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A new large one-piece frame for the Cadillac CTS Integrated Sunroof Module has been manufactured using the MuCell process.
Inalfa Roof Systems is one of the world’s biggest providers of vehicle roof systems, and was responsible for the design of the part. Typically sunroof openings for mid-size and large-size cars are multi-part assemblies as dimensional stability and stiffness are difficult to achieve in large one-piece designs. What makes this part special is that the front beam, rear beam and two guides were consolidated into one part, measuring one metre by one metre in dimension.
It is also the first integrated sunroof module assembly manufactured using short-glass-filled polypropylene, and features wall lengths that vary from 150mm to 300mm. As a consequence, the integrated Sunroof Module achieved a weight savings of 12% versus a traditional four-piece design and Inalfa also estimates that since the number of tools was dramatically reduced along with the reduction of checking fixtures too the capital investment required to produce the part was reduced by almost half, along with a considerable reduction in the cost of components.
MuCell technology involves the use of precisely metered quantities of atmospheric gases (nitrogen or carbon dioxide) in any of the three most common thermoplastic conversion processes (injection moulding, extrusion, blow moulding) to create millions of nearly invisible microcells in the end product.
Assembly time was reduced by more than a minute (67 sec.), and as a result of utilizing the MuCell process, a molding cycle time reduction of 15 seconds was realized as well. At the same time, the finished part has excellent fit and finish characteristics, along with improved dimensional stability.
How They Did It
Felice Bonucci, Director, Value Management for Inalfa Roof Systems, had discovered during a prior trial in Europe that the MuCell process helped to minimize warp in a large one-piece sunroof frame by some 60% versus solid moulding. Now based in the U.S., Bonucci decided to utilize the MuCell process for the Cadillac Integrated Sunroof Module, a difficult, tight tolerance sunroof frame scheduled for a fast development cycle.
Finished part quality and excellent dimensional stability were judged to be the key criteria for this new sunroof design, but Bonucci knew from experience that several iterations, or loops were typically required to optimize the tool design to achieve the results Inalfa was looking for. “”Each loop is expensive and time-consuming,”” said Bonucci, and multiple loops could take three months to accomplish adding excessive time and cost.
After the initial design was completed by Inalfa, Bonucci assembled his team of suppliers, which included representatives from the material supplier, the moulder and Trexel. An added element of complexity was that the tool builder was located in China. However, the moulder maintained excellent communication with the tool builder, which allowed the program to move along smoothly.
In addition, the decision was made to utilize a hybrid tool, with inserts to be utilized as dictated by the results of the mould flow analysis. The goal of the mould flow analysis was to minimize distortion to a number which, from prior Inalfa experience would allow for the elimination of cooling fixtures during actual production. This would result in significant piece part cost savings during the production cycle.
To compensate for the effects of the MuCell process on mouuld flow simulations, Trexel engineers supplied a significant amount of technical data to the material supplier on how to simulate the process during the mold flow study. At this point, the tool builder commissioned an additional moldflow analysis, which confirmed the earlier results and allowed tool construction to begin. After just one and a half iterations, or loops, the tool printed in all measured functional points. “”Inalfa history is that it would normally take several loops, so this is a huge result for us,”” said Bonucci.
The tool was then moved to North America and an additional trial confirmed the China results. “”This is a difficult, complex part,”” said Bonucci. “”In addition to its size, there are metal inserts, along with four metal tubes and this makes production difficult. The tubes do not allow for material shrinkage as the parts warp,”” he said. Bonucci concluded, “”Total time from initial design to finished parts was less than a year, and we saved a significant amount of time. I was sure the MuCell process would help us, and it has. I am happy, my company is happy and we intend to use the MuCell process again.””
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