Ink testing

Ink Testing: Methods and Standards for Quality Assurance

Ink testing evaluates the physical, chemical, and optical properties of printing inks to ensure consistent quality, proper performance on press, and compliance with industry standards. Key tests include tack measurement, viscosity analysis, pigment strength evaluation, and adhesion testing, with methods standardized by organizations like ASTM and ISO.

Why Ink Testing Matters

Printing ink performance directly affects print quality, production efficiency, and product compliance. Inconsistent batches can cause press problems including poor trapping, set-off, dot gain issues, and web breaks. For packaging applications, traceable codes must remain legible throughout the entire product lifecycle—from manufacturing through distribution to consumer use.

Industries with strict coding requirements include:

• Pharmaceuticals: Expiration dates and lot codes must survive condensation and handling
• Food and beverage: Codes must withstand retort processes, refrigeration, and moisture
• Electronics: Markings need to resist heat, chemicals, and abrasion
• Military equipment: Codes must remain permanent throughout the item's normal life expectancy per MIL-STD-130M

Inkometer Tack Testing (ASTM D4361)

Tack measures the force required to split an ink film between two rollers. The Inkometer test is the industry standard for assessing ink tack at different press speeds.

Test parameters:

• Measured at 800, 1200, or 2000 RPM
• Batch-to-batch tolerance: ±1 point at 1 minute
• Tack separation between process colors: minimum 1.5 gram-meter drop from cyan to magenta to yellow
• Stability requirement: tack increase should not exceed 2.0 gram-meters per minute

Proper tack hierarchy (cyan highest, then magenta, then yellow) ensures good trapping and prevents back-trap contamination on press.

Printing Tack Tests

Printing Tack on Acetate (P&I Method)

This test uses a non-absorbent substrate to simulate ink tack at a film thickness closer to actual printed conditions—approximately 30 times thinner than Inkometer samples. The measurement combines both tack and viscosity properties.

Specifications:

• Batch-to-batch tolerance: ±25 points
• Tack separation between CMY: 25-50 point difference
• Hierarchy: cyan (highest) > magenta > yellow

Printing Tack Set Rate Test

This measures how quickly paper absorbs thin oils from ink, causing the ink to set. The test generates a setting curve, with the slope reported as the set rate.

• Fast-setting inks: formulated with hydrocarbon oils
• Slow-setting inks: contain heavy oxidation-type oils
• Standard reference: "Fast Set" cyan ink or standard coated paper

Viscosity Testing Methods

Brookfield Viscosity

Measures dynamic viscosity using a cone and cup instrument after 5 minutes at 90 degrees, simulating press conditions. This method is suitable for all ink types from flexographic to electron beam (EB) offset inks.

Duke Viscosity (Falling Rod)

An automated system utilizing falling rod technology for monitoring paste ink consistency. Provides precise viscosity measurements for quality control.

Viscosity affects ink transfer, drying time, and print consistency. Both static and dynamic testing methods help predict press performance.

Pigment Strength and Transfer Testing

Wedge Prints

This test evaluates the printed density achievable at various ink film thicknesses, considering both pigment strength and transfer characteristics. Unlike a simple bleach test, wedge prints reveal how viscosity and transfer affect actual press performance.

Changes between batches correlate directly to:

• Dot gain on press
• Ink mileage variations
• Transfer efficiency

Solid Prints and Side-by-Side Comparisons

Prints made at controlled 0.05 mils film thickness allow comparison of:

• Strength and hue differences between batches
• Ink gloss measurements
• Delta E and Color LAB values for color consistency

Dry Rub Resistance Testing

Evaluates ink adhesion and durability after drying under controlled conditions.

Sheetfed inks:

• Dried in simulated lift for 24 hours
• Tested with Sutherland Rub tester: 20 sweeps with 4 lb weight

Web inks:

• Printed, dried, and chilled in GATE HeatSet tester to specified web exit temperature (recommended: 269°F)
• Tested: 40 sweeps with 4 lb weight

Coated samples: Evaluated with Taber Abrasion tester for enhanced durability assessment.

Percent Volatiles Testing

Measures volatile oils that evaporate at 155°F (68°C) in a still oven. This distinguishes between:

• Volatile materials: Hydrocarbon oils
• Non-volatile materials: Soya, linseed, and other fixed oils

Monitoring volatile content is critical for raw material quality control and troubleshooting press issues related to drying and setting.

