CA2460664A1

CA2460664A1 - Process for producing three-dimensional objects by means of microwave radiation

Info

Publication number: CA2460664A1
Application number: CA002460664A
Authority: CA
Inventors: Maik Grebe; Sylvia Monsheimer; Franz-Erich Baumann
Original assignee: Degussa GmbH
Current assignee: Evonik Operations GmbH
Priority date: 2003-03-15
Filing date: 2004-03-11
Publication date: 2004-09-15
Also published as: US7708929B2; CN1532043A; EP1459871A3; JP2004276618A; JP4455107B2; EP1459871A2; PL366190A1; CN100339208C; US20040232583A1; EP1459871B1

Abstract

The present invention relates to a process for bonding material to produce three-dimensional objects by means of selective heating via microwave radiation. Unlike selective laser sintering, the present process for producing three-dimensional objects yeas the type of simple microwave radiation available in any household. The selectivity of heating is achieved by applying a susceptor to certain subregions of a layer composed of a pulverulent substrate, and then heating the susceptor by means of microwave radiation. The heated susceptor transfers the energy present therein to a pulverulent substrate surrounding the susceptor, and the substrate is thereby melted, giving firm bonding within the substrate after cooling.

Description

O.Z. 6284 Process for producing three-dimensional objects by means of microwave radiation The invention relates to a process for producing three-dimensional objects from a pulverulent substrate by bonding, e.g. by fusion or sintering of parts of the substrate, the heat needed for the bonding of the substrate being generated by microwave radiation by way of a susceptor and being transferred by way of the susceptor to the subregions of the substrate.
A task often encountered in very recent times is the rapid production of prototypes. The prior art firstly describes the stereolithography method, whicli has the disadvantage of needing .
complicated support structures during the preparation of the prototypes from a liquid (resin), and the disadvantage that the resultant prototypes have relatively poor mechanical properties, these being attributable to the limited number of starting materials.
The other process which is often mentioned in the prior art and which has good suitability for rapid prototyping is selective laser sintering (SLS) which has now become widespread. In this process, polymer powders in a chamber are selectively irradiated briefly with a laser beam, the result being melting of the particles of powder on which the laser beam falls.
The molten particles coalesce and solidify again relatively rapidly to give a solid mass.
Complex three-dimensional bodies can be produced simply and rapidly by this process, by repeatedly applying 2o fresh layers and irradiating these.
The process of laser-sintering (rapid prototyping) to realize moldings composed of pulverulent polymers is described in detail in the patent specifications US 6,136,948 and (both DTM Corporation). The SLS processes described in the prior art have the disadvantage of expensive laser technology needed for this process. The laser functioning as energy source is extremely expensive and sensitive, as also is the optical equipment needed for the provision and control of the laser beam, such as lenses, expanders, and deflector mirrors.
Other processes have been developed for rapid prototyping, but cannot (yet) be introduced into the market. WO 01/38061 describes a process for producing prototypes which is based on the use of sinter inhibitors that inhibit sintering, initiated by introduction of energy, of pulverulent substrate in selected regions. This process can operate without any complicated laser O.Z. 6284

2 technology. However, specific introduction of heat is impossible with this process. By way of example, a disadvantage of this process is that the surrounding powder which was not melted comprises the inhibitor and cannot therefore be recycled. In addition, this process requires development of new software, specifically because it is the surrounding area that is printed, and not, as in other cases, the cross section of the part. For undercuts and changes in cross section, larg~surface-area application of inhibitors is needed. In addition, there is the risk of heat build-up.
US 5 338 611 describes the use of microwave radiation for the melting of polymers, use being made of pulverulent polymers and nano-scale carbon black. No production of prototypes is described. DE 197 27 677 generates prototypes by exposing selected regions of pulverulent layers to a focused microwave beam. Exposure to the controlled microwave beam bonds the pulverulent substrates within the layer, and also to the pulverulent substrates in the layer situated thereunder, via adhesive bonding, sintering, or fusion. This process, too, needs ~5 complicated technology in order to ensure that the microwave radiation reaches only the selected regions.
All of the prototype-production processes known from the prior art use relatively complicated technology. In particular, the use of lasers or of focused microwave radiation requires high precision and therefore requires apparatus which is expensive and susceptible to failure.
Although the known processes are suitable for producing prototypes, these processes are however unsuitable for rapid manufacturing applications, or for applications in the home.
It was an object of the present invention, therefore to provide a process which can produce three-dimensional objects and which can be carried out using simple apparatus which has low cost and is not susceptible to failure. The components should preferably be of robust design, and it should be possible here to utilize components from apparatus which is in everyday use.
Surprisingly, it has been found that three-dimensional objects can be produced from pulverulent substrates relatively simply by means of microwave radiation, e.g.
even by means of microwave kitchen equipment, by applying, to those regions to be bonded in a layer composed of a pulverulent substrate which absorbs microwave radiation only poorly or not at O.Z. 6284

