WO2023237882A1

WO2023237882A1 - Inorganic water-soluble binder system

Info

Publication number: WO2023237882A1
Application number: PCT/GB2023/051488
Authority: WO
Inventors: Vincent HAANAPPEL
Original assignee: Foseco International Limited
Priority date: 2022-06-08
Filing date: 2023-06-08
Publication date: 2023-12-14
Also published as: JP2025518382A; EP4536424A1; MX2024014988A; KR20250022054A; CN119300929A

Abstract

A composition for making a core for use in a moulding or metal casting process. The composition comprises a particulate refractory material; and a binder composition. The binder composition comprises at least one alkali metal silicate, potassium carbonate; and at least one pozzolanic additive. The cores formed from the composition have high strength and are water-dispersible.

Description

Inorganic water-soluble binder system

Technical field

The present invention relates to a composition for use as a core in a casting or moulding process, a core comprising the composition, casting moulds comprising a core, and a method for producing an article using a core. In particular, the present invention relates to a core which can be washed out of the internal cavity of a cast or moulded article using water.

Background

In a gravity casting process, molten metal (or metal alloy) is poured into a pre-formed mould cavity which defines the external shape of the casting, with the molten metal filling the mould cavity under the force of gravity. The shape of hollow sections or internal cavities in the casting may be defined by a disposable core, which is typically made from hardened, resin-bonded sand. During casting, the extreme heat from the molten metal thermally decomposes the resin binder in the core, allowing the core to break down and be easily shaken out of the finished casting when the molten metal has cooled and solidified. The use of hardened, disposable, resin-bonded sand cores is also commonly applied in low pressure or gravity sand casting processes, and also in low pressure die casting.

However, in die casting processes where the metal is cooled very quickly or the walls of the casting are relatively thin, the core may not be exposed to enough heat during the casting process for the binder to decompose, making the core very difficult to remove after the casting has solidified. Examples of such die casting processes include high pressure die casting, semi-solid casting (such as rheocasting and thixocasting), and squeeze casting. In such processes, the core will be exposed to much lower temperatures and for only a limited amount of time, which is insufficient to thermally decompose conventional resin binders.

In traditional liquid high pressure die casting (HPDC), molten metal is injected at high speed into the mould cavity at high pressure and held in place by a compressive force until the metal solidifies. Moulds for use in die casting can be made from metal (such as steel) to withstand the high pressures and metal velocities, and are typically reusable. In addition to the extremely fast molten metal filling time, the thinner casting walls and the use of active cooling systems in the tools combine to result in a very fast solidification time.

Semi-solid casting is a variation of HPDC, wherein the metal (or metal alloy) is injected into the die in a semi-solid state rather than a fully molten state which improves kinematic viscosity and metal flowability and thus enables even thinner wall designs, reduced porosity and improvements in surface finish, elongation, fatigue, and tool life, among other advantages. In rheocasting, the metal is cooled from a fully molten state to a semi-solid state before injection, whereas in thixocasting solid metal is partially melted to a semi-solid state.

The use of a sand core to form the internal geometry in die casting applications is desirable, but problematic. The low thermal decomposition of the binder at the lower times and temperatures of HPDC makes sand removal after casting more difficult. Conventional sand cores may not have sufficient strength to withstand the pressures and metal velocities involved in many applications and may shatter during such use. Conventional resin-bonded sand cores are therefore unsuitable for use in most high pressure die casting processes. Although alternative methods for forming internal cavities in HPDC exist, they are not highly adopted due to process complexity and/or cost.

One alternative technology is to form the cores from a soluble salt. However, this technology is cost prohibitive for all but very niche applications. A further solution to the above problems of core strength and removability makes use of a moulding or core sand and an organic polymeric binder. Although it is possible to achieve cores and moulds with good strengths and water-solubility, a common problem is the generation of Volatile Organic Compounds (VOCs) as the organic binder thermally decomposes, particularly in moulding processes which require longer cycle times. VOCs introduce risks for the foundry workers and have other negative environmental impacts, and can be expensive to mitigate.

