Patent Publication Number: US-8535036-B2

Title: Method and apparatus for combining particulate material

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This is a divisional of co-pending patent application Ser. No. 10/564,575 filed Jan. 13, 2006, which is a national phase application filed under 35 USC 371 based on International Application No. PCT/GB2004/003142 filed Jan. 10, 2005, and claims priority under 35 USC 119 of United Kingdom Patent Application No. GB 0317387.9 filed Jul. 25, 2003. 
    
    
     Embodiments of the present invention relate to a method of selectively combining particulate material and/or apparatus for combining particulate material. 
     Rapid Prototyping is widely used to form prototype components, and a number of methods are currently available for carrying out rapid prototyping. In one method, a computer generated three dimensional model of the component is initially produced using computer assisted drawing (CAD) software. The three dimensional model is then sliced into a number of virtual layers, and a device used to form and combine the layers to create the three dimensional component. 
     It is known to form the respective layers by combining particulate material using a laser that sinters the particulate material. However, this method can be disadvantageous since the laser must pass over the entire surface of each layer, which can be time consuming. As an alternative, infra-red radiation can be provided on selected portions of a layer of particulate material to combine it. However, the accuracy of the components produced using this method may be unsatisfactory. 
     According to a first aspect of the present invention, there is provided a method of selectively combining particulate material, comprising the steps of:
         (i) providing a layer of particulate material;   (ii) providing radiation over the layer of particulate material; and   (iii) varying the absorption of the provided radiation across a selected surface portion of the layer to combine a portion of the material of the layer.       

     According to a second aspect of the present invention, there is provided apparatus for combining particulate material, the apparatus comprising a controller for enabling the exposure of a surface portion of a layer of particulate material to radiation, wherein the controller is arranged to control the variation of radiation absorption across said surface portion. 
     Preferred features of the invention are defined in the accompanying claims. 
     The surface portion that receives variable radiation absorption may be a part, and not the whole, of the surface of the layer of particulate material. 
     The provision of variable radiation absorption across the surface portion may require the existence of multiple areas in which the absorption of radiation is different and greater than zero. 
     According to one embodiment, the variable radiation absorption across the surface portion may be provided by varying the intensity of the provided radiation incident on the surface portion of the layer of particulate material. 
     According to a different embodiment, the variable radiation absorption across the surface portion may be provided by varying the absorptive properties of the particulate material across the selected surface portion of the layer. 
    
    
     
       Embodiments of the present invention will now be described by way of example only and with reference to the accompanying drawings, in which:— 
         FIG. 1  is a diagrammatic illustration of a first embodiment of apparatus for combining particulate material; 
         FIG. 2  is a diagrammatic plan view of a surface portion of a layer of particulate material; 
         FIG. 3  is a diagrammatic illustration of a second embodiment of apparatus for combining particulate material; 
         FIG. 4  is a diagrammatic view of a third embodiment of apparatus for combining particulate material; 
         FIG. 5  is a diagrammatic view of a fourth embodiment of apparatus for combining particulate material; 
         FIG. 6   a  is a further diagrammatic plan view of a surface portion of a layer of particulate material; 
         FIG. 6   b  is a side view of the layer of particulate material of  FIG. 6   a;    
         FIG. 7  is a diagrammatic schematic view of apparatus for combining particulate material being used to form a three dimensional object; and 
         FIG. 8  is a diagrammatic view of the apparatus of  FIG. 1  being used to combine different types of particulate material. 
     
    
    
     Referring to the drawings, there is shown generally apparatus for combining particulate material, for example plastics material by sintering, the apparatus comprising a controller C for enabling the exposure of a surface portion of a layer  10  of particulate material to radiation, for example infra-red radiation provided by a radiation source  12 , the controller C being arranged to control the variation of radiation absorption across the surface portion. 
     In more detail,  FIG. 1  illustrates a first embodiment of apparatus for sintering particulate material in which an obscurer  14  (i.e. a mask) is provided for selectively obscuring the radiation provided by the source  12  on the surface portion of the layer  10  to thereby vary the intensity of the radiation incident on the surface portion of the layer  10 . The obscurer  14  comprises a radiation transmissive substrate  16 , such as a glass plate, which carries a varying amount of radiation reflective material  18 , such as aluminium oxide. The amount and pattern of material  18  deposited on the substrate may be varied to selectively vary the intensity of radiation incident on the surface portion of the layer  10 , as will be described hereinafter. 
