Patent Description:
Applications for forming three dimensional objects from particulate material, such as so-called "print and sinter" or laser sintering processes, are receiving increased interest as they are moving towards faster throughput times and become industrially viable. In these processes, the object is formed layer-by-layer from particulate material that is spread in successive layers across a build surface. Each successive layer of particulate matter is fused, or sintered, over defined regions to form a cross-section of the three-dimensional object. Typically such processes require accurate temperature control of the surface that is being processed to achieve high-quality uniform objects with well-defined properties. Temperature control requires use of a temperature sensor, such as a pyrometer or thermal camera, that detects the temperature of the surface of the layer being processed (the build bed surface). Detection may be continuous or intermittent during processing. For reliable process control from build to build or between different apparatus, it is desirable to have accurate knowledge of the onset of fusion (or sintering), or of the temperature at which fusion occurs and that is being measured since the process of fusion depends on the material characteristic melting point of the material that is being fused. In "print and sinter" processes for example, often some of the powder is recycled and reused and the powder properties are liable to change as a result of ageing. It is therefore important to be able to identify which temperature measurements correspond to the onset of fusion of a given particulate material used in the apparatus.

The onset of fusion may be determined from the characteristic change in the rate of heating when monitoring time-temperature behaviour of a reference area within the build bed surface upon a heating procedure, or by a characteristic change in optical properties when monitoring the reference area using an optical sensor, as the material transitions through a change of phase from the solid phase into the liquid phase. This procedure may thus be carried out in-situ on the apparatus. It is however challenging to apply a suitable combination of process parameters and conditions that achieve a level of control accurate enough to allow detection of this characteristic change of heating rate, specifically where the combination of absorptive properties of the particulate material and the radiation spectrum of the heat source is tuned to achieve rapid fusion, as is typically the case. In a conventional object build process a heat source is passed over the build bed to apply radiation to selectively fuse a cross-section of the object. The selectivity of the heat source is achieved by choosing a spectrum of radiation that overlaps with or comprises the absorption spectrum of the material to be fused. For example, a radiation-absorbing fluid may be deposited over the build bed surface to define the cross-section of the object. The radiation-absorbing fluid readily absorbs radiation from the heat source if it is tuned to the absorber fluid absorption spectrum, leading to fusion of the defined cross-section.

Using such a tuned heat source for the calibration process poses several problems. The rate of heating by the lamp has to be reduced significantly and the heat source passed repeatedly over the build bed surface to suitably reduce the rate of heating to allow a detectable onset of fusion. This leads to intermittent heating steps and the rate of heating has to be smoothed to detect the onset of fusion. In addition, a temperature sensor is typically located above the build bed surface and thus the area to be monitored is repeatedly obscured by the passing heat source, meaning that the onset of fusion will most likely be obscured by the heat source as it passes.

Most apparatus will further comprise a static overhead heater that is non-selective and provides preheating to the entire layer of particulate material. With such a heat source the heating process is less efficient and to reach fusion, the areas not to be fused also receive significant heat, which is likely to result in a decay of material properties and low recyclability. Calibration procedures that effectively and accurately detect the onset of fusion of the particulate material are therefore needed to address the aforementioned problems.

Background art is provided in <CIT>, <CIT> and <CIT>.

<CIT> discloses an additive manufacturing device comprising a sensor, a moveable radiation source and a controller. The controller is configured to determine an output of the sensor at a point at which build material melts by causing the moveable radiation source to periodically move over a layer of the build material to provide radiation to the layer of build material, and by monitoring the output of the sensor.

<CIT> discloses an additive manufacturing system comprising a dispensing device, an applicator, a thermal energy source, a thermal imaging device, and a controller. The controller is configured to cause the dispensing device to deposit a layer of build material, and to cause the applicator to apply a fusing agent to form an object portion and to apply a detailing agent to form a reference portion in the layer of build material. The controller is further configured to cause the thermal energy source to heat the reference portion and to heat and fuse the object portion, and to cause the thermal imaging device to measure a temperature of the reference portion. The controller is further configured to regulate a power level of the thermal energy source based on a comparison between the temperature of the reference portion and a set-point for the reference portion, which is based on a target temperature for the object portion.

<CIT> discloses a method of controlling temperature in an apparatus for generating a three-dimensional object, the method comprising: performing a calibration test on a sample of build material that is to be used in generating a three-dimensional object, calibrating at least one temperature point from the calibration test, and using the at least one calibrated temperature point during subsequent temperature control of the apparatus.

Aspects of the invention are set out in the appended independent claims, while particular embodiments of the invention are set out in the appended dependent claims.

The following disclosure describes, in one aspect, a method for determining a set point for measurements from a temperature sensor of an apparatus for the layer-by-layer formation of a three-dimensional object from particulate material, the apparatus having a moveable heat source, the method comprising:.

According to another aspect of the disclosure, there is provided a method for calibrating the measurement of a temperature sensor of an apparatus for the layer-by-layer formation of a three-dimensional object from particulate material, the method comprising:.

An apparatus for the manufacture of a three-dimensional object by layer-by-layer deposition of particulate material is defined in independent claim <NUM>.

Reference is now directed to the drawings, in which:.

In the Figures, like elements are indicated by like reference numerals throughout.

<FIG> illustrates a schematic cross-sectional front view of a typical print and sinter apparatus that will be used to describe the improved calibration methods according to embodiments of the invention, and their associated apparatus and controllers.

In a typical process for the generation of a three-dimensional object <NUM> from particulate material, successive layers of particulate material are distributed to form the build bed surface <NUM> which is processed to form successive cross-sections of the object <NUM>. The apparatus <NUM> comprises a powder container system which comprises a build bed <NUM> supported by container walls <NUM> and a build bed floor <NUM>, and comprising the formed object <NUM>. The build bed floor <NUM> is arranged to move vertically within the container wall <NUM> to lower or raise the build bed surface <NUM>; for example it may be moved up and down by a piston located beneath the build bed floor.

A carriage <NUM> is movably arranged on one or more rails <NUM> to allow it to be moved back and forth across the build bed surface <NUM>. The carriage <NUM> comprises a printing module <NUM> for selectively depositing absorption-modifying fluid across the build bed surface <NUM>; and a heat source module <NUM> (not shown in <FIG>). The same or a different carriage (not shown) comprises a distributor module for distributing the particulate material across the build bed <NUM>. A powder dosing module arranged to dose fresh powder to be distributed is also present but not specifically shown. The same or a different carriage <NUM> further supports a moveable heat source <NUM>.

In a typical build sequence of an object cross-section, the build bed floor <NUM> is lowered by a layer thickness and a dose of particulate material is distributed across the build bed surface <NUM>. Printheads of the fluid deposition module <NUM> deposit fluid, for example containing radiation-absorbing material such as carbon black, at selected locations defined by the cross-section of the object to be formed within the specific layer of distributed powder. After this, the moveable heat source, for example an infrared lamp spanning the width of the build bed surface <NUM>, supported on a carriage <NUM>, is passed across the build bed surface <NUM> to selectively heat and consolidate the powder that has received the radiation-absorbing fluid. In <FIG>, this is the defined area <NUM> defined by the radiation-absorbing fluid and that is to be fused. In this illustrative process, the steps of lowering the build bed floor, distributing particulate material to form a new layer, printing and fusing the defined area <NUM> happen sequentially. Depending on the number of carriages and the modules they support, suitable sequences of travel of the carriages may be applied to support the steps of layer formation and selective consolidation. An overhead heater <NUM> may be provided in the apparatus <NUM> to preheat the build bed surface <NUM> and to maintain the build bed surface at a preset temperature below the onset of fusion. The heater may be operated at least partially based on temperature measurements of a sensor <NUM> monitoring the temperature of part or all of the build bed surface. The sensor <NUM> may be located within the area of the heater <NUM>, as is shown for the illustrative apparatus <NUM> of <FIG>.

The particulate material may be a polymeric material such as nylon PA11, a bioplastic polyamide powder. During build procedures, the level of measurements of the sensor may change, for example due to contamination of a protective window protecting the sensor from the environment of the working space, or due to the temperature of the sensor environment; or the properties of the build material may change due to thermal cycling or provision of a new composition of particulate material. The sensor's relative scale of temperature may be calibrated against the onset of fusion as determined from the temperature-time characteristics of the particulate material. The relative scale may in addition be set to an absolute scale if the onset of fusion as determined with the sensor is provided with an absolute value measured separately.

The apparatus <NUM> is suitable to carry out any improved calibration routines based on determination of the onset of fusion from measured temperature-time characteristics, as will now be described with reference to <FIG>.

