"Platinum OA" basically means that the publication is openly available to readers from the publisher's site (just as in Gold OA) and that no fees have to be paid on the side of the authors (just as in "Green OA").
I do not know of any platinum open-access journals that have impact factors. Every single one I know transitioned to gold open-access as soon as they got accepted to Expanded Citation Index and some of the fees are insane (e.g., IEEE Access, Scientific Reports) and some are more reasonable. There are a few engineering journals I know that are platinum open-access and indexed in Scopus - are you interested in that?
Joshua M Pearce These are the platinum open-access journals (no fee open access) I know of that are indexed (none of them have impact factors but they are indexed in Scopus (and sometimes ESCI)). All are published by faculties of major research universities and academies of science in various countries (not predatory or commercial publishers) and seem to be generally good quality. Of course, you will have to look these over and make sure they are up to your standard. Several of the MDPI journals are still platinum open access for the next few months but I do not think any of those particular journals are indexed in ESCI or Scopus yet since they are new.
There were others on my list, but I just checked on all of them again and they all converted to gold open access since I last looked except these. I have a paper forthcoming in December in #1 and I was happy with the process - the review was very thorough and rigorous, just a little slower than I am used to with other journals (it took about 10 months from submission to acceptance but they did ask for two significant revisions in that time - the review was definitely more careful and rigorous than those I have seen from Elsevier recently).
4. International Journal of Extreme Manufacturing (ISSN: 2631-8644) (the indexing of this one is a little sketchy and I cannot find it on Scopus now - but it looked good in the past so maybe worth checking out. It is published by the Chinese Academy of Engineering and several major research universities. I did confirm that it is indexed in DOAJ currently).
Some extra that aren't quite what you asked for but might be interested in:
Check out The STEAM Journal (ISSN: 2327-2074): Not indexed that I can find but it is published by a high-profile group at Claremont Colleges in California and some of the papers have a ridiculous number of citations for an arts journal. The topic is kind of the interaction between engineering, art, and education - based on some of the papers I have seen from your lab, you might be interested.
Link: https://scholarship.claremont.edu/steam/
Finally, there is a low-cost (250 Euros per paper) gold open access journal from a university in Greece that I have heard good things about. I don't know much about it but it has been indexed in ESCI and DOAJ since 2015 so might be worth checking out if you need more options:
6. Engineering, Technology, & Applied Science Research (ISSN: 2241-4487)
Joshua M Pearce One of my Brazilian collaborators recently told me about another good platinum open access journal called Revista Materia that is definitely legit, publishes in English, is indexed in Scopus, DAOJ, and Scimago (I just checked to make sure - see attachment). Seems to have a good editorial board and reasonable review time.