Some efforts have been made to use traditional sand core technology in HPDC applications and to some degree it is possible to achieve the required strengths to resist the forces during filling and solidification. However, these cores are extremely difficult to remove through high mechanical impact loading to the part. The thin wall and weak initial casting strength of the part can result in damages or cracks on the casting and a high residual debris from the core remaining in the cavity.

The present invention seeks to mitigate or ameliorate the abovementioned problems associated with removing an internal core from a cast or moulded article, or at least to provide a useful alternative.

Summary of the invention

Composition

According to a first aspect of the invention, there is provided a composition for making a core for use in a moulding or metal casting process. The composition may comprise a particulate refractory material. The composition may comprise a binder composition. The binder composition may comprise at least one alkali metal silicate. The binder composition may comprise potassium carbonate. The binder composition may comprise at least one pozzolanic additive.

The inventors of the present invention have found that cores made from the composition of the first aspect have sufficient strength to withstand the forces experienced during casting or moulding processes but can be washed out of an internal cavity of a cast or moulded article using only water. In particular, the composition does not require mechanical forces to remove the core from the moulded part and provides a high quality surface finish and a high level of cleanliness of the internal cavity. For the avoidance of doubt, as used herein, the term ‘solubility’ refers to the solubility of the binder composition. In reference to the cured cores formed from the composition, discussions of the ‘solubility’ are intended to refer to the waterdispersibility of the cured cores. It would be understood that the particulate refractory materials are typically non-soluble.

In some embodiments, a core made using the composition is water soluble even after being heated to at least 200°C. In some embodiments, a core made using the composition is water soluble after being heated to a temperature from 200 to 350°C. The binder may comprise 1.5 to 35wt% relative to the weight of the particulate refractory material. In some embodiments, the composition comprises at least 2, at least 3, at least 4, at least 5, at least 6, at least 7, at least 8, at least 9, at least 10, at least 11 , at least 12, at least 13, at least 14, at least 15, at least 16, at least 17, at least 18, at least 19, or at least 20 wt% binder composition based on the weight of particulate refractory material. In some embodiments, the composition comprises no more than 35, no more than 30, no more than 25, no more than 20, no more than 15, no more than 10, no more than 5, or no more than 2, wt% binder composition based on the weight of particulate refractory material. In some embodiments, the binder composition comprises from 0.5 to 30, from 1 to 20, from 2 to 15, or from 2 to 10 wt% binder composition based on the weight of particulate refractory material.

The binder composition may be an inorganic binder composition. Such embodiments are advantages since they avoid the generation of VOCs due to the binder burning or pyrolysing in contact with molten metal.

Alkali Metal Silicate

The at least one alkali metal silicate may comprise 1 to 15wt%, relative to the weight of the particulate refractory material. The at least one alkali metal silicate may comprise at least 2, 3, 4, 5, 6, 7, 8, 9, 10, 11 , 12, 13, or at least 14wt% relative to the weight of the particulate refractory material. In some embodiments the at least one alkali metal silicate may comprise no more than 14, 13, 12, 11 , 10, 9, 8, 7, 6, 5, 4, 3, or 2wt% or the particulate refractory material.

In some embodiments the at least one alkali metal silicate comprises sodium silicate. In some embodiments, the at least one alkali metal silicate comprises potassium silicate. In one series of embodiments, the at least one alkali metal silicate comprises sodium silicate and potassium silicate.

The alkali metal silicate may be in aqueous solution. The aqueous solution may have a solids content of between 30 and 50 wt%. In some embodiments, the solids content may from 32 to 48 wt%, from 34 to 46 wt%, from 35 to 45 wt%, from 36 to 44 wt% or from 38 to 42%. The solids content may be approximately 40wt%. In some embodiments, the binder composition may further comprise 1 to 3 wt% of water relative to the particulate refractory material.

