Clamp time and press scheduling for PUR

The catalogue gives 1–4 hours cure for KA505. That range is not a tolerance; it is the span between a warm dry board and a cold damp one. Scheduling to the bottom of the range in winter is how joints spring back.

What moves the clock

• Temperature. Reaction rate roughly doubles per 10 °C; a cold press hall doubles the time.
• Moisture. Wet timber cures faster but foams; dry timber cures slower and weaker.
• Section. Heat and moisture move slowly through thick sections, so a 200 mm glulam needs longer than a 45 mm lamella.
• Spread rate. More adhesive means more water demand and a slower, thicker line.

A practical schedule

Condition	Clamp time to plan
20–25 °C, 8–12% MC, thin section	1–2 h
15–20 °C, 8–12% MC	2–3 h
Below 15 °C or thick section	3–4 h, verify by test

These are planning numbers derived from the catalogue range, not a guarantee. The release test is a broken sample joint: cohesive wood failure means the bond is ready; a clean glue-line failure means it is not.

Pressure and squeeze-out

Even pressure matters more than high pressure. A starved joint from over-clamping is as weak as one from under-clamping. Aim for a continuous squeeze-out bead along the joint, not a flood.

The press cycle as a timeline

Clamp time is one number inside a sequence, and the sequence is what a press operator actually schedules.

assembly time    spread to press closed     5–20 min (shorter in warm weather)
open time        spread to joint closed     30–70 min (seasonal)
clamp time       pressure applied           1–4 h (temperature dependent)
handling         press released to machining  after clamp + cooling
full cure        machining and load          24 h to several days, section dependent

Confusing the third and fourth rows is common: a beam released after three hours is ready to move, not ready to machine. A released joint that is machined immediately can open along the glue line, because the adhesive is still developing strength in the core.

What thick sections do to the clock

Heat and moisture move slowly through wood. A 45 mm lamella reaches press temperature in minutes; a 200 mm glulam section takes hours, and the core may never reach the press hall temperature during a winter shift.

Two practical consequences:

• Clamp time is set by the slowest part of the section, not by the average.
• A cold core in a warm room is the classic spring-back case: the surface cures, the press is released, and the core continues to move.

For thick sections, the release test is not optional. A broken sample joint from the same batch is the only evidence that the core is ready. If the sample shows a glossy glue line, the batch waits — regardless of the clock, and regardless of the delivery date.

Squeeze-out tells you about pressure, not about cure

A continuous bead of squeeze-out along the joint means the pressure was even and the spread was sufficient. It says nothing about whether the adhesive has cured.

The two checks are separate:

• Squeeze-out at pressing: pressure and spread.
• Broken sample joint at release: cure.

Confusing them is how a batch gets released on the evidence of a good squeeze-out and fails in machining the next morning.

What this means for buyers

1. Schedule the press by temperature, not by the fastest number in the data sheet.
2. Break a sample joint at release; do not rely on the clock.
3. Keep the press hall above 15 °C in winter, or add time.
4. Record clamp time and temperature per batch — it explains future failures.