Artificial sandstone moulding produced by additive manufacturing, and the production thereof

Abstract

The invention relates to a method for producing a consolidated artificial sandstone moulding by additive manufacturing, and to the consolidated artificial sandstone moulding produced by means of the method. According to the method, the artificial sandstone moulding is subjected, after additive manufacturing, to a heat treatment and to an infiltration treatment followed by a curing step. The artificial sandstone moulding is suitable in particular for use as a prototype model or forming model, for example as a casting model for producing lost casting moulds, and as a plastics article for decorative purposes.

Description

[0001] Additively manufactured artificial sandstone molding and its production

[0002] The invention relates to a method for producing a solidified, additively manufactured artificial sandstone molded body, as well as the solidified, additively manufactured artificial sandstone molded body produced by the method. The artificial sandstone molded body is particularly suitable for use as a master or forming model, for example, as a casting model or core box for the production of lost-die casting molds; it is also suitable as a permanent mold and as an art and decorative object.

[0003] The additive manufacturing of molded bodies, commonly referred to as 3D printing, can be carried out using materials from almost all material groups. For example, it is possible to produce molded bodies additively from sand and a binder using the powder bed process. In this process, the sand particles, applied layer by layer as a bulk material, are bound to predetermined positions in the respective sand layer by means of a specifically applied binder; the three-dimensional molded body is created by building up the material layer by layer. The material thus created from a binder and an aggregate, such as sand, forms an artificial stone; if sand is used as the aggregate, an artificial sandstone. Organic binders, such as synthetic resins, or inorganic binders, such as alkali silicates, can be used as binders for the process.

[0004] DE 10 2011 105 688 A1 or EP 4 019 161 A describe the additive manufacturing of molded bodies made of sand and binders, i.e., additively manufactured artificial sandstone molded bodies, which are used in particular as lost molds, for example, as casting molds and casting cores, in the foundry industry. WO 2007 / 110091 A1 discloses a binder system for producing a sand casting mold by additive manufacturing.

[0005] The additively manufactured artificial sandstone molds possess inherently limited mechanical stability, which is why they are very well suited as lost molds for foundries, but only of limited use for multiple or long-term applications. Such multi-use components in the foundry industry include, for example, casting patterns, which are used to mold frequently used casting molds. Casting patterns are generally produced from solid material, such as wood, using abrasive processes; however, this custom production is usually quite complex. The use of an additively manufactured casting pattern would therefore be economically attractive, especially for the production of casting molds in small series.However, the casting patterns additively manufactured from known materials either do not offer the required mechanical stability, as in the case of the additively manufactured artificial sandstone moldings described above, or are not economically viable compared to conventionally manufactured casting patterns. Since the additive manufacturing of artificial sandstone moldings is extremely economical due to the low-cost starting material, i.e., sand, and the low rework effort, it would be desirable to produce casting patterns additively from artificial sandstone, which has sufficient mechanical stability for casting patterns.

[0006] The conventional production of artificial stone from an inorganic substrate material and a polymer matrix is ​​disclosed in DE 10 2017 107 591 A1. According to this document, crushed inorganic substrate material is mixed with a reactive polymer resin to form a raw mass, shaped, compacted, and the polymer resin is then cured by a polymerization reaction. Additional Si compounds are added to the raw mass mixture of polymer resin and substrate material to improve crosslinking during the polymerization reaction. These are preferably silanes; however, silicone resins can also be used. The Si compounds are preferably introduced by homogeneous mixing into the raw mass; alternatively, they can also be introduced—before curing—by application to the surface of the pre-formed or pre-compacted raw mass.

[0007] The object of the invention is to provide a mechanically more stable, additively manufactured artificial sandstone molding compared to the prior art, and a method for its production. This object is achieved by a method for producing a consolidated, additively manufactured artificial sandstone molding according to claim 1 and by a consolidated, additively manufactured artificial sandstone molding according to claim 9. Advantageous developments of the invention are set forth in claims 2 to 8, as well as 10 and 11.

[0008] According to the invention, in the first step of the process for producing the solidified, additively manufactured artificial sandstone molding, a - still unsolidified - artificial sandstone molding is additively manufactured in a generally known manner from sand and a binder. Additive manufacturing is preferably carried out using the powder bed process. To produce the artificial sandstone molding, sand with a defined grain size is bound layer by layer using the binder at predefined positions. The layered structure creates the artificial sandstone molding, which, since it is in an unsolidified state immediately after additive manufacturing, is also referred to as a green body in the context of this disclosure. The green body produced in this way has a more or less pronounced open porosity between the sand particles bonded by the binder, which runs through the unsolidified artificial sandstone molding, i.e.The sand particles are embedded in a partially formed matrix of binder permeated with pores.

[0009] The additively manufactured artificial sandstone body is preferably made of quartz sand, with the grain size of the quartz sand grains primarily ranging from 0.05 mm to 0.30 mm. Quartz sand with an average grain size of 0.13 mm to 0.14 mm is particularly suitable for the additive manufacturing of artificial sandstone bodies.

