WO2022220824A1

WO2022220824A1 - Controlling curing processes in additive manufacturing

Info

Publication number: WO2022220824A1
Application number: PCT/US2021/027293
Authority: WO
Inventors: Macia SOLE PONS; Juan Carlos Ramos; Kelly Ronk
Original assignee: Hewlett Packard Development Co LP
Current assignee: Hewlett Packard Development Co LP
Priority date: 2021-04-14
Filing date: 2021-04-14
Publication date: 2022-10-20
Anticipated expiration: 2023-10-14

Abstract

An apparatus is disclosed. The apparatus comprises a first sensor and a processing unit. The first sensor is to measure a concentration of volatile organic compounds, VOCs, in vapors that have been extracted from a fabrication chamber of an additive manufacturing apparatus during a curing process to cure a three-dimensional object formed from build material in the fabrication chamber. The processing unit is to determine a concentration of VOCs present in the vapors; and responsive to determining that the concentration of VOCs present within the sampling region indicates that a first defined threshold condition is met, generate a control signal to end the curing process. A method and a machine-readable medium are also disclosed.

Description

CONTROFFING CURING PROCESSES IN ADDITIVE MANUFACTURING

BACKGROUND

[0001] Additive manufacturing may be used to generate three-dimensional objects on a layer-by-layer basis, by processing successive layers of a particulate build material. Print agent, such as binder agent, may be selectively applied to portions of layers of build material to cause those portions to coalesce and/or solidify, for example through chemical bonding. In some examples, metallic build material may be used to generate metallic three-dimensional objects.

[0002] One stage of an additive manufacturing process may involve curing an object formed using build material, to cause evaporation of some liquids and to cause particles of build material to bind together.

BRIEF DESCRIPTION OF DRAWINGS

[0003] Examples will now be described, by way of non-limiting example, with reference to the accompanying drawings, in which:

[0004] Figure 1 is a schematic illustration of an example of an apparatus;

[0005] Figure 2 is a schematic illustration of a further example of an apparatus;

[0006] Figure 3 is a flowchart of an example of a method of controlling a curing process of an additive manufacturing apparatus;

[0007] Figure 4 is a flowchart of a further example of a method of controlling a curing process of an additive manufacturing apparatus; and

[0008] Figure 5 is a schematic illustration of an example of a processor in communication with a machine-readable medium.

DETAIFED DESCRIPTION

[0009] Additive manufacturing techniques may generate a three-dimensional object through the solidification of a build material. In some examples, the build material may be a powder-like granular material, which may for example be a plastic, ceramic or metal powder. The properties of generated objects may depend on the type of build material and the type of solidification mechanism used. Build material may be deposited, for example on a print bed and processed layer by layer, for example within a fabrication chamber. According to one example, a suitable build material may be PA 12 build material commercially known as V1R10A “HP PA12” available from HP Inc. In other examples, metal powder, such as steel powder, may be used as the build material. For example, powdered steel alloy known as 316L and 17-4PH may be used for generating metal objects.

[0010] In some examples, print agent may be selectively applied to the build material, and may be liquid when applied. For example, a binder agent, or binding agent, may be selectively distributed onto portions of a layer of build material in a pattern derived from data representing a slice of a three-dimensional object to be generated (which may for example be generated from structural design data). The binding agent may have a composition such that, when energy (for example, heat) is applied to the binder agent, a change occurs that enables the chemical bonding of particles of build material together. [0011] Different types of binding agent may be used. In some examples, thermally curable binding agents may be used. When heat is applied to build material to which thermally curable binding agent has been applied, particles (e.g. latex particles) in the binding agent are caused to bind together, thereby binding the build material together in a binder matrix. In other examples, an ultraviolet (UV) curable binding agent may be used. When UV energy is applied to build material to which UV curable binding agent has been applied, components in the binding agent are caused to polymerize, thereby binding the build material together in a binder matrix.

[0012] According to one example, a suitable binding agent may be an ink-type formulation. In one example, such a binding agent may additionally comprise a UV light absorber. In other examples, a water-based binding agent may be used. Such a binding agent may be formulated such that, when energy (e.g. heat energy or UV energy) is supplied to the binding agent, the binding agent may be activated, and particles of metal build material are caused to bind together at positions where the binding agent is delivered. Capillary forces pull the binding agent into small interstices between the metal particles to produce a uniform binder distribution. [0013] As noted above, additive manufacturing systems may generate objects based on structural design data. This may involve a designer generating a three-dimensional model of an object to be generated, for example using a computer aided design (CAD) application. The model may define the solid portions of the object.

