Air Coupled Ultrasonic Guided Wave Measurement for Board Crease Integrity

Pulse

Air-coupled ultrasonic testing sends high-frequency acoustic waves across an air gap into structured paperboard, avoiding liquid couplants that damage cellulosic fibers. Piezocomposite transducers set in a pitch-catch geometry generate flexural Lamb waves within the sheet plane. The transmitter emits compressive air pulses at incident angles calculated from Snell’s law to match the phase velocity of the fundamental antisymmetric wave mode.

Energy moves through the paperboard matrix, crosses the score line, and radiates back through ambient air to a focused receiver on the opposite side of the score.

Acoustic impedance governs air coupling. Dry atmospheric air presents an acoustic impedance near 413 Rayl, whereas dense paperboard plies range from 1.5 to 2.5 MRayl. This severe impedance mismatch reflects more than 99 percent of incident sound pressure at the first boundary.

Modern transducers overcome transmission loss using high-sensitivity ferroelectric elements, optimized matching layers, and narrow-band excitation driving voltages between 100 V and 400 V. Operating frequencies spanning 40 kHz to 450 kHz balance spatial resolution across tight crease geometries against attenuation losses in air.

A dynamic amplitude attenuation of 14.2 dB across a 1.4 mm score line at 120 kHz signals structural shear separation within middle board plies under standard ISO 187 conditioning.

Guided wave propagation across a paperboard crease depends on physical properties established during sheet formation and finishing passes, as plies separate under tension.

• Transducer Incident Angle sets the phase velocity matching required to selectively excite either symmetric or antisymmetric Lamb wave modes without mode conversion.
• Excitation Frequency Selection dictates the acoustic wavelength relative to paperboard caliper and score width, controlling spatial resolution across the deformation zone.
• Air Gap Distance determines the geometric beam spreading and atmospheric attenuation of sound pulses traveling between transducer faces and sheet surfaces.
• Sheet Moisture Content alters the viscoelastic damping coefficient of paperboard fibers, directly influencing wave amplitude decay independent of mechanical creasing damage.

Score lines created during converting alter local material density and structural continuity. When male-to-female dies make proper contact with the board matrix, middle plies delaminate intentionally to form a flexible mechanical hinge. This internal ply separation disrupts the wave guide, scattering acoustic energy and delaying pulse arrival time.

Insufficient impression leaves internal plies intact, passing acoustic waves with minimal attenuation, while excessive die pressure ruptures outer linerboard fibers, creating acoustic scattering signatures distinct from pure internal delamination.

Converter make-ready teams rely on acoustic transmission profiles to confirm how die pressure shapes the delamination zone and forms the hinge before running high-speed folding equipment.

Shear

Creasing paperboard requires controlled z-direction shear deformation to collapse internal ply bonds while leaving top and bottom linerboards unruptured. The male creasing rule forces sheet caliper down into the female channel, inducing high tensile stress on the outer liner and severe shear stress through middle thermo-mechanical pulp plies. In solid bleached board, uniform fiber orientation permits consistent internal shear sliding.

Recycled cartonboard containing short, highly refined fibers exhibits lower z-direction shear strength, increasing the risk of complete sheet rupture during scoring.

Mechanical failure at the crease shows up as micro-cracking along outer liner coatings or unwanted delamination spreading outside score boundaries. Air-coupled ultrasonic guided waves detect these internal structural transitions continuously. As fundamental antisymmetric waves traverse a score line, internal delamination voids act as mechanical impedance discontinuities.

Acoustic energy splits into reflected and transmitted components, causing a measurable drop in transmitted signal amplitude proportional to the total delaminated area inside the score channel.

When surface liners crack under excessive tensile strain, acoustic guided waves undergo broad-frequency scattering that alters signal shape, producing high attenuation across all spectral bands.

Proper creasing creates uniform internal delamination that reduces bending resistance without cutting outer sheet fibers.

Evaluation of score integrity depends on isolating internal ply shear from surface liner fractures. The male rule penetration depth fixes the degree of middle ply delamination. If the clearance ratio between male rule and female channel drops below 110 percent of total board caliper, shear stresses concentrate sharply at the female die shoulders, where high attenuation marks fiber breakage.

This localized stress concentration generates micro-cracks along the print surface, destroying ink film continuity and allowing moisture penetration into raw fiber layers during packaging storage.

Suboptimal tooling geometry weakens box stacking strength. Oversized female channels leave ply delamination incomplete, forcing high bending resistance on automated cartooning lines and causing panel bulging, corner pop-outs, and jam-ups on high-speed gluer lines.

Signal

Where Does Signal Attenuation Correlate with Crease Stiffness?

Signal analysis algorithms compare wave amplitude, phase velocity, and frequency spectrum variations between uncreased board regions and completed score lines. The fundamental antisymmetric mode, designated A0, exhibits high displacement perpendicular to the sheet plane, making it sensitive to delamination and caliper variations. Phase velocity dispersion curves show that A0 wave velocity drops alongside the frequency-thickness product.

In a creased region where effective board caliper decreases and internal void fraction rises, the A0 mode slows down significantly, introducing a measurable phase delay in the received time-domain signal.

Zero Group Velocity resonance modes provide localized structural mapping. At specific frequency-thickness product thresholds, the group velocity of higher-order Lamb wave modes drops to zero, trapping acoustic energy within the thickness of the sheet directly under the sensor heads. Measuring the resonant frequency shift of these localized modes enables accurate thickness and stiffness profiling across a 1 mm score channel without spatial averaging blurring from adjacent uncreased material.

Setting up an air-coupled ultrasonic inspection system on a converting line requires systematic spatial and electronic alignment to achieve repeatable signal levels.

