Spirex Corp. has created a better way to purge old plastic material from screws and barrels for injection molding.
PowerPurge closes off the barrel, then the screw rotates so that high-pressure backflow flushes a special purging compound backward. Spirex said that closed-barrel scrubbing action cleans the screw and barrel much better than standard purging that only extrudes through the barrel. The company said PowerPurge is especially effective in cleaning the leading edge of the screw flight — which often is missed by traditional purging.
In most cases, molded parts are 100 percent acceptable with the first shot, according to Spirex, which manufactures screws and barrels in Youngs-town, Ohio.
“You've got frost-free refrigerators. Then you have your self-cleaning oven. Now you have a self-cleaning injection unit,” said Bruce Kozak, vice president of sales and marketing.
Kozak and Spirex President Paul T. Colby explained PowerPurge during an interview at Plastics News' office in Akron.
PowerPurge was invented by Joel Thompson, who also invented the Twinshot coinjection molding process for Community Products LLC of Rifton, N.Y. Spirex is manufacturing both systems under license from Community Enterprises. A patent is pending.
For PowerPurge, Spirex supplies the components, including a special nonreturn valve and a purge plug.
The purging compound is supplied by Asaclean-Sun Plastech Inc. of Parsippany, N.J., a unit of Japan-based Asahi Kasei Chemicals Corp. Spirex and Asaclean are the distributors.
In standard purging, a purging compound is poured in while the screw is rotated. The material extrudes out of the barrel until the old color is gone. Kozak said many times a molder will have to run 10, 20 or even 30 pounds of purging compounding.
Even then, some old material often remains in the screw and barrel, because traditional purging is a one-way operation: the material is pushed forward only until it exits the barrel. That can mean taking apart the injection unit and manually scrubbing it clean, a time-consuming process.
PowerPurge starts the same way, by regular extrusion. After the injection unit is moved away from the mold, the screw is rotated to remove as much resin as possible, using the proprietary compound. Then a purge plug is installed, which plugs the end of the barrel.
The PowerPurge valve is a three-piece design consisting of a retainer body and check ring, with a special lock. By running rapid decompression, also known as rapid suck-back, the check ring moves into a locked-open position. When the screw is injected forward, the ring remains open — and the purging compound is pushed backward under abnormally high-pressure backflow. That causes turbulent, agitated scrubbing and cleaning under pressure. Each time the screw starts to rotate again, it pushes the ring into normal operation, moving the material forward. The process is repeated a number of times.
The PowerPurge valve again works like a standard non-return valve as the new resin is introduced and the press returns to normal operation.
Colby said an injection molder could do the PowerPurge manually, or set it up into the machine's controller for one-touch operation.
Demag Plastics Group is the first injection press manufacturer to offer PowerPurge as an option on its machines. Spirex said it is working out details with Toshiba Machine Co. America to offer the system to its customers.
Contact information for Spirex: Tel. (330) 726-4000, fax (330) 726-9437, e-mail [email protected]