[0014] An additive manufacturing process may be performed in several stages using an additive manufacturing apparatus, and the stages of an additive manufacturing process performed according one example are discussed below.

[0015] In this example, metal powder is used as the build material, so as to generate a metallic three-dimensional object. In a build material depositing stage, a layer of build material (e.g. metal powder) is deposited onto a build platform of a fabrication chamber of an additive manufacturing apparatus. In a binding agent depositing stage, binding agent is distributed onto the layer of build material using an agent distributor. The binding agent is deposited at positions where it is intended for the build material to be bound together. Thermal energy is applied to the build material to increase the temperature of build material. During an evaporation stage the thermal energy applied to the build material causes water or other carrier fluids from the binding agent to evaporate from the build material. Evaporating water from the build material can help to prevent leaching of binding agent into other regions of build material. In some examples, however, the evaporation stage may be omitted. After a defined period of evaporation, the build material depositing stage, the binding agent depositing stage and the evaporation stage are repeated, with subsequent layers of build material being deposited on top of the previous layer of build material, until a sufficient number of layers have been deposited to create what is referred to as a "pre-green part".

[0016] Once the intended number of layers have been deposited, energy (e.g. thermal energy) is applied to the build material to cause particles of build materiel to bind together. This stage may be referred to as a curing stage, a curing process or an annealing process/stage. For example, curing may be achieved by increasing the amount of thermal energy provided in the fabrication chamber. The curing process causes residual water and solvents in the binding agent to evaporate, and causes particles in the binding agent to cure (e.g. to polymerize) and particles of build material to bind together. Vapors caused by the evaporation of water and components of the binding agent, such as solvents, may be removed from the fabrication chamber along a conduit (e.g. a pipe), for example using a pump (e.g. a vacuum pump). Following the curing process, the build material to which binding agent has been applied becomes bound or solidified, forming what is referred to as a "green part", which is a generally weakly bound matrix of particles of build material. The green part is formed once all of the build material in the fabrication chamber has been heated to a temperature exceeding the curing temperature of the binding agent for a sufficient duration.

[0017] Following the curing process, a "de-caking" process is performed, which involves removing any loose, residual build material from the green part (i.e. the bound portion of build material). The time taken to cure a pre-green part can depend on a large number of factors including, for example, the size and/or shape of the three-dimensional object to be formed, the number and arrangement of objects to be formed in the fabrication chamber, the nature of the build material and binding agent used in the additive manufacturing process, and the like. An expected curing time may be estimated based on several factors, such as those mentioned above. However, if the time allowed for curing is too short, then some of the solvents, co- solvents and/or surfactants from the binding agent may not be evaporated and removed from the fabrication chamber and their presence may impact the quality of the object being manufactured, for example by resulting in a reduced- strength green part. Weaker green parts may lead to three-dimensional objects that include cracks or broken edges, resulting in reduced yields. Furthermore, incomplete curing may lead to reduced build material recyclability if solvents and other components of the binding agent accumulate in portions of build material that are not intended to form part of the manufactured object. Components of the binding agent may also cause build material to attach to the green part, making the de-caking process more difficult, and affecting yield and productivity of the additive manufacturing apparatus. Conversely, if the time allowed for curing is too long, then heat may be provided into the fabrication chamber even after a sufficient amount of solvents from the binding agent have been removed. In this case, energy may be wasted due to the unnecessary heating of the fabrication chamber, and productivity and yield of the additive manufacturing apparatus may be reduced.

[0018] It has been recognized that a more accurate determination can be made of when the curing stage of an additive manufacturing process has been completed to a sufficient level (e.g. when a sufficient amount of water and solvents from the binding agent have been removed from the fabrication chamber). In this way, the curing process may be ended when it is determined, using a sensor, that sufficient solvents have been removed, rather than ending the curing process at a time based on an estimation.

[0019] More specifically, examples of the present disclosure make use of the understanding that, during a first stage of the curing process, vapors that are removed from the fabrication chamber tend to include evaporated water and solvents from the binding agent and, once the water and solvents have been removed, during a second stage of the curing process, vapors that are removed from the fabrication chamber tend to include primarily volatile organic compounds (VOCs). According to examples disclosed herein, curing of the green part may be considered to be completed once all (or nearly all) of the VOCs have been removed from the fabrication chamber.

