EP0316978B1 - Moulding device with variable porosity for making foundry sand moulds, and method for its manufacture - Google Patents

Abstract

PCT No. PCT/EP88/00942 Sec. 371 Date Aug. 9, 1989 Sec. 102(e) Date Aug. 9, 1989 PCT Filed Oct. 20, 1988 PCT Pub. No. WO89/03736 PCT Pub. Date May 5, 1989.A gas-permeable form tool for manufacturing casting and core moulds from hardenable moulding sand includes a heteroporous, open-pore material. The wall of the tool contains a first fine-pore layer region adjacent to the moulding sand with a thickness of about 0.2-2 mm and a material density of about 75% to 95% of theoretical specific density and a pore diameter of about 50 mu m. The first fine-pore layer comes in contact with a second, large-pore supporting skeleton having a theoretical material density of less than 80% of theoretical specific density and a median pore diameter of more than 100 mu m.

Description

The invention relates to a gas-permeable mold for the production of casting and core molds from hardenable molding sand, as well as a method for its production and an advantageous use of such tools.

Molds made from molding sand are widely used in the manufacture of cast metal parts. These are solid or bowl-shaped shapes that can only be used once. To produce the casting mold, fine-grained molding sand is provided with hardenable binder additives, placed in a mold via a sand inlet opening and cured there. The curing takes place thermally - high energy expenditure - or more recently alternatively also by means of reaction gases, which are pressed under pressure through the molding sand in the molding tool. In the latter variant, the gas is pressed into the sand at the sand inlet opening and must exit the molding tool through bores, nozzles or other channels and openings mechanically introduced into the mold wall.
According to a known embodiment (DE 24 03 199, DE 30 39 394), bores in the wall of the molding tool on the outside of the mold are closed by pressure relief valves. Such molds have the disadvantage of high tool costs. The valves often become clogged with sand grains entrained in the gas and must be cleaned. Above all, however, the mold wall does not have a homogeneous gas permeability, so that the reaction gases cannot flow homogeneously through the molding sand and consequently the molding sand does not harden uniformly. Core forms can only be produced in solid form.

DE 30 02 939 describes a molding tool with a wall into which ribs and slots of different dimensions are mechanically introduced. The reaction gas entering the molding sand through an inlet is drawn off through the slots.
But the slots are sanding. In addition, the production is very expensive and does not allow the production of a really close-knit network of slots and bores. The reaction gas also flows through the sand only unevenly in this embodiment of a mold. In addition, reaction gas is consumed in excess, that is, in much larger amounts than required by the stoichiometry of the desired reaction.

It has also already been required to manufacture the mold from porous and gas-permeable materials. The implementation of this requirement has so far failed due to the expected technical difficulties to implement complex geometries of castings in a mold made of porous materials, while meeting both the requirement of a homogeneous gas permeability of the wall in the micro range and its mechanical strength and at the same time ensuring that the molding sand does not clog the mold pores or even pass through the pores of the mold wall when pressurized with reaction gas.

The object of the present invention is therefore to produce a mold with a wall which is homogeneously gas-permeable in the micro range. The methods and techniques described at the beginning are thus ruled out. In particular, the task is to create a heteroporous mold wall using a suitable combination of techniques known per se for producing porous materials, which has a suitable microporosity in its area adjacent to the molding sand and forms a coarse-pored, skeletal support structure in its adjacent area . The molding tools produced in this way are intended to permit the production of casting molds from molding sand in large numbers, in particular also as a non-massive, shell-shaped casting mold. For this purpose, the surface of the molding tool exposed to the molding sand must be particularly wear-resistant. Pore clogging by molding sand should no longer be a major cause of failure of the molding tool. Pores which may be blocked by molding sand have to regenerate with little effort, i.e. H. get exposed again.

The object of creating a gas-permeable molding tool is achieved according to the invention in that the tool consists of heteroporous, open-pore material, the wall of the molding tool having a first, fine-pored layer area of 0.2-2 mm thickness, 75-95, adjacent to the molding sand % theoretical material density and pore diameter <50 µm, to which a second, massive area in the form of a large-pore support skeleton with <80% theoretical material density and an average pore diameter> 100 µm is materially adjacent.

For the gas-permeable mold, as well as processes for their production and their advantageous use, designs according to subclaims 2 to 10 have proven particularly useful.

The molding tools include both casting and core forms, i. H. both molds for the production of massive and hollow castings.

