In this way, the PVA is able to become part of the chain without actually doing anything. It's the passenger, and the PVOH is the driver. The PVA becomes part of the chain through its PVOH coating. The structure that is created is greater than the sum of its individual parts. To prevent any weak links in the chain, all of the diazo must be spent.
Underexposure creates weak links that lead to pinholing, screen breakdown, and difficult reclaiming.
If the emulsion was not thoroughly dried before exposing, the diazo will crosslink with the water as well as the PVOH, and the emulsion will not be sufficiently hardened. In fact, the resulting stencil may contain as many weak links as one that was simply underexposed. Heat from exposure units will not dry the emulsion during exposure, as was theorized a few years ago. Stencil problems caused by moisture in the emulsion are the same as those caused by underexposure. Like their advantages, the inherent limitations of diazo-sensitized direct emulsions come from their basic chemical makeup.