ASTM D6531 Standard Practice for Relative Tinting Strength of Aqueous Ink Systems by Instrumental Measurement

Get Certified ASTM D6531 Testing for Reliable Aqueous Ink Quality Control

A water-based ink that comes out weaker or stronger tint-wise than the reference standard requires an operator to use more or less ink to reach the same color, which affects a print’s cost and color throughout production runs. ASTM D6531 offers an objective instrumental analysis to compare against a reference standard versus relying on visual assessment. Producers use the ASTM D6531 practice to ensure the final lot matches the base formula before delivery, and print operators use it to identify a color-matching issue before printing.

The scope of ASTM D6531 includes-

• Material type: Water-based printing inks and bases intended primarily for flexographic and gravure printing applications.
• Purpose: Determination of relative tinting strength of a water-based ink system compared to a reference standard for the purposes of quality control and color matching.
• Properties evaluated: Relative tinting strength defined by the spectrophotometric analysis of a tinted sample compared to a reference standard.
• Testing method: A technician places a tinted sample in a cuvette for spectrophotometric measurement, and an analyst compares the resulting reading against a reference standard containing a pigment of the identical color index name and number.
• Results: Relative tinting strength expressed as a percentage derived from the spectrophotometric analysis compared to a reference standard using SI units of measurement as defined by the standard practice.
• Test limitations: The practice applies only to single, non-fluorescent pigmented colors and black ink systems for which a reference standard containing a pigment of the identical color index name and number exists. Thick wet drawdowns are inappropriate because aqueous systems can produce severe floating problems.
• Use conditions: Testing places tinted samples in a cuvette rather than applying wet drawdowns, since the aqueous system’s tendency to float and separate makes drawdown methods unreliable for this measurement.
• Applications: Quality control of production batches of ink bases and finished inks, and color matching between batches or against a target standard.

What are the Uses of ASTM D6531 Testing?

ASTM D6531 testing provides ink producers and printers with a comparable relative tinting strength value that visual assessment cannot always supply. The standard practice-

• Establishes the relative tinting strength value a production lot should meet compared to the reference standard.
• Supports quality control of ink bases and finished inks across production runs.
• Enables color matching between an ink batch and a target reference standard.
• Replaces visual judgment with an instrumental, repeatable spectrophotometric comparison.
• Flags batch-to-batch variation in pigment strength before ink reaches the press.
• Supports formulation troubleshooting when a batch’s color output drifts from expectation.
• Provides data usable alongside other water-based ink quality assessments.

Which materials can be tested under ASTM D6531?

ASTM D6531 applies to water-based printing inks and bases primarily for flexographic and gravure printing that contain nonfluorescent colorants and black inks for which a reference standard with identical pigment identity is available for comparison. An analyst should verify the pigment color index, the reference standard availability, and whether the system contains fluorescent colorants before testing, since these considerations fall outside the scope of the standard practice, and an analyst should ensure the material is a single-pigment system before testing commences.

Why is ASTM D6531 Important?

Tinting strength is an essential property of printing ink dispersions, since it determines how much color a given quantity of ink actually delivers once printed. Results on bulk tints do not guarantee equivalency of dry printed films, but they provide a useful, repeatable parameter for quality control of production batches of bases and finished inks. Tinting strength is an essential property of printing ink dispersions. Results on bulk tints do not guarantee equivalency of dry printed films, but they provide useful parameters for quality control of production batches of bases and finished inks. The results may also support color matching. ASTM D6531 uses cuvette-based spectrophotometric measurement because thick wet drawdowns are inappropriate for aqueous systems due to severe floating problems.

ASTM D6531 Testing Process and Data Collection

An analyst selects a representative water-based ink sample and confirms a matching reference standard exists for the comparison. A technician places the tinted sample in a cuvette, avoiding wet drawdown preparation, and measures it on the computer-aided spectrophotometer. A technician then measures the reference standard under identical instrumental conditions. An analyst compares the two spectrophotometric readings, calculates the relative tinting strength as a percentage against the reference, and records the sample identification, pigment identity, and calculated result.

Common Challenges and Troubleshooting

A technician should avoid using thick wet drawdowns in favor of the cuvettes specified in the ASTM D6531 standard practice since aqueous systems tend to float and separate when measured using a thick wet film. Incompatible reference standards that do not have an identical pigment color index generate inaccurate comparative results, and an analyst should verify reference standard availability for the intended analysis. Fluorescent systems and samples that do not meet the standard practice’s single-pigment scope are outside the practice’s scope, and an analyst should confirm that the sample is a single-pigment system before commencing with testing. Spectrophotometer instrumental calibration inconsistency affects test result accuracy, and regular calibration checks against reference standards are crucial in maintaining instrument precision.

• The technician records sample identification, pigment color index name and number, and the reference standard used for comparison.
• The analyst reports the relative tinting strength calculated from the spectrophotometric measurement against the reference standard.
• The report identifies the measurements and units used, with SI units regarded as standard under the practice.
• The report states that results on bulk tints do not guarantee equivalency of dry printed films, consistent with the limitation the practice itself sets.
• The analyst notes that test results may also support color matching between the sample and reference, beyond the tinting strength figure alone.

Link to ASTM D6531