Shiny Spots and Burn Marks on Calendered Nonwoven: Causes, Diagnosis and Roll Specification

Quick Answer

Shiny spots and burn marks on calendered nonwoven are both energy-dose failures at the web surface. Gloss appears when surface fibres melt, flow and are pressed flat; burn marks appear when the same surface passes from melting into oxidation. In a calender nip the web is in contact for only 2–5 ms and heat penetrates 13–20 µm, so the outcome is set by three variables: the temperature of the steel at the contact line, the local contact pressure, and the dwell time. Hold the cross-face profile within ±1.5 °C, match crown to the real line force, and keep the nip open during stops — the defect normally disappears within one shift.

Key Takeaways

· Gloss is an optical surface change; it does not by itself prove structural over-bonding. Tear and elongation tests settle that.

· Cross-face deviation above ±2 °C is a finding; above ±4 °C it is the cause of a visible band.

· A 0.03 mm crown error over a 2,500 mm face can drop edge nip pressure below the bonding threshold.

· Dwell halves when line speed doubles, so the temperature correction operators apply is usually too large and creates the next defect.

· Carbonised polymer on the shell insulates, runs hotter than the steel around it and prints a mirror image into the web.

· A stop with the nip closed and rolls hot leaves a burn patch matching one roll circumference exactly.

· Uniformity, crown at load, hot runout and balance belong in the purchase order, verified by documents shipped with the roll.

Summary Table: Defect Signature and First Check

Definition

A gloss defect is a localised rise in specular reflection, measurable as increased 60° gloss units against a reference sample, caused by fibre flattening, film formation or pattern transfer at the nip. A burn defect is a colour change — yellow, amber, brown or black — with embrittlement, caused by thermal oxidation or chain scission, sometimes with carbonised residue transferred from the roll. Gloss is surface reorganisation; burn is chemical degradation.

Working Principle

Bonding energy is the product of contact temperature, contact pressure and contact time. Contact time is nip width divided by line speed: an 8 mm nip at 150 m/min gives 3.2 ms, and at 300 m/min it gives 1.6 ms. Heat diffuses by roughly the square root of diffusivity × time, which for polyolefins at 0.1 mm²/s is about 18 µm at 3.2 ms and 12.6 µm at 1.6 ms — a 2 denier fibre is around 17 µm across.

Only the outer skin of the fibres touching the roll reaches bonding temperature. Bonding therefore depends on the roll surface, not on the average web temperature, and any local variation in surface temperature or contact pressure shows up directly as variation in bond quality and appearance. The energy window between "not bonded" and "glazed" is often only 10–15 °C of surface temperature at fixed dwell, which is why a modest profile error produces a visible defect on an otherwise acceptable roll.

Benefits of Correct Profile Control

· Higher first-quality yield, because defects are removed at source rather than sorted at the winder.

· A wider process window, so speed changes do not require a recipe rewrite.

· Longer cleaning intervals, since uniform surface temperature reduces local degradation.

· Lower energy use, because a uniform roll does not need a raised setpoint to mask a cold edge.

· A documented baseline for every later maintenance decision.

Applications and Sensitivity

Hygiene spunbond and SMS are most sensitive to gloss because the defect is visible to the consumer. Medical fabrics reject discoloration outright. Filtration media react through permeability, since a glazed patch has collapsed porosity. Geotextiles tolerate appearance variation but are vulnerable to burn at low speeds where dwell is long. Automotive interior grades read pattern gloss variation as colour mismatch in the finished part.

Material Comparison

Application Comparison