3 all, a susceptor which can absorb the microwave radiation and passes the energy absorbed in the form of heat to the substrate surrounding the susceptor, the result being that the substrate of the layer or, where appropriate, of a layer situated thereunder or thereover, is bonded in the regions mentioned via fusion or sintering. The susceptor may be applied using a printing head, similar to that of an ink Set printer.
1. The present invention therefore provides a process for producing a three-dimensional object, which comprises the following steps:
1 o a) providing a layer of pulverulent substrate, b) selectively applying at least one microwave-absorbing susceptor to regions to be treated of the layer from a), where the regions to which the susceptor is applied are selected in accordance with the cross section of the three-dimensional object, and specifically in such a way that the susceptor is applied only to the regions which make up the cross section of the three-dimensional object, and c) treating the layer at least once with microwave radiation, so that the regions of the layer which have been equipped with the susceptor, and also, where appropriate, the regions of the layer situated thereunder which have the susceptor, are bonded to one another via fusion or sintering, and also provides moldings produced by this process.
2. The present invention also provides an apparatus for the layer-by-layer production of three-dimensional objects, which comprises - a movable apparatus for applying layers of a pulverulent substrate to an operating platform or to a layer of a treated or untreated pulverulent substrate (2) which may by this stage be present on the operating platform, an apparatus (3) movable in the x,y plane, for applying a susceptor (4) to selected regions of the layer composed of pulverulent substrate, and - a microwave generator suitable for generating microwave radiation in the range from 300 MHz to 300 GHz (5).
The inventive process has the advantage that it does not use any complicated directed radiation,

4 such as laser radiation or narrowly focused microwave radiation. The controlled exposure of certain locations of the layer, or of a matrix composed of two or more layers, to energy is achieved via the susceptor excited by microwave radiation, the susceptor being applied to the desired regions of the layer or of the layers of the matrix.
The inventive process is a simple way of permitting layer-by-layer automated build-up of a three-dimensional object by using microwave radiation in combination with a suitable susceptor. Powder not treated with susceptor may readily be reused, this being impossible in the case of processes which use inhibitors.
The simplicity of operation of the apparatus is similar to that of a conventional ink jet printer, and the apparatus may therefore, by way of example, be linked to a PC, particularly if the microwave irradiation is then carried out in the microwave equipment present as a matter of course in most households. 3D printing can be afforded and operated even by a normal household. Another advantage of the inventive process is that the surrounding material can easily be reused. In addition, specific properties, such as electrical conductivity or colors, can be "printed" concomitantly. The part may thus be provided concomitantly with carefully selected properties.
The functional principle of the present inventive process for producing three-dimensional objects is based on the principle used in all of the other rapid-prototyping processes. The three-dimensional object is built up layer-by-layer. The method of build-up is that parts of liquid layers (stereolithography) or powder layers (laser sintering) are fixed or bonded to one another or to parts of layers situated thereunder, by supplying energy to these parts of the layers. Those parts of the layers to which no energy was introduced remain in the form of liquid or powder. A three-dimensional object is obtained layer-by-layer via repetition of the application and bonding or fixing of powder or liquid. Removal of the unconverted powder or of the unconverted liquid gives a three-dimensional object, the resolution of which (in relation to the outlines) depends on the layer thickness and on the particle size of the pulverulent substrate used.
In contrast to the processes known hitherto, the energy is not supplied directly to the substrates O.Z. 6284 to be bonded, but by way of a susceptor, which absorbs the energy and transfers it in the form of heat to the substrate surrounding the susceptor. The inventive process introduces the energy to the susceptor in the form of microwave radiation, wluch is absorbed by the susceptors, converted into heat, and transferred to the directly adjacent pulverulent material of the

5 substrate, this material being incapable of absorbing microwave radiation, or of absorbing it to a sufficient extent. In the present case, "not to a sufficient extent" means that absorption of microwave radiation cannot heat the pulverulent substrate sufficiently for it to enter into bonding via fusion or sintering with adjacent substrate particles, or that the time needed for this is excessive. However, the heat transferred from the susceptor is sufficient to bond the pulverulent substrate adjacent to the susceptor to it, and also to the susceptor, via fusion or sintering. The inventive process thus produces three-dimensional objects via fusion or sintering of a pulverulent substrate.
As in laser sintering and the other rapid-prototyping processes, the bonding of the substrate in certain regions within the layer, again takes place via bonding, in particular fusion or sintering, of the pulverulent substrate. The functional principle of rapid prototyping may be found in US 6,136,948 and WO 96/06881, for example.
As a consequence of the application of the susceptors in step b), which is usually computer-controlled, using CAD applications to calculate the cross sections, only treated pulverulent substrates are bonded in a subsequent treatment step c). The susceptor is therefore applied only to selected regions of the layer of a), these being within the cross section of the three-dimensional object to be produced. An example of a method for the application process itself is the use of a printing head equipped with nozzles. Once the treatment step c) has been concluded for the final layer, the inventive process gives a matrix with, in part, bonded powder material, and this matrix reveals the solid three-dimensional object once the unbonded powder has been removed.
The inventive process is described below by way of example, but there is no intention that the invention be restricted thereto.
The inventive pmcess for producing a three-dimensional object comprises the steps of O.Z. 6284

6 a) providing a layer of pulverulent substrate, b) selectively applying at least one microwave-absorbing susceptor to regions to be treated of the layer from a), where the regians to which the susceptor is applied are selected in accordance with the cross section of the three-dimensional object, and specifically in such a way that the susceptor is applied only to the regions which make up the cross section of the three-dimensional object, and c} treating the layer at least once with microwave radiation, so that the regions of the layer which have been equipped with the susceptor, and also, where appropriate, the regions of the layer situated thereunder which have the susceptor, are bonded to one another via 1 o fusion or sintering.
Step c) may be carried out on each occasion when steps a) and b) have been executed x times, where x is from 1 to the number of steps a) and b) carried out. This method can take into account the material-dependent penetration depth of the microwave radiation, as required by the powder material used. For example, depending on the powder material and on the number of steps a), a single treatment with microwave radiation may not be sufficient to bond all of the regions treated with susceptor in the layers of -powder material present in the construction chamber. In this type of case, it can be advantageous to carry out step c), by way of example, after 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, i3, 14, 15, 20, 30, 40, or 50 repetitions of steps a) and b}.
2o It can also be advantageous to delay carrying out step c) until steps a) and b) have been executed at least twice, because this method achieves a more secure bond between the layers. In one particular embodiment of the inventive process, steps a} and b) are repeated until all of the cross sections which compose the three-dimensional object are present within a matrix, and the outer limits of the object are formed by the boundary between powder material with susceptor applied and untreated powder material, and step c) is then carried out. This method requires only a single treatment with microwave radiation, and thus significantly lower energy cost.
In another embodiment of the inventive process, at the start of production of the three-dimensional object, step c) is initially carried out once after step a) has been carried out once 3o and step b) has been carried out and then step a) has again been carried out once, and then the further steps are earned out in the sequence b), a), and c). In this embodiment, an untreated powder layer covers each susceptor-treated powder layer. In step c}, therefore, the particles of O.Z.6284

7 the uppermost layer are not bonded, e.g. via melting or sintering, but the particles of the layer situated thereunder are bonded, and the particles of the two layers are indeed bonded at the boundary between the layers. This method can achieve a particularly durable bond between the layers. In addition, the transitions from one layer to the next layer in the finished object become softer. Step c) may in turn be caxried out on each occasion when steps b) and a) have been executed x times, where x is from 1 to the number of steps b) and a) carried out, the advantages achieved here being those mentioned above.
If step c} is executed on each occasion after steps a) and b), or b) and a), have been executed one or more times, it may take place directly in the construction chamber. If only one step c) treatment takes place, this may take place in the lower construction chamber or in another suitable place within the apparatus. Step c) may also be carried out in an apparatus other than the apparatus for carrying out steps a) and b). By way of example, the matrix generated by means of steps a) and b) and composed of treated powder layers may be transferred, by way of ~5 example, into commercially available food-preparation microwave equipment, where step c) is carried out. These possibilities make the inventive process particularly suitable for applications in the home.
By way of example, the pulverulent layer may be provided by applying a powder material as 2o substrate to a base plate, or to a layer which is present at this stage and has been treated in step b) or c). The method of application may be doctoring, rolling, or broadcasting and subsequent stripping, or a similar method. The single precondition with which the provision of the layer has to comply is that the layer has uniform height. The height of the layer provided in step a) is preferably less than 1 mm, with preference from 50 to 500 ~.m, and particularly 25 preferably from 100 to 200 Vim. The height of the layers here determines the resolution and therefore the smoothness of the external structure of the three-dimensional object produced.
The base plate, or else the apparatus for providing the layer, may be designed with adjustable height so that after a step b) or c) has been earned out, either the resultant layer can be lowered by the height of the layer to be applied next or the apparatus can be raised by the difference in 3o height of the next layer over the preceding layer.
The height of the layer provided in step a) depends, inter alia, on the median particle size or on O.Z. 6284