Pozzolanic additives

In the invention, the composition comprises at least one pozzolanic additive. The pozzolanic additive is typically a fine, powdered material. The at least one pozzolanic additive may comprise 0.5 to 10wt%, relative to the weight of the particulate refractory material. In some embodiments, the at least one pozzolanic additive comprises at least 1 , 2, 3, 4, 5, 6, 7, 8, or 9wt%, relative to the weight of the particulate refractory material. In some embodiments, the at least one pozzolanic particulate refractory material comprises no more than 9, 8, 7, 6, 5, 4, 3, 2, or 1wt%, relative to the weight of the particulate refractory material.

The inventors have found that a small amount of pozzolanic additive increases the strength of the cores produced from the composition. Even more desirably, the present composition has a significantly lower proportion of pozzolanic additive than conventional binder systems.

The at least one pozzolanic additive may be selected from the group consisting of: silica fume, fly ash, rice husk ash, diatomaceous earth, volcanic ash, metakaolin, and mixtures thereof. The at least one pozzolanic additive may comprise spherical particles and/or cenospheres. In a preferred embodiment, the at least one pozzolanic additive comprises silica fume.

In some embodiments, the at least one pozzolanic additive has a D50 particle diameter of no more than 20 pm, no more than 10 pm, no more than 5 pm, no more than 2 pm, no more than 1 pm, no more than 0.5 pm or no more than 0.1 pm. In some embodiments, the at least one pozzolanic additive has a D50 particle diameter of at least 0.01 pm, at least 0.05 pm, at least 0.1 pm, or at least 0.5 pm. In some embodiments, the at least one pozzolanic additive has a D50 particle diameter of from 0.01 pm to 50 pm, from 0.01 to 20 pm, from 0.01 to 10 pm, from 0.01 pm to 5 pm or from 0.01 pm to 2 pm. Potassium carbonate

The potassium carbonate may comprise 0.125 to 10wt%, relative to the weight of the particulate refractory material. In some embodiments, the potassium carbonate may comprise at least 0.2, 0.3, 0.4, 0.5, 0.75, 1 , 2, 3, 4, 5, 6, 7, 8, or 9 wt%. In some embodiments, the potassium carbonate may comprise no more than 9, 8, 7, 6, 5, 4, 3, 2, 1 , 0.75, 0.5, 0.4, 0.3, or 0.2wt%. The inventors have found that the addition of potassium carbonate to the composition improves the solubility of cores formed from the composition.

Surfactants

In one series of embodiments, the composition further comprises at least one surfactant. The at least one surfactant may be selected from group consisting of anionic, cationic, non-ionic and amphoteric surfactants, and mixtures thereof. Types of surfactant suitable for use in the present invention include sulphates, methosulphates, linear alcohol sulphates, sulphonates, sulphosuccinates, phosphate esters, glucosides, and mixtures thereof. In particular, the at least one surfactant may be selected from the group consisting of 2-ethylhexyl sulphosuccinate, 2-ethylhexyl sulphate, dodecylbenzene sulphonate, nonylphenol sulphate, sodium laureth sulphate, 3-ethylhexyl phosphate ester, undecyl amido propyl trimethyl ammonium methosulphate, alkyl polyglycol ether ammonium methosulphate, 2-ethylhexyl glucoside, hexyl glucoside, and mixtures thereof. In a preferred embodiment, the surfactant comprises sodium ethyl hexyl sulphate.

The inventors of the present invention have found that a binder composition with high surface tension can reduce the flowability of the composition, and that adding a small amount of surfactant can significantly increase the flowability of the composition by reducing the surface tension of the binder composition. In turn, the increased flowability of the composition results in a core having improved strength, for the reasons mentioned previously.

Particulate refractory material The particulate refractory material may be a natural refractory material. The refractory material may be a synthetic refractory material. In some embodiments, more than one refractory material may be used.

In some embodiments, the particulate refractory material comprises sand. The sand may be any type of sand suitable for use in refractory applications, such as quartz sand. In some embodiments, the particulate refractory material may comprise any one or more conventional refractory materials, such as oxides, carbides, nitrides etc of silicon, aluminium, magnesium, calcium and zirconium and other elements. Suitable refractory materials include but are not limited to quartz, olivine, chromite, zircon, and alumina. In some embodiments, the particulate refractory material comprises spherical particles and/or cenospheres, such as fly ash. In some embodiments, the particulate refractory material comprises a mixture of sand and spherical particles and/or cenospheres, such as a mixture of sand and fly ash.

