Method for the layered construction of bodies by 3D printing with a water-glass-containing binder and a processing additive

Abstract

The invention relates to a method for the layered construction of moulds and cores, comprising a fire-proof mould base material and a binder containing at least water glass in the form of an aqueous alkali silicate solution and containing at least one processing additive for increasing the nozzle open time or pressure stability. The invention also relates to the binder used in the method and to the moulds or cores thus produced.

Description

[0001] Process for the layer-by-layer construction of bodies by 3D printing with a water glass-containing binder and a processing additive

[0002] The invention relates to a method for the layered construction of bodies, in particular molds and cores, from a base material and a binder containing at least water glass in the form of an aqueous alkali silicate solution and a processing additive to improve printability and nozzle open time. For the layered production of molds and cores using 3D printing, it is necessary to apply a refractory base material in layers as part of the building material mixture and to selectively print each layer using the binder. Furthermore, the invention relates to the binder used in the method, a kit, and molds or cores produced in this way.

[0003] State of the art

[0004] Casting molds essentially consist of cores and molds, which represent the negative molds of the casting to be produced. These cores and molds consist of a refractory material, such as quartz sand, and a suitable binder that gives the casting mold sufficient mechanical strength after removal from the mold. For the sake of simplicity, cores and molds are referred to individually and collectively as casting mold or casting molds. To produce casting molds, a refractory mold base material is used, which is provided with a suitable binder. The refractory mold base material is preferably in a free-flowing form so that it can be poured into a suitable hollow mold. The binder creates a strong bond between the particles / grains of the mold base material, giving the casting mold the required mechanical stability.

[0005] Casting molds must meet various requirements. During the casting process itself, they must first exhibit sufficient strength and temperature resistance to accommodate the liquid metal in the cavity formed by one or more casting molds. After the solidification process begins, the mechanical stability of the casting is ensured by a solidified metal layer that forms along the walls of the casting mold. Furthermore, the casting mold must be gas-permeable to dissipate heated gases and binders that evaporate or decompose due to the heat during the casting process.

[0006] The material of the casting mold must now decompose under the influence of the heat emitted by the metal in such a way that it loses its mechanical strength, i.e., the cohesion between individual particles / grains of the refractory material is eliminated. The desired result is that the casting mold disintegrates into a fine sand that can be easily removed from the casting.

[0007] The term "rapid prototyping" refers to various methods for producing three-dimensional objects using layered construction. One advantage of these processes is the ability to produce even complex, one-piece objects with undercuts and cavities. With conventional methods, these objects would have to be assembled from several individually manufactured parts. Another advantage is that 3D printers are capable of producing the objects directly from CAD data without the need for molds.

[0008] 3D printing processes are placing new demands on binders that hold the mold together when the binder or a binder component is to be applied through the nozzles of a print head. These binders must not only provide a sufficient level of strength and good disintegration properties after metal casting, as well as exhibit sufficient thermal and storage stability, but must also be "printable." This means that, on the one hand, the print head nozzles must not become clogged with the binder, and, on the other hand, the binder should not be able to flow directly out of the print head, but rather form individual droplets.

[0009] Furthermore, there is an increasing demand that, as far as possible, no emissions in the form of CO2 or hydrocarbons are produced during the manufacture of the casting moulds, as well as during casting and cooling, in order to protect the environment and limit the odour nuisance of the surroundings caused by hydrocarbons, mainly aromatic hydrocarbons.

[0010] In order to meet these requirements, inorganic binder systems have been developed or further developed in recent years. Their use leads to the avoidance or at least significant minimization of emissions of CO2 and hydrocarbons in the production of metal molds.

[0011] EP 2392424 B1 discloses an inorganic binder system that makes it possible to produce casting molds with sufficient stability. This binder system is particularly suitable for thermal curing in a core shooter, in which a premixed molding material mixture (a mixture of at least refractory material and binder) is forced into the heated mold by pressure.

[0012] WO 2012 / 175072 A1 discloses a method for the layered construction of models, which utilizes an inorganic binder system. The layered, particulate material comprises a particulate building material and a spray-dried alkali silicate solution. Selective activation of the curing process occurs using a water-containing solution added via the print head. Both pure water and modified water containing rheological additives are disclosed. Examples of rheological additives include thickeners such as glycerin, glycol, or phyllosilicates, with particular emphasis on phyllosilicates. WO 2012 / 175072 A1 discloses the use of non-aqueous alkali silicate solutions. The binder or water glass solution is not metered via the print head, but is already contained in solid form as alkali silicate in the layered, particulate material.

[0013] DE 102011053205 A1 discloses a method for producing a component using deposition technology, in which, among other things, water glass is used as a printing fluid, among many other possibilities. The water glass can thus be dosed using a print head and applied to a predetermined portion of the respective uppermost layer. However, DE 102011053205 A1 does not provide any information on which water glass compositions can be used. WO 2013 / 017134 A1 discloses an aqueous alkali metal silicate solution with a viscosity at 20°C of 45 mPas or less, which has a solids content based on the alkali metal silicate of 39 wt.%. The ratio between SiO2 and M2O (M2O is Na2O and K2O) is given as a weight ratio. The narrowest limits of this weight ratio are between 1.58 and 3.30.The examples in WO2013 / 017134 A1 indicate that the viscosity of water glass binders can be reduced using a ball mill. However, such a process is very complex and costly.

[0014] DE 102014118577 A1 describes a process for the layered construction of casting molds comprising a refractory mold base material and a binder containing at least one aqueous alkali silicate solution and furthermore a phosphate or a borate, or both. The disclosed binder is said to be highly printable.

