Abstract:
The invention relates to a casting mold, in particular for use in cold casting methods, which is produced with the aid of a powder-based layering method, the final casting mold having a treated surface.

Description:
CLAIM OF PRIORITY 
       [0001]    This application is a national phase filing under 35 USC §371 of PCT Application serial number PCT/DE2015/000378 filed on Jul. 30, 2015, and claims priority therefrom. This application further claims priority to German Patent Application Number DE 10 2014 001 236.5 filed on Aug. 2, 2014. PCT Application Number PCT/DE2015/000378 and German Patent Application Number DE 10 2014 001 236.5 are each incorporated herein by reference in its entirety. 
     
    
     FIELD 
       [0002]    The present invention relates to a casting mold that is produced with the aid of a powder-based layering method, a use of the casting mold and a method for the production thereof. 
       BACKGROUND 
       [0003]    A method for producing three-dimensional objects from computer data is described in the European patent specification EP 0 431 924 B1. In this method, a particulate material is applied in a thin layer to a platform, and a fluid is selectively printed on the particulate material with the aid of a print head. In the area onto which the fluid is printed, the particles bind to each other, and the area solidifies under the influence of the fluid and, if necessary, an additional hardener. The platform is then lowered by a distance of one layer thickness into a build cylinder and provided with a new layer of particulate material, which is also printed as described above. These steps are repeated until a certain, desired height of the object is reached. A three-dimensional object is thereby produced from the printed and solidified areas. 
         [0004]    After it is completed, this object produced from solidified particulate material is embedded in loose particulate material and is subsequently removed therefrom. This is done, for example, using an extractor. This leaves the desired objects, from which powder deposits are removed, for example by manual brushing. 
         [0005]    Of the layering techniques, 3D printing based on powdered materials and the supply of fluids with the aid of a print head is the fastest method. 
         [0006]    This method may be used to process different particulate materials, including natural biological raw materials, polymers, metals, ceramics and sands (not an exhaustive list). 
         [0007]    The strengths of the submitted method lie in the high volume capacity and the cost-effective production. However, the material properties often lag behind those known from conventional production. 
         [0008]    For example, a material may be produced which uses sand particles as the base material and is bound by cement. This material is a type of concrete. The strength of a material of this type is, however, much lower than that of conventionally produced concrete, due to its porosity. 
         [0009]    Sand particles having other binding systems may also be processed by the powder-based 3D printing process. This includes, among other things, cold resin binding, which is used in foundry practice as well as in 3D printing. 
         [0010]    Inorganic binders are also state of the art in this area. These are the most environmentally friendly alternative to cold resin binders in foundry practice. 
         [0011]    These materials also do not directly achieve strengths that are relevant, e.g., for construction. In principle, only a few materials may be processed into dense and high-strength materials with the aid of the powder-based 3D printing method. These materials are essentially polymers. 
         [0012]    The use of two-stage methods is one way around this limitation in 3D printing. In this case, casting in 3D-printed molds is one option. This method is state of the art in the area of metal casting. 
         [0013]    In the area of concrete materials or cold-castable polymers, processing with the use of 3D-printed molds is not common practice for casting methods. On the one hand, this is due to the lack of the breakout and core removal capability, which is due to the absence of the solidity-reducing effect of heat in the cold casting process. On the other hand, an undesirable bonding of the casting material to the mold may build up during cold casting, since a 3D-printed casting mold has a relatively high porosity for cold casting methods, into which the casting material may penetrate. 
         [0014]    It is known from the prior art that reusable molds may generally be used for casting concrete parts. 
         [0015]    For example, coated wooden boards are used as formwork when casting straight walls. These boards are additionally pretreated with a formwork oil, thereby preventing the concrete from sticking to the multiple-use boards. 
         [0016]    To remove the formwork, these boards are usually peeled away from the concrete part by pounding them with a hammer after casting. 
         [0017]    More complex shapes made of concrete are often produced with the aid of silicone molds. In this case, the silicone mold is in the form of a negative model. The mold must then be laboriously produced using a model, which acts as a positive mold for the silicone casting. A model of this type may be produced, for example, using conventional techniques, such as milling in wood or plastic or using additive methods. To lend the silicone casting mold the necessary strength, an additional substructure is necessary, which makes the method even more complex. Like with the formwork boards, a mold release agent must frequently be applied to the silicone casting mold prior to casting. 
