How to calculate cavity pressure in injection molding?

Cavity pressure is one of the most important process variables in injection molding. Here's a comprehensive breakdown:

The core formula

The simplified relation is:

P_cavity ≈ P_injection × (1 / intensification ratio) × flow losses

But in practice, cavity pressure is calculated through a pressure drop analysis across the flow path:

P_cavity = P_inject − ΔP_sprue − ΔP_runner − ΔP_gate − ΔP_flow

Pressure drop at each stage

For each flow segment, pressure drop is calculated using the Hagen-Poiseuille equation adapted for polymer flow:

Circular channel (sprue/round runner):

ΔP=πR48ηLQ

Rectangular channel (tab/edge gate):

ΔP=WH312ηLQ

Where:

• η = apparent viscosity (Pa·s) — shear-rate dependent

• L = channel length (m)

• Q = volumetric flow rate (m³/s)

• R = radius (m)

• W = width, H = depth (m)

Viscosity & shear rate

Polymer viscosity isn't constant — it drops with shear rate (shear thinning). Use the Power Law model:

η = K × γ̇^(n−1)

• K = consistency index (material-specific)

• γ̇ = shear rate (s⁻¹)

• n = power law index (0.2–0.6 for most polymers; Delrin ≈ 0.4)

Shear rate in a circular channel: γ̇ = 4Q / πR³

Rule-of-thumb estimates

For quick hand calculations:

A commonly used approximation:

P_cavity ≈ 0.3 × P_injection

(i.e. roughly 30% of machine injection pressure reaches the end of the cavity — though this varies widely with part geometry and material.)

Clamp force from cavity pressure

Once you have cavity pressure, clamp force follows directly:

F_clamp = P_cavity × A_projected

• A_projected = projected area of the part + runners on the parting line (m²)

• Add 10–20% safety margin

• Result in Newtons; divide by 1,000 for kN, by 1,000,000 for MN

Example — Delrin gear, 80cm² projected area:

• P_cavity = 45 MPa = 45 N/mm²

• A = 80 cm² = 8,000 mm²

• F = 45 × 8,000 = 360,000 N = 360 kN ≈ 36 tonnes clamp

In practice

Most engineers don't hand-calculate — they use:

• Moldflow / Cadmould / Sigmasoft — full FEA simulation of cavity pressure distribution

• In-mold pressure sensors — piezoelectric sensors mounted flush in the cavity wall give real-time data

• Process monitoring — cavity pressure profiles (fill, pack, hold) are used to set consistent process windows and detect short shots, flash, and sink marks

The peak cavity pressure and the area under the pressure-time curve are both used as quality indicators — consistent curves mean consistent parts.