Appliance Design - August 2008 - (Page 23)

PLASTICS Wall thickness is thinnest using only water-assist methods. Graphic: BASF In BASF testing, the interior of the part is more fully cored when the resin is optimized for use in water-assist injection molding. the part. To employ WAIM, the water-assist equipment should have up to 300 bar pressure capability along with 50 liters per minute of water volume. Depending on the manufacturer, water units are either pressure or volume controlled. Most WAIM units will have the necessary hydraulic or pneumatic controls to operate the water nozzle and mold actuators. The water nozzle is an additional component to the injection mold, and must be capable of supplying a high volume of water. Correct placement of the water injector is critical for success, says Fleck. WAIM also can be employed in many of the same ways as gas-assist so the learning curve should not be as great for the molders and part designers that are familiar with gasassist molding methods. The WAIM processes include the short-shot method, full-shot with overflow, full-shot with water flow through, and the melt push back method. With short-shot, the mold cavity is filled with 60 to 70 percent polymer resin and after it reaches that point, water is injected. The water cores out the part, and finishes filling out and packing it. After a hold time of a few seconds to let the resin solidify, the pressure is released off the water and it drains out of the hole where the nozzle was inserted. (Preferably, the hole in which the water is injected should be at the bottom of the mold to facilitate draining. In some cases www.applianceDESIGN.com where the nozzle is inserted in an area in which gravity will not purge the water, forced gas or forced air can be used. These fluids can also be used to ensure that the interior is completely free of moisture.) An advantage of the short shot method is that there is little or no regrind with which to contend, but a disadvantage is that it could cause hesitation lines on the exterior of the part caused by the switchover from melt injection to water injection. In a full-shot method, the mold cavity is packed and held for several seconds. The water is injected and the displaced resin goes into an overflow channel. After a holding time, the water is released. The resins in the overflow can be collected and reground in secondary operations. The advantage of the full shot method is that the best exterior surface finish is attainable, but it does require secondary operations to remove the overflow and deal with regrind. The melt pushback process is similar to full-shot, accept with this method the displaced polymer is pushed back into the barrel. This method makes the process more economical by not having to deal with resin waste or regrinding. Using these methods, WAIM has a number of advantages. As compared to gas, WAIM can reduce cooling times by 50 percent or more, and create up to 25 percent thinner wall sections depending on the appli- Wall thicknesses can be reduced by using water-assist, gas-assist and a gas/water-assist injection molding process, but water-assist creates the thinnest walls, according to BASF testing. cation, according to Bernd Herzog, manager of application and process development for PME Fluidtec. In addition, the wall thickness can be more uniform over the entire length of the channel, and molded parts can have larger cross sections. With gas, Herzog says, there can be a problem when the wall thickness gets bigger than 4 mm because the resin doesn’t crystallize quickly enough and the material can flow down. Water, on the other hand, cools the resin faster, and as a result the materials do not flow downward. The resulting wall is smoother and more consistent in terms of thick- ness, he says. The largest part molded by PME is a lid for a plastic trash bin that measured 43.5 in. x 50 in. x 7.5 in. and weighed about 22 lbs. To verify a reduction in wall thickness, PME cored out 60 mm to 80 mm diameter glass filled nylon and polypropylene pipes using both the gas-assist and water-assist method. The company discovered that the wall thickness of the gas-assist part was two to three times thicker than the water-assist molded part, Herzog says. Other studies confirm these findings. Harold Colwell, BASF Engineering Plastics, technical development engiAugust 2008 23 applianceDESIGN 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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