Polymer Film Micro Fracture Mechanisms under High Strain Shear in Folding Boxboard Creases
High strain shear during boxboard creasing causes polymer film micro fractures when local elongation exceeds film yield capacity along the outer bend radius.
Web Strain
Tension Deformation
Continuous tension forces applied during roll-to-roll processing cause fractional elongation of paper and film webs along the machine direction. Monitoring web strain assesses the proportional dimensional change experienced by continuous material webs under mechanical tension during printing and laminating operations. Elastic deformation occurs within Hooke’s law limits, allowing the substrate to recover original dimensions upon tension release.
Exceeding elastic boundaries causes permanent plastic stretch or web breaks.
Elastic Stretch
Web tension control systems vary pull-roll speeds to maintain constant stress levels across continuous paperboard webs. Calculating web strain involves dividing the change in web length by the original un-tensioned length, expressed as a percentage or micro-strain value. High web strain induces transverse web contraction through Poisson ratio effects, narrowing web width and altering slit edge alignment.
Moisture variations significantly alter the elastic modulus of paper, causing wet web sections to elongate further than dry sections under identical line tension. Load cells mounted under idler rolls measure line tension, allowing automatic drive controllers to adjust motor torque and stabilize strain. Polyethylene coatings stretch more readily than paperboard bases, creating internal shear stresses that cause web curling after strain relaxation.
Maintaining low, uniform strain prevents web wrinkling and misregistration across multi-color printing decks.
Register Stability
Precision printing and converting require consistent web dimensions to achieve accurate color registration and crease placement. Excessive web strain causes image distortion and die-cutting misalignment on continuous converting lines. Online tension sensors provide continuous feedback control to prevent strain fluctuations during roll changes and speed transitions.
Process stability depends on balancing web tension against substrate modulus of elasticity.