[0015] The prior art described above does not disclose anything regarding the nozzle open time of the printing fluid, i.e., its printing behavior over an extended period of use. In this case, the binding agent is the printing fluid. The nozzle open time of a printing fluid is particularly crucial for longer printing processes in order to ensure consistent quality across the entire component height. Furthermore, a long nozzle open time enables reduced cleaning effort for the print head during the process, which leads to time savings and reduced consumption of cleaning substances. According to a prevailing theory in the prior art, during the printing process, air convection causes water to diffuse into the surrounding air at the print head nozzles. This leads to a local increase in the viscosity of the printing fluid directly at the nozzle opening.This viscosity increase can occur within seconds to minutes and is more pronounced the longer the waiting time between two printed drops (the period without drop application). The resulting increased viscosity of the printing fluid at the nozzle can lead to the corresponding nozzle either being able to form drops only late or no drops being able to be applied at all, thus clogging the nozzle and requiring further cleaning. This leads to areas to be printed being printed late, and thus incompletely, or not at all. This results in defects in the manufactured bodies. The object of the invention.

[0016] The inventors therefore set themselves the task of developing a process for 3D printing casting molds in which a water glass binder is selectively dosed directly onto the spread building material mixture via a print head, whereby the water glass binder has good printability and a long nozzle open time and, on the other hand, requires the smallest possible amount of organic substances in the binder in order to keep emissions during casting and cooling as low as possible.

[0017] Summary of the invention

[0018] This object is achieved by a method having the features of the independent claims. Advantageous further developments of the method according to the invention are the subject of the dependent claims or are described below. The invention further relates to a binder used as a printing fluid for 3D printing and to a kit consisting of the binder and the building material mixture.

[0019] The method for the layered construction of bodies comprises at least the following steps: a) spreading at least one layer of a building material mixture with a layer thickness of 0.05 mm to 3 mm, preferably 0.1 mm to 2 mm and particularly preferably 0.1 mm to 1 mm of the building material mixture, b) printing selected areas of the at least one layer with a binder comprising water glass and at least one processing additive, and c) repeating at least steps b) and c) several times); wherein the processing additive comprises one or two members, preferably both members selected from the group comprising: i) a salt of a lactam having 3 to 6 ring carbon atoms, in particular with

[0020] 4 or 5 ring carbon atoms, and a carboxyl group; and ii) a salt of a hydroxycarboxylic acid having 2 to 6, in particular 3 or 4, carbon atoms and having 1 to 4, in particular 1 or 2, hydroxy groups and having 1 to 3, in particular 1 or 2, carboxylic acid groups. The sodium salts are preferred in each case.

[0021] The processing additive is processing additive i) or processing additive ii) or the sum of processing additive i) and processing additive ii). The mass ratio of processing additive i) to ii) is in particular from 3:1 to 1:3, preferably 2:1 to 1:2, and particularly preferably 2:1 to 1:1.

[0022] In particular, the processing additive comprises at least i) the salt of a lactam having 3 to 6 ring carbon atoms, in particular having 4 or 5 ring carbon atoms, and a carboxyl group and optionally additionally ii) the salt of the hydroxycarboxylic acid.

[0023] A preferred example of a lactam (cyclic amide) with a carboxyl group is pyrrolidonecarboxylic acid, or the lactam salt is the sodium salt of pyrrolidonecarboxylic acid (sodium PCA). Pyrrolidonecarboxylic acid has 4 ring carbon atoms (processing additive i)).

[0024] A preferred example of the hydroxycarboxylic acid is 2-hydroxypropanoic acid, or of the salt of the hydroxycarboxylic acid is the sodium salt of 2-hydroxypropanoic acid (sodium lactate) (processing additive ii)). The amount of the processing additive in the binder is, individually when i) and ii) are used individually, or in total when i) and ii) are used together, preferably from 0.1 wt.% to 10 wt.%, more preferably from 0.5 wt.% to 5 wt.%, and particularly preferably from 1 to 4 wt.%, in each case based on the binder.

[0025] The binder contains the processing additive to increase print stability and nozzle open time. The addition of the processing additive increases the time during which the water glass binder can remain in the print head without droplet application, without clogging or constricting nozzles, and after which trouble-free application of the binder via the print head is possible. The time period without droplet application can also be very short, e.g., a few seconds, such as the time elapsed during the spreading of the thin layer during which no droplet application occurs.

[0026] The body can, after removal of the unbound areas of the building material mixture, be a core or a mold (here individually and collectively also referred to as casting molds).

[0027] When one or more processing additives are used in the binder, the process or the bodies produced with it have the following properties:

[0028] 1. High pressure and process stability

[0029] 2. Reduced use of cleaning processes in the process

[0030] 3. High homogeneity and long-term stability of the binder

[0031] 4. Good strength, especially after thermal curing

[0032] 5. Very good storage stability

[0033] 6. Good disintegration properties after metal casting

[0034] 7. Minimal emission of CO2 or other organic pyrolysis products during the casting and cooling process, as only a minimal amount of organic components is used.

[0035] Surprisingly, it was found that the processing additive has a very high compatibility and long-term stability in the binder and drastically increases the die open time of the binder.

[0036] Detailed description of the invention

[0037] The binder according to the invention is intended, among other things, for the 3D printing of casting molds. The binder serves as a printing fluid with which a material applied in layers, such as a refractory mold base material (e.g., quartz sand) and optionally one or more additives, collectively referred to as a building material mixture, is selectively printed. The building material mixture does not yet contain the binder. Typically, after the layer-by-layer application of the building material mixture in one or more layers (e.g., two or three), in particular one layer, a selective printing process follows. This process is repeated until the entire printing process is complete and the casting mold can be obtained after removing the unbound areas of the building material mixture. The curing of the binder can be carried out in the usual way.On the one hand, it is possible to add one or more water glass hardeners to the layered building material mixture, which cause the immediate hardening of the printed water glass-containing binder by chemical means, preferably using heat for thermal hardening.