[0020] Referring to the drawings, Figure 1 is a schematic illustration of an example of an apparatus 100 which may, for example, comprise an apparatus capable of controlling a curing process in additive manufacturing. The apparatus 100 (denoted by a dashed box) comprises a processing unit 102 and first sensor 104 in communication with the processing unit. The first sensor 104 is to measure a concentration of volatile organic compounds, VOCs, in vapors passing through a sampling region, the vapors having been extracted from a fabrication chamber 106 of an additive manufacturing apparatus during a curing process to cure a three-dimensional object formed from build material in the fabrication chamber. The three-dimensional object may also be formed from a binder agent, or binding agent, as discussed above. Thus, during the curing process in which heat is supplied to build material in the fabrication chamber, components including VOCs are evaporated from the binding agent, forming vapors that are extracted from the fabrication chamber, for example via a conduit 108 with the aid of a pump (not shown in Figure 1). The vapors are directed to a sampling region, which may for example be a region within the conduit where the first sensor is able to measure a concentration of the VOCs in the vapors.

[0021] The processing unit 102, which is in communication with the first sensor 104, is to determine, using the first sensor, a concentration of VOCs present within the sampling region. For example, the processing unit 102 may receive from the first sensor 104 a measurement of the concentration of VOCs at a particular instance or multiple measurements of concentrations of VOCs at multiple instances, for example over a period of time.

[0022] Responsive to determining that the concentration of VOCs present within the sampling region indicates that a first defined threshold condition is met, the processor 102 is to generate a control signal to end the curing process. For example, if it is determined, based on the determined concentration of VOCs present within the sampling region, that the first defined threshold condition is met, the processor 102 may generate a control signal to reduce an amount of heat provided within the fabrication chamber, thereby reducing or preventing further curing of the green part.

[0023] As noted above, according to some examples, the curing process may be considered complete once all or nearly all of the VOCs have been removed from the fabrication chamber. Thus, the first defined threshold condition may be met - such that the curing process may be considered complete - when the duration of VOCs extracted from the fabrication chamber falls to a sufficient level or when the concentration of VOCs extracted from the fabrication chamber changes (e.g. reduces) by less than a defined amount, or stops changing altogether, over a defined period of time. In other words, the processing unit 102 may determine that the concentration of VOCs present within the sampling region indicates that a first defined threshold condition is met by making a determination selected from a group comprising: i) determining that the concentration of VOCs present within the sampling region is below a defined threshold concentration; and ii) determining that a rate of change of the concentration of VOCs present within the sampling region over a defined duration is below a first defined threshold rate. For example, if the processing unit 102 determines, based on a measurement made using the first sensor, that the concentration of VOCs in vapors within (e.g. passing through) the sampling region is below 100 ppm, then it may be determined that the curing process has been completed, or the green part has been cured to a sufficient degree, and the processor may generate a control signal to end the curing process. Similarly, if the processing unit 102 determines that a rate of change of the concentration of VOCs passing through the sampling region over a defined duration (e.g. 0.5 seconds) is below 25 ppm, then it may be determined that the curing process is completed, or the green part has been cured to a sufficient degree, and the processor may generate a control signal to end the curing process.

[0024] The processing unit 102 may be in communication with the first sensor 104 and/or with a component (e.g. a heat source) 110 or multiple components associated with the fabrication chamber 106 via a wired connection or a wireless connection. The component(s) associated with the fabrication chamber 106 may comprise a component or a plurality of components responsible for or used as part of the curing process.

[0025] Figure 2 is a schematic illustration of a further example of an apparatus 200, which may, for example, comprise an apparatus capable of controlling a curing process in additive manufacturing. The apparatus 200 is similar to, and may comprise components of, the apparatus 100 discussed above. In particular, the apparatus 200 includes the processing unit 102 and the first sensor 104, and may further include the fabrication chamber 106 and the component(s) 110. In some examples, the component(s) 110 may comprise an energy source, such as a heat source, to supply thermal energy to build material in the fabrication chamber 106 during the curing process. The energy source 110 may, for example, comprise a heat lamp or a series of heat lamps arranged on, within and/or around the fabrication chamber to direct heat towards the build material. The control signal to be generated by the processing unit to end the curing process may comprise a control signal to reduce the amount of thermal energy supplied by the energy source 110.