In order to overcome or reduce the above problems, improved calibration routines are provided that determine the onset of fusion from measurements of the temperature of a reference area at least until it begins to fuse, with improved accuracy. In several variants of a first embodiment the calibration routines employ at least one reference area comprising radiation absorber while reducing the rate at which the reference area is heated, using conventional apparatus configured to carry out the corresponding configurations of the calibration routines. This may be achieved in several ways; for example, where a heat source having a radiation spectrum that is preferentially absorbed by the radiation-absorbing material is used, the procedure of a normal build process may be adapted to address the problems by, in one implementation, increasing the distance between the moveable heat source and the build bed surface (and thus reference area) so as to reduce the intensity of radiation that impacts the build bed surface. For example, the build bed may be lowered during the calibration routine, such as by operating a piston supporting the build bed floor and arranged to lower or raise it.

In another variant, the reference area is irradiated by a fixed overhead heater. In yet another variant, the moveable heat source may be repeatedly passed over the build bed surface so as to gradually provide sufficient energy to the reference area that the particulate material of the reference area fuses. In addition, the distance of the lamp with respect to the build bed surface may be improved for better accuracy of measurement; the duty cycle of the heat source for achieving fusing of an object build layer and/or the speed of travel of the heat source and/or the amount of radiation absorber per unit area in the reference area may be adjusted so as to further modify the time before fusion is achieved. Such an improved process may however still require a degree of extrapolation of the time-temperature data to account for the repeated passes of the heat source during which the reference area is cyclically heated. Furthermore, the measurements of the temperature sensor that is typically arranged in a fixed position above the build bed surface need to be adapted for the moving heat source intermittently obstructing the field of view.

The first embodiment will now be described in detail with reference to <FIG>, <NUM><NUM> and <NUM>, and their corresponding variants.

<FIG> are schematic views of a cross-section through an apparatus <NUM> (substantially as previously described with reference to <FIG>) configured to carry out the calibration routine. <FIG> is a plan view of the build bed surface <NUM> of <FIG> shows a cross-section through the build bed <NUM> having a build bed floor <NUM> that may be lowered for a new layer to be distributed across the build bed surface during a normal build procedure. For the calibration routine steps illustrated in <FIG>, the build bed floor <NUM> may further be lowered by a larger distance than a layer, so as to increase the distance between the build bed surface <NUM> and the heat source <NUM> mounted on the moveable carriage <NUM>.

<FIG> shows a calibration layer <NUM> that has been distributed across the build bed <NUM> to form the build bed surface <NUM>. The moveable printing module <NUM>, which is provided on the carriage <NUM> and moveable back and forth across the build bed surface <NUM> along a process direction x, is arranged to define a reference area <NUM> by selectively depositing a radiation-absorbing fluid over the reference area <NUM>. The carriage <NUM> supporting the heat source <NUM> is moveable back and forth across the build bed surface <NUM> along the process direction x.

Next, the build bed floor <NUM> is lowered to move the build bed surface <NUM> to a calibration depth. This is shown in <FIG>. The carriage <NUM> is next moved to locate the heat source <NUM> at a calibration position. <FIG> shows the heat source <NUM> in the calibration position, and further illustrates that the lowering of the build bed floor <NUM> by a calibration depth defines a calibration distance D between the build bed surface <NUM> and the heat source <NUM>. The calibration distance D is a distance that is greater than a normal build process distance between the moveable heat source <NUM> used to fuse the particulate material and the build bed surface <NUM> when an object is being built, and is the minimum distance between the heat source <NUM> and the build bed surface <NUM>. The calibration distance D may be determined so as to sufficiently reduce the heat impact of the heat source on the build bed surface <NUM>, to prevent particulate material from densifying or fusing in regions other than the reference area <NUM>. It should be noted that the order of steps of lowering the build bed surface to the calibration depth and of moving the moveable heat source to the calibration position is interchangeable.

Next, the heat source <NUM> is operated to heat the reference area <NUM> at least until the material of the reference area begins to fuse. Meanwhile, a temperature sensor <NUM>, such as an infrared camera, positioned above the build bed surface <NUM>, is configured to monitor the temperature of the reference area <NUM> as it is being heated. The sensor may be mounted in a fixed position, and the calibration position of the heat source <NUM> and/or the calibration distance D are chosen so as not to obstruct the reference area <NUM> from the field of view of the sensor <NUM>. This is illustrated in <FIG>, in which the sensor <NUM> is centrally mounted above the build bed surface <NUM>, and above the moving envelope of the moveable heat source <NUM>. The calibration position and the calibration distance D are chosen such that the field of view (FOV) of the sensor encompasses a "direct sub-reference area" directly underneath the heat source <NUM>, where the direct sub-reference area is comprised within the reference area <NUM>. In the apparatus shown, the calibration position is near one end of the build bed surface <NUM>. Other calibration positions may be possible depending on the arrangement and design of the carriage and the moveable heat source. In some implementations, the sensor <NUM> may be moveable.

The moveable heat source <NUM> positioned in the fixed calibration position can continuously provide energy to the reference area <NUM> without interrupting the heating process of the reference area <NUM>, and the time-temperature behaviour from the sensor measurements do not have to be smoothed to compensate for the effects of intermittent heating or any other effects due to a passing heat source. Thus the sensor measurements over the duration of the calibration routine can be used to accurately determine the onset of fusion over the reference area <NUM>.

The rate of heating of the reference area <NUM> may be reduced and the accuracy of determining the onset of fusion may be improved suitably by adjusting the calibration distance D in the vertical direction (along z). Additionally, the distance between the heat source <NUM> and the reference area <NUM> in the lateral direction (along x) may be adjusted, and/or the power output of the heat source <NUM>. This variant of the first embodiment provides several parameters that allow tuning the calibration procedure to achieve the desired accuracy. The rate of heating may be controlled by one or more of the following:.

The heat source <NUM> in <FIG> and <FIG> is elongate and spans the width of the build bed surface <NUM>, so that the reference area <NUM> receives substantially the same level of heating from the heat source <NUM> along a line of constant distance to the heat source <NUM>. The reference area <NUM> may extend sufficiently along the process direction x so that sub areas of the reference area <NUM> closest to the heat source <NUM> will fuse first and sub areas furthest away will fuse last, allowing a progressive onset of fusion to be monitored against distance of sub areas of the reference area from the heat source <NUM>.

<FIG> indicates that more than one reference area may be defined and fused in the same calibration routine while the sensor monitors the temperature of all the defined reference areas; in this case reference areas 50_1 and 50_2. The reference areas may be arranged near the edges of the build bed or at regular intervals covering part or all of the build bed width along the direction of elongation of the heat source <NUM>.

The sensor measurements are used to determine the onset of fusion in the reference areas <NUM>, using the characteristic change in the rate of heating that indicates the onset of fusion.

The onset of fusion is an important set point for the build process to the particulate material and may be used to calibrate the temperature measurements of the temperature sensor that is to be used for subsequent build processes.

<FIG> illustrates a second variant of the above embodiment of <FIG>. In this variant, the reference layers 50_1 and 50_2 have been defined with radiation-absorbing material within the build bed surface <NUM>. Instead of the moveable heat source <NUM> of <FIG> located at a fixed calibration position during the calibration routine, in <FIG> a fixed overhead heater <NUM> is the heat source operated to heat the reference areas. The carriage <NUM> is not shown in this Figure as it has been moved out of the field of view of the overhead heater <NUM> above the build bed surface. The reference areas 50_1 and 50_2 can thus be heated and monitored in an unobstructed manner by the overhead heater <NUM> and by the fixed temperature sensor <NUM>.

Where the fluid deposited to define the reference areas <NUM> comprises radiation absorber and the particulate material in regions surrounding the references areas does not comprise absorber, the calibration layer <NUM> may be supported on a base layer <NUM> of particulate material comprising one or more base reference areas <NUM>. <FIG> shows two base reference areas 54_1, 54_2 that have been fused after being defined by depositing radiation-absorbing material over them and fusing them, for example by passing a moveable heat source (not shown but analogous to heat source <NUM> in <FIG>, for example) over the build bed surface <NUM> to apply heat to the base reference areas 54_1, 54_2. After this, the calibration layer <NUM> is distributed over the base layer <NUM> to form the build bed surface <NUM>. In cases where the radiation emitted by the overhead heater <NUM> is not selective and thus may be slow to heat the reference area <NUM>, the fused base reference areas 54_1, 54_2 may be used to raise the base temperature of the reference areas 50_1, 50_2 at the beginning of the heating step, to reduce the temperature difference to the onset of fusion. The method may be further optimised by adjusting the amount of radiation-absorbing fluid that is deposited per unit area over the base reference area with respect to the reference area(s). The amount of radiation-absorbing fluid deposited per unit area over the reference area <NUM> may for example be lower than that used to fuse the base reference area <NUM>, so as to reduce the rate of heating and improve accuracy of detection of the onset of fusion of the reference area.

