EP1513670A1

EP1513670A1 - Laser sintering method with increased process precision, and particles used for the same

Info

Publication number: EP1513670A1
Application number: EP03759859A
Authority: EP
Inventors: Rolf Pfeifer; Jialin Shen
Original assignee: DaimlerChrysler AG
Current assignee: Mercedes Benz Group AG
Priority date: 2002-06-18
Filing date: 2003-06-16
Publication date: 2005-03-16
Also published as: JP2005536324A; WO2003106146A1; US20060159896A1

Abstract

In the rapid prototyping method of selective laser sintering, temperature gradients occur inside and between individual layers, leading to component deformation which is intolerable at least for high-quality components. The aim of the invention is to provide a method for selective laser sintering, whereby the temperature inside the built-up particle cake is as homogeneous as possible. To this end, particles containing at least one material having a maximum softening temperature of approximately 70^° C are used.

Description

Laser sintering process with increased process accuracy and particles for use

The invention relates to a selective laser sintering method according to the preamble of claim 7 and particles for use here according to the preamble of claim 1. Such methods and particles are already known from DE 690 31 061 T2.

Selective laser sintering (SLS) is a rapid prototyping process in which a platform that can be lowered into a construction space (construction space floor) carries a particle layer that is heated by a laser beam in selected areas, so that the particles form a first layer merge. The platform is then lowered by about 20 to 300 μm (depending on particle size and type) into the installation space and a new particle layer is applied. The laser beam again traces its path and fuses the particles of the second layer with one another and the second with the first layer. This gradually creates a multi-layered particle cake and a component in it, for example an injection mold.

Within the installation space, certain areas - depending on the geometry of the component to be manufactured - are heated by the load for a longer or shorter period of time. jet while others are not heated at all. In addition, only the uppermost particle layer is heated by the laser, the lower layers give the absorbed heat to their surroundings and cool down. The result is inhomogeneous temperature distributions and thermal stresses within the particle cake, which can lead to component distortion.

DE 690 31 061 T2 already suggests preheating the particle layers so that the energy beam only has to introduce a small amount of energy in order to bond the particles. At the same time, this measure has the effect that the temperature differences between irradiated and non-irradiated parts of a layer are reduced - even if this is not disclosed in DE 690 31 061 T2.

However, temperature gradients continue to occur within and between the individual layers, the former in particular leading to component distortion, which cannot be tolerated, at least in the case of high-quality components.

As a corrective measure, DE 101 08 612 A1 therefore proposes to replace the usual three-dimensional temperature gradient with an approximately one-dimensional one (in the direction of the installation space floor) by means of a segmented installation space jacket heater.

The invention is based on the object of specifying a further method and particles for selective laser sintering, in which the temperature within the piled-up particle cake is as homogeneous as possible.

This object is achieved in that the absolute temperature difference between the irradiated areas and their end temperature, that is to say the room temperature, by using suitable ter materials is reduced. Suitable materials are those that have a softening temperature of less than about 70 ° C. The term softening temperature is not to be interpreted narrowly, but it is clear to the person skilled in the art that this is to be understood as a temperature at which the particles form a bond with adjacent particles. Partial melting may be necessary for this, but softening (below the glass transition temperature), for example in the case of polymers, may also suffice or it is also conceivable that the activation energy for a chemical bond is exceeded.

With regard to the particles to be used and the method to be created, the invention is represented by the features of patent claims 1 and 7. The further claims contain advantageous refinements and developments.

The object is achieved according to the invention with regard to the particles to be created in that they are suitable for use in selective laser sintering (SLS) (that is to say their diameter is less than approximately 300 μm) and contain them

- A core made of at least a first material

an at least partial coating of the core with a second material (further components are optional), the second material having a lower softening temperature than the first material, and the softening temperature of the second material being less than approximately 70 ° C.

Suitable second materials can be lower alloys

Softening temperature used, for example, in fuses (see e.g. JP2001143588A), also saturated linear carboxylic acids with chain length> 16 (e.g. hepta-decanoic acid, melting point 60-63 ° C) or polymers in the broadest sense (see following definition and examples ). The softening temperature of the second material of approximately 70 ° C or less enables laser sintering compared to previously used particles at much lower temperatures and thus also a much lower temperature difference between irradiated particles and the usual room temperature in the order of 20 ° C. Tests show that with the lower maximum temperature difference, the temperature homogeneity of the entire installation space is also improved.

Materials with significantly higher softening temperatures require greater temperature inhomogeneities and therefore lower component accuracy, which is no longer sufficient for precision applications. Materials with significantly lower softening temperatures can only be stored for a relatively long time, since it must be ensured that they do not unintentionally bond. In summer, however, temperatures of over 30 ° C in the shade and over 50 ° C in the sun can also be reached in Germany, and this could lead to unintentional material softening and connections. It is therefore advantageous to use second materials with softening temperatures> 30 ° C, preferably greater than 50 ° C.

