Injection Molding Machine Selection and Tonnage

Putting a mold on the wrong machine is one of the most expensive routine mistakes in injection molding. Too small and the mold flashes, the part is under-packed and the tool is damaged; too large and the part is over-clamped, the cycle is inefficient and the energy cost is wrong. Yet machine selection is often reduced to "it fits in the tie bars," which checks one constraint out of five.

Figure 1. Five machine selection checks with cavity pressure factors and shot size band.

Selecting a machine correctly means verifying five things at once: clamp tonnage, shot size, injection pressure and rate, tie bar and platen capacity, and the practical process window. This guide walks each one with the formulas and the safety margins that keep a mold out of trouble.

1. Clamp tonnage

Clamp tonnage must exceed the force the cavity pressure exerts to open the mold. That force is the projected area of the part (plus runners) multiplied by the average cavity pressure.

Clamp force (tonne) = projected area (cm²) × cavity pressure (t/cm²) / 1000 ... expressed in the units you use consistently.

A practical working form:

Required clamp tonnage = projected area (cm²) × effective cavity pressure (MPa) / 9.81 / 1000 ... adjusted by the factor below.

Because that is easy to get wrong, most shops use a rule of thumb with a cavity-pressure factor:

Compute the projected area of every cavity plus the runner, multiply by the factor, and add a safety margin of roughly 10–20%. Then check the result against the machine's rated tonnage — and note that the rating is usually quoted as the maximum, which should not be approached in normal production.

Projected area includes everything the melt pushes against. That means all cavities, the runner system, and any overflow. A four-cavity mold has four times the projected area of a single cavity, which is why multi-cavity tools need much larger machines.

2. Shot size

The injection unit must be able to deliver the total shot weight — part, runner and any cushion — while keeping a healthy residual cushion in the barrel.

• Total shot weight = (part weight + runner weight) × number of cavities.
• Required machine shot capacity: the shot should normally be 20–80% of the machine's maximum shot size.
• Below about 20%, the residence time becomes long and the melt degrades; above about 80%, the screw cannot plasticize consistently and the cushion is unreliable.
• Convert to volume using the resin density, because machines are often rated in grams of polystyrene; a denser or lighter resin changes the usable capacity.

Ignoring the density difference between the machine's reference resin and the actual resin is a classic error: a machine rated in grams of PS may be over- or under-capacity for a filled PA or a foamed PP.

3. Injection pressure and rate

The machine must deliver enough pressure to fill the part, and enough rate to fill it in the time the process needs.

• Injection pressure must exceed the pressure required at the gate plus all losses, with margin. Thin walls and long flow paths raise the requirement sharply.
• Injection rate (fill time) drives whether the part fills before the gate seals and whether weld lines and flow marks form. Look at the machine's maximum injection speed and its ability to hold that speed under load, not just the peak number.
• Screw and barrel must suit the resin: correct compression ratio, suitable wear resistance for filled grades, and correct shot-to-barrel ratio.
• Plasticizing capacity must keep up with the cycle time; an injection unit that fills fine but plasticizes slowly caps the cycle.

4. Tie bar, platen and daylight

The mold must physically fit and be supportable:

• Tie bar spacing must exceed the mold width and height with clearance for clamping and extraction.
• Platen size must support the mold with adequate clamping area; a mold larger than the platen will bow.
• Daylight (maximum open distance) must allow the part to be ejected and, if needed, the mold to accommodate a hot-runner system and robot access.
• Mold height (thickness) must fall within the machine's minimum and maximum mold height, after accounting for the clamping system.
• Ejector system — stroke, force and position — must match the mold's knockout requirement. Ejector stroke matters for deep parts and unscrewing or slide mechanisms.
• Mold weight must be within the crane and platen limits.

5. The practical process window

Beyond the mechanical checks, the machine has to run the part well:

• Repeatability of shot size and cushion at the required speed.
• Clamp and injection profile control adequate for the part's tolerance.
• Barrel and screw suited to the resin's thermal sensitivity and abrasiveness.
• Dryer, temperature control and material handling matched to the resin.
• Energy and cycle efficiency — an oversized machine wastes energy every cycle.

A worked way to check a mold against a machine

1. Compute the projected area of all cavities plus runner.
2. Multiply by the cavity-pressure factor for the resin; add 15% margin. Compare with the machine's usable tonnage (not the maximum).
3. Compute total shot weight; check it lands in the 20–80% band of the machine's capacity, converted for the actual resin density.
4. Estimate the required injection pressure; compare with the machine's capability at the required rate.
5. Confirm the mold fits: tie bars, platen, mold height, daylight, ejector stroke, weight.
6. Confirm plasticizing capacity and drying capability suit the cycle and the resin.

Only when all six pass is the machine actually suitable for the mold.

Common mistakes

• Sizing on clamp tonnage alone. Shot size, pressure and rate can rule a machine out.
• Using the machine's maximum tonnage as the target. Production should run well below the rating; the rating is a ceiling, not a set point.
• Forgetting the runner and the overflow. Small additions to projected area and shot weight add up across cavities.
• Ignoring resin density when the machine is rated in grams of polystyrene.
• Under-cushioning. A short shot that is "fixed" by a bigger cushion hides a machine that is too small.
• Choosing a big machine "to be safe." Over-clamping warps molds and wastes energy; a machine much larger than needed is its own defect.
• Overlooking ejector stroke until the mold is on the platen.

Figure 2. Quick reference: the key checks and reference values from this guide.

Checklist

• Projected area computed for all cavities plus runners
• Cavity-pressure factor selected for the resin and part type, with margin
• Clamp tonnage compared against usable tonnage, not the machine maximum
• Total shot weight inside the 20–80% capacity band, converted for resin density
• Injection pressure and rate capability verified against the part's fill requirement
• Plasticizing capacity and drying matched to the cycle and resin
• Tie bar spacing, platen size, mold height, daylight and weight all confirmed
• Ejector stroke and force matched to the mold's knockout design
• Machine repeatability adequate for the part tolerance
• Energy and cycle efficiency acceptable for the production volume

Machine selection is a five-way fit problem, not a single-number lookup. When clamp tonnage, shot size, pressure and rate, physical fit and the practical process window are all checked before the mold goes on the platen, the part runs on the right machine the first time — and the tool is spared the damage that comes from forcing it onto the wrong one.