In the second variant of <FIG>, the sensor <NUM> is fixedly located within the area of the overhead heater <NUM>. The overhead heater <NUM> may comprise individually controllable heating elements arranged in a regular or irregular array over the heater area. While the overhead heater <NUM> applies radiation to the build bed surface <NUM> during the calibration routine, the area surrounding the reference areas heats up more slowly than the reference areas 50_1, 50_2, and the reference areas 50_1, 50_2 are fused before the area surrounding the reference areas.

<FIG> illustrates a third variant of the first embodiment that may be used to carry out an improved calibration routine. Unlike the first two variants, this variant uses a moving heat source <NUM> to achieve fusion in the reference layer <NUM>. As before, the reference layer <NUM> is defined by depositing absorber such that, during irradiation by the moving heat source <NUM>, the reference area <NUM> absorbs heat faster than the surrounding area.

The heat source is mounted to a movable carriage <NUM> arranged to move back and forth across the build bed surface <NUM>, and may be one or the same that, during a normal build process, carries out the fusing step of the cross-sections of the object, or it may be a preheat lamp used to preheat a newly deposited layer of powder before and/or after the deposition of modifying fluid, such as absorber fluid, and before the fusing step, but operated at a different power to achieve fusion.

In this variant, the time to fusion may be controlled by one or more of the following:.

In the variant in <FIG>, the heat source <NUM> is mounted in a preferable position so that it extends along one end of the carriage <NUM>. In some apparatus the end of the carriage extends along the width of the build bed surface. Depending on the direction of travel across the build bed surface <NUM> (as indicated by the arrows in <FIG>) the heat source <NUM> extends along the leading or trailing end of the carriage <NUM>. The effect of the periodic obstruction of the field of view of the sensor <NUM> located above the build bed surface <NUM> and above the heat source <NUM> is that the onset of fusion is obstructed from the sensor's view by the heat source at least. Thus only an indirect detection is possible by extrapolation of the measured temperatures. The field of view may be improved by increasing the distance between the heat source and the build bed surface.

In some implementations, this distance may be adjusted dynamically in synchronisation with the movement of the heat source, while adjusting the power output of the heat source dynamically to achieve a consistent heating effect by the heat source (e.g. the larger the distance between the heat source and the build bed surface, the higher the duty cycle). In this arrangement using a moving heat source <NUM>, multiple reference areas 50_n located at different locations across the build bed surface may be heated and measured in a relatively quick procedure (in comparison to multiple procedures necessary to capture similar locations when involving a moveable heat source in a fixed calibration position).

In the above variants, the calibration procedure is stopped once the reference area(s) <NUM> (or the direct sub-reference areas comprised within the reference area(s)) have begun to fuse, or soon after. In any case it desirable to discontinue the calibration routine before the area surrounding the reference areas fuse.

In the described variants of <FIG>, <FIG>, <FIG> and <FIG>, different combinations of radiation spectra and absorptive properties of the calibration layer and of the reference area <NUM> may be suitable. For example, the selectivity of the particulate material to a particular radiation spectrum may be achieved by depositing onto the calibration layer a radiation-absorbing fluid to define the reference area <NUM>. During the calibration routine, the radiation spectrum of the heat source <NUM> is chosen so that it is preferentially absorbed by the radiation-absorbing fluid. The reference area <NUM> thus heats up more rapidly than the area surrounding the reference area <NUM> and fuses ahead of the surrounding area.

Alternatively, the particulate material may comprise radiation-absorbing material that preferentially absorbs radiation from the spectrum of the heat source, and the fluid that is deposited inhibits absorption of the radiation spectrum emitted by the heat source. Such an absorption inhibitor may be deposited over the area surrounding the reference area <NUM>, so that the reference area <NUM> heats up more rapidly than the area surrounding the reference area <NUM> and fuses ahead of the surrounding area. Thus, generally, different combinations of absorption-modifying fluid providing selectivity to absorption of radiation over certain areas of the build bed surface <NUM> and the absorptive properties of the particulate material may be chosen, and for these the herein-described calibration procedures are equally applicable.

While the variants herein are illustrated using a temperature sensor to determine the onset of fusion, an optical sensor <NUM> as shown in <FIG> may instead be used to detect the onset of fusion, or any sensor arranged to detect a characteristic change in properties of at least part of the reference area that indicates the onset of fusion. The onset thus detected may then be used to calibrate the temperature scale of a temperature sensor in the apparatus to the onset of fusion, so that subsequent processes may be carried out using a temperature sensor calibrated to the onset of fusion of the particulate material.

Therefore, a method for determining a set point for measurements from a temperature sensor <NUM> of an apparatus for the layer-by-layer formation of a three-dimensional object from particulate material is provided, the method comprising:.

The expression "absorption-modifying fluid" refers to fluids that, when applied to particulate material, modify the absorption of radiation from the heat source by the particulate material, and thus the rate of heat absorption and fusion. For example, the absorption-modifying fluid may be a radiation-absorbing fluid that promotes the absorption of heat by the particulate material when exposed to radiation of the heat source. Alternatively, the absorption-modifying fluid may be an absorption-inhibiting fluid, which inhibits the absorption of heat by the particulate material when exposed to radiation of the heat source.

The heat source may for example be located over the build bed surface <NUM>, at a fixed or non-fixed position. For example, in some variants such as those described with respect to <FIG>, <FIG> and <FIG>, the heat source is a moveable heat source <NUM>. When using a heat source <NUM> moveable across the build bed surface <NUM> to heat the reference area <NUM>, the step of applying heat to the reference area <NUM> may be preceded by a step of lowering the build bed surface <NUM> by a calibration depth. The calibration depth defines a calibration distance D between the moveable heat source <NUM> and the build bed surface <NUM> that is greater than a build distance at which the object is being built in a normal operational state of the apparatus during which an object is formed across the build bed surface. The calibration distance may be greater than <NUM>. By setting the calibration depth to the calibration distance, the energy of the moveable heat source at the build bed surface is reduced compared to the normal operating distance. Depending on other properties used to lower the energy at the build bed surface, and at the reference area, such as reducing the power of the heat source by adjusting the duty cycle, or by adjusting (reducing) the amount of radiation absorber provided to the reference area for example, the calibration distance may be up to <NUM>. The calibration distance is the minimum distance between the heat source <NUM> and the build bed surface <NUM>. Preferably, the calibration distance D may be between <NUM> and <NUM>, or <NUM> and <NUM>, or between <NUM> and <NUM>, or between <NUM> and <NUM>.

Optionally, as for example described for the calibration routine illustrated in <FIG>, after the step of lowering the build bed surface <NUM> and before the step of applying heat to the reference area <NUM>, the moveable heat source <NUM> may be moved to a calibration position and remain stationary for the duration of the steps of applying heat to the reference area <NUM> and measuring, using the temperature sensor <NUM>, the temperature increase of the reference area <NUM> (and/or when using the optical sensor <NUM>, taking readings in respect of the surface of the reference area) over the duration of time. In such a variant, the field of view of the sensor <NUM> arranged to monitor the reference area <NUM> is not obstructed by the heat source <NUM> during the step of heating, and the onset of fusion may thus be accurately captured by the sensor <NUM>. This provides improved accuracy of determining the onset of fusion compared to using a moving heat source. For example, the moveable heat source <NUM> may be located off to one side of the reference area <NUM>, or the moveable heat source <NUM> may be located in a calibration position with the build bed surface at a calibration depth defining a calibration distance D that, in combination, allow the sensor <NUM> to continuously view a direct sub-reference area of the reference area <NUM> that is located substantially directly underneath the moveable heat source.

In some apparatus, where the sensor is located centrally above the build bed surface, the calibration position of the moveable heat source may be near an end of the build bed surface.

