BIZOMAT MAKEOS · Manufacturing
Injection Moulding Clamp, Shot, Cooling and Cycle Estimate
What clamp force and shot size does this mould need, how long will it take to cool, and how many parts per hour will it make?
First estimate for an injection moulding job: clamp force from projected area of the cavities and runner and the cavity pressure with a margin, shot volume and mass against the barrel capacity, cooling time for the wall thickness from the plastic’s thermal diffusivity and the melt, mould and ejection temperatures, cycle time and output per hour.
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How this tool works
Inputs, units and where to find them
| Input | Unit | Typical source |
|---|---|---|
| Mould | ||
| Projected area per part | cm² | Projected area of one part in the direction of mould opening. |
| Cavities | Number of cavities. | |
| Runner projected area | cm² | Projected area of the runner system. |
| Cavity pressure | MPa | Mean pressure in the cavity (about 20–40 MPa for easy-flow materials and thick walls, 40–80 for thin walls and stiff materials). |
| Clamp margin | % | Margin on clamp force. |
| Part volume | cm³ | Volume of one part. |
| Runner volume | cm³ | Volume of the runner (0 for a hot runner). |
| Melt density | g/cm³ | Density of the melt. |
| Cooling | ||
| Wall thickness | mm | Largest wall thickness of the part. |
| Thermal diffusivity of the plastic | mm²/s | Thermal diffusivity of the plastic (about 0.07–0.12 mm²/s). |
| Melt temperature | °C | Melt temperature. |
| Mould temperature | °C | Mould surface temperature. |
| Ejection temperature | °C | Temperature at which the part can be ejected. |
| Other cycle time (fill, hold, open, eject, close) | s | Time for filling, holding, mould movements and ejection. |
| Machine | ||
| Machine clamp force | kN | Clamp force of the machine considered. |
| Machine shot capacity | cm³ | Maximum shot volume of the machine. |
What each input means
Open an input to see what it is, its units, the worked example and the common mistakes, with a small diagram.
Mould
What this means: Projected area per part
- What it is
- The area of one part seen along the direction the mould opens: its shadow on the parting plane, which the melt pressure pushes against to open the mould.
- Units and format
- Enter a number in cm²: square centimetres. The form accepts 0.000001 cm² to 100000 cm². Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 50 cm² as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Clamp force rises in proportion to the total projected area.
- Get it right
- Use the projected area, not the total surface area of the part; holes through the part reduce it.
What this means: Cavities
- What it is
- How many identical parts the mould makes in each shot. It multiplies the projected area, the shot volume and the output per hour.
- Units and format
- Enter a plain number with no unit. The form accepts 1 to 256. Use a whole number. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 4 as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- More cavities raise clamp force, shot size and parts per hour together.
- Get it right
- Count only cavities that will run; a blanked-off cavity adds neither area nor output.
What this means: Runner projected area
- What it is
- The projected area of the sprue, runners and gates on the parting plane, which also feels the melt pressure and adds to the clamp force needed.
- Units and format
- Enter a number in cm²: square centimetres. The form accepts 0 cm² to 100000 cm². Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 20 cm² as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Adds to the total projected area, so clamp force rises with it.
- Get it right
- A hot runner kept inside the mould plates adds little or no area at the parting line; check the mould design.
What this means: Cavity pressure
- What it is
- The average melt pressure acting inside the cavity while it is packed. It depends on the material’s flow, wall thickness and flow length, and differs from the injection pressure at the machine.
- Units and format
- Enter a number in MPa: megapascals (1 MPa = 1 N/mm²). The form accepts 0.000001 MPa to 300 MPa. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 40 MPa as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Clamp force is directly proportional to cavity pressure.
- Get it right
- Use average cavity pressure, not the hydraulic or injection pressure on the machine display; a mould flow study gives the best figure.
What this means: Clamp margin
- What it is
- An extra allowance added on top of the calculated clamp force to cover pressure peaks and uncertainty in the estimate.
- Units and format
- Enter a number in %: percent: the number is read as a percentage, so type 8 for 8 %. The form accepts 0% to 100%. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 10% as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Clamp force is multiplied by (1 + margin ÷ 100).
- Get it right
- Type the percentage itself, not a fraction; your moulding team’s practice sets the figure.
What this means: Part volume
- What it is
- The volume of plastic in one finished part, from the CAD model. Times the cavities, plus the runner, it makes the shot volume.
- Units and format
- Enter a number in cm³: cubic centimetres. The form accepts 0.000001 cm³ to 1000000 cm³. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 30 cm³ as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Shot volume, shot mass and barrel use rise with it.
- Get it right
- Use the part volume, not its bounding box; CAD gives it directly from the solid model.
What this means: Runner volume
- What it is
- The volume of the cold sprue and runner system that is moulded with each shot and then removed or reground.
- Units and format
- Enter a number in cm³: cubic centimetres. The form accepts 0 cm³ to 1000000 cm³. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 10 cm³ as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Adds directly to shot volume and barrel use.
- Get it right
- Use 0 for a hot runner, where the melt in the runner stays molten and is not part of each shot.
What this means: Melt density
- What it is
- The density of the plastic in its molten state, used to turn the shot volume into a shot mass.
