CROSS-REFERENCE TO RELATED APPLICATION

This application is a national stage application under 35 U.S.C. § 371 of PCT/EP2019/086161, filed Dec. 19, 2019, which claims the benefit of European Application No. 18214397.4, filed Dec. 20, 2018, each of which is incorporated herein by reference.

FIELD

The present invention relates to a method of creating an article by means of additive manufacturing from various pulverulent components, wherein multiple pulverulent components having different melting points are simultaneously laid in specific locations and the powder layer is then subjected to thermal treatment. The present invention further relates to an article produced by such a method.

BACKGROUND

Additive manufacturing methods refer to methods by which articles are assembled layer by layer. They thus differ fundamentally from known material-removing methods, for example machining or drilling, which take on their ultimate geometry via removal of material from a larger blank. On the basis of the polymers predominantly used nowadays in additive manufacturing methods, however, it is usually “only” possible to obtain articles having resulting mechanical properties that differ fundamentally from the other plastics processing methods, for example injection molding. When processed by additive manufacturing methods, the thermoplastic materials used lose their known specific characteristics.

Nylon-12 (PA12) is the material currently most commonly used for powder-based additive manufacturing methods, for example laser sintering. PA12 is notable for high strength and toughness when it is processed by injection molding or by extrusion. A commercial PA12, for example, after injection molding has an elongation at break of more than 200%. PA12 articles that are produced by the laser sintering method, by contrast, show elongations at break around 15%. The component is brittle and therefore can no longer be regarded as a typical PA12 component. The same is true of polypropylene (PP), which is supplied in powder form for laser sintering. This material too becomes brittle as a result of the production process and hence loses the tough, elastic properties that are typical of PP. The reasons for this are to be found in the morphology of the polymers in conjunction with the production method. During the melting operation by means of laser or IR and especially in the course of cooling, a proportion of the polymer chains forms crystalline, tightly packed structures. The result is regions having semicrystalline morphology that are characterized by a high level of order. During melting and cooling, these crystallites grow irregularly at the boundaries of the incompletely molten particles and at the former grain boundaries of the powder particles and on additives present in the powder. This morphology promotes the formation of cracks under mechanical stress. Residual porosity, which is unavoidable in powder-based additive methods, makes a further contribution to crack propagation. As a result, the components thus produced exhibit comparatively brittle properties overall.

Even the pulverulent elastic polymers based on block copolymers that are used in laser sintering show a profile of properties of the finished components that is untypical of the polymers used. Thermoplastic elastomers (TPE) are used in laser sintering. Articles that are produced from the TPEs now available have high residual porosity after solidification, and the original strength of the TPE material does not correlate with the strength of the article manufactured therefrom. In practice, these porous components are therefore subsequently infiltrated with liquid hardening polymers in order to establish the profile of properties required. In spite of this additional measure, strength and elongation remain at a comparatively low level. The additional method complexity—as well as the still-inadequate mechanical properties—leads to poor economic viability of these materials and of the overall method.

Various solutions have been proposed in the prior art for the processing of these polymer classes within additive processing methods.

US 2005/0080191 A1 relates to a powder system for use in solid freeform fabrication methods, comprising at least one polymer having reactive and meltable properties, wherein the at least one polymer is selected in order to react with a liquid binder and to be meltable at a temperature above the melting point or glass transition temperature of the at least one polymer. The at least one polymer may comprise at least one reactive polymer and at least one meltable polymer, and the at least one meltable polymer may have a melting point or glass transition temperature in the range from about 50° C. to about 250° C.

DE 10 2016 207 283 A1 discloses a method of additively manufacturing a component, especially for a motor vehicle. The method has the following steps:—creating a shaped body from a multitude of mutually superposed shaped body layers of at least one first layer material and at least one second layer material, wherein an arrangement of the first layer material corresponds or corresponds essentially to a shape of the component to be created,—heating the shaped body by means of a heating device in such a way that the first layer material is melted, with fusion of adjacent first layer material elements to one another and without melting of the second layer material,—cooling the shaped body in such a way that the first layer material is solidified and—removing the excess second layer material from the shaped body to create the component.

US 2016/0243764 A1 describes a method of producing an article, comprising: laying a first layer of an object material on a support substrate electrode; applying a conductive material to the first layer; laying a first layer of a charge powder onto an intermediate substrate; transferring the first layer of the charge powder onto the intermediate substrate proximally to the first layer of the first object material, and applying an electrical transfer field in order to bring about transfer of the first layer of the charge powder from the intermediate substrate to the first layer of the substrate support electrode, such that a first powder layer is formed on the first layer of the substrate support electrode. Multiple powder layers may be laid on the first layer. The method may further include the fusing of the powder layer(s) to form a first fused layer on the substrate support electrode.

WO 2015/197515 discloses a thermoplastic pulverulent composition comprising 0.02% to 0.5% by weight, based on the total amount of composition, of plasticizer and pulverulent thermoplastic polyurethane, where at least 90% by weight of the composition has a particle diameter of less than 0.25 mm, where the thermoplastic polyurethane is obtainable from the reaction of components a) at least one organic diisocyanate, b) at least one compound having groups reactive toward isocyanate groups and having a number-average molecular weight (Mn) of 500 g/mol to 6000 g/mol and a number-average functionality of the totality of the components under b) of 1.8 to 2.5, c) at least one chain extender having a molecular weight (Mn) of 60-450 g/mol and a number-average functionality of the totality of the chain extenders under c) of 1.8 to 2.5, in the presence of d) optionally catalysts, e) optionally auxiliaries and/or additives, f) optionally one or more chain terminators. The thermoplastic polyurethane has a melting range (DSC, differential scanning calorimetry; second heating at heating rate 5 K/min) of 20 to 170° C. and a Shore A hardness to DIN ISO 7619-1 of 50 to 95. At a temperature T, the melt volume flow rate (MVR) to ISO 1133 is 5 to 15 cm3/10 min. The change in the MVR in the event of an increase of this temperature T by 20° C. is less than 90 cm3/10 min. The powder serves for production of articles in powder-based additive manufacturing methods.

There is still a need in the prior art for additive manufacturing methods capable of delivering components having satisfactory mechanical and homogeneous material properties overall.

SUMMARY

It is therefore an object of the present invention to at least partly overcome the disadvantages known from the prior art. More particularly, it is an object of the present invention to provide a way in which high stability of the components manufactured, especially also parallel to a layer direction, and/or homogeneous component properties overall are enabled. Furthermore, the disadvantages of long thermal stress as exist in the art of standard sintering methods are to be avoided.

What is therefore proposed is the method of the invention as per claim1. Advantageous developments are specified in the dependent claims. They may be freely combined, unless the context clearly suggests otherwise.

