I would suggest simax tubing. I have not had this issue with kimble but the quality and consistency of their products overall was poor. I am not suggesting that the brand of tubing is to blame for this though. I would suggest you try again and box the piece when it is hotter and the overall heat base is as even as you can achieve. Then if that doesnt work next place to look is in your annealing schedule. There are a lot of folks i know who use kimble tubing without any problems. I buy most of my boro from ABR.
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