         [0018]    The silicone mold may then be removed after the casting material solidifies by simply pulling the part away from the mold. However, the substructure must first be removed for this purpose. 
         [0019]    These two methods cannot be used for molds which have been produced by powder-based 3D printing with the aid of cold resin binder. 
         [0020]    Due to the porosity of the molds, conventional mold release agents are unable to prevent the casting material from penetrating the mold. Instead, the mold release agents infiltrate the molded part and enter the interior without having any effect on the surface. 
       SUMMARY 
       [0021]    A number of terms in the invention are explained in greater detail below. 
         [0022]    Within the meaning of the invention, “3D printing method” relates to all methods known from the prior art which facilitate the construction of components as three-dimensional molds and are compatible with the method components and devices described. 
         [0023]    Within the meaning of the invention, “selective binder application” or “selective binder system application” may take place after each particulate material application or irregularly, depending on the requirements of the molded body and for the purpose of optimizing the production of the molded body, i.e., non-linearly and not in parallel after each particulate material application. “Selective binder application” or “selective binder system application” may thus be set individually and during the course of producing the molded body. 
         [0024]    “Molded body” or “component” within the meaning of the invention are all three-dimensional objects that are produced with the aid of the method according to the invention and/or the device according to the invention and which have a nondeformability. 
         [0025]    All materials known for powder-based 3D printing, in particular sands, ceramic powders, metal powders, plastics, wood particles, fibrous materials, celluloses and/or lactose powders, may be used as “particulate materials.” The particulate material is preferably a dry, free-flowing powder. However, a cohesive, firm powder may also be used. 
         [0026]    “Build space” is the geometric place in which the particulate material feedstock grows during the build process by repeated coating with particulate material. The build space is generally delimited by a base, the building platform, by walls and an open cover surface, the build plane. 
         [0027]    “Casting material” within the meaning of this invention is any castable material, in particular materials in which no temperatures arise during processing which could weaken a cold resin binding and which thus promote breakout from the mold. 
         [0028]    A “concrete material” within the meaning of this invention is a mixture of an additive (e.g., sand and/or gravel or the like) and a hydraulic binder, the mixing water being used up by the solidification reaction. A concrete material is also a possible casting material. 
         [0029]    “Porosity” within the meaning of the invention is a labyrinthine structure of cavities which occur between the particles bound in the 3D printing process. 
         [0030]    The “seal” acts at the geometric boundary between the printed mold and the cavity to be filled. It superficially closes the pores of the porous molded body. 
         [0031]    “Black wash” designates a fluid which contains particles and does not seal the porosity but only reduces the pore diameter on the surface of the mold. 
         [0032]    “Hydrostatic pressure” is used as a general term for all pressures which arise by the action of the fluid column of the casting material. 
         [0033]    “Low strength” in terms of sealing means that the seal does not resist any strong forces during breakout from the mold. 
         [0034]    “Cold casting methods” are understood to be, in particular, casting methods in which the temperature of the casting mold and the core do not reach the decomposition or softening temperature of the molding material before, during and after the casting process. The solidity of the mold is not influenced by the casting process. The opposite thereof would be metal casting methods, in which the mold is, in general, slowly destroyed by the hot casting compound. 
         [0035]    The term “treated surface” designates a surface of the casting mold, which is treated in a preferably separate step after the mold is printed and cleaned. This treatment is frequently an application of a substance to the surface and thus also in the areas of the mold or core near the surface. All conceivable methods may be considered for application. 
         [0036]    It is desirable from an economical point of view to implement casting molds for cold casting by means of 3D-printed molds, in particular for more complex molds. 
         [0037]    The object of the present invention is to provide a casting mold, in particular for use in cold-casting methods, which is produced with the aid of a powder-based layering method, the final casting mold having a treated surface. 
         [0038]    The treated surface may, for example, prevent the casting material from penetrating the molded body, due to the hydrostatic pressure or capillary effects. 
         [0039]    Moreover, it could also be that the forces are low during the breakout of porous bodies from the mold due to the surface treatment, since the seal may be drawn out of the pores of the porous molded material by the mold breakout process, and air may then subsequently flow through the porous mold. 
