COMSOL Helps Commercialize Molding Technology
José Feigenblum, RocTool, Le Bourget du Lac, France
Mathematical modeling is going far beyond the R&D lab and is starting to make a real difference in manufacturing processes. For instance, it’s hard to imagine that RocTool’s primary process, our rapid composite molding technology, would be here today without COMSOL Multiphysics. In fact, the success story of our company is linked closely to the capabilities of that software.
The process that makes up virtually all of our business today did not exist just five years ago. And it was only with the help of COMSOL that we were able to discover, understand and commercialize our Cage System technology. Today it would be virtually impossible to adapt our process to each client’s requirements without the software.
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Figure 1: Example of a molded part and the molding apparatus used to create it. RocTool licenses its technology, which is used for many different types of molding. The materials, shapes and applications all differ greatly. |
RocTool is an innovative company that specializes in licensing its rapid molding technologies for the composites industry. Its customers include major automobile manufacturers plus their Tier 1 suppliers, the aircraft industry, as well as sports and leisure companies—all where lightweight yet strong composite materials are key components (Figure 1). The company does no manufacturing; rather, it helps clients set up production lines that integrate the Cage System inductive-heating method. With 15 employees and growing, RocTool last year had sales near 1.5 million euros, all from the licensing of its patents and in consulting.
Better Control of Tooling-Surface Heat
When making composite parts in Resin Transfer Molding (RTM), a mold must be hot enough to cure the material, but not too hot during the injection phase. The mold generally consists of top and bottom surfaces separated by a few millimeters. Traditionally, molding sections are made of solid metal; these are heated by sending hot oil or water through small shafts bored through the mold, or by sitting the mold on a heating plate. This means that large volumes of metal are being continuously heated and, depending on the application and mold, the process can require 20 to 50 kW or more of power for 24 hours a day, 7 days a week. A complete injection/curing cycle varies with the requirements of the material being formed; for instance, with a thermoset such as an epoxy resin it can take 15 to 20 minutes at a constant temperature of 90°C.
We at RocTool knew there must be a better, faster way. I learned about COMSOL Multiphysics’ capabilities during my PhD work and, when I joined RocTool five years ago, one of the first things I did was to tell engineering management that we had to have it. This turned out to be an excellent decision.
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Figure 2: The molding process. The composite material is placed within the mold that then presses down on the material while induction currents heat the two surfaces of the mold. When the final shape has been taken, water flows through pipes to cool down the material. The final shape can then be removed. |
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With COMSOL, we developed the Cage System, which we subsequently patented in 2004. This method involves surrounding both parts of the metal mold with an induction coil that is driven by a high-frequency signal of between 15 and 100 kHz. Because of skin effects, induced current stays within the outer 0.1 or 0.2 mm of the mold sections. The mold has a high magnetic permeability which induces strong eddy currents, and the material’s high resistance results in locally heating the tooling surface in desired places. The other conductive surfaces on the exterior of the mold are made of nonmagnetic materials and do not contribute noticeably to the heating. Heating of the tooling surface starts immediately when the coil is activated, and requires 200 kW or more of power to quickly get from 40°C to 140°C. As a consequence the heating/cooling cycle drops to between 2 and 5 minutes (Figure 2).
The Cage System also gives process engineers much better control of the temperature cycle. For instance, the classical method to mold material such as resin epoxies uses a lower constant mold temperature during both injection and curing. This lower mold temperature avoids cross-linking during injection, but the curing cycle takes a long time. On the other hand, the Cage System preheats the mold to 40°C so that the epoxy resin has good viscosity for being injected, and the temperature can be quickly increased for the curing stage. In this way, it is possible to cut cycle times by a factor of two or three for many processes. Further, the Cage System is applicable to many molding process that require fast heating and cooling, and it can be used in virtually all types of molding, such as blow, extrusion, injection, or compression molding.
A problem with using induction currents to heat the mold surfaces is the possibility of hot spots. The mold surface’s shape can influence the magnetic field and thus the induced current, and for many shapes it is possible to see considerable temperature differences across the tooling surface. The material being transformed is therefore not heated homogeneously and a sharp curve can cause overheating where it is convex and underheating where it is concave. This is due both to higher current density on the convex radius and the fact that heat is concentrated in the area near the radius. However, with the Cage System, engineers can place surface inserts with lower magnetic properties in certain areas to closely regulate the heat distribution and to eliminate hot spots. Modeling is an integral part of determining the placement of these inserts.
