Why an Automotive Stamper Replaced Metal Springs with Urethane for Wet Stamping

Wet stamping operations present a challenging environment for mechanical tooling components. The continuous spray of water-soluble coolants and synthetic lubricants protects stamping dies from zinc buildup but causes rapid corrosion in standard tool steel springs. This case study details how an automotive body stamper resolved chronic coil spring breakages in a drawing die by upgrading to Shore A90 polyether-based polyurethane blocks, eliminating corrosion failures and reducing annual maintenance costs by 72%.

Challenge: Rust Pitting and Stress Corrosion Cracking

An automotive stamping facility was running a 2-stage transfer die to draw galvanized chassis reinforcement panels. Stamping galvanized steel requires high volumes of water-soluble coolant (95% water, 5% oil emulsion) to prevent zinc transfer and galling on the draw die surfaces.

The draw ring required 18,000 daN of holding force to control metal flow during drawing. The die was originally designed with 24 heavy-duty mechanical coil springs (ISO 10243 yellow series) installed in pockets beneath the draw ring.

Within weeks of starting production, the coolant spray washed away the grease and wore off the springs' protective lacquer coating. This exposed the bare chrome silicon steel to moisture and oxygen, resulting in rapid corrosion.

This environment led to two primary issues:

• Stress Corrosion Cracking (SCC): The combination of tensile stress during spring compression and exposure to corrosive coolant caused rapid Stress Corrosion Cracking. Rust pits on the coil surfaces acted as stress concentrators, leading to sudden spring fractures after just 50,000 to 80,000 cycles.
• Part Scratching from Debris: When a coil spring broke, steel fragments fell into the draw pocket. Some of these fragments found their way onto the draw ring, scratching the cosmetic surfaces of the galvanized panels. This resulted in a $3.1%$ scrap rate and required polishing the die steels after every stoppage.
• Lubricant Degradation: The coolant emulsified with the spring lubricants, forming a sticky residue that clogged the spring coils. This residue limited spring travel, leading to unbalanced drawing pressure and wrinkling on the panel flanges.

Replacing the broken springs required stopping the press line for an average of 2 hours, costing $$1,500$ per hour in lost production. The tool experienced an average of 18 unscheduled spring failures per year, costing over $$54,000$ in downtime.

Solution: Upgrading to Polyether Urethane Blocks

To eliminate these corrosion failures, the tooling engineering team decided to replace the metal coil springs with polyurethane elastomer blocks.

The selection of the polyurethane formulation was critical.

Polyurethane elastomers are generally divided into two families: polyester-based and polyether-based. Polyester urethanes offer high mechanical strength but degrade rapidly in water, moisture, or coolants due to hydrolysis—a chemical reaction that breaks the ester bonds in the polymer chain.

The engineering team specified MISUMI Shore A90 Polyether-based Urethane Die Blocks (UDB-90). Polyether formulations are highly resistant to hydrolysis and oils, allowing them to maintain their mechanical strength and elasticity in wet environments.

The conversion process involved:

1. Pocket Enlargement: The original spring pockets were widened to provide a 15% diametral clearance (bulge allowance) around the urethane cylinders. Because polyurethane is virtually incompressible, it must expand laterally (bulge) under load. Proper clearance is necessary to prevent the urethane from rubbing against the pocket walls, which would cause friction heating and split the elastomer.
2. Center Hole Core Venting: The team selected hollow urethane cylinders. The center hole allowed the blocks to bulge inward as well as outward, reducing the shape factor and lowering internal shear stresses during compression.
3. Direct Drop-In Assembly: The 24 urethane blocks were installed directly over guide pins in the modified pockets.

Results and Cost Comparison

The upgraded drawing die returned to production. Over the next 12 months, the tool ran 1.2 million cycles. The table below compares the tool's performance and operating costs before and after the urethane upgrade.

"We were skeptical about using 'rubber' blocks for draw pad pressure, but the results speak for themselves. We haven't had a single broken spring or scratched panel in over a year, and we no longer have to stop the press to clean out metal shards."

— Plant Tooling Manager, Automotive Body Stamper

Engineering Discussion: Material Chemistry and Damping

The success of this upgrade is explained by two engineering principles: material chemistry and vibration damping.

First, the chemical structure of polyether-based polyurethane makes it highly stable in wet stamping environments. Unlike polyester urethanes, which break down when exposed to moisture (hydrolysis), polyether urethanes feature ether linkages (R-O-R) that are highly resistant to water cleavage. This stability allows the blocks to maintain their mechanical properties and elasticity even when submerged in water-soluble stamping coolants.

Second, polyurethane has excellent vibration damping properties. Unlike steel coil springs, which transfer vibration energy directly through the tool, polyurethane absorbs and dissipates this energy as heat. This damping effect stabilizes the draw ring during punch impact, reducing drawing shock waves and vibration. This stabilization helps maintain even holding pressure, preventing material wrinkling and improving panel quality.