         [0040]    Preferred specific embodiments are illustrated below. 
         [0041]    According to one preferred specific embodiment of the invention, the treated surface (or the surface treatment) comprises a sealing, a coating, a processing and/or another treatment. 
         [0042]    The treated surface should preferably have a lower porosity than the casting mold after it is produced. 
         [0043]    In another aspect, the invention relates to a use of the casting mold according to the invention to produce cold-cast parts as a lost-wax casting mold and/or as a continuous casting mold. 
         [0044]    In particular, the casting molds according to the invention may be used to produce concrete cast parts and/or cold-cast polymer components. 
         [0045]    In yet another aspect, the present invention relates to a method for producing casting molds, in particular for use in cold-casting methods, the casting mold being built with the aid of a powder-based layering method, and the surface of the casting mold being treated. 
         [0046]    A powder bed-based 3D printing method is preferably used for the layering method, and a cold resin binding system is even more preferably used. 
         [0047]    If the surface is additionally sealed with a hydrophobic material, as needed, the penetration of the casting material into the pores of the casting mold may be effectively limited. 
         [0048]    Another possibility is to modify the porosity of the surface of the casting mold with the aid of an infiltrate. 
         [0049]    This may be done, for example, with the aid of an epoxy resin, a polyurethane, an unsaturated polyester, a phenol/resol resin, an acrylate and/or a polystyrene. 
         [0050]    It is furthermore possible to modify the porosity of the surface with the aid of a black wash and/or dispersion, in particular a zirconium oxide-, aluminum oxide-, calcium oxide-, titanium oxide-, chalk- or silicic acid-based black wash and/or a plastic-, cellulose-, sugar-, flour- and/or salt-based solution. 
         [0051]    The porosity of the surface may furthermore be modified or sealed with the aid of a grease, oil, wax and/or hot water-soluble substances. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0052]    Brief description of the figures, which represent preferred specific embodiments: 
           [0053]      FIG. 1 : shows a schematic representation of the components of a powder-based 3D printer in a sectional isometric view; 
           [0054]      FIG. 2 : shows a representation of a simple cast part without undercuts; 
           [0055]      FIG. 3 : shows a representation of a complex cast part, including undercuts; 
           [0056]      FIG. 4 : shows a representation of a set of multiple-use molds for producing the simple cast part; 
           [0057]      FIG. 5 : shows a sectional representation of ejectors and assembly aids for a multiple-use casting mold; 
           [0058]      FIG. 6 : shows a representation of a set of single-use molds for producing the complex cast part; 
           [0059]      FIG. 7 : shows a sectional view of a component treated according to the invention; 
           [0060]      FIG. 8 : shows an illustration of the mold breakout process with a treatment according to the invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0061]    One example of a device for producing a molded part according to the present invention includes a powder coater ( 101 ). Particulate material is applied thereby to a building platform ( 102 ) and smoothed ( FIG. 1 ). The applied particulate material may be made from a wide range of materials; according to the invention, however, sand is preferred for reasons of its low cost. This sand is precoated, for example, with an activator component. The height of powder layers ( 107 ) is determined by the building platform ( 102 ). It is lowered after one layer has been applied. During the next coating operation, the resulting volume is filled and the excess smoothed. The result is a nearly perfectly parallel and smooth layer of a defined height. 
         [0062]    After a coating process, a fluid is printed onto the layer with the aid of an ink-jet print head ( 100 ). The print image corresponds to the section of the component in the present build height of the device. The fluid strikes and slowly diffuses into the particulate material. 
         [0063]    The fluid reacts with the activator in the particulate material to form a polymer. The latter binds the particles to each other. 
         [0064]    In the next step, the building platform ( 102 ) is lowered by the distance of one layer thickness. The steps of layer construction, printing and lowering are now repeated until the desired component ( 103 ) is completely produced. 
         [0065]    The component ( 103 ) is now complete, and it is located in the powder cake ( 114 ). In the final step, the component is freed of the loose particulate material and cleaned with compressed air. 
         [0066]    A component produced in this manner forms the basis for the present invention. The use of these molds may be divided into two areas: single-use molds and multiple-use molds. According to the present invention, they may be used in cold-casting methods. 