In order to achieve the required material and structural properties of the mold wall according to the present invention, a number of individual methods are available to the person skilled in the art for the production of porous materials, which can be combined in a sensible manner.

In principle, metallic and / or ceramic materials and / or plastics come into consideration as materials for the mold wall. Up to 60,000 sand molds can be produced in a single mold of known designs. For reasons of economy, the sand is filled into the mold at high speed and under high pressure. The wear requirements for the surface of the mold coming into contact with the molding sand are correspondingly high. This fact must be taken into account by the selection of the material for the fine-pored layer of the molding tool. Wear-resistant steel grades as well as wear-resistant ceramics as well as metallic and non-metallic hard materials, e.g. As silicon nitride, boron nitride, titanium carbide, titanium nitride, silicon carbide.

The heteroporous wall of the molding tool can either be formed by viscous, foamed and then solidified material, or the wall is formed by means of a powdery starting material to be solidified.

The layer of the mold wall that comes into contact with the molding sand can be formed by isostatically pressing powder onto a jig mold corresponding to the casting. The powder, mixed with a volatile solvent, can be applied as a paste or sprayed onto the gauge. Galvanic processes and gas deposition processes (PVD processes) for forming such layers have also proven successful. Finally, the layer can be placed on the gauge shape in the form of a flexible metallic or ceramic film. The flexibility of such foils is given by volatile, highly flexible thermoplastic components in solid form during subsequent heat treatment. Otherwise, the foils consist of powdery metals, hard materials or ceramics.

The gauge form covered with the layer material is then either foamed or, after embedding in a corresponding outer form, backfilled with coarse-grained powder material and preferably isostatically pressed.

The finished composite body is produced by thermal or chemical curing, firing or sintering of the compacted composite materials.

For the manufacture of the open-pore support structure, it has proven useful to first coat sand, glass or ceramic grains with a thin plastic layer by immersing them in appropriate dispersions or solutions.

The granules pretreated in this way can be poured into a mold and / or pressed and then chemically or thermally cured.

The techniques for obtaining fine-pore or coarse-pore and open-pore materials are known. For example, in the manufacture of diaphragms for electrodes in electrochemistry, techniques have been developed using special pore formers, which result in a material structure with a defined gas permeability, as is also required in the present case. Techniques for the production of coarse and open-pore materials have been developed in the wide range of applications of mechanical filters as well as for example in the field of self-lubricating plain bearings or in the field of electrical contact materials, consisting of a porous skeleton of a material A, into which a material B is infiltrated.

Molding tools according to the present invention have a number of advantages.

They have an open-pore wall structure with a defined pressure drop that is completely homogeneous down to the micro range. This allows an even gas passage through the wall and thus homogeneous hardening of the molding sand. The pores in the fine-pored area of the mold wall are designed so that grains of sand can only settle in the mold wall in exceptional cases. It is crucial, however, that these grains of sand can usually be removed from the pores again with little effort by blowing air under high pressure, possibly in connection with solvent vapors, from the direction of the coarse-pored skeleton of the mold wall through the fine-pored wall layer.

In contrast to known methods, in which the gas hardening of the molding sand is carried out by blowing gas through the sand inlet opening, when using the molding tools according to the invention, the molding sand enclosed in the molding tool can be pressurized through the heteroporous wall. By appropriately setting the gas pressure and time, it is possible to effect curing of the enclosed molding sand only in an edge zone to a desired depth. An even finer dosage can be achieved by soaking the mold with a suitable liquid. As a result, a defined capillary pressure builds up in the fine pores of the tool wall, which releases the reaction gas only when this pressure is exceeded. The core of the enclosed sand remains free-flowing if the gas is dosed stoichiometrically and can be removed and reused after the edge zone has hardened through the sand inlet opening.

A major advantage of molds according to the present invention lies in the possibility of adapting their surface facing the molding sand to the desired casting mold, but the rear surface of which with a few flat surfaces, e.g. B. cuboid or cylindrical. Due to the gas loading of the molding sand through the porous wall of the molding tool, a fine gas layer regularly forms between the wall of the molding tool and the molding sand. This prevents the molding sand from sticking to the mold wall during the sand curing process. The sand mold easily detaches from the mold after the hardening process. Special measures against the gluing of molding sand and molding tool (spraying the molding tool wall, inserting a film), as are required in known tools and processes for the production of casting molds, can therefore generally be avoided. The technique of successively applying fine-pored layers and skeletal materials to the gauge form allows the mold to be given the final shape, surface finish and wear resistance immediately. It is therefore neither a cost-intensive mechanical reworking of the surface of the mold wall to produce the desired geometry and surface roughness, nor a post-treatment, in particular thermal hardening processes, to achieve the required surface hardness or wear resistance - in contrast to the previous manufacturing processes for molds, which are not start from porous materials.

