Three people suffered burns when a plastics extruder exploded during trial production of a material Feb. 14 at San Diego Custom Extrusions in Santee, Calif. The midday blast moderately damaged the plant, shook equipment in adjacent offices and was heard blocks away, said Bill Cruzen, a detective with the arson and explosives unit of the San Diego County sheriff's department.
According to Cruzen, independent inventor Peter Nopper of Leesburg, Fla., asked the firm "to mix three chemicals to make a semisolid material that he wanted to use as an ignitable [backfire] material for forestry fire fighting."
Nopper said he had combined the materials previously on a smaller extruder, Cruzen said.
The materials — polyethylene granules, raw sulfur powder and potassium nitrate granules — had "a lot of the characteristics of black powder," Howard Rayon, division chief with the Santee Fire Department, said in a telephone interview. "The customer has his own recipe."
The materials were heated but never emerged from the 15-year-old extruder's 21/2-inch barrel. Instead, the cast-iron extruder's side chamber exploded, fracturing a half-inch-thick metal hopper and launching projectiles as far as 50 feet, Cruzen said.
A 20-pound chunk—10 inches square by 2 inches thick—passed through both sides of a refrigerator, moved a workbench about 1 foot, bounced off a galvanized vent hood and landed on a conveyer, bending metal brackets.
Longtime employee Thomas Thompson was feeding materials into the hopper and was blown off his perch on a ladder, Cruzen said. Nopper and company owner Steve Schrenk were standing on the extruder's motor side, away from the blast's direction but within range of a brief flash fire.
Thompson and Schrenk experienced face, shoulder and arm burns. Nopper also was burned.
Thompson and Nopper were hospitalized overnight, and Schrenk, for two nights, according to a spokeswoman for the regional burn unit of the University of California, San Diego.
Plastics News' most recent listing of North American pipe, profile and tubing extruders said San Diego Custom Extrusions employs 17, operates three lines and has annual sales of $1.2 million.
Seven employees were present at the time of the explosion.