Strand, water-ring, underwater or die-face: which pelletizing system fits your extruder?

The extruder melts and mixes; the pelletizer decides what your customer actually receives. Pellet shape, dust, moisture and how often an operator has to step in all come from the pelletizing system. VJJ twin-screw extruders can be supplied with all four common systems, so the choice comes down to your material, your output and your budget.

Strand pelletizing

The melt leaves the die as strands, runs through a water bath, is dried by an air knife and cut into cylindrical pellets by a strand pelletizer.

• Best for: most engineering plastics, colour and additive masterbatch, glass-fibre compounds, and lab or small production lines.
• Strengths: the lowest investment, simple to run and maintain, easy to switch between products.
• Limits: strands break when the material is brittle or very low in viscosity, and someone has to re-thread them. Highly filled compounds are hard to run this way.

Water-ring pelletizing

Knives rotate against the die face and cut the melt as it exits; a spinning ring of water catches and cools the pellets, which are then dewatered.

• Best for: PE and PP compounds and recycled PE and PP, especially materials that do not form good strands.
• Strengths: no strands to break, less labour, compact, and usually cheaper than underwater systems.
• Limits: less suited to very low-viscosity or very sticky melts; the knife-to-die setting needs care.

Underwater pelletizing

The die face sits inside a water-filled chamber. The melt is cut under water, and the water carries the spherical pellets to a centrifugal dryer.

• Best for: high outputs and soft or sticky materials, such as TPE and some polyolefin grades.
• Strengths: uniform spherical pellets with very little dust, fully closed, high throughput.
• Limits: the highest investment and the most complex start-up; the die can freeze if the start-up sequence is wrong.

Die-face (hot-cut) pelletizing in air

Knives cut the melt at the die face in air, and the pellets are cooled by air or on a vibrating conveyor, with no water contact.

• Best for: highly filled masterbatch such as CaCO3 filler masterbatch, and PVC compounds.
• Strengths: no moisture pick-up and no water system; handles brittle, highly filled melts that cannot form strands.
• Limits: not suited to materials that stay sticky while hot.

Quick comparison

What to send for a quotation

• Material and formulation, including filler or fibre content
• Target output in kg/h
• Pellet size and shape your customers expect
• How often you change products or colours
• Available water, floor space and plant power supply

See the VJJ specifications on the twin-screw extruders page, and the guide to filler masterbatch and specific torque.