Inline Breaker Bar Geometry Optimization and Reverse-Side Fluid Dampening Dynamics
Breaker bar geometry and reverse fluid dampening control film lamination curl by balancing plastic yield strain and z-direction substrate moisture gradients.
Viscoelastic Yield
Deformation Threshold
Transition point where a material ceases to return to its original shape after the removal of a load marks the end of purely elastic behavior. In the context of paper and polymers, viscoelastic yield occurs when the applied force is sufficient to cause permanent flow within the fiber matrix. Unlike metals which have a sharp transition, paper shows a gradual shift into plastic deformation.
This property determines how much a sheet can be stretched during a converting process before it permanently distorts.
Structural Permanence
Permanent changes to the fiber network happen because the bonds are broken and reformed in new positions, resulting in a material that no longer possesses its initial structural tension. Once the viscoelastic yield is surpassed, the sheet will exhibit a permanent set or a change in dimensions. This is critical in embossing where the goal is to exceed this limit to create a lasting pattern.
If the force is too low, the pattern will fade as the fibers spring back. The temperature of the process can lower the force required to reach this state.
Testing Procedure
Tensile testing machines identify this limit by plotting the stress against the strain. The curve deviates from a straight line at the onset of yield. This data allows packaging designers to calculate the maximum safe load for a carton.
It also informs the settings for tension control on a press.