Additively manufactured artificial sandstone mold body and its production

Abstract

The invention relates to a method for producing a hardened, additively manufactured artificial sandstone mold body, as well as to the hardened, additively manufactured artificial sandstone mold body produced by the method. According to the method, the artificial sandstone mold body undergoes heat treatment and an infiltration treatment followed by a curing step after additive manufacturing. The artificial sandstone mold body is particularly suitable for use as a master or forming pattern, for example, as a casting pattern for the production of expendable molds, as well as for use as an art object for decorative purposes.

Description

[0001] The invention relates to a method for producing a hardened, additively manufactured artificial sandstone mold body, as well as the hardened, additively manufactured artificial sandstone mold body produced by the method. The artificial sandstone mold body is particularly suitable for use as a primary or secondary mold, for example as a casting pattern or core box for the production of expendable molds; it is also suitable as a mold and as an art and decorative object.

[0002] The additive manufacturing of shaped objects, commonly referred to as 3D printing, can be carried out with materials from almost all material groups. For example, it is possible to additively manufacture shaped objects from sand and a binder using powder bed fusion. In this process, sand particles, applied layer by layer as a bed of sand, are bound at predetermined positions within each sand layer by means of a precisely applied binder; the three-dimensional shaped object is created layer by layer. The resulting material, consisting of a binder and an aggregate such as sand, forms an artificial stone, or, in the case of sand being 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 in this process.

[0003] In DE 10 2011 105 688 A1 or in EP 4 019 161 A, the additive manufacturing of shaped bodies made of sand and binders, i.e. additively manufactured artificial sandstone shaped bodies, is described, which are used in particular as lost molds, for example as casting molds and casting cores, in the foundry industry.

[0004] The additively manufactured artificial sandstone molds inherently possess limited mechanical stability. Therefore, while they are very well suited as expendable molds for foundries, they are only conditionally suitable for multiple or continuous applications. In the foundry industry, such frequently used components include, for example, casting patterns, which are used to create molds for commonly used castings. These patterns are generally produced from a solid material, such as wood, using subtractive manufacturing processes; however, this individual production is usually quite complex. For the production of molds in small batches, the use of an additively manufactured casting pattern would therefore be economically attractive.However, additively manufactured casting patterns made from known materials either lack the required mechanical strength, as in the case of the described additively manufactured artificial sandstone molds, or are not economically viable to produce compared to conventionally manufactured casting patterns. Since the additive manufacturing of artificial sandstone molds is extremely economical due to the cost-effective starting material, i.e., sand, and the minimal post-processing required, it would be desirable to additively manufacture casting patterns from artificial sandstone that possesses sufficient mechanical stability for casting purposes.

[0005] The object of the invention is therefore to provide a mechanically more stable, additively manufactured artificial sandstone mold body compared to the prior art, as well as a method for its production.

[0006] This problem is solved by a method for producing a hardened, additively manufactured artificial sandstone mold body according to claim 1 and by a hardened, additively manufactured artificial sandstone mold body according to claim 9. Advantageous embodiments of the invention are set out in claims 2 to 8 and 10 and 11.

[0007] According to the invention, in the first step of the process for producing the solidified, additively manufactured artificial sandstone mold, an unsolidified artificial sandstone mold is additively manufactured from sand and a binder in a generally known manner. The additive manufacturing is preferably carried out using a powder bed process. To produce the artificial sandstone mold, sand with a defined grain size is bound layer by layer at predefined positions by means of the binder. The layer-by-layer construction creates the artificial sandstone mold, which, since it exists in an unsolidified state immediately after completion of the additive manufacturing process, is also referred to as a "green body" within the scope of this disclosure. The green body produced in this way exhibits a more or less pronounced open porosity between the sand particles bound by the binder, extending throughout the unsolidified artificial sandstone mold.The sand particles are embedded in a partially formed, porous matrix of binder.

[0008] Preferably, the additively manufactured artificial sandstone mold is formed from quartz sand, with the grain size of the quartz sand grains primarily in the range of 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 the artificial sandstone molds.

