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FEA Software Speeds Plastic Product Design |
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Several of the original Magnum platens from plastic thermoforming machines, first manufactured in 1986, cracked after the plastics industry switched from foam materials to heavier plastics. |
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You see them at the deli counter, in the delivery of take-out food, in cafeterias, and grocery stores --plastic food containers created by machines called thermoformers. Irwin Research & Development in Yakima, WA, manufacturer of the best-selling web-fed foam thermoformer in the U.S., is using software from Pittsburgh-based ALGOR, Inc. to interface finite element capabilities with the SolidWorks CAD software Irwin Research already was using. By using analysis in conjunction with design software, Irwin Research has been able to produce stronger thermoformer components that keep up with changes in the plastics industry, and ensure that consumer products roll off the production lines and into grocery stores and restaurants. Irwin Research manufactures a variety of equipment for the plastics industry, specializing in equipment for thermoforming, a process by which flat sections of thermoplastic are molded into three-dimensional products. First, the thermoplastic passes through a heat tunnel, where it becomes soft and malleable. The thermoplastic may either be unwound from a roll (also called a web) or fed into the heat tunnel as a continuous, flat sheet. The plastic then passes into the former, where it is vacuum-stamped between two molds. The molded shape is then trimmed from the excess plastic, which is recycled to make new sections of plastic. Irwin's web-fed thermoformers use rolls of plastic to create small items such as cups, plates, and bowls. More than 700 of their machines operate in 26 countries.
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"At Irwin Research & Development, we strive to produce components that never fail, even after years of use," said Frederic Pasche, a mechanical engineer. "In 1986, the platen would have been prototype-tested. Now we use finite element analysis to determine where the highest stress will occur, and discover how to fabricate the platen to prevent future failures." Based on the platen's geometry, Pasche created a finite element model with nearly 3,000 elements using ALGOR's Superdraw II. To speed processing without significantly affecting results, Pasche omitted small features such as mounting holes. An applied pressure of 250,000 pounds over the surface elements of the finite element model reflected the rating of the press. Boundary conditions fixed the model on all four corners to replicate the platen's attachment to the bushings on which it is lowered and raised. Pasche used the material property values for steel from the Material Library included with ALGOR software. Pasche then conducted a linear stress analysis and compared the Von Mises stresses to published yield stress for steel. He discovered that high stresses were occurring where sections of a platen had been welded together. "We set a goal to reduce the Von Mises stresses to below one-third of the yield stress," said Pasche.
To reinforce the platen, Pasche added thickness to the platen's geometry. First, he created a model of the platen in SolidWorks. Since performing the first platen analysis, Irwin Research had purchased ALGOR's Houdini software, which enabled Pasche to automatically create a finite element mesh from a SolidWorks IGES file. Houdini offers SolidWorks users three choices for solid FEA meshing of existing designs: tetrahedral, all-brick, or hybrid, with bricks on the surface and tetrahedra inside.
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First, Houdini's Merlin Meshing Technology created a well-shaped surface mesh from the SolidWorks model. After the surface mesh was created, Houdini meshed a solid model from the surface in and put the highest-quality elements on the surface. The surface is where loads and boundary conditions are applied and where stress levels tend to be the highest. This principle works with tetrahedral, brick, or hybrid models. Pasche used Houdini's brick meshing with Hexagen with the standard option for a hybrid mesh. Again, Pasche omitted small features such as mounting holes to speed processing without significantly affecting results. The model was analyzed using the same loading and initial conditions as used for the original model. Von Mises stress results revealed a much stronger platen. "The new design is approximately four times as strong as the original design," said Pasche. "The added strength will help to prevent fatigue failures." As an added measure, Irwin Research is considering fabricating the platen so that the location of the weld does not coincide with the areas of highest stress. According to Pasche, "Performing finite element analysis helps with design. We no longer have to apply the trial-and-error method to prototypes that are expensive and time-consuming to produce." Irwin Research & Development relies on finite element analysis because of the demanding time constraints of the consumer plastics industry. As the company prepares the Magnum platen for the 21st century demands of the industry, Pasche is including 21st- century engineering software technology in his design cycle. The company plans to interface SolidWorks with ALGOR's Accupak/VE Mechanical Event Simulation software for Virtual Prototyping with Linear and Nonlinear Stress Analysis on projects in the future. |
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Accupak/VE replaces physical testing with virtual testing by replicating mechanical events with a computer, helping to determine the behavior of a product in its real-world, worst-case scenario. Irwin Research & Development can predict motion and impact, perform stress analysis for each instant as the event unfolds, determine flexibility, intrinsically determine forces, handle complex shapes and nonlinear behavior, and test strength of materials, all with one package. For more information on ALGOR software, contact the company at: |