The invention is explained in more detail with reference to FIG. 1 and by means of two exemplary embodiments.

Figure 1 shows the design of a half-shell of a mold, in section, and devices for producing the mold according to a preferred method. 1 shows the model plate -1- with the gauge shape for the half-shell of a molding tool. The area of the model plate which gives off the sand inlet opening of the molding tool -1a- during later use is particularly marked. A sealing plate -2- lies on the model plate, or is screwed or clamped to it. It has a central cutout corresponding to the geometric shape of the molding tool to be produced. The fine-pore layer area -3- of the mold adjoining the molding sand has a constant layer thickness over the entire surface area, with the exception of a narrow area at the separating surface of the two half-shells. The open-pore support skeleton -4- is materially adjacent to the fine-pore layer area of the molding tool. The external geometric shape of the mold is specified by a mold box -5- or mold frame screwed onto the model plate. There are manufacturing variants possible where the molding box is not completely filled with the material, but where an air space -6- remains between the support skeleton and the top of the molding box when filling in a flowable or spreadable material.

example 1

According to the technique shown in FIG. 1 (for the production of the molding tool), a model plate with the gauge shape of one half of the cast part to be manufactured is first produced from a metallic and / or ceramic material or from plastic by customary methods. In the majority of cases, it is advisable for core and casting molds to produce the mold from two half-shells. After applying a release agent, a sealing plate, preferably made of steel or ceramic, is applied to the model plate and screwed to the model plate. The central recess in the sealing plate is to be dimensioned such that in the area of the separating surface of the two half-shells of the mold between the gauge surface (model plate) and the sealing plate there remains a gap at least as thick as the fine-pored layer area of the mold.

The fine-pored layer of the molding tool is first applied to the gauge surface of the model plate - if necessary after applying a release agent on the gauge surface. For this purpose, a paste is spread or sprayed on. The paste consists of fine-grained, corrosion-resistant ceramic powder with an average grain size of 10 - 100 µm, to which 10 - 20% by volume titanium carbide powder (measured by the proportion of ceramic powder) of approximately the same grain size is added to increase the surface wear resistance of the mold. The powder is mixed with a volatile or thermally evaporable binders processed into a paste. If appropriate, non-volatile metallic and / or non-metallic components and / or pore formers are added to the binder. The fine-pore layer is advantageously applied in several layers until the desired total layer thickness is reached. The layer application according to FIG. 1 also takes place over the edge of the sealing plate.

The fine-pored layer applied in this way is dried or cured. Then a molding box or molding frame according to FIG. 1 is screwed onto the model plate or sealing plate and the material for forming the wall area is introduced into the molding box with an open-pore support skeleton. It is a coarse-grained ceramic powder to which volatile pore-forming materials have been added, such as are used, for example, in the production of porous ceramic filters. The ceramic powder is mixed with volatile binders to form a paste, which is then brushed into the molding box and cured there. The mold is then separated from the model plate and sintered or fired in high-temperature furnaces. In this way, wear-resistant, mountable mold half-shells with flat parting surfaces are obtained. As a rule, the mold surface does not require any surface treatment. The area of the sand inlet opening of the mold is finally sealed with a pore filler, so that no reaction gas can pass through this area of the mold wall during later operation and the molding sand cannot harden in this area.

The testing of molds produced in this way with a wall structure according to the invention has shown that a pressure difference of 1-2 bar can be built up at the boundary between coarse and fine-pored layers. Here lies the Fluctuation range of the absolute gas pressure in front of the border in the coarse-pored part of the wall in different sections of the mold wall or in different molds produced according to the same process between 0.1 - 0.2 bar and is therefore largely independent of how thick the coarse-pored supporting skeleton the mold wall is actually. The said jump in gas pressure at the boundary between the coarse and fine-pore layer occurs practically solely because of the structure of the fine-pore layer. This pressure jump can still be stabilized by impregnating the molding tool with a suitable sealing liquid, as a result of which a very homogeneous capillary pressure builds up in the pores of the fine-pored layer over the entire surface area of the molding tool.
The production of a casting mold from hardenable molding sand using a molding tool according to the present invention then proceeds as follows. After the molding sand has been filled in, the molding tool is pressurized with reaction gas from the outside at a pressure of> 2 bar. This presses the liquid out of the capillaries of the fine-pored layer of the molding tool and reaches the molding sand or an edge zone of the sand mold with precisely metered gas pressure. This enables the molding sand to harden to a desired, easily metered depth. The core area of the filled molding sand remains free-flowing. It can be removed and reused via the sand inlet after curing is complete. When the gas pressure drops below 2 bar, the barrier liquid is drawn back into the pores of the fine-pored layer by wicking. This means short production times for the individual sand molds as well as low susceptibility to faults and reject rates.

