FR2845987A1 - Ceramic mixture with a refractory plaster base and some mineral charges containing synthetic fibres for the fabrication of refractory blocks for the precision casting of low melting point alloys - Google Patents

Abstract

Ceramic mixture with a refractory plaster base and some mineral charges for the precision casting industry contains upto 1.0 % by wt of synthetic fibres with respect to the weight of the refractory plaster.

Description

The present invention relates to a ceramic mixture based on plaster

  refractory and mineral fillers intended for the foundry of

  Lost wax precision by the mold-block process.

  The mold-block process is currently commonly used for the production of generally complex shapes and shapes.

  precise dimensional tolerances without machining.

  This technique consists schematically in making a refractory block around wax models by vacuum casting then taking a ceramic mixture based on refractory plaster and mineral fillers called coating by the specialists.

  Given the decomposition temperature of the plaster (from 1200 ° C.) the mold-block process is mainly used for the casting of low-melting point metal alloys such as aluminum and magnesium alloys in the field of automotive, electronics, aeronautics, tire industry, footwear industry, etc.

  This technique also extends to jewelery, goldsmiths and crafts for the casting of gold, silver and copper alloys, and

  the crystal factory for making pieces of glass paste.

  The block mold technique is also used by jewelers and dental technicians for the casting of high melting alloys such as steel, titanium, platinum,

  In this case, the plaster, which constitutes the binder phase of the ceramic mixture, is replaced by a phosphatic bond, resistant to higher temperature.

  More precisely, during the implementation of the block mold process, the ceramic mixture based on plaster is premixed with water, under vacuum, so as to obtain a slip. In order for the final casting parts to be free of surface defects, it is essential that such a slip be homogeneous and

does not contain air bubbles.

  The models of the casting pieces are obtained by injecting liquid wax into elastomer or metal molds according to the sector of activity concerned.

  In the case of very complex technical parts, with fine-sectioned and often sinuous cavities, the interior forms

  future metal parts are obtained through the use of

ceramic cores.

  These cores, composed of a mixture of fine granular refractories and a binder, are shaped by injection or pressing.

  After heat treatment, they are positioned in the

  model molds into which the wax will be injected.

  The wax models, with or without ceramic core are then checked, possibly retouched, assembled in clusters and positioned in a metal frame serving as support for the ceramic mixture during the casting step of this mixture.

  It should be noted that, in the context of the mold-block process, the wax has numerous advantages, in particular due to its low melting temperature (below 70 ° C) and its excellent fluidity, which make

its easy use.

  In addition, the wax makes it possible to carry out retouches or repairs of the models by withdrawal or addition of material and does not melt with the contact

moisture or water.

  When the setting of the ceramic mixture is complete, the resulting refractory block is placed in an oven or autoclave so as to remove the wax.

  This operation, called "dewaxing" by the specialists, consists of rapidly melting the wax and flowing it out of the refractory block to leave a hollow impression into which the liquid material, in particular the molten metal or glass, will be subsequently poured. .

  This dewaxing step must be as short as possible to prevent the wax from expanding before it melts and to avoid cracking of the refractory block; it can be used in an autoclave

  under steam pressure or in a dry dewaxing furnace.

  The refractory block is then fired in an oven to about 650 ° C to remove the carbon traces left by the wax and dehydrate the ceramic mixture, then is brought to the casting temperature.

  liquid metal or molten glass.

  The impression left by the wax is then filled with molten metal or molten glass.

  This casting can be carried out by gravity or by centrifugation, under vacuum or under pressure.

  After solidification of the metal or glass, the refractory block must generally be destroyed to recover the molded parts; this

  operation is named "stall" by the specialists.

  During this, the destruction of the refractory block can be obtained either by sending jets of water under pressure to the cold block, or by sudden immersion thereof in a cold water bath while it is

still hot.

  In the latter case, the multi-cracking caused by the thermal shock makes it possible to reduce the mud coating; it should be noted that this technique is practiced mainly in

jewelery and crafts.

  The quality of the parts obtained by the mold-block process depends to a large extent on the composition of the ceramic mixture or constituent coating of the refractory block and the core, and which must be adapted to each particular field of use.

  The specialists have already proposed a wide range of such coatings which are generally constituted by mixtures of plaster, silica and / or alumina and / or silico-aluminous compounds and

various additives.

  These mixtures must satisfy multiple requirements: they must in particular have good dimensional stability and excellent reproducibility, and in addition be chemically inert with respect to the cast material, have a good mechanical strength both before cooking the mold only during dewaxing and casting, as well as good resistance to thermal shock, and at the same time

  be easily destroyed during the shakeout.

  These mixtures must also be sufficiently fluid, have a sufficient working time, do not risk giving rise to a settling of their various constituents before taking the refractory block and allow easy evacuation of air bubbles.

