Device for producing three-dimensional objects layer by layer

The present invention relates to an apparatus for layer-by-layer production of three-dimensional objects in a powder bed fusion method, and to a powder bed fusion method.

The rapid provision of prototypes or small batches is a problem that has frequently been encountered in recent times. Methods that enable this are called rapid prototyping, rapid manufacturing, additive fabrication methods, or else simply 3d printing. Particularly suitable methods are those in which the desired structures are produced layer by layer, by selective melting and/or consolidation of pulverulent materials. The methods that work according to this principle are summarized in ISO/ASTM 52900 or ISO 17296-2 using the umbrella term “Powder Bed Fusion”.

One example of powder bed fusion methods is laser sintering, which is described in detail in specifications US 6136948 and WO 96/06881. Further examples of powder bed fusion methods are described in documents US 6531086 and EP 1740367 (US 2007/238056).

In the powder bed fusion methods, powders composed of metallic, ceramic or else polymeric materials are used. For minimization of warpage of the three-dimensional objects to be produced, it is usually necessary to control the temperature in the construction space. DE 10108612 (US 2002/158054) describes an apparatus in which, by means of shell heating of the construction space, warpage of the three-dimensional objects is to be avoided. However, the heating of the shell of the construction space has the great disadvantage that the powder in the construction space container is then exposed to thermal stress for a longer period. Moreover, the temperature distribution to be established in the shell of the construction space which is described in this document is implemented by means of separate temperature control of regions, which requires a complex apparatus. However, prolonged thermal stress specifically in the case of polymeric materials leads to unwanted changes in the powder material, such as thermooxidative damage, or to a significant increase in molecular weight. Both effects are unwanted since these adversely affect the recyclability of the powder.

The problem addressed by the present invention was therefore that of minimizing the warpage of the three-dimensional objects produced and, at the same time, the thermal stress on the powder material. The powder should undergo a minimum increase in molecular weight.

In the case of apparatuses which are used in powder bed fusion methods, the direction of construction progress (direction of lowering of the construction space platform on which the three-dimensional object is formed) is defined as the z axis. The z axis thus stands vertically on the plane of the construction field. One problem in the prior art is that the mechanical characteristics in z direction are highly dependent on the position of the object in the construction field. Elongation at break in particular is significantly poorer in the edge regions than in the middle of the construction field. A further problem addressed is therefore that of improving the mechanical characteristics of the parts of the three-dimensional object that are manufactured in the edge region (outer region) of the construction field. As a result, the mechanical characteristics of the three-dimensional objects should be at the same level irrespective of the position in the construction field.

What has accordingly been found is a novel apparatus for layer-by-layer production of three-dimensional objects in a powder bed fusion method. The apparatus comprises a construction space, at least one energy source, a construction field having a construction space platform, and a construction space container that laterally bounds the construction space platform. The construction space platform has an upper side facing a powder and a lower side remote from the powder. The upper side of the construction space platform comprises a material having a thermal conductivity of at least 20 W/(m·K), and the lower side of the construction space platform comprises a material having a thermal conductivity of not more than 0.5 W/(m·K). In this way, a construction space platform is provided, the upper side of which comprises a material of good thermal conductivity and the lower side of which a material of poor thermal conductivity. Preferably, the upper side of the construction space platform consists of a material having a thermal conductivity of at least 20 W/(m·K), and the lower side of the construction space platform of a material having a thermal conductivity of not more than 0.5 W/(m·K).

The apparatus of the invention comprising an aforementioned construction space platform gives three-dimensional objects which exhibit lower warpage and good and homogeneous mechanical characteristics. Moreover, there is lower thermal stress on the powder.

Preferably, the construction space platform includes devices which enable cooling of the upper side of the platform. For this purpose, one or more flow machines that are driven from the outside may be integrated on the lower side of the construction platform, Additionally or alternatively, the construction space platform may exhibit at least one inlet for a cooling medium and at least one outlet for a cooling medium. Preferably, the inlet and outlet are each arranged on the lower side of the platform.

In a preferred embodiment of the invention, the construction space container has an outer face that faces the construction space platform, wherein the outer face comprises or preferably consists of a material having a thermal conductivity of not more than 0.5 W/(m·K), Preferably, the outer face has a wall thickness of at least 10 mm. The wall thickness is preferably at least 40 mm, more preferably at least 80 mm.

With the design of the outer face of the construction space container as a thermal insulator, it is possible to dispense with the complex temperature control of the construction space container and additionally to reduce the thermal stress on the powder and to further reduce the warpage of the three-dimensional objects produced.

The material of the outer face of the construction space container and the material of the lower side of the construction space platform may be the same or different, Preferably, the material in each case has a thermal conductivity of less than 0.3 W/(m·K), more preferably less than 0.1 W/(m·K) in each case and most preferably less than 0.05 W/(m·K) in each case, The material for the upper side preferably has a thermal conductivity of at least 80 W/(m·K), preferably of at least 140 W/(m·K).

Thermal conductivity is determined at 23° C. according to ASTM E1461 (LFA457 Micro Flash from Netzsch, sample thickness 2 mm, sample conditioning at 23° C./50% for 48 h). Suitable materials that exhibit these heat-insulating properties are, for example, foam glass, foam ceramic, expanded perlite, porous concrete, wood, high-temperature polymers such as polyetheretherketones or thermally stable thermosets.

The construction space platform may be configured such that cooling of the powder provided (powder cake) is possible during the construction process or after the construction process. The construction space platform is preferably cooled by conduction of heat and/or convection. The cooling output can be kept at the desired value by means of closed-loop control.

