Plastic Hardness Testing: How to Measure It With ASTM D2240, D785, and ISO 868

Elena runs quality control at an injection molder in northern Italy. In March she sent pellets from one lot of POM to two accredited labs for hardness verification. The first lab reported Shore D 76; the second reported Shore D 83, citing the same standard on both reports.

Her customer’s specification read “Shore D 78 ± 3.” One result passed. The other failed, on the same material.

Neither lab made a mistake. Plastic hardness testing is sensitive to time as much as to material: one lab took its reading the instant the durometer touched the surface; the other waited 15 seconds. Plastic is viscoelastic, so the indentation keeps recovering while the indenter rests on it, and on a semi-crystalline resin that wait is worth roughly seven points of Shore D.

It’s the most frequently performed test in plastics QC, and the most frequently misinterpreted. This guide covers the procedure for each method, the specimen rules that decide whether your number is valid, and the part equipment vendors leave out: how to read a hardness value on the datasheet or COA you were handed. For the broader picture, start with our complete guide to plastics hardness.

What Plastic Hardness Testing Actually Measures

Every method works the same way at bottom. You push a defined indenter into the surface under a defined force and measure how far it sinks or rebounds.

That makes hardness empirical. It has no meaning outside the specific indenter geometry, force, and timing that produced it. Two materials can only be compared if both were tested on the same scale by the same rules. There’s no reliable conversion between Shore A and Shore D, or between any Shore scale and any Rockwell scale, and MatWeb’s reference pages state plainly that such conversions should not be used for design specifications.

Hardness is nonetheless sensitive to what actually varies between resin lots: molecular weight, crystallinity, filler loading, plasticizer content, and crosslink density. That’s why it works as a consistency check.

Every valid hardness value needs four things

A number alone is not a result. A correctly reported value carries:

• The scale: Shore D, Rockwell M, Barcol.
• The method and standard: ASTM D785, ISO 868, and the edition.
• The specimen condition: dry-as-molded or conditioned, and at what temperature and humidity.
• The reading time: instantaneous, 1 second, or 15 seconds.

Drop any one and the value can’t be compared to anything, including the same material measured next month in the same lab.

What a hardness test cannot tell you

Hardness is not wear resistance, scratch resistance, impact strength, or tensile strength; a softer material can outlast a harder one in sliding wear. What hardness gives you is a fast, cheap, non-destructive signal that the material in front of you matches the material you qualified, which is why plastic hardness testing belongs in incoming inspection.

How to Measure Plastic Hardness: The Five Tests and When Each Applies

Five methods cover almost every plastic part you will test. Match the method to the material’s stiffness, not to the instrument you happen to have.

Shore durometer (ASTM D2240 / ISO 868)

Shore is the default for elastomers, flexible plastics, and semi-rigid thermoplastics. A spring-loaded indenter presses into the surface and the depth of penetration reads on a 0–100 dial. Type A uses a truncated-cone indenter under an 822 gf spring; Type D uses a pointed cone under a 4,550 gf spring.

That difference in geometry and force is why the scales are not convertible. See the ASTM D2240 standard for the full method, and our Shore A versus Shore D guide for the scale logic.

The switch rule: if a Type A reading climbs above about 90, move to Type D. If a Type D reading falls below about 20, move to Type A.

Rockwell hardness for plastics (ASTM D785 / ISO 2039-2)

Rockwell presses a steel ball in two stages and measures the permanent depth left after the major load is removed. Because the minor load is applied first and used to zero the instrument, Rockwell largely cancels the surface-roughness effects that disturb a durometer reading, which is why labs treat it as the more reproducible choice for rigid plastics. The plastic scales are R (60 kgf, ½ in ball), L (60 kgf, ¼ in ball), M (100 kgf, ¼ in ball), and E (100 kgf, ⅛ in ball); our guide to Rockwell hardness testing of plastics covers scale selection and the hardness formula.