[0038] It is also possible to harden the applied water glass using acidic gases such as CO2 - although this option is less preferred.

[0039] On the other hand, thermal curing can also occur. For example, it is possible for thermal curing to occur after completion of one or every second or third printing process (immediately before, during, or after the next layer of the building material mix is ​​applied). This can be done by irradiating the mixture of building material mix and binder, for example using infrared light. During this layer-by-layer curing, the infrared light can be guided along the print head, for example in the form of a spot. It is of course also possible to carry out this type of thermal curing in stages after several layers have been applied. It is also possible to carry out thermal curing only after the last printing process has been completed - the steps of "applying a layer of the building material mix" and the subsequent "printing process" alternate until the last layer required to completely produce the casting mold has been printed.For this purpose, the applied and partially printed layers remain in a so-called "job box," which can then be transferred to a microwave oven or a convection oven, for example, for thermal curing. Thermal curing is preferably carried out using microwaves and preferably after the entire printing process in the microwave oven.

[0040] The binder, within the meaning of the present invention, is the printing fluid with all its components as it is transported through the nozzle(s) of the print head. Preferably, all components of the binder are in dissolved or liquid form.

[0041] The binder contains water glasses, which are produced, for example, by dissolving glassy lithium, sodium, and / or potassium silicates in water. Water glasses containing at least sodium are preferred, the content of which is expressed as Na2O.

[0042] Preferably, the Na2Ü / M2O ratio (where M = Na, K, and Li, respectively) in the binder is greater than 0.4, preferably greater than 0.5, more preferably greater than 0.6, and particularly preferably greater than 0.7, where M2O represents the sum of the molar amounts of lithium, sodium, and potassium, calculated as oxide, in the binder. In a preferred embodiment, M2O is equal to Na2Ü. For example, the molar ratio Na2Ü / M2O is from greater than 0.4 to 1 or greater than 0.7 to 1, in particular from 0.98 to 1.

[0043] According to one embodiment, the binder has a molar modulus SiO2 / M2O of greater than 1.4, preferably greater than 1.6, more preferably greater than 1.8, more preferably greater than 1.9. The water glass preferably has a molar modulus of less than 2.8, preferably less than 2.6, more preferably less than 2.5, particularly preferably less than 2.4, where M2O stands for the sum of the molar amounts of lithium, sodium and potassium ions, each calculated as oxide. For example, the molar modulus of the water glass is from greater than 1.4 to less than 2.8 or greater than 1.8 to less than 2.4 for water glass.

[0044] According to one embodiment, the binder has a solids content of greater than 20 to less than 42 wt. %, preferably greater than 24 to less than 38 wt. %, more preferably greater than 27 to less than 37 wt. The solids content is determined by carefully evaporating the liquid, thus drying the binder, and then heating it at 600°C for 1 h in an air atmosphere. The remaining oxidic material is weighed to determine the solids content. "Solids content" does not necessarily mean that solids are present in the binder. On the contrary, the solids determined in this way are preferably all dissolved in the binder.

[0045] Irrespective of this, the molar amount of SiO2 and M2O (calculated as mol%) in the binder is generally less than 16 mol%, preferably less than 15 mol%, more preferably less than 14 mol%, and particularly preferably less than 13.5 mol%. Furthermore, this molar amount is generally greater than 7 mol%, preferably greater than 8 mol%, preferably greater than 9 mol%, particularly preferably greater than 10 mol%, and particularly preferably greater than 10.5 mol%. The binder must not be too thin, but also not too thick. At a temperature of 25°C, the binder according to the invention, according to a preferred embodiment, has a viscosity of less than 45 mPas, preferably less than 20 mPas, preferably less than 16 mPas, and particularly preferably less than 14 mPas.Independently of this, at a temperature of 25°C, the binder according to one embodiment has a viscosity of greater than 2 mPas, preferably greater than 4 mPas, preferably greater than 7 mPas and particularly preferably greater than 8 mPas.

[0046] The viscosity is measured using a Brookfield rotational viscometer with the measuring geometry spindle 18 at a viscosity up to 16 mPas and a speed of 200 rpm and at a viscosity below 16 mPas with the measuring geometry spindle UL adapter at a speed of 50 rpm.

[0047] According to a preferred embodiment, the density of the binder (at 25°C) is less than 2.5 g / cm 3 , preferably less than 2.0 g / cm 3 and particularly preferably less than 1.5 g / cm 3 . Independently of this, according to a preferred embodiment, the density is greater than 1.0 g / cm 3 , preferably greater than 1.05 g / cm 3and particularly preferably greater than 1.1 g / cm 3 The density is measured using the oscillating U-tube method, for example.

[0048] According to a preferred embodiment, the surface tension of the binder is less than 60 mN / m, preferably less than 50 mN / m, and particularly preferably less than 45 mN / m. Independently of this, the surface tension of the binder is also greater than 15 mN / m, preferably greater than 20 mN / m, and particularly greater than 25 mN / m. The surface tension is measured using the Du Noüy ring method (at 25°C).

[0049] The binder should be a clear solution and as free as possible from particles that are smaller than 25 pm at their greatest dimension and that could originate from impurities, for example. Commercially available water glass solutions generally contain these coarser particles. The particle sizes in the water glass and also in the binder are determined using dynamic light scattering in accordance with DIN / ISO 13320 (e.g. Horiba LA 950, Fraunhofer method). The determined Dgo value (in each case based on the volume) is a measure of the larger particles - it means that 90% of the particles are smaller than the specified value. The water glass according to the invention has in particular a Dgo value (determined by dynamic light scattering) of less than 20 pm, preferably less than 10 pm and particularly preferably less than 5 pm.

