Since in most situations lid stock materials have a heat-seal coating applied to them and the films used
in pharmaceutical packaging do not, effective seal performance of a film typically relates to the
aforementioned topics of minimizing residual stress and thickness variation in the flange of the package.
Note, however, that as the chemistry of the film changes with selection of the type, this may sometime
require changes to the type of heat-seal coating used with it. In some cases, a suitable combination of
heat-seal coating and film contact layer may not exist, therefore necessitating the use of possibly
another film or orienting the layers properly in order to make a seal. With regard to processing the
materials, when using commonly available materials in the market, most tend to work well together
when applying heat through the lid stock to make the heat-seal coating flow while keeping the film
cool to preserve its dimension. Most compatible heat-seal coatings tend to flow and make good
contact with the film when heated to 150°C with post seal cooling. As mentioned before, try to avoid
the use of universal or “bell” shaped forming tools when forming blisters to eliminate many common
problems associated with sealing related to web dimensional stability, wrinkling, and stress.