ATLANTA — The biggest challenge to some molders using the gas-assist injection process in the next few years could come from a clear, odorless substance that doesn't seem very threatening — water.
Several injection press makers, including Battenfeld GmbH, are expected to introduce water injection molding systems this October at the K 2001 plastics trade show in Dusseldorf, Germany, according to Juergen Ehritt, manager of process engineering for Battenfeld of America's sales and service center in Auburn Hills, Mich.
The process puts pressurized water into the melt after the plastic, rather than injecting inert nitrogen gas, he said. Ehritt spoke at the Structural Plastics 2001 conference, held April 1-3 in Atlanta. The event is sponsored by the Society of the Plastics Industry Inc.'s Structural Plastics Division.
The technology, developed two years ago by a German university, is being shared with several other injection press makers, including Milacron Inc. and Engel Vertriebsgesellschaft GmbH. It is being tested exclusively in Europe, but commercial applications could begin appearing in the next few months, he said.
"Water-assist will not replace gas-assist," Ehritt said. "That is very important. It will replace gas-assist [only] on very large parts."
Water-assist molding has one distinct advantage over its gas counterpart — much faster cycle times, he said. A part that takes 80-110 seconds to cool with gas molding will take 20-30 seconds with water, he said.
That is because parts made with water can be cooled from both the exterior and interior of the part, while gas parts cool only from the outside because the gas must stay hot, he said.
Water-assist has other advantages, such as smoother surfaces and much more even wall thicknesses, he said.
But water-assist has some potential safety hurdles, he said. For example, the molder needs to do a good job removing the steam from the mold in a controlled way and keeping water off the shop floor. The water generally is pressurized to 2,000-4,000 pounds per square inch, he said.
Molders who heard his presentation also said that removing water is key because customers will not want parts that retain any water.
Ehritt said water injection can be done on traditional presses with special equipment that costs about what gas-assist equipment costs, according to Battenfeld, a long-established gas-assist supplier. A mold made for gas-assist molding can be modified to support water, he said. He declined to talk about specific costs.
Generally, water injection technology is being tested with polypropylene, polystyrene and ABS. Ehritt said he is not sure if it will work with polycarbonate.
The technology will be used in applications such as handles, armrests and media ducts, he said. Another application: a German shopping cart maker is expected to introduce a model made with water injection. The company originally wanted gas-assist molding but found the cart's heavy handle had too long a cycle time, so it switched to water technology, Ehritt said.
The process was developed by the Institut fur Kunststoffverarbeitung in Aachen, Germany. The technique was mentioned in patent applications in the 1970s, but IKV was the first organization to really test water injection, Ehritt said. IKV tried water after unsuccessfully trying to find a gas that was significantly better than nitrogen, he said.