METHOD AND DEVICE FOR PRODUCING 3D MOULDED PARTS BY MEANS OF A LAYER CONSTRUCTION TECHNIQUE

Abstract

The invention relates to a method and an apparatus for producing three-dimensional models by layering in a high-speed sintering process.

Claims

1. A method of producing 3D moulded parts, comprising the steps of: applying particulate construction material onto a construction field in a defined layer by means of a coater; selectively applying one or more absorbers on the defined layer, wherein the absorber is a liquid or a particulate material; heating the defined layer in a first heating step to a basic temperature of the particulate construction material, which is within a sintering window of the particulate construction material; heating the defined layer in a second heating step which is a sintering step that leads to selective solidification, by heat input, of the areas printed with absorber, at a sintering temperature above the melting temperature of the particulate construction material, wherein the areas with the selectively applied absorber heat up more than the areas without absorber, and thus a temperature difference is set between areas with and without absorber; and lowering the construction field by one layer thickness or raising the coater by one layer thickness; wherein these steps are repeated until the desired 3D moulded part is produced.

2. The method according to claim 1, wherein additional absorber is printed around the 3D moulded part in order to produce at least one jacket.

3. The method according to claim 1, wherein heating takes place such that only the areas printed with absorber connect by partial melting or sintering, or wherein the construction material is used in the form of a powder or dispersion, or wherein the layer is heated by radiation in a planar or sweeping manner, or wherein the temperature of the construction field and/or the construction material applied is controlled, or wherein the same or different absorbers are used in the 3D moulded part and in the jacket, or wherein the same or a different absorber is used for the jacket in an amount of 50-100%, or wherein the absorber used for the jacket prevents sintering of the construction material, or wherein the absorber comprises radiation-absorbing components, plasticizers for the particulate construction material or/and one or more substances interfering with recrystallization.

4. The method according to claim 1, wherein one source of radiation is used for each absorber, preferably using two absorbers with two sources of radiation, preferably wherein the sources of radiation preferably emit infrared radiation in the wavelength range from 1 to 20 μm and/or the source of radiation is a short-wavelength IR emitter made of quartz glass.

5. The method according to claim 1, wherein the jacket is constructed with a wall thickness of 1 to 10 mm, and is not solidified by the process, or wherein the jacket is constructed with a wall thickness of 1 to 10 mm, has a gap of 0.3 to 2 mm, separating it from the constructional part and solidifies through the process in a similar manner as the constructional part.

6. The method according to claim 1, wherein a temperature profile is generated in the applied construction material, said temperature profile being characterised by a temperature distribution of low:higher:still higher (T1<T2<T3) in the following areas: area outside the jacket:jacket area:area within the jacket (jacket<jacket area<area within the jacket) according to FIG. 6, or/and wherein an absorber is used for the jacket which has a higher boiling point than the absorber used for the 3D moulded part, or/and wherein the amount of the absorber or absorbers is regulated via greyscale values of the print head or via dithering methods, or wherein the liquid is selectively applied by means of one or more print heads, or wherein the print head or print heads selectively apply liquid in one or both directions of movement, or wherein the particulate construction material is selectively solidified, or wherein the applied construction material is cyclically heated and cooled off within a predetermined temperature band, or wherein, due to the selection of the construction material, said temperature band extends below a given melting point within a range of from 0 to −50K, 0 to −25K, 0 to −15K and 0 to −10K, or wherein the temperature difference between areas with and without absorber is within a range of from 0.5 to 10K, or wherein the temperature in the area printed with absorber or in the area within the jacket is maintained substantially constant until completion of the printing process and a cooling step, or is set within a temperature range of from 0 to −30K.

7. A 3D moulded part, manufactured using an absorber, said 3D moulded part being laterally surrounded, substantially along its entire circumference, by a jacket, said jacket having been constructed using an absorber, there being unsolidified particulate construction material between the 3D moulded part and the jacket, wherein the jacket preferably has a strength which is greater, substantially the same as or lower than that of said part, preferably lower, or 3D moulded part, manufactured using an absorber, said 3D moulded part being laterally surrounded, substantially along its entire circumference, by a jacket, said jacket having been constructed using an absorber, and said jacket being easily removable after the construction process by powder blasting or an air jet.

8. A device suitable for carrying out a method according to claim 1, or which is temperature-controllable and preferably comprises an insulation on a construction platform downward and preferably laterally, preferably comprising a resistance heating, preferably comprising a construction platform in a construction container, said construction container preferably being temperature-controllable, preferably comprising a heating means disposed above the construction platform, preferably an overhead lamp, preferably comprising a movable heating means, preferably a sintering lamp.

