Injection Molding Cycle Time Calculator

What this calculator computes

This tool estimates how long one injection-molding cycle takes, and where the time goes. From the part’s wall thickness and the plastic’s thermal properties it computes the cooling time — the dominant part of the cycle — using the Ballman–Shusman equation, then adds the injection, packing, and mold-reset times you supply to give the total cycle time. It also reports what share of the cycle is cooling and, from the cavity count, how many parts the mold makes per hour. It carries presets for common polymers so you can start from representative properties and refine them, and it plots how the cooling time changes with wall thickness.

The reason cycle time matters is that it is the single biggest driver of the cost and output of a molded part. The machine hourly rate is largely fixed, so the number of parts per hour — set by the cycle time and the number of cavities — determines the cost per part and the capacity of the machine. And within the cycle, cooling usually accounts for 60 to 80 percent of the time, so the cooling estimate is the estimate that matters most. Understanding what drives it, above all the wall thickness, is how a designer or process engineer shortens the cycle and lowers the cost.

What sets this calculator apart is that it combines the real cooling physics with the full cycle and the productivity output, rather than doing only one. It uses the established Ballman–Shusman cooling equation, not a guessed constant; it adds the other cycle stages for a complete cycle time; it reports the cooling share and the parts per hour with cavities; it includes material presets so it is usable without a data sheet; and it plots the wall-thickness effect. Everything runs in your browser with nothing stored.

How to use this calculator, step by step

Start by choosing the material from the preset list, which fills in a representative thermal diffusivity and the melt, mold, and ejection temperatures for that polymer; choose Custom to enter your own. Then enter the part’s wall thickness — use the nominal wall, or the thickest section if you want a conservative estimate, since the thickest part governs when the whole part can be ejected. The temperatures can be adjusted to match your process: the melt temperature from your barrel setting, the mold temperature from your coolant, and the ejection temperature from the material’s heat-deflection temperature.

Next enter the non-cooling cycle stages: the injection (fill) time, the packing and holding time, and the mold reset time to open, eject, and close. These come from your machine and mold, or you can start from the default couple of seconds each. Set the number of cavities in the mold. The calculator opens on a worked example — a 2.5 mm wall in ABS at 240 °C melt, 60 °C mold, 90 °C ejection, with 2, 4, and 3 seconds for the stages — giving a cooling time of about 12 seconds and a cycle of about 21 seconds, and every field recomputes live as you type.

Read the result panel, which leads with the total cycle time, then lists the cooling time, the full cycle time, the cooling share as a percentage, and the parts per hour for your cavity count. The chart plots cooling time against wall thickness so you can see the strong, squared effect of thickness and judge how much thinning the wall would save. You can download or share the result, all locally.

The cooling time equation

Cooling dominates the cycle because it is the slow step: injection and mold movement take a second or two each, but the plastic must lose enough heat to become rigid before it can be ejected, and heat leaves a thick plastic part slowly. The Ballman–Shusman equation models this as one-dimensional heat conduction out of a flat plate of thickness s, cooling from the melt temperature toward the mold-wall temperature, and needing to reach an ejection temperature at its centre. The result is cooling time = (s² / (π²·α)) × ln[(4/π)·(T_melt − T_mold)/(T_eject − T_mold)], a compact expression whose every term has a clear physical meaning.

The first factor, s² divided by π² times the thermal diffusivity, sets the timescale of the conduction.

The thickness is squared because heat has to diffuse from the centre to the surface, and diffusion time grows with the square of the distance; the thermal diffusivity α, in the denominator, is how fast the material conducts heat away, so a higher diffusivity shortens the time.

The logarithmic factor captures the temperatures: it is large when the melt is much hotter than the mold and the part must cool to only a little above the mold temperature, and small when the ejection temperature is close to the melt. Together they give the time for the centre of the plate to reach the ejection temperature.

The squared thickness is the headline. Because s appears as s², the cooling time is extremely sensitive to wall thickness: reducing a wall from 3 mm to 2 mm cuts the cooling time by a factor of about (3/2)², or roughly 2.25, more than halving it. This is why thin, uniform walls are the first rule of design for injection molding, and why a thick boss or rib can dominate the cooling time of an otherwise thin part. The calculator makes this concrete by plotting the cooling time across a range of wall thicknesses, so the curve’s steepness is visible at a glance.

From cooling time to full cycle and output

The cooling time is the largest piece, but the full cycle includes the other stages, and adding them gives the number that actually sets the output. The injection or fill time is how long the screw takes to push the melt into the mold, usually a second or two for a typical part and longer for a large or thin-walled one.