As a further advantage, the use of particles according to the invention enables a significantly higher process speed. The usual SLS devices can still be used (see, for example, DE 102 31 136 AI), but because of the lower softening temperatures, only a significantly lower energy input is required for sintering. With the same laser power, this can be achieved with a higher travel speed of the laser scanner and thus a higher process speed. In addition, the sintered component cools to room temperature much faster. The coating can be produced using the customary coating methods for powder particles. The coating is preferably applied in a fluidized bed reactor or a spray dryer.

In the fluidized bed reactor, the cores are fluidized (swirled) and the second material is supplied by spraying or atomizing a solution (in a suitable solvent), suspension or dispersion. Likewise, the second material can also be metered in as a solid in the same way as the powder material and agglomerate with the cores.

Depending on the dwell time of the particle material in the coating device, the particles (of a single first material or a material mixture) can be coated individually, or can be built up into granules by means of the second material as a binder phase. The layer thickness of the applied coating can be set, for example, via the concentration of the second material in the sprayed solution / suspension / dispersion, the residence time and the temperature in the reactor or spray dryer. Preferred layer thicknesses are between 0.1 and 10 percent of the mean particle radii.

In an advantageous embodiment of the particle according to the invention, the coating contains a polymer, preferably a thermoplastic polymer. The term polymer should again be interpreted broadly. It is not limited to the typical plastics, but also includes polyolefins (waxes), polyacids and bases, organometallic polymers, polymer blends and polymers in the broadest sense, whose softening temperatures do not exceed 70 ° C. It is advantageous if these are in the solid state at room temperature. The group defined in this way is large enough to to be able to select chemically and / or physically adapted coatings for any core materials. For example, the polarity can be specifically selected or the steric polymer structure. For special requirements, however, the coating can have further components, for example surfactants to improve the flow properties, adhesion promoter to the core, microsinter particles for a second sintering step and other components.

In a particularly advantageous embodiment of the particle according to the invention, the coating contains a polyvinyl acetal, preferably a polyvinyl butyral (PVB). On the one hand, the softening temperature can be specifically selected based on the degree of acetalization (there are a number of unsuitable polyvinyl acetals and butyrals with softening temperatures above 100 ° C, but also a large number of suitable ones with softening temperatures below 70 ° C. On the other hand, the polyvinyl acetals in most are organic Solvents are insoluble and therefore a component connected in this way is generally very durable. On the other hand, it is suitable for investment casting, in particular of cores, since it can be burned out with almost no residual ash. In general, for investment casting of the SLS components, it is advantageous if the coating at least is low in residual ash.

Other suitable coating materials can be found in suitable databases such as BEILSTEIN or GMELIN: For example, poly (alkylene di- or tri-sulfides) are suitable, for example poly (methylene trisulfide) with softening temperatures between 55 and 70 ° C, poly (ethylene glycols) , in particular pol (ethylene glycol) amines or amides with softening temperatures between 50 and 65 ° C, or copolymers of ethylene and linear alkene (di, tri) - oils with chain length> 8 (e.g. poly (ethylene-co-10-undecen- l-ol), melting point about 66 ° C). In a further advantageous embodiment of the particle according to the invention, the coating is not hygroscopic, preferably hydrophobic. This ensures that the particles absorb little or no water and can therefore be stored for a long time without unintentionally clumping.

In an advantageous embodiment of the particle according to the invention, the core contains at least one element from the material group metal, ceramic, polymer. The terms have to be interpreted broadly again. Metal also includes semi-metals, ceramics also sand and the like, and polymer as defined above. Particles with such cores and the coatings described above enable the SLS production of components with practically any physical, in particular mechanical properties.

For investment casting applications, in particular particles with a polymethacrylate core, preferably polymethyl methacrylate

(PMMA) core, and a polyvinyl acetal, preferably polyvinyl butyral, coating are advantageous since such particles can be burned out almost without residual ash.

It is also advantageous if the core contains at least two parts from the material group metal, ceramic, polymer in loose or solid connection. This can be at least two parts of the same group element or different group elements. The parts can be loosely connected (agglomerate) or solid (coating / alloy / chemical compound, etc.). This further increases the selection options with regard to the physical properties of the SLS component to be manufactured. With regard to the SLS method to be created, the object is achieved according to the invention in that it has the following steps:

- applying a layer of particles on a target surface,

Irradiating a selected part of the layer, corresponding to a cross section of the object, with an energy beam, so that the particles in the selected part are connected,

- repeating the steps of applying and irradiating for a plurality of layers so that the joined parts of the adjacent layers join to form the article, wherein

- Particles are used that contain at least one material whose softening temperature is less than approximately 70 ° C.

This achieves the advantages mentioned above with regard to the homogeneity of the temperature gradient and the resulting component quality and the process speed. These advantages occur not only in the case of particles according to the invention, but also in the case of particles which consist only of a single material or are composed homogeneously, as long as they contain only at least one material whose softening temperature is less than approximately 70 ° C.

