No. 42 - Underwater Pelletizing

When new plastics appeared, suppliers had to prepare them in a form handy for further processing. PVC could be diced from sheet with rubber-chipping equipment, but PS and PE held too much heat and had to be cooled in water before cutting. So resin producers invented strand pelletizing through a water bath. However, only 50 to 100 lb/hr of plastic could pass through each hole in the die face. Higher throughput required more holes and more strands, but too many sticky, molten strands were hard to manage.

ICI in the U.K. is credited with inventing the underwater pelletizer around 1955 and obtained the original patent. ICI extruded PE upward into a standpipe of water, where pellets floated to the top and were skimmed off. ICI licensed its invention to Farrel, Black Clawson, Berstorff, Werner & Pfleiderer, and Union Carbide.

Farrel made its first pelletizer prototype in 1956 and launched it commercially in 1957 to fit LDPE extruders up to 12 in. diam. It extruded horizontally through a vertical die plate.

In 1960, Black Clawson, which then built paper machinery, introduced an underwater pelletizer as its first entry into plastics. The design was (and still is) closest to ICI’s original invention—extruding up through a horizontal die.

Berstorff built its first underwater unit, designed for 22,000 lb/hr, in 1968. W&P delivered its first underwater model in 1969. Today it claims to have the world’s largest, with proven capacity for 167,000 lb/hr of LDPE and design capacity up to 200,000 lb/hr.

Until the early ’70s, underwater pelletizers were generally large systems for use by resin companies. But Gala was the first to foresee a market for smaller units for compounders with volumes of 2000 to 4000 lb/hr.

Next came water-ring designs, in which the die plate isn’t submerged, so it’s easier to heat. Pellets cut in the open air drop into a stream of water that takes three or four spins inside the housing and exits with the quenched pellets. Water rings cost less than underwater units but have lower thoughput. They also aren’t good for sticky, low-viscosity resins because pellets can stick together before they hit the water.

W&P is credited with the first water-ring pelletizer in the early 1960s. However, Welding Engineers patented a die-face unit in 1952 that cut pellets in air and dropped them into a circling stream of water. But the patent didn’t mention use of water.

In 1977, Berstorff patented the first water-ring model that extruded down through a horizontal die plate to better handle low-viscosity resins.