Ball indentation, Barcol, and Vickers

Three narrower methods fill the gaps. Ball indentation (ISO 2039-1), common in European specifications, presses a 5 mm ball in under 49 N, 132 N, 358 N, or 961 N and reports the result in N/mm², a genuine stress unit, with the recovered depth calibrated to land between 0.15 mm and 0.35 mm. The Barcol hardness test (ASTM D2583) is a portable impressor for rigid, reinforced, and finished parts that cannot be cut into a coupon, with a dimensionless 0–100 scale and a roughly 3 mm thickness minimum. Vickers (ASTM E384) uses a diamond pyramid under 0.01 gf to 1,000 gf for hard, brittle polymers, composites, and coatings rather than routine QC, and IRHD (ISO 48) is its rubber-industry counterpart.

Method selection at a glance

For a side-by-side view of how the common engineering grades sit relative to each other, see our engineering plastic hardness comparison.

Shore Durometer Test Procedure, Step by Step

Shore is the method most people mean when they search for plastic hardness testing, and every step below exists to control a variable that would otherwise move the number.

Step 1: Select the scale. Choose Type A or Type D from the material’s expected stiffness, then honour the switch rule.

Step 2: Prepare the specimen. Under ASTM D2240, the specimen must be at least 6 mm thick and the reading taken at least 12 mm from any edge. Under ISO 868, the minimums are 4 mm and 9 mm. The specimen must be flat, with parallel faces and a clean, smooth surface.

Step 3: Condition before you test. The standard laboratory atmosphere is 23 ± 2 °C and 50 ± 5 % RH per ASTM D618. Bring specimens to equilibrium there, and check whether the material specification overrides it.

Step 4: Verify the instrument. Confirm the dial agrees with a check block to within ±1 point before the session begins. See the calibration section below for why this is verification, not calibration.

Step 5: Apply the presser foot and read. Seat the durometer flat and firm. Take the reading either instantaneously or after 15 seconds of firm contact, but decide in advance and record which you used.

Step 6: Take five readings and average. Space readings at least 6 mm apart, respect the edge distance, and average the set. Discard readings below 10 or above 90 and switch scales instead.

Record on the test report: scale, standard and edition, specimen thickness, conditioning history, reading time, number of readings, and the average. Anything less cannot be reproduced.

The 3 mm part that “failed”

Raj, a design engineer at a medical device developer, measured Shore D 92 on a production housing against a specification of D 78. The material was fine; the 3 mm wall was the problem. The indenter’s stress field reached the rigid support beneath the part, so it read falsely hard. A 6 mm plaque from the same lot gave Shore D 79, in spec.

How to Run a Rockwell Hardness Test on Plastics

The load sequence. Apply the minor load (about 10 kgf) and hold it while you zero the dial, which limits creep before measurement. Apply the major load for the specified dwell, release it, let the material recover, and read the remaining depth.

Procedure A reads while the major load is still applied; Procedure B reads after it is removed. The two give different numbers on the same specimen, so state which you used.

Validity. Values should land roughly between 50 and 115; outside that band, re-run on a more suitable scale. The specimen must be at least 6.4 mm (¼ in) thick, and the M scale is invalid below 1.27 mm (0.05 in). Dirt, scale, or roughness on the underside lowers the reading, because the specimen can’t seat flat on the anvil, so clean both faces.

The ASTM D785 method defines the load sequence, and we work through the HR = 130 − (penetration ÷ 0.002 mm) formula in our guide to Rockwell hardness testing of plastics.

Specimen Preparation and Conditioning: The Rules That Decide Validity

Specimen preparation is where plastic hardness testing most often goes wrong, and the requirements differ by method and by standard. This table consolidates the limits.

Why a thin specimen reads falsely hard: the anvil effect

This is the most common source of invalid results, and most guides state the rule without explaining it. When the specimen is too thin, the stress field from the indenter travels through it and reaches the rigid anvil underneath. The material is then backed by steel instead of by more polymer, so it cannot deform the way the test assumes, and the dial reports a hardness the material does not have at that thickness.