The packing and holding time keeps pressure on the melt after filling, while more material is forced in to compensate for shrinkage and until the gate freezes; it is also a few seconds. The mold reset time covers opening the mold, ejecting the part, and closing again, and depends on the machine and the mold size.

The full cycle is the sum of all four: cooling plus injection plus hold plus reset.

Because cooling is usually 60 to 80 percent of that total, the calculator reports the cooling share so you can see whether your process is cooling-dominated, as most are, or whether the other stages are unusually large. It then converts the cycle time to parts per hour, dividing 3600 seconds by the cycle time and multiplying by the number of cavities.

The cavity count is the direct multiplier of output: a four-cavity mold makes four times as many parts per hour as a single-cavity mold at the same cycle time, which is why multi-cavity tooling is the standard way to raise volume without shortening the cycle.

Reading the parts-per-hour output against your production target tells you whether the cycle and the tooling are adequate.

Five worked examples you can follow

Example 1: the default ABS part

A 2.5 mm wall in ABS, melt 240 °C, mold 60 °C, ejection 90 °C. The cooling time is (2.5²/(π²·0.11))·ln[(4/π)·(180/30)] ≈ 11.7 seconds. Adding 2 s injection, 4 s hold, and 3 s reset gives a cycle of about 20.7 seconds, and cooling is about 57 percent of it. A single-cavity mold makes about 174 parts per hour.

Example 2: thinning the wall

Reduce the wall to 1.5 mm. The cooling time falls with the square, to about (1.5/2.5)² of 11.7, roughly 4.2 seconds, and the cycle drops to about 13 seconds — lifting output to about 275 parts per hour. Thinning the wall is the most powerful lever on cycle time.

Example 3: a thick section

Raise the wall to 4 mm, as a thick boss might. The cooling time climbs to about 30 seconds and the cycle to about 39 seconds, cutting output to about 92 parts per hour. A single thick feature can dominate the cycle of an otherwise thin part.

Example 4: a hotter, slower-cooling material

Switch to polycarbonate: melt 300 °C, mold 90 °C, ejection 130 °C, diffusivity 0.12. At the 2.5 mm wall the cooling time is a similar order, but the higher temperatures and material behaviour shift it; the preset fills the values so you can compare materials at the same geometry.

Example 5: multi-cavity output

Return to the default ABS part and set the mold to eight cavities. The cycle time is unchanged at about 20.7 seconds, but the output multiplies to about 1,390 parts per hour. Cavitation, not cycle reduction, is how high-volume parts hit their numbers.

Three expert tips for reliable results

The mathematics behind the results

The Ballman–Shusman equation is a solution of the one-dimensional heat-conduction equation for a plate.

When a plate of thickness s starts at a uniform melt temperature and its surfaces are held at the mold temperature, the temperature at its centre falls over time as a series of decaying exponentials; keeping the dominant term gives the centre temperature as a function of time, and inverting it for the time to reach the ejection temperature yields the formula.

The factor s²/(π²·α) is the characteristic diffusion time of the plate, and the logarithm of (4/π) times the ratio of temperature differences is the number of those characteristic times needed to cool from the melt to the ejection temperature. It is an approximation, but a good one for the plate-like sections that dominate most parts.

The remaining calculations are straightforward. The total cycle time is the sum of the cooling time and the injection, hold, and reset times you enter — there is no hidden model, just addition, because those stages are set by the machine and mold rather than by heat conduction. The cooling share is the cooling time divided by the cycle time, as a percentage. The parts per hour is 3600 divided by the cycle time in seconds, times the number of cavities, converting a per-cycle rate into an hourly rate and accounting for the parallel production of a multi-cavity mold.

Two assumptions frame the results and are worth keeping in mind.

The cooling model assumes a uniform wall of the entered thickness, constant material properties, and a mold wall held at a fixed temperature; real parts vary in thickness, have corners and ribs that cool in more than one dimension, and see material properties that change with temperature, so the true cooling time can differ, usually being longer where thick or three-dimensional features hold heat.

And the non-cooling times are yours to supply accurately. The equation captures the dominant physics and the all-important wall-thickness effect correctly, which makes it excellent for design and comparison; for a production-critical figure, confirm it with a mold-flow simulation or a trial shot.

Where cycle time calculations are used

Cycle time is central to molding economics and design, and this tool completes the manufacturing-processes silo by covering the molding side alongside machining and forming.

In part design, the cooling estimate turns the wall-thickness decision into a cycle-time and cost decision, showing a designer the productivity price of a thick wall or the payoff of thinning it; this is the earliest and cheapest place to shorten a cycle.

In mold design, the cooling time is the target the cooling-channel layout must achieve, and the ejection temperature it implies sets how much heat the coolant must remove. In process engineering, the cycle time and parts-per-hour drive machine selection, scheduling, and the quoted price of a part.

