The female access key melted because of how saidar had been used as well as the massive amount of the Power it was channeling. Rands' understandable lack of experience channeling saidar and how he forced it into contact with saidin put the female sa'angreal under a far greater strain than the forces the male Choedan Kal was handling. Remember that he yanked control of saidar from Nynaeve and proceeded to force it into a particular shape, while saidar itself kept trying to fall into other patterns?