an external heat source, and allowing the water to evaporate until at least so much water has evaporated from the mixture that it is no longer sticky.

10. A process according to claim 8 or 9 , wherein 0 the green article is prepared by

(c) providing the particles covered with water glass in a mould,

(d) ensuring the presence of water for the activation of water glass in the particle mass, and 5 (e) curing the particles covered with water glass in the mould into a green article by supplying energy from a source thereof .

11. A process according to claim 8, wherein step

(e) is followed by a sintering step in which the green 0 article is heated to such a temperature that the particles are sintered together so that a direct contact between the articles is obtained.

12. A process according to any of the claims 8 to 11, wherein the liquid material for the infiltration is 5 a solution of water glass.

13. A process according to claim 12, wherein the solution of water glass is aqueous and the water glass has a weight module of 3.0 to 4.0.

14. A process according to any of the claims 8 to 30 13 comprising the steps of contacting the green article with a solution of water glass so that part of the solution is absorbed therein, heat-treating the green article, wherein a sol- 35 ution of water glass has been absorbed so that 25 the water glass is solidified, and, optionally, repeating the preceding steps in order to obtain an infiltrated article.

15. A process according to claim 14, wherein the solution of water glass contains solidiefied metal or metallic oxide powder having a diameter that is smaller than the pore diameter of the green article.

16. A process according to any of the claims 8 to 15, wherein the heat treatment is carried out at a temperature from 100 to 250┬░C, preferably at a temperature form 120 to 180┬░C.

17. A process according to any of the claims 8 to 11, wherein the material for the infiltration is a metal or a metal alloy, e.g. copper or a cupriferous alloy.

18. A process according to claim 8 or 17 comprising the steps of contacting the green article with a melted metal or a melted metal alloy so that melted metal or metal alloy is absorbed in the article, cooling the article comprising melted metal or metal alloy to solidify, the metal or the metal alloy, and, optionally, repeating the preceding steps in order to obtain an infiltrated article.

19. A process according to claim 18, wherein, when contacted with the melted metal or the melted metal alloy, the article essentially has the same temperature or a higher temperature than the melting temperature of the metal or the metal alloy.

20. An infiltrated article obtainable according to any of the claims 8 to 19.

21. The use of the article, according to claims 1-7 or 20 as a core or a mould for iron or metal casting or for die-casting of plastics.