Appliance Design - January 2008 - (Page 42)

PLASTICS Another major benefit of EGM as compared to conventional injection molding is that a part does not require as large of a press to make the part, says Kincaid. Typically, 3 to 5 tons of clamping pressure per square inch of part is required to hold the plastic inside the mold. A 100-square inch part would require a 300- to 500-ton machine to hold it closed. With EGM, the required pressure is only that which is needed to fill the mold and inject the gas. After that, the pressurized gas will exert much of the needed pressure. “The tonnage can be reduced to about 1-ton per square inch,” says Kincaid. “That part that needed a 300 to 500 ton press can now be made with a 100 or 150 ton press, which can save significant press costs.” For instance, one EGM project completed by Mack Molding featured a 195.5-sq.-in. part that was molded in a 300-ton press of PC/ABS resin. By comparison, conventional molding would have required a 600-ton to 850-ton press, says Kincaid. The part was a front fascia for an automated teller machine manufactured by Triton, a Dover Co. based in Long Beach, Miss. This was the first commercial use of EGM, says Kincaid. Previously, Triton had used structural foam to make the 11.5 in. by 17 in. by 4 in. fascia. The part required strength to withstand direct impact and day-to-day wear, and had to match the other front panels on the unit. “Using the structural foam, they had to paint the outside of the part to get a good look,” says Kincaid. “They wanted to reduce the amount of paint that they needed without reducing the big, heavy features on the inside of the part. External gas molding was a way to mold a nice looking part on the outside with heavy mounting features, ribs and structural features on the inside. The result was a solid part with no gas holes, no voids and no read-through from gas lines on the show surface,” says Kincaid. In another application, Mack Molding used EGM and internal gas assist to develop a Class A surface component. The part was the base of an external refrigeration unit for semi-trailer trucks that sits between the truck cab and the front of the trailer. It measured 76 in x 22.4 in. x 0.18 in., and weighed 15.3 lbs. The part previously had to be produced by twin-sheet thermoforming of ABS/PC with high gloss on one of the two sheets. In the field, the base suffered cracking and other failures and Mack Molding was charged with eliminating the cracking and 42 applianceDESIGN January 2008 A comparison of a large part. On the left are sink marks that are eliminated with external gas molding. Photo: Steve Ham Plastics. producing a Class A surface. Because of the size of the part, and the internal ribs and bosses, conventional injection molding was ruled out. Kincaid and his crew developed a process that combined internal and external gas-assist in the same cycle to mold the large structural part and produce a high-gloss, Class A surface. Twenty-five seconds after injecting a full shot of resin into the tool, internal gas was injected to core out the part’s 2.5-in-thick rim and bosses. This displaced the material and created a gas channel of 1.6 in. diameter around the rim. The rim sealed off the periphery of the part, preventing gas seepage during the external gas phase. External gas was applied on the core side, forming a pillow of nitrogen that packed out the part and resulted in a 90-gloss rating on the cavity or show side, right out of the mold. Kincaid says that this is an example of the technique’s overall potential. “Features can be designed into the mold tool to isolate the gas in just the areas where you need it, making it possible to take advantage of the benefits of both internal and external gas in the same part.” < For more information, enter: Cinpres Gas Injection Ltd . . . . . . . . . . . . . . . . . . . . . . . 304 Or email: enquiries@cinpres .com Mack Molding Co . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 305 Or email: kkincaid@mackmolding .com Steve Ham Plastics . . . . . . . . . . . . . . . . . . . . . . . . . . . 306 Or email: Steve .ham@dnet .net A rectangular shaped test component measuring about 280 mm by 190 mm by 50 mm with apertures and various ribs of different heights and thicknesses. The ‘A’ surface of the mold has gloss, matte and grained finishes. Photo: Steve Ham Plastics. External gas pressure is shown in green during the gas-packing phase. The still molten plastic shown in blue was completely injected into the mold in the prior phase. Gas can be introduced through individual gas pins or through a gas manifold system; either approach puts gas on the back side to allow uniform gas packing pressure. Integral seals in the piece part geometry provide the sealing. Another sealing technique is to “O” ring the parting line. The gas pressure “inflates” and holds the plastic against the cavity half of the mold during cooling. With other processes the plastic shrinks down on the core. www.applianceDESIGN.com http://www.appliancedesign.com

Table of Contents Feed for the Digital Edition of Appliance Design - January 2008

Appliance Design - January 2008 Contents Editorial Shipments/Forecasts News Watch Makers of Commercial Refrigeration Equipment Turn to Variable-Speed Motors to Improve Energy Efficiency. New Motor Design and Control Scheme Makes Brushless Motors More Economically Feasible for a Wider Range of Applications. External Gas Molding, a Relatively New Technique, Solves Many of the Challenges Associated with the Injection Molding of Large Plastic Parts. Plastic Housings Offer Many Benefits: EMI Protection Isn't One of Them. But, New Materials and Techniques are Solving that Problem. Choosing the Optimal Strategy for Effective EMI Shielding Depends on Both Mechanical and Electrical Requirements Ethernet-Enabled Microcontroller Makes Networking Schemes Easier and More Cost Effective to Implement The Ever-Changing Array of Microcontroller Memory Options Makes it Imperative to Keep Abreast of What's Available to Determine the Optimal Choice for an Application. A Vast Range of Components and Finished Products will be Exhibited at the Country's Preeminent HVAC/R Show, AHR Expo, which is Scheduled for Jan. 22-24 in New York. Advertiser's Index Association Report: PRBA

Appliance Design - January 2008

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