Hi @ADouyon,
- You're right, since you add few factors to screen, I recommended you to add quadratic effect for time, since chemical or biological process are rarely linearly dependent over time, so it may be best to have this quadratic term directly in the model. And since you had "constraints" with this time factor (for decimals), you choose a discrete numeric type with 3 levels for this factor, which automatically adds the quadratic terms in the model with the estimability "if possible", so you should be ok with these settings.
- These buckets are not entirely separated from each others, it's more a help and to guide you during your DoE creation process, to self-assess at which stage you are. Depending on your experimental budget, it can be tempting to do only one big DoE, but it may be a lack of ressources, as most of the effects won't probably be significant or have a big influence on the response. This is why I tend to describe the use of DoE during a study with these 3 steps : screening, optimization and prediction, as it helps to figure out what you already know about the system, and what you would like to know to further understand and predict your system. I used in previous DoE trainings the figure I have inserted as a screenshot here. But as you say, depending on the situation it may look a bit over-simplistic and all studies using DoE don't go through these 3 stages, as previous knowledge, domain experience or historical data can help have a fair understanding of the system.
In your case, your number of factors is quite low, but you're unsure about the significance of some effects (X4 and interactions/quadratic effect involving X4). So I would choose a D-optimality criterion in order to assess parameters estimates as precisely as possible, and have the possibility to sort significant effects from non-significant ones.
Your design does make sense, and the 24-runs DoE ("i_tv5_Iopt_changedEst_remov2_24runs") done before replication looks like a good compromise between runs size and screening of effects. And due to the presence of 5 replicate runs, you should be able to estimate noise in your response and lack-of-fit.
It may not be already a predictive model (depending on the precision you want to have), but it's a first good step in assessing which terms are significant, and from there, you can augment the design to improve the prediction precision.
I hope this answer will help you,
Victor GUILLER
"It is not unusual for a well-designed experiment to analyze itself" (Box, Hunter and Hunter)