8 the maximum particle size. It is therefore clearly apparent that firm and compact layers of uniform height cannot be produced using particles whose size is 150 ~,m, because the volume between the particles would undergo very great shrinkage in step c).
s The powder material used as pulverulent substrate particularly preferably has a median grain size (due) of from 10 to 150 wm, particularly preferably of from 20 to 100 ~.m and very particularly preferably of from 40 to 70 ~,m. Depending on the application, however, powder materials comprising particularly small particles, and also comprising particularly large particles, may be used with advantage. In order to realize three-dimensional articles with 1o maximum resolution and maximum surface smoothness, it can be advantageous to use particles whose median particle size is from IO to 45 Vim, preferably from 10 to 35 Vim, and very particularly preferably from 20 to 30 Vim.
It is very difficult to process fine materials smaller than 20 ~,m, in particular smaller than 15 IO Vim, because it does not flow, and the bulk density falls drastically, and this can cause more production of cavities. To ease operations, it can be advantageous to use particles whose median size is from 60 to 150 ~,m, preferably from 70 to 120 Vim, and very particularly preferably from 75 to 100 pm.
2o The pulverulent substrate used preferably comprises powder material which was prepared by milling, precipitation, and/or anionic polymerization, or by a combination of these, especially precipitation of a somewhat excessively coarse powder, and subsequently milling, or precipitation and subsequently classifying.
25 The particle size distribution may be selected as desired for the stated median grain sizes of the powder materials. It is preferable to use powder materials which have a broad or narrow grain size distribution, preferably a narrow grain size distributior.~. Particularly preferred pulverulent materials for use in the inventive process have a particle size distribution in which, based on the median particle size, a particle size deviation of more than 50% is present in not more than 30 20% of the particles, preferably 15%, and very particularly preferably not more than S% of the particles. The particle size distribution may be adjusted by conventional classification methods, e.g. pneumatic separation. Maximum narrowness of particle size distribution in the inventive O.Z. 6284

9 process gives three-dimensional objects in which the surface is very uniform and any pores present are very uniform.
At least a part of the pulverulent material used may be amorphous, crystalline, or semicrystalline. Preferred powder material has a linear or branched structure.
Particularly preferred powder material used in the inventive process has,'at least in part, a melting point of from 50 to 350°C, preferably from 70 to 200°C.
Substrates suitable in the inventive process are substances whose susceptibility to heating by 1o microwave radiation is smaller than that of the selected susceptors. The pulverulent substrate used should also exhibit sufficient flowability in the heated state.
Particular pulverulent substrates which may be used are polymers or copolymers, preferably selected from polyester, polyvinyl chloride, polyacetal, polypropylene, polyethylene, polystyrene, polycarbonate, poly(N-methylmethacrylimide) (PMMI), polymethyl methacrylate (PMMA), ionomer, polyamides, copolyester, copolyamides, terpolyme:rs, acrylonitrile-butadiene-styrene copolymers (ABS), or a mixture of these.
A powder material particularly preferably used as pulverulent substrate in the inventive process is one in which at least one nylon-6, nylon-I1, andJor nylon-12, or a copolyester, or a 2o copolyamide, is present. The use of polyamides can give three-dimensional moldings with particular dimensional stability. It is particularly preferable to use nylon-12 powder, preferably prepared as described in DE 197 08 946, or else as described in DE 44 21 454, and which particularly preferably has a melting point and an enthalpy of fusion as stated in EP 0 911 142.
Preferred copolyamides or copolyesters used are those obtainable from Degussa AG with the trademark VESTAMELT. The melting point of part;icularly preferred copolyamides, determined by means of differential scanning calorimetry (DSC) is from 76 to 159°C, preferably from 98 to 139°C, and very particularly preferably from I10 to 123°C. By way of example, the copolyamides may be prepared by polymerizing mixtures of suitable monomers, e.g. selected from laurolaetarn and/or caprolactam as bifunetional component, suberic acid, 34 azelaic acid, dodeeanedioic acid, adipic acid and/or sebacic acid as component bearing an acid function, and l,s-hexanediamine, isophoronediamine, and/or rnethylpentamethylenediamine as diamine.

O.Z. 6284 In order to improve proeessability of the pulverulent substrates, it can be advantageous to use a powder material which comprises additives. By way of example, these additives may be flow aids. The pulverulent substrate used particularly preferably comprises from 0.05 to 5% by 5 weight, with preference from 0.1 to 1 % by weight, of additives. By way of example, flow aids may be fumed silicas, stearates, or other flow aids known from the literature, e.g. tricalcium phosphate, calcium silicates, A12O3, MgO, MgC03, or ZnU. By way of example, fumed silica is supplied by Degussa AG with the trademark Aerosil~. In addition, it can be advantageous for the pulverulent substrate used to comprise laser-activatable additives. By way of example, these

10 additives permit the three-dimensional objects to be subsequently inscribed or equipped with electrical conductor tracks. DE 4402329 describes additives which may be used, by way of example.
Alongside, or instead of, these, in some case inorganic, flow aids, or other additives, a pulverulent substrate used according to the invention may also comprise inorganic fillers. The use of these fillers has the advantage that they retain their shape to a substantial extent through the treatment and thus reduce the shrinkage of the three-dimensional object.
In addition, the use of fillers permits, by way of example, alteration of the plastic and physical properties of the objects. For example, the use of powder material which comprises metal powder can adjust not only the transparency and color of the object but also its magnetic or electrical properties.
Examples of fillers which may be present in the powder material are glass particles, ceramic particles, or metal particles. By way of example, typical fillers are granulated metal, aluminum powder, steel shot, or glass beads. It is particularly preferable to use powder materials which comprise glass beads as filler. In one preferred embodiment, the inventive powder material comprises from 1 to 70% by weight, preferably from 5 to 50% by weight, and very particularly preferably from 10 to 40% by weight, of fillers.
Alongside, or instead of, inorganic flow aids or fillers, a pulverulent substrate used according to the invention may also comprise inorganic or organic pigments. These pigments may be not only color pigments which determine the perceived color of the three-dimensional body to be produced, but also pigments which affect other physical properties of the three-dimensional articles to be produced, e.g. magnetic pigments, or conductivity pigments, for example O.Z. 6284

