POWDER BED FUSION APPARATUS AND METHODS

Abstract

A method of filtering gas in a powder bed fusion apparatus, wherein an object is built layer-by-layer by selective solidification of a powder bed, and a powder bed fusion apparatus for executing the method. The apparatus includes a build chamber housing the powder bed, a gas circuit for recirculating the gas, including passing the gas over the powder bed within the build chamber, multiple filter assemblies in the gas circuit for filtering process emissions from the recirculated gas and a valve system regulating gas flow to each filter assembly. The method may include controlling the valve system to divide the gas flow between the filter assemblies. The method includes controlling the valve system such that a first one of the filter assemblies is connected with at least one second one of the filter assemblies such that the gas passes through the filter elements of both filter assemblies.

Claims

1. A method of filtering gas in a powder bed fusion apparatus, in which an object is built layer-by-layer by selective solidification of a powder bed, the powder bed fusion apparatus comprising a build chamber for housing the powder bed, a gas circuit for recirculating the gas, including passing the gas over the powder bed within the build chamber, a plurality of filter assemblies in the gas circuit for filtering process emissions from the gas recirculated through the gas circuit, the filter assemblies connected or connectable in parallel in the gas circuit, and a valve system operable to regulate a flow of the gas to each one of the filter assemblies, the method comprising controlling the valve system to divide the gas flow between a first one of the filter assemblies housing an unused filter element and at least one second one of the filter assemblies housing a used filter element such that less gas flows through the first filter assembly than the or each second filter assembly.

2. The method according to claim 1, comprising controlling the valve system such that, after an initial time period, the flow of the gas to the first and second filter assemblies is altered by increasing the gas flow to the first filter assembly and/or reducing or stopping the gas flow to the second filter assembly.

3. The method according to claim 2, comprising a blended switchover between the second and first filter assemblies, with the gas flow to the first filter assembly progressively increased and the gas flow to the second filter assembly progressively decreased.

4. A method of filtering gas in a powder bed fusion apparatus, in which an object is built layer-by-layer by selective solidification of a powder bed, the powder bed fusion apparatus comprising a build chamber for housing the powder bed, a gas circuit for recirculating the gas, including passing the gas over the powder bed within the build chamber, and a plurality of filter assemblies in the gas circuit for filtering process emissions from the gas recirculated through the gas circuit, the filter assemblies connectable such that ones of the filter assemblies are connected in series in the gas circuit, and a valve system operable to regulate a flow of the gas to each one of the filter assemblies, the method comprising controlling the valve system such that a first one of the filter assemblies housing an unused filter element is connected in series in the gas circuit with at least one second one of the filter assemblies housing a used filter element such that the gas passes through the filter elements of both the first and second filter assemblies.

5. The method according to claim 3, comprising controlling the valve system such that, after an initial time period, the gas flow through the at least one second filter assembly is reduced or stopped altogether.

6. The method according to claim 5, comprising a blended switchover, wherein the gas flow to the second filter assembly is progressively decreased.

7. The method according to claim 2, wherein the initial time period is a period during which powder is solidified using the powder bed fusion apparatus and process emissions are carried away by the gas flow.

8. The method according to claim 7, wherein the initial time period includes a period in which one or more layers of a build are completed.

9. A method of filtering gas in a powder bed fusion apparatus, in which an object is built layer-by-layer by selective solidification of a powder bed, the powder bed fusion apparatus comprising a build chamber for housing the powder bed, a gas circuit for recirculating the gas, including passing the gas over the powder bed within the build chamber, a plurality of filter assemblies in the gas circuit for filtering process emissions from the gas recirculated through the gas circuit and a valve system operable to regulate a flow of the gas to each one of the filter assemblies, the method comprising controlling the valve system to direct the gas flow to a first one of the filter assemblies during a first period of a build of an object resulting in a first partially used filter element, switch the valve system to direct the gas flow to a second one of the filter assemblies during a second period of the build resulting in a second partially used filter element and switch the valve system to divide the gas flow between the first filter assembly housing the first partially used filter element and a second filter assembly housing the second partially used filter element during a third period of the build.

10. A controller for controlling a powder bed fusion apparatus, in which an object is built layer-by-layer by selective solidification of a powder bed, the powder bed fusion apparatus comprising a build chamber for housing the powder bed, a gas circuit for recirculating gas, including passing the gas over the powder bed within the build chamber, a plurality of filter assemblies in the gas circuit for filtering processing emissions from gas recirculated through the gas circuit and a valve system operable to regulate a gas flow to each one of the filter assemblies, the controller comprising a processor arranged to carry out the method of claim 1.

11. A powder bed fusion apparatus, in which an object is built layer-by-layer by selective solidification of a powder bed, the powder bed fusion apparatus comprising a build chamber for housing the powder bed, a gas circuit for recirculating gas, including passing the gas over the powder bed within the build chamber, a plurality of filter assemblies in the gas circuit for filtering processing emissions from gas recirculated through the gas circuit, a valve system operable to regulate a gas flow to each one of the filter assemblies and a controller for controlling the valve system according to claim 10.