The effect is well documented. Durometer-vendor literature frequently describes thin specimens reading substantially harder than the true value, in some cases approaching twice the correct figure. If your geometry is thinner than the minimum, test a thicker plaque from the same lot.

Stacking, moisture, and temperature

Standards permit stacking thin specimens to reach the required thickness, provided the plies are in intimate contact and the stack is flat and parallel. Report when you have stacked; plied specimens behave differently from solid ones.

Polyamides absorb moisture, and absorbed water plasticises them. PA66 tested dry-as-molded reports a higher Rockwell value than the same grade conditioned to equilibrium, and our PA66 hardness breakdown, covering Rockwell R120, Shore D, and conditioned values, carries both sets of numbers. Never compare a dry-as-molded value with a conditioned one.

Temperature matters for the same reason. Thermoplastics soften toward their glass transition or melting point, so a specimen tested warm reads lower than the same specimen at 23 °C. If the material specification requires a different atmosphere, follow it and record it; the material specification takes precedence.

Running an incoming-inspection program? Send us your hardness requirement and application details and we will confirm which grade meets it. Request a grade recommendation and get a response within 24 hours.

Calibration, Verification, and the Mistakes That Invalidate a Result

Most disagreements in plastic hardness testing trace back to instrument verification and operator technique, not to the resin.

Check blocks verify; they do not calibrate

A check block is a reference specimen with a stamped hardness value. Pressing the durometer onto it confirms the instrument reads what it should, to within ±1 point. It doesn’t adjust the instrument, and it isn’t a calibration.

The trap is treating a check block as a permanent standard. Its rated value shifts with age, temperature, and exposure to light, so a block that read 60 when new may not represent 60 today. Every instrument also has an indenter-extension specification: 2.50 ± 0.04 mm for standard durometers and 1.25 ± 0.02 mm for Type M. Verify the extension with gauge blocks, and confirm the indenter protrudes 0.098–0.100 in below the presser foot under load.

Hand-held versus test stand

Operator technique dominates the error budget. A hand-held instrument produces variable descent speed, contact angle, and dwell: the three variables the standard exists to control. An operating stand applies the load at a constant rate for a constant time. When two labs disagree, ask how each applied the instrument before you question the resin.

Seven mistakes that invalidate a test

• Testing below the standard’s minimum thickness.
• Reading too close to an edge or a previous indentation.
• Using the wrong scale for the material’s stiffness.
• Hand-holding the instrument instead of using a stand.
• Ignoring creep by not specifying reading time.
• Skipping conditioning, or testing at the wrong temperature.
• Treating a check-block field check as a calibration record.

Why you average five readings

Five is the minimum for a reported durometer value, and ASTM E691 defines the practice for quantifying how much two labs scatter on the same material.

A few points of disagreement between labs is normal. Seven or more usually has a physical cause, whether reading time, thickness, conditioning, or instrument setup. Find it before you reject the lot.

How to Read Plastic Hardness on a Datasheet or COA

What a correctly stated value looks like

A properly reported value names the scale, the number, the standard, and the condition. Rockwell M78, ASTM D785, dry-as-molded is complete and usable. Hardness: 78 is not.

When you write a purchase specification, spell out the scale letter, the standard, the conditioning state, and the reading time. That one line prevents the two-lab disagreement this article opened with.

Branded-grade baselines

These are comparison baselines, not acceptance criteria. Always test against the grade’s own data sheet. For grade-level detail behind the numbers above, see our dedicated guides: polycarbonate hardness, Rockwell M70 and scratch resistance for PC (Makrolon®), ABS plastic hardness, Shore D, Rockwell R and grade selection for ABS, and HDPE hardness, Shore D 60–70 and why abrasion is the real spec for HDPE.

