# Clamp time and press scheduling for PUR

> How temperature, moisture and section thickness move the 1–4 hour catalogue range, and how to schedule a press without gambling on the release time.

Updated: 2026-11-10 · Source: https://karmindadhesive.com/resources/pur-adhesive-clamp-time/

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.