[0009] Synthetic resins, especially furan, phenolic, or polyurethane resins, are preferably used as binders in additive manufacturing. Alternatively, inorganic substances, such as water glass, can also be used as binders.

[0010] In the next process step after the additive manufacturing of the artificial sandstone mold, the mold, still in its unhardened state (i.e., as a green body), undergoes heat treatment. 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. Subsequently, the artificial sandstone mold is cooled, typically to room temperature.

[0011] In the next process step, the artificial sandstone mold, cooled after heat treatment, undergoes an infiltration treatment by applying a synthetic resin. For this purpose, the synthetic resin, preferably an epoxy or silicone resin, is applied to the surface of the artificial sandstone mold, where it is absorbed by the mold until the absorption capacity of the resin-wetted surface is exhausted. The resin applied to the surface of the artificial sandstone mold penetrates the open pores of the mold, i.e., it infiltrates or saturates the mold in an infiltration zone adjacent to the surface of the mold to a predetermined infiltration depth. The resin application process is continued until, under normal operating conditions, the applied resin no longer migrates into the interior of the mold.The process continues until no more resin is absorbed by the artificial sandstone mold, i.e., until the infiltration zone is saturated. The formation of a surface-applied resin film can serve as an indicator of this saturation point. The resin can be applied to the surface using standard application methods familiar from painting techniques, such as spraying, brushing, dipping, or similar methods. The infiltration treatment is typically carried out under normal atmospheric conditions, i.e., without the aid of overpressure or underpressure.

[0012] In the subsequent process step following the infiltration treatment, the resin-infiltrated artificial sandstone mold is air-cured, resulting in the formation of a solidified, additively manufactured artificial sandstone mold. For this purpose, the artificial sandstone mold is stored in an ambient air environment, causing the resin in the infiltration zone to cure and thus solidify the mold. Like the infiltration treatment, air curing typically takes place under normal atmospheric conditions, i.e., without overpressure or underpressure. Depending on the impregnating agent, the curing time ranges from 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 in this case is typically 12 to 24 hours.

[0013] It may be possible to repeat the infiltration treatment and subsequent air curing process steps to fill any remaining open porosity. Preferably, the infiltration and subsequent curing treatment steps are repeated once; however, they can also be repeated multiple times.

[0014] As an intermediate step after heat treatment and before infiltration treatment, the artificial sandstone mold can be cleaned, for example by cleaning jets with air or compressed air.

[0015] The strength of the additively manufactured green body artificial sandstone mold is significantly increased by the consolidation steps following additive manufacturing. The hardened, additively manufactured artificial sandstone mold produced according to the inventive method has an infiltration zone adjacent to the mold surface, which is permeated with cured resin to a predetermined infiltration depth. Typically, the infiltration depth is in the range of 2 mm to 1.5 cm, and often in the range of 4 mm to 1 cm. Thin areas of the artificial sandstone mold are therefore completely permeated with cured resin. Thicker areas, on the other hand, retain the material structure remaining after heat treatment in their core zone.The artificial sandstone molded bodies according to the invention therefore regularly possess a solid, resin-impregnated outer layer in the near-surface region and a somewhat softer, but more flexible core zone. This combination of properties leads to the high mechanical stability of the artificial sandstone molded body, namely to high strength coupled with good toughness. That is to say, the artificial sandstone molded bodies produced according to the process not only possess improved tensile, compressive, and flexural strength, but also improved resistance to impact and shock loads.

[0016] Furthermore, the resin-impregnated surface zone is generally liquid-tight, meaning the artificial sandstone molds are surface-sealed. Artificial sandstone molds according to the invention, 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 resulting from the surface sealing.

[0017] The artificial sandstone mold bodies are particularly suitable for primary and secondary forming patterns because, in addition to the required mechanical stability, they exhibit inherently high wear resistance due to their production from sand and their sealed surface. The possibility of quickly and individually producing mold bodies through additive manufacturing therefore makes the inventive method particularly well-suited for the production of casting patterns, especially those used for sand casting mold production for small and medium-sized casting series.