What does and does not appear on a resin COA

A certificate of analysis typically carries lot-specific values for melt flow index, density, moisture content, and sometimes filler content. Hardness is often not a routine COA line item, because it’s a part-level property influenced by molding conditions rather than a pure resin property.

If hardness is critical to your application, confirm it against the grade’s published data sheet and ask your supplier to state a lot-specific figure in writing, distinguishing lot-specific lines from nominal ones on the COA.

Hardness as an incoming-inspection screen

Here is where testing plastic hardness earns its place in procurement. Set up a Rockwell or Shore check on one coupon per incoming lot and record it against lot number. You’re not building a specification; you’re building a trend line.

The quality team at a Spanish injection molder did exactly this with PC. Their incoming Rockwell M readings sat in a tight band for eleven months, then a shipment arrived consistently 6 to 8 points low. The hardness result triggered a COA review, which showed an out-of-band melt flow index consistent with regrind or a substituted grade. Hardness cost them minutes and caught a problem that full mechanical testing would have found after the material was already in the press.

Lot-to-lot hardness drift points to something real: an off-spec lot, regrind content, a crystallinity shift, filler dispersion problems, or moisture pickup. Pair the trend with the COA lines that corroborate it, namely melt flow index, density, and lot number, and you have a supplier-qualification signal that costs almost nothing to run. Our POM resin range ships with full lot traceability.

Plastic Hardness Testing: Frequently Asked Questions

How is plastic hardness measured?
By pressing a defined indenter into the surface under a defined force and measuring the depth of penetration or the rebound. Because the result depends on the indenter shape, force, and timing, the method and scale must always be reported with the number.

How thick does a plastic specimen need to be?
At least 6 mm under ASTM D2240 for Shore testing, 4 mm under ISO 868, 6.4 mm for Rockwell, and roughly 3 mm for Barcol. The Rockwell M scale is invalid below 1.27 mm. A thinner specimen reads falsely hard because the indenter’s stress field reaches the support beneath it.

How long do you hold a durometer on plastic?
Either instantaneously or for 15 seconds of firm contact. The 15-second reading is lower because the material keeps recovering. The standard permits either but requires you to state which you used.

What is the difference between ASTM D2240 and ISO 868?
They are the American and international versions of the same durometer method, with different specimen limits. ASTM D2240 requires 6 mm thickness and 12 mm edge distance; ISO 868 requires 4 mm and 9 mm. A thin part that is marginal under D2240 may be testable under ISO 868.

Can you convert Shore A to Shore D, or Shore to Rockwell?
Not reliably. The scales use different indenter geometries and spring forces. Approximate tables are useful for intuition only, never for a specification or an acceptance decision.

Should hardness be tested on the part or a molded specimen?
Both. A molded plaque under controlled conditions gives the material’s baseline and is the right basis for a specification. Testing parts catches weld lines, gate-adjacent regions, and wall-thickness effects, but only if the geometry meets the thickness minimum.

The Bottom Line

Plastic hardness testing is fast, cheap, and non-destructive, which is precisely why it is so often run badly and read worse. Five points carry the article:

• Hardness is empirical. A number without a scale, standard, condition, and reading time is not a result.
• Specimen thickness decides validity: 6 mm under ASTM D2240, 4 mm under ISO 868, 6.4 mm for Rockwell.
• Check blocks verify; they do not calibrate. Their rated value drifts with age, temperature, and light.
• Cross-scale conversion is not valid. Treat every conversion table as intuition, never as specification.
• Two labs disagreeing by a few points is normal. Seven or more usually has a physical cause worth finding.

Run the test correctly, record the four required facts, and keep a hardness trend on every incoming lot. It’s the cheapest early-warning system in plastics QC.

If your specification calls for a hardness band you need to hold across lots, send it to us. Suzhou Yifuhui New Material supplies certified, traceable engineering resins from BASF, Celanese, Covestro, Polyplastics, and Formosa with full COA documentation and lot-level traceability, and we will confirm the grade against your hardness requirement before you order. Request a quote for a response within 24 hours.