The only time I had it happen was when I used mystery foam and the melting temp of the foam was below the 275ish I heat the kydex to (I used foam for a couch when it melted to the kydex). Normally for knifes I use the cheap blue camping pads from Walley World and cut my foam squares from them you will get some foam deformation but I can normally get 5-10 presses from a piece before I need to replace it.
Sorry it took so long to respond the normal thicknesses I use are .060 for sheaths and .090 for clips. But when I'm doing larger knives or holsters I prefer the 0.090 for all of it. I did a sheath for a 6.5in blade and the .060 seemed too flimsy but I break alot of stuff any way so I prefer to overbuild if I can.
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