A preferred powder is a polymer powder. Suitable polymers of the polymer powders are selected from polyamides, polyolefins such as polyethylene and polypropylene, polyesters and polyaryletherketones (PAEK) such as polyetheretherketone. Suitable polyamides may be conventional and known polyamides. Polyamides comprise homopolyamides and copolyamides. Suitable polyamides or copolyamides are selected from nylon-6, 11, 12, 10,13, 10,12, 6.6, 4.6, 6.13, 10.6, 11/10.10, 12.12 and 12/10.12. A preferred polyamide is selected from nylon-11, 12, 10.13, 10.12, 66, 6.13, 11/10.10, 12.12 and 12/1012, more preferably nylon-11 or 12 and most preferably nylon-12.

The present invention likewise provides a method for layer-by-layer production of three-dimensional objects in a powder bed fusion method. The method comprises the repeating steps of a) providing a powder (2), b) adjusting a temperature of the upper side (12) of the construction space platform (6) to at most 15° C. below a process temperature, c) adjusting a temperature in the construction space (1) below the melting temperature of the powder (process temperature), d) optionally applying a melting aid by means of inkjet to sites in the powder (2) that are to be sintered, e) applying electromagnetic energy to the powder for selective sintering by means of energy source (5), f) lowering the construction platform by one layer thickness, g) applying further powder (2) and h) repeating steps c to g until the three-dimensional object has been completed. In the first provision of a powder, it is preferable to provide a powder layer thickness of 4 to 10 mm.

The process temperature is the temperature of the powder in the construction field. The process temperature is preferably 10 to 20° C. below the melting point of the powder, preferably polymer powder.

It is a feature of the process that, after commencement of the production method, the temperature of the upper side (12) of the construction space platform (6) is lowered by at least 5° C. per 10 mm of construction progress until the upper side (12) of the construction space platform (6) has reached a temperature of not more than 50° C. The temperature of the construction space platform here is already being lowered during the construction process, After the construction process has commenced, the temperature of the upper side (12) of the construction space platform (6) is preferably lowered by at least 7° C. per 10 mm, more preferably 10° C. per 10 mm, of construction progress, until the upper side (12) of the construction space platform (6) has reached a temperature of not more than 50° C. Construction progress shall be considered here to mean the layer-by-layer lowering of the construction platform during the construction process. The temperature of the upper side of the construction space platform can be lowered in a linear manner or preferably in a greater than proportional manner with increasing construction progress. The effect of the latter case is that the temperature of the upper side (12) of the construction space platform (6) is lowered more quickly with increasing duration of the construction process.

1 construction space

2 powder

3 sliding apparatus

4 construction field

5 energy source

6 construction space platform

7 three-dimensional object

8 construction space container

9 powder cake

10 construction field plane

11 outer face of the construction space container

12 upper side of the construction space platform

13 lower side of the construction space platform

14 outlet for cooling medium

15 flow machine driven from the outside

16 inlet for cooling medium

17 cooling channels

The apparatuses described in the examples were used to create three-dimensional objects. For the production of the three-dimensional objects, a PA12 powder having the properties listed in Table 1 was used. For this purpose, in all examples, a powder layer of 6 mm was placed onto the construction space platform and the entire construction space was preheated to a temperature of 168° C. for 180 minutes. The construction process was started (process temperature 174° C., layer thickness 0.15 mm), and a total of 36 tensile specimens (DIN SO 527, exposure parameter set: speed, position and alignment of the objects identical in all examples) were constructed. 12 tensile specimens in each case were positioned in z direction (vertically) at the edge and in the middle of the construction field. The remaining 12 tensile specimens were positioned in x direction (horizontally) in the construction field. The height of the powder bed at the end of the construction process was 320 mm in each case. The duration of the construction process in the examples was 18 h 57 min. After the construction process had ended, the heating was switched off and the construction space container with the powder present therein was stored in a laser sintering machine for 72 h. Thereafter, the three-dimensional objects created were removed from the powder bed and tested. The powder was likewise removed from the construction space container and homogenized by means of a mixer. Subsequently, the solution viscosity (ISO 307, Schott AVS Pro, solvent: acidified m-cresol, volumetric method, double determination, dissolution temperature 100° C., dissolution time 2 h, polymer concentration 5 g/l, measurement temperature 25° C.) of the powder thus homogenized was measured.

A PA12 powder having the material characteristics from Table 1 was processed in an EOSINT P395 laser sintering machine from cos GmbH. The withdrawal chamber temperature was set to 130° C.

A PA12 powder having the material characteristics from Table 1 is processed in an EOSINT P395 laser sintering machine from eos GmbH. The heating of the withdrawal chamber was switched off.

The outer face of the con r n space container consisted of foam glass having a wall thickness of 40 mm.

The construction space platform was configured according to FIG. 4. The upper side of the construction space platform consisted of aluminium, and the lower side of foam glass. Compressed air was blown onto the upper side, and the air rate was adjusted under closed-loop control in accordance with the cooling output necessary. The construction space platform was adjusted to a temperature of 161° C. at the start of the construction process. After the construction process had commenced, the temperature of the upper side of the construction space platform was lowered by at least 7° C. per 10 mm of construction progress until the upper side of the construction space platform had reached a temperature of 49° C.

Tables 2 to 4 list the test results for the components created in each case and the solution viscosity of the homogenized powder. It can be seen that the three-dimensional objects have similar properties but, in the inventive example, the homogenized powder from the construction space container has undergone a significantly smaller increase in molecular weight. Moreover, the standard deviation of the mechanical characteristics of the components in z direction in Inventive

Example 2 is distinctly smaller, which means more homogeneous component quality.