The calculation connects to the wider set of manufacturing tools. Like the machining and forming calculators in this silo, it relates a material property and a geometry to a time and a cost: where the machining time calculator turns cutting parameters into a cycle, this turns thermal properties and wall thickness into one.

It feeds costing directly, since cycle time times the machine rate, divided by cavities, is the machining-equivalent cost of the molding operation, to which material and tooling amortisation are added. And it informs capacity and make-versus-buy decisions, because the parts-per-hour it produces, multiplied across a run, gives the machine time a job requires.

Return to the Manufacturing Processes hub for the companion tools across machining, forming, and molding.

Beyond the individual part, cooling and cycle estimates support continuous improvement and tooling investment. A shop chasing shorter cycles looks first at cooling — better mold temperature control, conformal cooling channels, or a design change to thin a wall — because that is where most of the time is.

A decision to add cavities or buy a faster machine is justified by the parts-per-hour the change delivers. And the same wall-thickness discipline that shortens the cycle also improves part quality by reducing sink marks and internal stress, so the cooling estimate ties productivity and quality together.

Accurate cycle estimates are what let a molder quote, schedule, and improve with confidence.

What shortens the cooling time

Because cooling dominates the cycle, the levers that shorten it are the levers that shorten the cycle, and the equation shows exactly what they are. The strongest by far is wall thickness, through the square law: any reduction in the governing wall thickness pays back twice over in cooling time, which is why thinning and, crucially, keeping walls uniform — so no single thick section holds up the whole part — is the first move. Coring out thick sections, replacing a solid boss with a ribbed one, and avoiding thick-to-thin transitions all attack the same driver.

After geometry, the mold temperature is the next lever. A lower mold-wall temperature increases the temperature difference driving heat out and shortens the cooling time, which is why chilled mold cooling shortens cycles — within limits, since too cold a mold hurts surface finish and can freeze the gate early.

The material’s thermal diffusivity matters too, but it is usually fixed by the part’s functional requirements rather than chosen for cycle time; where a choice exists, a higher-diffusivity grade cools faster. Finally, the ejection temperature can sometimes be raised if the part is stiff enough to eject warmer, shortening the cooling, though at the risk of distortion.

The calculator lets you test each of these by changing the input and watching the cooling time and the cycle respond.

The injection and holding stages in more detail

Although cooling dominates, the injection and holding stages deserve a closer look because they set the two non-cooling times you enter and because they overlap with cooling in reality. The injection, or fill, stage is when the screw drives the melt through the sprue, runners, and gates into the cavity; its time depends on the shot volume and the injection speed, and it is short for a small part but can be significant for a large or very thin-walled one where the melt must be pushed fast before it freezes. Filling too slowly risks the melt solidifying before the cavity is full, a short shot; filling too fast can cause defects, so the fill time is a balance rather than simply the minimum.

The packing and holding stage follows the fill: the machine holds pressure on the melt so that additional material is forced in to compensate for the shrinkage that begins as the plastic cools, and it continues until the gate freezes and no more material can enter.

This stage overlaps the start of cooling — the part is already losing heat while it is being packed — which is one reason the simple sum of stage times is an approximation rather than an exact accounting.

For the estimate, the holding time is entered as a distinct few seconds; in practice a process engineer sets it from a gate-freeze study, increasing it until the part weight stops rising, which signals the gate has frozen and further holding adds only cycle time without adding material.

Amorphous and semi-crystalline plastics cool differently

Plastics fall into two families that behave differently on cooling, and knowing which you have sharpens the estimate. Amorphous plastics — ABS, polycarbonate, polystyrene, acrylic — have no ordered crystal structure; they simply stiffen as they cool through their glass-transition region, so the ejection temperature is taken near that region and the cooling follows the conduction model cleanly. Their cooling time is reasonably well predicted by the equation with a representative diffusivity, and they tend to shrink less and more predictably, which is why they are common for dimensionally precise parts.

Semi-crystalline plastics — polypropylene, polyethylene, nylon, acetal — form crystals as they cool, and the crystallisation releases latent heat that the simple conduction model does not account for, so their real cooling can run longer than a first estimate and their shrinkage is larger and more sensitive to cooling rate.

In practice this is handled by using an effective diffusivity and an ejection temperature drawn from the material’s data or from experience with the grade, which is what the presets approximate.

The calculator’s equation still captures the dominant wall-thickness effect for both families; the family difference mostly shifts the effective properties you enter, and is a reason to lean on data-sheet values for semi-crystalline grades and to treat the estimate as conservative rather than exact.

