you can use the “synthetic” setting on clothes irons for polyester and nylon (flags won’t really be made of a different plastic bc these are the cheapest reliable cloth alternatives). i think u can use higher heat on them than silk and satin. p sure the synthetic setting is half of the max heat

edit: just checked my clothes iron and the “nylon” setting is actually below the “silk” setting. regardless, u can use the “nylon” setting on a flag no problem as long as you use the steam often enough