ISO 48-3:2018

Overview

ISO 48-3:2018 - "Rubber, vulcanized or thermoplastic - Determination of hardness - Part 3: Dead‑load hardness using the very low rubber hardness (VLRH) scale" specifies a dead‑load indentation method for measuring the hardness of very soft vulcanized or thermoplastic rubbers. The standard defines the test principle, required apparatus, calibration and test piece requirements, a VLRH conversion table and the linear relationship between indentation depth and the VLRH scale (VLRH = 100 − 0.1·D).

Key topics and technical requirements

• Test principle: Measure the difference D between two indentation depths (under a small contact force and a larger total force); convert D to VLRH using Table A.1 or the formula VLRH = 100 − 0.1D.
• Apparatus essentials:
  • Vertically guided plunger with spherical tip (ball diameter 2.50 ± 0.01 mm).
  • Annular foot (diameter 6.0 ± 0.5 mm; hole 3.0 ± 0.1 mm) rigidly connected to the depth gauge.
  • Indenting/contact forces: contact ~8.3 ± 0.5 mN, indenting ~91.7 ± 0.5 mN, total ~100.0 ± 1.0 mN; foot load ~235 ± 30 mN.
  • Indentation measurement either in mm (max uncertainty 0.001 mm) or directly in VLRH (resolution ≥ 0.5 VLRH).
• Test procedure highlights:
  • Condition test pieces per ISO 23529 at standard laboratory temperature (minimum 3 h).
  • Place test piece on flat measuring table; apply contact force for 5 s, then additional indenting force for 30 s; read VLRH or convert from measured D.
  • Make at least three measurements at prescribed separations and report the median.
• Test pieces: flat, smooth, parallel surfaces; minimum thickness 6 mm, with the thinnest layer ≥ 2 mm; spacing and edge distances defined for reproducibility.
• Calibration & traceability: Annex B and references (ISO 48-9, ISO 18899) cover calibration/verification schedules and good laboratory practice.

Applications

• Measurement of very low rubber hardness in materials where traditional IRHD/durometer scales lack discrimination.
• Typical uses include low‑modulus bearings, soft roller coverings, printing rubbers, cushioning materials and any application requiring accurate characterization of very soft elastomers.
• Useful in quality control, incoming material inspection, R&D and comparative material selection.

Who should use this standard

• Rubber test laboratories, manufacturers of soft elastomers, product designers and QA/QC engineers working with very soft vulcanized or thermoplastic rubbers, and instrument manufacturers producing VLRH hardness testers.

Related standards

• ISO 48 series (overview grouping of hardness methods)
• ISO 48-2 (IRHD methods), ISO 48-4 (AO durometer for soft rubbers)
• ISO 48-9 (calibration/verification of hardness testers)
• ISO 23529 (preparing and conditioning test pieces)
• ISO 18899 (guide to calibration of test equipment)

Keywords: ISO 48-3:2018, VLRH, very low rubber hardness, rubber hardness testing, dead-load method, indentation depth, hardness calibration, soft elastomers.