In embodiments where the particulate refractory material and the pozzolanic additive both comprise spherical particles and/or cenospheres, the particulate refractory material and pozzolanic additive may both comprise the same type of spherical particles and/or cenospheres, e.g. fly ash. Alternatively, the particulate refractory material and pozzolanic additive may comprise different types of spherical particles and/or cenospheres, e.g. the particulate refractory material may comprise fly ash while the pozzolanic additive comprises silica fume. It will be understood that, in embodiments where the particulate refractory material and the pozzolanic additive both comprise the same type of spherical particles and/or cenospheres, the D50 particle size of the particulate refractory material will be larger than the D50 particle size of the pozzolanic additive, such that the particulate refractory material is distinct from the pozzolanic additive.

In some embodiments, the particulate refractory material has a D50 particle diameter of at least 20 pm, at least 50 pm, at least 100 pm, at least 250 pm, or at least 500 pm. In some embodiments, the particulate refractory material has a D50 particle diameter of no more than 2 mm, no more than 1mm or no more than 500 pm. In some embodiments, the particulate refractory material has a D50 particle diameter of from 20 pm to 2 mm, from 50 pm to 2 mm or from 50 pm to 1 mm. Cores

According to a second aspect of the invention, there is provided a core for use in a moulding or metal casting process comprising the composition of the first aspect of the invention.

The core may be coated with a surface coating. The surface coating may be any coating suitable for refractory applications, The surface coating may comprise one or more of boron nitride, silicate, titania, alumina, zirconia, alumina, aluminium silicates, or mixtures thereof. In some embodiments, the surface coating may be a sealant.

The core may be water-dispersible. The core may be configured such that the solidified core composition degrades in water such that a cylinder of the solidified core composition having a maximum height of 80 mm and a maximum diameter of 50 mm disintegrates in less than 10 minutes when immersed in water. Preferably, the solidified core disintegrates in less than 5 minutes, 4 minutes, 3 minutes, 2 minutes, 1 minute, 45 seconds, 30 seconds, or 15 seconds. The water may be at a temperature of 20°C. In some embodiments, the water may be at least 30, 40, 50 or 60°C. In some embodiments, the water may be less than 100°C e.g. less than 90, 80, 70, 60 or 50°C. The water may be stirred at a speed of at least 60 rpm. The water may be stirred at less than 150rpm.

In some embodiments, the solidified core composition may have a flexural strength of at least 250 N/cm². In some embodiments, the solidified core composition has a flexural strength of at least 500 N/cm², at least 750 N/cm³ or at least 1000 N/cm². Higher flexural strengths are preferred, where possible, to reduce the chances of the core cracking during casting.

In some embodiments, the cylinder of solidified core composition disintegrates in water in less than 10 minutes after being heated to a temperature from 200 to 350 °C.

Moulds

According to a third aspect of the invention, there is provided a mould comprising the core of the second aspect. The mould may be for producing an article by metal casting and the core is for defining an internal cavity of the article. The mould may be for high pressure die casting, semi-solid casting and/or squeeze casting.

Methods

According to a fourth aspect of the invention, there is provided a method for producing an article by high pressure die casting or semi-solid casting. The method may comprise mixing a composition according to the first aspect to form a mixture. The method may comprise moulding and hardening the mixture to produce a core in the shape of an internal cavity of the article. The method may comprise assembling the core with a mould for high pressure die casting or semi-solid casting, such that the mould and the core together define a casting cavity. The method may comprise injecting molten or semi-solid metal into the casting cavity until the cavity is filled. The method may comprise cooling and solidifying the molten or semi-molten metal to form the article, the core being contained within the internal cavity of the article. The method may comprise removing the article containing the core from the mould. The method may comprise removing the core from the internal cavity by flushing out with water.

The method may further comprise a step of coating the core with a surface coating prior to assembling the core with the mould.

