Appliance Design - January 2008 - (Page 41)

PLASTICS The image illustrates how the internal gas-assist molding process cored out the thick rim and bosses on this large truck panel, resulting in a structurally strong, yet light part. Photo by Mack Shows the flip side or cosmetic side of the same truck panel, illustrating the high-gloss, Class A surface that was achieved by using external gas-assist molding technology. the manufacture of hollow parts. The technique requires that an internal gas channel be designed into the mold to deliver the gas and that parts have a large enough gap to contain the gas bubble. Internal gas assist also often requires over-flow channels to be incorporated into the mold to help distribute weld lines and make room for plastic movement when the gas enters the mold cavity. “EGM has none of these problems,” says Ham. The gas injected in the EGM process is used for packing pressure and works especially well for large, flat parts and flat panels. The pressure eliminates the sink marks, which are small depressions on the part’s surface. They typically appear opposite thick areas of plastic such as the supporting ribs on the underside of the part. “Initially,” says Ham, “I was intrigued by the sink mark elimination feature, but as I dug into it I found that there was so much more to it and designers had more freedom in creating their parts.” For instance, when injection molding, Ham says that designs can be constrained by the ratio of the wall thickness to the thick- ness of the projections that are allowed off of that wall thickness. Conventional injection molding needs to have the profile of the ribs reduced by 0.7:1 or 0.8:1 (70 t o 80 percent of the wall thickness) to keep the mass down and eliminate the sink mark. Internal gasassist molding can have thicker ribs, but the projections need to be much thicker in order to fully encapsulate the gas bubble. With external gas molding, the ratio of wall thickness to rib base thickness can be 1:1. In essence, the rib-to-wall ratio can be increased for more strength and the ribs can be placed where the designer wants to place them. Another benefit includes reducing moldedin stress. One of the main issues of injection molding is dealing with the plastic shrinkage that happens inside the steel mold throughout the cycle time and the stress that builds up. In general, the plastic linearly shrinks 5 to 20 thousandths of an inch per inch of the part. On a volumetric basis, that amounts to approximately 5 to 10 percent of the mold volume that is reduced by plastic shrinkage and for which it must be compensated. “With normal injection molding, additional The image from Mack Molding illustrates how large this truck panel is by photographing it with a ruler alongside. Photo by Mack www.applianceDESIGN.com material is used,” says Ham. “The machine will try and ram more material in there after the initial shot trying to compensate for that shrinkage,” says Ham. Reducing shrinkage in the mold can also help the finished product. Studies have been conducted on in-mold pressures that indicate that in an injection molding 50-second cycle, the pressure on the cavity side of the mold begins to drop after 10 seconds. Thus, the plastic starts to shrink inside the mold and it begins to pull away from the mold’s cavity side, says Ham, adding that EGM can solve this problem. For example, if injection molding a 12-in. cubed box, made from a material like polypropylene that shrinks at 20 thousandths inch-per-inch, the part will try to shrink approximately 1/4 in. in all directions. By inflating the mold from the backside, the plastic is held in constant contact with the cavity throughout the cycle time. “It is still going to shrink when it comes out,” says Ham, “but now it is going to shrink uniformly throughout the part instead of trying to shrink more in the corners and less in other places causing the part to twist and warp. This way, it will shrink much more uniformly.” The part will also eject from the mold easier, says Ken Kincaid, technical engineering manager, Mack Molding Co., Southern Division. In conventional injection molding, a molded part will often need ejector pins to push the part out and that can leave pin marks. With EGM, once the gas pressure is removed, the part loosens and is ready to eject from the mold. applianceDESIGN January 2008 41 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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