[0026] While the first sensor 104, which may be referred to as a VOC sensor, can be used to determine when the curing process is deemed to have completed based on the concentration of VOCs in vapors extracted from the fabrication chamber 106, some examples of the present disclosure may use an additional sensor. A VOC sensor can become saturated or damaged if it encounters vapors containing particularly high concentrations of VOCs. Vapors extracted from the fabrication chamber during the early stages of the curing process may contain sufficiently high concentrations of VOCs to impair or damage a VOC sensor and, therefore, in some examples, use of such a VOC sensor may be limited to a later stage of the curing process. A different sensor may be employed to monitor vapors extracted from the fabrication chamber during the first stage of the curing process, as discussed below. [0027] As shown in the example of Figure 2, the apparatus 200 may comprise a receptacle 202 to receive solvents extracted from the fabrication chamber 106 during the curing process. Solvents may be present in vapors extracted from the fabrication chamber 106 during the early stages of the curing process. In some examples, the vapors extracted during the early stages of the curing process may also contain water, co-solvents, surfactants, and the like. In order to collect solvents in the receptacle 202, vapors may be extracted from the fabrication chamber 106 via the conduit 108. As the vapors pass along the conduit 108, they are cooled (e.g. using a heat exchanger, not shown in Figure 2), and the condensate formed from the vapors is received in the receptacle 202. The apparatus 200 may further comprise a second sensor 204 to measure a volume of solvents received in the receptacle 202. The second sensor 204, which may comprise a liquid level sensor, may for example comprise an electrode-type sensor or a time-of-flight (TOF) type sensor to measure movement of a floater 206 floating on the surface of the liquid (e.g. solvents) in the receptacle 202. Alternatively, an optical device such as a laser may be used to measure the volume of solvents in the receptacle 202. In other examples, some other mechanism may be employed for measuring the volume of liquid and/or solvents in the receptacle 202. The second sensor 204 may be in communication (e.g. wired or wireless communication) with the processing unit 102.

[0028] Responsive to determining that the measured volume of solvents indicates that a second defined threshold condition is met, the processing unit 102 may generate a control signal to cause the first sensor to measure the concentration of VOCs in vapors within the sampling region. Thus, the apparatus 200 may first measure the volume of solvents that have been received in the receptacle 202 and, once it has been determined that the second defined threshold condition has been met, monitoring of the VOC concentration within the sampling region may be initiated. For example, the processor 102 may determine, based on the measurements made using the second sensor 204, that the second threshold condition has been met, and may then generate the control signal to commence measurement of the VOC concentration using the first sensor 104.

[0029] The second defined threshold condition may be considered to be met when it is determined that all, or nearly all, of the water and solvents have been removed from the fabrication chamber 106. Thus, the processing unit 102 may determine that the measured volume of solvents indicates that a second defined threshold condition is met by determining that a change of the volume of solvents received in the receptable over a defined duration is below a second defined threshold volume. Thus, the second sensor 204 may measure the volume of solvents received in the receptacle over a defined duration (e.g. over a period of between 30 seconds and 60 seconds) and, if it is determined that the amount by which the volume of solvents has changed over that duration is less than the second defined threshold volume, then it may be understood that all or nearly all of the solvents have been removed from the fabrication chamber 106, such that measurements using the first sensor 104 (e.g. the VOC sensor) can commence. [0030] As noted above, during the early stages of the curing process, the vapors extracted from the fabrication chamber 106 contain large concentrations of VOCs. If vapors containing such large VOC concentrations were to come into contact with the first sensor (e.g. a VOC sensor) 104, then the first sensor may become saturated or even damaged. Therefore, during the first stage of the curing process, the vapors, largely containing water and solvents, are condensed and received in the receptacle 202. Once most or all of the water and solvents have been extracted from the fabrication chamber 106, the vapors that are then extracted from the fabrication chamber contain much lower concentrations of VOCs, and these vapors can come into contact with the first sensor 104 with a much lower risk of causing damage to the sensor.

[0031] The first sensor 104 may be prevented from coming into contact with vapors during the early stages of the curing process in a number of different ways. Referring again to Figure 2, the apparatus 200 may comprise a sensor engagement mechanism 208 to enable and/or restrict contact between the first sensor 104 and the vapors (i.e. vapors containing the solvents and/or the VOCs) extracted from the fabrication chamber 106.