9. A 3D printing method, characterised in that two sintering lamps with different wavelength spectrums or wavelengths or energy input are used, preferably two sintering lamps or one sintering lamp whose spectrum is characterised by being composed of two different blackbody radiation spectrums, or being characterised by having a spectrum which differs from the blackbody radiation spectrum.

10. A device for carrying out a method according to claim 9.

11. The method of claim 1, wherein the first heating step to the basic temperature is effected by an emitter with a wavelength of about 3 to about 8 μm, and the sintering step is effected by an emitter with a wavelength of about 0.5 to about 1.5 μm.

12. The method of claim 11, wherein the first heating step to the basic temperature is effected by an emitter with a wavelength of about 5 μm, or the sintering step is effected by an emitter with a wavelength of about 0.9 to about 1.2 μm.

13. The method of claim 11, wherein the wavelength of an emitter is about the peak wavelength of a blackbody radiation.

14. The method of claim 13, wherein the absorber is a liquid (preferably an oil-based in containing carbon particles); and the particulate construction material has an average particle size of 50-60 μm.

15. The method of claim 1, wherein the absorber is applied before the first heating step and the areas with the absorber heat up to a higher temperature than the areas without the absorber in the first heating step.

16. The method of claim 1, wherein the particulate construction material includes a polyamide.

17. The method of claim 11, wherein the particulate construction material has a melting temperature of 180-190° C., or the particulate construction material has a recrystallization temperature of 140-150° C., or both.

18. The method of claim 17, wherein the basic temperature is set to 145° C. to 186° C., or the sintering temperature is set to 175° C. to 220° C., or both.

19. The method of claim 1, the temperature or heat input, respectively, is achieved via an emitter or a thermolamp, at a distance from the construction field surface of 10 to 50 cm.

20. The method of claim 5, wherein a higher temperature is generated in the jacket as compared to the 3D moulded part.

Description

[0180] Preferably, the following sequence of process steps is carried out by the device after the heating phase: A powder layer is formed by the coater (101) on the construction platform (FIG. 1a). Optionally, depending on the design of the machine, the new layer can be additionally heated by the sintering lamp (109). Next, this layer is printed on by one (100) or several inkjet print heads (100 and 508) (FIG. 1b). Then, the construction platform (102) is lowered (1d). Now, the printed layer is heated by the sintering lamp (109) and then covered with powder again.

[0181] This operation is repeated until completion of the components (103) in the construction container (110). Then the cooling phase follows. This phase preferably takes place in the construction container which is then supplied with energy outside the device.

[0182] FIG. 2 presents temperature diagrams. FIG. 2a schematically shows the profile of the energy emitted by the powder when it is heated and cooled again in one cycle. During heating, significant absorption of energy occurs at a certain temperature. This is where the material melts or sinters (sintering temperature). For polyamide 12, which is suitable for laser sintering, this temperature is approx. 185° C. During cooling, there is also a significant point considerably below the sintering temperature (recrystallization temperature). This is where the molten material solidifies.

[0183] FIGS. 2b and 2c show the temperature profile during a process run according to a prior art method. FIG. 2b shows the temperature profile on the unprinted surface. Using the sintering radiation source produces heating and cooling phases in the otherwise constant profile. In the unprinted area, the temperature never reaches the sintering temperature.

[0184] FIG. 2c shows the profile in the printed area. Here, the variations are more marked. The process is controlled at least such that the sintering temperature is briefly exceeded, so that part of the powder is melted and remains molten. Excessive heating may cause all of the powder to melt in this area, resulting in massive warping. Excessive cooling of the printed area must also be avoided, because otherwise recrystallization will start, and then all shrinkages due to the now possible power transmission will lead to geometric warping (curling), which may make the further process impossible.

[0185] FIG. 8 describes an advantageous combination of assembly parts by which the advantageous process conditions according to the invention can be achieved. Further details thereof are illustrated in embodiment example 4.

[0186] The further FIGures show the aspect of at least two sintering lamps or of at least two emitter types in one sintering lamp or thermolamp.

[0187] FIG. 9 shows a construction, wherein in addition to the heating element (507) with reflector (502) another heating element (509) with reflector (510) having a different radiation spectrum is used.

[0188] FIG. 10 shows a second variant of the construction, wherein the second heating element is arranged on the left, next to the oscillating blade.

[0189] FIG. 11 shows a construction, wherein both heating elements having different spectra are mounted together, preferably inside a quartz glass bulb.

[0190] FIG. 12 shows a construction wherein further heating elements, optionally having different peak wavelengths, are mounted on the print head axle system.

[0191] FIG. 13 shows a construction, wherein the overhead-heating unit comprises an additional type of emitter whose emitted spectrum differs from that of the first one.

EXAMPLES

Example 1

Device Comprising an Inkjet Print Head with a Binary Droplet Size

[0192] The injket print heads common in 3D printing deposit one droplet on a dot in the raster of the print area. The size of said droplet is adjusted once.