Users indicate that while a molding machine using this process might cost 15 to 20% more than one using classical methods, manufacturing efficiencies then lower the cost of the final by 15 to 20%. For instance, the production of 20,000 automotive roofs using classical methods requires 2 production units and leads to a cost of 90 euro/ piece, which decreases to 45 euro/piece using the Cage System production unit. This is mostly due to a drastic reduction in the time cycle—by a factor 5 in this case (4 min as opposed to 20 min).
A Model for Every Client
Figure 3: The magnetic flux (streamlines) and temperature distribution (color plot) from the part shown in Figure 1 (symmetry meant that only a quarter of the geometry had to be modeled). The purple lines indicate the position of the coils.
In the composite-molding business there is no such thing as a standard product; each mold must be designed individually based on the customer’s material and product specifications. This is where COMSOL Multiphysics plays a crucial role. After reviewing the application with a client, RocTool creates a model that simulates the magnetic fields, eddy currents and Joule heating on the molding surface. The aim of the 3D model is to define the optimal inductor configuration and surface layer that creates efficient and homogeneous heating. We pay close attention to the selection of materials and the mold geometry, which we can determine effectively only through simulation. We use the AC/DC Module to first compute a quasi-static magnetic solution for the field due to the coil, and then use those results to find the heat distribution on the tooling surfaces (Figure 3).
With this model, engineers can determine the optimal size of the coil, its placement, heating material properties and the optimized cycle time. We quickly learn how the number and placement of the turns in the inductor coil is crucial for homogeneous heating. Further, if the inductor is not long enough compared to the mold, cold areas can arise on the mold extremities where the magnetic field density is lower. After modeling, RocTool engineers can tell the client that the process will result in a given temperature cycle, that molding will take a given amount of time, and they can also give a reasonable estimate of what the parts will cost to manufacture.
Our latest research effort, which we are still in the process of commercializing, concentrates on determining how to best work with self thermally regulating materials (STRMs) to control hot spots. These materials are useful because when they reach their Curie temperature they become nonmagnetic and thus are no longer subject to inductive heating. Depending on the amount and placement of these materials, we can gain even better control of the temperature profile over the tooling surfaces. To model these materials in COMSOL, we create a 3D geometry and first solve for the magnetic field, then determine the materials’ temperature. When these materials reach the Curie point, we re-run the magnetic field calculations and find the new temperatures.
CAD Import for a Variety of Clients
Figure 4: Close-up of a cross-section of the model of a mold. The ‘hot’ color scale shows the amount of current density inducing heat (A/m2), while the ‘jet’ color scale shows the localized temperature in the mold (K).
Another important aspect of the software is the CAD Import Module as the tool designers must work with CAD data supplied by our clients. A large number of them, especially those in the automotive industry, work with the CATIA CAD system and provide geometry data in either the IGES or STEP formats. Work with these imported geometries, from which the tooling engineers create the heating surfaces for the mold, is supported through the CAD Import Module. Part geometries can sometimes be quite complex, and the modelers use defeaturing tools to simplify the geometry to make the modeling more practical. They must strike a balance between model solution times and the detail of the results.
RocTool engineers are also very impressed with COMSOL’s graphics capabilities, which make it easy to show results (Figure 4). For instance, a client once visited us and had some questions about how to improve his existing process. Within an hour, an engineer was able to create and solve a model and graphically show the improvements in the heating process – and before the customer left, he had made a commitment to order the next process improvement.
In general, though, the most attractive aspect of COMSOL Multiphysics is that it sets no limits on our R&D. It lets us see new approaches and opportunities, such as when we used it to develop the technology upon which RocTool’s success depends. We can get by with far fewer prototypes because the models allow us to understand all the 3D phenomena going on inside this sophisticated process. Besides, running these COMSOL models is actually a lot of fun, especially when we can impress our clients as much as we do.
Author Biography
The modeling grouop at RocToll: from left, Damien Perrier, Rémi Hemous and José Feigenblum
Dr José Feigenblum has been the Research and Development Manager of RocTool in Le Bourget du Lac Cedex, France since 2004, after having acquired a Ph.D. in inductive processes. He is in charge of all technical developments at the company in areas such as induction, thermal, mechanical and mainly processes for plastic transformation.
Read the research paper at:
www.comsol.com/industry/papers/2577/