Appliance Design - August 2008 - (Page 24)

PLASTICS neering manager, says that his company’s internal studies, in which they tested an automotive cooling pipe test mold, confirmed that water-assist can generate thinner walls than either gas-assist or a combination of gas-assist and water-assist. In the study, nylon 6 with 40 percent mineral and glass fill can create a wall thickness of 2.9 mm using water-assist, while gas-assist was 4.2 mm, and a combination of gas-assist and water-assist was 3.2 mm. The water-assist method compares favorably to gas because of water’s innate properties. Water’s thermal conductivity is 40 times greater than that of gas, and water’s heat capacity is four times greater than gas. In addition, cooling times, and typically the resulting cycle times, are reduced in part because of the dual jobs that water is doing in the mold machine. When water is injected into the mold cavity, it is not just coring out the part, it is also cooling the part from the inside, according to Fleck. In his studies, Fleck says that parts molded with gas-assist continue to get hotter, while WAIMmolded parts dropped in temperature after the part was ejected from the mold. In tests, 15 seconds after mold ejection both the gas-assist and water-assist parts were around 280 DegF. After 30 seconds, however, the temperature of the gas-assist had increased while the water assist part had decreased. Most likely, Fleck says, this is because the gasassist part was still cooling on the inside whereas with waterassist the water more quickly cooled the inside of the part. A potential problem with the gas-assist part only being cooled from the mold tool side, is that the differential cooling could cause molded in-stress and warping, he adds. Faster cooling times have also been shown to reduce cycle times. In BASF’s 18-in. test handle mold, the company’s 15-percent glass filled PET resin was molded and cycle times tested. The gas took 62 seconds to cycle, while water took 40 seconds, and water flow through, in which the water flows through the hollow part and exits at the end of the part, took about 30 seconds, Fleck says. But water can work too well. Without the right process or material, problems can occur. The water can freeze the resin too quickly and sometimes that can lead to parts that are not internally smooth or can create shrinkage voids in the part wall. The water can push aside the cooling melt front and create new channels in the part, or cause other internal defects. (The water’s temperature seemingly has no part to play in this crystallization process. Researchers have injected water at temperatures up to 140 DegF without it affecting the crystallization rate.) The problem has been solved with both material and process changes. Resin suppliers have created several specially formulated materials for water assist A handle for a small appliance made with PME Fluidtec waterassist injection molding. molding. To date, most of the resin material has been in the polyamide (nylon) grades 6 and 6/6, although other materials such as ABS, and filled polyesters have been used. Nylons were the initial resin targeted for WAIM, but other materials, including lower cost resins, are under development. Fleck adds that even with hygroscopic resins, which nylon is one, the water injection process has no adverse effect on the material properties. Fleck surmises that because water solidifies the resins so quickly, water doesn’t penetrate the material. In addition to resins specifically formulated for WAIM applications, processing methods have been improved including the use of gasassist and water-assist in tandem to get the benefits of smoother internal walls and faster cooling times, says Brian Brookshaw, deputy managing director, Cinpres Gas Injection Ltd. In this instance, a bubble of nitrogen would be injected first and begin the coring process, helping to create a smoother interior surface. Water is then injected into the cavity, compressing the gas against the resin, and the One of the earliest and largest applications for water-assist injection molding is the lid to this trash receptacle. The polyethylene lid has been made with PME Fluidtec technology since 2001. 24 applianceDESIGN August 2008 www.applianceDESIGN.com http://www.appliancedesign.com

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Appliance Design - August 2008 Contents Editorial Shipments/Forecasts News Watch Plastics Switches Noise & Vibration Control Joining New Products Design Marts Association Report: CEA Advertiser’s Index

Appliance Design - August 2008

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