In some variants, such as the one described with reference to <FIG>, instead of being moved to a calibration position, for the duration of the steps of applying heat to the reference area <NUM> and measuring, using the temperature sensor <NUM>, the temperature increase of the reference area <NUM> over time (and/or when using the optical sensor <NUM>, taking optical readings in respect of the surface of the reference area), the moveable heat source <NUM> may repeatedly be moved across the reference area <NUM> to apply heat to the reference area. The sensor may be located fixedly above the build bed surface <NUM> and the distance between the build bed surface and the moveable heat source <NUM> may be adjusted to a calibration distance D by lowering the build bed surface <NUM> by a calibration depth. The calibration distance D may remain constant for the duration of the heating step. In other implementations of this variant, the calibration distance D may be adjusted dynamically as the moveable heat source moves across the reference area. In other words, the calibration depth D is adjusted synchronously with the motion of the moveable heat source. This may improve the instances over which the sensor's field of view encompasses the direct sub-reference area located substantially directly underneath the moving heat source, where the direct sub-reference area is comprised within the reference area <NUM>. Calibration depth adjustments may be based on position information of the heat source <NUM> over the build bed surface, for example from the carriage encoder.

A dynamic adjustment of the calibration distance may in turn be synchronised to a dynamic adjustment of the power output of the heat source <NUM>. Specific combinations of calibration depth and power output may both be based on encoder position information.

The heat source <NUM> may be an elongate heat source spanning the length of the end of the carriage <NUM>, and/or spanning the width of the build bed surface <NUM>. Furthermore, the heat source <NUM> may be a near-infrared lamp, and one of the absorption-modifying fluid and the particulate material may comprise a near-infrared radiation-absorbing material.

Alternatively, the particulate material may comprise radiation-absorbing material and the absorption-modifying fluid may comprise absorption-inhibiting material that is applied to the area surrounding the reference area so as to inhibit fusion of the particulate material upon heating.

In variants such as the one illustrated in <FIG>, instead of using a moveable heat source <NUM> to apply heat to the reference area <NUM>, the heat source may be an overhead heater <NUM> fixedly located above the reference area <NUM>.

In some of the above variants, a further level of control over the rate of heating may be provided by first distributing a base layer <NUM> of particulate material before distributing the calibration layer <NUM>. Before distributing the calibration layer <NUM>, the base layer <NUM> may be modified by defining and fusing a base reference area <NUM> comprised on the base layer <NUM>.

The reference area <NUM> may overlap with or be coincident with the fused base reference area <NUM>. This may provide an elevated base temperature to the reference area <NUM> of the calibration layer <NUM>.

In such variants, both the reference area <NUM> and the base reference area <NUM> are provided with radiation-absorbing fluid in the case where the particulate material does not comprise radiation absorber.

Therefore, optionally, in some variants in which the absorption-modifying fluid is radiation-absorbing fluid that has been deposited over the reference area <NUM>, before the step of distributing the calibration layer <NUM>, the method further comprises the steps of: distributing a base layer <NUM> of particulate material over the build bed to form the build bed surface <NUM>; applying radiation-absorbing fluid to the base reference area <NUM>, wherein the base reference area <NUM> overlaps at least partially with the reference area <NUM>, and fusing the base reference area <NUM>, using a heat source <NUM>, <NUM>, before the calibration layer <NUM> is distributed. The heat source <NUM>, <NUM> may be the same heat source <NUM>, <NUM> that is used for fusing the reference area <NUM>, or it may be a different heat source. The amount of radiation-absorbing fluid per unit area applied to the reference area <NUM> may be different to the amount of radiation-absorbing fluid per unit area applied to the base reference area <NUM> so as to modify the amount of heating of the reference area <NUM>. For example, by reducing the amount of absorber content in the base reference area <NUM> compared to the reference area <NUM>, the effect of heating of the reference area provided by the base reference area <NUM> may be reduced. In this way the impact of the radiation spectrum of the heat source <NUM>, <NUM> may be tuned to the rate of heating required for accurate determination of the onset of fusion.

The method steps may be carried out by the apparatus <NUM> as described in the Figures and comprising a controller <NUM> configured to determine the onset of fusion of the particulate material from the measured temperature increase over time of the reference area <NUM> (and/or when using the optical sensor <NUM>, from readings by the sensor of an optical property of the surface of the reference area); and to optionally apply the onset of fusion as a set point for the temperature measurements of the temperature sensor <NUM>.

The controller <NUM> may further be configured to control the heat source <NUM>, <NUM> so as to initiate the step of heating the reference area <NUM>. The controller <NUM> may further be configured to control the temperature sensor <NUM> to initiate the step of monitoring the temperature of the reference area <NUM> and to provide temperature measurements by the sensor <NUM> to the controller <NUM> (and/or to control an optical sensor <NUM> to initiate the step of taking readings of the surface of the reference area and to provide the reading to the controller <NUM>). Furthermore, the controller <NUM> may be configured to control the build floor depth so as to lower the build floor to the calibration depth, and to control the position of the moveable heat source <NUM>, for example by controlling the position of the carriage <NUM> supporting the heat source <NUM>, for example to move the moveable heat source <NUM> to the fixed calibration position. The controller may further be configured to control the deposition module to deposit absorption-modifying fluid over the calibration layer <NUM> so as to define the reference area <NUM>. The controller may further be configured to control a distribution module to distribute a new layer (a base layer or a calibration layer) over the build bed <NUM>.

The steps carried out by the controller <NUM> configured to control the apparatus <NUM> during the calibration routine according to the first embodiment are illustrated in the block diagram of <FIG>. At step <NUM>, the controller <NUM> is configured to initiate the calibration routine. For the variants of the first embodiment, at step <NUM>, the controller <NUM> is configured to control a distribution module to distribute a calibration layer <NUM> across the build bed <NUM> to form the build bed surface <NUM>.

At step <NUM>, the controller <NUM> is configured to receive data defining the reference area <NUM> or a plurality of reference areas 50_n (e.g. 50_1 and 50_2).

At step <NUM>, the controller <NUM> is configured to control a fluid deposition module to deposit absorption-modifying fluid selectively onto the build bed surface <NUM> so that only the reference area <NUM> is heated and such that at least a sub-reference 50A area comprised within the reference area <NUM> is heated to at least the onset of fusion during the heating step <NUM>. During the step of heating the reference area, the controller is further configured at step <NUM> to control a sensor to monitor the reference area (or a sub-reference area comprised within the reference area) for the duration of time over which the reference area is being heated. The sensor may be a temperature sensor <NUM> monitoring the temperature increase of the reference area or sub-reference area, and/or or it may be an optical sensor <NUM> to take readings of the surface colour or reflectivity.

The absorption-modifying fluid may be radiation-absorbing fluid applied to the reference area <NUM>, or it may be radiation-inhibiting fluid applied to the area surrounding the reference area <NUM> in the case where the particulate matter comprises radiation-absorbing material.

At step <NUM>, the controller may be configured to control one or more of the build bed height, the heat source position, motion and/or duty cycle to modify the heat impact on the reference area during the step <NUM> of heating the reference area until it fuses.

For example, for the variants of <FIG>, <FIG> and <FIG>, the controller may be configured to control a means for lowering and raising the build bed floor, such that the build bed surface <NUM> is lowered to a calibration depth and the distance between the build bed surface <NUM> and the heat source is a calibration distance D. This reduces the heat impact on the reference area <NUM> compared to the normal operating distance between the build bed surface <NUM> and the heat source <NUM>, <NUM> when an object is built. This variant of modifying the heat impact is illustrated in <FIG> with focus on step <NUM> in which the build bed surface is lowered. Optionally, at step <NUM>, the heat source <NUM> is a moveable heat source and the controller is configured to move the heat source to the calibration position where it remains stationary for the duration of time over which the reference area is being heated. Furthermore, other measures may be applied to further reduce the heat impact, such as lowering the duty cycle of the heat source <NUM>, <NUM>. The controller <NUM> may further be configured to control a carriage <NUM> to move the moveable heat source <NUM> to the calibration position.

For the variants of <FIG>, the controller <NUM> may be configured to modify the heat impact on the reference area <NUM> as necessary. With regard to <FIG>, the controller <NUM> may be configured to modify the duty cycle of the heat source <NUM>, <NUM>. The controller may additionally, or instead, be configured to dynamically modify the distance between the heat source <NUM> and the build bed surface <NUM> (i.e. the 'calibration distance D') in synchronisation with the position of the moveable heat source <NUM> as the moveable heat source passes across the build bed surface, and across the reference area <NUM>.

With regard to the variant of <FIG>, the effectiveness of heating by the stationary overhead heater <NUM> may be improved by providing a fused base reference area <NUM> underneath the reference area <NUM> before starting the step of heating at step <NUM>, thus increasing the impact of the radiation by the heat source <NUM>, <NUM>. The necessary steps in this case will have to precede the distribution of the calibration layer <NUM>, and are indicated as step <NUM> in dashed outline of providing a base layer before the calibration layer <NUM> is deposited.