     Referring also to  FIG. 2 , the surface portion of the layer  10  is logically divided by the obscurer  14  into a number of areas including a combination portion  20 , which is to be exposed to radiation to combine the particulate material, and a non-combination portion  22  which is to be shielded, or at least substantially shielded, from radiation to prevent combination of the particulate material by sintering. Full shielding of the non-combination portion  22  is not essential, provided that the intensity of radiation transmitted to the non-combination portion  22  is such that the particulate material is not heated to its sintering temperature. In some circumstances, transmission of low intensity radiation onto the non-combination portion  22  to heat the material can be desirable and can result in improved accuracy of the finished component. This is because heating material in the non-combination portion  22  reduces the thermal gradient between the material in the combination portion  20  and the non-combination portion  22 . 
     The combination portion  20  is logically divided by the obscurer  14  into a central portion  24  and an edge portion  26 , and reflective material  18  is deposited onto the substrate  16  such that a greater amount of the material  18  is provided on the central portion  24  than on the edge portion  26  where no reflective material  18  may be provided. Consequently, the intensity of radiation provided across the surface of the combination portion  20  increases from a minimum value at the central portion  24  to a maximum value at the edge portion  26  where the surface of the layer  10  of particulate material is fully exposed to radiation provided by the radiation source  12 . 
     The layer of reflective material is schematically illustrated in  FIG. 1 . The variation of thickness of the layer in the figure does not illustrate a variation of thickness of the layer in practice but illustrates a variation in the amount of the material. Where the layer is thick in the figure, in practice there will be a large amount of the material present. 
     Although the combination portion  20  has been shown to have only one edge portion  26  such that the central portion  24  is located at the centre of the combination portion  20 , it should be appreciated that the combination portion  20  may for example be of annular configuration such that the central portion  24  is bounded on two sides by edge portions  26 . Moreover, it is not essential that the central portion  24  is located at the centre of the surface portion of the layer  10  of particulate material. 
     The controller C is arranged to control a motor  28  for moving the obscurer  16  from an obscuring position in which it overlies the layer  10 , as shown in  FIG. 1 , to a non-obscuring position in which it does not overly the layer  10 . The controller C is also arranged to control a deposition device, such as a printing head  30 , for depositing the reflective material  18  onto the substrate  16 . The controller C controls the amount of material  18  deposited by the head  30  onto each part of the substrate  16 . In the embodiment shown in  FIG. 1 , the head  30  remains stationary and deposits reflective material  18  onto the substrate  16  as the motor  28  moves the substrate  16  past the head  30 . In an alternative embodiment (not shown), the substrate  16  may remain stationary, overlying the layer  10 , and the motor  28  may move the printing head  30  over the substrate  16  to deposit reflective material  18  thereon. 
     In the illustrated embodiment, the reflective material  18  is contemporaneously printed onto the substrate  16  during operation of the apparatus. The amount of material  18  printed onto the substrate  16  by the head  30  may be varied by the controller C according to the surface temperature of the layer  10 . The surface temperature of the layer  10  may be measured by a temperature measuring device, such as, for example, a pyrometer P or a thermal imaging camera, and surface temperature measurements are communicated in real time to the controller C. A wiping arrangement (not shown) may be provided for removing reflective material  18  from the substrate  16 , so that it can be re-used. Different amounts of material  18  can be deposited onto the substrate  16 , in dependence on the desired radiation intensity profile at the substrate surface. 
     Alternatively, the reflective material  18  may be pre-printed onto the substrate  16  prior to operation of the apparatus and the same pre-printed substrate  16 , or a number of pre-printed substrates  16 , may be used, one for each layer  10  of particulate material. In this case, measurement of the surface temperature using pyrometer P may not be needed. The use of a plurality of pre-printed substrates  16  is particularly advantageous when there is a need to produce a large quantity of the same component since it reduces the time taken to sinter each layer of material and hence produce the prototype component, increases repeatability and leads to a reduction in the cost of producing the components. 
