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%
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.
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| Figure 1: The Effect of Pressure on Thermoplastic Viscosity. |
Thanks for reading & #findoutaboutplastics
Greetings,
Herwig
Literature:
[1] Rao Natti - Design Formulas for Plastics Engineers
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