Most injection molding tonnage selection errors fall into two opposite categories: undersizing, which causes flash and short shots, and oversizing, which wastes capital and energy for the entire equipment life. The 6-step tonnage selection method at NBT starts from projected area, not shot weight, because clamp force is the structural requirement that prevents the mold from opening under injection pressure — not a function of how much plastic the part uses.

The formula is straightforward: clamp force F ≥ P × A × safety factor. P is the cavity pressure (typically 30-50 MPa for unfilled PP/PE, 60-100 MPa for glass-filled engineering plastics), A is the projected area of the part plus runner, and the safety factor of 1.2-1.5 accounts for pressure spikes during packing and holding phases. For a typical 38g thin-wall container with 320 cm² projected area in PP at 40 MPa cavity pressure: F = 40 × 320 × 1.3 = 16,640 kgf ≈ 17 tons, which puts the calculation in the 50-90 ton machine class.

The 6-step method below builds on that formula with the practical considerations that engineering calculations miss. Browse the injection molding machines by tonnage range to match clamp force to part size, and consider the servo energy-saving injection molding machine lineup for energy-cost-sensitive programs. For specific tonnage sizing on your part geometry, request tonnage sizing help with your projected area and resin specification.

NBT injection molding machine — clamp force range 50-4000 ton, servo energy-saving configuration available. Browse by tonnage →

The Mold Setter Who Faced a 250-Ton Shortage on a 38-Gram Part

The tonnage selection conversation almost always starts after the part is designed and the mold is being ordered. The mold setter picks a machine based on shot weight recommendations from the resin supplier, runs the first samples, and either gets acceptable parts or faces flash and short shots that trace back to clamp force. When a 38g thin-wall container fails on a 50-ton machine but succeeds on a 90-ton machine, the problem is not the resin — it is the projected area × cavity pressure calculation that was never run before the machine was specified.

This scenario played out in our floor with a thin-wall yogurt container program in 2024. The mold arrived, the supplier recommended a 50-ton machine based on shot weight, and the first samples had flash at the parting line — 2-3 mm overflow that triggered cosmetic scrap. Recalculating: 320 cm² projected area × 45 MPa cavity pressure during high-speed filling of thin walls × 1.3 safety factor = 18,720 kgf ≈ 19 tons calculated, but with cavity pressure spikes during packing the peak reached 60 MPa, putting the actual requirement at 25 tons calculated — still within 50-ton nameplate, but the pressure spike margin was zero.

The fix was to move up to the 90-ton machine class, which gave the pressure spike margin to operate cleanly. The mold ran for the next 18 months on the 90-ton machine without a single flash event, and the energy cost cost of operating a 90-ton versus 50-ton machine was marginal compared to the scrap rate reduction. This is the conversation that tonnage selection enables — not the abstract engineering calculation, but the operational outcome.

For mold setters and factory engineers new to the tonnage calculation, the formula is the foundation, but the practical considerations are what determine whether the production runs cleanly. The 6-step method below handles both.

Step 1 — Calculate the Projected Area (Including Runner System)

Projected area is the footprint of the part and runner system on the mold parting plane. For a single-cavity mold, the projected area is the part footprint plus the runner footprint, measured in cm². For a multi-cavity mold, the projected area is the sum of all cavity footprints plus the runner system. The runner system typically adds 10-20% to the projected area, which is significant for high-cavitation molds with cold runners.

Three measurement approaches, in order of accuracy:

1. CAD measurement (most accurate): most 3D CAD packages can calculate the projected area of a part on the parting plane automatically. For complex geometries with draft angles and curved surfaces, this is the only reliable method.
2. Drawing footprint calculation (typical): for rectangular or near-rectangular parts, the projected area is approximately length × width. For cylindrical parts, it is approximately π × (diameter/2)².
3. Weight-based estimation (rough): for parts with consistent wall thickness, projected area can be estimated from shot weight and part wall thickness using the polymer density. Less accurate, used only for first-pass sizing.

Common error in projected area calculation: excluding the runner system. The runner footprint adds 10-20% to the projected area in cold-runner molds, and 30-50% in hot-runner molds with substantial manifold cross-section. For programs where the runner is not separately accounted for, the calculated clamp force is 10-50% low, and the undersized machine shows up as flash.

