Understanding Hot Melt Adhesive Coat Weight
Hot melt adhesive coat weight means the adhesive mass applied to one square meter. It is usually expressed in grams per square meter, or gsm. This value affects bond strength, flexibility, cost, and curing behavior. Too little adhesive can create dry spots and weak seams. Too much may cause squeeze-out, stiffness, or longer cooling time. The correct target depends on the substrate, application pattern, and end-use demands.
Control begins with stable process conditions. Pump speed changes adhesive flow, while line speed changes the amount deposited. A wider die gap usually increases coat weight, but excessive adjustment can damage coating uniformity. Temperature also matters because hotter adhesive flows more easily. Operators should verify the actual adhesive temperature near the coating head, not only at the melting tank. Substrate tension and surface texture can also change the measured result.
Measure it regularly. Cut a known area, remove the adhesive, and weigh the difference with a calibrated balance. Check samples across the web, not just at the center. A single reading may look acceptable while the edges remain under-coated. In production trials, operators sometimes trust machine settings too quickly. That is a mistake worth revisiting. Coating weight can drift as filters load, viscosity changes, or the roll warms. Keep records beside the line, compare results with bond tests, and adjust one variable at a time. Small changes are easier to understand.
| Control Dimension | Typical Reference Value | Effect on Coat Weight | Recommended Control Method | Measurement or Check |
|---|---|---|---|---|
| Coat weight unit | g/m² (GSM) | Provides a consistent way to compare adhesive usage across web widths and production speeds. | Calculate the target mass per square metre before setting the applicator. | Record adhesive mass and coated area for each production sample. |
| Web or substrate speed | 20–150 m/min | At a constant pump output, increasing line speed normally reduces coat weight; reducing speed normally increases it. | Synchronize pump output with line speed and use closed-loop speed control where available. | Verify actual line speed against the machine set point during sampling. |
| Adhesive flow rate | 0.2–6.0 kg/min | Higher adhesive flow generally increases coat weight when substrate width and line speed remain unchanged. | Adjust the gear pump, piston pump, or metering system in small increments. | Collect adhesive for a measured time and weigh it to determine actual flow. |
| Coating width | 100–1,600 mm | With the same total flow, a wider coating width produces a lower coat weight per square metre. | Match die width, slot opening, spray pattern, or roller coverage to the usable web width. | Check edge-to-edge coverage and calculate GSM using the actual coated width. |
| Formula for target coat weight | GSM = Flow × 60 ÷ (Speed × Width) | Links adhesive flow, line speed, and coating width in one operating calculation. | Use flow in kg/min, speed in m/min, and width in metres. The result is approximately kg/m², numerically equal to g/m². | Example: 1.5 kg/min × 60 ÷ (60 m/min × 1 m) = 1.5 g/m². |
| Film or web lamination | 10–30 g/m² | Lower values may reduce bond reliability, while excessive values can increase cost, squeeze-out, and cooling demand. | Set the minimum weight that meets peel, shear, and appearance requirements on the actual substrates. | Use a gravimetric test and confirm bond performance after conditioning. |
| Nonwoven or textile assembly | 2–15 g/m² | Required weight depends strongly on surface porosity, open area, bonding pattern, and mechanical load. | Control spray density, bead spacing, or slot coverage rather than relying only on total pump output. | Inspect coverage uniformity and test peel or tensile strength at several web locations. |
| Packaging bead application | Typically 0.2–2.0 g per metre | Bead size and continuity influence bond strength, set time, and material consumption. | Control nozzle diameter, pressure, pump speed, and application distance. | Weigh adhesive collected from a fixed-length bead and divide by bead length. |
| Adhesive temperature | 120–180°C | Higher temperature lowers viscosity and can increase flow; excessive heat may cause degradation or stringing. | Operate within the adhesive supplier’s recommended processing window and stabilize the system before sampling. | Measure temperature at the tank, hose, and applicator; do not rely on only one reading. |
| Adhesive viscosity | 500–10,000 mPa·s | Viscosity changes affect pump delivery, die flow, spray pattern, and the consistency of the applied layer. | Keep temperature stable and use the same adhesive grade throughout a controlled comparison. | Compare viscosity at a specified temperature and time, using a consistent test method. |
| Coating gap or die opening | 0.1–1.0 mm typical starting range | A larger opening may increase wet film thickness but can also cause streaks, edge build-up, or unstable flow. | Adjust gradually while maintaining constant temperature, pressure, and web tension. | Check transverse coat-weight profile across the full web width. |
| Sampling frequency | Start-up, every 30–60 minutes, and after changes | Frequent checks identify drift caused by temperature changes, filter loading, pump wear, or speed variation. | Use a written sampling plan with defined upper and lower control limits. | Record GSM, line speed, flow rate, temperature, lot information, and operator adjustments. |
| Gravimetric coat-weight test | Sample area: 0.01–0.10 m² | Direct weighing provides a practical verification of average adhesive deposition. | Weigh the uncoated substrate, apply or remove a known-area adhesive sample, and calculate the mass difference. | Coat weight = adhesive mass in grams ÷ sample area in square metres. |
| Typical process tolerance | ±5–10% of target GSM | Tighter tolerance improves material efficiency but may require better temperature, flow, and speed control. | Define the tolerance according to product performance, substrate variation, and application technology. | Use average, minimum, maximum, and standard deviation from repeated samples. |
Important: The reference ranges are general starting points rather than universal specifications. The final coat-weight target should be validated through bond strength, peel, shear, appearance, set time, and aging tests using the actual adhesive, substrates, equipment, and production conditions.
It is the adhesive mass applied to one square meter, usually measured in grams per square meter. It affects bond strength, flexibility, cost, and cooling time.
Too little adhesive can leave dry spots and weak seams. Too much may squeeze out, make the material stiff, or slow cooling.
Pump speed, line speed, die gap, adhesive temperature, and substrate tension all matter. A small temperature shift can change how easily adhesive flows.
Cut a defined sample and weigh the substrate before and after coating. Use a calibrated balance, then calculate the difference per square meter.
A center sample can look fine while the edges are under-coated. Check several positions, such as the left edge, center, and right edge.
Check it near the coating head, not only at the melting tank. The temperature can differ along the line.
Loaded filters, changing viscosity, and a warming roll can affect the result. Machine settings may stay the same. Output may not.
Record coat weight, temperature, and speed beside the line. Change one variable at a time and compare results with bond tests. I would not trust settings alone.
Hot Melt Adhesive Coat Weight Control is essential for achieving reliable bonding performance, efficient material use, and consistent product quality. Coat weight refers to the amount of adhesive applied over a specific area, and the correct target depends on factors such as substrate type, surface condition, application speed, adhesive temperature, coating pattern, and required bond strength. Selecting an appropriate coat weight requires balancing coverage and adhesion with production cost, flexibility, and drying or cooling behavior.
A stable coating process begins with properly setting and calibrating the equipment, including pump output, slot or roller settings, line speed, and temperature controls. During production, operators should measure coat weight at regular intervals using accurate sampling and weighing methods. If results drift from the target, adjustments to adhesive flow, coating gap, web tension, speed, or temperature can help restore consistency. Recording these measurements and responding to changes systematically supports dependable performance and reduces waste.