[0193] During printing, in the method according to the invention, a respective cross-sectional image of the desired components (103) is printed using absorber, said image being adapted to the construction height. In this case, the image is printed in an intensity ensuring definite sintering of the particles during the pass of the sintering lamp. As described above, unprinted areas will remain unsintered. The necessary amount of liquid imprinted per raster dot in this case will be considered black in the following.

[0194] In the process, a jacket (301) is printed around the component, said jacket (301) representing a greyscale value, i.e. containing less absorber based on the local average. As preferred according to the invention, the jacket (301) is determined from the raw data during layer calculation. In this case, e.g. for the .stl file format, the jacket area is generated by an offset of the triangular areas.

[0195] The greyscale value can be achieved in the raster area by mathematical methods. For this purpose, the coverage of the jacket area with printed dots is controlled such that the desired greyscale value is achieved on average in a certain local viewing area. An example of such a mathematical method is the so-called error diffusion method. In this case, an area is used for averaging, and the corresponding raster dots are placed as a function of the greyscale value. In a simple example, in order to achieve a greyscale value of 30% in an area of 10 by 10 dots, 30 dots must be printed “black” and 70 dots remain unprinted.

[0196] Using this method, a conventional print head (100) will suffice. The device does not differ from the prior art devices. Also, the same data paths can be achieved, because the information can be stored in one single monochromatic raster bitmap.

[0197] The above-described temperature curves now need to be regarded in three areas on the construction field: In the component area (103), the same conditions apply as shown in FIG. 2b. The temperature in that area is raised above the sintering temperature for brief periods at a time. In contrast thereto, FIG. 5a shows the temperature in the unprinted area. It may even be considerably below the recrystallization temperature because the peripheral area (301) is not raised above the sintering temperature and, thus, does not enable any power transmission. The curve in FIG. 6a shows the temperature in the peripheral area (301) of the component. Here, the temperature should be above the recrystallization temperature and below the sintering temperature so that the heat transfer cannot cause the component area to drop below the recrystallization temperature and said area can be easily removed after the construction process.

Example 2

Greyscale Print Head

[0198] A device comprising a greyscale print head is more precise in use. These print heads are common and well-known in the print media sector. They allow the apparent resolution to be increased in this sector, thus achieving a better image quality.

[0199] In 3D printing, this increase in resolution does not have a direct effect. However, according to the invention, this technique can be used to introduce different amounts of liquid in the component (103) and jacket (301) areas. For instance, the amount of liquid introduced in the jacket area may be set to 50% of that introduced in the component area. Data transmission is effected by the use of a polychromatic raster bitmap. As a minimum, another raster bitmap can be defined which contains the data for the peripheral area. The electronic system of the print head then considers the respective bits as greyscale information.

Example 3

Material System Comprising a Separating Agent

[0200] Further essential degrees of freedom result for the invention if several different liquids are used for printing.

[0201] For this purpose, the device according to the invention has to be extended. A second print head (508) is used which can print the second absorber. In this case, the data path in the control unit of the machine need not be changed. The data is divided up electronically before the respective print head. This print head need not be a separate physical unit, but can be part of the print head (100) of the device.

[0202] The second absorber liquid may contain an oil, for example, which serves as a separating agent. This separating agent deposits between the individual particles and prevents their contact with the molten base material. In the case of polyamide 12 as base material, a certain amount of silicone oil in the printing liquid may serve as separating agent. This oil must be maintained as a suspension with the rest of the ink which additionally includes the absorber.

[0203] An example of such composition is: [0204] 80% propylene glycol [0205] 14% polyethylene glycol 400 [0206] 5% nanoscale graphite [0207] 1% emulsifier

[0208] Since the oil has a high boiling point, it does not evaporate during the sintering operation. In the jacket area (301), no sintering of the particles is possible. However, the oil produces an peripheral layer which adheres to the component and is easy to remove when unpacking the components. It can be removed separately from the rest of the powder. This avoids major contamination of the powder during further cycles.

[0209] This liquid is imprinted in the peripheral area (301) in the same amount as the liquid in the component area (103). In this example, the liquid in the component area (103) is composed of: [0210] 95% propylene glycol [0211] 5% nanoscale graphite.

Example 4

An Advantageous Combination of Constructional Features

[0212] 1) An assembly consisting of 6 thermal radiation-emitting elements of the FTE Ceramicx type based on ceramics, having a peak wavelength of 5 μm, and mounted at a distance of 175 mm above a 330 mm×230 mm construction platform, i.e. elements with a basic power of 300 Watt and a size of 245 mm×60 mm×31 mm each, which are operated at a power of 50%. The assembly provides a constant basic temperature of the untreated powder of 175° C. on the surface. The elements are mounted centrally above the edge of the construction platform, thus ensuring the homogeneity of the temperature over the entire area of the powder present on the construction platform and preventing cooling of the edges of the construction platform.

