Technical Article
Frozen Die Holes in Underwater Pelletizing: Causes and Prevention
A frozen die hole is a hole in which the polymer has solidified. In underwater pelletizing the cutting face of the die plate sits in cooling water while the melt inside it must stay molten, so every die plate works close to the point where holes can freeze. This article explains the main causes and what die plate design and operation can do about them.
Published 2026-09-25 by JINFENG engineering, Shenyang Jinfeng Special Equipment Co., Ltd.
Why Die Holes Freeze
Melt leaves the die holes directly into water. Heat flows from the melt and the heating medium toward the cooled face, and if a hole loses more heat than it receives, the polymer in the hole solidifies. JINFENG's simulation work shows that uneven temperature distribution or a die plate temperature that is too low leads to frozen holes at the discharge end of the die holes.
The simulation below also shows that the solidification front inside a die hole moves with the flow rate: at low flow through a hole, the polymer solidifies further inside the hole; at higher flow, the melt stays molten to the exit.
Common Causes
- Uneven die plate heating. Holes far from the heating channels, or in areas with poor heating-medium flow, run colder than the rest.
- Fouled or blocked heating channels. Deposits or leaks in the heating channels reduce heat transfer to part of the plate.
- Low throughput per hole. Running well below design rate, or with more open holes than the throughput needs, lowers the flow through each hole.
- Start-up sequence. Water reaching the die face before melt flows steadily through the holes cools them before they are running.
- Cold process water for the grade. EVA and POE run in chilled water at about 7–15 °C, compared with about 40–70 °C for PP and PE, which makes these grades more sensitive.
Always follow the pelletizer and extruder manufacturer's start-up and operating instructions; the points above are general guidance.
How Die Plate Design Reduces Frozen Holes
The heating-channel layout, the path of the heating medium, and the insulation between the cooled face and the heated body all affect temperature uniformity. For new designs JINFENG builds a three-dimensional thermo-mechanical model of the die plate and uses ANSYS Fluent to simulate the heating medium, melt flow, and polymer solidification, then optimizes the structure before machining.
After manufacture or repair, heating channels are pressure and leak tested, and the hard face temperature uniformity can be measured.
What to Check if Holes Keep Freezing
- Map which holes freeze. Holes that freeze in the same area point to a heating or insulation problem in that zone.
- Check the heating medium supply temperature and flow, and inspect the heating channels for fouling or leaks at the next shutdown.
- Compare the operating throughput with the design throughput of the die plate.
- Review the start-up sequence against the equipment manual.
- If the problem persists, have the die plate design and condition assessed.
Frequently Asked Questions
What is a frozen die hole?
A die hole in which the polymer has solidified, so no melt passes through it. The remaining holes then carry more flow, pellet size becomes uneven, and output across the die plate is no longer balanced.
Can frozen die holes be cleared during operation?
Sometimes, by restoring die plate temperature and throughput, but holes that stay frozen usually have to be cleared at a shutdown. If the same holes freeze repeatedly, the heating layout or heating-channel condition should be checked.
Does a die plate design change reduce frozen holes?
It can. Temperature uniformity across the die plate depends on the heating-channel layout and insulation. JINFENG checks this with thermal and melt-flow simulation when designing new die plates.
Having Problems with Frozen Die Holes?
Send the die plate drawing or photos, your line data, and a map of the frozen holes for assessment by JINFENG.
Contact: [email protected] WhatsApp: +86 133 0983 7263