     It should also be noted that it is within the scope of the present invention to utilise a plurality of pre-printed substrates  16 , or to contemporaneously print different amounts of reflective material  18  onto the same substrate  16 , and to use these to expose the same layer  10  of material to different radiation intensity profiles in multiple exposure steps. 
       FIG. 3  illustrates a second embodiment of apparatus for combining particulate material, in which corresponding elements are given corresponding reference numerals. The apparatus of  FIG. 3  is similar to that shown in  FIG. 1 , except that instead of the reflective material  18  being deposited onto a substrate  16 , the reflective material  18  is deposited, using the printing head  30 , directly onto the surface portion of the layer  10  of particulate material. 
     In the apparatus of this embodiment, the printing head  30  is again controlled by the controller C which controls both the movement of the head  30  across the surface of the layer  10  and the rate of deposition of reflective material  18  onto the layer  10 . Again, real time measurement of the surface temperature of the layer  10  may be carried out using a temperature measurement device, for example, a pyrometer P or thermal imaging camera, the temperature measurement being used by the controller C to determine the amount of reflective material  18  to be printed by the head  30  onto the surface portion of the layer  10 . 
     The layer of reflective material is schematically illustrated in  FIG. 3 . The variation of thickness of the layer in the figure does not illustrate a variation of thickness of the layer in practice but illustrates a variation in the amount of the material. Where the layer is thick in the figure, in practice there will be a large amount of the material present. 
       FIG. 4  illustrates a third embodiment of apparatus for combining particulate material which is similar to the first and second embodiments and in which corresponding elements are given corresponding reference numerals. In this embodiment, the controller C is arranged to selectively redirect the radiation provided by the source  12  and thereby vary the radiation intensity incident across the surface portion of the layer  10 . Selective redirection of the radiation is achieved by controlling, using the controller C, a plurality of mirrors  34  which form a Digital Mirror Device (DMD)  36 . Each mirror  34  is adjustable by the controller to an operative position, in which radiation is fully redirected onto the surface portion of the layer  10 , or to an inoperative position in which radiation is fully redirected away from the surface portion. By providing an array of mirrors  34 , the surface portion of the layer  10  can be effectively divided into an array of segments, as discussed hereinafter, and the intensity of the radiation incident on each segment can be varied, according to a bitmap image, by selectively varying the frequencies at which individual mirrors  34  are moved between the operative and inoperative positions. 
     Use of a temperature measurement device, such as a pyrometer P, although optional is particularly advantageous with the apparatus of this embodiment as the position of each mirror  34  can be instantaneously controlled, in real time, by the controller C in response to instantaneous temperature variations across the surface portion of the layer  10 . 
       FIG. 5  illustrates a fourth embodiment of apparatus for combining particulate material which is similar to the embodiments described above and in which corresponding elements have been given corresponding reference numerals. 
     The apparatus of  FIG. 5  is most similar to the apparatus of  FIG. 3  in that material is deposited directly onto the surface portion of the layer  10  of particulate material. However, according to the fourth embodiment, the material is a radiation absorbent material  50 , for example a material such as carbon black in powder form. In use, radiation provided by the radiation source  12  is absorbed by the radiation absorbent material  50  where it is present on the surface, causing the radiation absorbent material  50  to heat up. Heat from the radiation absorbent material  50  is radiated to the underlying particulate material raising the temperature of individual particles of the particulate material. As the particles are heated to a temperature approaching their melting temperature, they neck and coalesce with adjacent heated particles. As the temperature subsequently decreases, the particles form a coherent mass of combined particulate material. 
     The deposition of a radiation absorbent material  50  directly onto the surface portion of the layer  10  enables the radiation absorptive properties of the particulate material to be varied and carefully controlled, as desired. In particular, varying the amount of the radiation absorbent material  50  on the surface enables the variation of the radiation absorptive properties of the surface portion of the underlying layer  10  of particulate material. In areas where there is a greater amount of the radiation absorbent material  50 , a greater amount of the radiation provided by the radiation source  12  is absorbed. This provides for a greater amount of heat transfer to the underlying particulate material thereby heating it to a higher temperature and causing it to combine more rapidly. In areas where there is less absorbent material  50 , there is lower radiation absorption and hence less heat transfer to the underlying particulate material, causing it to combine at a slower rate. 