12. The powder bed fusion apparatus according to claim 11, wherein the valve system is capable of regulating a proportion of the gas flow flowing to each one of the filter assemblies, the valve system comprising at least one valve capable of being maintained in a plurality of positions, wherein in each position the valve provides a different sized opening for gas flow to at least one of the filter assemblies.

13. A data carrier having instructions thereon, which, when executed by a processor of a controller for controlling a powder bed fusion apparatus, cause the controller to carry out the method of claim 1.

Description

DESCRIPTION OF THE DRAWINGS

[0022] Embodiments of the invention will now be described, by example only, with reference to the following drawings, in which:—

[0023] FIG. 1 is a schematic of a powder bed fusion apparatus according to an embodiment of the invention; and

[0024] FIG. 2 is a perspective view of a filter assembly of the powder bed fusion apparatus shown in FIG. 1.

DESCRIPTION OF EMBODIMENTS

[0025] Referring to FIGS. 1 and 2, a powder bed fusion apparatus according to an embodiment of the invention comprises a build chamber 101 sealable from the external environment for maintaining a controlled atmosphere at a working surface 104a of a powder bed 104. Contained within the build chamber 101 is a build platform 102 for supporting the powder bed 104 and an object 103 built by selective laser melting powder of the powder bed 104. The platform 102 is lowerable in a sleeve 117 as successive layers of the object 103 are formed. Layers of powder 104 are formed as the object 103 is built by a powder dispensing apparatus and a wiper (not shown). For example, the powder dispensing apparatus may be apparatus as described in WO2010/007396. A laser module 105 generates a laser for melting the powder 104, the laser directed as required by a scanner in the form of optical module 106. The laser enters the build chamber via a window 107.

[0026] A gas circuit is provided for generating a gas flow across the powder bed formed on the build platform 102. The gas circuit comprises a gas nozzle 112 and gas exhaust 110 arranged either side of the build sleeve 117 for generating a gas knife across the powder bed 104. The gas nozzle 112 and gas exhaust 110 are arranged to produce a laminar gas knife local to the working surface 104a of the powder bed 104. It will be understood that more than one gas inlet 112 may be provided in the build chamber 101. In this embodiment, an array of apertures 150 are provided in a roof of the build chamber 101 to provide a steady downwards flow of gas away from the window 107. The process emissions generated by the laser melting process are carried away by the gas flow to the exhaust 110.

[0027] The gas circuit is completed by a gas recirculation loop 111, which re-circulates the gas from the gas exhaust 110 to the gas nozzle 112. The gas recirculation loop 111 comprises a pump 113 for driving the gas around the gas circuit and a filter system 114, upstream of the pump 113, for filtering particles from the gas flow. In this embodiment, the filter system comprises a cyclone separator 115, which separates larger particles of the process emissions from smaller “condensate” particles, and a pair of filter assemblies 230a, 230b arranged in a parallel relationship in the gas circuit. The cyclone separator 115 is located upstream of the filter assemblies 230a, 230b such that the larger particles of the process emissions are removed from the gas flow before the gas flow reaches the filter assemblies 230.

[0028] Downstream of the pump is a cooling device 118 for cooling the gas before it re-enters into the build chamber 101.

[0029] Referring to FIG. 2, each filter assembly 230a, 230b comprises a filter housing 260 having a gas inlet 210 and a gas outlet 220 and a filter element 250 located within the filter housing 260 either side of the gas inlet 210 and gas outlet 220. The filter housing 260 is formed from separable upper 231 and lower 232 portions. The upper portion is illustrated as transparent in FIG. 2 in order to clearly show the filter element 250 and deflector 240 inside. The housing is substantially cylindrical and the upper portion 231 and the lower portion 232 are securely clamped, when in use, at clamping rim 233. The clamping rim incorporates screws for affecting clamping of the two portions and an O-ring for sealing the housing when assembled.

[0030] The flow deflector 240 is incorporated in the upper portion 231 of the filter housing. The flow deflector 240 presses down on the end of and directs gas flow to the sides of a cylindrical filter element 250. The filter element 250 is located by a spigot surrounding the gas outlet in the lower portion 232 of the housing and securely clamped in place by pressure exerted from the flow deflector 240 of the upper portion 231 when the housing is assembled.

[0031] Referring back to FIG. 1, each filter assembly 230a; 230b further comprises valves 121a, 122a; 121b, 122b for sealing the gas inlet 210 and gas outlet 220 to gas flow and allowing the filter assembly to be removed from the gas circuit in a sealed condition. The dotted lines indicate where the filter assemblies 230a, 230b can be detached from the gas circuit. In this way, the filter element 250 can be replaced. Replacement of the filter element 250 may be carried out in the manner described in WO2010/026396 A2.

[0032] The gas circuit comprises a valve system for regulating the gas flow to each of the filter assemblies 230a, 230b. The valve system comprises valves 123a, 124a, 123b, 124b under the control of controller 125. The valves 123a, 124a; 123b, 124b are operable to shut-off the gas flow when the corresponding filter assembly 230a, 230b is not in use, for example when it is detached from the gas circuit, and regulate an amount of gas flow to the corresponding filter assembly 230a, 230b. The valves 123a, 123b can be set to a plurality of partially opened positions to regulate an amount of gas flow to the corresponding filter assembly 230a, 230b and can be opened at the same time such that gas flows simultaneously through both filter assemblies 230a, 230b. Sensor 226 is provided for measuring the gas pressure differential across the filter system 114.

