Predicting Delamination Behaviour in Multi Layer Paperboard Creasing

Shear

When a male creasing tool penetrates a multi-ply sheet, it forces local out-of-plane compression and inter-ply displacement. Paperboard substrates used in folding cartons consist of distinct functional layers: bleached chemical top liners, bulky mechanical middle plies, and unbleached or bleached back liners. Creasing acts as a localized pre-conditioning step to break internal bonds across specific plies before folding occurs.

As the male crease rule depresses the board into a female matrix groove, the material undergoes severe out-of-plane transverse shear strain and localized thickness reduction.

Clean folding requires controlled internal ply separation within the crease bead zone. This delamination lowers bending stiffness along the crease score, creating a series of thin, flexible parallel laminates that buckle independently during a ninety or one hundred and eighty degree fold. Controlled separation prevents excessive tensile elongation on the printed top liner and compression buckling on the back liner.

Improper stress distribution across the sheet thickness leads to structural failure modes during flatbed die-cutting or subsequent high-speed gluer operations:

• Top liner cracking occurs when internal plies fail to separate, forcing excessive strain onto the outer bleached kraft layer during score folding.
• Back liner bursting develops under severe tool indentation depths that exceed the shear strain limit of the bottom ply.
• Board rupture happens when matrix channel width is undersized, causing punch shear instead of controlled ply separation.
• Asymmetric crease beads stem from misalignment between the male tool and female matrix die, inducing uneven stress distribution across the board thickness.

Effective scoring creates multiple internal delamination cracks so the board folds as a set of independent thin plies rather than a rigid beam.

Delamination prevents top liner rupture. When internal plies hold together as a solid block during score bending, the neutral axis remains near the geometric midpoint of the sheet thickness. This positioning generates maximum tensile stress on the outer printed surface, causing visible white lines or coating rupture along the hinge line.

Correct internal splitting shifts the stress state, allowing outer plies to bend without exceeding their ultimate tensile strain limits.

Selecting incorrect creasing conditions transforms a high-speed packaging line into a continuous source of split carton edges and jammed folder-gluer feeds.

Fibre

Structural raw material selection and inter-ply bonding agents define the mechanical fracture envelope of paperboard substrates. Folding Boxboard, abbreviated as FBB, contains thermomechanical or chemithermomechanical pulp in its middle plies, sandwiched between chemical pulp outer layers. The coarse, unrefined mechanical pulp fibers create a high-bulk core with low internal bond energy.

This low bond strength allows FBB to delaminate smoothly under transverse shear during creasing, providing wide converting windows across variable press speeds.

Solid Bleached Sulfate board, designated as SBS, utilizes refined chemical softwood and hardwood pulps throughout every layer. Chemical refining creates high hydrogen bonding density between fibers, raising the energy needed to initiate and propagate delamination. SBS requires precise creasing geometry and higher indentation forces to force ply separation, making it susceptible to score line stiffness issues if internal bond levels are too high.

Coated Recycled Board, known as CRB, contains secondary fibers with shortened length distributions and variable ash contents, yielding fluctuating inter-ply strength across web positions.

Laboratory Measurement of Inter Ply Bond Mechanics

Standardized physical tests isolate the forces needed to separate internal paper layers prior to high-speed converting. Z-direction tensile strength evaluation measures out-of-plane tensile resistance under static loads, while Scott Bond testing quantifies dynamic energy absorption during rapid ply separation.

1. Condition all paperboard test specimens at 23 degrees Celsius and 50 percent relative humidity for 24 hours under ISO 187 directives.
2. Cut double-sided adhesive tape and apply it uniformly to both surfaces of a 25 by 25 millimeter board sample.
3. Clamp the prepared sample between two heavy metallic blocks within an ISO 15754 Z-direction tensile testing apparatus.
4. Apply a constant crosshead strain rate of 10 millimeters per minute until complete internal plane separation takes place.
5. Record the peak force and divide by the specimen surface area to establish the Z-directional tensile failure threshold.

Variations in ambient water content alter hydrogen bonding and plasticize starch spray layers applied between plies during wet-end sheet formation. High humidity softens starch bonds, while lower moisture levels increase internal bond rigidity, requiring deeper crease punch penetration to trigger internal separation.

A virgin folding boxboard rated at 350 grams per square meter requires a Z-direction tensile strength between 250 and 350 kilopascals when conditioned at 23 degrees Celsius and 50 percent relative humidity to ensure stable delamination during die-cutting.

Localized score cracking on recycled board grades originates from ambient humidity fluctuations in the converting plant rather than uneven starch distribution across internal plies.

Simulation

Modern computational mechanics relies on non-linear finite element modeling and continuum damage formulations to predict out-of-plane shear fracture. Physical creasing experiments provide valuable endpoint data, but numerical simulations reveal the instantaneous stress distributions within internal plies during punch penetration. Modeling multi-layer paperboard requires treating individual plies as orthotropic elastic-plastic solids while placing cohesive zone interface elements between adjacent layer boundaries.

Cohesive Zone Modeling, abbreviated as CZM, represents inter-ply adhesion using bilinear traction-separation constitutive laws. Initial linear elasticity governs interface stiffness until stress levels reach the damage initiation threshold. Once the failure criterion is met, material softening begins, reducing interface stiffness until complete delamination occurs.

Mode I fracture energy governs normal tensile separation, while Mode II fracture energy governs longitudinal shear sliding between plies.

What Criteria Govern Fracture Initiation during High Speed Scoring?

Critical strain energy release rates under combined tension and sliding modes dictate when internal micro-cracks coalesce into continuous delamination planes. Mixed-mode fracture criteria, such as the Benzeggagh-Kenane parameter, combine Mode I and Mode II energy release rates to predict failure under complex stress states induced by the male creasing rule.

