Process Know-how
Strand vs. Water-Ring vs. Underwater Pelletizing
Three ways to turn melt into pellets, each suited to a different output range, material and level of pellet-shape control.
Strand vs. Water-Ring vs. Underwater Pelletizing
Key takeaways
- Strand pelletizing cools the polymer as strands and cuts them after cooling — easy to inspect, suited to lower output.
- Water-ring pelletizing cuts at the die face with water cooling and suits most recycled polymers.
- Underwater pelletizing cuts at the die face under water and is used where tighter pellet shape control is required.
- The choice affects pellet shape, output range and how the downstream cooling and conveying is arranged.
- Die-face cutting in water needs the pellet to be cooled fast enough not to stick together — that is a material question as much as a machine question.
The three arrangements
All three take molten polymer and turn it into pellets. They differ in where the polymer is cut relative to where it is cooled, and that single difference explains almost all of their behaviour.
Strand pelletizing
Polymer is extruded through a die into strands, the strands are cooled — usually in a water bath — and then cut into pellets by a strand pelletizer.
Strengths:
- The strand is visible before it is cut, so melt quality problems are easy to see.
- It is simple, robust and easy to maintain.
- Start-up and shutdown are straightforward; there is no die-face cutting to keep synchronised.
Limitations:
- Output is limited by how many strands can be cooled and handled.
- Strands break, particularly with low-melt-strength material, which stops the line.
- It is labour-intensive relative to die-face cutting.
Strand pelletizing is well suited to lower output and to situations where visual inspection of the melt is valuable.
Water-ring pelletizing
The die face is sprayed or swept with water while rotating blades cut the polymer directly at the die. The pellets are thrown into a water ring, cooled and carried away.
Strengths:
- Cuts at the die face, so there is no strand handling to break.
- Handles a wide range of recycled polymers.
- Moderate output with reasonable pellet shape.
Limitations:
- Pellet shape depends on the interaction between melt temperature, cut speed and water cooling —
it needs to be tuned rather than simply switched on.
- Pellets can agglomerate if cooling is not fast enough.
Water-ring pelletizing is the common choice for most recycled polymers where a good general pellet is required.
Underwater pelletizing
Cutting takes place at the die face but under water, with the die itself submerged. The pellet is quenched immediately as it is cut.
Strengths:
- The most uniform pellet shape of the three, because quenching happens instantly and uniformly.
- Suits materials and output rates where shape control is critical.
- Very short distance between cutting and cooling, which reduces sticking.
Limitations:
- More complex, with tighter tolerances and more demanding maintenance.
- Requires careful water temperature and flow control.
- Higher capital cost.
It is selected where the end product genuinely requires tight pellet geometry — not as a general upgrade.
How to choose
| Consideration | Points toward |
|---|---|
| Lower output, visual melt inspection valuable | Strand |
| General recycled polymer, moderate output | Water-ring |
| Tight pellet shape requirement | Underwater |
| Low melt strength material that breaks strands | Die-face cutting (water-ring or underwater) |
| Simplest maintenance and start-up | Strand |
| Highest pellet uniformity | Underwater |
What sits downstream
The pelletizing method determines what comes after it. Water-ring and underwater systems carry the pellets away in water, so a dewatering and drying stage follows. Strand systems produce pellets that are already cool and largely dry, so the downstream arrangement is simpler.
Either way, pellet cooling, conveying and storage are part of the system rather than separate purchases, because pellets that are still warm when they reach a silo will block it.
Related: Single-Stage vs. Two-Stage Pelletizing · Screen Changers and Melt Filtration · Plastic Pelletizing Systems
ABOUT THE AUTHOR
Plastic Recycling Process & System Engineering
The Retech Engineering Team works across material analysis, process design, recycling line configuration, factory testing and project commissioning.
Questions
Strand vs. Water-Ring vs. Underwater Pelletizing — FAQ
Which pelletizing method is suitable for my material?
It depends mainly on bulk density and contamination. Low bulk density film and fibre usually need compaction before extrusion, while flakes and regrind can often be fed directly. The cutting method is then chosen against the pellet shape the end product requires.
What is the difference between water-ring and underwater pelletizing?
Both cut at the die face. In water-ring pelletizing the cut pellets are thrown into a ring of water and cooled in flight; in underwater pelletizing the die face itself is submerged, so the pellet is quenched immediately as it is cut. Underwater gives tighter shape control at higher complexity.
Why does strand pelletizing stop when a strand breaks?
The strands must stay intact from the die to the cutter. Low-melt-strength material, or material with contamination that disrupts the strand, breaks it and interrupts the line. This is one reason die-face cutting is preferred for difficult recycled streams.
More answers in the <a class="retech-x-link-arrow" href="/faq/">full FAQ</a>.
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