The results section describes experiments that were completed before the paper was written. Therefore, the simple past tense is the natural choice when describing the results obtained.
Example: Overall, there was a significant reduction in the blood pressure of more than 60% of the patients.
However, you should use the present tense to refer to tables, figures, and graphs that you are using to present your results.
Example: Table 4 shows the bold pressure levels of patients before and after administration of the recommended dose of the drug.
In the discussion section, the past tense is generally used to summarize the findings. But when you are interpreting the results or describing the significance of the findings, the present tense should be used. Often, a combination of both the past and the present tense is used in sentences within the discussion section.
Example: 63% of the children demonstrated an elevated level of at least one risk factor, indicating that are children with obesity are at an increased risk of cardiovascular diseases.
Note that the first part of the sentence refers to the results; hence the past tense has been used for this part. On the other hand, the present tense has been used for the second part as this part explains what the result means.
You may also need to use the future tense in the discussion section if you are making recommendations for further research or providing future direction.
Example: The methods reported here will open up avenues for further research in the field.
For further reading, you can refer to these posts: