How to Calculate Gas Spring Force for Mold Ejection — Pressure, Stroke & Safety Factor

The Basic Gas Spring Force Equation

A nitrogen gas spring generates force by pressurizing nitrogen gas against a piston. The fundamental equation is straightforward: F = P × A, where F is the output force in Newtons, P is the internal nitrogen pressure in MPa, and A is the effective piston cross-sectional area in mm².

For a gas spring with a 25mm bore diameter: A = π × (25/2)² = 490.9 mm². At 15 MPa (150 bar) nitrogen pressure: F = 15 × 490.9 = 7,363N initial force. This is the force the spring delivers at the start of its stroke, before any compression occurs.

The standard for gas spring dimensions and force ratings is ISO 11901 (Gas Springs for Press Tools and Moulds), which defines bore sizes, stroke lengths, and nominal initial force specifications.

Force Progression During Stroke

Unlike coil springs (where force increases linearly with compression), gas springs exhibit a non-linear force increase governed by Boyle's Law. As the piston compresses the nitrogen, the gas volume decreases and the pressure increases:

The exact progression ratio depends on the gas spring's gas volume ratio (gas chamber volume / total cylinder volume). Springs with larger gas chambers have flatter force curves. This ratio is typically published in the manufacturer's catalog as the "K-factor" or "progression factor."

Calculating Required Force for Mold Ejection

The required ejection force depends on the part geometry, material, and mold surface:

• Stripping force: F_strip = Part projected area × Injection pressure × Friction coefficient. For a typical ABS part with 100 cm² projected area: F_strip = 10,000 mm² × 0.3 MPa (residual pressure) × 0.3 (friction) = 900N
• Core pull force (if applicable): Additional force needed to pull the part off core pins or ribs. Depends on draft angle, depth, and material shrinkage.
• Safety factor: Apply 1.5× safety factor to the calculated stripping force: F_required = 900 × 1.5 = 1,350N minimum gas spring initial force
• Temperature compensation: Gas pressure increases approximately 0.3% per °C above the catalog rating temperature (typically 20°C). At 60°C mold base: force increases by ~12%. At 80°C: ~18%.

Worked Example — Multi-Cavity Mold

A 4-cavity mold producing ABS housings needs gas spring ejection. Each cavity has 80 cm² projected area:

• Total stripping force: 4 × (8,000 × 0.3 × 0.3) = 2,880N
• With 1.5× safety factor: 2,880 × 1.5 = 4,320N required initial force
• Available pocket: 40mm bore, 80mm stroke length
• Selected gas spring: 40mm bore at 10 MPa = π × 20² × 10 = 12,566N initial force
• Quantity needed: 1 spring provides 12,566N > 4,320N required — one spring is sufficient

However, using 2 springs at 25mm bore (2 × 4,909N = 9,818N) provides better force distribution across the ejector plate and redundancy if one spring leaks. Always verify the end-of-stroke force against the mold structure capacity. Reference Boyle's Law for the gas pressure-volume relationship.

Common Sizing Mistakes

• Ignoring force progression: Sizing based only on initial force and forgetting that end-of-stroke force can be 30-60% higher. This can damage ejector pins or distort thin parts at the end of the ejection stroke.
• No temperature compensation: At 80°C mold base temperature, gas spring force is ~18% higher than catalog rating. This is additive with the progression factor — end-of-stroke force at 80°C can be 50-90% above the catalog initial force.
• Undersizing the safety factor: Using 1.0× instead of 1.5×. Gas springs lose 5-10% force between nitrogen recharges, so the safety factor ensures adequate force throughout the maintenance interval.
• Using full stroke: Never operate a gas spring to 100% of its rated stroke. Leave 2-5mm unused at the end of stroke to prevent piston impact and seal damage. Design for 90-95% of rated stroke.