ASTM D7734 Open Time Testing for Oxidative Printing Inks
ASTM D7734/D7734M describes a finger transfer method for determining the stay open time of oxidative drying printing inks and overprint varnishes. A wet ink/varnish film is defined as open. Oil-based sheetfed offset, letterpress, and screen inks are examples of inks and overprint varnishes that dry through oxidation.
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- Overview
- Scope, Applications, and Benefits
- Test Process
- Specifications
- Instrumentation
- Results and Deliverables
Overview
ASTM D7734 provides a standard test method for measuring the open time of oxidative-drying printing inks—the period during which the ink remains workable on a printing plate or substrate before it begins to skin over or dry. Open time is a critical processability parameter for lithographic, letterpress, and other oxidative ink systems.
Ink formulators and printers use open time data to optimize press wash-up schedules, manage ink train drying behavior, and select inks for specific press configurations and production run lengths.
Scope, Applications, and Benefits
Scope
ASTM D7734 evaluates:
- Open time of oxidative drying inks under defined conditions
- Effect of temperature and humidity on ink open time
- Comparative open time of different ink formulations
- Track development as a function of time after ink application
Applications
- Lithographic and letterpress ink formulation development
- Press wash-up interval optimization
- Ink quality control for commercial printing operations
- Cold-set and heat-set ink open time differentiation
- Printing ink supplier qualification
Benefits
- Standardized, reproducible open time measurement
- Enables ink formulation optimization for specific press conditions
- Reduces press downtime from premature ink drying
- Supports specification writing for printing ink procurement
- Applicable to a wide range of oxidative ink systems
Test Process
Ink Application
A defined quantity of ink is applied to a smooth, clean glass or metal plate at a controlled film thickness using a drawdown bar or ink knife.
1Open Time Monitoring
At defined time intervals, a fresh substrate or test probe is applied to the ink film; the ink's ability to transfer or resist skinning is assessed and recorded.
2End-Point Determination
The open time end-point is defined as the time at which the ink no longer transfers cleanly or begins to show skin formation, as specified in ASTM D7734.
3Reporting
Open time (minutes), test temperature, humidity, and ink film thickness are recorded and reported; comparative data for multiple inks are tabulated.
4Technical Specifications
| Parameter | Details |
|---|---|
| Applicable Inks | Oxidative drying lithographic, letterpress inks |
| Test Conditions | 23 °C ± 1 °C, 50% ± 5% RH |
| Film Thickness | As specified per ink type |
| Time Intervals | Every 15–30 minutes (typical) |
| End-Point Criterion | Ink no longer transfers or shows skin formation |
Instrumentation Used for Testing
- Drawdown bar or ink knife (calibrated film thickness)
- Smooth glass or metal test plates
- Environmental chamber (temperature and humidity control)
- Timer and periodic transfer substrate
- Tack meter (optional, for tack-development tracking)
Results and Deliverables
- Open time value (minutes) with test conditions
- Transfer behavior at each time interval
- Comparative open time table for multiple formulations
- Temperature/humidity condition log
- Full test report per ASTM D7734
Frequently Asked Questions
Temperature, humidity, ink film thickness, substrate porosity, and ink formulation (drier type and level, oil viscosity, resin content) all influence open time. Higher temperatures and lower humidity generally shorten open time.
Printers need to know the open time to schedule wash-up intervals, manage ink train behavior on long production runs, and select inks appropriate for their press speed and ambient conditions, avoiding toning or drying in on-press.
No. Open time is the period during which the ink remains workable and transferable. Drying time (or set time) is the time for the ink to fully cure or solidify on the substrate. Open time is always shorter than or equal to full drying time.
No. ASTM D7734 is specific to oxidative drying inks. UV-curing inks dry by photoinitiated polymerization and do not have an open time in the same sense. UV ink processability is evaluated by different test methods.
Thicker ink films take longer to oxidize through their depth, generally extending open time. Standardized film thickness is therefore critical for reproducible open time measurements between labs and formulations.
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