[0050] Independently of this, the water glass according to the invention has a Dwo value of in particular less than 25 pm, preferably less than 20 pm and particularly preferably less than 10 pm with respect to the solid contained therein.

[0051] The water glasses described above or the binder containing water glasses can be obtained, for example, by suitable filtration – for example, filters with a sieve diameter of 25 pm, preferably 10 pm, and particularly preferably 5 pm are suitable. Preference is given to water glass or a binder containing particles with a maximum size of 1 pm, preferably containing no particles at all.

[0052] In one embodiment, the binder according to the invention can contain lithium ions. The molar ratio of Li2O / M2O can vary widely, for example, between 0.01 and 0.3. The ratio is preferably in the range between 0.03 and 0.17, more preferably between 0.035 and 0.16, and particularly preferably between 0.04 and 0.14.

[0053] In one embodiment, the binder according to the invention can contain potassium ions. The molar ratio of K2O / M2O can vary widely, for example, between 0.01 and 0.3. The ratio is preferably in the range between 0.01 and 0.17, more preferably between 0.02 and 0.16, and most preferably between 0.03 and 0.14.

[0054] The addition of network formers other than water glass / silicate can both increase thermal stability and reduce reactivity. Therefore, a network former from the phosphate group can be added to the binder and dissolved in the binder. Alkali phosphates (e.g., sodium hexametaphosphate or sodium polyphosphates) have proven particularly beneficial.

[0055] Among the alkali phosphates, alkali orthophosphates such as trisodium phosphate (NasPO4) are not preferred. Sodium polyphosphates and / or sodium metaphosphates are particularly preferred. Other network formers that can be added to the binder alternatively or additionally are borates, especially alkali borates, e.g., disodium tetraborate decahydrate. These are also dissolved in the binder.

[0056] The amounts of alkali metals resulting from the proportions of alkali borates and / or alkali phosphates in the total amount of the binder (including diluent) are calculated as oxides and contribute to the total amount (i.e., the sum of the individual amounts) of lithium, sodium, and potassium oxide in the entire aqueous solution. Consequently, according to this specification, the addition of alkali borates and / or alkali phosphates also influences the molar modulus SiO2 / M2O.

[0057] The borate content in the binder, in particular the content of alkali borates, is calculated as B2O3. The molar ratio of B2O3 / SiO2 can vary over wide ranges, for example from 0 to 0.5. This ratio is preferably less than 0.3, more preferably less than 0.2, more preferably less than 0.1, especially preferably less than 0.08 and most preferably less than 0.06. This ratio is preferably greater than or equal to 0. In a further embodiment, this ratio is preferably greater than 0.01, especially preferably greater than 0.02. Borates in the sense of the invention are boron compounds in oxidation state III which are only directly bonded to oxygen, ie oxygen atoms are the direct bonding partners of the boron in the compound.

[0058] The phosphate content in the binder, in particular the content of alkali phosphates, is calculated as P2O5. The molar ratio of P2O5 / SiO2 can vary over wide ranges, for example from 0 to 0.5. This ratio is preferably less than 0.4, more preferably less than 0.3, more preferably less than 0.25, particularly preferably less than 0.2 and most preferably less than 0.15. This ratio is preferably greater than 0, preferably greater than 0.01, particularly preferably greater than 0.02. Phosphates in the sense of the invention are phosphorus compounds in oxidation state V which are only directly bonded to oxygen, ie oxygen atoms are the direct bonding partners of the phosphorus in the compound.

[0059] In another embodiment, the binder can also contain aluminum, in which case the aluminum content is calculated as Al2O3. Typically, the Al2O3 content is then less than 2 wt.%, based on the total mass of the binder.

[0060] In a preferred embodiment, surfactants can be added to the binder to influence the surface tension of the binder. The proportion of these surfactants is generally between 0.01 and 4.0 wt.%, preferably between 0.1 and 3.0 wt.%.

[0061] Suitable surface-active substances in the binder are described, for example, in DE 102007051850 A1, including preferably anionic surfactants bearing a sulfate and / or sulfonate group, especially C8 alkyl sulfates. Other suitable surface-active substances include polyacrylate salts (e.g., sodium salts - for example, Dispex N40 - Ciba) or silicone surfactants for aqueous systems (e.g., Byk 348, Altana). Surface-active substances based on trisiloxane or glycol (e.g., polyethylene glycol) can also be used.

[0062] In a further embodiment, polyethylene glycols can be added to the binder to make it somewhat more "benign" or easier to apply. These glycols are preferably polyethylene glycol, with low-molecular-weight polyethylene glycol such as PEG 200 being particularly preferred. The polyethylene glycol used has an average molecular weight (Mw) of less than 1000 g / mol, preferably less than 500 g / mol, and particularly preferably less than 400 g / mol and / or greater than 150 g / mol, preferably greater than 200 g / mol, or particularly preferably greater than 200 g / mol.

[0063] The addition of polyethylene glycols, based on the binder, is in the range from 0.01 wt.% to 2 wt.%, preferably from 0.1 wt.% to 1 wt.%, and particularly preferably from 0.2 wt.% to 0.7 wt.%. In a further embodiment, polyols can also be added to the binder to make the binder easier to apply. The polyols are tri- or tetrahydric alcohols having 2 to 4 carbon atoms and optionally an ether group, with glycerol being particularly preferred. The addition of the polyols, or glycerol, based on the binder, is in the range from 0.01 wt.% to 10 wt.%, preferably from 0.5 wt.% to 3 wt.%, and particularly preferably from 1 wt.% to 2 wt.%.

