How to Select Urethane Spring Hardness — Shore A70 vs A90 for Die Applications

Understanding the Shore Hardness Scale in Die Elastomers

Polyurethane elastomer springs are widely used in press dies as a high-density, fragmentation-free alternative to mechanical coil springs. Unlike steel springs that operate under linear Hookean elastic principles, polyurethane springs exhibit highly non-linear, viscoelastic behavior. Hardness, measured on the Shore A durometer scale per ASTM D2240, is the primary physical property that dictates a spring's compressive stress profile, load capacity, and maximum allowable deflection.

Durometer hardness directly correlates with the shear modulus ($G$) and Young's modulus ($E$) of the material. A Shore A90 compound has a substantially higher modulus than a Shore A70 compound. This means that for a given volume of elastomer, the A90 spring will require significantly higher forces to compress, while generating more internal thermal energy (hysteresis) under high-speed cycling. Designing with these materials requires understanding how these physical properties govern performance limits.

Shore A90 Polyurethane: The Heavy-Duty Stamping Workhorse

Shore A90 is the default hardness grade specified for most heavy industrial metal stamping dies. Commonly sold as MISUMI A or C series or DANLY Formathane Heavy Duty springs, Shore A90 polyurethane provides the highest force-to-volume ratio of all solid elastomer options. It is engineered to withstand the extreme shock loads associated with high-tonnage blanking and piercing operations.

At Shore A90, the material resists wear from metal shavings, hydraulic oils, and standard coolants. However, the high material stiffness restricts the maximum safe stroke of the spring. When subjected to continuous production cycling, the deflection of a Shore A90 spring must be limited to prevent premature material breakdown. The mechanical properties of Shore A90 include:

• Load Capacity: High load density, providing up to 5 times the force of Shore A70 for the same footprint.
• Deflection Limits: Recommended continuous cycle deflection is limited to 15% of the free length. Absolute maximum peak deflection is 25%.
• Cycle Speed: Best suited for low-to-medium cycle speeds (under 80 strokes per minute) due to high hysteresis and heat build-up.
• Durability: Excellent tear strength and resistance to localized stress concentration from punch shoulders or stripper bolts.

Typical applications include heavy-gauge steel blanking, compound piercing dies, and high-pressure stripper pads where space is extremely limited but high stripping force is critical. When designing with Shore A90, large clearance bores must be integrated into the tool plate to accommodate the radial expansion of the compressed elastomer.

Shore A70 Polyurethane: Medium Load, High Deflection

Shore A70 polyurethane springs (such as MISUMI E series or DANLY Formathane Medium/Light Duty) are selected when longer travel, softer landing, or cosmetic part protection is required. With a lower shear modulus, a Shore A70 spring compresses more easily, resulting in a shallower progressive force curve.

Because the material is more flexible, it undergoes less internal friction during compression. This lowers thermal generation, allowing Shore A70 springs to run at higher cycle rates than Shore A90 variants without melting or developing permanent compression set. The mechanical properties of Shore A70 include:

• Load Capacity: Moderate load density, ideal for light-to-medium pressure requirements.
• Deflection Limits: Recommended continuous cycle deflection is 20% of the free length, with a peak deflection limit of 35%.
• Cycle Speed: Better suited for faster stamping presses (up to 120 strokes per minute) due to lower hysteretic energy losses.
• Surface Protection: Softer surface contact prevents marking or burnishing on pre-painted steel, aluminum, and copper parts.

Common applications include aluminum sheet forming, drawing die cushion pads, and stripper plates for cosmetic outer panels (e.g., automotive body parts or consumer electronics enclosures). The lower surface pressure helps prevent cosmetic defects on the stamped material while still providing sufficient force to pull the metal sheet off the punches.

Porous Polyurethane Foam: High Deflection with Zero Bulge

For applications requiring long strokes and high deflection rates that exceed the limits of solid elastomers, porous polyurethane foam (such as the MISUMI PA/PLA series or Lamina Marshmellow springs) is the optimal choice. Unlike solid polyurethane, which has a Poisson's ratio of approximately 0.5 (making it practically incompressible in volume, requiring it to expand laterally), porous foam is highly compressible.

