Dynamic Mechanical Analysis Caliper Retention Testing for Multi-Layer Boxboards

Ply

Multi-layer paperboard architectures derive bending resistance from spacing dense surface fibers away from the neutral axis. The structural efficiency of folding boxboards relies on a layered distribution of furnish components. Bleached chemical pulp layers form the smooth outer skins, while middle layers utilize high-yield thermo-mechanical pulp, chemithermomechanical pulp, or secondary recycled fibers.

This construction maximizes total sheet thickness while minimizing basis weight. Bending stiffness remains directly proportional to the elastic modulus of the outer skins multiplied by the third power of total thickness. A minor degradation in central layer thickness causes a severe reduction in structural performance.

Z-direction mechanical stability determines how effectively the central structure resists permanent consolidation during processing and storage. Chemical pulp skins maintain high tensile strength and elastic recovery under load. In contrast, mechanical pulp cores rely on the physical bulk of unrefined, stiff fibers and hollow lumen geometry to maintain volumetric space.

When dynamic forces press down on the sheet surface, internal air voids compress and inter-fibre bonds experience high shear stresses. High-yield pulps preserve initial bulk under ambient conditions, yet their cellular walls succumb to collapse when dynamic mechanical limits are exceeded. Core density determines thickness loss.

A ten percent reduction in middle layer thickness decreases total package stiffness by nearly twenty-seven percent.

Recycled fibers in coated recycled board grades introduce further structural vulnerability under mechanical stress. Repeated repulping cycles shorten cellulosic fibers, hornify cell walls, and destroy internal lumen spaces. Consequently, recycled core layers require additional mineral filler loadings and starch binder additives to achieve target initial thickness.

Under dynamic loading, these unbonded filler particles slip past shortened fibers, resulting in lower z-axis elastic recovery compared to virgin mechanical furnish.

• Crease cracking occurs when the score line loses internal z-direction thickness and forces outer chemical skins to take tensile strain beyond their elongation limit.
• Bulge deformation appears on filled cartons when sidewall central layers collapse under internal contents hydrostatic pressure during transport.
• Corner collapse develops when vertical edge columns sustain permanent thickness reduction from dynamic top-load stacking forces.
• Delamination splitting results when internal shear forces exceed the lower interlaminar bond strength of compressed mechanical core plies.

Evaluating multi-layer boxboards requires analyzing the viscoelastic response of each individual furnish layer rather than treating the sheet as a single homogeneous mass. Standard static thickness gauges apply fixed presser foot forces for set durations, masking the time-dependent plastic strain occurring within internal mechanical plies. Ignoring the dynamic loss of thickness within middle layers leads directly to specifying excessive basis weights to compensate for structural failures that arise during high-speed converting.

Amplitude

Measuring thickness retention under sinusoidal compression requires sweeping oscillatory forces across defined frequency ranges. Dynamic Mechanical Analysis applies a controlled cyclic stress or strain in the thickness direction, decoupling the elastic energy stored by the fibrous network from the viscous energy dissipated through polymer rearrangement. The complex compressive modulus defines total material resistance.

The real component, known as storage modulus, quantifies elastic thickness retention, while the imaginary component, known as loss modulus, represents permanent viscous deformation. The ratio of loss modulus to storage modulus yields tan delta, indicating internal damping.

Fibre orientation governs sheet performance. Testing within the linear viscoelastic region requires identifying the maximum strain threshold where the storage modulus remains independent of applied force amplitude. Standard test procedures apply sinusoidal compression at frequencies ranging from zero point one to one hundred Hertz under precise environmental conditioning according to ISO 187 standards.

Small strain amplitudes preserve internal fibre network bonds, isolating fundamental material properties. Large strain sweeps force structural cell wall collapse, exposing the transition point where irreversible plastic deformation begins.

At eighty-five percent relative humidity and thirty degrees Celsius, mechanical pulp core plies lose forty-two percent of their dry compressive storage modulus.

Relative humidity exerts a profound effect on the storage modulus of multi-ply boxboards. Water molecules penetrate amorphous cellulose regions and hemicellulose matrices within fiber walls, breaking intermolecular hydrogen bonds. This plasticization process shifts the glass transition temperature of internal polymers downward into ambient operating ranges.

Under dynamic mechanical testing, elevated moisture levels cause tan delta to spike sharp peaks during cyclic compression. Moisture accelerates internal polymer collapse. The loss of elastic storage capacity in the core layer accelerates overall caliper loss.