The sensor engagement mechanism 208 may, in a first configuration, prevent the first sensor 104 from encountering vapors in the sampling region and, in a second configuration, enable the first sensor to encounter vapors in the sampling region to thereby measure the concentration of VOCs in the vapors. Thus, the apparatus 200 may further comprise a conduit 108 along which the solvents and the vapors are able to travel from the fabrication chamber 106. In some examples, such as the example shown in Figure 2, the sensor engagement mechanism 208 may comprise a valve which, in the first configuration, prevents vapors in the conduit from reaching the first sensor 104 and, in the second configuration, allows vapors in the conduit to reach the first sensor. In other examples, other mechanisms may be used to allow and/or prevent vapors from reaching the first sensor 104. Thus, in the example shown in Figure 2, during the early stages of the curing process, the valve 208 may be closed, such that vapors extracted from the fabrication chamber 106 are caused to pass along the conduit 108, where they are cooled and caused to condense so that they can be collected in the receptacle 202. Once it is determined, based on a measurement made using the second sensor 204, that the level of solvents collected in the receptacle 202 has stabilized or substantially stabilized, the processor 102 may open the valve 208, allowing vapors extracted from the fabrication chamber 106 to pass along a conduit 210, towards a sampling region where the concentration of VOCs in the vapors can be measured using the first sensor 104.

[0032] In some examples, conduit heating elements 212 may be provided at various positions along the conduits 108, 210 to provide heat to the conduits and to the vapors (including water, solvents and/or VOCs) passing along the conduits. The conduit heating elements 212 may heat the contents of the conduits 108, 210 to cause any liquids within the conduits to be vaporized, or to keep the contents at such a temperature that they do not condense until intended. The conduit heating elements 212 may be in operable communication with the processing unit 102, such that the processing unit can separately operate each conduit heating element to control heating of different parts of the conduit. The apparatus 200 may further comprise a pump 214 to pump vapors from the fabrication chamber 106. In some examples, the apparatus 200 may comprise a pump 216, which may be referred to as a purge pump, and which may be used to remove vapors from the conduit 210 and/or from the first sensor 104. The pumps 214, 216 may be in operative communication with the processing unit 102, such that the processing unit can separately operate each pump.

[0033] The apparatus 100, 200 and may, in some examples, comprise a stand-alone unit which can be connected to a fabrication chamber 106 in order to receive vapors extracted from the fabrication chamber. In some examples, the apparatus 100, 200 may be incorporated into an additive manufacturing apparatus. Thus, in some examples, the apparatus 100, 200 may be, form part of, or comprise an additive manufacturing apparatus, and may further comprise the fabrication chamber 106 in which layers of build material are to be processed to form a three-dimensional object during an additive manufacturing process. The first sensor 104 is to measure a concentration of VOCs in vapors extracted from the fabrication 106. The processing unit 102 may be to determine the concentration of VOCs present within the sampling region based on a plurality of concentrations of VOCs measured in vapors extracted from the fabrication chamber. The processing unit 102 may generate the control signal responsive to determining that: a measured concentration of the plurality of measured concentrations of VOCs is below a defined concentration threshold; or a rate of change of the measured concentrations of VOCs is below a defined rate of change threshold.

[0034] According to other examples disclosed herein, a method is provided. Figure 3 is a flowchart of an example of a method 300, which may comprise a method of controlling a curing process in additive manufacturing. The method 300 comprises, at block 302, measuring, using a first sensor 104, a concentration of volatile organic compounds,

VOCs, in a stream of vapors removed from a fabrication chamber 106 of an additive manufacturing apparatus during a curing process in which a three-dimensional object formed from build material in the fabrication chamber is cured. At block 304, the method 300 comprises, responsive to determining, using processing apparatus (e.g. the processing unit 102), that the measured concentration of VOCs in the vapor stream indicates that a first defined threshold condition is met, operating a component of the additive manufacturing apparatus to end the curing process.

[0035] In some examples, operating the component of the additive manufacturing apparatus to end the curing process, at block 304, may comprise controlling a heat source to reduce an amount of heat directed from the heat source towards build material in the fabrication chamber 106. The heat source may, for example, comprise the component or energy source 110 discussed above.

[0036] In some examples, determining that the measured concentration of VOCs in the vapor stream indicates that a first defined threshold condition is met, at block 304, may comprise determining that: the concentration of VOCs in the stream of vapors is below a defined threshold concentration; or a change in the concentration of VOCs present within the sampling region over a defined duration is below a defined threshold change. Thus, if it is determined that the concentration of VOCs in the stream of vapors is below the defined threshold concentration, then it may be assumed that all, or at least the majority, of the VOCs have been extracted from the fabrication chamber 106, signaling that the curing process has been completed, and that the additive manufacturing process can proceed to the next stage. Similarly, if the change in the concentration of VOCs within the sampling region over the defined duration is below the defined threshold change, then it may be assumed that the concentration of VOCs is unlikely to reduce further, indicating that the curing process has been completed.