     In areas where no radiation absorbent material  50  is provided and pure particulate material is exposed to the radiation provided by the radiation source  12 , there will be insufficient absorption of the radiation to heat the particulate material to its melting temperature. Thus, there will be no combination of the particulate material in areas where no radiation absorbent material  50  is provided. 
     The layer of radiation absorbent material  50  is schematically illustrated in  FIG. 5 . The variation of thickness of the layer in the figure does not illustrate a variation of thickness of the layer in practice but illustrates a variation in the amount of the material. Where the layer is thick in the figure, in practice there will be a large amount of the material present. 
     As with the embodiments of  FIGS. 1 and 3 , it is desirable to provide for a greater amount of radiation absorption at the edge portion  26  of the combination portion  20  than at the central portion  24 . Accordingly, the amount of the radiation absorbent material  50  decreases from a maximum value at the edge portion  26  to a minimum value at the central portion  24 . 
     As illustrated, no radiation absorbent material  50  is provided on the surface portion of the layer  10  of the particulate material in the non-combination portion  22 . For the reasons explained above, there will be no combination of the particulate material in the non-combination portion  22  when the layer  10  is exposed to radiation. There may however be some heating of the particulate material in the non-combination portion  22 , and this can be advantageous to minimise the thermal gradient between the particulate material in the combination portion  20  and the non-combination portion  22 , as already discussed. 
     As with the embodiment of  FIG. 3 , the printing head  30  is operable to deposit desired amounts of the radiation absorbent material  50  onto the surface portion of the layer  10 , and the movement of the printing head  30  and the amount of material  50  deposited by the head  30  is controlled by the controller C. Again, the pyrometer P or a thermal imaging camera may be used to measure the surface temperature of the layer  10 , the amount of radiation absorbent material  50  deposited being varied by the controller C in accordance with the temperature measurements. 
     The applicant has appreciated that when the particulate material is combined by sintering at a slow rate, the combined material has good material properties, for example high strength, but has poor definition at the edge portion  26 . The poor edge definition arises because as the particulate material combines, there is some shrinkage which causes unwanted movement of uncombined particulate material from the non-combination portion  22  towards the combination portion  20 . On the other hand, when the particulate material is combined by sintering at a rapid rate, the combined material has inferior material properties, but has good edge definition since the particulate material in the edge portion  26  is rapidly combined and locked in position, thereby minimising unwanted movement of surrounding uncombined particulate material. 
     In order to provide a layer  10  of combined particulate material having good material properties and good definition at the edge portion  26 , it is thus desirable to cause the particulate material in the combination portion  20  to combine at a slow rate to provide good material properties, and to cause the particulate material at the edge portion  26  to combine rapidly to provide good edge definition. 
     One method by which this can be achieved is to use the apparatus according to the different embodiments of the invention described above to provide for greater absorption of radiation at the edge portion  26  than over the remainder of the combination portion  20 . This can be achieved by varying the intensity of the radiation incident on the selected surface portion of the layer  10  using the apparatus according to the first, second or third embodiments, or by varying the absorption of the radiation across the selected surface portion by providing a variable amount of radiation absorbent material  50  across the surface portion. In all of the above cases, radiation is provided over the layer  10  in a single exposure step. 
     Using the apparatus according to the fourth embodiment of the invention, similar results may be achieved by providing radiation over the layer  10  of particulate material in multiple exposure steps, as will now be discussed. 
     According to a first method, a constant first amount of radiation absorbent material  50  is provided over the combination portion  20 , and radiation is then provided over the layer  10 , using the radiation source  12 , to cause the underlying particulate material in the combination portion  20  to combine. The first amount of radiation absorbent material  50  is selected to be a relatively low amount so that the underlying particulate material combines at a slow rate and has good material properties. 
     After the particulate material has been combined, further particulate material is added to the layer  10  at the edge portion  26  where there will have been shrinkage. A second amount of the same radiation absorbent material  50 , which is greater than the first amount, is then provided over the edge portion  26 , and radiation is again provided over the layer  10  using the radiation source  12 . The second amount of material is selected to be a relatively high amount so that the underlying particulate material is caused to combine at a rapid rate. Due to the increased amount of radiation absorbent material  50  present at the edge portion  26 , and hence the rapid combination of the underlying particulate material, material shrinkage is minimised thus providing the resultant layer  10  of combined material with good definition at the edge portion  26 . 