- Units and format
- Enter a number in g/cm³: grams per cubic centimetre (1 g/cm³ = 1000 kg/m³). The form accepts 0.000001 g/cm³ to 3 g/cm³. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 0.95 g/cm³ as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Changes only the shot mass shown; clamp, barrel use and cycle are unaffected.
- Get it right
- Melt density is lower than the solid density usually printed first on data sheets; take the melt value from the material maker.
Cooling
What this means: Wall thickness
- What it is
- The thickest wall of the part, which cools last and so sets how long the part must stay in the mould before ejection.
- Units and format
- Enter a number in mm: millimetres. The form accepts 0.000001 mm to 50 mm. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 2.5 mm as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Cooling time rises with the square of wall thickness.
- Get it right
- Use the thickest section, such as a boss or rib junction, not the nominal wall.
What this means: Thermal diffusivity of the plastic
- What it is
- How quickly heat spreads through the plastic: its conductivity divided by density times specific heat. A higher value lets the wall cool faster.
- Units and format
- Enter a number in mm²/s: mm²/s. The form accepts 0.000001 mm²/s to 1 mm²/s. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 0.09 mm²/s as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Cooling time is inversely proportional to diffusivity.
- Get it right
- Check the unit is mm²/s; data in m²/s must be multiplied by one million.
What this means: Melt temperature
- What it is
- The temperature of the molten plastic as it enters the cavity, set by the processing conditions for the material.
- Units and format
- Enter a number in °C: degrees Celsius. The form accepts 0.000001 °C to 450 °C. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 230 °C as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- A hotter melt has more heat to lose, so cooling time rises.
- Get it right
- It must be above the ejection temperature. Use the actual melt temperature, not a barrel zone setting.
What this means: Mould temperature
- What it is
- The temperature of the mould cavity surface that the plastic touches, held by the mould temperature controller.
- Units and format
- Enter a number in °C: degrees Celsius. The form accepts 0.000001 °C to 200 °C. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 40 °C as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- A warmer mould slows cooling, so cooling time rises.
- Get it right
- It must be below the ejection temperature. Use the cavity surface temperature, not the coolant setpoint, which is usually lower.
What this means: Ejection temperature
- What it is
- The temperature the centre of the wall must fall to before the part is stiff enough to eject without distorting.
- Units and format
- Enter a number in °C: degrees Celsius. The form accepts 0.000001 °C to 300 °C. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 90 °C as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- A higher ejection temperature shortens the cooling time.
- Get it right
- It must sit between the mould and melt temperatures; the material maker’s processing data gives it.
What this means: Other cycle time (fill, hold, open, eject, clos…
- What it is
- All the parts of the cycle other than cooling: injection, hold, mould opening, ejection and closing.
- Units and format
- Enter a number in s: seconds. The form accepts 0 s to 600 s. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 8 s as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Adds directly to the cycle time, lowering parts per hour.
- Get it right
- Do not repeat the cooling time here; the tool calculates it separately and adds it.
Machine
What this means: Machine clamp force
- What it is
- The rated clamping force of the moulding machine being considered, from its specification plate or data sheet.
- Units and format
- Enter a number in kN: kilonewtons (1 kN = 1000 N). The form accepts 0.000001 kN to 100000 kN. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 1500 kN as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- It does not change the clamp force needed, only whether the machine passes.
- Get it right
- Check the rating unit: machines are often rated in tonnes force, which must be converted to kN.
What this means: Machine shot capacity
- What it is
- The largest volume of melt the machine’s screw and barrel can inject in one shot, from the machine specification.
- Units and format
- Enter a number in cm³: cubic centimetres. The form accepts 0.000001 cm³ to 1000000 cm³. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 300 cm³ as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Barrel use is the shot volume divided by this capacity.
- Get it right
- Use the swept volume in cm³, not the shot weight in grams of polystyrene that some data sheets quote.
Method and formulas
- Clamp force = (cavities × A_part + A_runner) × cavity pressure × (1 + margin).
- Shot = cavities × V_part + V_runner; barrel use = shot ÷ capacity (20–80% preferred).
- Cooling time t = s² ÷ (π² a) × ln[(4 ÷ π)(T_m − T_w) ÷ (T_e − T_w)] (plate cooling to ejection at the mid-plane).
- Cycle = cooling + other times; output = cavities × 3600 ÷ cycle.
- Example: 4 cavities need 968 kN of clamp and cool in 11.093 s, giving 754.2 parts per hour.
Limitations
- Estimates only; mould flow simulation gives cavity pressure and fill time properly.
- Cooling time assumes a flat wall cooled from both sides.
- Example values are illustrations. Replace them with values for your actual task.
Typical uses
- Choose a moulding machine for a new mould
- Estimate cycle time and output for a quote
- See how wall thickness drives cycle time
Who it is for
- Moulding engineers
- Estimators
- Product engineers
What you get
- Clamp force against the machine
- Shot volume and barrel use
- Cooling time, cycle and output
Interpreting the result
- Cooling time rises with the square of wall thickness: thinner, even walls pay back on every shot.
References
- Machinery’s Handbook, 29th edition (Industrial Press, 2012), printed pp. 584–592. Topic reference; no source tables or recommended settings are reproduced.
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Open the Injection Moulding Clamp, Shot, Cooling and Cycle Estimate calculator