Injection Molding Fill Pressure & Cooling Time Calculator
Enter runner geometry, melt viscosity, and part thickness to instantly compute fill pressure, cooling time, clamping force, and cycle time. Watch the animated polymer flow fill the mold cavity in real time.
Material Presets
Runner & Part Geometry
Runner Length L
mm
Runner Radius R
mm
Flow Rate Q
cm³/s
Melt Viscosity η
Pa·s
Wall Thickness t
mm
Melt Temp T_melt
°C
Mold Temp T_mold
°C
Ejection Temp T_ej
°C
Results
Results
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Fill Pressure [MPa]
—
Cooling Time [s]
—
Clamping Force [kN]
—
Cycle Time [s]
Polymer Flow Animation
Mold
Theory & Key Formulas
Fill pressure (Hagen-Poiseuille):
$\Delta P = \dfrac{8\eta Q L}{\pi R^4}$
Cooling time (slab solution):
$t_c = \dfrac{t^2}{\pi^2\alpha}\ln\!\left(\dfrac{4}{\pi}\cdot\dfrac{T_m - T_{mold}}{T_{ej}- T_{mold}}\right)$
What is Injection Molding Pressure & Cooling?
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What exactly is "fill pressure" in injection molding? Is it just the pressure at the machine nozzle?
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Basically, it's the pressure needed to push the molten plastic from the machine nozzle, through the runner system, and into the empty mold cavity. It's not constant—it's highest at the gate. In this simulator, the main equation calculates the pressure drop across the runner. Try increasing the "Runner Length (L)" or "Melt Viscosity (η)" sliders above; you'll see the required fill pressure shoot up, which makes sense because you're pushing thick fluid through a longer pipe.
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Wait, really? So the runner size has a huge effect? And what happens after the mold is full? Why do we need to calculate cooling time separately?
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Great questions! The runner radius is incredibly powerful—it's to the fourth power ($R^4$) in the equation. Halving the radius increases the pressure drop by 16 times! After filling, the part must cool and solidify enough to be ejected without deforming. That's where the cooling time equation comes in. It depends heavily on part thickness. For instance, a thin phone case cools in seconds, while a thick automotive panel can take a minute. Adjust the "Wall Thickness (t)" slider and watch the cooling time change dramatically.
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So the total cycle time is just fill time plus cooling time? And how do these numbers connect to choosing an actual injection molding machine?
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In practice, yes, cycle time is roughly fill + cooling + a bit for opening/closing. The fill pressure is critical for machine selection. The machine must provide that pressure, and its "clamping force" must be high enough to keep the mold closed against that internal pressure. If the fill pressure from our simulator is 80 MPa, the cavity pressure might be 50 MPa. Multiply that by the part's projected area to get the clamping force needed. A common case: a large, thin part might need high fill pressure (thin runners) but low clamping force (low area), while a thick part needs the opposite.
Physical Model & Key Equations
The pressure required to fill the runner system is modeled using the Hagen-Poiseuille equation for laminar flow of a Newtonian fluid in a pipe. This is a foundational concept in fluid dynamics applied to polymer processing.
$$\Delta P = \dfrac{8\eta Q L}{\pi R^4}$$
Where:
$\Delta P$ = Pressure drop across the runner (Pa)
$\eta$ = Melt viscosity (Pa·s) – try changing this based on material!
$Q$ = Volumetric flow rate (m³/s)
$L$ = Runner length (m)
$R$ = Runner radius (m) – Note the powerful R⁴ dependence!
The cooling time is estimated using the 1D heat conduction solution for a slab (the part wall). It calculates the time for the centerline to cool down to a safe ejection temperature.
Where:
$t_c$ = Cooling time (s)
$t$ = Part wall thickness (m) – the most critical factor.
$\alpha$ = Thermal diffusivity of the plastic (m²/s)
$T_m$ = Melt temperature at injection (°C)
$T_{mold}$ = Mold temperature (°C)
$T_{ej}$ = Ejection temperature (°C)
The logarithmic term represents the required temperature drop ratio.
Frequently Asked Questions
This is because, according to the Hagen-Poiseuille equation, pressure loss is inversely proportional to the fourth power of the radius. If the radius is halved, the pressure increases by a factor of 16, so even minor design adjustments have a large impact. Try changing the values in the simulator to verify.
The calculation assumes ideal heat conduction, but in reality, factors such as uneven mold temperature, cooling circuit layout, and the set ejection temperature have an effect. Additionally, if the wall thickness is not uniform, the cooling time of the thickest section becomes dominant, so please input that value.
They visualize the pressure distribution during the resin filling process. Red indicates high pressure, blue indicates low pressure, and you can observe in real time how the pressure decreases from the runner inlet to the end of the molded part.
Please use it as a reference. The calculated value is a theoretical value derived from the filling pressure and projected area, but in practice it varies depending on the resin flowability and the venting condition of the mold. We recommend allowing a margin of 20–30% when selecting a machine.
Real-World Applications
Machine Selection & Tonnage: The calculated fill pressure directly determines the required injection pressure capability of the machine. More importantly, it is used to estimate the clamping force (in tons) needed to prevent the mold from flashing. Engineers use this to choose the correct size press, balancing cost and capability.
Runner System Design: This calculation is vital for designing balanced runner systems in multi-cavity molds. Engineers adjust runner lengths and diameters (the R⁴ factor!) to ensure all cavities fill simultaneously and at the same pressure, guaranteeing consistent part quality across all cavities.
Cycle Time Optimization: In high-volume production (e.g., bottle caps, automotive connectors), saving even one second of cooling time per cycle translates to thousands of extra parts per year. Engineers use this model to find the minimum safe wall thickness and optimal cooling temperature to maximize output without causing warpage or ejection failures.
Material Selection & Processing Windows: By inputting different melt viscosities (η) and thermal diffusivities (α), engineers can compare how different plastics (like Polypropylene vs. ABS) will behave in the same mold. This helps predict if a material switch will require a higher-pressure machine or lead to longer cycle times before any material is purchased.
Common Misconceptions and Points to Note
There are a few key points you should be aware of when starting to use this tool. First, the calculated cooling time is not an absolute value. The formula assumes ideal plate cooling, but actual molded parts have ribs, bosses, and curved surfaces where heat escapes less easily. For example, the ribs for buttons on the side of a smartphone case will cool slower than the main body, even if the nominal wall thickness is 1mm. Therefore, treat the calculated value as a "baseline" and consider applying a safety factor of 1.2 to 1.5 times based on practical experience—that's the wisdom from the shop floor.
Next, don't assume viscosity is a fixed value. The tool uses representative viscosities for each resin type, but the actual viscosity of molten resin changes dramatically with shear rate (flow speed) and temperature. For instance, during high-speed filling, friction heats the resin, lowering its viscosity and allowing flow at lower pressure than calculated—a phenomenon called "shear heating." Conversely, if the mold temperature is too low, the resin can start solidifying quickly, causing a sharp viscosity increase and requiring much higher pressure than calculated.
Finally, the idea that "increasing the runner radius lowers pressure, so it's safe" is dangerous. While pressure does decrease, a thicker runner takes longer to cool and wastes more material. If the runner itself doesn't cool sufficiently, its interior might still be molten after the part is ejected, causing "overpack" where it mixes into the next shot. Always be mindful of the trade-off between pressure reduction and material loss/cooling time.