[0064] Depending on the application and the desired strength level, the waterglass-based binder is preferably used in a concentration of between 0.5 wt.% and 7 wt.%, preferably between 0.75 wt.% and 6 wt.%, particularly preferably between 1 wt.% and 5.0 wt.%, and especially preferably between 1 wt.% and 4.0 wt.%, based on the base material. All data refer to the total amount of binder, including the (particularly aqueous) solvent or diluent and the (possible) solids content (total = 100 wt.%).

[0065] Common and well-known materials can be used as refractory mold base material for the production of casting molds. Suitable examples include quartz sand, zirconium sand, or chrome ore sand, olivine, vermiculite, bauxite, chamotte, and artificial mold base materials such as glass beads, glass granules, and / or hollow aluminum silicate microspheres, in particular more than 50 wt.% quartz sand based on the refractory mold base material. To keep costs low, the proportion of quartz sand in the refractory mold base material is advantageously greater than 70 wt.%, preferably greater than 80 wt.%, and particularly preferably greater than 90 wt.%.

[0066] It's not necessary to use only virgin sand. To conserve resources and avoid landfill costs, it's actually advantageous to use as much reclaimed sand as possible, such as that obtained from used molds through recycling.

[0067] A refractory mold base material is understood to be a substance that has a high melting point (melting temperature). The melting point of the refractory mold base material is preferably greater than 600°C, preferably greater than 900°C, particularly preferably greater than 1200°C, and especially preferably greater than 1500°C. The refractory mold base material preferably makes up more than 80% by weight, in particular greater than 90% by weight, and particularly preferably greater than 95% by weight, of the building material mixture.

[0068] A suitable refractory mold base material, which can also be used as a component of the building material mixture, is described, for example, in WO 2008 / 101668 A1 (= US 2010 / 173767 A1). Reclaimed materials obtained by washing and subsequently drying crushed used molds are also suitable. As a rule, the reclaimed materials can make up at least approximately 70% by weight of the refractory mold base material, preferably at least approximately 80% by weight, and particularly preferably greater than 90% by weight.

[0069] In one embodiment of the invention, it may be advantageous for certain applications to use reclaimed material obtained by purely mechanical treatment. Mechanical treatment means that at least a portion of the binder remaining in the used sand is removed from the sand grain by a grinding or impact principle. These reclaimed materials can be used as required. The proportion of these reclaimed materials can, for example, be greater than 5% by weight, preferably greater than 20% by weight, more preferably greater than 50% by weight, particularly preferably greater than 70% by weight, and especially preferably greater than 80% by weight of the refractory mold base material. Such reclaimed materials are used, for example, to achieve (pre- or partial) hardening of the applied binder.

[0070] The average particle size of the refractory mold base material is generally between 30 pm and 500 pm, preferably between 40 pm and 400 pm, more preferably between 50 pm and 250 pm and particularly preferably between 100 pm and 200 pm. The particle size can be determined, for example, by sieving in accordance with DIN 66165 Part 2. Particular preference is given to particle shapes / grains with a ratio of greatest longitudinal expansion to smallest longitudinal expansion (at right angles to one another and in all spatial directions) of 1:1 to 1:5 or 1:1 to 1:3, i.e. those which are not fibrous, for example. The refractory mold base material is free-flowing. In a preferred embodiment, the building material mixture can contain a proportion of particulate amorphous silicon dioxide in order to increase the strength level of the casting molds. Increasing the strength of the casting molds, especially increasing the hot strength, can be advantageous in the automated manufacturing process.Synthetically produced amorphous silicon dioxide is particularly preferred.

[0071] The average particle size (including any agglomerates) of the amorphous silicon dioxide is preferably less than 300 pm, more preferably less than 200 pm, particularly preferably less than 100 pm. The sieve residue of the particulate amorphous SiO2 when passing through a sieve with a mesh size of 125 pm (120 mesh) is preferably between 0.1 and 10 wt.%, more preferably not more than 5 wt.%, and most preferably between 0.15 and 2 wt.%. Irrespective of this, the sieve residue on a sieve with a mesh size of 63 pm is less than 10 wt.%, preferably less than 8 wt.%. The sieve residue is determined according to the machine sieving method described in DIN 66165 (Part 2), with a chain ring additionally used as a sieving aid.

[0072] The particulate amorphous silicon dioxide preferably used according to the present invention has a water content of less than 15 wt.%, in particular less than 5 wt.% and particularly preferably less than 1 wt.% (drying to constant mass at 105°C).

[0073] The particulate amorphous SiO2 is used as powder (including dusts).

[0074] Both synthetically produced and naturally occurring silicas can be used as amorphous SiO2. The latter are known, for example, from DE 102007045649 A1, but are not preferred because they generally contain significant crystalline fractions and are therefore classified as carcinogenic. Synthetic refers to non-naturally occurring amorphous SiO2, i.e. its production involves a deliberately performed chemical reaction such as that initiated by humans, e.g. the production of silica sols by ion exchange processes from alkali silicate solutions, precipitation from alkali silicate solutions, flame hydrolysis of silicon tetrachloride, or the reduction of quartz sand with coke in an arc furnace during the production of ferrosilicon and silicon. The amorphous SiO2 produced by the last two processes is also referred to as pyrogenic SiO2.

[0075] Occasionally, synthetic amorphous silicon dioxide refers only to precipitated silica (CAS No. 112926-00-8) and flame-hydrolytically produced SiO2 (pyrogenic silica, fumed silica, CAS No. 112945-52-5), while the product resulting from ferrosilicon or silicon production is referred to simply as amorphous silicon dioxide (silica fume, microsilica, CAS No. 69012-64-12). For the purposes of the present invention, the product resulting from ferrosilicon or silicon production is also understood as particulate amorphous SiO2.