The individual steps are illustrated in the block diagram in <FIG> in dashed outline and may be inserted between steps <NUM> and <NUM> of <FIG>.

Thus, with reference to <FIG>, steps <NUM> to <NUM> provide for the controller <NUM> being configured to: control the distribution module to distribute a base layer of particulate material at step <NUM>; receive data defining a base reference area <NUM> at step <NUM>; at step <NUM> to control a fluid deposition module <NUM> to apply radiation-absorbing fluid to the base layer <NUM> to define the base reference area <NUM>; and at step <NUM> to control a heat source <NUM> to fuse the base reference area <NUM>.

In variants in which a fused base reference area <NUM> is provided as part of the calibration routine, the absorption-modifying fluid comprises absorber and the particulate material does not.

In all variants therefore, the step <NUM> of modifying the heat impact on the reference area <NUM> may be a combination of suitable measures to adjust the rate of heating of the reference area <NUM>, and may comprise step <NUM>.

Following step <NUM>, or where step <NUM> is replaced by step <NUM> proceeding directly from step <NUM> of selectively applying absorption-modifying fluid, the controller <NUM> is configured to simultaneously control the steps <NUM> of heating the reference area <NUM> and of controlling the sensor to acquire data such as temperature measurements from the temperature sensor <NUM>, and/or to acquire optical readings from an optical sensor <NUM>.

The step <NUM> of heating the reference area <NUM> may either comprise the controller <NUM> being configured to: control the heat source <NUM> so as to heat at least a portion of the build bed comprising the reference area <NUM> (for example with respect to the variant of <FIG> having a moveable heat source <NUM> positioned in the calibration position, and optionally for the variant of <FIG> having a moving heat source <NUM>); or to control a heat source <NUM>, <NUM> so as to heat the build bed surface <NUM> (for example with respect to the variant of <FIG> having a static overhead heater and optionally for the variant of <FIG>).

At step <NUM> the controller <NUM> is configured to receive temperature data from the sensor <NUM> and at step <NUM> to determine the onset of fusion from the temperature data (and/or to receive optical readings from the optical sensor <NUM> to determine the onset of fusion). These steps may be concurrent with the step of heating and acquiring data from the sensor(s), or they may happen subsequent to a timed step <NUM> of heating the reference area <NUM>.

Once the onset of fusion has been determined, the controller <NUM> may be configured to apply the onset of fusion as a set point for the temperature measurements of the temperature sensor <NUM> at an optional step <NUM>. For example, where the onset of fusion is detected for a specific temperature measurement, the onset of fusion corresponds to a specific point of the temperature scale of the sensor measurement, and the onset of fusion may be used as a set point to calibrate the temperature scale for any subsequent measurements made by the temperature sensor.

In a second embodiment, the reference area is an area substantially free of radiation absorber such that it does not preferentially fuse over the surrounding area in the absence of other measures being taken. A 'blank' reference area may be used to reduce the rate at which the reference area is heated by the heat source if the absorption spectrum of the reference area does not substantially coincide with the radiation spectrum of the heat source.

In a suitable calibration routine, selectivity of the reference area for the preferential absorption of heat is provided by a fused base reference area <NUM> defined within a base layer <NUM> that is to support the calibration layer <NUM>.

<FIG> illustrate an apparatus <NUM> (substantially as previously described with reference to <FIG>) configured to carry out stages of an improved calibration routine according to the invention. <FIG> shows a base layer <NUM> distributed over the build bed <NUM> to form the build bed surface <NUM>. The base layer <NUM> is next modified by defining and fusing a base reference area <NUM> comprised within the base layer <NUM>. The base reference area <NUM> may for example be defined by radiation-absorbing fluid. This may be achieved by a printing module (not shown) supported on the carriage <NUM> which deposits radiation-absorbing fluid over the base reference area <NUM> as the carriage passes over the build bed surface <NUM>.

Next, the apparatus may use a moveable heat source <NUM> supported on the carriage <NUM> to heat the base reference layer to achieve fusion, in any suitable manner. For example, the heat source <NUM> may pass across the build bed surface <NUM> while the heat source <NUM> is operated to apply heat.

In <FIG>, the build bed <NUM> is lowered by a layer thickness by lowering the build bed floor <NUM>, and a fresh layer of particulate material applied, which is to represent the calibration layer. The surface of the calibration layer forms the build bed surface <NUM>. In this embodiment the reference area <NUM> is coincident with, and defined by, the fused base reference area <NUM>.

In <FIG>, the build bed is lowered to a calibration depth, by lowering the build bed floor <NUM>. The build bed <NUM> may be lowered by more than a layer thickness, for example by several millimetres or tens of millimetres, so as to create a minimum calibration distance D between the heat source <NUM> and the build bed surface <NUM>. The minimum calibration distance D may range from <NUM> to <NUM>, or preferably from <NUM> to <NUM>, from <NUM> to <NUM>, or more preferably still from <NUM> to <NUM>, when the heat source is located above the build bed <NUM>.

In <FIG>, the heat source is moved, similar to the step of the variant of the first embodiment described with respect to <FIG>, to a calibration position where it remains stationary for the duration of the step of heating the reference area <NUM>. The calibration position is located off to one side of the reference area so as not to obscure the field of view of the sensor <NUM> as it monitors the temperature of the reference area <NUM>. In some implementations, the calibration distance and the calibration position may be adjusted such that the reference area <NUM> extends to an area underneath the moveable heat source <NUM> while still being visible to the sensor <NUM>.

During the heating step, the fused base reference area <NUM> provides an elevated base temperature to the reference area <NUM> within the calibration layer. Furthermore, the base reference area <NUM> provides a degree of selectivity to the reference area <NUM> over the area surrounding the reference area and which is not supported by a fused based layer area, and aids in heating the blank reference area so that upon irradiation by the heat source, the reference area <NUM> will reach the fusion temperature ahead of the surrounding area. The amount of absorber deposited over the base reference area per unit area may be adjusted to adjust the rate of heating of the reference area <NUM> during the heating step.

Similar to the variant of <FIG> of the first embodiment, the radiation from the moveable heat source <NUM> positioned in the fixed calibration position can continuously provide energy to the reference area <NUM> without interruption, and the time-temperature behaviour from the sensor measurements do not have to be extrapolated to compensate for the effects of the passing heat source <NUM>. Thus the sensor measurements over the duration of the calibration routine can be used to accurately determine the onset of fusion over the reference area <NUM>.

The rate of heating of the reference area <NUM> and the accuracy of determining the onset of fusion may be adjusted suitably by adjusting the distance between the heat source <NUM> and the build bed surface <NUM> in the vertical direction (along z), and/or between the heat source <NUM> and the reference area <NUM> in the lateral direction (along x), and/or by adjusting the duty cycle of the heat source <NUM>. This embodiment therefore also provides several parameters that allow tuning the calibration procedure by slowing down the rate of heating to achieve the desired accuracy.

The heat source <NUM> in <FIG>, in analogy to the one shown in <FIG>, is elongate and spans the width of the build bed surface <NUM>, so that the reference area <NUM> receives the same level of heating from the heat source <NUM> along a line of constant distance to the heat source <NUM>. The reference area <NUM> may have a sufficiently large dimension over which the distance between the build bed surface <NUM> and the heat source varies linearly, which in analogy to <FIG> is along the direction orthogonal to the direction of elongation of the heat source (along the x-direction). The regions of the reference area <NUM> closest to the heat source <NUM> will fuse first and the regions furthest away will fuse last, and a progressive onset of fusion may be monitored against distance over the reference area <NUM> away from the heat source <NUM>.

More than one reference area may be defined and fused in the same calibration routine while the sensor monitors the temperature of all the defined reference areas. The reference areas may be arranged near the edges of the build bed <NUM> or at regular intervals covering the build bed width along the direction of elongation of the heat source <NUM>.

Therefore an alternative calibration method for calibrating the measurement of a temperature sensor <NUM> of an apparatus <NUM> for the layer-by-layer formation of a three-dimensional object from particulate material, comprises the steps of:.

The reference area <NUM> in this embodiment is defined by the base reference area <NUM> and thus is coincident with the base reference area <NUM>. Steps (a) to (c) may be repeated to provide more than one base reference layer and a corresponding stack of more than one fused base reference area. The heat source may be located over the build bed surface <NUM>, and may be a moveable heat source <NUM>.

