Softness Testing Instrument Usage: Principles, Methods, and Applications

A softness testing instrument is a precision device used to quantify the softness, flexibility, and bending stiffness of materials such as tissue paper, nonwovens, textiles, leather, and plastic films in accordance with ISO, ASTM, EN, and GB standards. This technical guide explains the operating principles, sample requirements, test procedures, and practical applications of softness testing equipment for laboratory and quality-control environments. Understanding correct instrument usage ensures repeatable results, supports product development, and helps manufacturers meet international performance specifications.

Technical Overview

A softness testing instrument measures the tactile softness or mechanical flexibility of flexible sheet materials by applying a controlled deformation and recording the resulting force or displacement response. The most widely adopted design is the slot-and-plunger (blade) principle, in which a specimen is draped over a slotted orifice plate and a blunt blade or indenter presses the sample into the slot at a constant speed. The peak resistance force recorded during this stroke is converted into a softness value or handle index. This method underpins national and international procedures such as GB/T 8942 for tissue paper softness and forms the conceptual basis of most handle-feel instruments used for nonwovens and hygiene materials.

Complementary instrument types extend the measurement range to other flexibility characteristics. Cantilever bending testers, used in ASTM D1388 and ISO 9073-7, determine bending length and flexural rigidity by allowing a fabric strip to bend under its own weight as it slides over a platform edge. Two-point bending testers conforming to ISO 2493-1 and ASTM D5342 measure the bending moment of paperboard, while leather softness testers following ISO 17235 measure the distension of a circular specimen pressed by a calibrated indenter. A typical modern instrument integrates a precision load cell, a stepper-motor-driven measuring head, a rigid test platform with interchangeable orifice plates, an operator interface or PC software, and data-processing functions that report peak force, mean values, standard deviation, and statistical outliers.

Key performance parameters include force range (commonly 0 to 2,000 mN), measurement resolution, test speed (typically 1 to 50 mm/s), stroke depth, and orifice slot width. Instruments intended for accreditation are calibrated against certified reference weights and verified with standardized reference samples to guarantee traceability and inter-laboratory reproducibility.

Testing Samples

Tissue Paper and Hygiene Products

Toilet tissue, facial tissue, kitchen towels, napkins, sanitary napkins, and diapers are the primary materials for softness instruments. Samples are tested as finished plies or as specified ply combinations to reflect the layered construction of the product, and embossed or printed areas are treated according to the relevant product standard.

Nonwoven Fabrics

Spunbond, meltblown, thermally bonded, hydroentangled, and SMS composite nonwovens used in medical gowns, wipes, and hygiene back sheets are tested for handle and bending stiffness using slot-type instruments and cantilever methods referenced in ISO 9073-7.

Textiles and Apparel Fabrics

Woven, knitted, and coated apparel and home-textile fabrics are evaluated for bending length and flexural rigidity per ASTM D1388 and GB/T 18318.1, providing objective data that correlate with subjective hand-feel panel ratings.

Leather and Synthetic Leather

Genuine leather for footwear, upholstery, and garments, together with PU and PVC artificial leathers, is measured with an ISO 17235 softness tester that records millimeters of distension under a standardized indenter load.

Plastic Films and Flexible Packaging

Polyolefin films, laminates, and label stocks are characterized for rigidity using procedures aligned with ASTM D2923 and ISO 2493-1, which guide material selection for form-fill-seal lines and converting operations.

Foams and Padding Materials

Thin foam sheets, batting, and cushioning layers used in automotive interiors and upholstered goods can be assessed with adapted fixtures to compare compression and surface softness between suppliers and lots.

Testing Items

Depending on the instrument configuration and the governing standard, softness testing instruments are used to determine the following measurable quantities:

• Peak softness force (mN or gf) obtained with the slot-and-blade method
• Softness index or graded softness value for tissue products per GB/T 8942
• Bending length (mm) of fabric and nonwoven strips
• Flexural rigidity (mg·cm or µN·m) calculated from bending length and mass per unit area
• Bending stiffness (mN·m) and bending moment of paper and board per ISO 2493-1
• Leather softness expressed as distension in millimeters per ISO 17235
• Handle index and machine-direction versus cross-direction flexibility ratios
• Statistical parameters including mean, range, standard deviation, and coefficient of variation

Testing Methods

Sample Preparation and Conditioning

Cut specimens to the dimensions required by the applicable standard, typically using a precision cutter to guarantee parallel edges. For slot-method softness testing, common specimen sizes are 100 mm by 100 mm or standard-width strips; for cantilever testing, strips of 25 mm width with lengths of 150 to 300 mm are typical. Condition all specimens in a standard atmosphere of 23 ± 2 °C and 50 ± 5 % relative humidity for a minimum conditioning period specified by the standard, generally at least 24 hours for paper and 16 hours for textiles, because moisture content strongly influences flexibility results.