In the case of particles according to the invention, it is particularly advantageous with regard to component accuracy if the injected radiation energy is dimensioned in such a way that it only leads to softening of the coating and thereby to connection of the irradiated particles without melting the core material. It is also advantageous if at least the particle layer to be irradiated is additionally heated, preferably to a temperature level of approximately 2-3 ° C. below the lowest softening temperature of the particle materials used. As a result, temperature inhomogeneities within and from a layer are further reduced. The laser power to be entered is also further reduced.

A segmented installation space heater according to DE 101 08 612 AI can also be used for the highest precision requirements.

With connected particles according to the invention and / or objects manufactured according to the method according to the invention, their actual geometry shows only minimal deviations due to shrinkage compared to their specified target geometry.

The particle according to the invention and the method according to the invention are explained in more detail below with reference to FIGS. 1 and 2 and several exemplary embodiments:

Figure 1 does not show the particles according to the invention according to a first embodiment to scale. These are used in an otherwise customary laser sintering process for the production of objects. The particles have a core 1 made of a PMMA with a softening temperature of approximately 124 ° C. and a coating 2 made of a PVB with a softening temperature of approximately 66 ° C. The laser beam is guided (power »10 watts (less if the strength is low), feed speed» 5 m / s, laser spot diameter «0.4 mm) in such a way that the radiant energy injected to soften the coating 2 and thereby connect the irradiated ones Particles leads without melting the core material. The particles have ben an average diameter of about 35 microns, the coating has a thickness of about 0.3 to 0.7 microns.

In such a method using these particles, the particles are only connected via the surface-softened coatings. There are only slight temperature inhomogeneities, which cause low shrinkage and thus high component accuracy. 2, in which the connected particles 1 are shown hatched. The coatings, which were thickened for the sake of better representability, were not softened to the surface in the connection areas to such an extent that the particles were connected.

The accuracy is further increased if the particle layers are preheated to around 60 ° C, since then the temperature inhomogeneities will decrease significantly further. The laser power and / or feed rate is adjusted accordingly. The preheating is carried out by means of IR radiation on the surface or, in the case of even higher accuracy requirements, by means of the segmented jacket heating according to DE 101 08 612 AI.

According to a further exemplary embodiment, 1-component particles made of pure PVB with a softening temperature of approximately 66 ° C. and an average diameter of approximately 80 μm are used. Particles with average diameters of around 50 - 100 μm are also suitable. The resulting components have lower mechanical resilience and can mainly be used as models or as so-called lost cores, especially in investment casting applications.

For applications that have to meet higher physical, in particular mechanical requirements, particles with metallic and / or ceramic cores and preferably also metallic coatings are used. As loading Layers are particularly suitable for all alloys, in particular non-toxic bismuth-lead-indium alloys with a low melting point, which are known to the person skilled in the art, for example, as fuses according to JP2001143588A, or solder alloys such as the bismuth-lead-tin alloy PAD-165-851 the Stan Rubinstein Assoc. , Foxboro, MA 02035 USA (cf.http: // www.sra-solder. Com / pastesp.htm).

In the case of metallic particles, the mean diameters are preferably 40-150 μm, for special accuracy requirements also less, for ceramic particles mostly less than 150 μm, preferably 15 to 40 μm, for special requirements also up to 5 μm.

Claims

claims

1. Containing particles for use in selective laser sintering (SLS) - a core 1 made of at least one first material

- An at least partial coating 2 of the core 1 with a second material, the second material having a lower softening temperature than the first material, so that the softening temperature of the second material is less than approximately 70 ° C.

2. Particles according to claim 1 that the coating 2 contains a polymer, preferably a thermoplastic polymer.

3. Particle according to claim 2, that the coating 2 contains a polyvinyl acetal, preferably a polyvinyl butyral.

4. Particle according to one of the preceding claims, characterized in that the coating 2 is not hygroscopic, preferably hydrophobic.

5. Particle according to one of the preceding claims, that the core 1 contains at least one element from the metal, ceramic, polymer material group.

6. Particle according to claim 5, so that the core 1 contains at least two parts from the material group metal, ceramic, polymer in loose or solid connection.

7. A method for producing a three-dimensional object by means of SLS comprising the following steps:

- applying a layer of particles on a target surface,

Repeating the steps of applying and irradiating for a plurality of layers so that the joined parts of the adjacent layers join to form the object, so that the object is made,

- that particles are used that contain at least one material whose softening temperature is less than approximately 70 ° C.

Method according to claim 7, characterized in that particles according to one of claims 1 to 6 are used.

9. The method according to claim 7, wherein the at least the particle layer to be irradiated in each case is additionally heated, preferably to one

Temperature level of about 2-3 ° C below the lowest softening temperature of the particle materials used.

10. Object made of interconnected particles, characterized in that it was produced from particles according to one of claims 1 to 6 and / or that it was produced by a method according to one of claims 7 to 9.