     According to a second method, a constant amount of a first radiation absorbent material  50  having a first natural radiation absorbency is provided over the combination portion  20 , and radiation provided over the layer  10 , using the radiation source  12 , to cause the underlying particulate material in the combination portion  20  to combine. The first radiation absorbent material  50  is selected to have a low natural radiation absorbency so that a relatively low amount of the radiation is absorbed and so that the underlying particulate material combines at a slow rate and has good material properties. 
     After the particulate material has been combined, further particulate material is added to the layer  10  at the edge portion  26  where there will have been shrinkage. A second different radiation absorbent material  50 , having a second natural radiation absorbency, is then provided over the edge portion  26 , and radiation is again provided over the layer  10  using the radiation source  12 . The second radiation absorbent material  50  is selected to have a high natural radiation absorbency, which is higher than the absorbency of the first radiation absorbent material  50 , so that a high amount of the radiation is absorbed and so that the underlying particulate material in the edge portion  26  combines at a rapid rate. 
     According to a third method, a first radiation absorbent material  50  capable of absorbing a first wavelength of radiation is provided over the combination portion  20 , and radiation of a first wavelength is then provided over the layer  10 , using the radiation source  12 , to cause the underlying particulate material in the combination portion  20  to combine. 
     After the particulate material has been combined, further particulate material is added to the layer  10  at the edge portion  26  where there will have been shrinkage. A second radiation absorbent material  50 , capable of absorbing a second different wavelength of radiation, is then provided over the edge portion  26 , and radiation of a second wavelength is provided over the layer  10  using the radiation source  12 . 
     In order to provide the desired material properties in the combination portion  20 , the radiation at the first wavelength may be selected to have a relatively low intensity so that the first radiation absorbent material  50  is heated at a slow rate thereby causing the underlying particulate material to combine at a slow rate. In order to provide good definition at the edge portion  26 , the radiation at the second wavelength may selected to have a relatively high intensity so that the second radiation absorbent material  50  is heated rapidly thereby causing the underlying particulate material to combine at a rapid rate. 
     Alternatively, a greater amount of the second radiation absorbent material  50  than the first radiation absorbent material  50  may be provided, as described above with reference to the first method, and the radiation of the first and second wavelengths provided by the radiation source  12  selected to have the same intensity. 
     As a further alternative, the second radiation absorbent material  50  may be selected to have a higher natural radiation absorbency than the first radiation absorbent material  50 , as described above with reference to the second method, and the radiation of the first and second wavelengths provided by the radiation source  12  selected to have the same intensity. 
     If desired, the third method could be adapted so that the first and second radiation absorbent materials  50  are simultaneously applied to the surface of the layer of particulate material, and the radiation of the first and second wavelengths provided in separate steps. 
     It is possible that the first, second and third methods described above could be modified so that the particulate material at the edge portion  26  of the layer  10  is initially caused to combine at a rapid rate to lock the edge portion  26 , and the particulate material in the remainder of the combination portion  20  is subsequently caused to combine at a slow rate to provide the desired material properties. 
     Referring now to  FIGS. 6   a  and  6   b , the apparatus according to the invention allows the surface portion of the layer  10  of particulate material to be logically divided into an array of segments  32 . The controller can control the amount of radiation absorption on each segment  32  independently and a bitmap image can be used to specify the amount of radiation that should be absorbed at the surface portion. The greyscale of each segment  32  of the bitmap image is individually adjustable, and in the case of the first and second embodiments of the apparatus, the amount of reflective material  18  deposited onto each segment of the substrate  16  or surface portion of the layer  10  is individually adjustable, according to the bitmap image, to provide any desired radiation intensity profile over the surface portion of the layer  10 . When the apparatus of the third embodiment is employed, the mirrors  34  are adjusted to vary the intensity of radiation incident on each segment  32  of the array. When the apparatus of the fourth embodiment is used, the amount of radiation absorbent material  50  deposited onto each segment of the surface portion of the layer  10  is individually adjustable, according to the bitmap image, to provide any desired radiation absorption profile over the surface portion of the layer  10 . 