[0037] Figure 4 is a flowchart of a further example of a method 400. The method 400 may comprise a method of controlling a curing process in additive manufacturing, and may comprise blocks of the method 300 discussed above.

[0038] The method 400 may comprise, at block 402, prior to measuring the concentration of VOCs in the vapor stream (at block 302), receiving, in a vessel (e.g. the receptacle 202), condensate of vapors (e.g. a liquid condensate) extracted from the fabrication chamber 106. The condensate of vapors extracted from the fabrication chamber and received in the vessel may comprise water and solvents. The condensate may also comprise large amounts of VOCs which, as noted above, could impair or damage the first sensor 104 if allowed to come into contact with the sensor at such high concentrations.

At block 404, the method 400 may comprise measuring, using a second sensor 204, a volume of condensate received in the vessel. Responsive to determining that the volume of condensate indicates that a second defined threshold condition is met, the method 400 may comprise, at block 406, operating the first sensor 104 to measure the concentration of VOCs in vapors in the vapor stream. Thus, the first sensor 104 may be used once it has been determined, using the second sensor 204, that the amount of condensate collected in the vessel has met a defined threshold condition. The second defined threshold condition may be met if it is determined that the volume of condensate received in the vessel exceeds a defined threshold volume or if the volume of condensate stabilizes. For example, the second defined threshold condition may be met if the rate of change in the volume of condensate received in the vessel is determined to be below a defined threshold rate. Once the second defined threshold condition has been met, it may be accepted that any remaining VOCs in the vapors extracted from the fabrication chamber 106 will be present in sufficiently low concentrations that the first sensor 104 is unlikely to be damaged when it comes into contact with the vapors.

[0039] Operating the first sensor 104 to measure the concentration of VOCs in vapors in the vapor stream (block 406) may, in some examples, be achieved by operating (e.g. opening) a valve to enable the vapor stream to come in contact with the first sensor.

When the valve is in its closed position, the vapor stream may be prevented from coming into contact with the first sensor 104.

[0040] Over time, VOCs and other particles within the vapors that come into contact with the first sensor 104 may adhere to the first sensor, affecting measurements made using the first sensor. Thus, the first sensor 104 may measure a concentration of VOCs in the sampling region when vapors from the application chamber 106 are prevented from reaching the sampling region and the first sensor. This may provide a baseline measurement for the first sensor 104, which may be used, for example, to calibrate the first sensor. The method 400 may therefore comprise, at block 408, measuring, using the first sensor 104 prior to measuring the concentration of VOCs (block 302), a first baseline concentration of VOCs outside of the stream of vapors. At block 410, the method 400 may comprise measuring, using the first sensor 104 after operating the component of the additive manufacturing apparatus to end the curing process (block 304), a second baseline concentration of VOCs outside of the stream of vapors. Baseline concentration measurements may be taken, for example, then the valve 208 is closed and when the vapors containing VOCs have been pumped or flushed out of the conduit 210 The first baseline concentration and the second baseline concentration may be used to calibrate the first sensor 104. In some examples, just a first baseline concentration may be measured using the first sensor 104, and the first baseline concentration may be used to calibrate the first sensor 104.

[0041] Measurements of the first baseline concentration and/or the second baseline concentration may be used to determine when a buildup of VOCs on the first sensor 104 have reached such a level that maintenance (e.g. cleaning) of the first sensor should be carried out, or even such a level that the first sensor should be replaced. Thus, the method 400 may further comprise, at block 412, responsive to determining that the first baseline concentration and/or the second baseline concentration exceeds a threshold baseline concentration, generating an alert signal to indicate that the first sensor 102 is to undergo maintenance. The alert signal generated at block 412 may be provided for presentation (e.g. audibly or visually) to an operator, for example.

[0042] The methods 300, 400 may include other blocks corresponding to functions performed using the apparatus 100, 200 as disclosed herein.

[0043] According to other examples disclosed herein, a machine-readable medium is provided. Figure 5 is a schematic illustration of an example of a processor 502 in communication with a machine-readable medium 504. The machine-readable medium 504 comprises instructions which, when executed by a processor, cause the processor to perform various functions. The machine-readable medium 504 comprises instructions (e.g. VOC concentration measurement instructions 506) which, when executed by a processor, cause the processor to receive, from a first sensor, an indication of a measurement of concentration of volatile organic compounds, VOCs, in vapors removed from a fabrication chamber 106 of an additive manufacturing apparatus during a curing process in which a three-dimensional object formed from build material in the fabrication chamber is cured. The machine-readable medium 504 comprises instructions (e.g. control signal generation instructions 508) which, when executed by a processor, cause the processor to, responsive to determining that the measured concentration of VOCs indicates that a defined threshold condition is met, generate a control signal to control a component of the additive manufacturing apparatus to end the curing process.