Trapping Efficiency Testing

Trap measures how well a wet ink film accepts the next ink printed on top of it, calculated using Preucil's formula:

% Trap = {(Density of overprint – Density of 1st ink) / Density of 2nd ink} × 100

Target values:

• Above 75% on paper
• Above 50% on tape (without paper aid)

Trap affects secondary color values and overall color gamut. Poor trap leads to color shift in reds, greens, and blues.

Water Pickup and Contamination Testing

Measures ink's ability to absorb fountain solution using a Duke emulsification tester over five minutes. Results correlate to press performance issues including:

• Ink-water balance problems
• Tinting and scumming
• Poor print quality

Conductivity testing of fountain solution runoff detects calcium salt contamination from insufficiently washed flush pigments, which can ruin fountain solution effectiveness.

Fineness of Grind (ASTM D1316-06)

Evaluates pigment dispersion quality using a grindometer. Optimum pigment size improves:

• Pigment dispersion in the vehicle
• Color strength and consistency
• Plate and blanket wear characteristics

Larger pigment particles can cause increased abrasion on printing plates, leading to premature image wear.

Ink Mileage Evaluation

Ink consumption depends on strength, transfer ability, and setting characteristics. The evaluation combines:

• Paper & Ink Stability Test on standard paper
• Pigment Strength and Transfer test

Results enable batch-to-batch comparison and benchmarking against industry standards for cost optimization.

Commercial Ink Testing Standards

PTS-DF 103/2011 (Drying Time Test)

Developed by the Paper Technology Foundation (PTS) for thermal inkjet (TIJ) inks:

• Creates GS1 data matrix codes at 300 DPI
• Automated wipe unit tests smudge resistance at predetermined intervals
• Records accurate drying times and degradation patterns

ISO 18935 (water resistance test)

Standardized water resistance evaluation:

• Creates GS1 data matrices at 300 DPI
• Submerges codes in deionized water for 1 hour
• Dries codes on glass plate
• Evaluates readability with code reader

Additional Standard Tests

• Heat resistance: For retort processes and high-temperature applications
• Alcohol resistance: For pharmaceutical and personal care packaging
• Sunlight/UV resistance: For outdoor storage and display
• Thermochromic properties: For temperature-sensitive applications
• Ink migration: Critical for food packaging safety

Advanced analytical testing

For formulation development and problem solving, advanced techniques include:

Chemical Analysis

• NMR Spectroscopy for molecular structure
• Mass Spectrometry (MS, MS/MS, LC-MS/MS) for impurity identification
• Gel Permeation Chromatography for polymer analysis
• Inductively Coupled Plasma (ICP-AES/ICP-MS) for heavy metal detection

Physical Characterization

• Differential Scanning Calorimetry (DSC) for UV-cure cross-linking assessment
• FTIR Spectroscopy for residual functional groups
• Transmission and Scanning Electron Microscopy (TEM/SEM) for pigment and coating analysis
• Interferometry for surface and multilayer system characterization

Paper and Ink Interaction Testing

Printability depends on both ink properties and paper characteristics. Critical interaction tests include:

Set-Off Testing

Measures ink transfer from freshly printed surface to another paper without friction. Tested at 0.35 seconds (high-speed press) and 3 seconds (low-speed press). Faster-setting inks and more porous papers reduce set-off.

Print Through

Measures ink visibility on the reverse side of printed paper at standard density. Lower print through indicates better opacity and quality.

Strike Through

Evaluates ink penetration into paper substrate, affecting drying time and print quality.

Quality Control Best Practices

Batch Consistency

• Document all test results for each batch
• Compare against established tolerances and previous batches
• Track statistical variance in optical density and color values

Substrate Compatibility

• Test inks on actual production substrates whenever possible
• Ensure surface cleanliness to avoid adhesion issues
• Consider environmental conditions (humidity, temperature) during testing

Documentation and Standards

• Follow ASTM D5010-01 guidelines for testing printing inks
• Reference ISO 12647-3 for newsprint specifications
• Maintain traceability to international standards

Choosing the Right Tests

Application	Priority Tests
Offset lithography	Tack, viscosity, trap, set rate
Flexographic printing	Viscosity, pigment strength, adhesion
Packaging codes	Dry rub, water resistance, adhesion
UV-curable inks	Cure testing, FTIR, DSC
Food packaging	Migration testing, heavy metals, regulatory compliance

Effective ink testing programs combine routine quality control tests with advanced analytical methods as needed. Partner with qualified testing laboratories and follow established standards to ensure consistent, compliant results.