Numerical models must incorporate precise non-linear material properties to output accurate delamination predictions:

• Mode II fracture energy must receive primary calibration weighting because out-of-plane shear forces drive ply separation in the crease channel.
• Cohesive traction limits require reduction in core layers containing CTMP furnish to reflect lower internal fiber cohesion.
• Non-linear ply compression demands hyperelastic constitutive material models for mechanical pulps subjected to severe punch indentation.
• Friction coefficients between sliding internal surfaces must be set between 0.25 and 0.40 to prevent artificial interface locking during crease folding.

Deviations exceeding five percent in male rule tip radius alter the localized shear stress distribution sufficiently to invalidate finite element delamination predictions established under DIN 55437-1 guidelines.

Finite element simulations show that peak shear stress concentrations develop near the edges of the male creasing rule, propagating inward along the starch bonding planes. If the cohesive strength of the interface elements exceeds the transverse compressive yield strength of the bulk plies, the paperboard crushes instead of delaminating. This localized crushing destroys sheet bulk without providing the thin, flexible laminates needed for low-torque carton folding.

Whether dynamic moisture migration across the web during multi-stage high-speed creasing alters the Mode II fracture energy threshold remains an unresolved problem in computational paperboard mechanics.

Rule

Die-cutting tooling parameters, specifically punch thickness and female die channel dimensions, directly govern the physical strain imparted on the board core. The male creasing rule width must match sheet caliper to deliver adequate transverse displacement without pinching the surface liners. Female die channels, formed using cut creasing matrices or milled counter-die plates, establish the geometric boundary conditions for the underlying crease bead.

Geometric relationships define the target channel width and depth. The standard industry calculation sets the die channel width equal to the male rule thickness plus one point five to one point eight times the nominal paperboard thickness. Channel depth typically equals board thickness.

Deviating from these proportional guidelines alters the shear angle, preventing full ply separation or forcing premature outer liner fracture.

Tooling Optimization and Matrix Calibration

Correct selection of counter-die channel width prevents surface scuffing while enforcing deep internal shearing within the middle plies. A narrow matrix channel clamps the board tightly, restricting transverse deformation and causing the rule to shear clean through the top liner. An oversized channel allows the board to sink without generating sufficient localized shear stress to break inter-ply bonds.

Mill test documentation must confirm physical board consistency prior to high-speed die-cutting runs to avoid mass scoring failures:

• Caliper uniformity records specifying board thickness tolerances within plus or minus three percent across the entire master roll width.
• Z-direction tensile values certified under ISO 15754 to verify consistent inter-ply bond strength before delivery.
• Scott Bond energy ratings documented across all reel positions to guarantee core delamination performance under high-speed converting.
• Moisture content certificates confirming sheet conditioning within five point five to six point five percent relative to total dry weight.

Undersized creasing channels increase the force needed to fold score lines, causing panels to bow outwards on automatic cartoning lines.

Inserting ISO 12647-2 tolerance limits into die-cutting supply contracts transfers financial liability for score cracking directly to the converter when die wear exceeds zero point zero five millimeters.

Ledger

Financial yield in packaging converting is determined by the balance between raw material ton costs and waste generation during folder-gluer operations. Substrates are purchased by weight but consumed by surface area. High-bulk paperboard grades provide higher sheet yields per metric tonne while offering wide delamination margins during scoring.

Lower-cost recycled boards often exhibit higher structural density, lower sheet yield, and narrower creasing windows that increase operational scrap rates.

Economics of Grade Substitution and Crease Scrap Rates

Material cost savings achieved by switching from virgin folding boxboard to cheaper recycled grades frequently disappear when score cracking increases line downtime, as secondary fibers reduce bond energy. Consider a manufacturing run requiring 100,000 finished folding cartons produced from a 700 by 1000 millimeter sheet format.

A virgin Folding Boxboard at 350 grams per square meter has a bulk of 1.6 cubic centimeters per gram and a landed substrate cost of 1,450 Euros per tonne. One metric tonne yields 4,081 sheets, making the material cost per sheet 0.355 Euros. Under standard die-cutting conditions, this virgin FBB maintains a total defect scrap rate of 0.5 percent due to predictable core delamination, resulting in a net substrate cost of 357.00 Euros per 1,000 good sheets.

A substitute Coated Recycled Board at 380 grams per square meter provides equivalent stiffness but has a lower bulk of 1.25 cubic centimeters per gram and a landed price of 1,120 Euros per tonne. One metric tonne yields 3,759 sheets due to higher density, giving a raw material cost per sheet of 0.298 Euros. Inconsistent inter-ply bond distribution in the recycled furnish increases score cracking and surface bursting during high-speed folding, raising the gluer reject rate to 3.8 percent.

Net good yield requires additional press make-ready time and extra raw material sheets to fulfill the order.

Operating a folder-gluer at 300 meters per minute costs approximately 220 Euros per hour in machine time and direct labor. Frequent score ruptures on the CRB run force operators to lower line speed by 25 percent and cause 1.5 hours of cumulative machine downtime due to jam clearances. The total extra converting time adds 412.50 Euros in labor and overhead costs to the 100,000 carton batch.

When adjusted for yield reduction, scrap waste, and gluer downtime, the effective cost per 1,000 good cartons on CRB rises from an initial material estimate of 298.00 Euros to 321.40 Euros. The original 16 percent raw material cost advantage narrows substantially once converting friction and delamination performance enter the financial yield ledger.

High bulk virgin boards maintain structural rigidity at lower grammages while offering wide creasing windows. The financial advantage of choosing high performance furnish becomes obvious when folder gluer speeds exceed four hundred meters per minute without a single split score line.