Instrument Setup and Calibration

Before testing, verify the mechanical zero of the measuring head, confirm that the correct orifice plate or platform accessory is installed, and perform force calibration with certified dead weights traceable to national metrology institutes. Select the test parameters prescribed by the method, including blade or indenter travel speed, stroke depth, and slot width, and enter specimen identification data into the control software. Daily verification with a certified reference sample is recommended to detect drift before production release testing.

Test Procedure

For the slot-and-blade method, place the conditioned specimen flat over the orifice without tension or wrinkles, start the automatic test sequence, and allow the blade to press the sample into the slot at constant speed while the load cell records the resistance curve. Record the peak force, remove the specimen, and rotate or replace it for the next measurement. Test a minimum of five to ten specimens per direction or per ply configuration as required by the standard, covering different positions across the web to capture cross-machine variability. For cantilever and two-point bending methods, align the strip specimen in the clamp, advance it over the sensing edge until the strip bends to the specified angle, and record the overhang length from which bending length and flexural rigidity are calculated. For leather softness, clamp the specimen in the ring, apply the indenter, and read the distension directly.

Data Processing and Reporting

The instrument software calculates the arithmetic mean, standard deviation, and coefficient of variation for each test series and flags statistical outliers for review. Report the test standard, instrument model and capacity, orifice or fixture geometry, conditioning atmosphere, number of specimens, individual and mean results, and any deviations from the standard method. Results should be traceable to calibration certificates, which is essential when data are used for certification by accredited third-party laboratories or for regulatory submissions.

Applications

Softness testing instruments are used throughout the value chain of flexible materials manufacturing:

• Incoming inspection of tissue, nonwoven, film, and leather rolls at converting and assembly plants
• In-process quality control to monitor the effect of refining, calendaring, embossing, creping, and softness additives
• Research and development benchmarking for new hygiene products, medical textiles, and apparel fabrics
• Supplier qualification and lot acceptance testing against contractual softness specifications
• Claims substantiation and comparison advertising support for consumer product positioning
• Automotive interior material approval, including trim panels, headliners, and seating surfaces
• Compliance demonstration for EN, ISO, ASTM, and GB product standards in trade and certification contexts

By converting the subjective perception of softness into a reproducible numerical value, these instruments allow manufacturers to set objective tolerances, reduce customer complaints related to harshness or stiffness, and optimize raw material usage without compromising perceived quality.

FAQ

Which standards govern softness testing instrument operation?

Commonly referenced methods include GB/T 8942 for tissue paper softness, ISO 17235 for leather softness, ASTM D1388 and GB/T 18318.1 for fabric bending stiffness, ISO 9073-7 for nonwoven bending length, and ISO 2493-1 and ASTM D5342 for the bending stiffness of paper and board. Always follow the standard specified in the product specification or customer agreement.

How often should the instrument be calibrated?

Force verification with certified weights is recommended daily or before each testing session, with full calibration performed at least annually by a qualified metrology service. Instruments used in accredited third-party laboratories or for certification purposes must maintain an unbroken calibration traceability chain.

What is the difference between softness and stiffness measurements?

Softness methods, such as the slot-and-blade test, measure the resistance of a draped specimen to indentation and correlate with tactile perception, whereas stiffness methods, such as the cantilever and two-point bending tests, measure the force or moment required to bend a strip and characterize structural rigidity. The two properties are related but not interchangeable, and the choice depends on the end-use requirement.

How can test variability be minimized?

Strictly control conditioning temperature and humidity, use precisely cut specimens, avoid handling deformation before testing, standardize ply count and orientation, apply consistent placement over the orifice, and follow the specified number of replicates with statistical outlier review in the reporting software.

Can a single instrument test all material types?

Many instruments are modular, offering interchangeable orifice plates, indenters, and cantilever accessories to cover tissue, nonwovens, textiles, leather, and film within one platform. Verify that the selected configuration and force range are validated for each material class and standard before use.