[0018] The effectiveness of the infiltration treatment is ensured, among other things, by the preceding heat treatment. This heat treatment removes or degasses residual substances, particularly residual moisture, from the existing open pores, thus creating a favorable initial condition for infiltration. Thanks to this combination of process steps, infiltration can be carried out without under- or overpressure. Costly process steps, such as vacuum or pressure impregnation, are possible but unnecessary.

[0019] According to one embodiment of the process, the solidified artificial sandstone mold body can be surface-treated after final air curing to improve the surface quality, i.e., in particular to reduce the roughness of the mold body surface, by abrasive processes such as grinding, vibratory finishing, blasting or polishing.

[0020] It can further be provided that the hardened artificial sandstone mold is coated with a final finish in a concluding process step. Pigmented alkyd resin nitrocellulose lacquers, polyisocyanate lacquers, and urea coatings have proven suitable for the final finish. Preferably, a lacquer system is used that comprises a primer of pigmented alkyd resin nitrocellulose lacquer and a topcoat of polyisocyanate lacquer.

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

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

[0023] The invention is explained in more detail below with reference to exemplary embodiments and the schematic drawings; these show: Fig. 1: an additively manufactured artificial sandstone mold body in the form of a casting model in perspective view, and Fig. 2: an additively manufactured artificial sandstone form in the shape of a sculpture in longitudinal section.

[0024] In Fig. Figure 1 shows a typical casting pattern for sand casting, produced using the described method. The additively manufactured artificial sandstone mold body forming the casting pattern is finished with a final coating for this application.

[0025] The Fig.Figure 2 illustrates the typical material structure of the additively manufactured artificial sandstone formwork, which is shaped like a sculpture and has been solidified. The artificial sandstone formwork therefore has the cross-hatched infiltration zone 1 in its near-surface edge zone, which surrounds the core zone 2, which remains largely unchanged from the additive manufacturing process. QUOTES INCLUDED IN THE DESCRIPTION

[0000] This list of documents cited by the applicant was automatically generated and is included solely for the reader's convenience. The list is not part of the German patent or utility model application. The DPMA accepts no liability for any errors or omissions. Cited patent literature

[0000] DE 10 2011 105 688 A1

[0003] EP 4 019 161 A

[0003]

Claims

[1] Process for producing a solidified, additively manufactured artificial sandstone body, characterized by the procedural steps: a) Additive manufacturing of an artificial sandstone body made of sand and a binder; b) heat treatment of the artificial sandstone body at a heat treatment temperature in the range of 50 °C to 150 °C for a holding time of at least 15 minutes; c) infiltration treatment of the artificial sandstone body cooled after the heat treatment by applying a synthetic resin, whereby the synthetic resin is applied to the surface of the artificial sandstone body with absorption in the artificial sandstone body until the absorption capacity of the surface of the body 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] Method according to claim 1, characterized by that the process steps of infiltration treatment and subsequent air curing are repeated at least once. [3] Method according to claim 1 or 2, characterized by that after the heat treatment and before the infiltration treatment, the artificial sandstone molded body is cleaned by blasting with compressed air. [4] Method according to one of claims 1 to 3, characterized by 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 bythat the solidified artificial sandstone molded body is coated with a final coating in a final process step, wherein the final coating comprises a primer of pigmented alkyd resin nitrocellulose varnish and a top coat of polyisocyanate varnish. [6] Method according to one of claims 1 to 5, characterized by that the additively manufactured artificial sandstone molded body is formed from quartz sand, wherein the grain size of the quartz sand grains is in the range from 0.05 mm to 0.30 mm, and wherein the binder is a furan resin, a phenolic resin, a polyurethane resin or water glass. [7] Method according to claim 6, characterized bythat the synthetic resin for the infiltration treatment is an epoxy resin, wherein the epoxy resin has 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] Method according to claim 6, characterized by that the resin used for 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] Solidified, additively manufactured artificial sandstone molded body according to claim 9, characterized by that the infiltration depth is in the range of 2 mm to 1.5 cm. [11] Solidified, additively manufactured artificial sandstone molded body according to claim 9 or 10, characterized by that the solidified artificial sandstone molded body is a master mold, a forming model, a core box, a mold, an art object or a decorative object.

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