Mold cooling design and the mold temperature

The equation treats the mold wall as held at a fixed temperature, and achieving that in a real mold is the job of the cooling system, which is where much of the cycle is won or lost. Cooling channels drilled through the mold carry a coolant — water or oil — that removes the heat the plastic gives up; the closer and more uniform the channels are to the cavity surface, the more effectively they hold the mold wall near the set temperature and the faster and more evenly the part cools. A mold with sparse or poorly placed channels cannot hold the wall temperature the calculation assumes, so the real cooling time runs longer and the part may cool unevenly, warping.

This is why conformal cooling — channels that follow the contour of the cavity, made possible by additively manufactured mold inserts — has become a lever for shorter cycles: by keeping the whole surface at a uniform, lower temperature, it both shortens the cooling time in the equation, through a lower and steadier mold temperature, and improves part quality. The mold temperature you enter in the calculator should be the temperature the cooling system can actually hold at the cavity surface, not the coolant inlet temperature, which is lower. Setting a realistic mold temperature, and recognising when the cooling design cannot achieve it, keeps the cycle estimate honest.

Cycle time and the cost of a molded part

Cycle time matters because it converts almost directly into cost. The machine hourly rate — covering the press, the operator, energy, and overhead — is largely fixed, so the molding cost per part is the machine rate divided by the parts produced per hour, and parts per hour is set by the cycle time and the cavity count. A shorter cycle or more cavities lowers the per-part molding cost in proportion; to that molding cost are added the material cost per part and the amortised cost of the mold spread over the production volume. On a high-volume part the cycle time, through the parts-per-hour it sets, is often the largest controllable cost after material.

This connection is why the wall-thickness and cooling levers in this calculator are cost levers, not just engineering ones. Thinning a wall that shortens the cooling time by a third lowers the molding cost of every part made for the life of the tool; adding cavities raises output and lowers per-part cost at the expense of a more expensive mold, a trade-off the parts-per-hour output helps quantify. Used at the design stage, the calculator lets a team see the cost consequence of a geometry or material choice before the tool is cut, which is the cheapest time to change it, and lets an estimator turn a cycle time into a defensible quote.

Common mistakes to avoid

A few errors recur in cycle-time estimates. Watch for them.

• Using the average wall instead of the thickest. The part ejects only when its thickest section is rigid. Basing the cooling time on the average wall underestimates it wherever a thick feature exists.
• Treating the estimate as exact. The equation is a one-dimensional approximation. Corners, ribs, and varying thickness cool differently; confirm a production-critical cycle with simulation or a trial.
• Guessing the ejection temperature high. Too high an ejection temperature shortens the predicted cooling but risks warped parts. Use the heat-deflection temperature for the grade.
• Ignoring the non-cooling stages. Cooling is most of the cycle but not all of it. Leaving out injection, hold, and reset undercounts the cycle and overstates parts per hour.
• Forgetting cavities. Parts per hour scales with cavities. Estimating output from the cycle alone, for a multi-cavity mold, undercounts production several-fold.
• Confusing diffusivity with conductivity. The equation uses thermal diffusivity, not conductivity; diffusivity is conductivity divided by density times specific heat. Using conductivity directly is wrong by orders of magnitude.

Reference: typical properties for common polymers

These representative values seed the material presets. Thermal diffusivity and the process temperatures vary with grade, filler, and supplier, so use your data sheet for a firm estimate; the table is a starting point.

Read these alongside the wall thickness, which matters more than the material differences for cooling time. Two parts of the same wall in different polymers cool in a broadly similar time; the same polymer at twice the wall takes four times as long. The presets get you a credible cycle quickly; a data-sheet diffusivity and a grade-specific ejection temperature refine it for a real job.

Input format and quick reference

Choose a material preset or Custom, then enter the wall thickness, the thermal diffusivity and temperatures, and the injection, hold, and reset times with the cavity count. The reference below explains each output.

Frequently asked questions

Sources, disclaimer and editorial transparency

This calculator uses the Ballman–Shusman cooling-time equation t_cool = (s²/(π²α))·ln[(4/π)·(T_melt−T_mold)/(T_eject−T_mold)]; the cycle as cooling + injection + hold + reset; and parts per hour = 3600/cycle × cavities, consistent with standard injection-molding and plastics-processing references.

Typical thermal diffusivity and process temperatures are representative published values for general guidance. This calculator and guide are created and reviewed by the OpsCalculators team; see our Editorial Policy for how each tool is researched, built, and tested.

Results are accurate estimates for design, quoting, and education, not a substitute for a mold-flow simulation, verified material data, or a trial shot. The cooling model is a one-dimensional approximation and real parts vary in thickness and geometry. See our full Disclaimer. OpsCalculators.com is operated by MAFHH INTERNATIONAL LTD. Your data is processed in your browser and never stored; see our Privacy Policy.