It is OK to mix tenses:
eg. “From these results, Vogelsang et al. concluded that MEH-PPV/PMMA films absorb solvent and swell during SVA, and that the annealing process lowers the glass-transition temperature of the film to below ambient temperature, which increases chain mobility.”
- When other people’s opinions are reported: use past tense.
- But as for a ‘reproducible results” : use present tense. OR Indicating a fact
- New results in the current paper should be in present tense since they are waiting to be peer-reviewed.
My Journal 