  However, it has not heretofore been proposed ceramic mixture likely to meet all these requirements.

  One of the most delicate phases of the implementation of the mold-block process is in particular the cooking phase of the fracture block.

  Indeed, during this phase, there is a risk of cracking due in particular to the presence of mixing water and also dewaxing water when the latter operation was implemented

under pressure of water vapor.

  To avoid these risks, it is necessary to effectively remove the water present in the refractory block, without trapping water vapor to the inner part thereof: in fact, by expanding, this water vapor

  could generate cracks, and at worst cause the explosion of the block.

  As a result, the temperature rise of the refractory blocks must be sufficiently slow and progressive.

  However, and for economic reasons, the cooking cycle must, at the same time, be as short as possible so as not to delay the provision of the refractory block for the casting of the liquid metal or

molten glass.

  The object of the present invention is to propose a ceramic mixture based on refractory plaster and mineral fillers intended for the precision foundry by the mold-block process of nature to satisfy

these imperatives.

  According to the invention, this ceramic mixture is characterized in that it contains up to 1.0% by mass of synthetic fibers, relative to the mass of refractory plaster.

  These synthetic fibers generally have a length

less than 12 mm.

  They can advantageously be constituted by polypropylene fibers.

  Such fibers, which are evenly distributed in the refractory block, soften when it reaches a certain temperature (1280 ° C. for polypropylene) and finally burn when this temperature rises (at 165 ° C. for polypropylene). in order to create within the block an organized network of microscopic capillary channels which promote the evacuation of water in the form of steam and reduce the risks of explosion due to thermal stresses.

  According to the invention it is essential that the dosage of the fibers, that is to say the number of synthetic fibers per cm 3 of ceramic mixture is chosen so that these fibers are sufficiently in contact with each other to create a distribution homogeneous capillaries 35 throughout the mass of the refractory block.

  It should be noted that the synthetic fibers also serve to contribute to the mechanical cohesion of the ceramic mixture

before baking the mold.

  According to another characteristic of the invention, the mixture contains from 15 to 50% by weight, preferably from 18 to 35% by weight of

refractory plaster.

  This plaster is generally present in the form of a mixture of plaster ci and plaster P. Plaster ca is obtained by wet-gypsum firing and is characterized by compact grains requiring small amounts of mixing water ; this type of plaster develops mechanical resistance

the highest.

  The plaster 3 is obtained by dry gypsum firing. The grains obtained are more porous than in the case of plaster

  oc, leading to higher amounts of mixing water.

  According to a first variant of the invention, the ceramic mixture is intended for industrial applications, in particular for the casting of low-melting point metal alloys such as aluminum and magnesium alloys, especially in the field of

  automotive, electronics or aeronautics.

  According to this first variant, such a mixture contains in proportions by mass approximately 30% of refractory plaster, 30 to 70% of silicas or aluminas, up to 50% of silico-aluminous compounds, in particular of chamottes and of 0, 05 to 0.1% of synthetic fibers.

  According to a second variant of the invention, the ceramic mixture is intended for casting low-melting metal alloys, especially aluminum alloys in the tire or footwear industry.

  According to this second variant of the invention, such a ceramic mixture contains, in mass proportions, approximately 20% of refractory plaster, 20 to 25% of silicas or aluminas, preferably of quartz, approximately 60% of silico-aluminous compounds, including chamotte, and from 0.05 to 0.1% of synthetic fibers.

  According to a third variant of the invention, the ceramic mixture is intended for crystal and craft industries, in particular for

  production of pieces made of glass paste.

  According to this third variant, such a mixture contains, in mass proportions, approximately 30% of refractory plaster, from 60 to 65%

  silicas or aluminas, 5 to 10% of silico-aluminous compounds, especially chamottes, and 0.05 to 0.1% of synthetic fibers.

  According to the invention, the silica contained in the ceramic mixture may be amorphous (electro-fused silica) or crystalline (quartz,

  cristobalite or more rarely tridymite).

  Quartz and electro-fused silica act as inert inorganic fillers for the overall particle size arrangement.

(permeability, flowability, ...).

  Cristobalite has crystalline properties which can be very interesting in the formulation of a refractory block. Indeed, during heating, cristobalite undergoes an allotropic transformation at 220 ° C theoretical; this network change

  crystal is accompanied by a strong dimensional expansion.

  This reaction is reversible, so that on cooling, cristobalite retracts to its original crystal lattice.

  This property can be used for the dimensional adjustment of the refractory block and the blocking of the ceramic mixture in the

metal frame.

  In addition, in some applications, cristobalite may contribute to the improvement of shakeout conditions.

  Alumina, which may be present in the form of electro-fused alumina, tabular, calcined, or bauxite, ... allows

  to increase the refractoriness and the conductivity of the refractory block.