The step of operating the heat source <NUM> to apply heat to the reference area <NUM> may be preceded by the steps of lowering the build bed surface <NUM> to a calibration depth; and positioning the moveable heat source <NUM> at a calibration position above the build bed surface <NUM>; wherein the calibration depth defines a calibration distance D between the heat source <NUM> and the build bed surface <NUM>. The calibration distance may be greater than <NUM>, and may range from <NUM> to <NUM>, and the step of applying heat by the moveable heat source <NUM> to the reference area <NUM> is carried out while the moveable heat source <NUM> remains in the calibration position. Preferably, the calibration distance D ranges from <NUM> to <NUM>, <NUM> to <NUM>, <NUM> to <NUM> or from <NUM> to <NUM>. As before, the calibration distance depends on other properties that may also be used to reduce the energy input to the build bed surface, such as reducing the power of the heat source and/or reducing the amount of infrared radiation absorber applied to the base reference area <NUM>.

It should be noted that the order of steps of lowering the build bed surface and positioning the heat source in the calibration position are interchangeable; in other words the build bed surface <NUM> may be moved to the calibration depth after the moveable heat source <NUM> is positioned in the calibration position above the build bed surface <NUM>.

The reference area may comprise a direct sub-reference area located directly underneath the moveable heat source <NUM>, and the calibration position and the calibration distance D may be arranged to allow the sensor <NUM> to view the direct sub-reference area. The field of view over which the reference area is not obstructed by e.g. the moveable heat source <NUM> may be improved by increasing the distance between the heat source and the build bed surface, analogous to the description of <FIG> which equally applies to the arrangement of <FIG>.

Additionally, or instead, the reference area <NUM> may be located off to one side of the calibration position, or the reference <NUM> may comprise a sub-reference area located off to one side of the direct sub-reference area when the heat source is in the calibration position, so that the sensor is measuring the temperature of an area within the reference area that is not located directly underneath the moveable heat source <NUM>.

In a second variant of the second embodiment, instead of keeping the moveable heat source at a fixed calibration position for the duration of the heating and measuring steps, the step of operating the heat source to apply heat to the reference area is preceded by the step of lowering the build bed surface to a calibration depth, wherein the calibration depth defines a calibration distance D between the heat source <NUM> and the build bed surface <NUM> and may range from <NUM> to <NUM>, or from <NUM> to <NUM>; and wherein the step of applying heat by the moveable heat source <NUM> to the reference area <NUM> is carried out while the moveable heat source is repeatedly moved across the reference area <NUM> to heat the reference area at least until it begins to fuse.

Additionally, instead of maintaining a constant calibration distance D during the heating and measuring steps, the calibration distance D may be dynamically adjusted as the heat source is moved across the reference area, optionally while the power output of the heat source <NUM> is dynamically adjusted with the variation in calibration distance D. The calibration distance D and optionally also the power output of the heat source may thus be dynamically adjusted as the heat source is repeatedly moved across the reference area. For example, dynamic adjustment may be based on encoder position information of the carriage to which the moveable heat source is mounted. By adjusting both calibration distance D and the power output of the heat source, a consistent heating effect by the heat source may be achieved (e.g. the larger the distance between the heat source and the build bed surface, the higher the duty cycle) while allowing the sensor to detect the direct sub-reference area underneath the moving heat source <NUM>, where the direct reference area is comprised within the reference area <NUM>. In this arrangement using a moving heat source <NUM>, multiple reference areas 50_n located at different locations across the build bed surface may be heated and measured in a relatively quickly procedure compared to multiple procedures necessary to capture similar locations when involving a moveable heat source in a fixed calibration position.

The moveable heat source <NUM> may be a near-infrared lamp with a spectrum that overlaps at least partially with the absorption spectrum of the radiation-absorbing fluid. The moveable heat source <NUM> may be the same as the heat source used to fuse the base reference area, and operated at a reduced duty cycle of below <NUM>% for example. The moveable heat source <NUM> may for example be operated to output a power of <NUM> Watts or less, or <NUM> Watts or less. As with the variant of <FIG>, the heating effect on the build bed surface may be modified by adapting the speed at which the heat source travels across the build bed surface, or by adjusting the amount of radiation-absorbing fluid in the base reference area.

The methods of the second embodiment may be carried out by an apparatus <NUM> for the manufacture of a three-dimensional object by layer-by-layer deposition of particulate material having a controller <NUM> configured to carry out any of the steps of the calibration routine described.

<FIG> is a block diagram illustrating the capability of such a controller <NUM>. The controller is configured to, at step <NUM>, initiate the calibration routine.

At step <NUM> the controller is configured to control the apparatus to provide a base layer <NUM> having a fused base reference area 54_n, the individual steps of which are illustrated in the block diagram in <FIG> and are inserted between steps <NUM> and <NUM> of <FIG>. Thus, with reference to <FIG>, at step <NUM> the controller is configured to control a distribution module to distribute the base layer <NUM> of particulate material.

At step <NUM>, the controller is configured to receive data defining a base reference area <NUM> that is to coincide with the reference area <NUM>, and at step <NUM> to control a fluid deposition module <NUM> to apply radiation-absorbing fluid to the base layer <NUM> to define the base reference area <NUM> based on the data received. At step <NUM>, the controller is configured to control a heat source <NUM> to apply heat to the base reference area <NUM> so as to fuse it.

Returning to <FIG>, next, at step <NUM>, the controller <NUM> is configured to control the distribution module again to distribute a calibration layer <NUM> of particulate material across the build bed <NUM> to form a new build bed surface <NUM>.

At an optional step <NUM>, the controller may be configured to control one or more of the build bed height, the heat source position and heat source duty cycle to modify the heat impact on the reference area during the step <NUM> of controlling the heat source to heat the reference area at least until it, or at least until a sub-reference area comprised within the reference area, begins to fuse. For example for the variant of <FIG>, the controller may be configured to control the height of the build bed floor so as to lower the build bed surface <NUM> to a calibration depth and to control the position of the heat source <NUM> (for example by controlling the carriage <NUM>) so as to locate it at the calibration position, such that the distance between the build bed surface <NUM> and the heat source is a calibration distance D. This reduces the heat impact on the reference area <NUM> compared to the normal operating distance between the build bed surface <NUM> and the heat source when an object is built. This variant is illustrated in <FIG> and the same description for steps <NUM> and <NUM> apply. Furthermore, other measures may be applied to further reduce the heat impact, such as lowering the duty cycle of the heat source.

Following optional step <NUM>, the controller is configured to simultaneously control the heat source <NUM>, <NUM> and the sensor <NUM> to carry out step <NUM> of heating the reference area <NUM> and step <NUM> of acquiring data from the temperature sensor <NUM> and/or the optical sensor <NUM>.

The step <NUM> of heating the reference area <NUM> may either comprise the controller <NUM> controlling the heat source <NUM>, <NUM> so as to heat at least a portion of the build bed surface <NUM> comprising the reference area <NUM> (as with the variant of <FIG> having a moveable heat source <NUM> positioned in the calibration position) or controlling the fixed overhead heat source <NUM> to heat the build bed surface <NUM>.

At step <NUM> the controller <NUM> is configured to receive temperature data from the sensor <NUM> and at step <NUM> to determine the onset of fusion from the temperature data. These steps may be concurrent with the step <NUM> of heating and step <NUM> of acquiring data, or they may happen subsequent to the duration of time over which step <NUM> of heating the reference area <NUM> is applied.

Once the onset of fusion has been determined, the controller <NUM> may further be configured at step <NUM> to apply the onset of fusion as a set point or calibration reference for the temperature measurements of the sensor <NUM>. For example, where the onset of fusion is detected for a specific temperature measurement, the onset of fusion corresponds to a specific sensor measurement, for which the onset of fusion may be used as a set point to calibrate the temperature scale of any subsequent measurements made by the sensor.

For the calibration methods described with reference to <FIG>, <FIG>, <FIG> and <FIG>, in which the build bed surface <NUM> is lowered and the heat source is a moveable heat source <NUM>, the apparatus <NUM> may be controlled by a controller <NUM> that is configured to:.

The moveable heat source may be mounted to a carriage <NUM> arranged to move across the build bed surface <NUM>, and the controller <NUM> may further be configured to, before controlling the heat source to heat the reference area at least until the reference area begins to fuse: control the carriage <NUM> to move the heat source <NUM> to a calibration position above the build bed surface <NUM>, wherein the calibration depth defines a calibration distance D between the heat source and the build bed surface <NUM>; and control the heat source at the calibration position to heat the reference area <NUM> at least until it begins to fuse.

As before, the calibration distance D may range from <NUM> to <NUM>, and may preferably range from <NUM> to <NUM>, or more preferably from <NUM> to <NUM>, from <NUM> to <NUM>, or from <NUM> to <NUM>. The droplet deposition module and the heat source may be supported on the same carriage <NUM>.