The step of moulding and hardening the mixture may include drying the mixture. The step of moulding and hardening the mixture may include compacting the mixture into a core mould. The step of moulding and hardening the mixture may be performed using a core-shooting apparatus. The step of moulding and hardening the mixture to produce a core includes producing the core by an additive manufacturing or 3D printing process.

In a further aspect of the invention, there is provided the use of a composition according to the first aspect of the invention in a moulding process or a metal casting process.

Examples

Example 1 - Comparative examples The following example compositions were initially prepared:

Table 1

a wt% relative to total weight of binder composition (excluding pozzolanic additive), ^b wt% relative to weight of particulate, ^c wt% relative to total weight of pozzolanic additive, ^d aqueous solution, 40% w/v solids content, ^e aqueous solution, 20% w/v solids content, ^f aqueous solution, 35-45% w/v solids content

¹ BASF GmbH, Ludwigshaven, Germany; ² Intercol BV, Ede, the Netherlands; ³ Brenntag BV, Enschede, the Netherlands; ⁴ PQ Corporation BV, Eysden, the Netherlands; ⁵ Mercur Handel GmbH, Dusseldorf, Germany; ⁶ Vivochem BV. Almelo, the Netherlands; ⁷ AVEBE BV, Veendam, the Netherlands; ⁸ Foseco BV, Enschede, the Netherlands; ⁹ Vivochem BV, Almelo, the Netherlands; ¹⁰ Cofermin Chemicals GmbH, Essen, Germany; ¹¹ Staus-Perlite GmbH, St. Poelten, Austria; ¹² Columbian Carbon GmbH, Hannover, Germany; ¹³ Xatico Benelux-France SarL, Troisvierges, Luxemburg

Comparative compositions C1 to C3 are examples of compositions as described in international application PCT/EP2021/079512. Composition 1 was formed by first mixing the alkali metal silicate with water and subsequently mixing this solution with a mixture of the pozzolanic additive, potassium carbonate and the particulate. Cores were then formed using a Laempe L1 Core Shooter at a shooting pressure of 4 bar and shooting time of 1 second and a core box temperature of 140°C. All cores were cured with hot air (120°C) and with various purging times depending on the size and shape of the produced cores.

Binder solubility was evaluated by immersing and rotating the cured cores in water at the temperature and rotational speed shown in the above table. The binder solubility was determined as ‘excellent’ if the core dissolved within 5 seconds when immersed in water irrespective of its temperature. Cores which dispersed within 5 to 45 seconds in water were deemed ‘good’, within 45-120s ‘moderate’, within 120s and 600s ‘poor’, and >600s ‘non-soluble’.

Comparative compositions C1 and C3 were found to have excellent flexural strengths of around 800 N/cm². C1 also had good solubility - in tests in water at 20°C and rotated at 60rpm, the binder dissolved in 20-40s, and within 15 seconds at 65°C and 150rpm. C3 had good solubility, dissolving in 15 to 45s in water at 65°C and at 150rpm, although solubility at 20°C and at 60rpm was poor, typically taking 240-260s.

Comparative composition C2 had a comparable flexural strength of around 800 N/cm², but the binder composition failed to dissolve in water within 600 seconds. It was thus determined to be unsuitable for use in forming cores for moulding processes, since it would not be possible to efficiently remove the cores from the mouldings.

The flexural strength and solubility of C1 and C3 make them acceptable for use within HPDC and other casting processes. However, each required a relatively high content of a pozzolanic additive in order to achieve the necessary strengths. Silica Fume A is a byproduct from the production of zirconia and is known to contain heavy metal contaminants. In particular, the presence of lead and nickel can, in some moulding processes, lead to waste water contamination during clean-up of the moulding sands, thus requiring additional processes to prevent water pollution.

Example 2 - Effect of alkali metal silicate content

The following compositions were prepared to assess the impact of the alkali metal silicate content on the core properties.

Table 2

^a H33 is a quartz sand (Quarzwerke GmbH, Haltern, Germany); ^b wt% relative to weight of particulate.