[0044] The machine -readable medium 504 may comprise further instructions which, when executed by a processor, cause the processor to perform functions corresponding to functions performed using the apparatus 100, 200 as disclosed herein and/or functions corresponding to blocks of the methods 300, 400 disclosed herein.

[0045] Examples in the present disclosure can be provided as methods, systems or machine readable instructions, such as any combination of software, hardware, firmware or the like. Such machine readable instructions may be included on a computer readable storage medium (including but is not limited to disc storage, CD-ROM, optical storage, etc.) having computer readable program codes therein or thereon.

[0046] The present disclosure is described with reference to flow charts and/or block diagrams of the method, devices and systems according to examples of the present disclosure. Although the flow diagrams described above show a specific order of execution, the order of execution may differ from that which is depicted. Blocks described in relation to one flow chart may be combined with those of another flow chart. It shall be understood that each flow and/or block in the flow charts and/or block diagrams, as well as combinations of the flows and/or diagrams in the flow charts and/or block diagrams can be realized by machine readable instructions.

[0047] The machine readable instructions may, for example, be executed by a general purpose computer, a special purpose computer, an embedded processor or processors of other programmable data processing devices to realize the functions described in the description and diagrams. In particular, a processor or processing apparatus may execute the machine readable instructions. Thus functional modules of the apparatus and devices may be implemented by a processor executing machine readable instructions stored in a memory, or a processor operating in accordance with instructions embedded in logic circuitry. The term ‘processor’ is to be interpreted broadly to include a CPU, processing unit, ASIC, logic unit, or programmable gate array etc. The methods and functional modules may all be performed by a single processor or divided amongst several processors.

[0048] Such machine readable instructions may also be stored in a computer readable storage that can guide the computer or other programmable data processing devices to operate in a specific mode.

[0049] Such machine readable instructions may also be loaded onto a computer or other programmable data processing devices, so that the computer or other programmable data processing devices perform a series of operations to produce computer-implemented processing, thus the instructions executed on the computer or other programmable devices realize functions specified by flow(s) in the flow charts and/or block(s) in the block diagrams.

[0050] Further, the teachings herein may be implemented in the form of a computer software product, the computer software product being stored in a storage medium and comprising a plurality of instructions for making a computer device implement the methods recited in the examples of the present disclosure. [0051] While the method, apparatus and related aspects have been described with reference to certain examples, various modifications, changes, omissions, and substitutions can be made without departing from the spirit of the present disclosure. It is intended, therefore, that the method, apparatus and related aspects be limited only by the scope of the following claims and their equivalents. It should be noted that the above- mentioned examples illustrate rather than limit what is described herein, and that those skilled in the art will be able to design many alternative implementations without departing from the scope of the appended claims. Features described in relation to one example may be combined with features of another example.

[0052] The word “comprising” does not exclude the presence of elements other than those listed in a claim, “a” or “an” does not exclude a plurality, and a single processor or other unit may fulfil the functions of several units recited in the claims.

[0053] The features of any dependent claim may be combined with the features of any of the independent claims or other dependent claims.

Claims

1. An apparatus comprising: a first sensor to measure a concentration of volatile organic compounds, VOCs, in vapors passing through a sampling region, the vapors having been extracted from a fabrication chamber of an additive manufacturing apparatus during a curing process to cure a three-dimensional object formed from build material in the fabrication chamber; and a processing unit in communication with the first sensor, the processing unit to: determine, using the first sensor, a concentration of VOCs present within the sampling region; and responsive to determining that the concentration of VOCs present within the sampling region indicates that a first defined threshold condition is met, generate a control signal to end the curing process.

2. An apparatus according to claim 1, wherein the processing unit is to determine that the concentration of VOCs present within the sampling region indicates that a first defined threshold condition is met by making a determination selected from a group comprising: determining that the concentration of VOCs present within the sampling region is below a defined threshold concentration; and determining that a rate of change of the concentration of VOCs present within the sampling region over a defined duration is below a first defined threshold rate.

