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A selective laser solidification apparatus including: a powder bed onto which a powder layer can be deposited, a gas flow unit for passing a flow of gas over the powder bed along a gas flow direction, a laser scanning unit for scanning a laser beam over the powder layer to selectively solidify at le

A selective laser solidification apparatus including: a powder bed onto which a powder layer can be deposited, a gas flow unit for passing a flow of gas over the powder bed along a gas flow direction, a laser scanning unit for scanning a laser beam over the powder layer to selectively solidify at least part of the powder layer to form at least one object and a processing unit for selecting a scanning sequence of the laser beam based on the gas flow direction.

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1. A selective laser solidification apparatus, comprising: a build carrier for supporting a powder bed onto which a powder layer can be deposited;a gas flow unit for passing a flow of gas over the powder bed along a gas flow direction;a laser scanner for scanning a laser beam over the powder layer t

1. A selective laser solidification apparatus, comprising: a build carrier for supporting a powder bed onto which a powder layer can be deposited;a gas flow unit for passing a flow of gas over the powder bed along a gas flow direction;a laser scanner for scanning a laser beam over the powder layer to selectively solidify at least part of the powder layer to form one or more objects; anda processor for selecting a scanning sequence of the laser beam based on the gas flow direction. 2. A selective laser solidification apparatus according to claim 1, wherein the processor is arranged to select the scanning sequence such that debris produced during a scan of the laser beam in accordance with the scanning sequence is carried away from areas of the powder layer which are yet to be scanned. 3. A selective laser solidification apparatus according to claim 1, wherein the processor is arranged to select the scanning sequence such that a first area is scanned before a second area, the first area being located downwind in the gas flow direction of the second area. 4. A selective laser solidification apparatus according to claim 1, wherein: the one or more objects are formed through solidification of separate islands in the powder layer, andthe processor is arranged to select an order in which the separate islands are formed based upon (i) relative location of the separate islands in the powder layer and (ii) the gas flow direction. 5. A selective laser solidification apparatus according to claim 4, wherein the order in which the separate islands are formed is selected such that debris produced by forming a first one of the separate islands is carried away from an area of the powder layer in which a second one of the separate islands is to be formed. 6. A selective laser solidification apparatus according to claim 4, wherein the processor is arranged to select the scanning sequence such that at least part of a first island of the separate islands is formed before at least part of a second island of the second islands, the at least part of the first island being located downwind in the gas flow direction of the at least part of the second island. 7. A selective laser solidification apparatus according to claim 4, wherein, for an arrangement of the separate islands in which a first island of the separate islands is located wholly downwind of a second island of the separate islands, the processor is arranged to select to form the first island completely before forming the second island. 8. A selective laser solidification apparatus according to claim 4, wherein, if a first island of the separate islands is located to at least partially surround a second island of the separate islands such that a first part of the first island is downwind and a second part of the first island is upwind of the second island, the processor is arranged to select to form at least part of the second island in between forming the first and second parts of the first island. 9. A selective laser solidification apparatus according to claim 1, wherein the processor is arranged to determine an order in which a plurality of areas of the powder layer should be scanned by projecting a debris fallout zone that would be created when solidifying a first area of the plurality of areas and determining whether a second area of the plurality of areas to be solidified falls within the debris fallout zone, the processor selecting to solidify at least a part of the second area that falls within the debris fallout zone before solidifying the first area. 10. A selective laser solidification apparatus according to claim 1, wherein the processor is arranged to select a location on the build platform for one of the objects, the processor selecting the location based upon a debris fallout zone that would be created when solidifying a second one of the objects whose location on the build platform has already been selected. 11. A selective laser solidification apparatus according to claim 1, wherein the processor is arranged to select the scanning sequence for the laser beam such that areas of the powder layer are progressively solidified in a direction opposed to the gas flow direction. 12. A processor for use with a selective laser solidification apparatus that comprises (i) a build platform for supporting a powder bed onto which a powder layer can be deposited, (ii) a gas flow unit comprising a gas inlet and a gas outlet located to generate a flow of gas over the powder bed in a gas flow direction, and (iii) a laser scanner for scanning a laser beam over the powder layer to selectively solidify at least part of the powder layer, the processor being arranged to: receive geometric data defining at least one object to be built using the selective laser solidification apparatus;select a scanning sequence of the laser beam to form the at least one object based on the gas flow direction; andgenerate instructions for controlling the selective laser solidification apparatus to build the at least one object based upon the scanning sequence. 13. A selective laser solidification apparatus, comprising: a build platform for supporting a powder bed onto which a powder layer can be deposited;a gas flow unit comprising a gas inlet and a gas outlet located to generate a flow of gas over the powder bed in a gas flow direction;a laser scanner for scanning a laser beam over the powder layer to selectively solidify at least part of the powder layer to form one or more objects; anda processor arranged to select a scanning sequence for the laser beam based on the gas flow direction. 14. A selective laser solidification apparatus according to claim 13, wherein the processor is arranged to select the scanning sequence for the laser beam such that areas of the powder layer are progressively solidified in a direction opposed to the gas flow direction. 15. A selective laser solidification apparatus, comprising: a build platform for supporting a powder bed onto which a powder layer can be deposited;a gas inlet and a gas outlet, a gas flow direction being defined by a location of the gas inlet and the gas outlet;an optical module comprising steerable optics for scanning a laser beam over the powder layer to selectively solidify at least part of the powder layer to form one or more objects; anda processor arranged to select a scanning sequence for the laser beam based on the gas flow direction. 16. A selective laser solidification apparatus according to claim 15, wherein the processor is arranged to select the scanning sequence for the laser beam such that areas of the powder layer are progressively solidified in a direction opposed to the gas flow direction.