The alkali metal silicate, MTR 6099/17 is a mixture of sodium silicate (90wt% Crystal 230, PQ Corporation BV, Eysden, the Netherlands), potassium silicate (5.0 wt% Kasil 1841 PQ Corporation BV, Eysden, the Netherlands), water (4.5 wt%) and DSK40 (0.5 wt% Brenntag BV, Enschede, the Netherlands). Without wishing to be bound by theory, it is believed that the reduced surface tension due to the presence of a surfactant improves the flowability of the compositions, and thus improves the compaction, density and mechanical strength of the resulting cores.

Cores were formed as per example 1. The tests show that increasing the alkali metal silicate content leads to small increases in bending strength. At very high contents of binder relative to the particulate, a loss of structure was observed. At 20wt% of alkali metal silicate the composition was a wet paste and was impossible to form a core with.

In all cases, the solubility of the cores in hot tap water (approx. 65°C) was good and unaffected by the increasing strength with increasing content of alkali metal silicate.

Example 3 - Effect of K2CO3 content

The following compositions were prepared to assess the impact of the K2CO3 content on the core properties.

Table 3a

a H33 is a quartz sand (Quarzwerke GmbH, Haltern, Germany); ^b wt% relative to weight of particulate.

In the compositions shown in Table 3a, it is shown that small amounts of potassium carbonate still provide improved binder solubility over the prior art. Increasing potassium carbonate did not appear to improve the strength or the solubility of the cores. At the highest level of potassium carbonate tested (10 wt%) the cores showed defects. Without wishing to be bound by theory, it is believed that the lower flowability of Composition 8 due to the high potassium carbonate level lead to such defects and accordingly, higher levels of potassium carbonate were not tested.

Table 3b

a LA32 is a quartz sand from Sibelco, France (AFS = 53) b wt% relative to weight of particulate. In the compositions in Table 3b, water was added in equal parts to the potassium carbonate in order to dissolve the potassium carbonate prior to mixing with the alkali metal silicate, pozzolanic additive and particulate. Cores were formed as per Example 1.

The tests show that the presence of potassium carbonate greatly improves the solubility of the binder composition and thus the water-dispersibility of the respective cores formed from the compositions, but as supported by Table 3a, increasing K2CO3 levels did not increase the solubility further. It was found that even low potassium carbonate contents (e.g. of 0.5 wt%) would still demonstrate the improvement in solubility over compositions not containing potassium carbonate.

A further benefit found was the increase in bending strength even at relatively low levels of potassium carbonate. Without wishing to be bound by theory, it is understood that the presence of potassium carbonate can improve the chemical bonding between the individual sand grains. This effect was also observed with H33 sand (see Table 3a above) - firstly an increase in strength up until approximately 3wt%. Higher concentrations lead to a less stable situation with lower observed strength values.

Example 4 - Effect of pozzolanic additive

The following compositions were prepared to assess the impact of the pozzolanic content and composition on the core properties.

Table 4a

^a H33 is a quartz sand (Quarzwerke GmbH, Haltern, Germany); ^b wt% relative to weight of particulate; ^d Silica Fume A; ^e MS - Microsit MS971 U (a microsilica - Elkem Microsilica, Norway); ^f H10 - Microsit H10 fly ash (BauMineral GmbH, Herten, Germany);

The tests show that both silica fume and microsilica have a significant positive impact on the bending strength of the cores. Fly ash was found to have a less pronounced effect on strength compared to the example with no pozzolanic additive, with the values of each being broadly the same within the respective margins of error. The sample weight of the core comprising no pozzolanic additive was the lowest of those tested, indicating the poorest compaction and thus lowest density of the core.

Table 4b

^a H33 is a quartz sand (Quarzwerke GmbH, Haltern, Germany); ^b components are in wt% relative to weight of particulate; ⁰ XSW - XSW-95U (a silica fume - Possehl Erzkontor GmbH, Lubeck, Germany); ^h SFP - Denka SFP (a silica fume - Denka Company Limited, Tokyo, Japan); ¹ MS ZZ - Microsilica ZZ (Moertelshop GmbH, Germany)

Compositions 16 to 18 show that the high strength achievable with the use of silica fume and microsilica is repeated with alternative silica fume and microsilica products, and is not limited to a single supplier or product. Compositions 19 and 20 show significantly increased flexural strength with increased content of pozzolanic additive. However, the increased pozzolanic content significantly reduces the solubility of the cores.

