Recycled PET Flakes

Recycled PET arrives as a flake with a shape that nobody designed for conveying.

The particle is thin, curved and variable, which means the bulk behaves by interlocking rather than by the friction rules that work for granules.

Bridging and hopper design

• Outlet size driven by flake length, not by equivalent particle diameter.
• Steep walls and smooth liners, since a shallow cone simply holds the arch.
• Live bottom or agitated discharge where the silo is tall and consolidation is real.

A bridge in flake material often looks stable enough to walk on, which makes silo entry one of the more dangerous tasks in a recycling plant.

Static and light weight

Dry flake generates and holds static, so it clings to sight glasses, filter bags and conveying line walls.

Grounding is necessary but will not fix the clinging; controlled conveying velocity and attention to line layout do more.

Moisture and drying

PET hydrolyses in the extruder if moisture is present, and the result is low intrinsic viscosity and a weak product.

Drying to the required dew point and protecting the dried material from re-absorption between dryer and extruder are part of the handling design, not a separate process step.

Contamination control

Label fragments, adhesives, aluminium and foreign polymers all survive washing to some degree.

Metal detection, air classification and inline screening before the feeder protect both the extruder screw and the food-grade claim of the pellet.

What to establish before buying equipment

• Flake size distribution including the longest particles, which set the outlet.
• Bulk density range, which is lower and more variable than for pellets.
• Required dryness at the extruder inlet.
• Acceptance limits for foreign material and the detection method.

PET flakes interlock and bridge, and the variability of recycled material makes it worse.

Flake size, thickness and residual label and glue content vary between deliveries, so the same silo behaves differently from one batch to the next. Moisture is a process problem: PET must be dried before extrusion or it degrades in the melt.

• Size the outlet for the worst flake geometry, not the nominal one.
• Keep flakes dry and accept that drying is a process step with its own equipment.
• Expect residual contaminants and verify the upstream separation rather than trusting it.

Bridging and static define the handling.

Flat flakes interlock across an opening and charge readily, so a plain cone with a slide gate is unreliable. Agitated or live-bottom discharge is normal, and conductive components reduce the clinging.

• Live-bottom or agitated discharge specified with the silo.
• Conductive, earthed components in transfer lines.
• Metal detection and separation close to the process inlet.

Polymer dust is combustible and the recycled stream adds unknowns.

• Explosion protection where fines accumulate, especially in dust collection.
• Verified contaminant removal; recycled streams can carry anything the collection allowed.
• Dust capture at transfer points, where flake abrasion produces fines.