Last updated: 2026-09-05 · 5 min read

TPE Shore Hardness — How Do You Choose the Right Hardness for Your Part and What Does the Number Actually Mean?

Thermoplastic elastomer (TPE) Shore hardness is measured on the Shore A scale (soft, rubbery) from 10A to 90A, or the Shore D scale (firm, plastic-like) from 10D to 80D. A typical TPE grip runs 40–60A, a gasket 30–50A, a soft-touch button 70–85A and an overmolded bumper 50–70A. The hardness determines compression set, tear strength, rebound and surface feel — and also affects gate size, draft angle, injection speed and cooling time.

Key Facts

  • Shore A measures indentation depth with a flat-tipped durometer indenter under 1 kg spring force — used for TPE below 90A. When the material is harder than 90A, the indenter bottoms out and the reading becomes unreliable. At that point, switch to Shore D (sharp-tipped indenter, 5 kg force). A 90A TPE and a 40D TPE may have the same actual hardness — the scales overlap. Always specify which scale: “60A” and “60D” are completely different materials.
  • Hardness correlates with compression set — softer TPE (20–40A) has higher compression set (30–50% after 22 hours at 70°C) because the polymer chains are more mobile and cannot fully recover after sustained compression. Harder TPE (70–90A) has lower compression set (15–25%) and better seal retention over time. For static gaskets that must maintain seal force for years, 50–70A is the practical range — below 40A, the gasket will relax and leak within months. See the TPU vs TPE comparison for how resin chemistry interacts with hardness selection.
  • Hardness affects processing: softer TPE (below 40A) requires lower injection pressure (40–60 MPa), larger gates (2–3mm) and longer cooling times (15–25 seconds) because the material has lower thermal conductivity and higher elasticity. Harder TPE (70–90A) processes more like rigid plastic — higher injection pressure (80–120 MPa), smaller gates (1–1.5mm) and shorter cooling times (8–15 seconds). The mold must be designed for the specific hardness range — a mold optimized for 80A will flash or short at 30A.

How Does Shore Hardness Map to Real-World Applications Across the TPE Range?

Hardness Feel Typical Application Compression Set
10–20A Very soft gel Medical cushions, padding 50–70%
30–40A Soft rubber Gaskets, seals, vibration mounts 30–50%
50–60A Firm rubber Grips, overmolded handles, bumpers 20–35%
70–80A Leather-like Soft-touch buttons, keyboard keys, wheels 15–25%
85–90A Rigid rubber Gears, rollers, structural elastomer 10–20%

Shore hardness — a measure of a material’s resistance to indentation, defined by ASTM D2240. Shore A uses a flat-tipped indenter under 1 kg force (for rubber-like materials up to 90A); Shore D uses a sharp-tipped indenter under 5 kg force (for harder plastics above 90A). The two scales overlap around 90A — always specify which scale.

The hardness selection is a trade-off between feel/softness and structural performance. Softer TPE feels better but sacrifices compression set, tear resistance and dimensional stability. Harder TPE performs better mechanically but loses the “soft-touch” appeal. The engineer must define the primary function first — seal, grip, cushion or structural — and select hardness accordingly.

What Three Design Rules Govern TPE Hardness Selection for Overmolded Parts?

  1. Adhesion to the substrate depends on hardness — softer TPE bonds better. TPE overmolding onto rigid substrates (PP, ABS, PC, nylon) relies on both mechanical interlock and chemical adhesion. Softer TPE (30–50A) flows into substrate texture and micro-undercuts more easily, creating stronger mechanical bonds. The peel strength for 40A TPE on PP is typically 8–12 N/mm; for 80A TPE on PP it drops to 3–6 N/mm. If the bond must withstand repeated flexing or impact, keep the TPE below 60A and add mechanical features (through-holes, undercuts) to the substrate. The TPE overmolding bond strength guide covers substrate preparation and peel-test methods.
  2. Wall thickness minimum scales with hardness — softer TPE needs thicker walls. A 30A TPE requires minimum wall thickness of 1.5mm to fill without hesitation — below 1.0mm the material freezes before the cavity is full because the low-viscosity melt loses heat too quickly. A 70A TPE can fill walls as thin as 0.6mm because the higher-viscosity melt retains heat longer. For overmolded grips and seals, 2.0–3.0mm is the practical range regardless of hardness — this provides enough material for the TPE to compress and recover without tearing.
  3. Shrinkage varies with hardness — softer TPE shrinks more. TPE shrinkage ranges from 0.5% (90A) to 2.5% (20A) — softer grades have higher shrinkage because the polymer chains are less constrained and relax more after molding. This means the mold dimensions must be compensated for each hardness grade. If the same mold is used for multiple hardness levels (e.g., 40A and 70A for the same grip design), the 40A part will be 0.3–0.5% smaller than the 70A part — acceptable for grips but not for precision seals where the O-ring groove clearance is ±0.1mm.

