FR2779425A1 - Refractory powder composition e.g. for wax foundry, for dental applications, for silversmith's trade - Google Patents

Abstract

A refractory powder to be used with an aqueous binder comprises 1-70 wt.% of a cellular particle-like refractory material of particle size distribution below 200 mu m and a density below 0.9. An Independent claim is also included for a process for preparing a casting with the above powder and comprising mixing the above refractory powder with an aqueous binder to obtain a liquid paste, pouring into a mold containing a wax mold printing, waiting for the mixture to set, heating, the wax is evacuated from the block to form a mold with a cavity, removing the mold from the oven, injecting a metal or a melted ceramic into the cavity, cooling and breaking the mold to obtain the casting.

Description

AQUEOUS BINDER REFRACTORY POWDER COMPOSITION

AND ITS APPLICATIONS

  The invention relates to an aqueous binder refractory powder composition and its applications in the field of lost wax foundry, and in particular in dentistry, goldsmithing and industry. One of the current techniques for replacing a tooth is to place on a tooth stump a thick crown adapted to the contours of the adjacent teeth that hold it in place. To make this thick crown, one takes a wax impression of the stump by conforming the impression to the adjacent teeth and giving the impression of a rod of manipulation in wax. This impression is placed in a standard frustoconical or cylindrical shape, for example that referenced 3 X (Degussa type), into which a composition of refractory powder and aqueous binder is poured which is allowed to set before being baked to obtain a refractory mold

  wherein the melting and the flow of the wax leave an empty cavity.

  The material intended to constitute the final crown, for example a metal, ceramic or glass, is injected in the molten state in this

  cavity while the refractory mold thus obtained is still hot.

  After cooling, the mold is broken to release the crown.

  Conventional refractory powder contains, in general, 4 to 10% magnesium oxide, 20 to 40% monobasic ammonium orthophosphate, the remainder being essentially silica. The granulometry of the silica is a function of the surface condition necessary for the final part. It goes from 10 / m to make crowns with a very smooth surface, to 300 gm for removable prostheses which can be rougher. A larger particle size can still be

  used in areas other than dentistry.

  Additives are optionally present in an amount generally from 0 to 3%. It may be pigments such as iron oxide, aluminum oxide, titanium oxide, or additives such as zirconium oxide which improves the fluidification of the powder mixed with the aqueous binder, the citric acid which allows to reduce the amount of aqueous binder required but which causes problems of aging, or

  again borax to delay the setting.

  The aqueous binder is water and / or a suspension of colloidal silica, the proportion of water relative to the total of the binder ranging from 0% to 100% by weight as a function, in particular, of the final temperature and the baking time of the mold. The baking temperature of the mold is itself a function of the material that will ultimately be injection molded: if it is a metal, it will be between 550 C (for gold) and 1000 C (for steels) and if it is a ceramic

  melting at 1200 C, it will be greater than 1200 C.

  For example, to make a gold crown, the aqueous binder lo contains 30% water and 70% of a 30% silica suspension.

  colloidal particle size 50 nm.

  For 150 g of refractory powder, 30 to 40 ml of aqueous binder are used and the mixture is mixed until a flowable liquid paste having the texture of a liquid yoghurt is obtained. The paste is cast in the form, for example a standard 3 X cone frustum, containing the wax imprint, and allowed to set for 15 to 30 minutes or more, including the 10 minute return time. The return time is the time during which the dough, which has solidified by increasing its volume, stabilizes. Optionally, after extraction out of shape, the solid block containing the wax impression is baked in a low oven (0-250 ° C) and then the temperature is raised, either continuously (20 to 50 C / min) or in steps (for example 0-350 C, 45

  minutes at 350 C, 350-700 C, 45 minutes at 700 C, 700 C -

  final temperature). The wax melted towards 80 C - 100 C leaving a

cavity in the refractory block.

  The resulting refractory is allowed to stabilize at the final temperature for about 30 minutes or more depending on the size of the refractory mold and the thickness of the wax imprint, before it is taken out of the oven and placed on a device. for casting the desired material in the molten state into the cavity of the refractory block, which constitutes the mold. Casting in the mold is usually done under force

  centrifugal or under pressure or depression.

  After cooling in air, the refractory mold is broken

  mallet and we recover the desired part that is stripped by sandblasting.