The following sequence outlines the standard calibration and execution steps required for quantifying z-axis dynamic mechanical properties across relative humidity transitions:

1. Mount a thirty-millimeter diameter circular test specimen between parallel titanium compression platens inside the environmentally controlled DMA chamber.
2. Apply a static pre-load force of zero point five Newtons to establish uniform contact across the sheet surface without exceeding the elastic limit.
3. Set the environmental control system to establish baseline conditions at twenty-three degrees Celsius and fifty percent relative humidity for two hours.
4. Execute an amplitude sweep from zero point zero one to fifty micrometers at a fixed frequency of one Hertz to establish the linear viscoelastic boundary.
5. Apply an oscillatory force within the linear range while stepping the environmental chamber humidity from fifty percent to eighty-five percent relative humidity at one percent per minute.
6. Record continuous measurements of dynamic displacement, storage modulus, loss modulus, and tan delta throughout the humidity ramp.
7. Calculate the permanent displacement offset after returning the specimen to baseline humidity conditions and unloading the compressive force.

Dynamic mechanical testing proves that caliper retention varies dramatically across distinct commercial packaging grades, as shown in the laboratory comparison data below.

Mill technical sales teams often attribute post-converting bulk loss to minor variations in ambient mill humidity during winding. They claim swatch-book caliper tolerances apply only to unprinted, uncalendered raw board straight off the reel, disclaiming performance drops occurring on converting equipment.

Nip

High-speed converting rollers subject paperboard webs to millisecond compression pulses that alter fibre density. Modern sheet-fed offset printing presses, flexographic coaters, and rotogravure units apply high dynamic nip pressures to transfer inks and coatings evenly. A typical printing press impression cylinder creates local z-direction pressures between one point five and three point five Megapascals.

Dwell times inside the contact area range from two to fifteen milliseconds depending on line velocity. This transient pulse exceeds the static yield stress of mechanical core plies, compressing the sheet rapidly.

How Does Dynamic Nip Dwell Time Alter Caliper Recovery?

Compressive energy imparted during rapid roller contact forces air out of internal core pores and drives individual fiber segments into tight alignment. Viscoelastic flow reduces elastic recovery. When the compressed sheet emerges from the nip exit, chemical surface layers recover their dimensions almost instantaneously.

The mechanical middle layer recovers at a much slower rate governed by time-dependent viscoelastic relaxation. If the machine line speed is increased, the reduced dwell time limits viscous flow, yet higher peak dynamic pressure spikes can crush hollow CTMP fiber lumens permanently.

Dynamic force destroys pore volume. Higher web speeds reduce dwell. Converting lines equipped with inline calendering or multi-station print units compound caliper degradation across sequential processing stations.

Passing board through four consecutive print nips generates cumulative plastic deformation, reducing total sheet thickness long before the web reaches the creasing matrix and die-cutting station.

Engineers manage converting settings using structured verification steps:

• Threshold stress evaluation establishes the dynamic storage modulus limit on laboratory DMA instruments prior to setting press impression pressures.
• Line speed adjustments balance dynamic compression dwell time against throughput targets to minimize core ply crushing.
• Anvil clearance setting optimizes die-cutting cylinder impression gaps to prevent localized board destruction adjacent to score lines.
• Moisture window verification ensures press room atmospheric control remains strictly within forty-five to fifty-five percent relative humidity.

Whether modern high-speed printing nips induce micro-fractures in starch-bound mechanical core layers that continuously degrade long-term box compression strength remains an open question across packaging research laboratories.

Creep

Warehouse pallets stacked four high deliver continuous static loads compounded by transport vibration. Stacking weight forces z-axis strain. Boxboard containers stored in unconditioned fulfillment facilities sustain chronic compressive stress over weeks or months.

This static top-load stress, combined with dynamic transit vibrations ranging from five to fifty Hertz, induces compressive creep within the middle layer. Under continuous load, the fiber network undergoes structural sliding and micro-buckling, reducing the thickness dimension continuously over time.

Cyclic humidity changes accelerate creep strain dramatically, a phenomenon recognized as mechano-sorptive deformation. As moisture content within the board cycles between ambient extremes, the internal stress distribution reorganizes under load. Each moisture absorption cycle increases fiber wall flexibility, allowing applied vertical loads to collapse internal structural voids further.

Pallet vibration breaks web bonds. When the relative humidity drops, the newly consolidated structure dries in its compressed state, locking in permanent caliper reduction.

