Home / Guide / Injection Molding Machine Tonnage Calculation

Injection Molding Machine Tonnage Calculation

Injection molding machine tonnage is the clamping force needed to keep the mold closed during injection. Too low = flash; too high = mold damage and wasted energy.

The Formula

Tonnage (tonnes) = Projected Area (cm²) × Injection Pressure (bar) / 1000 × Safety Factor

Quick Reference

Material	Typical Pressure (bar)	Tonnes/cm²
PP, PE	300-500	1.5-3
ABS, HIPS	400-600	2-4
PC, PMMA	500-800	3-5
PA6, POM	400-600	2-4
PA66-GF30	600-900	3.5-6
PEEK, LCP	800-1200	5-8

Step-by-Step

Calculate projected area (L×W in cm). Add 10-20% for runner/sprue.
Multiply by number of cavities.
Select injection pressure from table or Mold Flow.
Divide by 1000, multiply by 1.2-1.5 safety factor.

Example

PP part 20×15cm = 300 cm². 4 cavities = 1,200 cm². PP pressure ≈ 400 bar. 1,200 × 400 / 1000 = 480T. × 1.3 = 624T → 650T machine.

Buyer's Tip: The most common mistake: running a mold on a much larger machine because the right one is busy. This causes higher energy costs, longer plastic residence time (material degradation), and potential mold damage from over-clamping. Always specify the machine tonnage range on your mold design sheet.

Verification Checklist

Ask the factory: what machine tonnage will this mold run on?
Get the machine specification: tie-bar spacing, max shot weight, screw L/D ratio
Verify tie-bar spacing accommodates the mold dimensions
Check shot weight: the shot should be 25-70% of machine capacity
Request a tonnage readout during tryout — don't trust the machine label

What This Means for Your Project: Include the machine tonnage requirement in your mold specification document. For a medium-complexity mold, provide both the required tonnage (calculated) and acceptable range. A good rule: the mold should run on a machine where the shot size is 30-60% of the machine's maximum shot capacity. This ensures consistent melt quality and stable process parameters.

MoldKey.com — Your Gateway to China Manufacturing