Why Coating Thickness Matters

Coating thickness, usually expressed as coating weight in grams per square metre, determines the moisture barrier, strength, and cost of a laminated bag. Too thin and the bag fails to protect its contents and looks weak; too thick and you waste resin on every metre, raising cost with no benefit. Because thickness is set by a balance of machine variables, controlling it precisely is the fastest way to protect both quality and margin on a woven fabric coating line.

The Variables That Set Thickness

1. Screw speed and extruder output

Screw speed controls how much molten resin the extruder delivers. Raising screw speed increases output and coating thickness; lowering it reduces both. For stable results, keep screw speed steady and make fine adjustments in small steps, watching the measured coating weight respond.

2. Line speed

Line speed is the single most powerful lever. At a fixed extruder output, increasing line speed spreads the same resin over more fabric, so coating weight falls; slowing the line raises coating weight. Always adjust line speed and screw speed together to reach the target without starving or flooding the die.

3. Die gap and lip setting

The die gap sets the initial thickness of the molten curtain, and the die lips shape its cross-web uniformity. A correctly set die gives even coating across the width; a worn or misadjusted die causes thick and thin bands that no speed change can fix.

4. Air gap and nip

The air gap between the die and the nip affects how much the molten film draws down before it bonds, which changes thickness. Nip pressure then controls how firmly the film is pressed into the fabric. Keep both consistent to avoid thickness drift.

Measuring and Adjusting

You cannot control what you do not measure. Use inline beta-gauge or thickness sensors where available, and cross-check with periodic lab samples of coating weight. Establish a standard recipe for each product, record the screw speed, line speed, and temperatures, and adjust one variable at a time so you always know what caused a change. Re-verify after every grade change and after any maintenance that touches the die or extruder.

Best Practices for Consistent Thickness

Keep resin dry and consistent, since moisture and grade changes alter melt behavior and output. Maintain stable unwind and rewinding tension so the fabric does not stretch or bunch. Service the die and screw on schedule to preserve flow, and train operators to watch the trend, not just the instant reading. With disciplined settings and regular measurement, coating thickness stays on target and resin waste drops sharply.

Common Mistakes to Avoid

Two mistakes cause most thickness problems: adjusting line speed and screw speed at the same time, which makes the response impossible to read, and ignoring resin moisture, which changes melt behavior and output. Avoid both by changing one variable at a time and storing resin under dry, stable conditions. Keep the die clean and correctly set, and never chase the target by feel, always verify coating weight with a measurement.