11 conductivity modified titanium dioxide or tin oxide, which alter the magnetic properties and, respectively, the conductivity of the article. However, the powder material to be used particularly preferably comprises inorganic or organic color pigments selected from chalk, ochre, umber, green earth, burnt sierra, graphite, titanium white (titanium dioxide), white lead, zinc white, lithopone, antimony white, carbon black, iron oxide black, manganese black, cobalt black, antimony black, lead chromate, mennium, zinc yellow, zinc green, cadmium red, cobalt blue, Prussian blue, ultramarine, manganese violet, cadmium yellow, Schweinfurter green, molybdate orange, molybdate red, chrome orange, chrome red, iron oxide red, chromium oxide green, strontium yellow, metallic-effect pigments, pearlescent pigments, luminescent pigments using fluorescent and/or phosphorescent pigments, umber, gamboge, animal charcoal, Cassel brown, indigo, chlorophyll, azo dyes, indigoids, dioxazine pigments, quinacridone pigments, phthalocyanine pigments, isoindolinone pigments, pexylene pigments, perinone pigments, metal complex pigments, alkali blue pigments, and diketopyrrolopyrrole. By way of example, further information relating to pigments which may be used may be found in Rompp I,exikon Chemie [Rompp Chemical Encyclopedia] - Version 2.0, StuttgartlNew York: Georg Thieme Verlag 1999, and in the references given therein.
The particle sizes of the pigments used may be those described for the powder material.
However, the pigments frequently have particle sizes significantly smaller than the median 2o grain sizes of the polymers used. By way of example, the pigments may be applied in a manner similar to that for the susceptors, via nozzles, such as those used in printing heads, or may be present in the pulverulent substrates used, in particular in the polymer particles. The inventive powder material particularly preferably comprises polymer particles which comprise one or more of the pigments mentioned - preferably with the except of white pigments alone. 'The proportion of the pigments in the powder material is preferably from 0.01 to 25% by weight, with preference from 0.1 to 10% by weight, and particularly preferably from 1 to 3% by weight.
The possibility of using pigmented substances is a further advantage of the inventive process over laser-sintering processes, in which color pigments of metallized pigments impede or attenuate the laser beam and thus prevent processing of such materials.
The powder material used may also comprise substances which may be regarded as a specific form of the abovementioned fillers or pigments. In this type of powder material, the powder O.Z. 6284

12 comprises grains composed of a first material with a size which is smaller than the abovementioned dimensions of the powder material. The grains have been coated with a layer of a second material, the selection of the thickness of the layer being such that the powder material composed of a combination of a grain of the first material and a coating of the second material has the size stated above. The grains of the first material preferably have a size which deviates by less than 25%, preferably by less than 10%, and particularly preferably by less than 5%, from the size of the powder material. The second material, which is the coating of the grains, is a material less susceptible than the selected susceptors to heating by microwave radiation. The second material should also exhibit sufficient flowability in the heated state and 1o should be capable of melting or sintering on exposure to heat, the heat being that provided by the susceptor. Coating materials which may be used are the pulverulent substrates (the powder materials), in particular the abovementioned polymers or copolymers, preferably selected from polyester, polyvinyl chloride, polyacetal, polypropylene, polyethylene, polystyrene, polycarbonate, poly(N-methylinethacrylimide) (PMMI), polymethyl methacrylate (PMMA), ionomer, polyamides, copolyester, copolyamides, terpolymers, acrylonitrile-butadiene-styrene copolymers (ABS), or a mixture of these, or phenolic resins. By way of example the first material of this specific form of the powder material may encompass grains of sand, ceramics, metal, andlor alloys. Particularly preferred powder material of this type is phenolic-resin-coated or thermoplastic-coated sand, known as molding sand.
If the susceptor is capable of transferring a sufficient amount of heat, it is also possible for the powder material used to comprise metal powders, in particular powders of low-melting metals, e.g. lead or tin, or alloys which comprise, by way of example, tin or lead.
This powder material, too, preferably has the abovementioned dimensions. (If metal powder is used, a check first has to be made as to whether the metal is suitable for microwave treatment or whether sparking occurs, or irreversible damage to the microwave generator. This check can be earned out by simple preliminary experiments.) 'The inventive process can therefore produce three-dimensional objects which may be equipped 3o with one or more functionalized layers. By way of example of a functionalization, the entire molding is equipped with conductive properties, or else only certain regions are equipped therewith, through application of appropriate pigments or substances in a manner similar to that O.Z. 6284