Example 2

Analogous to example 1, a jig mold or model plate is produced for a half-shell of a molding tool. Similarly to Example 1, a sealing plate is clamped onto the model plate. The mold wall material for the fine-pored layer is placed on the gauge shape in the form of a flexible metallic foil. The separately manufactured metallic foil consists of a homogeneous mixture of corrosion-resistant steel particles with a grain size distribution of 10 - 100 µm, possibly enriched with a few volume percent wear-resistant titanium carbide particles of comparable grain size, possibly supplemented by powdered fillers and pore-forming materials as well as a thermoplastic that evaporates at higher temperatures Plastic. Using techniques known for isostatic powder tube pressing, a rubber or plastic "tube" is then clamped to the bottom of the model plate and filled with a coarse-grained powder mixture consisting of alloyed iron powder and pore former - covering the fine-pored layer. The inside of the hose is then evacuated and the hose is closed. The complete unit is cold isostatically pressed. The green compact of the molding tool that is produced in this way can be separated from the model and processed further using customary sintering processes. The sintered molding tool can - if necessary - be mechanically processed and, for example, be dimensionally adapted for inclusion in tool holders.
In isostatic powder pressing in plastic or rubber casings, it is common to give the rubber casing the rough shape or contours of the molded part to be pressed. Correspondingly, half-shells of shaped tools with approximately homogeneous mold wall thickness can be achieved in the present case.

As already mentioned above, a large number of techniques are known for producing porous and / or large-pored molded articles based on powdery materials, in accordance with the broad field of application for porous molded articles. The description for the production of molds according to the present invention is therefore not conclusive with reference to those product groups.

Claims (12)

1. Gas-permeable form tool for manufacturing casting and core moulds made of hardenable moulding sand,
   characterised in that
   the tool is made of an open-pored material of variable porosity, with the wall of the form tool having adjacent to the moulding sand a first fine-pored layer region, which is 0.2 - 2 mm thick, 75 - 95% of the theoretical material density and has a pore diameter < 50 µm, adjacent to which in a material-locking manner is a second solid region in the form of a large-pored support skeleton of < 80% of the theoretical material density and with a mean pore diameter > 100 µm.
2. Form tool according to claim 1, characterised in that the material is an open-pored, hardened foam.
3. Form tool according to claim 1, characterised in that the form tool is made of a ceramic material.
4. Form tool according to claim 1, characterised in that the form tool is made of a metallic material.
5. Form tool according to claim 1 to 4, characterised in that the first and/or the second region comprise(s) two or more layers of an intrinsically homogeneous structure and material composition.
6. Form tool according to claim 1 to 5, characterised in that the wall regions of varying pore size are made of different materials.
7. Form tool according to one of claims 1 to 6, characterised in that the inner tool surface has the complex geometry of the cast part to be produced and the outer tool surface comprises a few plane surfaces.
8. Form tool according to claim 1 to 7, characterised in that the form tool comprises two or more parts.
9. Process for manufacturing a form tool according to one of claims 1, as well as 3 to 8, characterised by the following process steps:

   - application of a layer of a fine-grained powder of material A onto a template mould of the casting body,

   - application, in particular pressing on, of a coarse-grained powder of material B onto the layer,

   - hardening of the laminated body thus formed in a single working process, in particular under gradually changing process conditions.
10. Process for manufacturing a form tool according to claim 9, characterised in that a pore-forming agent is added to at least one of the powdery materials prior to application.
11. Process for manufacturing a form tool according to claim 10, characterised in that hardening of the laminated body is effected by sintering.
12. Use of the form tool according to one of claims 1 to 11 to manufacture non-solid, shell-shaped casting moulds made of moulding sand.

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