The controller may further be configured to receive at least one calibration depth and one calibration position, and optionally one sensor position, that are determined such that when a direct sub-reference area comprised within the reference area <NUM> is located substantially directly underneath the moveable heat source, and the sensor has a field of view that comprises the direct reference area. In other words, in this arrangement the direct sub-reference area is not obstructed from the view of the sensor and the sensor <NUM> can continuously capture temperature measurements (and/or the optical sensor <NUM> can take readings continuously) from the direct sub-reference area during the calibration procedure.

The controller <NUM> may further be configured to receive position dependent temperature measurements from the sensor that correspond to different positions measured within the reference area. For example, the controller may receive measurements from the sensor that correspond to a sub-reference area located off to one side of the calibration position of the heat source, where the sub-reference area is comprised within the reference area <NUM>. Additionally or instead, the controller may be configured to receive position dependent temperature measurements (or optical readings) from the sensor that correspond to different positions measured within multiple reference areas 50_n.

In variants in which a moveable radiation source is used to repeatedly move across the reference area <NUM> to heat it until it begins to fuse, the controller may further be configured to control the depth of the build bed surface <NUM> (in the vertical direction) so as to define a minimum calibration distance D between the moving heat source <NUM> and the build bed surface <NUM>, and to control a carriage <NUM> supporting the moveable heat source <NUM> and moveable across the build bed surface <NUM> to repeatedly move the moveable heat source <NUM> back and forth across the reference area <NUM> to heat at least a sub-reference area comprised within the reference area until the sub-reference area begins to fuse. The calibration distance is greater than a normal operating depth during which an object is built in the apparatus and may range from <NUM> to <NUM>, or from <NUM> to <NUM>, and may remain fixed for the duration of controlling the heating and measuring of the reference area <NUM>.

The controller <NUM> may additionally be configured to dynamically adjust the calibration distance D and the power output of the heat source <NUM> as the heat source is moved across the reference area <NUM>, in synchronisation with the position of the heat source <NUM> above the build bed surface <NUM>. The controller may for example be configured to receive position information from the carriage encoder that relates to the position of the heat source above the build bed surface, and to adjust the calibration depth and the power output of the moveable heat source such that the calibration distance and the power output of the heat source are synchronised with the position of the heat source above the build bed surface.

The controller <NUM> may cause the sensor <NUM> to measure the temperature of one or more direct sub-reference areas comprised within the reference area <NUM> and that are located periodically directly underneath the moving heat source as it passes back and forth over the reference area <NUM>, and to receive measurements from the one or more direct sub-reference areas. The controller <NUM> may further be arranged to extrapolate the measurements received from the sensor <NUM> to determine the onset of fusion from the one or more direct sub-reference areas.

In some variants of the calibration routine, the controller may be arranged to control a fluid deposition module <NUM> to apply absorption-modifying fluid to the reference area <NUM>. The fluid deposition module <NUM> may for example deposit radiation-absorbing material over the reference area <NUM> so that upon heating with the heat source, the reference area fuses ahead of the area surrounding the reference area.

In addition, the controller <NUM> may be arranged to control the apparatus <NUM> to provide a base layer <NUM> comprising a fused base reference area <NUM> ahead of controlling the distribution module to distribute the calibration layer <NUM>. In this case, the controller <NUM> is configured to control the distributor to distribute, at the start of the calibration routine, a base layer <NUM> of particulate material, and to control the deposition module <NUM> to apply radiation-absorbing fluid to a base reference area <NUM> within the base layer, for example based on data received by the controller that defines the location of the reference area <NUM>. The controller <NUM> may further be configured to control the moveable heat source <NUM> to pass over the base reference area <NUM> and to heat the base reference area <NUM> so as to fuse it.

In the embodiments and their variants described herein, it is not necessary for the area to be monitored by the sensor to be the same area as defined to represent the reference area. For example, the reference area as defined by radiation absorber may comprise a sub-reference area that is monitored by the sensor while the reference area is being heated. The sub-reference area may comprise the direct sub-reference area; alternatively the direct sub-reference area may be an area removed from the sub-reference area while both are comprised within the reference area. Defining a reference area <NUM> larger than a sub-reference area 50A to be monitored may be useful to allow a rough alignment of the heat source <NUM> above the build bed surface <NUM> once the build bed surface <NUM> has been lowered to the calibration depth, as the alignment of the sensor array with respect to the build bed surface at the normal build depth is liable to deviate in the x- and y-direction. In some cases however, the sub-reference area 50A may be the same (and coincident with) the reference area <NUM>.

It is therefore also not necessary to apply heat to the reference area by the heat source at least until the particulate material within the reference area <NUM> begins to fuse; it is sufficient that at least some of the particulate material within of the sub-reference area begins to fuse. In some variants, the process of heating is determined by the direct sub-reference area fusing and its eventual temperature.

Furthermore, the sensor used to monitor the reference area <NUM>, or the sub-reference area 50A comprised within the reference area <NUM>, may alternatively be an optical sensor <NUM> as indicated in <FIG> and arranged to detect a physical property such as a colour change or reflectance change of the surface of the sub-reference area (or reference area) while it is being heated over the duration of time. As the area undergoes fusion, a sudden change to a particular colour (in the case of where the radiation modifying fluid contains the absorber carbon black, a change to deep black) may be detected. Alternatively the sensor may be positioned to detect the specular reflection of a light beam directed at the area, the sensor detecting a change in the reflection response as the area undergoes fusion and turns from an unfused, rough surface into a liquid, reflective surface. The onset of fusion as detected by an optical sensor <NUM> may be used to calibrate the temperature scale of a thermal sensor also present in the apparatus by simultaneously monitoring the temperature increase with the temperature sensor for the duration of the time over which the sub-reference area (or reference area) is being heated. By comparing the time stamps between the data recorded by the optical sensor <NUM> and the temperature measurements by the temperature sensor <NUM>, the onset of fusion can be correlated to the corresponding temperature measurement and the temperature scale of the temperature sensor can be calibrated to the onset of fusion. The use of an optical sensor, and in combination with the temperature sensor where a temperature sensor requires calibration to the onset of fusion, is applicable to all variants irrespective of the way in which the sub-reference area, or reference area, is being heated.

Further still, once the onset of fusion has been determined, the onset of fusion may be applied as a set point for subsequent measurements by the sensor <NUM>. This may comprise applying the set point to the scale of the corresponding temperature map of the sensor that indicated the onset of fusion to a reference temperature, so that any subsequent temperature measurements made by the sensor are relative to the onset of fusion. This calibrated scale takes into account changes in the properties of the particulate material due to recycling rate and/or ageing of at least some of the components of the particulate material.

The onset of fusion as measured by the temperature sensor of the apparatus may be set to an absolute temperature equal to the inherent onset of fusion temperature. The absolute fusion temperature may have been determined by a separate measurement outside of the apparatus using for example a calorimeter, such as a differential scanning calorimeter, to calibrate the sensor to an absolute temperature scale.

The temperature sensor <NUM> may be a thermal camera that captures time-dependent (time-stamped) temperature maps of the build bed surface over the duration of the calibration routine. The thermal maps are analysed and from the time-temperature behaviour in the reference areas the onset of fusion may be determined. The temperature corresponding to the onset of fusion may applied as a set point to the temperature scale of the temperature maps of the sensor <NUM>.

In some apparatus, the heat source may be directly visible to the sensor. For example, the user may have removed the lamp housing, or the lamp housing may comprise one or more openings in its roof so as to be directly visible to the sensor. Direct vision of the heat source by the sensor may be beneficial as follows. In routines in which the build bed surface is lowered, variation in the position of the build bed surface with respect to the sensor may occur due to the container walls <NUM> not being perfectly vertical, leading to a change in the x and y positions of the build bed surface with respect to the sensor. After lowering the build bed surface to the calibration distance, it is desirable to determine which of the sensor pixels are to monitor a region within the direct sub-reference area that is located substantially directly underneath the heat source and that will experience the highest power from the heat source due to being closest to it.