  The silico-aluminous compounds which may be present in the form of refractory chamois with 40 to 60% of alumina, kaolin, mullite, etc. are often preferred to silica sand because of their great

  dimensional stability (absence of quartz point).

  The use of refractory chamottes obtained by firing high temperature refractory clay can in particular

  prove particularly advantageous.

  During cooking, these clays dehydrate, and give rise above 970 ° C, to the formation of mullite and amorphous silica.

and sometimes crystalline.

  Chamottes available on the market contain 40 to 65% mullite and up to 25% cristobalite.

  Mullite, of chemical formula 3AI203-2SiO2 is the form

  so-called stoichiometric (stable) silico-aluminous; it contains 76% alumina and is characterized by its excellent resistance to thermal shock.

  The resulting chamottes are then crushed, crushed and sieved to be available in a wide range of particle sizes.

  However, the use of chamottes having a particle size of less than 0.5 mm is particularly advantageous in the formulation of ceramic mixtures intended for industrial applications since it makes it possible to obtain a better mechanical reinforcement than quartz sand and can advantageously replace the latter thanks to its better dimensional stability: indeed, the low content of quartz and cristobalite of 10 chamottes mitigates the risk of cracking due to the expansion of the quartz

  at 5730C and cristobalite at 220 ° C during cooking.

  The ceramic mixtures in accordance with the invention may, of course, contain various additives, such as, for example, set retarders, setting accelerators, fluidizers, wetting agents, anti-foaming agents, anti-settling agents, foaming agents, thickeners, and fibers. of glass,...

  It should be noted that the ceramic mixtures in accordance with the invention also have the advantage of allowing the use of refractory blocks of large dimensions (of the order of 1 m 3) whereas until now the process mold - block was limited to blocks of

250 mm3.

  The composition of various ceramic mixtures according to the invention is detailed in the table below.

  In this table: - Example 1 corresponds to a ceramic mixture for casting aluminum in the industrial field; - Example 2 corresponding to a ceramic mixture for casting aluminum alloys in the industrial field; Example 3 corresponds to a ceramic mixture for the casting of magnesium alloys in the industrial field; Example 4 corresponds to a ceramic mixture for the casting of aluminum alloys in the industrial field; Example 5 corresponds to a ceramic mixture for casting aluminum alloys in the tire or footwear industry; Example 6 corresponds to a ceramic mixture for casting glass

  in the field of crystal and crafts.

Board

Example

Example

Example

Example

Example

Example

  Electro-Alumina 5-15 30-35 melted 0/1 mm Silica 0/1 mm 30-40 30-35 6070 20-30 60-70 Alumino-silky 30-40 25-30 35-45 55-60 5-15 0 / 0.5 mm Plasters 30-35 30-35 30-35 30-35 15-20 30-35 Glass fibers <12 vee <0, 25 <0.25 <0.25 <0.25 <0.25 < 0,25 <12mm Synthetic fibers <0,0,1 <, 1 <0,0,1 <, 1 <0 <0,1 <0,1 <12mm Additives <2 <2 2 <2 <2 <2 < 2

Claims (4)

Refractory gypsum ceramic and mineral fillers for precision casting by the mold - block process, characterized in that it contains up to 1.0% by mass of synthetic fibers in relation to the mass of refractory plaster.   2) ceramic mixture according to claim 1, characterized in that   the synthetic fibers have a length of less than 12 mm.   3) ceramic mixture according to any one of claims 1 and 2,   characterized in that the synthetic fibers are polypropylene fibers. 15   4) ceramic mixture according to any one of claims 1 to 3,   characterized in that it contains from 15 to 50% by weight, preferably from 18 to 35% by weight of refractory plaster.    Ceramic mixture according to any one of claims 1 to 4, intended for industrial applications, in particular for the casting of low-melting point metal alloys such as aluminum and magnesium alloys, especially in the field of automotive, electronics 25 or aeronautics, characterized in that   it contains, in mass proportions, approximately 30% of refractory plaster, 30 to 70% of silicas or aluminas, up to 50% of silicoaluminous compounds, especially chamottes, and from 0.05 to 0.1% of synthetic fibers. tick.   Ceramic mixture according to any one of claims 1 to 4 for the casting of low-melting metal alloys, especially aluminum alloys in the tire or shoe industry,   characterized in that it contains in mass proportions about 20% of refractory plaster, 20 to 25% of silicas or aluminas, preferably quartz, about% silico-aluminous compounds, especially chamottes, and 0, 05 at 0.1% of synthetic fibers.   7) ceramic mixture according to any one of claims 1 to 4, 5 for the crystal and crafts, especially the realization   of glass paste parts, characterized in that   it contains, in mass proportions, approximately 30% of refractory plaster, 60 to 65% of silicas or aluminas, 5 to 10% of silicoioaluminous compounds, especially chamottes, and 0.05 to 0.1% of fibers. synthetic.