Find out about.......Plastics, Polymer Engineering and Leadership: Polymer Injection Molding – The Effect of Pressure on Viscosity [infographic]

Hello and welcome to a new blog post.

How much does pressure really affect polymer viscosity in injection molding?

When we talk about melt viscosity, most of us immediately think about temperature and shear rate. But pressure also matters — and for some polymers, it matters a lot more than many people expect.

The infographic below highlights a key point: the viscosity of thermoplastics depends not only on temperature, shear rate, and pressure, but also on chemical structure and physical conditions.

A good example is the difference between amorphous and semi-crystalline polymers:

Polystyrene (PS): at 200 bar, viscosity can increase by about 22%
Polyethylene (PE): at the same pressure, viscosity increases by only about 3–4%

Calculation in detail

The pressure dependence of polymer viscosity, often analyzed using methods like those proposed by Dudvani and Klein [2,3], is significant for polystyrene (PS) at high pressures.

Exponential Model: The effect of pressure P on the viscosity Eta of PS is generally described using the exponential formula: Eta = Eta_0 exp (P + Alpha).

Pressure Coefficient (Alpha): For atactic and syndiotactic polystyrene, studies show the average pressure coefficient Alpha is in the range of 1–3 x10^(-8) Pa^-1.

At 200 bar (200 x 10^5 Pa) with Alpha of 10^-8 Pa, viscosity increases to 1.2214 (22%).

Mechanism: Increased pressure decreases the free volume available for polymer chain movement, increasing the intermolecular friction and thus the viscosity.

Conclusions

That is an important reminder for injection molding, where pressure effects can strongly influence filling behavior and process stability. In extrusion, by contrast, the effect of pressure on viscosity is often much less relevant.