Machine tonnage is the spec everyone asks about first. It’s also the one most often misunderstood, over-specified, or mismatched to actual requirements. Tonnage refers to clamping force, not machine size or capability in any general sense. Understanding what tonnage actually means, how to calculate requirements, and how it fits within the broader machine specification enables better machine selection and utilization.
What Tonnage Actually Means
Tonnage measures the clamping force that holds mold halves together during injection. When molten plastic fills the mold cavity under pressure, that pressure tries to push the mold halves apart. The clamping system must generate enough force to resist this separation or the mold opens, material escapes, and flash results.
Units vary by region. In the United States, tonnage typically uses US tons (2,000 pounds each). European machines often specify kilonewtons (kN), where 1,000 kN equals approximately 102 metric tons. Some specifications use metric tons directly. Converting between systems prevents specification errors when working across regions. A machine advertised as 5,000 kN is roughly equivalent to a 500 US ton machine, not a 5,000 ton machine.
Clamping force is not injection force. A 500-ton machine doesn’t inject with 500 tons of force. Injection force depends on hydraulic or electric system design and is typically much smaller than clamp force. Tonnage specifically addresses the mold-holding function. Confusing these forces leads to specification errors.
Relationship to separation force: During injection, pressure inside the mold cavity acts on the projected area (the mold footprint visible from the clamping direction). This pressure times area equals the force trying to separate mold halves. Clamp tonnage must exceed this separation force or problems result.
Dynamic versus static loading: The separation force isn’t constant throughout the cycle. Peak force occurs during the injection and pack phases when cavity pressure is highest. Some clamping systems handle dynamic loads better than others. Toggle clamps lock mechanically and maintain force without continuous hydraulic pressure. Direct hydraulic clamps must maintain pressure throughout, consuming energy but offering easier force adjustment.
Calculating Required Tonnage
A systematic calculation determines the minimum machine tonnage for a given mold. Guessing or using rules of thumb risks either under-sizing (causing flash and mold damage) or over-sizing (wasting capacity and energy).
The basic formula:
Required Tonnage = Projected Area × Clamp Factor
Projected area is the total area visible when looking at the mold from the clamping direction. This includes all cavities, runners, and sprue. For a rectangular part, projected area is simply length times width. For complex shapes, the outline area applies. CAD software can extract projected area directly from part models. When in doubt, use the bounding rectangle as a conservative estimate.
Multi-cavity molds multiply single-cavity projected area by cavity count, then add runner system area. Runner area typically adds 5 to 15 percent depending on system design; hot runners eliminate runner area from the calculation.
Clamp factor represents the pressure inside the cavity during molding, expressed as tons per square inch (or kN/cm²). This factor varies by material and processing requirements.
| Material Type | Typical Clamp Factor |
|---|---|
| PP, PE | 2-3 tons/in² |
| ABS, PS | 3-4 tons/in² |
| PC, Nylon | 4-5 tons/in² |
| Thin wall (any material) | 5-7 tons/in² |
| Very thin wall | 6-8 tons/in² |
Safety factor adds 10 to 20 percent to calculated requirements. This margin accommodates material variation, process excursions, and conditions that increase cavity pressure beyond typical values. Higher safety factors apply to critical applications where flash would cause significant problems; lower factors suffice for applications tolerating occasional flash.
Example calculation:
Part dimensions: 6 inches × 8 inches = 48 in² projected area per cavity.
Mold: 4 cavities plus runner system (estimated at 10 percent of cavity area).
Total projected area: (48 × 4) × 1.10 = 211 in².
Material: ABS at 3.5 tons/in² clamp factor.
Required tonnage: 211 × 3.5 = 739 tons.
With 15 percent safety factor: 739 × 1.15 = 850 tons minimum machine size.
Common calculation errors include: forgetting to add runner area (leading to under-sizing), using clamp factors inappropriate for the material or wall thickness (typically under-estimating thin-wall requirements), and applying safety factors inappropriately (either omitting them entirely or stacking multiple safety factors).
The Full Machine Specification
Tonnage is necessary but not sufficient for machine selection. Several other specifications must match application requirements.
Shot size is the maximum material volume the injection unit can deliver in one cycle. Shot size must exceed part weight plus runner weight, typically with 20 to 80 percent barrel utilization (too small underutilizes capacity; too full may not leave adequate cushion). Shot size specifications often express weight of general-purpose polystyrene; actual capacity varies by material density.
Injection rate measures how fast the machine can deliver material, typically in cubic inches per second or grams per second. Thin-wall parts require high injection rates to fill before freezing. Large machines don’t automatically have proportionally higher injection rates; this specification requires separate verification.
Platen dimensions determine maximum mold size the machine can accommodate. Even with adequate tonnage, a mold must physically fit on the platens with clearance for mounting and connections.
Tie bar spacing limits mold width and height. The mold must fit between the four tie bars that transmit clamping force. Some machines offer removable tie bar options for oversized molds.
Daylight opening (or mold height range) specifies the minimum and maximum distance between platens. The mold must fit within this range when closed. Daylight opening when fully open must accommodate mold height plus part ejection clearance.
