That is an interesting question. Stainless is highly resistive and won’t conduct heat too far or fast but the plastic is only 3 mm away.
My gut says without some sort of extra cooling the plastic is likely to see some heat. I trial run is needed.
Is water an issue? Put a mist of water in the weld area as the part welds, similar to a seam welder just less water. Initiate the mister with the weld cycle.
Make sure the electrodes are cooled well to reduce time at heat. Minimized the size of the weld and weld nugget. Keep the electrodes in place after the weld to pull heat out.
GOOD LUCK!