Injection moulding tonnage: how to spec for your packaging mix

Who this is for

Plastics processors, packaging manufacturers, and entrepreneurs specifying injection moulding machines (IMMs) for an African project — closures, containers, thin-wall packaging, crates, housings, fittings, and technical parts. Also useful as a sanity-check before signing an IMM quote.

The one number that sizes the machine: clamp force

An injection moulding machine is sized by its clamp force — the force (in tonnes) that holds the mould closed against the pressure of molten plastic being injected. If the clamp force is too low, the mould blows open slightly during injection ("flash"), ruining parts. If it is far too high, you have paid for capacity and energy you never use.

Everything else — shot size, screw diameter, plasticising rate, tie-bar spacing — matters, but clamp force is the headline that determines which machine class you are in.

How clamp force is estimated

The working estimate is:

Clamp force (tonnes) ≈ Projected area (cm²) × Clamp factor (tonnes/cm²)

• Projected area is the part's shadow area in the direction the mould opens — including runners, and multiplied by the number of cavities.
• Clamp factor depends on the material and the part geometry. Indicative ranges: easy-flow commodity parts ≈ 2–4 t/cm²; engineering plastics or thin-wall/high-pressure parts ≈ 4–8 t/cm².

Worked example: a 4-cavity closure mould where each closure + runner share projects ~12 cm², total ~48 cm². At ~4 t/cm² for a thin-wall closure in PP, that's ~190 t of clamp force needed — so a 220–250 t machine (with margin) is the right class, not a 400 t.

This is a feasibility-level estimate. The final number comes from the part designer / mould-maker's flow analysis. But it is enough to know whether a quote is in the right ballpark.

Common tonnage bands and what they make

Spec for the mix, not the single part

Most processors run several products on one machine over its life. The right tonnage covers the most demanding part in that mix at acceptable cycle time — but over-buying to cover a hypothetical future part you may never make is just idle capex. Two practical patterns:

• One machine, a coherent part family — size for the largest projected-area part in the family, with margin. Don't stretch the family so wide that you need a much bigger machine for one outlier.
• Multiple machines, segmented by tonnage — common in established processors: a bank of 150–250 t for closures and small parts, a 350–650 t for containers, etc. Each part runs on the smallest machine that can make it well — the cheapest cost-per-part.

Running a small closure on a 650 t machine because "it was free that shift" wastes energy and clamp wear. Cost-per-part is lowest on the right-sized machine. Our plastics OEE retrofit case study shows how visibility data exposes exactly this kind of machine/part mismatch.

What else to check beyond tonnage

Clamp force gets you to the right class, but a few other specs decide whether the machine actually suits your parts:

• Shot size / shot weight — the machine must be able to inject the total shot (all cavities + runners) comfortably, ideally using 20–80% of its barrel capacity for stable processing.
• Tie-bar spacing / platen size — the mould must physically fit between the tie-bars. A high-cavity mould can be limited by platen size before clamp force.
• Plasticising rate — the machine must melt enough material per second to sustain your cycle time at your output.
• Injection pressure and speed — thin-wall and fast-fill parts need higher injection pressure and speed than commodity parts.
• Drive type — hydraulic, servo-hydraulic, or all-electric. Servo-hydraulic and electric machines cut energy substantially — important where power cost or grid stability matters.

Energy and the African grid reality

Injection moulding is energy-intensive, and energy is often the second-largest cost after material. In African projects with high tariffs or unstable supply, the drive technology matters:

• Standard hydraulic — lowest capex, highest energy draw.
• Servo-hydraulic — moderate capex premium, typically 20–60% energy saving versus fixed-pump hydraulic. Usually the best value in African conditions.
• All-electric — highest capex, lowest energy, fastest and most repeatable cycles; best for high-volume precision (closures, thin-wall) where the energy and cycle gains repay the premium.

For voltage-unstable sites, also budget surge protection and consider how the machine restarts cleanly after a grid event.

What gets under-budgeted on injection moulding projects

• Auxiliaries — chillers, mould-temperature controllers, dryers, granulators, material handling. These can add 20–40% to the bare machine cost and are essential, not optional.
• Tooling — moulds are often the largest single cost over the machine's life, and the most important quality determinant. A cheap mould on a good machine makes bad parts.
• Mould-temperature control — under-specified MTC is a common cause of cycle and quality problems.
• Material drying — many resins need drying; skipping it ruins parts.
• Visibility / OEE — knowing real cycle time, scrap, and per-mould performance. See how to measure OEE on an old line.

How to spec — a short sequence

1. List your part family — sizes, materials, cavity counts.
2. Find the most demanding part — largest projected area × highest clamp factor.
3. Estimate clamp force for that part, add 10–20% margin.
4. Check shot size and tie-bar clearance for that part.
5. Choose drive technology on energy cost and volume (servo-hydraulic is usually the African default).
6. Budget the auxiliaries and tooling as core scope, not extras.

What CISH does in this part of the process

For plastics and packaging projects, we help specify the right machine class for your part mix, choose drive technology for your energy reality, and scope the auxiliaries and tooling properly — then structure the sourcing split. See Plastics & packaging production lines, our OEE retrofit case study, and the import vs local framework.

Frequently asked questions