     In the arrangement shown in  FIGS. 6   a  and  6   b , a first amount of reflective material  18  has been deposited by printing head  30  onto the segments  32  defining the central portion  24  of the combination portion  20 . Accordingly, a first intensity of radiation, which is less than the maximum intensity, is incident on the surface portion of the layer  10  located beneath these segments  32 . The first intensity of radiation is sufficiently high to raise the temperature of the particulate material to cause it to combine. No reflective material  18  has been provided on the segments  32  which define the edge portion  26  of the combination portion  20 , thereby allowing a maximum intensity of radiation to reach the surface portion of the layer  10  located beneath these segments  32 . The maximum intensity of radiation causes the particulate material located beneath the segments  32  defining the edge portion  26  to combine more quickly than particulate material in the central portion  24 . 
     A second amount of reflective material  18 , which is greater than the first amount, is deposited by printing head  30  onto the segments  32  defining the non-combination portion  22 . A sufficient amount of material  18  may be provided to prevent transmission of any radiation to the surface portion of the layer  10  located beneath these segments  32 . Consequently, the particulate material located beneath these segments  32  does not combine. 
     Whilst variation of the radiation intensity on each individual segment  32  has been described with respect to the second embodiment of the apparatus, it is to be understood that the same effect can be achieved using apparatus according to the first embodiment, in which reflective material  18  is printed onto a substrate  16 , according to the third embodiment, in which mirrors  34  are used to vary the intensity of radiation incident on each segment  32 , or according to the fourth embodiment in which radiation absorbent material  50  is printed onto the surface portion of the layer  10  of particulate material. 
     The layer of reflective material is schematically illustrated in  FIG. 6   b . The variation of thickness of the layer in the figure does not illustrate a variation of thickness of the layer in practice but illustrates a variation in the amount of the material. Where the layer is thick in the figure, in practice there will be a large amount of the material present. 
     Referring now to  FIG. 7 , there is shown a diagrammatic illustration of the apparatus of  FIG. 3  being used to form a three dimensional object  38 . Again, elements of the apparatus which have been referred to above are given corresponding reference numerals. 
     The apparatus is used to form a three dimensional object  38  by combining a plurality of layers  10   a  to  10   e  of particulate material. A supply of particulate material, for example Nylon powder, is provided in a supply tank  40  and the controller C is arranged to control a motor M which can move particulate material from the tank  40  into a building device  42 , which includes a vertically movable platform  44 . Movement of the platform  44  is controlled by the controller C, such that the platform  44  is moved vertically downwards in discrete steps after each layer  10  has been formed. 
     Initially, with the platform  44  in an uppermost position, the controller C actuates the motor M to provide a first layer  10   a  of particulate material on the platform  44 . The controller C then actuates the printing head  30  to deposit a desired pattern of reflective material  18  onto the surface portion of the layer  10  of material. Alternatively, the reflective material  18  may be deposited by the printing head  30  onto a substrate  16 , as previously discussed, or the intensity incident at the surface may be controlled using digital mirrors. 
     The controller C then activates the radiation source  12  to provide radiation over a selected surface portion of the layer  10 , as defined by the reflective material  18 . As shown in  FIG. 7 , radiation is provided with varying intensity across the combination portion  20  and the material in this portion is combined. The reflective material  18  prevents, or at least substantially prevents, transmission of radiation to the surface portion of the material in the non-combination portion  22  where the material is not combined and remains in particulate form. The varying amount of reflective material  18  thus provides for variable intensity radiation across the combination portion  20  of the layer  10 . 
     After combination of the material in the combination portion  20  of the first layer  10   a  has been carried out, the controller C deactivates the radiation source  12  and lowers the platform  44  by a distance approximately equivalent to the desired layer thickness. The controller C then actuates the motor M to provide a second layer  10   b  of particulate material overlying the first layer  10   a  including a previously combined portion of material. The controller C then actuates the printing head  30  to deposit reflective material  18  onto the surface portion of the second layer  10   b . The amount and pattern of reflective material  18  deposited onto the surface portion of the second layer  10   b  may be the same as that provided on the first layer  10   a , or may be different, for example in response to design or surface temperature measurements carried out using the pyrometer P. The controller C then activates the radiation source  12  to provide radiation across the surface portion of the second layer  10   b , the reflective material  18  providing for variable intensity radiation across the surface portion. The material in the combination portion  20  of the second layer  10   b  is thus caused to combine, and also to combine with the previously combined portion of material in the first layer  10   a . The adjacent layers  10   a ,  10   b  are thus combined to form part of a coherent object  38 . 