Example 5 - Effect of refractory particulate material

The following compositions were prepared to assess the impact of the type of refractory particulate material on the core properties.

Table 5

a H33 is a quartz sand; ^b components are in wt% relative to weight of particulate; _c ‘Nissan’ is a synthetic sand - NISSAN, China, AFS 61 ; ^d Bauxite 65 - Ziegler & Co, GmbH, Germany, AFS 65; ^e Fosbeads - Foseco, the Netherlands, AFS 52; ^f Poraver (Dennert Poraver GmbH, Schlusselfeld, Germany) - glass beads - 0.1 - 0.3 mm

These tests show that similar results are repeatable with alternative selections of particulate refractory materials. Compositions 21 to 23 are synthetic sands commonly used within foundry applications where high strengths are required.

Without wishing to be bound by theory, it is believed that the presence of the pozzolanic additive aids flowability of the mixture and increases the strength since the smaller particle sizes of the pozzolanic additive give better packing and thus density of the cores. The synthetic sands tested are spherical and thus lower addition rates of the binder composition were required to achieve similar strengths. The use of the Poraver (small, spherical, expanded glass beads) failed to produce an effective core, believed to be due to the very low density. These test results show that synthetic sand and synthetic spherical ceramic particles can be used with the binder composition to provide desirable cores for moulding processes.

Example 6 - Effect of various types of alkali silicates

The following compositions were prepared to assess the impact of the type of alkali silicate on the core properties.

Table 6

^a Quartz sand H33 (Quarzwerke GmbH, Haltern, Germany); ¹ MTR 6099/17*: Foseco BV, Enschede, the Netherlands; ² Crystal 230M = Crystal 230 + 20 wt% water + 0.5 wt% DSK40 (surfactant); ³ KasiH 841 M = Kasil 1841 + 20 wt% water + 0.5 wt% DSK40 (surfactant); ⁴ Crystal 230M2 = Crystal 230 + 20 wt% water without DSK40

Table 6 shows the impact of various types of alkali silicates on the quality and watersolubility of the cores formed from the compositions. It was found that the use of sodium silicate based MTR 6099/17*, Crystal 230M and Crystal 230M2 all had excellent performance. Composition 29 comprised 5 parts by weight of Kasil 1841M, which comprises potassium silicate, 20wt% water and 0.5wt% of surfactant. The use of exclusively potassium silicate in Composition 29 resulted in a slightly lower mechanical strength as well as a lower water-solubility, but both were acceptable.

Compositions 28 and 30 differ only in the inclusion of a small amount of surfactant in Composition 28 (the alkali silicate component comprised 0.5wt% DSK40). It was found that in the absence of DSK40 the sample weight and the mechanical strength of the core formed from the composition was lower. Composition 27, comprising a mixture of both sodium silicate and potassium silicate showed good performance in the solubility test, and provided lower viscosity and better flowability than Compositions 28 and 30, which comprised only sodium silicate. Without wishing to be bound by theory, it is believed the lower viscosity and better flowability as a result the inclusion of a small amount of surfactant and potassium silicate leads to higher compaction and density of the formed cores, and thus a higher flexural strength.

Example 7 - Effect of reclaimed sand

The effect of reclaiming the particulate was investigated.

Table 7

Composition 31 used a fresh sample of LA32 which had not been used previously to act as a benchmark. In Composition 32 the particulate, LA32, had been reclaimed and recycled 5 times - in each use the same binder system has been used. These results show that the use of a reclaimed particulate did not significantly affect the weight or strength of the cores formed and thus did not negatively affect the performance of the binder system. Example 8 - Effect of storage conditions

A range of storage conditions and storage periods were investigated using a series of identical cores formed from Composition 1 above. The varying conditions were chosen to assess the impact of exposure under ambient conditions or storage in an airtight plastic bag. The sample cores all had an initial sample weight of 426.1g and an initial bending strength of 1009 ± 50N/cm².