  This process has certain disadvantages. One of them is that for a given volume of form, the mass of powder and the quantity of liquid are important, which poses problems of cost of product and cost of transport, and makes the process expensive. Another is that because of the large proportion of silica, it is difficult to unmould and the duration of sanding leads to additional costs of energy and manpower. It is already known that the demolding of a final piece of gold is improved when using a specific refractory powder called "plaster base": a brushing with water allows for roughing and microsandage gives the desired finish. But this refractory powder can not be used above 700 C and the problem remains for steels,

  o cobalt, nickel, iron and ceramics.

  It was also sought to eliminate the preheating stress by charging directly to the final temperature. For this, it is necessary to increase the phosphate content and bake during the

  return time as defined above, which is very restrictive.

  There was therefore a need to further improve this lost-wax casting technique in order, in particular, to improve

  demoulding operations taken as a whole.

  The invention solves the problem surprisingly by incorporating into a refractory powder composition a particulate alveolar material, with a particle size of less than 200 gim and a density of less than 0.9, in a weight ratio of between 1 and 70% by weight. relative to the total weight of the composition. As indicated above, the other main constituents of such a composition are generally magnesium oxide and a phosphate, such as monobasic ammonium orthophosphate, and

that, frequently, silica.

  The demolding can be obtained in a conventional manner by breaking the mold with the mallet but also, surprisingly, by quenching the hot mold in a water bath, which "explode" the mold and releases the molded part with a very good surface condition and keeping the dimensions of the room. If this quenching technique is applied to a refractory mold of conventional composition, there is a shrinkage deforming the molded part, which is not permissible. Because of the alveolar character of the particulate material and its density of less than 0.9, there is a reduction in the weight of refractory powder used to fill a given volume form, where, on the one hand, a reduction in cost of the refractory material by prosthesis or molded crown and, on the other hand, a reduction in the weight of refractory powder stored for a given production and finally a reduction in the cost of transporting the refractory powder from its place of manufacture to its place of manufacture. use. In addition, the proportion by weight of the aqueous binder with respect to the powder remaining substantially the same, the reduction in the weight of the powder used results in a reduction of

weight of aqueous binder.

  There is, in addition, a significant increase in the return time for charging hot. This becomes possible for a period of up to 40 minutes or more, which leaves a great room for maneuver technician. In addition, the temperature loss at the exit of the oven, which is conventionally 200 C in 20 to 30 seconds is slower with the composition of the invention and gives the technician several minutes to position the mold on the machine. injection of metal or ceramic in the molten state. In order not to weaken the molding part, the cellular particulate material is generally used in an amount of less than 70% by weight relative to the total weight of the refractory powder composition. Beyond the upper value indicated above, it can be seen, in addition, a decrease in the precision of the molding and the fluidity of the liquid paste to flow in the form

normalized.

  One way to obtain a compromise between lightening and mechanical strength of the molding part also consists in using at least two particulate cellular materials of substantially different particle size, and in particular one of relatively large diameter 30, so less dense, which lighten, and another of relatively small diameter, therefore denser but which improves the

mechanical resistance.

  The particulate alveolar material may be a natural expanded mineral powder of volcanic origin such as perlite. Tests carried out with a powder having an average particle size of 10 to 350 μm showing a weight gain of 10 to 12% with easier demolding for amounts of perlite of 3 to 5% by weight relative to the total weight of the composition of the composition. powder. The use of perlite gives particularly good results in jewelery and metal foundry

  industrial areas that require very high precision.

  Particularly advantageous cellular particulate material is microspheres also known as hollow microspheres made of heat resistant glass, especially sodium borosilicate and calcium based glass. They are preferably used in combination with a small amount of amorphous synthetic silica; the amorphous silica particles have a low average particle size, generally of the order of 12 nm and, especially on the surface, they fill the spaces between the microspheres avoiding air bubbles, resulting in a very good surface finish . The amount of amorphous silica used varies according to the granulometry of the microspheres: the lower the latter, the more silica

amorphous is weak or even nil.

  The invention thus provides an aqueous binder refractory powder composition advantageously having the following composition in weight%: Magnesium oxide 2-8 Ammonium orthophosphate 20-30 monobasic Silica (10 μm to 300 mm) 0-80 Particulate material The aqueous binder is, in any proportion defined according to the case, water and / or an aqueous suspension of colloidal silica (10-50 nm), for example a suspension at 30% by weight. of

silica.