3. An apparatus according to claim 1, further comprising: a receptacle to receive solvents extracted from the fabrication chamber during the curing process; and a second sensor to measure a volume of solvents received in the receptacle; wherein the processing unit is to: responsive to determining that the measured volume of solvents indicates that a second defined threshold condition is met, generate a control signal to cause the first sensor to measure the concentration of VOCs in vapors within the sampling region.

4. An apparatus according to claim 3, wherein the processing unit is to determine that the measured volume of solvents indicates that a second defined threshold condition is met by determining that a change of the volume of solvents received in the receptable over a defined duration is below a second defined threshold volume.

5. An apparatus according to claim 3, further comprising: a sensor engagement mechanism which, in a first configuration, prevents the first sensor from encountering vapors in the sampling region and, in a second configuration, enables the first sensor to encounter vapors in the sampling region to thereby measure the concentration of VOCs in the vapors.

6. An apparatus according to claim 5, further comprising: a conduit along which the solvents and the vapors are able to travel from the fabrication chamber; wherein the sensor engagement mechanism comprises a valve which, in the first configuration, prevents vapors in the conduit from reaching the first sensor and, in the second configuration, allows vapors in the conduit to reach the first sensor.

7. An apparatus according to claim 1, wherein the apparatus comprises an additive manufacturing apparatus, and further comprises: a fabrication chamber in which layers of build material are to be processed to form a three-dimensional object during an additive manufacturing process; wherein the first sensor is to measure a concentration of VOCs in vapors extracted from the fabrication chamber; and wherein the processing unit is to: determine the concentration of VOCs present within the sampling region based on a plurality of concentrations of VOCs measured in vapors extracted from the fabrication chamber; and generate the control signal responsive to determining that: a measured concentration of the plurality of measured concentrations of VOCs is below a defined concentration threshold; or a rate of change of the measured concentrations of VOCs is below a defined rate of change threshold.

8. An apparatus according to claim 7, further comprising: an energy source to supply thermal energy to build material in the fabrication chamber during the curing process; wherein the control signal to be generated by the processing unit to end the curing process comprises a control signal to reduce the amount of thermal energy supplied by the energy source.

9. A method comprising: measuring, using a first sensor, a concentration of volatile organic compounds, VOCs, in a stream of vapors removed from a fabrication chamber of an additive manufacturing apparatus during a curing process in which a three-dimensional object formed from build material in the fabrication chamber is cured; and responsive to determining, using processing apparatus, that the measured concentration of VOCs in the vapor stream indicates that a first defined threshold condition is met, operating a component of the additive manufacturing apparatus to end the curing process.

10. A method according to claim 9, wherein determining that the measured concentration of VOCs in the vapor stream indicates that a first defined threshold condition is met comprises determining that: the concentration of VOCs in the stream of vapors is below a defined threshold concentration; or a change in the concentration of VOCs present within the sampling region over a defined duration is below a defined threshold change.

11. A method according to claim 9, further comprising, prior to measuring the concentration of VOCs in the vapor stream: receiving, in a vessel, condensate of vapors extracted from the fabrication chamber; measuring, using a second sensor, a volume of condensate received in the vessel; and responsive to determining that the volume of condensate indicates that a second defined threshold condition is met, operating the first sensor to measure the concentration of VOCs in vapors in the vapor stream.

12. A method according to claim 9, further comprising: measuring, using the first sensor prior to measuring the concentration of VOCs, a first baseline concentration of VOCs outside of the stream of vapors; and measuring, using the first sensor after operating the component of the additive manufacturing apparatus to end the curing process, a second baseline concentration of VOCs outside of the stream of vapors; wherein the first baseline concentration and the second baseline concentration are used to calibrate the first sensor.

13. A method according to claim 12, further comprising: responsive to determining that the first baseline concentration and/or the second baseline concentration exceeds a threshold baseline concentration, generating an alert signal to indicate that the first sensor is to undergo maintenance.

14. A method according to claim 9, wherein operating the component of the additive manufacturing apparatus to end the curing process comprises controlling a heat source to reduce an amount of heat directed from the heat source towards build material in the fabrication chamber.

15. A machine-readable medium comprising instructions which, when executed by a processor, cause the processor to: receive, from a first sensor, an indication of a measurement of concentration of volatile organic compounds, VOCs, in vapors removed from a fabrication chamber of an additive manufacturing apparatus during a curing process in which a three-dimensional object formed from build material in the fabrication chamber is cured; and responsive to determining that the measured concentration of VOCs indicates that a defined threshold condition is met, generate a control signal to control a component of the additive manufacturing apparatus to end the curing process.