Table 8a

a H33 is a quartz sand (AFS 53) (Quarzwerke GmbH, Haltern, Germany); ^b wt% relative to weight of particulate Exposure tests were performed up to one week (168 hours) whereby sand cores were stored at 25°C and a relative humidity (RH) of 30% (corresponding to the maximum storage time and typical conditions in a foundry). One further batch (example E38) was stored in an airtight plastic bag. Water-solubility was determined with hot tap water (65°C) In all tests, the solubility of the cores were found to be good, showing that the desirable solubility characteristics are maintained over a useful product lifetime.

The experiments were then repeated using a composition comprising 7 wt% of the alkali metal silicate and 1.5 wt% of potassium carbonate, relative to the weight of the particulate. The initial sample weight was 419.2g and the initial bending strength was 950 ± 23 N/cm². The results are shown in Table 8b.

Table 8b

a H33 is a quartz sand (AFS 53) (Quarzwerke GmbH, Haltern, Germany); ^b wt% relative to weight of particulate

These tests show that irrespective the amount of potassium carbonate or storage conditions, and with a maximum storage period of 168 hours, the water-solubility of the produced cores was still acceptable. In all cases, sand cores with 3.0 wt% potassium carbonate showed slightly higher water-solubility rates. All tests were done with hot tap water (65 °C).

Claims

CLAIMS:

1. A composition for making a core for use in a moulding or metal casting process, the composition comprising: a particulate refractory material; and a binder composition comprising:

1 to 15 wt% of at least one alkali metal silicate:

0.125 to 10 wt% of potassium carbonate; and

0.5 to 10 wt% of at least one pozzolanic additive wherein the binder composition ranges are relative to the weight of the particulate refractory material.

2. The composition according to claim 1 , wherein the binder comprises 1.5 to 35wt% relative to the weight of the particulate refractory material.

3. The composition according to any preceding claim, wherein the binder composition is an inorganic binder composition.

4. The composition according to any preceding claim, wherein the at least one alkali metal silicate comprises sodium silicate and potassium silicate.

5. The composition according to any preceding claim, wherein the at least one alkali metal silicate is in aqueous solution and has a solids content of between 30 and 50 wt%.

6. The composition according to any preceding claim, wherein the at least one pozzolanic additive has a D50 particle diameter of no more than 20 pm.

7. The composition according to any preceding claim, wherein the at least one pozzolanic additive comprises silica fume.

8. The composition according to any preceding claim, wherein the potassium carbonate comprises 1 to 6wt%, relative to the weight of the particulate refractory material.

9. The composition according to any preceding claim, further comprising a surfactant, and optionally, wherein the surfactant is sodium ethyl hexyl sulphate.

10. The composition according to any preceding claim, wherein the binder composition further comprises 1-3 wt% of water relative to the weight of particulate refractory material.

11. A core for use in a moulding or metal casting process comprising the composition according to any one of the preceding claims.

12. The core of claim 11 , wherein the core is coated with a surface coating, and optionally, wherein the surface coating comprises boron nitride, silicate, titania, alumina, zirconia, alumina, aluminium silicates, or mixtures thereof.

13. A mould comprising the core of claim 11 or 12, wherein the mould is for producing an article by metal casting and the core is for defining an internal cavity of the article.

14. A method for producing an article by high pressure die casting or semi-solid casting, the method comprising the steps of:

(i) mixing a composition according to any one of claims 1 to 10 to form a mixture;

(ii) moulding and hardening the mixture to produce a core in the shape of an internal cavity of the article;

(iii) assembling the core with a mould for high pressure die casting or semi-solid casting, such that the mould and the core together define a casting cavity;

(iv) injecting molten or semi-solid metal into the casting cavity until the cavity is filled;

(v) cooling and solidifying the molten or semi-molten metal to form the article, the core being contained within the internal cavity of the article;

(vi) removing the article containing the core from the mould; and

(vii) removing the core from the internal cavity by flushing out with water.

15. Use of a composition according to any of claims 1 to 10 in a moulding process or a metal casting process.