In sinter metallurgy, the added polymeric binder provides improved adhesion to the metal powder. However, if the 1st heating phase is too quick, outgassing of the binder may cause micro-cracking. For the production process, this means finding a good balance between the following two factors:
- Slow heating increases the process time.
- Fast heating leads to quality defects due to intensive gas evolution during polymer decomposition.
Therefore, the optimum temperature profile over all zones of the tunnel kiln has to be determined to ensure a favorable relationship between process duration and product quality.
6 measurements at heating rates of 0.1, 0.3, 1, 3, 5 and 10K/min in nitrogen were carried out, and the kinetic model was found. The optimal rate for mass loss in order to maintain the best material quality is 0.05%/min during the whole process; for a higher decomposition rate, there is a loss in quality, and for a lower rate, there is an increase in time. Based on this optimization for constant mass loss, the optimal zone temperatures for production in the tunnel kiln were found.
Figure A shows six thermogravimetric measurements carried out at different heating rates in a nitrogen atmosphere. Using the three-step model presented here, it is possible to carry out a high-quality approximation of all measurements. The higher the fit quality, the more confident the predictions.
Figure B shows the optimum temperature profile for the polymer burnout under laboratory conditions. For the best material quality, a constant mass-loss rate of 0.05%/min should be maintained over the entire process.
Higher rates would improve the process time, but reduce the product quality. Temperature profiles for other mass-loss rates can be easily and quickly calculated.
Figure C depicts the mass-loss prediction in a 5-zone tunnel kiln for a temperature program corresponding to the optimum zone temperatures for the polymer burnout during the production process.