Online Thickness Measurement Technologies for Hot Melt Coating: Beta Gauges and NIR Sensors
Online thickness measurement (also called coat weight measurement) is essential for closed-loop control of hot melt coating processes. By continuously measuring the weight or thickness of the applied adhesive, the control system can adjust pump speed or die gap to maintain target coat weight within tight tolerances (±1-2%). The two dominant technologies are beta transmission gauges and near-infrared (NIR) absorption sensors. Beta gauges have been used for eight decades and are reliable, but they use radioactive isotopes, raising safety and disposal concerns. NIR sensors are non-nuclear, safe, and have improved significantly in recent years, making them increasingly popular for hot melt coating. A third option is X-ray fluorescence (XRF), used for very thin metallic coatings, but less common for adhesives. The traditional method for online measurement of extruded/cast films, extrusion coatings, solution and hot-melt coatings and nonwovens has been the use of one or more Beta Transmission Sensors. However, as the availability of certain isotopes has become uncertain and end-users are more concerned about radioactive sources, the desire to find alternative non-nuclear measurement solutions is at an all-time high. Recent improvements in NIR sensors have expanded their use into traditional beta-gauge applications with a good degree of success.
Beta transmission gauges work by measuring the attenuation of beta radiation (electrons) as it passes through the web. A source emits beta particles; a detector on the opposite side measures how many particles penetrate the web. The attenuation is proportional to the basis weight (mass per unit area) of the material. For a coating application, two beta sensors are required: one before and one after the coater, to perform a mass subtraction to calculate the net coating basis weight. This method requires precise same-spot synchronization of the scanners, and any stretching of the web or change in moisture will introduce measurement error. Available isotopes include Promethium-147 (Pm-147) for lightweight coatings (1-250 gsm), Krypton-85 (Kr-85) for medium weights, and Strontium-90 (Sr-90) for heavy coatings (200-600 gsm). However, Pm-147 is no longer available, and Kr-85 supplies have diminished. The energy level must be matched to the coating weight; using a Sr-90 source on a thin 20 gsm coating is unsafe and inaccurate. Beta gauges are highly accurate when calibrated correctly and are not affected by color or opacity, making them suitable for pigmented adhesives. However, the radioactive sources require licensing, regular leak testing, and secure disposal—significant burdens for many manufacturers. The diminishing availability of Pm-147 has accelerated the shift to non-nuclear alternatives.
Hot Melt Coating Machine - Hot Melt Adhesive Coating Machine
Near-infrared (NIR) absorption sensors offer a non-nuclear alternative that directly measures coat weight by analyzing the absorption of specific wavelengths of light. Different chemical bonds (C-H, O-H, N-H) absorb NIR light at characteristic wavelengths. By measuring the absorption at these wavelengths, the sensor can determine the amount of adhesive present. In similar manner to the measurement of aqueous coatings, the NIR sensor adapts itself very well to the measurement of dry coatings after an oven or 100%-solids coatings (solvent coatings, hot-melts, etc.). NIR sensors can be configured as single-point measuring heads (e.g., PrediktIR II) or traversing scanners (e.g., SlimTrak) that provide a cross-web profile. Typical measurement range is 1 to 250 gsm, with accuracy comparable to beta gauges (±0.5-1 gsm absolute) for clean, well-calibrated systems. NIR sensors have several advantages: no radioactive source, lower cost, no scanning head synchronization required (a single sensor can measure the coated web directly, though a substrate-only measurement is still needed for absolute coat weight; this can be obtained by measuring an uncoated edge of the web). NIR sensors are also faster than beta gauges in terms of response time, as there is no need for dual scanners. Implementation of NIR technology to measure hot melt coatings speeds startup, increases production, reduces scrap and allows for quality data archiving by roll or run. The Guardian-HD Web Profiling Series measures moisture, coat weight, adhesive thickness, and web temperature for all web-converting processes.
NIR sensors do have limitations. They are sensitive to the chemical composition of the adhesive; a change in resin type or pigment addition can alter the absorption spectrum, requiring recalibration. For clear or lightly colored adhesives on a dark substrate, NIR works well; for heavily pigmented (e.g., white titanium dioxide-filled) adhesives, the pigment may interfere with the measurement, though advanced sensors can compensate. NIR sensors also require a stable ambient environment; dust or moisture on the sensor optics can cause errors. Calibration is performed by taking offline samples of known coat weight (gravimetric measurement) and correlating them to the NIR signal; this calibration must be repeated whenever the adhesive formulation changes. Despite these limitations, NIR sensors are now the preferred choice for many hot melt coating lines, especially those producing clear or lightly colored PSAs for tapes and labels. The ability to avoid radioactive materials simplifies regulatory compliance and reduces long-term operating costs. For converters who must comply with ISO 14001 environmental standards, eliminating radioactive sources is a significant advantage. The MCT460 Online NIR Smart Sensor is used to continuously measure and control moisture, coat weight, film thickness, and product temperature during manufacturing, with applications including converting: hot melts, pressure sensitive adhesives. Control Metrix offers a range of non-contact NIR gauges that measure the thickness of many types of coatings including adhesive coatings on paper, film or adhesive tapes and hot melt adhesives on all types of substrates.
Scanning systems provide cross-web profile measurement. A traversing gauge moves across the web width (typically at 10-50 mm/s), taking measurements at regular intervals. The result is a 2D profile map showing coat weight at each position across the web. This data is used to control die bolts (manual or automatic) to flatten the profile. The SlimTrak scanning system provides a complete cross-section measurement across the web width; thickness, coat-weight, basis weight, and moisture can all be measured on narrow web lines. For wide webs (1600mm+), a scanning gauge is essential for maintaining uniformity. The gauge typically uses either a beta source (for legacy systems) or an NIR sensor (for new installations). Some advanced systems combine both: a fixed-point NIR sensor for fast feedback on average weight, and a traversing beta or NIR sensor for profile measurement at slower intervals. The measurement system is integrated with the line PLC to provide closed-loop control. The gauge sends a coat weight reading every 10-30 seconds; the PLC compares to target and adjusts pump speed (for average weight) or individual die actuators (for profile). The control loop must account for the transport delay between the coating die and the measurement gauge; a PID with feed-forward compensation is used. The typical accuracy of a well-calibrated system is ±1% of reading or ±0.5 gsm, whichever is greater. For medical tape production, where coat weight tolerance is often ±0.5 gsm on a 20 gsm target (2.5% variation), closed-loop measurement and control is mandatory. By selecting the appropriate online thickness measurement technology—beta for pigmented or variable adhesives, NIR for safety and simplicity—manufacturers achieve consistent, high-quality coated products with minimal waste.