13 for the susceptor, or through provision of a layer composed of a pulverulent substance which comprises these pigments.
The method for applying the susceptor may be based on the inhibitor application method described in WO 01/38061. The susceptor is preferably applied using an apparatus movable in the x,y plane. The apparatus is capable of transfernng liquid and/or pulverulent susceptors at defined sites on the layer provided in step a). By way of example, the apparatus may be the printing head used in an ink jet printer. The guiding cf the apparatus for positioning the printing head may likewise take place in identical fashion to the guiding of the printing head of 1o an ink jet printer. Using this apparatus, the susceptor is applied at those sites on the layer provided in step a) where the substrate is to be bonded by sintering or fusion.
Susceptors which may be used in the inventive process are any of those which are heated by microwave radiation. Among these are pulverulent substances, e.g. metal powders, metal compounds, ceramic powders, graphite, carbon black, or activated charcoal, or profit liquids selected from the group consisting of saturated mono- or polyhydric linear, branched, or cyclic aliphatic alcohols, undiluted or in a mixture with water, or water alone.
Preferred profit liquids used are glycerol, trimethylolpropane, ethylene glycol, diethylene glycol, or butanediol, or a mixture of these, undiluted or in a mixture with water. It is also possible to use a mixture of 2o solid, liquid, or solid and liquid, susceptors. It may also be advantageous to suspend solid susceptors in liquids which are not susceptors, in order to achieve better distribution of the solid susceptors over the entire depth of the layer provided. Another advantage may be achieved if the susceptor, in particular the liquid susceptor, is equipped with surfactants for better wetting of the substrate.
In this inventive process it is also possible to conceive of a large number of combinations of susceptors and substrate, but for the process there has to be a sufficiently large difference between susceptor and substrate in susceptibility to heating by microwave radiation, in order that the process finally gives a matrix which has a clear boundary between bonded (i.e.
3o susceptor-treated) substrate and unbonded substrate. This is the only way of ensuring that the three-dimensional object produced has a sufficiently smooth outline, and can be readily released from the unbonded substrate.

