Non Contact Thickness Gauging on Continuous Web Board Machines
Non-contact thickness gauging uses optical or nuclear sensors to deliver real-time caliper profiles, enabling basis weight down-gauging while meeting ISO 534 limits.
X-Ray Backscatter
Compton Scattering Metrology
Diffuse reflection of ionizing radiation directed at moving web surfaces provides non-contact measurement of coating thickness and inorganic pigment loading in paper conversion. Photons generated by low-energy X-ray tubes strike the substrate, undergoing Compton scattering where incident radiation interacts with light atomic nuclei and bounces back to solid-state detector arrays on the same side of the processing line. Continuous X-ray backscatter measurement evaluates barrier layers, silicone coatings, and mineral formulations on paperboard webs without requiring sensors beneath the sheet.
The measurement loses fidelity when base sheet caliper changes dramatically or when substrate moisture shifts change bulk scattering coefficients.
Scattering Mechanics
Incident photons penetrate surface applications and interact with atomic structures, with a portion of the energy deflected backward through incoherent scattering. Backscatter intensity depends on the average atomic number and electron density of the irradiated target material. Mineral pigments containing calcium carbonate, clay, or titanium dioxide scatter radiation with distinct energy signatures compared to the low atomic numbers found in cellulose and organic polymer coatings.
Sensors mount both the radiation generator and the detector crystal inside a single compact scanning head, operating without the precise geometric alignment required by through-transmission nuclear gauges. Calibrating the return signal requires isolating surface reflections from deep backscatter originating inside the core fibre sheet. Multi-channel energy analysis separates low-energy scatter produced by lightweight surface silicones from high-energy returns generated by thick bleached paperboard cores.
Thermal fluctuations inside scanner assemblies alter solid-state detector efficiency, requiring internal water-cooling circuits and automated reference calibration passes over internal standards during off-sheet carriage returns. Base paper density variations must be decoupled from the coating calculation using simultaneous thickness gauges or optical sensors.
Converting Layer Control
Production lines applying functional barriers to barrier boards require tight coat-weight control to achieve water and grease resistance without wasting raw chemicals. Real-time backscatter feedback connects to automated coating blade profiling actuators, maintaining uniform dry pick-up across six-metre web widths running at high production velocities. Detecting localized coating streaks prevents whole rolls from suffering barrier failure during subsequent food packaging conversion.
Instantaneous thickness metrics minimize chemical waste during start-up cycles and reduce raw material consumption on high-value release liner manufacturing lines.