         [0067]      FIG. 2  shows a simple cast part ( 200 ). It is economical to achieve multiple castings with the aid of one mold. A larger and more complex component is represented, for example, by a sink ( 300 ) in  FIG. 3 . The sink has a bowl-shaped formation ( 301 ) in its middle. An opening ( 303 ) for the later drain is situated in its center. Another opening ( 302 ) for the faucet is situated in the rectangular part of the basin. 
         [0068]    As a single-use mold ( 600 ), breakout is achieved by destroying the mold. The mold is expediently produced as a thin bowl. The structure is additionally reinforced by means of ribbing to withstand the hydrostatic pressures.  FIG. 6  shows a mold of this type. The mold is designed in two parts ( 600 ,  601 ). 
         [0069]      FIG. 4  shows a multiple-use mold. It comprises two halves ( 400 ,  401 ), each of which has a thick-walled design, and into which the cavity ( 402 ) for the casting material is introduced. A sprue ( 403 ) is also provided. 
         [0070]    The mold ( 400 ,  401 ;  600 ,  601 ) may be produced, for example, from a sand having an average grain size of 140 μm, which was premixed with a hardener for a so-called cold resin in the amount of 0.3 wt %. The binding process preferably takes place with a concentration of cold resin in the range of 1.0 to 2.5 wt %. 
         [0071]    After the printing process, the mold may be removed from the loose sand and cleaned. 
         [0072]    Different methods may be used to modify the pore size. For example, an infiltration with a two-component polymer is possible. However, the material must be used in such a way that, according to the invention, pores which facilitate easy mold breakout remain on the surface after treatment. For this purpose, the mold is treated, for example, with an adapted seal, which is processed at room temperature and does not develop high strengths. 
         [0073]    It is likewise possible to additionally use a black wash from the metal casting field. Smaller particles are applied to the surface in this case. The effective pore cross section is modified thereby. As a result, it is possible to prevent, for example, the mechanically weak seal according to the invention from being pressed into the mold due to high hydrostatic pressures. 
         [0074]    Grease may be used as a simple seal according to the invention. The grease may be applied to the mold by spreading or spraying it on. The grease muse be suitably selected for the task. Too heavy a grease may be difficult to process. Too thin a grease or oil infiltrates the mold and thus no longer provides a sealing function. 
         [0075]    After spreading or brushing, the grease may be additionally smoothed. A superficial application of heat is suitable for this purpose. This may be done, for example, with a hot air gun or a blowtorch. Thoroughly heating the mold is not desirable, since this may lead to the possibility of leaks in the seal. 
         [0076]    The use of wax is also possible according to the invention. The wax is expediently liquefied by heating for processing. The low viscosity must be increased by means of a thickener; for example, polystyrene microgranulates may be used for this purpose. It is also possible to use hydrophobic solvents, such as the alkanes or benzine, to create a wax solution whose viscosity may be effectively adjusted. 
         [0077]    A seal made from hot water-soluble polyvinyl alcohol may also be created. This material is dissolved in hot water and applied to the preheated mold. The mixing water of a concrete is unable to attack the seal. 
         [0078]      FIG. 7  shows the process on the microscopic level. The molded body is constructed with the aid of particles ( 700 ), which are bonded to each other. Fine particles ( 702 ) collect on the geometric component boundary ( 701 ) in the event of a black-washed component. The seal ( 703 ) seals the surface water-tight. 
         [0079]    The molds prepared in this manner are subsequently equipped with additional function components. 
         [0080]    For example, ejectors ( 500 ,  501 ) may be inserted into multiple-use molds for easier breakout from the mold. Depending on the expected breakout forces, the seat of the ejectors in the printed mold was reinforced in advance, e.g., using an epoxy resin infiltration. The ejectors may be designed as bolts ( 501 ), which engage with a nut ( 500 ), which may be countersunk into the printed part. A force is then generated between the mold and the cast part by applying a torque to the bolt. 
         [0081]    The mold may also be provided with centering pins. These pins minimize the offset between the mold halves and thus ensure a precise cast part. 
         [0082]    Some structures known from metal casting molds may be provided directly on the printed part. Thus, centering elements ( 603 ) may be implemented, and labyrinth seals ( 502 ) may be mounted for a better sealing action between the mold halves. 