[0010] Synthetic resin, especially furan, phenol, or polyurethane resin, is the preferred binder in additive manufacturing. Alternatively, inorganic substances such as water glass can also be used as binders.

[0011] In the next process step after the additive manufacturing of the

[0012] The artificial sandstone molded body is subjected to heat treatment while still unconsolidated, i.e., as a green body. The heat treatment temperature is preferably in the range of 50 °C to 150 °C. The heat treatment temperature should be maintained for a holding time of at least 15 minutes. The artificial sandstone molded body is then cooled, usually to room temperature.

[0013] In the next process step, an infiltration treatment of the artificial sandstone molding, which has cooled after heat treatment, is carried out by applying a synthetic resin. For this purpose, the synthetic resin, preferably an epoxy resin or a silicone resin, is applied to the surface of the artificial sandstone molding with absorption in the artificial sandstone molding until the absorption capacity of the molding surface wetted with the synthetic resin is exhausted. The synthetic resin applied to the surface of the artificial sandstone molding penetrates into the open pores of the artificial sandstone molding, i.e. it infiltrates or saturates the artificial sandstone molding in an infiltration zone adjacent to the molding surface up to a predetermined infiltration depth. The process of applying the synthetic resin is continued until the applied synthetic resin no longer seeps into the interior of the molding under normal practical conditions.until no more resin is absorbed by the artificial sandstone molding, i.e., until saturation of the infiltration zone occurs. The formation of a permanent surface resin film can be used as an indicator for reaching this saturation level. The surface application of the resin can be carried out using conventional methods known from painting technology, such as spraying, brushing, dipping, or similar methods. The infiltration treatment is generally carried out under normal atmospheric conditions, i.e., without the aid of positive or negative pressure.

[0014] In the process step following the infiltration treatment, the resin-infiltrated artificial sandstone preform is air-cured to form the solidified, additively manufactured artificial sandstone preform. For this purpose, the artificial sandstone preform is stored in an air environment, which causes the synthetic resin in the infiltration zone to harden and thus the artificial sandstone preform to solidify. Like the infiltration treatment, air-curing typically takes place under normal atmospheric conditions, i.e., without overpressure or underpressure. Depending on the impregnate, the curing time is approximately 12 hours to several days. Curing can also be carried out as oven curing with elevated air temperatures, for example, in the range of 200°C to 220°C; the curing time is typically 12 to 24 hours.

[0015] It may be necessary to repeat the infiltration treatment and subsequent air curing steps to fill any remaining porosity. The infiltration and subsequent curing steps are preferably repeated once; however, they can also be repeated multiple times.

[0016] As an intermediate step after the heat treatment and before the infiltration treatment, the artificial sandstone molded body can be cleaned, for example by blasting with air or compressed air.

[0017] The strength of the additively manufactured green body of artificial sandstone is significantly increased by the consolidation steps following additive manufacturing. The solidified, additively manufactured artificial sandstone that can be produced or is produced using the process according to the invention has an infiltration zone adjacent to the surface of the body that is penetrated with cured synthetic resin to a predetermined infiltration depth. Typically, the infiltration depth is in the range of 2 mm to 1.5 cm, frequently in the range of 4 mm to 1 cm. Thin areas of the artificial sandstone are therefore completely penetrated with cured synthetic resin. Thicker areas, in contrast, have the material structure remaining after heat treatment in the core zone.The artificial sandstone moldings according to the invention therefore generally have a solid, resin-permeated shell zone near the surface and a somewhat softer, yet more flexible core zone. This combination of properties results in the high mechanical stability of the artificial sandstone molding, namely high strength coupled with good toughness. This means that the artificial sandstone moldings produced according to the process not only possess improved tensile, compressive, and flexural strength, but also improved resistance to impact and shock loads. Furthermore, the resin-permeated surface zone is generally liquid-tight, meaning the artificial sandstone moldings are surface-sealed.Artificial sandstone moldings according to the invention, which are designed as art or decorative objects, for example as sculptures, can therefore be installed not only indoors but also outdoors - due to the good weather resistance provided by the surface sealing.

[0018] The artificial sandstone molds are particularly suitable for master and forming models because, in addition to the required mechanical stability, they exhibit inherently high wear resistance due to their sand-based construction and sealed surface. The ability of additive manufacturing to quickly and individually produce molds makes the process according to the invention particularly suitable for the production of casting models, especially those used for sand casting mold production for small and medium-sized casting series.

[0019] The effectiveness of the infiltration treatment is ensured, among other things, by the preceding heat treatment. The heat treatment removes or outgasses residual materials, especially residual moisture, from the existing open pores, thus creating a favorable initial condition for the infiltration. Due not least to this combination of process steps, the infiltration can be carried out without negative or positive pressure. Cost-intensive process steps, such as vacuum or pressure impregnation, are possible but unnecessary.

