The treatment of the face of the opposing jaw is very important in obtaining a perfect seal.
The thickness and hardness of the jaw face can make the difference between a perfect seal or cut and a worthless seal or cut.
The face of the opposing jaw must be soft enough to deform and compensate for any deviation or deformation in the alignment or flatness of the face of the heatseal jaw. In addition, the jaw facing must be covered with PTFE self-adhesive tape to assure a clean release of the film after sealing.
When sealing with a band such as a Tapered Band, the opposing jaw facing must be soft enough to conform to the shape of the band thereby assuring contact and equal force being distributed over the area to be sealed.
It must also be hard enough to present a firm enough face that the jaw force is directed principally on the band and not on the adjacent surfaces of the soft face. For general applications a strip of self-adhesive, 30° durometer, silicone rubber, 2 mm thick and 15 mm wide is satisfactory. For cutting, harder surfaces are more suitable. Durometers of up to 60° are often used and Durit material works well on thin plastic films. The selection is a little more difficult when using bands that both cut and provide a wider seal. Bands such as Beaded Bands and T-Profile Bands require a silicon pad on the opposite jaw that will deform enough to force the plastic film to conform the face of the band while concurrently providing sufficient firmness to allow the band to cut along the center line of the band. Depending upon the composition and the thickness of the film the silicone face may need to be up to 60° Durometer in hardness and perhaps up to 6 mm thick.
This is still a little more art than science, but the people at Jørgen Bork Electronic ApS can help you with your specific application.