# Windscreen adhesive cure time: why a bead stays soft > Why a bead is still soft the next morning: cold glass, dry air, thick sections and the four reasons a cure runs late, with the arithmetic that predicts it. Updated: 2026-11-10 · Source: https://karmindadhesive.com/resources/why-windscreen-adhesive-cures-too-slowly/ "the adhesive did not cure" is almost never an adhesive problem. Moisture-curing PU cures at a rate set by temperature, humidity and bead geometry — and the bay controls all three. ## Skin time is not cure time Skin time is how long the surface stays workable: 15 minutes for KA530F, 30 for KA518, 40 for KA520. Cure rate is how fast the bead hardens through its depth: 3.2 to 4.0 mm per day. A 10 mm bead at 4 mm/day is not fully cured in two hours. It is strong enough to hold the glass, which is a different claim. Confusing the two is how "two-hour drive-away" becomes "the glass moved". ## Reason 1: the glass is cold Reaction rate roughly doubles for every 10 °C, and below about 5 °C it effectively stops. A windscreen stored overnight at 2 °C will pull the bead temperature down for the first hour after installation, no matter what the workshop thermostat says. Fix: bring glass and cartridges to room temperature before use. Do not heat the cartridge with a hot air gun — you shorten pot life and can degrade the prepolymer. ## Reason 2: the air is dry Moisture-curing chemistry needs water. At 20% RH in a heated winter workshop, cure slows dramatically. A humidifier in the bay is a cheaper fix than a faster adhesive. ## Reason 3: the bead is too thick Cure progresses from the surface inward. A 15 mm bead takes roughly four days to cure through, while the surface skins in 20 minutes. If the joint needs a thick bead, either accept the longer cure or use a two-part system. ## Reason 4: the cartridge is old Shelf life is real. A cartridge stored warm for a year skins over in the nozzle and cures unevenly. Check the batch date, and store below 25 °C. ## How to diagnose it Cut a section of the bead after 24 hours and look at the cross-section: a soft core with a hard skin is normal for a thick bead. A uniformly soft bead points to cold, dry conditions or an expired cartridge. A crumbly, foamed bead points to moisture ingress before use. ## The temperature arithmetic, worked Reaction rate roughly doubles for every 10 °C, which turns a cure-rate figure into a table. ``` KA530F, 10 mm bead, 4.0 mm/day at 23 °C 23 °C 4.0 mm/day → full section in 2.5 days 13 °C 2.0 mm/day → 5 days 3 °C 1.0 mm/day → 10 days -7 °C effectively stopped ``` The same table applies to every grade; the fast one simply starts further ahead. This is why a winter bay that reports "the adhesive is still soft after two days" is usually reporting a glass temperature of 5 °C, not a bad batch. Two refinements matter: - **The bead temperature is not the air temperature.** A glass pulled from an outdoor rack at 0 °C holds the bead near freezing for the first hour, and that hour is when most of the skin forms. - **Humidity has a smaller effect than temperature on the core.** Below 30% RH the surface skins slowly; the core cure is diffusion-limited and barely notices. A dry workshop slows the skin, not the depth. ## Test cure by destruction, not by touch Thumb pressure measures the first two millimetres. The only field test that says something about the core is destructive: 1. Apply a bead of the same section to an offcut at the same time as the job. 2. At the drive-away time, cut the bead across and look at the section. 3. A cured shell with a soft core is normal; a uniformly soft bead means the conditions, not the product. Keep the offcut with the job card. When a bond fails six months later, the offcut is the only physical record of how that batch cured on that day. ## What this means for buyers 1. Condition glass and cartridges to room temperature before installation. 2. Watch humidity, not just temperature, in a winter bay. 3. Do not exceed the bead section the cure rate supports. 4. Keep batch dates; an expired cartridge is not a manufacturing defect.