Colour Penetration Test
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Colour penetration test of laser sintered components
The colouring of laser sintered components
The mass density of a laser sintered component can be used as an indicator of component quality. The density of a component determined using the buoyancy method can be used to draw conclusions about its mechanical properties. As local disturbances can occur during the construction process, the colouring of the laser sintered parts in a colour liquor is used for location-dependent density testing. The colour pigments diffuse better and faster in layers of lower density than in regions of higher density. As a result, weak points in the component are coloured more intensively (Fig. 1) [1, 2].
| Fig. 1: | Schematic illustration of the colour saturation in dependence on the density [1,3] |
Determining the colour saturation correlation with the mass density
The absolute values of colour saturation, which lie between 0 and 255, are determined using a computer-aided graphic evaluation based on the red–green–blue system (short RGB). The absolute colour saturation difference for – in this case – red can be derived from this.
The relative colour saturation thus results in:
| Fig. 2: | Pictures of ranges of coloured multi-purpose specimen produced of PA12 with different densities: low (a), middle density (b) and high density (c) [1, 3] |
To determine the relationship between density and relative colour saturation, multipurpose test specimens were produced with different laser sintering parameters on edge to the application direction of the wiper. The density was determined using the buoyancy method. The test specimens were immersed in a red colour liquor at 95 °C for five minutes. After removal, they were rinsed under running water. Photographic images of the test specimen surfaces were then taken using a digital camera after white balancing (Fig. 2) and these images were analysed using Origin image processing software. Furthermore, test specimens of different densities were first coloured and then mechanically separated in order to investigate the depth of colour penetration. This clearly shows that with low-density sintered components, the colour diffuses about 500 µm deep into the component within the specified immersion time (Fig. 3a). For components made from the optimised material, the ink penetration depth is not even 100 µm (Fig. 3b), which ultimately also affects the colour intensity [1].
| Bild 3: | Lichtmikroskopische Aufnahmen von Querschnittsflächen vorher eingefärbter Prüfkörper mit geringer (a) und hoher (b) Dichte |
The values determined for the relative colour saturation are plotted in Figure 4 as a function of the relative mass densities of the test specimens. In Figure 4, a value of ∆Frel = 1 was assumed for ∆Frel at a relative density of 0.79, the density value for structural integrity.
Correlation between density, crystallinity and mechanical properties
As the density (and thus the porosity) and the degree of crystallinity (or the amorphous proportion) of laser-sintered parts are interdependent, it is impossible to clearly assign which of the two structural parameters ultimately has the main influence on the mechanical properties.
The general rule is:
The greater the density ρ and the lower the crystalline content of laser-sintered components, the better their mechanical properties.
| Bild 4: | Relative colour saturation ∆Frel in dependence on the relative density ρ/ρ0 for PA12 |
See also
- Multipurpose test specimen
- Specimen for laser sintering
- Laser sintering
References
| [1] | Grießbach, S.: Korrelation zwischen Materialzusammensetzung, Herstellungsbedingungen und Eigenschaftsprofil von lasergesinterten Polyamid-Werkstoffen. Dissertation, Martin-Luther-Universität Halle-Wittenberg, Verlag Wissenschaftliche Scripten, Auerbach (2012) |
| [2] | Grießbach, S., Lach, R., Grellmann, W.: Structure–property Correlations of Laser Sintered Nylon 12 for Dynamic Dye Testing of Plastic Parts. Polymer Testing 29 (2010) 1026–1030 |
| [3] | Grießbach, S., Buschner N., Lach R., Grellmann, W.: Einsatz des Diffusionsfärbens zur zerstörungsfreien Bauteilprüfung an Lasersinterteilen. Polymerwerkstoffe (2010) 15.–17. September 2010, Halle, Tagungsband Beitrag PT 10, 1–7 |