[0033] The controller 115 operates the valves 123a, 124a; 123b, 124b as described hereinafter.

[0034] During a build of an object in the powder bed fusion apparatus, the valves 123a, 124a; 123b, 124b to one of the filter assemblies 230a, 230b are open whilst the valves 123b, 124b; 123a, 124a to the other filter assembly 230b, 230a are closed such that the gas flows through only one of the filter assemblies 230a, 230b. During the build, particles are filtered from the gas flow by the filter element 250 of the filter assembly 230a, 230b open to the gas flow and build up on a surface of the filter element 250. After a period of time, the filter element 250 can start to become blocked by the particles that have built up thereon. When the differential pressure across the filter assembly 230a, 230b as detected by sensor 226 exceeds a threshold (or when the pump 113 is no longer able to maintain the required gas flow rate through the filter assembly 230a, 230b/build chamber 101), a switchover of the filter assemblies 230a, 230b is initiated.

[0035] The switchover comprises progressively closing the valves 123a, 124a; 123b, 124b to the filter assembly 230b, 230a comprising a (partially) used filter element 250 whilst simultaneously progressively opening the valves 123b, 124b; 123a, 124a to the filter assembly 230b, 230a comprising the unused filter element 250. In this embodiment, the valves 123a, 124a; 123b, 124b are closed and opened in 10 steps of 9 degrees.

[0036] Accordingly, during the switchover, initially most of the gas flows through the used filter element 250 with only a relatively small proportion flowing through the unused filter element 250. In this way, the gas that flows through the new filter element 250 that may not be sufficiently filtered of process emissions by the new filter element is diluted by the larger volume of gas flowing through the used filter element 250 (which is sufficiently filtered). Due to this dilution a density of particles that are carried back through the build chamber 101 during a switchover is insufficient to have a detrimental effect on the build. Furthermore, it is believed that the lower gas velocity through the unused filter element 250 compared to the gas velocity when the corresponding valves 123a, 124a; 123b, 124b are fully open helps the filtration of particles from the gas flow further reducing an amount of particles that make it through the filter assemblies 230a, 230b compared to a non-blended switchover.

[0037] The time period for the blended switchover is based upon a cumulative laser firing time. This time period can be user set. However, it will typically be set to a value that is greater than a laser firing time for a single layer. For example, the cumulative laser firing time for the blended switchover may be more than 5, 10, 20, 30, 40, 50 or 60 seconds.

[0038] By using the blended switch-over, the unused filter element 250 is “preconditioned” by being coated with particles from the gas flow before the (now partially used) filter element 250 is exposed to the full/higher gas flow at the end of the blended switchover. This ensures that the particles required for satisfactory filtering of the gas flow are present when the filter element 250 is exposed to the full/higher gas flow.

[0039] Once full switchover between the filter assemblies 230a, 230b has been completed, the filter assembly 230a, 230b containing the “fully” used filter element 250 can be removed and the filter element 250 replaced using the process described in WO2010/007394.

[0040] The controller 115 may also operate the valves 123a, 124a; 123b, 124b to extend a useful life of a filter element 250. If no new filter element 250 is available for use (because a used filter element 250 has not been replaced), when the differential pressure across the filter assembly 230a, 230b as detected by sensor 226 exceeds a threshold, the controller 115 operates the valves 123a, 124a; 123b, 124b to fully open both filter assemblies 230a, 230b to the gas flow. The valves 123a, 124a; 123b, 124b may be switched to such a condition at maximum speed rather than the progressive switching which occurs for the blended switchover as no preconditioning is required. By opening both filter assemblies 230a, 230b to the gas flow, the flow velocity to the filter assemblies is halved, greatly reducing the differential pressure (by nearly a quarter) if both filters are dirty and near the end of life. This provides an alternative procedure to stopping the build when no more clean filter elements 250 are available and allows the build to continue until a point when the gas flow through both filter assemblies 230a, 230b in parallel exceeds the differential pressure threshold. This operation of the valve system may extend the useful life of the filter elements 250 sufficiently to complete a build such that replacement of the filter elements 250 during the build is not required. Non-ideal atmospheric conditions in the build chamber 101 at the start of a build when using one or more unused filter elements 250 may be acceptable as the start of the build may comprise the building of non-critical structures, such as supports, for which non-ideal melting conditions are acceptable. Accordingly, unused filter element(s) 250 present at the start of a build may have time to become preconditioned before the critical structures are built.

[0041] It will be understood that alterations and modifications may be made to the embodiments described above without departing from the invention as defined herein. For example, more than two filter assemblies may be used. The filter assemblies may be connectable in series as well as in parallel. The time period for a blended switchover may be based on a number of layers to be processed or a measurement of an amount of particles on the filter element rather than cumulative laser firing time.