         [0083]    The reinforcement is inserted into the mold cavity ( 402 ) before the molds are closed. It is expediently held at a distance relative to the mold with the aid of plastic or concrete supports. In this state, empty conduits may also be inserted into the mold for later introduction of electric lines or other media. 
         [0084]    The assembly of the molds may be facilitated by bores ( 503 ) in the molds. Bolts, which apply the compressive forces onto critical mold areas in a targeted manner, may be guided through these bores. Additional plates may also be screwed on, which reinforce the mold against the casting forces. 
         [0085]    Casting takes place through mounted sprues ( 403 ) or material shafts. Depending on the technique and casting material used, additional ventilation bores ( 602 ) may also be introduced. If a vibrator is to be inserted after casting to compress the casting material, an access is provided in the mold. Mold parts (e.g.  601 ), which are able to float, due to the pressure of the casting material, must be prevented from changing position, e.g. by being weighted down. 
         [0086]    After the casting process, the part rests for up to several days, depending on the binding time of the casting material. The demolding process then takes place. 
         [0087]    Due to the low strength of the seal, the latter is easily removed from the pores of the mold during breakout (see  FIG. 8 ). This process may be assisted by heating the mold together with the cast part. As a result of the low separating forces, even delicate cast parts may be safely broken out of the mold. 
         [0088]    If a single-use mold is used, the mold may be pre-damaged by hitting it with a hammer in a targeted manner. Depending on the wall thickness of the mold, the actual separation process is carried out with the aid of a putty knife or another flat tool. The mold may also be separated from the cast body by means of sand blasting. The selection of the blasting material and the pressure must be adapted according to the hardness of the casting material, so that the casting material is not damaged. 
         [0089]    The multiple-use mold is preferably placed in a furnace before breakout and heated overnight to a temperature of, for example, 60° C. Air circulation should be avoided to prevent drying out if concrete is used as the casting material. 
         [0090]    After the furnace process, the bond between the mold and cast part is stressed by tightening the bolts on the ejectors. The mold then usually opens with the aid of slight vibrations or hammer blows. 
         [0091]    After the casting process, the sealing medium ( 801 ) must be removed from the cast part ( 800 ). If grease is used, soaps and washing pastes for cleaning oils and greases are helpful. Hand washing paste that includes cleansing particles is particularly preferred in this case. 
         [0092]    After casting, the parts are further processed as in the case of conventional production methods. The usual methods such as grinding or sand blasting are used for surface modification. 
       LIST OF REFERENCE NUMERALS 
       [0093]      100  Print head 
         [0094]      101  Coater 
         [0095]      102  Building platform 
         [0096]      103  Component 
         [0097]      104  Build container 
         [0098]      105  Print head path 
         [0099]      106  Coater path 
         [0100]      107  Powder layers 
         [0101]      108  Direction of building platform movement\ 
         [0102]      109  Dosed droplets 
         [0103]      110  Powder roll 
         [0104]      111  Build space boundary 
         [0105]      112  Coater gap 
         [0106]      113  Coater stock 
         [0107]      114  Powder Cake 
         [0108]      200  Simple cast part 
         [0109]      300  Complex cast part, sink 
         [0110]      301  Bowl-shaped sink area 
         [0111]      302  Hole for faucet 
         [0112]      303  Hole for drain 
         [0113]      400  Casting mold, top box 
         [0114]      401  Casting mold, bottom box 
         [0115]      402  Cavity for casting material 
         [0116]      403  Sprue 
         [0117]      500  Nut 
         [0118]      501  Bolt 
         [0119]      502  Labyrinth seal 
         [0120]      503  Bore for assembly bolts 
         [0121]      600  Sink mold, bottom box 
         [0122]      601  Sink mold, top box 
         [0123]      602  Ventilation bores 
         [0124]      603  Mold centering element and contact point 
         [0125]      604  Mold core for drain 
         [0126]      605  Mold core for faucet 
         [0127]      606  Mold cores for wall mounting 
         [0128]      700  Particle 
         [0129]      701  Geometric mold boundary 
         [0130]      702  Particle of the black wash 
         [0131]      703  Seal 
         [0132]      800  Cast part 
         [0133]      801  Drawn-out seal