Compression set — the percentage of permanent deformation remaining after a material is compressed for a specified time and temperature (typically 22 hours at 70°C per ASTM D395). Lower compression set = better seal retention. For static gaskets: target ≤25% (achieved at 60–70A for most TPE grades). Below 40A, compression set rises to 30–50% and the gasket will relax over time.

TPE Shore A: 10–90A range. Soft (30–50A) for gaskets/seals, medium (50–70A) for grips/bumpers, firm (70–90A) for buttons/wheels. Compression set: 30–50% at 30A, 15–25% at 70A. Processing: low pressure for soft (40–60 MPa), high for firm (80–120 MPa). Adhesion: softer bonds better to substrate.

  • Primary function defined — seal (compression set governs), grip (feel governs), cushion (rebound governs) or structural (tear strength governs).
  • Scale confirmed — Shore A for 10–90A; Shore D above 90A; Shore OO below 20A. Never mix scales.
  • Substrate adhesion tested — peel strength ≥8 N/mm for 40A on PP; ≥3 N/mm for 80A. Add through-holes for dynamic loads.
  • Wall thickness adequate — 30A: min 1.5mm; 70A: min 0.6mm. Overmolded grips: 2.0–3.0mm regardless of hardness.
  • Shrinkage compensated in mold — 0.5% at 90A to 2.5% at 20A; precision seals need per-grade mold inserts.

Frequently Asked Questions

Can I use the same mold for different Shore hardness grades?

Partially — the cavity dimensions will produce different part sizes because shrinkage varies with hardness (0.5% at 90A to 2.5% at 20A). For non-critical dimensions (grips, bumpers), the same mold works across a 20–30A range. For precision dimensions (seals, O-rings, press-fit features), each hardness grade needs its own mold or at minimum a re-machined insert for the critical dimension.

Gate design also needs adjustment — softer TPE requires larger gates (2–3mm) to prevent freeze-off before the cavity fills. If the mold has a 1.0mm gate designed for 80A TPE, switching to 30A will cause short shots because the gate freezes too quickly. The runner system can stay the same; only the gate and cavity dimensions need modification.

Why does my TPE part feel harder or softer than the datasheet value?

Three causes: temperature, measurement technique and processing history. Shore hardness is temperature-dependent — a 50A TPE at 23°C reads 45A at 50°C and 55A at 0°C. If the part is measured hot from the mold (50–60°C), it reads 3–5 points softer than the datasheet value measured at 23°C. Wait at least 2 hours after molding for the part to reach thermal equilibrium before measuring.

The second cause is measurement technique — the durometer must be perpendicular to the surface, the presser foot must sit flat (no rocking) and the reading should be taken at 1 second (instantaneous) and 15 seconds (sustained). For curved surfaces, use a type A durometer with a curved adapter — a flat indenter on a curved surface reads falsely hard.

What is the difference between Shore A and Shore OO?

Shore OO uses a 5mm-diameter flat indenter with 0.5 kg force — it measures very soft materials (gels, foams, soft TPE below 20A) that the Shore A indenter would penetrate completely. Shore OO values range from 10 OO (very soft gel) to 90 OO (equivalent to approximately 30A on the Shore A scale). If your TPE is specified at 15A and the durometer reads off-scale (the indenter hits the bottom), switch to Shore OO for an accurate reading.

For injection-molded TPE parts, Shore OO is rarely needed — most molded parts fall in the 30–90A range. Shore OO is used primarily for extruded profiles, gels and foam-in-place gaskets.

Key Takeaways

  • Shore A: 10–90A for TPE. Below 10A → Shore OO. Above 90A → Shore D.
  • Soft (30–50A): gaskets, seals. Medium (50–70A): grips, bumpers.
  • Compression set: 30–50% at 30A, 15–25% at 70A — softer = worse seal retention.
  • Softer TPE bonds better to substrate but needs thicker walls and larger gates.

Send the part drawing with the required hardness range, the substrate material and the performance requirement (seal force, grip feel, compression set) — the quotation specifies the TPE grade, gate design and mold modifications needed to hit the target hardness and adhesion.

Sources: ASTM D2240 — Standard Test Method for Rubber Property — Durometer Hardness · Kraiburg TPE — Shore Hardness Selection Guide and Overmolding Adhesion Data · Plastics Technology — TPE Processing Parameters by Hardness Range · Society of Plastics Engineers (SPE) — Thermoplastic Elastomer Design Guidelines · MatWeb — TPE Material Property Database Sorted by Shore Hardness

— Arthur, Tooling Engineer at aCatMould