Stroke is the maximum mold opening distance. Deep-draw parts or molds with extended ejection requirements need adequate stroke.
Machine Types by Tonnage Range
Different tonnage ranges serve different market segments with typical applications and characteristics.
Small machines (under 100 tons) suit precision parts, medical components, electronic connectors, and other small, detailed applications. These machines often emphasize precision and repeatability over raw power. Electric machines dominate this segment because their precision advantages matter most at small scale. Typical shot sizes range from a few grams to perhaps 50 grams. Cycle times can be very short, sometimes under 10 seconds for small thin-wall parts.
Medium machines (100 to 500 tons) cover the broadest application range: general industrial components, consumer products, housewares, automotive interior parts, and technical components. This range offers the greatest machine variety and availability. Both hydraulic and electric options compete actively. Shot sizes typically range from 50 grams to 2 kilograms. Most custom injection molders concentrate their capacity in this range because it serves the widest variety of work.
Large machines (500 to 1,500 tons) serve automotive exterior components, large appliance parts, industrial containers, and material handling products. These machines require significant floor space, electrical infrastructure, and material handling capability. Hydraulic systems dominate because their power density advantages become significant at scale. Shot sizes commonly reach 2 to 5 kilograms. Not every molder operates machines in this range; projects requiring large tonnage may have limited supplier options.
Very large machines (1,500+ tons) handle automotive bumpers, large panels, pallets, and industrial products requiring substantial projected area. These specialized machines require purpose-built facilities with adequate floor loading, crane access, and electrical supply. Relatively few molders operate in this range. Shot sizes may exceed 10 kilograms. Cycle times extend due to the thermal mass involved, often reaching several minutes for thick-wall parts.
Oversizing vs. Right-Sizing
Customers frequently over-specify tonnage, requesting larger machines than parts require. This tendency creates costs that compound over production life.
Why customers over-specify:
Safety margin assumptions (“bigger is safer”)
Uncertainty about actual requirements
Flexibility expectations (“we might need larger parts someday”)
Lack of calculation confidence
Costs of oversizing:
Higher machine hour rates (larger machines cost more to operate)
Larger floor space requirements
Higher energy consumption (especially hydraulic systems that maintain clamping pressure)
Reduced scheduling flexibility (large machines aren’t available for small jobs)
Potential quality issues (small molds in large machines may have less consistent support)
Benefits of right-sizing:
Lower operating costs per machine hour
Better machine utilization (correct-sized machines can run a broader job mix)
More precise control (machines operating in their design range perform better)
Lower capital investment for equivalent capacity
Matching approach: Calculate required tonnage carefully, apply appropriate safety factor, then select the machine that provides adequate capacity without significant excess. A part requiring 400 tons belongs in a 450-ton machine, not a 750-ton machine “just in case.”
Fleet Considerations
For molders operating multiple machines, fleet composition affects flexibility and economics.
Size distribution should match job mix. A fleet weighted toward large machines struggles with small jobs. A fleet of only small machines cannot accept large opportunities. Analyzing current and anticipated job mix guides fleet composition. Historical job data reveals patterns: what percentage of jobs require under 200 tons, 200-500 tons, over 500 tons? Future fleet investments should align with where the work actually falls.
Standardization simplifies operations. Multiple machines of the same model share spare parts, training, and tooling auxiliaries. But over-standardization may not match actual job diversity. A balance between standardization benefits and application coverage usually works better than either extreme.
Investment timing distributes risk. Adding machines incrementally as demand justifies, rather than buying capacity for anticipated growth, matches investment to validated demand. The temptation to buy larger machines “for future flexibility” often results in underutilized capacity and higher operating costs for current work.
Utilization targets for individual machines typically aim for 70 to 85 percent of available time (accounting for setup, maintenance, and normal variability). Under-utilization suggests excess capacity; over-utilization signals need for additional machines or overtime costs. Tracking utilization by tonnage range reveals where capacity is tight and where investment would have most impact.
Machine hour rates vary by tonnage. Larger machines cost more to operate due to higher energy consumption, larger floor space allocation, and typically higher labor requirements. Understanding true machine hour rates by size enables accurate job costing and reveals when moving work to different-sized machines improves economics.
When to add tonnage range:
Consistent work turned away because no machine fits
Persistent over-utilization of a specific size range
Customer requests in segments not currently served
Strategic decision to enter new market segments
Tonnage selection is a systems decision. The right machine produces quality parts efficiently; the wrong machine compromises quality, costs, or both. Understanding tonnage in context of full machine specifications, actual part requirements, and fleet economics enables decisions that optimize long-term production capability.
Sources
- Rosato, Donald V. “Injection Molding Handbook.” Springer.
- RJG Inc. “Machine Selection Guidelines.” https://rjginc.com/
- Plastics Technology. “Machine Specification Guide.” https://www.ptonline.com/
- Engel. “Injection Molding Machine Selection.”
- Krauss Maffei. “Machine Tonnage Calculation Guidelines.”