1. Large surface roughness makes it difficult to measure dimensions accuracy;
2. Internal stresses can distort the product;
3. Incorrect laser processing parameters or machine settings can cause discrepancy between CAD and SLMed product dimensions;
4. Digitalisation (mostly for small features such as holes)
This is why understanding the machine and laser/material interactions is critical.
Processing parameters: understanding actual track width
It is critical to point out the impact of power/speed combination on the actual track width (and on the track depth, and on the melt pool maximum temperature and solidification rate hence on the density, microstructure and mechanical properties, etc but let’s keep that for later!). In other words, a laser track can be compared to a scalpel with a variable blade thickness that varies as a function of laser power and speed (function of layer thickness – usually 20/40um for maximum output power limited to 370W).
Contours and offset parameters
That’s where knowing the actual track width of the contour parameters (ie: power + scan speed scanned on the CAD contour of the component) comes into play. It is necessary to offset this contour scan by (usually) 30% to 70% of its width inwards to ensure that the overall dimensions of the components are within tolerances.
With this in mind, the parameters responsible for dimension accuracy accessible on the EOS M280 are: offsets values, contours parameters, track-width, skywriting and surface roughness parameters. More details next week.
 Lijun Han, Frank W. Liou, Srinivas Musti - Thermal Behavior and Geometry Model of Melt Pool in Laser Material Process, Journal of Heat Transfer, SEPTEMBER 2005, Vol. 127 / pp1005-1014