A scan track in the SLM 100 comprises of a series of discrete points, with the controlling parameters being the distance between the points and dwell time at each point, called point distance and exposure time, respectively. The laser remains on during the entire track and heats material between spots. This is seen in Figure 86, though the point distance is exaggerated, compared to values used during processing experiments, to show the scanning between spots. An experiment was carried out to learn the limits of these two parameters by measuring the time to scan a line with varying inputs. Alongside this, images were taken of various scanned lines (Table 2).
Each line ends with a larger shallower spot, possibly indicating a defocussing at the end of a hatch. The spot appears only as the final point in a scanned area, not on each hatch line. The thickness of the lines changes with heat input but this did not affect the final spot size. The discrete positioning of the spots is only evident where the point distance is at least 200 μm, while optimised density tests found the best results were with point distance below 100 μm. It may be the scanner mirrors cannot move fast enough to allow a complete dwell time and are always in motion. The mirrors will have periods of acceleration and deceleration between spots and these may alter the speed-time profile but it appears that up to 100 μm point distances can be interpreted as a continuous moving beam rather than the series of discrete points it should be.
The implication of this would be that within this range altering both point distance and exposure time would be redundant. It was not clear if this interpretation could be extrapolated to higher exposures. The optimised density test found the best exposure times were above 500 μs, for all three materials.
Figure 86 laser scan on thermal sensitive paper, exposure 200 μs and point distance 500 μm
The time duration of the line scans was recorded by connecting an oscilloscope to the laser control unit. The scan line length was set to be 10 mm for all scans; hatch exposure and point distance were varied to examine the limits of response of the laser control unit to test the machines sensitivity to these parameter changes. Various values were chosen to examine where the limits of these inputs are. Figure 87 shows that exposures below 20 μs and point distances below 10 μm do not affect the scan duration. These limits appear to be independent of each other, so tests with holding one variable constant will suffice to get the resolution and minimum value of the other.
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Table 2 Images of scanned tracks across varying exposure and point distance
Figure 87 Effect of hatch exposure and point distance on the scan duration of a 10 mm line using hatch scan strategy.
The lower limit of point distance was found using 50 μs exposure time. The smallest increment of change in point distance is 10 µm and smallest value that can be input is 10 µm. The limits of exposure were found using 50 μm point distance. The smallest
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increment of change in exposure is 1 µs and smallest value that can be input is 20 µs.
These values inform the parameters tested in chapter 4, to find the optimised density.
Figure 88 Point distance resolution and minimum input test
Figure 89 Exposure resolution and minimum input tests.
By understanding the limits of the parameters, it is possible to test how scan duration can be altered. If point distance is held constant then exposure has a proportional response to scan duration, and if exposure is constant then point distance has an inverse response to scan duration. These relationships were used to formulate an equation:
𝑺𝒄𝒂𝒏 𝑫𝒖𝒓𝒂𝒕𝒊𝒐𝒏, 𝒕𝑺𝑫 = 𝑪𝟏 𝒕𝒆𝒙𝒑𝒐
𝑷𝒅𝒊𝒔𝒕+ 𝑪𝟐 𝒕𝒆𝒙𝒑𝒐+ 𝑪𝟑 𝑷𝒅𝒊𝒔𝒕+ 𝑪𝟒
Equation 6
Where; texpo is exposure time, Pdist is point distance and C1, C2, C3 and C4 are constants.
Minitab was used to find the equation that best fitted this form:
𝑺𝒄𝒂𝒏 𝑫𝒖𝒓𝒂𝒕𝒊𝒐𝒏, 𝒕𝑺𝑫 = 𝟏𝟎. 𝟐𝟓 𝒕𝒆𝒙𝒑𝒐
𝑷𝒅𝒊𝒔𝒕+ 𝟒. 𝟐𝟓 𝒙𝟏𝟎−𝟒 𝒕𝒆𝒙𝒑𝒐+ 𝟏𝟖𝟔.𝟕𝟓
𝑷𝒅𝒊𝒔𝒕 + 𝟎. 𝟎𝟐
Equation 7
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Figure 90 shows the fit of the predicted scan duration compared to the measured values. The errors show that all predicted values are within 2% of the measure value.
Figure 90 Measured scan duration vs. residual error in scan duration as predicted by Equation 7.
Error in generating this equation could have arisen from the accuracy of the readings from the oscilloscope and the truncation of readings to three significant figures. The first constant C1 is likely to be the length of the line in mm, the second and fourth, C2
and C4, are insignificant. The relationship was not tested across lines of different length which could influence these constants. Dividing the line length by the scan duration gives the scan speed of the line. Using these values, scan speed can be expressed as:
𝐒𝐜𝐚𝐧 𝐒𝐩𝐞𝐞𝐝, 𝒗𝒔𝒄𝒂𝒏 = 𝑷𝒅𝒊𝒔𝒕 𝒕𝒆𝒙𝒑𝒐+ 𝟏𝟗. 𝟔𝟕
Equation 8
The predicted scan speeds are compared to the measured scan speeds in Figure 91.
The errors have increased compared to Figure 90, as minor terms have been omitted but all errors are still within 5% of the measured value.
Figure 91 Measured scan speed vs. residual error of the scan speeds as predicted by Equation 8
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This shows a different relationship then expected, as theoretically the denominator should only be the exposure time. It was not tested how geometry of the scan affects this relationship. It is possible that the denominator could be influenced by line length or by hatch spacing. The main conclusion from this investigation is that one must be aware that point distance and exposure time do not have the same influence on the scan lines, and so were not simplified to a scan speed parameter.