Permanent Set
Materials that undergo a permanent change in shape after being subjected to stress have reached their limit of elastic recovery. In the context of paperboard, plastic deformation occurs when the fibers are stretched or compressed so far that they cannot return to their original position. This is the goal of the creasing and embossing processes.
By forcing the material into a new shape, the converter creates a functional fold or a decorative relief that remains stable over time.
Fiber Rearrangement
High pressure causes the internal bonds between the cellulose fibers to break and reform in a new configuration. This movement is often aided by heat, which makes the lignin and moisture in the board more pliable. During embossing, the male die stretches the fibers until they take on the geometry of the female die.
Once the pressure is released, the material stays in that shape because the internal structure has been permanently altered.
Crease Retention
Folding a carton depends on the board staying bent at the correct angle. If only elastic deformation occurred, the box would spring open as soon as the glue was dry. Plastic deformation ensures that the crease acts as a permanent hinge.
The quality of this change is measured by how well the board holds its new form without cracking on the outer surface.