[0076] Precipitated silicas and pyrogenic silicon dioxide, i.e., silicon dioxide produced by flame hydrolysis or arc annealing, are preferred. Particularly preferred materials are amorphous silicon dioxide produced by thermal decomposition of ZrSiO4 (described in DE 102012020509 A1) and SiO2 produced by oxidation of metallic silicon using an oxygen-containing gas (described in DE 102012020510 A1). Also preferred is quartz glass powder (mainly amorphous silicon dioxide), which is produced by melting and rapidly cooling crystalline quartz, so that the particles are spherical and not splintery (described in DE 102012020511 A1).

[0077] The average primary particle size of the particulate amorphous silicon dioxide can be between 0.05 pm and 10 pm, in particular between 0.1 pm and 5 pm, most preferably between 0.1 pm and 2 pm. The primary particle size (like the particle size in general) can be determined, for example, using dynamic light scattering (e.g. Horiba LA 950) and checked by scanning electron microscope images (SEM images using, for example, a Nova NanoSEM 230 from FEI). Furthermore, the SEM images made it possible to visualize details of the primary particle shape down to the order of 0.01 pm. The silicon dioxide samples were dispersed in distilled water for the SEM measurements and then applied to an aluminum holder covered with copper tape before the water was evaporated. Furthermore, the specific surface area of ​​the particulate amorphous silicon dioxide was determined using gas adsorption measurements (BET method, nitrogen) according to DIN 66131.The specific surface area of ​​particulate amorphous SiO2 is between 1 and 200 m. 2 / g, especially between 1 and 50 m 2 / g, particularly preferably between 1 and 30 m 2 / g. If necessary, the products can also be mixed, e.g., to obtain mixtures with specific particle size distributions.

[0078] Depending on the manufacturing method and producer, the purity of the amorphous SiO2 can vary considerably. Types containing at least 85 wt.% silicon dioxide have proven suitable, preferably at least 90 wt.%, and particularly preferably at least 95 wt.%. Depending on the application and the desired strength level, between 0.1 wt.% and 2 wt.% of particulate amorphous SiO2 are used, preferably between 0.1 wt.% and 1.8 wt.%, particularly preferably between 0.1 wt.% and 1.5 wt.%, in each case based on the refractory mold base material.

[0079] Based on the total weight of the binder (including diluent or solvent), the amorphous SiO2 is preferably present in the building material mixture in a proportion of 1 to 80 wt.%, preferably 2 to 60 wt.%, particularly preferably 3 to 55 wt.%, and especially preferably between 4 and 50 wt.%. Or, independently thereof, preferably, based on the ratio of solids content of the waterglass-based binder (based on the oxides, ie, total mass of alkali metal oxides M2O and silicon dioxide) to amorphous SiO2 of 10:1 to 1:1.2 (parts by weight).

[0080] The amorphous SiO2 is added to the building material mixture. When using amorphous SiO2, the process according to the invention is thus further characterized by one or more of the following features:

[0081] (a) The amorphous silicon dioxide is only added to the building material mixture.

[0082] (b) The amorphous silicon dioxide has a BET surface area of ​​between 1 and 200 m 2 / g, preferably greater than or equal to 1 m 2 / g and less than or equal to 30 m 2 / g, particularly preferably less than or equal to 15 m 2 / g. (c) The amorphous silicon dioxide is selected from the group consisting of: precipitated silica, pyrogenic silicon dioxide produced by flame hydrolysis or in an electric arc, amorphous silicon dioxide produced by thermal decomposition of ZrSiO4, silicon dioxide produced by oxidation of metallic silicon by means of an oxygen-containing gas, quartz glass powder with spherical particles produced by melting and rapidly recooling crystalline quartz, and mixtures thereof, and is preferably amorphous silicon dioxide produced by thermal decomposition of ZrSiO4.

[0083] (d) The amorphous silicon dioxide is preferably used in amounts of 0.1 to 10 wt.%, particularly preferably 0.15 to 2 wt.%, in each case based on the building material mixture.

[0084] (e) The amorphous silicon dioxide has a water content of less than 5 wt.% and particularly preferably less than 1 wt.%.

[0085] (f) The amorphous silicon dioxide is particulate amorphous silicon dioxide, preferably having an average primary particle diameter determined by dynamic light scattering between 0.05 pm and 10 pm, in particular between 0.1 pm and 5 pm and particularly preferably between 0.1 pm and 2 pm.

[0086] In another embodiment, an inorganic hardener for waterglass-based binders is optionally added to the building material mixture before the binder is added. Examples of such inorganic hardeners include phosphates such as Lithopix P26 (an aluminum phosphate from Zschimmer und Schwarz GmbH & Co. KG Chemische Fabriken) or Fabutit 748 (an aluminum phosphate from Chemische Fabrik Budenheim KG). Other inorganic hardeners for waterglass-based binders include calcium silicates and their hydrates, calcium aluminates and their hydrates, aluminum sulfate, magnesium carbonate, and calcium carbonate.

[0087] The ratio of hardener to binder can vary depending on the desired properties, e.g., processing time and / or stripping time of the building material mixtures. The hardener content (weight ratio of hardener to binder and, in the case of water glass, the total mass of the silicate solution or other binders absorbed in solvents) is advantageously greater than or equal to 5 wt.%, preferably greater than or equal to 8 wt.%, particularly preferably greater than or equal to 10 wt.%, in each case based on the binder. The upper limits are less than or equal to 25 wt.% based on the binder, preferably less than or equal to 20 wt.%, particularly preferably less than or equal to 15 wt.%.

[0088] Irrespective of this, between 0.05 wt.% and 2 wt.% of the inorganic hardener are used, preferably between 0.1 wt.% and 1 wt.%, particularly preferably between 0.1 wt.% and 0.6 wt.%, in each case based on the mold base material.

