FAQ - Static Mixer - PSI-Polymer Systems

Static Mixer – FAQ

FAQs for Static Mixers in Plastics Extrusion

1. What is a static mixer, and how does it work in plastics extrusion?

A static mixer is a device with no moving parts that is installed in the extrusion line, typically just before the die. It works by continuously dividing and recombining the molten polymer as it flows through a series of geometric elements. This process eliminates variations in temperature, viscosity, and composition within the melt stream, resulting in a homogeneous product.

2. What is the primary purpose of using a static mixer in sheet extrusion?

In sheet extrusion, the main goal of a static mixer is to achieve thermal homogenization of the polymer melt. This means eliminating temperature gradients, which in turn eliminates viscosity variations across the melt. A uniform temperature and viscosity profile ensures an even flow of material through the die, which is critical for producing a sheet with consistent thickness.

3. What are the main benefits of using a static mixer?

The benefits of using a static mixer in plastics extrusion include:

Improved product quality: Static mixers produce a more uniform melt, leading to a streak-free product with improved surface quality and mechanical characteristics.
Tighter tolerances: By eliminating temperature and viscosity gradients, static mixers allow for a more even sheet thickness, enabling operation at minimum thickness specifications.
Reduced material usage: Improved color mixing can lead to a reduction in color additive consumption.
Faster color changes: Static mixers help to eliminate stagnant wall layers, leading to quicker color changeovers.
Improved regrind quality: They enhance the quality of products when using regrind material.

4. Where is a static mixer typically installed in an extrusion line?

A static mixer is usually installed just before the extruder die. It is placed after other equipment like the screen changer and gear pump to correct any irregularities introduced by them.

5. Do static mixers require power or have moving parts?

No, static mixers do not have any moving parts and do not require an external power source. The energy required for mixing is derived from the pressure drop of the polymer melt as it flows through the mixer’s elements.

6. Can a static mixer help with issues other than thermal homogenization?

Yes. Besides thermal homogenization, static mixers also help with:

Compositional homogenization: They ensure even distribution of colorants, additives, fillers, and regrind.
Viscosity homogenization: By evening out temperature, they also even out viscosity.
Elimination of wall-creep: They incorporate stagnant polymer from the pipe walls into the main flow, preventing material degradation.

7. Will a static mixer cause a significant pressure drop?

While a static mixer does introduce a pressure drop, it is designed to be efficient and can typically accommodate the pressure drop constraints of most extrusion systems. The benefits of a homogeneous melt often outweigh the impact of the pressure drop. PSI utilizes a proprietary pressure drop calculator specifically designed for our mixers to estimate the expected pressure drop.

8. How are static mixers cleaned?

Many modern static mixers are designed to be self-cleaning, with optimized flow paths that prevent dead zones where material could accumulate. For color or material changes, purging compounds can be run through the system to effectively clean the mixer. Some designs also feature removable elements for easy manual cleaning.

9. What are the signs that a static mixer might be needed in an extrusion line?

Common issues that indicate a need for a static mixer include:

Uneven sheet thickness.
Color streaks or blotches in the final product.
Inconsistent foam cell size and distribution in foam sheet applications.
Long color changeover times.
Flow lines or other surface defects.

10. Are there different types of static mixers?

Yes, there are several designs of static mixers available, each with its own advantages. Common designs include those with helical elements (supplied by PSI) and those with a series of intersecting bars.

PSI supplies helical element (bow-tie) mixers due to:

Low Pressure Drop: Helical mixers are known for their relatively low pressure drop compared to other designs, which can be a significant advantage in extrusion processes where pressure is a limiting factor.
Gentle Mixing: The smooth, continuous surfaces of the helical elements provide a gentle mixing action, which is ideal for shear-sensitive polymers that could be degraded by more aggressive mixing.
Good for Laminar Flow: This design is very effective for mixing fluids in the laminar flow regime, which is typical for high-viscosity polymer melts.