How to Choose the Right Spring for Your Injection Mold

Why Spring Selection Matters in Injection Molding

The springs inside your injection mold perform a critical but often overlooked function: they return the ejector plate, stripper plate, or runner plate to its home position after every cycle. A correctly selected mold spring delivers consistent return force for over one million cycles. A poorly selected spring — too weak, too stiff, or over-compressed — leads to short shots, stuck parts, and unplanned downtime.

Mold spring selection involves balancing three interdependent variables: the force required to return the plate, the stroke distance the plate must travel, and the pocket depth available in the mold base. This guide walks you through the selection process step by step.

Step 1 — Determine the Required Return Force

The return force must overcome three resistances:

• Ejector plate weight: The gravitational force of the ejector assembly (typically 5-50 kg depending on mold size).
• Friction resistance: The friction between ejector pins, guide bushings, and their bores. This is the dominant resistance in most molds, accounting for 60-80% of the total return force.
• Resin adhesion: In some cases, residual plastic on the ejector pin tips creates additional resistance during return.

A practical estimation method: total required return force = 1.5 × ejector plate weight × (1 + friction coefficient). For a 20 kg ejector plate with a typical friction coefficient of 0.15: F = 1.5 × 20 × 9.81 × 1.15 = 339 N. Divide this by the number of springs to get the force per spring.

Step 2 — Calculate Pocket Depth and Free Length

The spring pocket is the cylindrical bore in the mold plate that houses the spring. The pocket depth determines the maximum spring free length you can use:

Critical constraint: The working compression (S + P) must not exceed the spring's rated maximum deflection. For example, with a 50mm free length SWM spring (40% deflection): maximum compression = 50 × 0.40 = 20mm. If your stroke + preload exceeds 20mm, you need either a longer spring (deeper pocket) or a higher deflection grade.

Step 3 — Select the Load Grade

Mold springs are classified by load grade using an international color code system. Each grade represents a different balance between force capacity and allowable deflection:

The selection logic follows a simple upgrade path: start with SWM. If the force at maximum deflection is insufficient, move to SWH. If pocket depth limits you, switch to flat wire (SWU or SWY) which delivers 20-30% more force per unit depth.

Step 4 — Verify Against Fatigue Life Requirements

Spring fatigue life depends primarily on the operating deflection ratio — the ratio of working compression to free length. Operating closer to the maximum rated deflection dramatically reduces cycle life:

• At 30% of rated deflection: Expected life > 2,000,000 cycles
• At 50% of rated deflection: Expected life ~1,000,000 cycles
• At 80% of rated deflection: Expected life ~300,000 cycles
• At 100% of rated deflection: Expected life ~100,000 cycles

For production molds expected to run 1M+ cycles, design the spring to operate at no more than 50% of its rated maximum deflection. This provides both a safety margin and long fatigue life. The wire material also matters: SiCr alloy wire (SWOSC-V, per JIS G3561) maintains consistent properties up to 200°C, while standard carbon steel wire (SWP-A) degrades above 80°C.

Step 5 — Consider Temperature Effects

Standard carbon steel springs lose 10-15% of their rated load when operating above 80°C due to stress relaxation — the gradual loss of stored elastic energy at elevated temperatures. In injection molds processing engineering resins like PA (nylon), POM, PBT, PC, or high-performance polymers like PPS, PEEK, and LCP, mold base temperatures routinely reach 80-200°C.

Heat-resistant coil springs use oil-tempered SiCr alloy wire that maintains 95%+ of rated load at 200°C. The premium over standard springs is typically only 10-15%, making heat-resistant springs the default choice for any mold that may be used with engineering resins. Reference: ASTM A401 specification for chromium-silicon alloy steel wire.

Common Spring Selection Mistakes

After reviewing hundreds of mold spring failures, these are the most frequent errors:

• Over-compression: Compressing the spring beyond its rated deflection. This is the #1 cause of premature spring breakage. Always verify total compression (stroke + preload) against rated deflection.
• Wrong load grade: Using SWF (light) springs where SWM (medium) is needed, often because the spring "fits" dimensionally but doesn't deliver enough force.
• Ignoring temperature: Using standard carbon steel springs in heated molds. The springs work initially but lose force progressively, leading to increasingly inconsistent ejection over time.
• Missing guide pins: Springs with free length-to-OD ratios above 4:1 buckle without guide pins, causing uneven force and accelerated fatigue. See our guide on spring guide pins.
• Mismatched sets: Using springs of different ages or conditions in the same mold. Old springs with reduced set height deliver less force, causing uneven plate return.

Quick Selection Summary

For rapid spring selection without detailed calculations:

• Small mold (<300mm): SWM (Red), round wire, 40% deflection
• Medium mold (300-600mm): SWM or SWH, round wire, check pocket depth
• Large mold (>600mm): SWH or SWU flat wire, consider gas springs for very heavy plates
• High-temperature mold (>80°C base): Always use heat-resistant (200°C rated) springs
• Three-plate mold runner return: Gate cut springs (separate category, see gate cut springs)