Open time
Open time is the period after mixing during which a printable mortar remains pumpable and extrudable, and during which a deposited layer still bonds well to the layer placed on it.
Updated
Open time is the period after mixing during which a printable mortar remains fluid enough to be pumped and extruded, and, in a second and related sense, the interval between the deposition of one layer and the next within which the two still bond properly. Both senses matter on a print, because the same mix has to survive its journey through the line and still accept the following layer when the nozzle returns.
In practice
The material open time has to exceed the time the mix spends in the hopper, the pump and the hose, plus any pause caused by a nozzle change, a refill or a fault. The interlayer interval is set by the geometry: a long perimeter means a long return time, so the surface of the previous layer has more time to stiffen and dry. The literature consistently reports that interlayer bond strength falls as this interval grows, with the drop becoming pronounced once the surface has begun to skin over. Open time is controlled with retarders and accelerators and by the mix water, and it shortens noticeably in hot, dry or windy conditions, which is a practical constraint for summer printing.
Related terms
- Cold joint — a weak plane formed where a layer was placed on one that had already stiffened.
- Interlayer bond — the adhesion between two successive printed layers.
- Set on demand — triggering stiffening at the nozzle so open time in the line stays long.
Frequently asked questions
- How long is the open time of a printable mortar?
- There is no single value. For premixed printable mortars the workable window is typically in the range of tens of minutes, but it depends on the binder, the admixtures, the temperature and the humidity. The reliable figure is the one measured on the actual mix under the conditions of the print, not a catalogue number.