Troubleshooting Melt Filtration

1. Start with the Basics: Inspect the Material

Effective troubleshooting always starts with the raw material itself.

• Is the material lumpy?
  Lumpy pellets create uneven dosing, which leads to inconsistent polymer flow from the extruder.

• Is the material damp?
  Moisture reduces viscosity and can contribute to both lumping and process instability.

Material inconsistencies introduce problems long before the melt reaches the filtration system. Ensuring clean, dry, stable feedstock is the first step toward reliable filtration performance.

2. Why Piston Type Filtration Helps Prevent Problems

Piston-type filtration systems are widely used for good reason:

• They provide large filtration area for lower shear rates and reduced differential pressure.
• The piston cavity protects the system from damage caused by contaminants like metal particles.
• No dead zones mean melt doesn’t stagnate or degrade.
• Continuous flow stabilizes pressure and improves product quality.

However, because piston systems move during operation, every movement has the potential to influence pressure. Proper adjustment is essential.

3. When Pressure Fluctuations Occur: Key Areas to Check

Even a well adjusted system can occasionally show pressure fluctuations. Here’s where to look first.

Check for Material Leaks

A sealed system is crucial. Leaks can occur when:

• Adapter bolts aren’t tightened enough after heating.
• Screws are overtightened, causing misalignment and gaps.

A quick visual inspection can reveal polymer leakage — and with it, the likely source of pressure swings.

Verify Temperatures and Sensors

Temperature and viscosity are tightly linked. Too high or too low process temperatures can destabilize pressure. Check:

• Is the polymer overheated?
• Are temperature sensors working and calibrated?
• Is external airflow cooling the machine unintentionally?

Inaccurate or poorly placed pressure sensors can also falsely indicate pressure peaks.

Observe When the Issue Occurs

Understanding the timing of fluctuations is key:

• Does it happen during a screen change?
• Only when the piston moves?
• During backflushing?
• When the extruder or gear pump reaches maximum speed?
  
  

Operators’ observations and system pressure trends provide valuable clues. A change in material or process parameters might require updated filtration settings.

4. Screen Change: Slow and Steady Wins

During screen changes, the piston closes one cavity and the melt flows through the remaining area — naturally increasing differential pressure. After the new screen is inserted, the piston returns and the cavity is refilled with molten polymer.

If this happens too quickly, pressure drops can occur.

Solution: Reduce piston speed to allow smoother refilling and stabilize melt flow.

5. Backflushing: Adjust for Smooth, Efficient Cleaning

Backflushing reverses the melt flow to clean contaminants from the screen. While this increases screen life, it always affects pressure to some degree.

To keep fluctuations under control:

• Reduce material loss by refining piston parameters.
• Shorten intervals if screens clog too heavily.
• Use gentle, precise backflushing - especially for low-viscosity polymers.
• Consider impulse backflush options that divide one long backflush into several shorter bursts.

Advanced systems like the BKG® HiCon™ V-Type 3G use displacement pistons to maintain more constant pressure during backflush cycles.

6. Venting: Avoid Air Entrapment and Turbulence

Proper venting prevents air bubbles from entering the melt, which can cause holes, snaps, and defects.

Tips for effective venting:

• Clean all venting grooves after each screen change.
• Adjust piston speed so the cavity fills smoothly without turbulence.
• Pay close attention to the first venting position - getting this right significantly improves stability.
• Aim for a slow, controlled filling that keeps a stable melt/air boundary.

Low viscosity polymers especially benefit from careful venting and slower cavity filling.

7. Screen Packs: Small Component, Big Influence

The quality and design of the screen pack directly affect pressure differential.

A good screen pack should be:

• Precisely fitted to the cavity
• Symmetrically designed
• Made with high quality, consistent mesh weaves

This ensures even polymer distribution and helps maintain stable differential pressure.