Step 2 — Identify Cavity Pressure for Your Resin

Cavity pressure is the internal pressure in the mold cavity during the packing and holding phase of the injection cycle. It is the force that pushes the mold halves apart, which is why clamp force sizing is fundamentally a pressure-area calculation. The cavity pressure is not a fixed property of the resin — it varies with part geometry, wall thickness, packing time, injection speed, and melt temperature.

Typical cavity pressure ranges for the major resin categories:

For first-pass tonnage calculation, the mid-range value of the cavity pressure range is the typical starting point. For PP at 40 MPa, the calculation for the 38g thin-wall container (320 cm² projected area) with 1.3 safety factor: F = 40 × 320 × 1.3 = 16,640 kgf ≈ 17 tons calculated. This is the calculated minimum, not the machine specification — the machine specification rounds up to the next standard class (50 or 90 tons) to absorb pressure spikes and processing variability. Cavity pressure measurement methodology and processing window guidance for injection molding is standardized through ISO 294-1 injection moulding of test specimens and the related ASTM D3641 injection molding processing practices standard.

Cavity pressure can be measured directly with a pressure transducer mounted in the mold cavity. The transducer data validates whether the calculation matched reality, and it allows the safety factor to be tuned for subsequent production runs. Molders running high-cosmetic-finish parts or engineering plastic programs typically install cavity pressure transducers on production molds as part of the scientific molding approach.

Step 3 — Apply the Clamp Force Equation and Choose the Safety Factor

The clamp force formula F ≥ P × A × safety factor converts the cavity pressure and projected area into the clamp force requirement. The result is in kgf (kilogram-force), which converts to tons by dividing by 1000. The formula assumes consistent units: P in MPa (which equals N/mm² or 10 kgf/cm²), A in cm², and F in kgf. When P is in MPa and A in cm², F comes out directly in kgf because 1 MPa × 1 cm² = 10 kgf — verify your unit conversion before running the calculation.

Safety factor selection depends on the part characteristics and processing variability:

• 1.2 safety factor: well-characterized parts, stable processing, narrow cavity pressure variation. Use for commodity resins with consistent processing.
• 1.3 safety factor: standard specification for most production programs. Balances machine utilization against pressure spike margin.
• 1.4 safety factor: engineering plastics, glass-filled compounds, parts requiring high cosmetic finish, programs with rapid cycle targets where pressure spikes are common.
• 1.5+ safety factor: structural foam, parts with very thick sections, programs where processing window is narrow and pressure spikes are unpredictable.

Worked example for the 38g thin-wall container (320 cm² projected area, PP at 40 MPa cavity pressure, 1.3 safety factor): F = 40 × 320 × 1.3 = 16,640 kgf ≈ 17 tons calculated. The standard machine class above 17 tons is 50 tons, but the pressure spike margin during packing phase pushes the peak requirement to 25 tons, which puts the machine specification at 90 tons. The 50-ton machine would fail with flash under production pressure spikes; the 90-ton machine has margin to operate cleanly.

Worked example for a 250g engineering plastic automotive connector (180 cm² projected area, glass-filled PA at 80 MPa cavity pressure, 1.4 safety factor): F = 80 × 180 × 1.4 = 20,160 kgf ≈ 20 tons calculated, with packing pressure reaching 100 MPa peak, putting the requirement at 25 tons calculated. Standard machine class for 25 tons with margin: 50 tons or 90 tons. The narrower processing window of glass-filled PA rewards the higher machine class. Energy efficiency comparison methodology for injection molding machines follows the Euromap technical bulletins for plastics and rubber machinery, which is the European industry standard for measuring and reporting energy consumption across injection molding machine classes.

Steps 4–6 — Machine Selection, Servo vs. Fixed-Pump, and Quoting

Step 4 is matching the calculated clamp force to the standard machine tonnage class. Standard machine classes run in increments of approximately 50, 90, 120, 160, 200, 250, 300, 400, 500, 600, 800, 1000, 1250, 1600, 2000, 2500, 3000, and 4000 tons. The matched class is the next class up from the calculated clamp force, never the class below. Undersizing is the structural risk; oversizing is the capital efficiency risk.