  When the cellular particulate material consists of glass microspheres, the composition preferably contains from 0.02 to 1% amorphous synthetic silica having preferably

particle size of the order of 12 nm.

  Between 3 and 10% by weight of the silica may be replaced by charge additives such as, for example, zirconium oxide,

molochite or kaolin.

  The additives contain pigments and / or up to 0.1% citric or malic acid and / or up to 0.1% borax and / or up to 0.1%

of sodium silicate.

  Advantageously, the composition contains a mixture of two types of glass microspheres, one of relatively low (more dense) and the other of relatively large (less dense) diameter, and a mixture of glass microspheres with density between 0.1 and 0.15 and glass microspheres having

  a density of between 0.2 and 0.25.

  There is also provided a process for preparing a molded part by means of a refractory powder composition as defined above in which said refractory powder composition and an aqueous binder are mixed until a liquid paste is obtained. the paste is poured into a form containing a wax impression, the mixture is waited for to set, the block obtained is heated in an oven to the temperature at which the refractory is to be heated for cooking, the wax of the melting impression during said heating and being removed from the block, which forms a mold having a hollow, it is defourne when the firing is completed, is usually injected a metal or a ceramic melt in the hollow of the mold , let cool and break the refractory mold to

recover the molded part.

  The invention finds use in the foundry in lost wax

  all types of industrial products, in goldsmithing and dentistry.

  The following examples illustrate the invention without limiting it. Example 1: Use of Perlite as Particulate Cellular Material Refractory powders according to the invention are prepared with the compositions defined in Table 1, where the values are percentages given by weight relative to the total weight of the

composition.

Table 1

  Ingredients A B C D Magnesium oxide 4 4 5 6 Ammonium orthophosphate 20 25 22 20 monobasic Silica 70.5 60.3 61.8 66.3 (particle size in μm) (10) (50) (150) (300)

Zirconium oxide - 0,5 0,5 -

  Pigment (s) - 0.2 0.2 0.2 Citric acid 0.5 - 0.5 0.5 Perlite 5 10 15 7 (particle size in μm) (100) (150) (30) (200) perlite is an expanded natural volcanic material containing aluminum silicates of variable composition; it has a density of between 50 and 150 kg / m3 depending on the type chosen; we use the one

  sold by the company "NOBLE" under the name "Noblite".

  As magnesium oxide, for example, the magnesium and calcium oxide sold under the trade name lo "Lycal" is used by the British company "REDLAND MAGNESIA LTD", and / or the magnesium oxide sold under the name commercial

  "MagChem 10-325" by the company "MARTIN MARIETTA".

  With each composition, a refractory mold was prepared according to the procedure described above using 20 to 30 ml of aqueous binder per 100 g of powder. In the examples, a form is used

standardized 3 X (Degussa type).

  For compositions A and B for molding a gold part, 30% by weight of water and 70% by weight of a solution of colloidal silica (50 nm) at 30% by weight of silica were used as aqueous binder. ; for compositions C and D used for molding a steel part, only the silica solution was used as aqueous binder

colloidal defined above.

  In all cases, we obtain a reduction of 10 to 15% of the

  refractory mold obtained according to the percentage of perlite used.

  The demolding by quenching with water gives a surface state, which does not

  requires only a short stripping to the sand.

  Example 2: Use of glass microspheres as a material

particulate alveolar.

  Refractory powders according to the invention are prepared with the compositions defined in Table 2 where the values are

percentages given by weight.

Table 2

  Ingredients E F G H Magnesium oxide 4 5 8 8 Ammonium orthophosphate 20 22 20 20 monobasic

Silica 59.4 68.2 54.8 -

  (particle size in μm) (50) (10) (200)

  Zirconium oxide / molochite - - 5 -

Pigment (s) 0.5 0.2 0.2 -

0.1 0.1 citric acid - -

Sodium silicate - - 0,1

Borax - - 0,1 -

Glass microspheres

Type K1 7 2 3 -

Type S22 8 2 8 -

  Type VS 5500 - - - 71 Amorphous synthetic silica 1 0.5 0.8 1 The glass microspheres sold under the trademark "Scotchlite" of type K1 and S22 are supplied by the company "3M Company" and have the following characteristics ( manufacturer's instructions): Type of microspheres KI S22 "Scotchlite" Average density 0.125 0.22 nominal / lm% vol. im% vol. passing by Distribution of 125 100 125 100 granulometry 102 90 90 90