Compliance with ISO 187 conditioning guarantees static test accuracy but fails to predict dynamic thickness decay in unconditioned freight containers.

The loss of thickness directly impairs the Box Compression Test rating of finished cartons. Box compression strength depends on vertical panel bending stiffness and corner post column rigidity. As compressive creep reduces sheet thickness, the bending stiffness of the flat carton walls declines at a cubic rate.

The structural load shifts entirely to the carton corners, exceeding the compressive strength of the vertical edges and causing catastrophic pallet collapse during transit.

• Z-direction modulus profile provides continuous dynamic storage and loss modulus values across operating temperature and humidity spectrums.
• Dynamic humidity decay curve documents total thickness retention percentage during forced moisture cycling under constant compressive stress.
• Permanent thickness offset report details post-load dimensional recovery measured exactly twenty-four hours after releasing static pre-load.
• Batch statistical variance distribution declares upper and lower control limits for basis weight and bulk thickness across the manufacturing run.

Quality assurance contracts incorporate technical supply terms to enforce board structural integrity: “The substrate supplier warrants a minimum z-axis dynamic storage modulus of ten Megapascals measured under zero point five percent dynamic strain at eighty-five percent relative humidity, ensuring total sheet thickness retention stays within four percent of certified ISO 534 baseline values over a sixty-day static load duration.”

Tariff

Stock pricing tied to mill tonnage creates commercial tension against converted sheet yield. Paperboard mills sell substrate by weight, pricing orders per metric tonne. Buyers sell finished cartons by unit volume, requiring square meters of yield at precise caliper thresholds to satisfy filling line mechanical tolerances.

Choosing low-density, high-bulk folding boxboards reduces the total tonnage needed to produce a fixed number of cartons. Yield losses drive real costs. If a lower-grammage high-bulk board loses excessive thickness under converting dynamic stresses, the finished pack fails performance specifications, forcing buyers toward heavier grammage sheets.

Thicker skins maintain structural resistance. Heavy sheets increase freight billing. Substituting a virgin folding boxboard with a recycled grade often appears financially attractive on initial tonnage price quotes.

Recycled grades trade at lower base prices per tonne compared to virgin fiber stocks. However, recycled furnish exhibits lower compressive storage modulus and higher dynamic loss factors under DMA testing. To match the required bending stiffness of a three-hundred-micrometer virgin GC2 board, a buyer must select a significantly heavier recycled GD2 grade, completely erasing the nominal price per tonne savings through reduced sheet yield per ton.

Consider a practical purchasing calculation for a packaging run of one million folded cartons requiring a minimum post-converting thickness of three hundred and fifty micrometers. Sheet dimensions are seven hundred by one thousand millimeters. Virgin Folding Boxboard GC2 at two hundred and fifty grams per square meter delivers an initial caliper of four hundred micrometers.

Under press compression and folder-gluer loading, the sheet exhibits a twelve percent caliper loss, retaining three hundred and fifty-two micrometers and meeting specification. Total board mass required for one million sheets equals one hundred and seventy-five metric tonnes. At a price of one thousand seven hundred dollars per tonne, landed material cost totals two hundred and ninety-seven thousand five hundred dollars.

An alternative recycled GT2 board option provides an initial caliper of four hundred micrometers at a higher basis weight of three hundred and twenty grams per square meter. Due to a lower z-axis compressive storage modulus, the recycled furnish suffers a twenty-two percent caliper loss during processing, landing at a final thickness of three hundred and twelve micrometers. This fails the minimum specification.

To hold the three hundred and fifty micrometer post-converting threshold, the specification must jump to a three hundred and seventy gram per square meter GT2 grade. Total board mass required rises to two hundred and fifty-nine metric tonnes. Even at a lower price of one thousand six hundred dollars per tonne, the landed substrate cost rises to four hundred and fourteen thousand four hundred dollars.

The lower-priced recycled alternative increases total raw material expenditure by thirty-nine percent while adding eighty-four tonnes of excess shipping weight.

Specifying extra basis weight to recover lost thickness inflates freight charges and increases extended producer responsibility fees without restoring core elasticity.

Thickness retention protects carton value. Dynamic mechanical analysis isolates the precise viscoelastic boundaries of multi-layer furnish plies, converting converting risks into measurable material metrics. Specifiers who audit substrate storage modulus under dynamic conditions protect their production yields, holding convertors and paper mills to verified physical performance standards.