[0213] 2) A silicone-based heating mat with a maximum power of 400 W, mounted in a planar manner on the underside of the construction platform, and controlled to a constant temperature of 175° C., serves to reach the basic temperature for powder coating homogeneously and to keep said temperature constant over time.

[0214] 3) A halogen heat emitter, type QHM, manufactured by Freek GmbH, with a maximum power of 1.6 kW and a peak wavelength of 1 μm, mounted on the rear surface of the oscillating blade recoater at a distance of 55 mm above the powder layer. The power of the emitter is changed according to its position while passing over the powder coating. 1.5 kW while passing over the powder wetted with an infrared light-absorbing liquid, so as to increase the temperature of the powder to above the melting temperature, approx. 200° C. in this case; 0.3 kW during application of the next layer so as to keep it from cooling, otherwise in the deactivated state.

[0215] 4) Another ceramic-based emitter of the FTE Ceramicx type and identical in size serves to preheat the reservoir of the oscillating blade recoater which contains polyamide powder for coating. Thus, the temperature of the powder is adjusted to 70° C. This allows the temperature and flowability of the powder to be kept constant throughout the construction process. A temperature below the glass transition further guarantees constant flowability, thus ensuring smooth powder application by means of the oscillating blade recoater.

[0216] 5) The structure has two axle systems, each equipped with a drive and being able to pass over the construction platform with the powder coating. One axle, with a rest position on one side, on the left in this case, of the construction platform, includes the oscillating blade recoater on the left-hand side, as well as the halogen emitter on the right-hand side, i.e. the side facing away from the oscillating blade recoater. The second axle includes the print head, which can be additionally moved perpendicular to the axle system so as to ensure strip printing of the entire powder surface. The cyclic sequence of the layered printing process is structured as follows: [0217] 1) Recoater axle passes over the construction platform=sintering pass. Lamp power: 1.5 kW, speed: 60 mm/s. [0218] 2) Construction platform is lowered by the layer height of 150 μm [0219] 3) Recoater axle passes over the construction platform again, returning to its rest position, with simultaneous coating by means of the activated oscillating blade recoater. Lamp power: 0.3 kW, speed: 40 mm/s=recoating and heating pass [0220] 4) Print head axle passes over the construction platform into the printing start position. [0221] 5) Print head axle returns to its rest position. At the same time, type 1001 print head, manufactured by XAAR, is activated, thus wetting the powder surface with absorber in the desired locations.

[0222] The fluid used for wetting the powder surface consists of a commercially available, oil-based soot particle ink, e.g. IK821 manufactured by XAAR, whose absorption maxima are outside the wavelengths emitted by the ceramic heat emitters and is therefore heated to exactly the same extent as the unwetted powder. Since the absorption of the printing fluid increases considerably at wavelengths of less than 2 μm, the emissions of the halogen heat emitter are absorbed by the wetted powder to a much greater extent than by unwetted powder, because the latter reflects almost completely in the wavelength range below 2 μm. Any energy of the emitter absorbed nevertheless in the unwetted locations serves to maintain the basic temperature of the powder surface.

Example 5

Use of Printing Fluid with an Advantageous Absorption Spectrum

[0223] Another means of increasing the selectivity of the construction process, thereby maximizing both the strength of the structures to be generated and also the removability of unwetted particulate material, consists in using printing fluids with an advantageous absorption spectrum, so that particulate material wetted with them can be heated by means of a defined infrared emitter spectrum to the greatest possible extent. Preferably, a printing fluid is selected whose absorption spectrum differs considerably from the particulate material used. This enables more sensitive control both of the basic temperature of the powder surface and of the melting temperature of the wetted surface, without having to take the constructive effort of providing an infrared emitter specially tailored for the ideal spectrum.

[0224] An exemplary embodiment is characterised by: [0225] Printing fluid on the basis of [C47 H47 CI N2 O3 S] (CAS # 134127-48-3) with an absorption maximum of 815 nm, dissolved in methanol, e.g. ADS830AT manufactured by American Dye Source [0226] Infrared emitters in the form of commercially available quartz halogen emtiters with a peak wavelength of 0.9-1.2 μm, preferably as sintering lamp [0227] Infrared emitters in the form of carbon infrared emitters with a peak wavelength of 1.9-2.7 μm, preferably as overhead heat emitters, and/or additional sintering emitters, e.g. manufactured by Heraeus Noblelight.

[0228] The replacement of the overhead ceramic emitters with the carbon infrared emitters is advantageous here because the latter have a shorter reaction time, which makes the process temperature easier to control.