Detecting and measuring hot blisters in tires

A hot blister is a bulge in the surface of a cured tire caused by entrapped gasses or steam. Such blisters can be observed in the hot tires when they exit curing. Blisters often shrink as the gases migrate outward or are absorbed into the tire materials. There is concern, however, that the entrapped steam and gases present during curing can weaken the laminate structure of the tire. Air entrapment has been observed frequently by the author during capability studies conducted on tire-building machines. This paper presents cases showing entrapped air between the inner liner and body ply in a typical green tire carcass construction. Such air pockets can remain between the layers and migrate outward during tire curing to produce blisters. Air can also be absorbed into the tire structure. Steam and gases also can be introduced into the tire structure during curing and emerge as blisters after curing. This paper presents a method to detect blisters after curing and to detect entrapped air pockets during the tire-building stage. Scans of both cases are presented. These cases demonstrate that automatic monitoring systems can be implemented to detect these conditions. The method uses high-speed line laser sensors to digitize the sidewall and/or tread of the un-inflated tire. The data set is rendered as a 3D model. The data is post-processed and searched for blisters. This same sensor technology is employed to scan tire sidewalls for bulges and depressions.