DuPont Consulting Mechanical Engineer Joseph S. O’Hara PE made necessary reductions in the weight of a machine used to cut tough plastic with the assistance of a computerized engineering handbook. The 34,000 pound machine cuts polyvinyl butyral (PVB), the material used as an interlayer in automotive windshield to prevent shattering. An earlier version of the machine experienced weld failures due to the high forces generated when cutting this material. To help relocate the welds to lower stress areas, O’Hara used a computerized engineering handbook to perform closed form solutions and finite element analyses. The plant mezzanine where the machine was to be located, however, was unable to support the 40,000 pound weight of the initial design. So O’Hara performed multiple iterations of the analysis to optimize the cross-sections of structural members and reduce the weight of the machine within a very short period of time.

DuPont is one of the world’s largest chemical producers with revenues of $24.8 billion and net income of $4.7 billion. DuPont Butacite PVB film has been used in windshields since 1938 and has applications in residential and commercial construction for windows, skylights, atriums, partitions, curtain walls, doors and even roofs. Glass laminated with Butacite PVB will not shatter. Even if the glass is broken, the opening will not be penetrated because glass fragments adhere to the interlayer. PVB is a pliable but very tough material that also has a very high strain rate sensitivity which means that the faster it is cut the more strongly it resists. O’Hara experienced the unique properties of PVB firsthand when a chunk of concrete fell off a truck and was thrown right at his windshield. The windshield broke but the PVB interlayer prevented the concrete from penetrating the interior of the vehicle, saving his life.

Current processes for producing PVB sheet include recycling of some finished material as production input. DuPont product in the form of 48 inch cubic blocks of material that weigh about 2,000 pounds each. In order to be re-used, these blocks must be cut up into slices that are small enough to go into a shredder that reduces them to smaller chunks. The machine used at DuPont to slice the blocks is called a block guillotine. It has a large blade that is driven through the PVB blocks by a hydraulic cylinder driving a modified four-bar linkage. These cutting forces generate reaction forces many times higher on various places in the machine frame. The previous version experienced continual weld failures that required it to be shut down for repairs. Because a consistent flow of recycled material facilitates smooth for plant operation, shutting down the machine for any extended period was disruptive to the entire PVB Operation.

O’Hara was assigned to overcome these problems by redesigning the machine to be able to withstand the enormous cutting forces. Since the machine would be located on the plant mezzanine, however, it could weigh no more than the old machine. O’Hara recognized that his first task was quantifying stress levels throughout the structure of the machine so that the weld points could be relocated in lower-stress areas. He considered using the high-end computer aided design (CAD) and associated finite element analysis (FEA) program used by DuPont. But O’Hara knew that a considerable amount of time would be involved if he took this approach. First of all, he was not a regular CAD or FEA user so he would require some training before even beginning the project. Secondly, he would have to define the geometry of the machine to a high level of detail in the CAD program. Thirdly, he would have to build a finite element model of the machine which would require guesstimating stress transients in each area of the machine in order to size elements and very likely reconstructing his model if the guesses were not accurate.

Instead, O’Hara decided to turn to an entirely different type of program called a computerized engineering handbook that is designed to reduce the time required to solve over 5000 every-day engineering problems scattered in over 100 engineering reference books, while increasing accuracy and improving documentation quality. The Desktop Engineer, from Desktop Engineering Int’l Incorporated, Woodcliff Lake, New Jersey, makes it possible for engineers to simply select the type of problem using a graphical user interface, then enter the required parameters in response to prompts. Input is automatically verified by the program. The program then generates professional-looking documentation for the solution that shows all intermediate calculations. The Desktop Engineer includes over 50 modules grouped into the following categories: geometric analysis; static analysis; dynamic analysis, and buckling analysis. These categories are used to analyze structures including straight beams, curved beams, cables, circular arches, circular rings, columns, discrete systems, disks, foundations, frames, grillages, helical springs, plates, shafts, shells and solids. It even includes a small, easy to use, FEA module.

O’Hara began by modeling the existing machine. The first step was determining the properties of the huge structural members used in the machine. This was a simple as creating a simple cross-section model of the members in the thin-walled sections module and selecting the proper material and took only a minute or two. Next, using the frames module of The Desktop Engineer, O’Hara constructed a very simple stick-model of the machine, input the section properties determined previously and the forces exerted by the linkage. The program automatically generated a simple finite element model, ran a finite element analysis and provided output that showed the stress on each node of the simple model, in less than 5 minutes. O’Hara then identified the loads in each area of the structure and he was able to confirm that the welds that had failed were located in high-stress areas. The deflection output was used to optimize the cutting area of the machine, enhancing performance. All of this was accomplished in less than one day.

O’Hara, and his team, then began designing the new version of the machine. He relocated welds to areas of low stress and provided increased support in areas of high stress. Unfortunately, these changes raised the weight of the machine about 30% above acceptable levels. O’Hara realized that the only way to reduce the weight by that amount while maintaining structural integrity would be to resize the structural members. O’Hara had a considerable amount of flexibility in this area because even the original members had to be specially formed because of their unusual size and thickness. O’Hara took advantage of the redo function of the computerized engineering handbook to perform a parametric analysis on the machine frame. He simply reduced the cross-sectional properties of the members in the frames module of the program and then re-ran the analysis. After about 20 analysis runs that took about 15 minutes, he had determined the minimum properties that would provide the necessary strength and deflection criteria.

O’Hara then switched to the thin-walled cross-sections module of the handbook. His goal was to find a cross-section that would provide the required strength without exceeding the weight limitation. He tried about a half dozen different configurations. He finally settled on 12 inch by 12 inch square tubing with one inch thick walls for the base and top of the structure and a unique configuration consisting of three vertical and two horizontal members made of two-inch thick plate for the cross-members. Both of these members had to be specially formed. O’Hara also used the shaft section of the module to design the torque tube in the four-bar linkage and the beam section for the dead shaft. The final machine has a footprint of 10 feet by 12 feet and is about 16 feet tall.

O’Hara’s design met all the requirements of the application. It has now been in operation for over one year at DuPont’s Parkersburg, West Virginia plant and has not experienced any failures nor any significant downtime. The rigidity of the machine is significantly better than the old version, avoiding a problem that sometimes occurred in the past where deflections as small as 0.020 inch would cause the last slice of PVB to stick in the machine, resulting in unnecessary downtime. O’Hara credits the speed and versatility of the computerized engineering handbook with the fact that the machine went from initial concept to production in only 8 months, about the half the time required for the previous machine. "The program can be learned in a day so it’s perfect for an engineer that doesn’t perform analysis on a regular basis," O’Hara said. "Yet it is powerful enough to analyze hundreds of design alternatives and iterate to an optimized solution in well under a day. I have an enormous amount of confidence in this tool and have used it on over one hundred projects. All that power, and it’s cheap too!"

For more information contact Desktop Engineering Int'l Inc., 172 Brodway, Woodcliff Lake NJ 07677. Phone: 800-888-8680 or 201-505-9200 Fax: 201-505-1566