When to Switch from Coil Springs to Gas Springs in Your Injection Mold — Decision Criteria

Coil Springs vs Gas Springs — Fundamental Differences

Coil springs and gas springs serve the same function in injection molds — providing return force for ejector plates and stripper plates. The physics are different: coil springs store energy in wire deformation; gas springs store energy in compressed nitrogen gas. This difference creates distinct performance characteristics.

Coil springs produce force proportional to compression distance (Hooke's Law: F = k × x). The force starts low and increases as the spring compresses. Gas springs produce nearly constant force throughout their stroke because the nitrogen pressure decreases only slightly as the piston moves (Boyle's Law in a large-volume chamber).

Head-to-Head Performance Comparison

Five Situations That Require Gas Springs

Gas springs are not always necessary — coil springs handle 80% of injection mold applications. But certain conditions make gas springs the only practical choice:

• Insufficient pocket depth for required force: When you need more than 2,000N from a pocket shallower than 80mm, coil springs cannot deliver enough force even at the heaviest load grade (SWH). A single gas spring of 40mm diameter and 80mm length can deliver 5,000-15,000N.
• Ejection stroke exceeds 50mm: Coil springs are limited by their deflection percentage. A 40% deflection spring needs a 125mm free length for a 50mm stroke — too long for most ejector plate designs. Gas springs handle 100mm+ strokes in compact packages.
• High initial force needed: Parts with deep ribs or tight draft angles require high force at the start of ejection to break adhesion. Coil springs deliver minimum force at the start of stroke (only preload). Gas springs deliver full rated force from the first millimeter of travel.
• Space-constrained ejector plates: In multi-cavity molds with many ejector pins, there may not be enough room for 6-8 coil springs. Two gas springs can replace all of them with higher total force in a smaller footprint.
• Consistent force throughout stroke: Applications where the ejection force must be uniform — such as stripping thin-wall containers — benefit from the constant force output of gas springs. Coil springs deliver increasing force as they compress, which can distort thin parts.

When to Stay with Coil Springs

Do not switch to gas springs if your application meets these criteria:

• Adequate pocket depth: The spring bore can accommodate a coil spring at the required free length within rated deflection.
• Force under 2,000N total: Four medium-load (SWM) coil springs deliver 2,000N total at a fraction of the gas spring cost.
• Mold temperature above 150°C: Gas spring seals degrade above 150°C. Coil springs rated for 200°C (SWOSC-V wire) are the only option for high-temperature molds running PPS, PEEK, or LCP.
• Budget-constrained prototype molds: At $2-15 per spring vs $50-300 per gas spring, the cost difference is significant for short-run molds.

For gas spring sizing, the fundamental relationship is F = P × A where F is force, P is nitrogen pressure, and A is the piston cross-sectional area. For detailed calculations, refer to ISO 11901 (Gas Springs for Press Tools) which standardizes dimensions and force ratings.

Cost Comparison — Total Installed Cost

The per-unit cost comparison is misleading. A more accurate analysis considers the total installed cost including pocket machining, ejector plate design complexity, and replacement frequency:

At high production volumes (>500K shots/year), gas springs often break even or save money compared to coil springs due to fewer replacements and simpler pocket machining.