With reference to <FIG>, the moveable heat source <NUM> has been moved to the calibration position above the reference area <NUM> and the build bed surface <NUM> has been lowered to the calibration depth defining the calibration distance D. The temperature sensor <NUM> has a direct line of sight of the heat source, in other words when the heat source is operated and starts to warm up, and the sensor <NUM> is able to detect its temperature and thus its position within the thermal image. Next, the heat source is operated at a power sufficiently low so as to prevent significant heating of the reference area <NUM>, for example briefly at a low duty cycle, while the sensor <NUM> records an image of the build bed surface <NUM> with the heat source <NUM> above it. From the sensor pixels that correspond to the heat source position as imaged, a geometric conversion may be applied to determine which pixels are to be used to monitor a region within the direct sub-reference area at the calibration distance D underneath the heat source. In this way, the location of the direct sub-reference area in the sensor image based on the imaged location of the moveable heat source may be defined. Measurements of temperature, using the temperature sensor <NUM>, of regions within the direct sub-reference area may be used to control the duration of the step of heating by the heat source and to terminate the step once a threshold temperature has been detected. In other words, the duration of time of step (d) of applying heat to the reference area is controlled by terminating the step of heating the reference area when the monitored temperature of the direct sub-reference area reaches a threshold temperature. In this way, excessive fusion of particulate material beneath and potentially beyond the reference area may be avoided. The steps may be carried out by the controller <NUM>.

The size of the reference area <NUM> is chosen to ensure that, in the event of lateral deviations along the x-direction (perpendicular to the direction of elongation of the moveable heat source, for example) when the build bed surface is lowered to the calibration distance D, the heat source remains positioned above it, while still providing a region (sub-reference area 50A) sufficiently large enough to at least one side of the heat source <NUM> to be monitored and analysed to reliably determine the onset of fusion. This region may be chosen to fall within the reference area <NUM>. In other words, a sub-reference area 50A may be monitored that is comprised within the reference area <NUM>, and for the misalignment reasons discussed above may be smaller than the reference area. From the pixel, or pixel line, corresponding to the direct sub-reference area (in this example, due to the elongate heat source being an elongate area parallel to the axis of the heat source), a sub-array of sensor pixels may be defined to monitor the sub-reference area 50A, the sub-reference area 50A extending perpendicular to the direction of elongation of the direct sub-reference area over a distance Δx from one side of direct sub-reference area. Alternatively the sub-reference area 50A may extend to both sides of the direct sub-reference area along a direction perpendicular to the direction of elongation of the direct sub-reference area.

The temperature sensor <NUM> may monitor the temperature of the reference area <NUM> or of all or some of the build bed surface <NUM>. It may comprise an array of pixels where each pixel corresponds to a sub area of the build bed surface <NUM>, and the onset of fusion may be determined for each pixel corresponding to that sub-reference area or a group of pixels corresponding to a group of sub-reference areas.

The reference area <NUM> may be defined as part of a cross-section of the calibration layer to be fused by providing the apparatus <NUM> with a bitmap of layers to be processed and that define the reference area <NUM> and/or, where applicable, the sub-reference area 50A and/or base reference area <NUM>. In any of the above routines more than one reference area <NUM> may be arranged to be fused. A second reference area may or may not comprise radiation-absorbing material so as to be fused. More than one fused reference area <NUM> provides more than one position dependent (or sensor pixel dependent) measurement of the onset of fusion which may allow normalisation between sensor pixels or groups of sensor pixels based on their corresponding onset of fusion.

In some calibration routines, more than one reference area <NUM> may be defined on the build bed surface <NUM>. A second reference area <NUM> may overlie or coincide with a fused base reference area <NUM>. Alternatively, it may not overlie a fused base reference area <NUM> but instead be monitored alongside the reference area <NUM> to be fused. Any of the calibration routines described may therefore further comprise the step of measuring, during the step of measuring the temperature increase of the (first) reference area 50_1, the temperature increase of a second reference area 50_2 over time; and determining the onset of fusion of the particulate material from the measured temperature increase over time of the first reference area 50_1 and of the second reference area 50_2.

In the case where a base layer <NUM> is provided with a base reference area <NUM>, the one or more reference areas <NUM> may overlie the same base reference area <NUM> or they may overlap or coincide with individual corresponding base reference areas <NUM>. In addition, one or more second reference areas may be defined to be monitored that are not to fuse. For example, in cases where the particulate material does not comprise radiation-absorbing material, the second reference area may not be provided with radiation-absorbing material by deposition with a radiation-absorbing fluid deposited by a printing module. Where a base layer is provided, the second reference area may not overlie the base layer and thus is not selectively heated by a base reference area <NUM>.

Upon application of heat by the heat source <NUM>, <NUM>, the temperature sensor <NUM> may monitor the temperature of a plurality of reference areas <NUM> until at least one of them begins to fuse, and the onset of fusion may be determined based on the measurements of the plurality of reference areas <NUM>, for example based on a first reference area 50_1 that reached the onset of fusion and a second reference area 50_2 that may or may not have reached the onset of fusion. The temperature sensor <NUM>, such as a thermal camera, may be located above the build bed surface <NUM> to monitor the full area of the build bed surface <NUM>. In some arrangements, the thermal camera <NUM> may be located within the area of the fixed overhead heater <NUM>.

The moveable heat source <NUM> may be an infrared lamp emitting a near infrared spectrum of radiation, and the lamp may be elongate and span the width of the build bed surface <NUM>. The moveable heat source <NUM> may be supported on the carriage <NUM>, or it may be supported on a second carriage arranged to move across the build bed surface <NUM>. During the calibration routine, the moveable heat source <NUM> may be operated at a power output of <NUM> Watts or less, preferably at <NUM> Watts or less. For a 3kW heat source, this may correspond to a duty cycle of less than or equal to <NUM>%, or less than or equal to <NUM>%, so as to reduce the rate of heating of the reference area <NUM>. However the power output chosen will also depend on the calibration distance D, for example, and may be higher than <NUM> Watts for calibration distances D near the higher end of the range of <NUM> to <NUM>.

The power output of the heat source used to heat the reference area <NUM> may be variable and may be controlled based on measurements of the build bed surface <NUM> and/or based on measurement of the one or more reference areas <NUM>, or sub-reference areas comprised within a reference area <NUM>, by a sensor, such as by the sensor <NUM>. A controlled variable power output may be achieved by using any known closed or open loop temperature control methods, or combinations thereof. During such control methods, the power output of the heat source may vary about an average power output of <NUM> Watt or less, but this average may be higher depending on the distance between the heat source and the build bed surface <NUM>; for example it may be dependent on a constant or variable calibration distance D.

The absorption-modifying fluid may be a radiation-absorbing fluid with an absorption spectrum that at least partially overlaps with the radiation spectrum of the moveable heat source <NUM>, in which case the fluid deposition module <NUM> is arranged to apply the radiation-absorbing fluid to the reference area <NUM>. The fluid deposition module <NUM> may optionally be arranged to apply a reduced amount of absorption-modifying fluid to the reference area <NUM> compared to that applied during a normal build process, so as to reduce the rate of heating of the reference area <NUM>.

The overhead heater <NUM> may comprise a plurality of individually controllable heating elements arranged in a regular matrix or arranged in an irregular pattern over the overhead heater area. Each individual heating element has a corresponding area of influence of heat impact on the build bed surface <NUM>. Thus the reference area <NUM> may be heated with a subset of one or more of the plurality of individual heating elements of the fixed overhead heater <NUM>.

Claim 1:
A method for determining a set point for measurements from a temperature sensor (<NUM>) of an apparatus (<NUM>) for the layer-by-layer formation of a three-dimensional object (<NUM>) from particulate material, the apparatus (<NUM>) having a moveable heat source (<NUM>), the method comprising:
distributing (<NUM>) a calibration layer of particulate material over a build bed surface (<NUM>);
selectively applying (<NUM>) absorption-modifying fluid to a reference area (<NUM>) or to an area surrounding a reference area (<NUM>), on the build bed surface (<NUM>);
lowering (<NUM>) the build bed surface (<NUM>) to a calibration depth, wherein the moveable heat source (<NUM>) is moveable across the build bed surface (<NUM>), and wherein the calibration depth defines a calibration distance (D) between the moveable heat source (<NUM>) and the build bed surface (<NUM>) greater than a build depth at which the object (<NUM>) is built;
applying (<NUM>) heat to the reference area (<NUM>) using the moveable heat source (<NUM>) while measuring (<NUM>), using the temperature sensor (<NUM>), the temperature increase of a sub-reference area (50A) comprised within the reference area (<NUM>) over a duration of time, at least until the particulate material of the reference area (<NUM>) begins to fuse, and/or taking optical readings, using an optical sensor (<NUM>), of an optical property of the sub-reference area (50A) over the duration of time;
determining (<NUM>) the onset of fusion of the particulate material from the measured temperature increase and/or from a change in the optical property over the duration of time of the reference area (<NUM>); and
applying (<NUM>) the onset of fusion as the set point for subsequent temperature measurements of the temperature sensor (<NUM>).