O.Z. 6284

14 In order to permit a sufficient amount of, and a sufficient duration of, heat transfer from susceptor to the substrate, the boiling point of the susceptor, or in the case of a mixture of susceptors, the boiling point of at least one susceptor, should be higher than the melting point of the substrate used. The metering of the susceptor, and also the properties of the powder and of the susceptor, have to have been matched to one another in order that, in particular if a liquid susceptor is used, the susceptor does not run through the layers but is absorbed exclusively by the powder to be wetted. An example of a method for the matching uses viscosity adjustment and the amount used of the susceptor. The amount used here of the liquid susceptor is l0 particularly dependent on the layer thickness of the powder, on the porosity of the powder, and on the particle size. The ideal amount and viscosity for a particular combination of materials may be determined in simple preliminary experiments. To adjust the viscosity, use may be made of known thickeners, such as fumed silicas, or else organic agents. The susceptor may remain in the melt or in the molding. This may indeed be advantageous if there is reinforcement or if the susceptor adjusts other properties (electrical or magnetic conductivity).
The energy needed for heating the susceptor is introduced in the form of microwave radiation.
It may be advantageous to use introduction of heat to bring the layers to be sintered to an elevated temperature or to keep them at an elevated temperature, this temperature being below 2o the melting point or sintering point of the polymer us~i. This method can reduce the energy or power which has to be introduced in the form of microwave energy. However, a disadvantage of this design is that specific apparatus has to be used which is often not present in households, e.g. conventional ovens combined with incorporated microwave equipment.
However, if these devices become more widespread, household application of the inventive process also permits some of the sintering energy needed to be introduced by means other than microwave energy.
The treatment with microwave radiation in step c) may, as described above, take place after each step b), or else may be delayed until all of the layers have been treated with the susceptor.
1n particular if a liquid susceptor is used, it has proven advantageous to undertake the 3o microwave treatment directly after each treatment of a layer in step b), preferably directly in the construction chamber, because otherwise there is a risk that the liquid susceptor will also become dispersed into undesired parts of the layer or of the matrix composed of two or more O.Z. 6284 layers.
The microwave radiation required for the inventive process is generated by a, preferably external, microwave generator, and may lie within the frequency range from 300 MHz to 5 300 GHz. The frequencies nationally approved and used in industrial processes are generally from 430 to 6 800 MHz (Encyclopedia of Chemical Processing and Design, Vo1.30, p. 202 et seq., Marcel Dekker, N.Y., Basle, 1989). Microwave radiation in the frequency range from 430 to 6 800 MHz is therefore preferably used in the inventive process.
The radiation generated by the microwave generator may, where appropriate, be polarized and/or filtered.
Three-dimensional moldings can be produced by the inventive process. These three-dimensional objects produced layer-by layer are finally present, at the end of the inventive process, within a matrix, which is formed from two or more layers.
The object may be removed from this matrix, which is composed of bonded and unbonded pulverulent substrate, and also susceptor, while the unbonded substrate may be reused, where appropriate after treatment, e.g. by sieving. The inventive moldings may comprise fillers selected from glass beads, silicas, or metal particles.
The inventive process is preferably carried out in an inventive apparatus for the layer-by layer production of three-dimensional objects, which comprises - a movable apparatus fox applying layers of a pulverulent substrate to an operating platform or to a layer of a treated or untreated pulverulent substrate which may by this stage be present on the operating platform, e.g. a doctor, - an apparatus movable in the x,y plane, for applying a susceptor to selected regions of the layer composed of pulverulent substrate, e.g. a printing head, and - a microwave generator suitable for generating microwave radiation in the range from 300 MHz to 300 GHz, preferably from 430 to 6 800 MHz, and which permits heating of the susceptor to the extent that the substrate bonds via fusion or sintering in those regions where the susceptor was applied to the substrate.
The apparatus has preferably been equipped with two or amore feed vessels from which the pulverulent substrate to be processed can be introduced to the apparatus for generating the layers, and the susceptor{s) used can be introduced into the apparatus movable in the x;y plane for applying a susceptor to selected regions of the layer composed of pulverulent substrate. By using pressure heads with two or more nozzles and with provision of a mixer, the mixture of susceptors used at certain zones within the layer, e.g. at particularly filigree regions or, by way of example, at the margin of the object to be produced, may differ fram the mixture used in the core region of the object to be produced. This method permits different amounts of energy to be introduced at different positions within the layer.
The present invention also provides the powder material as described above, suitable for use in 1o the inventive process, and in particular having a median grain size of from 10 to 150 gm, and comprising at least one polymer or copolymer, selected from polyvinyl chloride, polyester, polyacetal, polypropylene, polyethylene, polystyrene, polycarbonate, PMMA, PMMI, ionomer, polyamides, copolyester, copolyamides, terpolymers, or ABS, or a mixture of these. 'The powder particularly preferably comprises nylon-1 l, nylon-12, copolyamide, or copolyester, or a mixture of these. The powder particularly preferably comprises polymer particles which have been colored and whose color is non-white.
The inventive process and the inventive apparatus are further illustrated using Fig. 1, but there is no intention that the invention be restricted to that embodiment.
Fig. 1 is a diagram of 2o the inventive apparatus. Untreated pulverulent substrate (2), which has previously been charged to a feed vessel (1) is built up on a movable base (6) to give a matrix (8). A
doctor is used to distribute the substrate to give thin layers on the movable base or on the previously applied layers. The susceptor (4) is applied to selected regions of the layer composed of pulverulent substrate, by way of an apparatus (3) movable in the x, y plane. After each treatment with a susceptor, a fresh layer of the pulverulent substrate is applied. 'The sites on the applied substrate which have been treated with the susceptor are bonded by means of a microwave generator suitable for generating microwave radiation in the range from 300 MHz to 300 GHz (5), to give a three-dimensional object, e.g. a cup (7).
3o The inventive process is further illustrated, using the following examples, but there is no intention that the invention be restricted thereto.

Example 1: Production of a cup from a copolyamide A model for a cup with an external diameter of 80 mm, a height of 60 mm, and a wall thickness of 1.5 mm, composed of a copolyamide powder (VESTAMELT 840, Degussa AG, Marl) is produced in the apparatus described by Fig.1. The susceptor used comprises a graphite-based s suspension which comprises 40% by weight of water, 40% by weight of graphite, and 20% by weight of isopropanol. The apparatus has an operating temperature of about 40°C. The frequency of the microwave generator is 2 450 MHz. The layer thickness is 0.15 mm. For each layer, the power introduced is 700 watts, in each case for 30 seconds. The dso of the powder is 60 gm.
io Example 2: Production of a tensile speefmen from nylon-12 A tensile specimen of length 160 mm and width 10 mm, a~~d depth 4 mm, is produced in the apparatus previously described from a nylon-12 powder (EOSINT P PA 2200, EOS
GmbH
Electro Optical Systems, Krailling, Germany). Ethylene glycol is used as susceptor. The

15 apparatus has an operating temperature of about 160°C. The frequency of the microwave generator is 2 450 MHz. The depth of the powder layers applied was 0.15 mm.
The power introduced per layer was 750 watts, for 45 seconds. The powder used had a d5o of 55 um.
*Trade-mark

Claims

CLAIMS:

1. A process for producing a three-dimensional object, which comprises:
a) providing a layer having a uniform height of a powder material which is incapable of absorbing microwave radiation to an extent sufficient to fuse but is capable of fusing when heated;
b) selectively applying at least one susceptor to regions to be treated of the layer of the powder material, wherein the susceptor absorbs microwave energy and transfers the energy in the form of heat to the powder material surrounding the susceptor and wherein the regions to which the susceptor is applied are selected in accordance with a cross section of the three-dimensional object in such a way that the susceptor is applied only to the regions which make up the cross section of the three-dimensional object; and c) treating the layer at least once with microwave radiation so that the regions of the layer which have been equipped with the susceptor, and also, where appropriate, regions of a layer situated under the susceptor, are heated and bonded to one another via fusion or sintering of the powder material.

