The crimps (You called them "Closed end caps") should be of a size suitable for the cable AND the amount of current the appliance will draw. Too small and they will melt. You must also secure them with a ratchet crimper to ensure a secure connection. Also as its a clothes iron you can not use "any old flex" it has to be cotton braided flex. (Anything else will mark the clothes, believe it or not)
This is a UK based forum, we today use mm squared to show cable sizes not American Wire Gauge so we really can't help. We have 0.75 mm, 1 mm, 1.5 mm, 2.5 mm, 4 mm and 6 mm cables likely you would buy 1 mm to 2.5 mm and that likely means red or blue crimps.
But if your using AWG cable I would assume your in USA and your voltage is different to ours so really you need a USA forum.
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