[0089] In a further embodiment, graphite is added to the building material mixture. Such graphite additives are, for example, glass powder graphite or graphite-silver powder. The amount used is preferably between 0.1 wt.% and 10 wt.%, and particularly preferably between 0.15 wt.% and 2 wt.%, in each case based on the building material mixture. The particle size of the graphite is preferably less than 300 μm, preferably less than 200 μm, particularly preferably less than 100 μm. The particle size of the graphite is preferably more than 1 μm, preferably more than 3 μm, particularly preferably more than 5 μm.

[0090] Once the strength permits, the unbound building material mixture can be removed from the mold and the mold can be sent for further processing, such as preparation for metal casting. The unbound and bound building material mixtures can be separated, for example, by means of a spout, allowing the unbound building material mixture to trickle out.

[0091] The bound building material mixture (casting mold) can be freed of residues of the unbound building material mixture using compressed air or by brushing.

[0092] The unbound building material mixture can be reused for a new printing process.

[0093] Printing is performed, for example, with a print head having a plurality of nozzles, wherein the nozzles are preferably individually and selectively controllable. According to a further embodiment, the print head is moved at least in one plane under computer control, and the nozzles apply the liquid binder layer by layer and selectively according to the available data. The print head can be, for example, a drop-on-demand print head with bubble jet or, preferably, piezo technology. Test examples

[0094] The invention is explained below using experimental examples, but is not limited to them.

[0095] Unless otherwise stated, all ratios and percentages are based on weight.

[0096] In order to compare the printing stability of different water glass binders, the quality of drop formation in the print head was investigated after a defined downtime.

[0097] The tests were conducted on a commercial printing system (VX 200 from Voxeljet AG). To determine a starting value, the print head of the printing system was first cleaned and then a test print was made on thermal paper (test print "Start"). The print head was then cleaned again, left in the system without printing for 20 minutes, and then another test print was made (test print "20 min"). The test prints were then digitized with a scanner at a resolution of 600 dpi. In each test print, a fixed image section (1640 x 520 pixels) was isolated and converted into a black and white image using the Imaged software (open source), so that only black and white pixels remain in the image.The software can now determine the number of black pixels representing the ink-wetted areas and compare this to the number of pixels across the entire surface (in this case, 1640 x 520 pixels). This yields an area fill for the test print. The test print after the 20-minute waiting time is processed in the same way. The relative loss of print quality is then calculated as a percentage using the formula 100. The following binders were compared: Binder 1: A commercially available binder with a viscosity of 11 mPas at 25°C (INOTEC™ Binder EP 5061 from ASK Chemicals) (not according to the invention)

[0098] Binder 2: A water glass binder with a viscosity of 10 mPas at 25°C (INOTEC Binder EP 5061 from ASK Chemicals diluted with water to obtain an approximately equal viscosity) (not according to the invention)

[0099] Binder 3: Binder 2 with a viscosity of 11 mPas at 25°C further containing 2.5 wt.% of the sodium salt of pyrrolidonecarboxylic acid (sodium PCA) as a processing additive (according to the invention)

[0100] Binder 4: Binder 2 with a viscosity of 11 mPas at 25°C, further containing 2.5 wt.% sodium PCA as processing additives and additionally 2.5 wt.% glycerol (according to the invention)

[0101] Binder 5: Binder 2 with a viscosity of 11 mPas at 25°C containing additionally 2 wt.% sodium PCA and 1 wt.% sodium lactate as processing additives and additionally 2 wt.% glycerol (according to the invention)

[0102] Binder 6: Binder 2 with a viscosity of 10.5 mPas at 25°C, further containing 1% sodium PCA and 0.5% by weight sodium lactate as processing additives and additionally 1% by weight glycerol (according to the invention)

[0103] Binder 7: Binder 2 with a viscosity of 11 mPas at 25°C further containing 2 wt.% sodium lactate as processing additives (according to the invention)

[0104] Binder 8: Binder 2 with a viscosity of 11 mPas at 25°C, further containing 2 wt.% sodium PCA and 1 wt.% sodium lactate as processing additives (according to the invention) Table 1: Relative loss of print quality after 20 min standstill time for different binders

[0105] The results in Table 1 show a significantly lower relative loss of print quality for the binders according to the invention compared to a commercial binder (Binder 1) or a binder without processing additives (Binder 2). These low quality losses of the binders according to the invention allow a significant reduction in print head cleaning cycles, resulting in time and material savings in the printing process. Furthermore, the high print stability ensures consistent component quality throughout the entire manufacturing process. This avoids weak points in the manufactured component.

Claims

Patent claims 1. A method for the layered construction of bodies, comprising at least the following steps: a) spreading at least one layer of a building material with a layer thickness of 0.05 mm to 3 mm; b) printing selected areas of the at least one layer of the building material with a binder comprising at least water, water glass, and a processing additive; c) repeating at least steps a) and b several times; wherein the processing additive is: i) a salt of a lactam substituted with a carboxyl group and having 3 to 6 ring carbon atoms, in particular 4 or 5 ring carbon atoms; or ii) a salt of a hydroxycarboxylic acid having 2 to 6 carbon atoms, in particular 3 or 4 carbon atoms, and having 1 to 4 hydroxy groups, in particular 1 or 2 hydroxy groups, and having 1 to 3 carboxylic acid groups, in particular 1 or 2 carboxylic acid groups; or a mixture of i) and ii).

2. The process according to claim 1 or 2, wherein, independently of one another, the processing additive is i) the salt of pyrrolidonecarboxylic acid, in particular the sodium salt; and / or ii) the salt of 2-hydroxypropanoic acid, in particular sodium lactate.

3. Process according to at least one of the preceding claims, wherein the amount of processing additive in the binder is from 0.1% by weight to 10% by weight, preferably from 0.5% by weight to 5% by weight and particularly preferably from 1 to 4% by weight, in each case based on the binder.