Porous foam contains microscopic gas-filled cells. When compressed, the air in these pockets is compressed, allowing the volume of the block to decrease. This results in a Poisson's ratio closer to 0.25 to 0.3. Because the material compresses inwards, it exhibits virtually zero lateral bulging. The mechanical properties of porous foam include:

• Deflection Limits: Excellent deflection tolerance, with continuous cycle deflection up to 35% and peak compression limits reaching 50% to 60%.
• Lateral Bulge: Negligible radial expansion, eliminating the need for oversized pocket clearance bores.
• Load Profile: Low initial spring rate, with a steep progressive curve in the last 15% of the stroke as the cells collapse fully.

Porous foam is highly recommended for progressive dies with deep forming stages where standard solid urethanes would fail due to excessive travel, or where tight pocket configurations prevent radial expansion. However, it exhibits lower overall tear strength and should not be used in environments saturated with highly abrasive metal chips.

Deflection Limits and Progressive Load Profiles

Selecting the correct hardness requires analyzing the force-deflection curve of the spring. Polyurethane springs do not obey Hooke's Law ($F = kx$). Instead, their stiffness increases progressively as they are compressed. The rate of stiffness acceleration is steeper for Shore A90 than for Shore A70.

Under continuous operation, compressing an elastomer spring beyond its recommended continuous deflection rate leads to a phenomenon known as "compression set." This is a permanent deformation where the spring fails to return to its original free length. This reduces the pre-load force and causes loose parts or stripping failures. Excessive deflection also generates internal heat exceeding the polymer's thermal limit (typically 80°C per ISO 10243 related thermal guidelines), resulting in thermal degradation, cracking, and melting.

Urethane Spring Material Comparison

The table below summarizes the key engineering parameters of Shore A70, Shore A90, and porous foam springs. Data is compiled based on standard testing methods under ASTM and JIS standards.

Engineering Design Guidelines: Bulging and Clearances

Because solid polyurethane springs (A70 and A90) maintain a constant volume during deflection, the axial compression must result in lateral expansion. Designers must calculate this bulging diameter ($D_bulged$) to size the pocket bore holes in the stripper or punch plates. If the pocket bore is too tight, the expanding urethane will bind against the steel wall. This increases the spring rate exponentially, restricts stroke, generates frictional heat, and rapidly destroys the spring.

The standard design formula for estimating the bulged diameter is:

Where $D_free$ is the outer diameter of the spring at rest, and $d$ is the percentage of axial deflection expressed as a decimal (e.g., 0.15 for 15%). For example, a Shore A90 spring with a 50mm OD compressed by 20% will expand to approximately 56mm. To prevent binding, the bore diameter should be machined to at least 58mm, ensuring a safety clearance buffer.

Step-by-Step Hardness Selection Protocol for Die Designers

To select the correct hardness and prevent premature failures, follow this sequence during the die design phase:

• Analyze the Stroke Requirements: Calculate the total stroke required for part forming and stripping. If the required stroke exceeds 20% of the maximum allowable space, solid Shore A90 cannot be used. You must specify Shore A70 (up to 35% peak) or shift to porous foam/nitrogen gas springs.
• Calculate Required Force: Determine the total tonnage required to strip the material from the punches. Use Shore A90 to pack the highest stripping force into a compact footprint. If the force is too high for the space, increase the spring diameter rather than exceeding deflection limits.
• Assess Part Surface Quality: If the workpiece is pre-finished, polished aluminum, or thin-gauge copper, solid Shore A90 will cause localized compression marks. Select Shore A70 or porous foam to distribute the load gently across a wider contact area.
• Calculate Bulge and Size Bores: Apply the bulging formula to size all pocket bores. Ensure the plate bores are chamfered to prevent sharp corners from cutting the expanding elastomer during cycling.