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Injection pressure
Pressure drives the melt
Pressure is the driving force that overcomes the resistance of polymer melt (see
Pressure-driven flow), pushing the polymer to fill and pack the mold cavity. If you place a
number of pressure sensors along the flow path of the polymer melt, the pressure
distribution in the polymer melt can be obtained, as schematically illustrated in Figure 1
below.

FIGURE 1. Pressure decreases along the delivery system and the cavity.

Equations
Based on a simplification of classic fluid mechanics theory, the injection pressure required to fill the
delivery system (the sprue, runner, and gate) and cavities can be correlated with several relevant
material, design, and processing parameters. In the following equations, P is the injection pressure
and n is a material constant (the power-law coefficient), which typically ranges from 0.15 to 0.36
(with 0.3 being a good approximation) for a variety of polymer melts. Figure 2 shows injection
pressure as a function of several of these parameters.
Circular channel flow
The melt flow in the sprue, runner, and cylindrical gates

Strip channel flow
Such as melt flow in a thin cavity

FIGURE 2. Injection pressure as a function of melt viscosity, flow length, volumetric flow rate, and part
thickness

Factors that influence injection pressure requirements
The following diagrams illustrate the design and processing factors that influence injection pressure