Spring Deflection Defined
Deflection percentage is the single most important specification when sizing a mold spring. It tells you how much you can safely compress the spring as a percentage of its free length:
Maximum compression = Free length × Deflection percentage
For a 50mm spring with 40% deflection: 50 × 0.40 = 20mm maximum compression. The spring can be shortened from 50mm to 30mm (its "solid height") without permanent deformation.
Exceeding this limit — even briefly — causes the wire to plastically deform. The spring takes a permanent set, loses free length, and delivers less force on every subsequent cycle. This is the #1 cause of mold spring failure.
How Deflection Percentage Affects Spring Selection
Springs with higher deflection percentages allow more compression from a shorter free length. This creates a trade-off between stroke capacity and force:
| Deflection Grade | Max Compression | Force Capacity | Fatigue Life | Best Application |
|---|---|---|---|---|
| 30% (SWN/SWH) | Low | Highest | Longest | Heavy loads, generous pocket depth |
| 35% (SWH) | Low-Medium | High | Very Long | Standard heavy-duty ejection |
| 40% (SWM/SWS) | Medium | Medium | Long | 80% of mold applications (default) |
| 50% (SWL/SWR) | Medium-High | Medium-Low | Moderate | Limited pocket depth, moderate force |
| 60% (SWF/SWU) | High | Low-Medium | Shorter | Minimum pocket depth applications |
| 65% (SWY) | Highest | Variable | Shortest | Extreme space constraints only |
Higher deflection = more stroke from a shorter spring, but shorter fatigue life and often lower absolute force. Lower deflection = less stroke but more force and longer life.
The Deflection-Life Relationship
Fatigue life drops exponentially as operating deflection approaches the rated maximum. Here is the relationship based on published spring fatigue data:
| Operating Deflection (% of Rated Max) | Expected Fatigue Life | Risk Level |
|---|---|---|
| 30% | > 2,000,000 cycles | Minimal — spring operates well within safe zone |
| 50% | ~1,000,000 cycles | Low — recommended design target |
| 70% | ~500,000 cycles | Moderate — acceptable for low-volume molds |
| 90% | ~200,000 cycles | High — replacement needed every 2-3 months in production |
| 100% | ~100,000 cycles | Maximum — spring at its design limit |
| 105%+ | < 50,000 cycles | Critical — permanent set and rapid failure |
These values are based on published fatigue curves for SWOSC-V wire at room temperature, consistent with data in ISO 13906 (Cylindrical Helical Compression Springs) and spring engineering handbooks.
Practical Calculation Example
A mold designer needs to select a coil spring for an ejector plate with the following parameters:
- Ejection stroke: 25mm
- Desired preload: 3mm
- Maximum pocket depth: 60mm
- Target life: 1,000,000+ cycles
Step 1: Total working compression = 25 + 3 = 28mm.
Step 2: For 1M+ cycle life, design at 50% of rated deflection. Required rated max compression = 28 / 0.50 = 56mm compression capacity.
Step 3: For a 40% deflection spring (SWM): Free length = 56 / 0.40 = 140mm. This exceeds the 60mm pocket depth — won't fit.
Step 4: For a 60% deflection spring (SWF): Free length = 56 / 0.60 = 93.3mm. Still too long for a 60mm pocket.
Step 5: Re-evaluate design target. At 70% of rated deflection (accepting 500K cycle life): Required max compression = 28 / 0.70 = 40mm. For SWM (40%): Free length = 40 / 0.40 = 100mm. Still too long.
Step 6: Consider SWU flat wire (60% deflection): Free length = 40 / 0.60 = 66.7mm. With 2mm bottom clearance: needs 68.7mm pocket — still too deep. The designer must either deepen the pocket or switch to gas springs which handle long strokes in compact packages.
This example demonstrates why understanding deflection percentage is essential: it determines whether a coil spring solution is physically possible within the mold's space constraints.
Best Practices for Mold Designers
- Always start with 40% deflection (SWM) as baseline: This is the most versatile grade for mold applications.
- Design for 50% of rated deflection: This ensures 1M+ cycle life and provides margin for spring set over time.
- Never exceed rated deflection: If the calculation requires more compression than the spring allows, change the spring — never force the existing spring to over-compress.
- Account for preload: Always add 2-5mm preload compression to the ejection stroke when calculating total compression.
- Include bottom clearance: Leave 1-2mm between the spring bottom and the pocket floor to prevent solid-height bottoming.
For comprehensive spring selection guidance, see our full guide on how to choose the right spring for your injection mold.