A method for determining a temperature set point for calibrating the thermal measurement scale of a thermal sensor positioned above a build bed surface in an apparatus for the layer-by-layer manufacture of a three-dimensional object from particulate material;
- the method comprising sequentially performing layer cycle steps (a) to (f) of:
- (a) distributing a layer of particulate material, the layer providing the build bed surface;
- (b) preheating the layer to a preheat temperature value by passing a preheat heat source across the layer while operating the preheat heat source to preheat the layer to a preheat temperature, wherein the preheat temperature value is lower than the melting temperature of the particulate material;
- (c) depositing an amount of absorption modifier over at least one of a test region and a surrounding area surrounding the test region;
- (d) heating the test region over a period of time by passing a second heat source across the layer while operating the second heat source, and wherein the period of time over which the test region is heated is determined by the speed of the second heat source relative to the test region;
- (e) measuring with the thermal sensor a temperature value TR within the test region;
- (f) after at least one or more of the layer cycle steps, measuring the temperature of the build bed surface using the thermal sensor;
- and operating a stationary third heat source provided above the build bed surface continuously throughout the layer cycle based on the measured temperature and a predefined target layer temperature between the solidification temperature and the melting temperature of the particulate material, so as to maintain the build bed surface at the target temperature;
- wherein the steps (a) of distributing each layer, (b) of passing the preheat source across each layer and (d) of passing the second heat source across each layer are carried out in the same direction;
- wherein the steps (a) to (f) of the layer cycle are repeated one or more times, wherein the repeating of step (a) includes distributing a further layer of material over a preceding layer of material, the further layer providing the build bed surface;
- wherein the step of distributing each further layer is initiated after a first time interval from initiating the step of heating with the second heat source, and wherein the step of heating with the second heat source is initiated after a second time interval from initiating the step of distributing the layer, and wherein the first and second time intervals are the same throughout the calibrating of the thermal measurement scale of the thermal sensor;
- wherein the repeating of step (c) comprises depositing an amount of absorption modifier over at least the test region, such that, compared to the preceding layer, the test region of each further layer reaches a higher temperature value TR, by:
- at step (c) depositing a different amount, or a different absorption modifier, per unit area over the test region, wherein the different amount or the different absorption modifier is capable of absorbing a different amount of energy of the radiation spectrum provided by the second heat source;
- wherein for at least one of the further layers the different amount or the different absorption modifier is sufficient to cause the particulate material of the test region to start to melt;
- identifying a characteristic in the layer-by-layer evolution of the measured temperature value TR measured for each layer;
- determining from the determined characteristic a temperature set point for the thermal sensor;
- and
- applying the determined temperature set point to subsequent measurements of the thermal sensor.