50 40 50

_ _ _ _ _ _ 35 10 18 10

  The microspheres sold under the trademark "Scotchlite" have chemical properties similar to those of a sodium borosilicate and calcium based glass. The VS 5500 microspheres are products of the 3M Company having an apparent density of 0.19 to 0.28 and the following granulometry: Volume distribution (/ m)% 50% 90% Maximum particle size

45 75 85

  Other types of microspheres are commercially available and the proportion to be used is a function of their characteristics and is easily determined by experiments.

routine.

  With each of these compositions, a mold is prepared using a standardized form "3 X Degussa"; 100 g of one of the powdery compositions E, F or G and 29 ml of an aqueous binder, which is an aqueous suspension with 30% of colloidal silica (12 nm), are used. For comparison, it is recalled that with a prior art powder, using the same standard form, g powder and 38 ml binder are required. As colloidal silica, the one sold under the trademark "Sigasol" by the company has been used.

GAZECHIM "

  The operating conditions for producing the mold are as follows. After preparation and casting of the paste in the normalized form, to embed the wax impression previously placed in said form, it is allowed to stand for 40 minutes for the setting to take place, then the resulting block is extracted out of shape. and is baked in the oven at 800 C. It is left at this temperature for minutes, the mold obtained is placed on a molding machine by centrifugation and then immediately is injected into the

  mold cavity of the molten metal between 800 C and 900 C.

  It is demolded by quenching in water, after suitable cooling to solidify the metal; molded parts are obtained that simply brush under a stream of water to remove any mold residue and have an excellent surface condition, with perfect respect for the shape of the wax print. A slight microsablage is enough for finishing.

Claims (9)

  1 - Refractory powder composition intended to be implemented with an aqueous binder, characterized in that it contains from 1 to 70% by weight of at least one particulate refractory honeycomb material having a particle size of less than 200 .mu.m and density less than 0.9.   2 - Composition according to claim 1, characterized in that the cellular particulate material is selected from the group formed   by hollow glass microspheres and perlite.   3 - Composition according to one of claims 1 or 2,   characterized in that it has the following composition, in% by weight: Magnesium oxide 2-8 Ammonium orthophosphate 20-30 monobasic Silica (10 / m to 300 / m) 0-80 Foam particulate material 1-70 Additives <2 4 - Composition according to Claim 3, characterized in that from 3 to 10% by weight of the silica may be replaced by oxide   zirconium, molochite or kaolin.   - Composition according to one of claims 3 or 4,   characterized in that the additives comprise pigments and / or up to 0.1% citric acid and / or up to 0.1% borax and / or   up to 0.1% sodium silicate.   6 - Composition according to one of claims 1 to 5, characterized   in that the aqueous binder is water and / or an aqueous suspension 30% colloidal silica.   7 - Composition according to claim 3, wherein the cellular particulate material consists of glass microspheres, characterized in that it contains from 0.02 to 1% of silica amorphous synthetic.   8-Composition according to one of claims 3 or 7,   characterized in that the silica content increases with the   particle size of the cellular particulate material.   9 - Composition according to one of claims 1 to 8, characterized   in that it contains a mixture of two types of glass microspheres, one of relatively low (more dense) average diameter and   the other of relatively large diameter (less dense).   Composition according to Claim 9, characterized in that a mixture of glass microspheres having a density of between 0.1 and 0.15 and glass microspheres having a   density between 0.2 and 0.25.   11 - Process for preparing a molded part by means of a refractory powder composition in which said refractory powder composition is mixed with an aqueous binder until a liquid paste is obtained, the paste is poured into a form containing a wax imprint, the mixture is expected to set, the block obtained is heated in an oven to the temperature at which it is desired to carry the refractory to ensure its cooking, the wax of the melting impression during said heating and being evacuated from the block, which forms a mold having a cavity, is defourne when the firing is completed, is injected in the usual manner a metal or a ceramic melt in the mold cavity, is allowed to cool and breaks the mold refractory for recovering the molded part, characterized in that the refractory powder composition is a composition according to one of claims 1 to 10.   12 - Use of a refractory powder composition according to   one of claims 1 to 10 in the lost wax foundry.