4. Process according to at least one of the preceding claims, wherein the mass ratio i) to ii) is from 3:1 to 1:3, preferably 2:1 to 1:2 and particularly preferably 2:1 to 1:

1.

5. The process according to at least one of the preceding claims, wherein the binder further comprises a polyol having 2 to 4 carbon atoms, 3 or 4 hydroxy groups, and optionally an ether group, preferably glycerol; in particular in an amount of 0.01 wt.% to 10 wt.%, preferably 0.5 wt.% to 3 wt.%, and particularly preferably 1 wt.% to 2 wt.%.

6. Method according to at least one of the preceding claims, wherein the binder is characterized by one or more of the following features: - the water glass has a molar modulus SiC>2 / M2O of greater than 1.4 to less than 2.8, preferably greater than 1.6 to less than 2.6, preferably greater than 1.8 to less than 2.5 and more preferably greater than 1.9 to less than 2.4, where M2O stands for the sum of the molar amounts of lithium, sodium and potassium ions, each calculated as oxide; - the binder has a dynamic viscosity at a temperature of 25°C of less than 45 mPas, preferably less than 20 mPas, particularly preferably less than 14 mPas; - the binder has a dynamic viscosity at a temperature of 25°C of greater than 2 mPas, preferably greater than 4 mPas, particularly preferably greater than 7 mPas, particularly preferably 7 to 14 mPas; - any particulate components in the binder have a Dgo value of less than 20 pm, preferably less than 10 pm and particularly preferably less than 5 pm; - the binder contains at least one phosphate or at least one borate or a phosphate and a borate, in particular alkali phosphates and particularly preferably sodium hexametaphosphate and / or sodium polyphosphates.

7. The method according to at least one of the preceding claims, wherein the binder further contains surface-active substances, preferably surfactants, in particular between 0.01 and 4.0 wt.%, preferably between 0.1 and 3.0 wt.%.

8. The method according to at least one of the preceding claims, wherein the binder further comprises polyethylene glycols, in particular - 0.01 wt% to 2 wt%, preferably from 0.1 wt% to 1 wt% and particularly preferably from 0.2 wt% to 0.7 wt%, based on the binder; and / or - the polyethylene glycols have an average molecular weight (Mw) of greater than 150 to less than 1000 g / mol, preferably greater than 200 to less than 500 g / mol, particularly preferably greater than 200 to less than 400 g / mol.

9. Method according to at least one of the preceding claims, wherein the building material mixture comprises a refractory molding base material and the refractory molding base material is preferably quartz sand, zircon sand, chrome ore sand, olivine, vermiculite, bauxite, chamotte, glass beads, glass granules, aluminum silicate micro hollow spheres and mixtures thereof.

10. The method according to at least one of the preceding claims, wherein the building material mixture or the refractory molding base material according to claim 9 has an average particle diameter of 30 pm to 500 pm, preferably from 40 pm to about 400 pm, and particularly preferably from 50 pm to about 250 pm.

11. A method according to claim 9 or 10, wherein greater than 80% by weight, preferably greater than 90% by weight, and particularly preferably greater than 95% by weight of the building material mixture is refractory molding material.

12. Method according to at least one of the preceding claims, wherein the building material mixture further comprises particulate amorphous silicon dioxide, in particular - between 0.1 wt.% and 10 wt.% and preferably between 0.15 wt.% and 2 wt.%, in each case based on the building material mixture and / or - the particulate amorphous silicon dioxide has a particle size of less than 300 pm, preferably less than 200 pm, particularly preferably less than 100 pm.

13. Method according to at least one of the preceding claims, wherein the building material mixture further comprises graphite, in particular - between 0.1 wt.% and 10 wt.% and preferably between 0.15 wt.% and 2 wt.%, in each case based on the building material mixture and / or - the graphite has a particle size of less than 300 pm, preferably less than 200 pm, particularly preferably less than 100 pm.

14. Method according to at least one of the preceding claims, further comprising the following steps: i) thermal curing of the building body after completion of the layered construction, if necessary in an oven or by means of a microwave, to obtain an at least partially cured building body, and subsequently ii) removing the unbound building material mixture from the at least partially cured building body.

15. Method according to at least one of the preceding claims, wherein the printing is carried out with a print head having a plurality of nozzles, wherein the nozzles are preferably individually selectively controllable, wherein the print head is in particular a drop-on-demand print head with bubble jet or piezo technology.

16. The method according to claim 15, wherein the print head is movable at least in one plane under computer control and the nozzles apply at least the binder layer by layer.

17. Mould or core producible by the process according to at least one of the preceding claims for metal casting, in particular iron, steel, copper or aluminium casting.

18. Binder comprising at least water, water glass and a processing additive, wherein the processing additive is: i) a salt of a lactam substituted with a carboxyl group and having 3 to 6 ring carbon atoms, in particular having 4 or 5 ring carbon atoms, or ii) a salt of a hydroxycarboxylic acid having 2 to 6 carbon atoms, in particular 3 or 4 carbon atoms, and having 1 to 4 hydroxy groups, in particular 1 or 2 hydroxy groups, and having 1 to 3 carboxylic acid groups, in particular 1 or 2 carboxylic acid groups; and or a mixture of i) and ii).

19. Binder according to claim 18, wherein the binder has a dynamic viscosity at a temperature of 25°C of less than 45 mPas, preferably less than 20 mPas, particularly preferably less than 14 mPas.

20. Binder according to claim 18 or 19, further characterized by one or more of the features of claims 2 to 8.

21. Kit comprising the binder according to at least one of claims 18 to 20 and, separately therefrom, a building material mixture comprising refractory mold base material and preferably further particulate amorphous silicon dioxide and / or graphite.

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