Step 5 is choosing between servo energy-saving and fixed-pump hydraulic systems. The technical difference: servo systems use a variable-speed servo motor driving the hydraulic pump, which delivers flow only when the cycle demands it. Fixed-pump systems run the pump at constant speed regardless of cycle demand, with overflow valves bleeding off excess flow. The practical difference: servo systems deliver 30-70% energy reduction on variable-load cycles. The payback calculation depends on annual machine utilization: above 4,000-5,000 hours per year, servo typically pays back within 18-36 months; below 2,000 hours per year, fixed-pump remains more cost-effective.

Three secondary considerations for servo vs fixed-pump:

• Cycle time stability: servo systems offer more consistent cycle times because the hydraulic flow matches demand precisely. Fixed-pump systems show small cycle time variation depending on oil temperature.
• Noise level: servo systems operate at significantly lower noise levels (60-65 dB vs 75-80 dB for fixed-pump), which matters for factory environments with strict noise regulations.
• Cooling circuit efficiency: servo systems generate less heat in the hydraulic oil, which reduces cooling circuit load and extends oil service life.

Step 6 is the validation trial on the actual target machine before placing the production order. The trial validates that the calculated tonnage produces flash-free parts at the target cycle time, with the cavity pressure transducer data confirming the actual pressure profile. Programs that skip the trial and commit directly to bulk production occasionally encounter pressure spikes or processing variability that the calculation did not predict — the trial catches these before the production run begins.

Three Production Examples From Our Floor

Three production programs from our recent book illustrate how the 6-step method plays out across different part categories and resin systems.

Example 1: 38g PP thin-wall yogurt container (320 cm² projected area). Cavity pressure 40 MPa typical, 60 MPa peak during packing. Safety factor 1.3. Calculated F = 16,640 kgf ≈ 17 tons minimum. With peak pressure margin, machine specification: 90 tons (servo energy-saving). Cycle time target: 3.5 seconds. Outcome: 18 months of production with no flash events, 18% energy reduction versus the 50-ton fixed-pump machine it replaced.

Example 2: 250g glass-filled PA automotive connector (180 cm² projected area). Cavity pressure 80 MPa typical, 100 MPa peak. Safety factor 1.4. Calculated F = 20,160 kgf ≈ 20 tons minimum. With peak margin, machine specification: 50 tons. Cycle time target: 28 seconds (thick wall section). Outcome: production runs cleanly with cavity pressure transducer monitoring, no flash, no short shots. The 50-ton machine class provides the pressure spike margin that the narrower glass-filled PA processing window requires.

Example 3: 1,200g PP structural foam pallet (1,400 cm² projected area). Cavity pressure 55 MPa typical, despite the foamed part structure. Safety factor 1.5. Calculated F = 115,500 kgf ≈ 116 tons minimum. With peak margin, machine specification: 160 tons. Cycle time target: 180 seconds (large part cooling). Outcome: production runs cleanly with the higher safety factor absorbing the foaming pressure variability. The 160-ton class is the right specification despite the lower density of the foamed part, because cavity pressure remains high.

The common thread across all three examples: the formula calculation produces the minimum clamp force, and the machine specification adds margin for pressure spikes, processing variability, and safety factor selection. Programs that specify machine tonnage purely on shot weight typically undersize the first two examples and oversize the third.

Where the 6-Step Method Breaks Down

Three scenarios where the standard 6-step method needs adjustment or extension.

Long-glass-fiber compounds: the standard cavity pressure values for glass-filled resins assume short-glass-fiber (length 1-3 mm) compounds. Long-glass-fiber compounds (length 10-25 mm) generate higher cavity pressure during packing because the longer fibers restrict flow. Use a 1.5 safety factor for long-glass-fiber compounds even when short-glass-fiber of the same resin would only need 1.4.

Multi-material / over-molding: when the part includes over-molded subcomponents (inserts, previously molded parts, fabric layers), the cavity pressure calculation needs to account for the insert pre-stress. Programs running insert molding typically use 1.4-1.5 safety factor regardless of the base resin.

Boss and rib heavy designs: parts with substantial boss and rib structure concentrate cavity pressure at the thick sections, which can cause local flash even when the average cavity pressure is within the calculated range. Programs running boss-heavy parts typically use a 1.4 safety factor even for commodity resins.

For these scenarios, the 6-step method provides the starting point, and the safety factor adjustment handles the engineering variability. Programs that skip the safety factor adjustment and use 1.2 universally typically encounter flash at the thick sections of complex parts.

Frequently Asked Questions