[0020] According to one embodiment of the process, the solidified artificial sandstone molding can be surface-treated after the final air curing to improve the surface quality, i.e., in particular, to reduce the roughness of the molding surface, by abrasive processes such as grinding, vibratory finishing, blasting, or polishing. Furthermore, it can be provided that the solidified artificial sandstone molding is coated with a final coat of paint in a final process step. Pigmented alkyd resin nitrocellulose paints, polyisocyanate paints, and urea coatings have proven suitable for the final coat. Preferably, a paint system comprising a primer of pigmented alkyd resin nitrocellulose paint and a top coat of polyisocyanate paint is used as the final coat.

[0021] According to one embodiment of the process, the synthetic resin used for the infiltration treatment is an epoxy resin. An epoxy resin having a bisphenol A-based epoxy resin content of 80 to 90 percent by mass, a bisphenol F-based epoxy resin content of 5 to 10 percent by mass, and an aliphatic glycidyl-based epoxy resin content of 5 to 10 percent by mass has proven particularly suitable.

[0022] When using a silicone resin for the infiltration treatment, alkyl silicone resins have proven suitable, especially alkyl silicone resins with alkoxy groups.

[0023] The invention is explained in more detail below using exemplary embodiments and with reference to the schematic drawings, in which:

[0024] Fig. 1 : an additively manufactured artificial sandstone molded body in the form of a casting model in perspective view, and

[0025] Fig. 2: an additively manufactured artificial sandstone molded body in the form of a sculpture in longitudinal section.

[0026] Figure 1 shows a typical casting pattern for sand casting, produced using the described process. The additively manufactured artificial sandstone body forming the casting pattern is provided with a final coating for this application.

[0027] Figure 2 illustrates the typical material structure of the solidified, additively manufactured artificial sandstone body, which is shaped like a sculpture. The artificial sandstone body has the cross-hatched infiltration zone 1 in the near-surface edge zone, which surrounds the core zone 2, which remains largely unchanged from the additive manufacturing process.

Claims

Patent claims 1 . A method for producing a solidified, additively manufactured artificial sandstone molding, characterized by the sequentially carried out method steps: a) additive manufacturing of an artificial sandstone molding from sand and a binder; b) heat treatment of the artificial sandstone molding at a heat treatment temperature in the range from 50 °C to 150 °C for a holding time of at least 15 minutes; c) infiltration treatment of the artificial sandstone molding cooled after the heat treatment by application of a synthetic resin, wherein the synthetic resin is applied to the surface of the artificial sandstone molding with absorption in the artificial sandstone molding until the absorption capacity of the molding surface wetted with the synthetic resin is exhausted; d) air curing of the artificial sandstone molding infiltrated with synthetic resin to form the solidified, additively manufactured artificial sandstone molding.

2. Process according to claim 1, characterized in that the process steps of infiltration treatment and subsequent air curing are repeated at least once.

3. A method according to claim 1 or 2, characterized in that after the heat treatment and before the infiltration treatment, the artificial sandstone molded body is cleaned by cleaning with compressed air.

4. Method according to one of claims 1 to 3, characterized in that the solidified artificial sandstone molded body is surface-treated by grinding, vibratory grinding, blasting or polishing after the final air curing in order to reduce the roughness of the molded body surface.

5. Method according to one of claims 1 to 4, characterized in that the solidified artificial sandstone molded body in a final process step is coated with a final coat of paint, the final coat comprising a primer of pigmented alkyd resin nitrocellulose varnish and a top coat of polyisocyanate varnish.

6. The method according to any one of claims 1 to 5, characterized in that the additively manufactured artificial sandstone molded body is formed from quartz sand, the grain size of the quartz sand grains being in the range from 0.05 mm to 0.30 mm, and the binder being a furan resin, a phenolic resin, a polyurethane resin or water glass.

7. The method according to claim 6, characterized in that the synthetic resin for the infiltration treatment is an epoxy resin, the epoxy resin having a bisphenol A-based epoxy resin content of 80 to 90 mass percent, a bisphenol F-based epoxy resin content of 5 to 10 mass percent, and an aliphatic glycidyl-based epoxy resin content of 5 to 10 mass percent.

8. The method according to claim 6, characterized in that the synthetic resin for the infiltration treatment is an alkyl silicone resin.

9. Solidified, additively manufactured artificial sandstone molded body, produced according to one of claims 1 to 8, wherein the solidified artificial sandstone molded body is penetrated with cured synthetic resin up to a predetermined infiltration depth in an infiltration zone adjacent to the molded body surface.

10. Consolidated, additively manufactured artificial sandstone molded body according to claim 9, characterized in that the infiltration depth is in the range from 2 mm to 1.5 cm.

11. Solidified, additively manufactured artificial sandstone molding according to claim 9 or 10, characterized in that the solidified artificial sandstone molding is a master mold, a forming model, a core box, a mold, an art object or a decorative object.

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