     The controller C continues to operate in this manner to provide further layers  10   c  to  10   e  of particulate material and combine them, until formation of the object  38  has been completed. Once the coherent object  38  has been formed, the platform  44  is raised by the controller C to eject the combined object  38  and any remaining uncombined particulate material surrounding the object  38  from the device  42 . 
     Again, it should be appreciated that the apparatus according to any of the other embodiments of the invention may be used to form a three dimensional object  38 . 
       FIG. 8  illustrates use of the apparatus of  FIG. 1  to combine different particulate materials P 1  and P 2  which are located adjacent to each other in a layer  10 . By way of illustration, the material P 1 , for example copper, may have a lower melting point than the material P 2 , for example steel, and may therefore combine by sintering at a lower temperature. The concentration of material P 2  decreases from right to left across a transition gradient region  19 . The concentration of material P 1  decreases from left to right across the transition gradient region  19 . 
     In order to ensure optimum material characteristics and minimise thermal stresses over the gradient region  19  between the materials P 1  and P 2 , the substrate  16  may be provided with a high amount of reflective material  18  on the portion overlying the material P 1  of the layer  10 , a low amount of reflective material on the portion overlying the material P 2  and an amount of reflective material over the gradient region  19  that decreases from left to right in the figure. By varying radiation intensity in this way, the materials P 1  and P 2  are heated to different temperatures using a fixed intensity radiation source  12  and are simultaneously combined to form a coherent layer. 
     The layer of reflective material  18  is schematically illustrated in  FIG. 8 . The variation of thickness of the layer in the figure does not illustrate a variation of thickness of the layer in practice but illustrates a variation in the amount of the material. Where the layer is thick in the figure, in practice there will be a large amount of the material present 
     Whilst the first embodiment of the apparatus has been described for use in combining the dissimilar particulate materials P 1  and P 2 , it will be readily appreciated that the second embodiment of the apparatus in which reflective material  18  is printed directly onto the surface portion of the layer  10 , the third embodiment of the apparatus which uses mirrors  34  to selectively redirect radiation, or the fourth embodiment of the apparatus in which radiation absorbent material  50  is printed directly onto the surface portion of the layer  10 , could alternatively be used. 
     In any of the above described embodiments, it may be desirable to add radiation absorbing material to the particulate material to increase the absorption of radiation. For example, a material such as carbon black may be used for this purpose. 
     Other particulate materials, such as ceramic filler powder, may be added to the particulate material to improve the material properties of the resultant component. 
     Where different radiation absorbent materials are employed, for example as described above with reference to  FIG. 5 , these may be of different colours to provide the resultant component with desired aesthetic properties. 
     Although embodiments of the invention have been described in the preceding paragraphs with reference to various examples, it should be appreciated the various modifications to the examples given may be made without departing from the scope of the present invention, as claimed. For example, although the use of infra-red radiation is described, radiation other than infra-red may be used, provided that it is able to elevate the particulate material to a temperature at which it combines by sintering. The source of radiation may be of any suitable type, for example, LEDs, a scanning laser or a halogen source. The particulate material that is combined by the above described embodiments may be any suitable material, such as a metal, ceramic etc. A device other than a motor M may be used to move particulate material from the supply tank  40  to the combination device  42 . The combination device  42  may be of a different configuration to that shown. Any number of different types of particulate material may be provided in a layer  10 . Alternatively, different types of particulate material may be provided in adjacent layers. Reflective material  18  may be deposited onto a lower surface of the substrate  16  rather than an upper surface, as illustrated. Different materials may be used for the reflective material  18  and the substrate  16 . Any suitable material may be used for the radiation absorbent material  50 . For example, a liquid suspension and/or a gas, for example carbon dioxide, could be employed instead of a powder material. The digital mirror device described in relation to  FIG. 4  could be replaced by a series of diffractive optics, one for each layer. 
     Whilst endeavouring in the foregoing specification to draw attention to those features of the invention believed to be of particular importance, it should be understood that the Applicant claims protection in respect of any patentable feature or combination of features hereinbefore referred to and/or shown in the drawings, whether or not particular emphasis has been placed thereon.