2. The process as claimed in claim 1, wherein step c) is carried out after each of step a) and step b) has been executed x times, where x is from 1 to 50.

3. The process as claimed in claim 1, wherein step c) is initially carried out once after carrying out step a) once, step b) and then step a) once again; and then the steps b), a), and c) are carried out in this sequence.

4. The process as claimed in any one of claims 1 to 3, wherein steps a) and b) are repeated until all of the cross sections which compose the three-dimensional object are present within a matrix, and outer limits of the object are formed by a boundary between the powder material with the susceptor applied and untreated powder material, and step c) is then carried out.

5. The process as claimed in claim 4, wherein step c) is carried out in a lower construction chamber of an apparatus.

6. The process as claimed in any one of claims 1 to 5, wherein step c) is carried out in an apparatus other than an apparatus for carrying out steps a) and b).

7. The process as claimed in any one of claims 1 to 6, wherein step c} is carried out in a commercially available food-preparation microwave equipment.

8. The process as claimed in any one of claims 1 to 7, wherein the powder material has a median grain size of from 10 to 150 um and is employed in step a) in such amount that the height of the layer is 50 to 500 µm.

9. The process as claimed in any one of claims 1 to 8, wherein the microwave radiation has a frequency in the range of from 430 to 6,800 MHz.

10. The process as claimed in any one of claims 1 to 9, wherein the susceptor is at least one member selected from the group consisting of a powder of a metal or a metal compound, a ceramic powder, graphite, activated charcoal, and a protic liquid selected from the group consisting of a saturated mono- or polyhydric linear, branched, or cyclic aliphatic alcohol, a mixture of the alcohol with water, and water alone.

11. The process as claimed in claim 10, wherein the susceptor is a erotic liquid that is an alcohol selected from the group consisting of glycerol, trimethylolpropane, ethylene glycol, diethylene glycol, butanediol, or a mixture thereof or a mixture of the alcohol with water.

12. The process as claimed in any one of claims 1 to 11, wherein the powder material is a polymer which is amorphous, crystalline or semicrystalline and has a melting point of 50 to 350°C.

13. The process as claimed in claim 12, wherein the powder material is polyester, polyvinyl chloride, polyacetal, polypropylene, polyethylene, polystyrene, polycarbonate, poly(N-methylmethacrylimide) (PMMI), polymethyl methacrylate (PMMA), ionomer, polyamide, copolyester, copolyamide, terpolymer, acrylonitrile-butadiene-styrene copolymer (ABS), or a mixture thereof.

14. The process as claimed in any one of claims 1 to 13, wherein the powder material also contains from 0.05 to 5% by weight of a flow aid selected from the group consisting of silica, stearate, tricalcium phosphate, calcium silicate, Al2O3, MgO, MgCO3 and Z n O.

15. The process as claimed in any one of claims 1 to 14, wherein the powder material also comprises an inorganic filler.

16. The process as claimed in claim 15, wherein the filler is glass beads.

17. The process as claimed in any one of claims 1 to 16, wherein the powder material also comprises an inorganic or organic pigment.

18. The process as claimed in any one of claims 1 to 17, wherein the powder material also comprises a laser-activatable additive.

19. The process as claimed in any one of claims 1 to 11, wherein the powder material is composed of a grain of a first material and a coating of the second material on the grain, in which the first material is selected from sand, ceramics, metal, allay, and a mixture thereof and the second material is a polymer having a melting point of 50 to 350°C.

20. An apparatus for a layer-by-layer production of a three-dimensional object, which comprises:
(A) a movable apparatus for applying layers of a powder material to an operating platform or to a layer of a treated or untreated powder material which may by this stage be present on the operating platform, (B) an apparatus movable in a x,y plane, for applying a susceptor to selected regions of the layer of the powder material, and (C) a microwave generator suitable for generating microwave radiation having a frequency in the range from 300 MHz to 300 GHz.

21. The apparatus according to claim 20, wherein the movable apparatus (B) is a printing head of an ink-jet printer.

22. The apparatus according to claim 20 or 21, wherein the microwave generator (C) generates microwave radiation having a frequency in. the range of from 430 to 6,800 MHz.

23. The apparatus according to any one of claims 20 to 22, which further comprises:
(D) feed vessels for introducing the powder material to the movable apparatus (A) and introducing the susceptor to the movable apparatus (B).

24. A molding produced by the process as claimed in claim 15, which comprises an inorganic filler selected from glass bead, silica and metal particles.