1. Position the piezocomposite pitch-catch transducer heads at the theoretical Snell angle calculated for the target sheet caliper and acoustic velocity.
2. Adjust the vertical air gap distance between transducer faces and paperboard surface to maintain a uniform 10 mm clearance.
3. Drive the transmitting transducer with a 5-cycle tone burst centered at 120 kHz using a dedicated power amplifier.
4. Capture the time-domain voltage trace on the receiving transducer using high-speed digital sampling at 10 MHz.
5. Apply a Hanning window over the primary A0 wave packet to isolate the direct path arrival from edge reflections.
6. Compute the Fast Fourier Transform of the windowed waveform to determine peak spectral energy attenuation relative to an uncreased reference sheet.
7. Extract the phase delay at peak frequency to calculate localized phase velocity reduction across the score line.

Signal noise stems from ambient converting floor vibration and atmospheric turbulence generated by fast-moving paperboard webs. Differential signal paths and digital bandpass filtering suppress mechanical machinery noise occurring below 20 kHz. Air gap temperature variations shift acoustic sound velocity in air by 0.6 meters per second per degree Celsius, altering transmitter angle alignment and corrupting time-of-flight measurements.

Operational systems incorporate real-time temperature compensation sensors positioned near the acoustic sound path to adjust transit time calculations dynamically.

What structural threshold separates acceptable internal ply delamination from destructive score fracturing across variable board densities?

Coating

Surface finishing passes alter the outer boundary conditions of the paperboard matrix, changing acoustic wave dispersion and absorption properties. Extrusion coating with low-density polyethylene adds a viscoelastic polymer skin layer that increases high-frequency acoustic attenuation. Pigmented mineral coatings consisting of calcium carbonate or kaolin clay bound with styrene-butadiene latex increase surface stiffness, elevating the phase velocity of symmetric S0 Lamb wave modes.

Metallic foil laminations introduce dense aluminum barriers that increase acoustic reflection at the air interface.

Polymer film lamination causes sheet curl when single-sided coatings shrink after heat application. This sheet curl alters the local angle of incidence between air-coupled transducers and the board surface, misaligning pitch-catch paths. A beam misalignment of two degrees reduces received amplitude by up to 8 dB, mimicking severe structural creasing damage when the underlying fiber matrix remains intact.

Automatic tilt-compensating transducer arrays adjust beam steering electronically to maintain optimal alignment against curling webs.

UV varnish layers embrittle under high intensity curing lamps, reducing strain-to-break limits on outer linerboards. During scoring, brittle varnishes develop micro-cracks before underlying cellulose fibers reach yield point.

ISO 5628 specifies bending resistance testing parameters, but non-contact ultrasonic evaluation captures non-destructive micro-structural integrity across score lines before conversion.

Surface variations complicate non-destructive evaluation by introducing non-linear signal attenuation unrelated to structural fiber defects.

• Coating Viscoelastic Damping absorbs high-frequency acoustic energy within polymer layers, broadening transmitted wave pulses.
• Varnish Interface Delamination creates secondary acoustic reflections that interfere constructively or destructively with primary guided wave arrival packets.
• Substrate Moisture Barrier Coatings prevent environmental moisture equilibrium, creating internal moisture gradients that skew sound velocity calibration curves.
• Metallic Foil Shielding increases external acoustic reflectance, requiring higher excitation voltages to drive sufficient sound energy into the underlying board plies.

Surface micro-crazing detected by high-frequency acoustic waves can appear as cosmetic coat cracking that leaves overall box compression performance intact; however, liner coat cracking allows atmospheric moisture ingress, weakening internal fiber hydrogen bonds and reducing long-term box stacking strength by up to 22 percent under humid warehousing conditions.

Yield

Deploying air-coupled ultrasonic crease measurement systems alters line yield economics by capturing defective scoring configurations before folding boxboard enters high-speed carton assembly. Traditional quality assurance relies on off-line destructive testing of score bending resistance using ISO 2493 punch-and-die meters. Off-line sampling evaluates less than 0.01 percent of total production runs, allowing thousands of sub-standard creased cartons to reach folder-gluers before operators detect tool wear or make-ready drift.

In-line ultrasonic monitoring measures every score line at web speeds reaching 400 meters per minute. Real-time amplitude and phase data feeds automated die-positioning actuators, maintaining consistent internal delamination across shift changes, moisture fluctuations, and board caliper variance. Preventing invalid score setup reduces blank scrap generated during make-ready by 60 to 80 percent, saving high-value printed and laminated substrate stock.

Tooling wear flattens male rule profile radii over extended production runs. As male rules wear down from continuous impact against abrasive mineral-coated board, peak shear stress drops while compressive deformation widens. Ultrasonic guided wave signals register this transition through a steady decrease in attenuation across the score center accompanied by broadening of the acoustic attenuation spatial footprint.

Plant maintenance teams set automated threshold alerts based on attenuation footprint expansion, scheduling die re-tooling before box compression strength drops below specification limits.

Score bending resistance correlates directly with final package performance on automated filling lines. Excessive bending resistance causes carton sidewalls to bow outward during flat-to-square opening, leading high-speed inserter pushers to misfeed and shut down packing operations. Insufficient score integrity weakens vertical corner posts, dropping box compression test ratings and causing bottom-carton collapse inside palletized freight stacks.

Inline acoustic verification converts destructive bench-sampling inspections into immediate process-control metrics across high-speed packaging runs.

Commercial contracts specify mechanical compliance limits for creased board shipments, including score ratio tolerances defined under DIN 55437. Integrating air-coupled ultrasonic data into batch qualification dossiers establishes verifiable proof of internal ply delamination consistency across every delivered pallet roll. Incorporating continuous acoustic integrity metrics into purchasing contracts overrides subjective visual inspection methods, preventing costly batch rejections and duty disputes at international destination ports.