Plastics with Unidirectional Reinforcement for the Series
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When it comes to lightweight solutions, materials like steel and aluminum have little more to offer in the way of optimization potential, but fiber- reinforced plastics do. Thermoplastics, in particular reinforced with tapes made of continuous fibers, offer great potential. A new process is finally paving the way for this material to enter mass production.
Beset with challenges ranging from Dieselgate to vehicle bans in German cities, the automotive sector is under immense pressure to cut vehicle emissions. No more than 95 g/km of CO2 is what the vehicles of the future will be allowed to emit - from as early as 2020, if the European Union has any say in it. To meet this target, automotive OEMs have two basic options at their disposal for conventional-drive vehicles: they can improve engine efficiency or reduce vehicle weight. With engineers working intensely on both issues, weight reduction is an area that still offers considerable potential - particularly given the advent of fiber-reinforced plastics. Lightweight and sometimes even cheaper than steel or aluminum, these plastics perform just as well as the established materials on stability. Manufacturing processes are still in their infancy but they are getting better all the time.
UD Tapes Add Stability to Components
The addition of reinforcing tapes has established itself as the ideal way of manufacturing parts made from fiber reinforced thermoplastics. These long, thin tapes consist of glass or carbon fibers oriented lengthwise along the tape and embedded in a thermoplastic polymer. Because of the alignment of the fibers, they are also referred to as unidirectional tapes (UD tapes). The aviation industry has already been using these tapes for a number of years to manufacture extremely lightweight yet stable components.
Fiber thermoplastic composite parts are manufactured in a highly automated process today. Cycle times of 2 to 3 min are common. But engineering firms and plastic parts manufacturers are working on shortening cycle times even further to save time and money. After years of intensive R&D, however, the levers for achieving any additional reductions are few and far between, and no-one had previously thought of shaking up the two-stage process of preforming and welding.
Two Become One
Now, however, a new innovation combines the two steps in one. Toray, a Japanese advanced materials technology company specializing, among others, in fibers, plastics and composite materials, has developed a system that does completely without the preforming stage. In its new hybrid injection molding system, there is no need for the preheating furnace or an extra mold in which to preform the UD tape. The preform is manufactured and the welding done in a single step in one tool. Besides Toray Engineering, Japan Steel Works was also involved in the development. The technology is already fit for mass production, as Toray demonstrated at the International Plastic Fair 2017 in Japan, when it ran the system for the five days of the fair with no problems whatsoever.
In-house tests have shown that cycle times can be dramatically reduced with the hybrid method. It took just 40 s to produce a component in the demo machine at the Plastic Fair. Bigger parts do take slightly longer - a car seat back frame, for example, takes 2 min. Compared with the conventional production process, which involves manufacturing and assembling numerous steel parts, this represents a dramatic improvement.
Added Material Improvements
The hybrid process, however, is just one of the elements that has led to this improvement. For the process to be possible in this form at all, the developers had to make a number of other optimizations. One of them concerns the UD tapes. Unlike conventional tapes, the tapes used here are already fully impregnated and can thus be placed straight in the mold. Users must additionally impregnate conventional tapes with thermoplastic before they are ready for preforming. Toray's tapes are impregnated with carbon fibers and a special plastic in a 50:50 ratio. For a 3-mm-thick sheet consisting of 2.4 mm of polyamide 6 with 45 % glass fibers and a piece of 0.3-mm-thick UD tape from Toray added on top and bottom, the flexural strength is around 50 % higher than for the same part with conventional UD tape. Irrespective of the specific production process, the structures are thus much more stable.
Less Weight, Same Costs
Toray's hybrid method offers a solution to this cost/benefit dilemma. Here, too, the seat back frame weighs 20 % less, but the costs are about the same as in the conventional steel option - even though the material costs actually amount to double the cost of the corresponding steel. It is the lower process costs and the easier assembly that create the significant savings.
Software Package Simplifies the Design Process
Before putting a part into production, the automotive industry normally runs a thorough check on its mechanical properties. In this Computer-Aided Engineering (CAE) phase, the engineers usually work with at least three programs: one software for the injection molding, another software to predict the material's properties and a third software to run the structural analysis. These programs cost more than just time and money. There is always the risk of problems occurring in one of the software packages that then impact the other steps in the program.
This offers clear advantages for design engineers: All of the preparatory work for the hybrid injection molding process is fast, easy, accurate and cost-effective, and the best solution for the problem at hand can always be found. Starting by inputting the requirements that the part needs to meet, the engineer can then model the part structure and choose the best thermoplastic matrix and the most suitable UD tape. The software also helps engineers decide how many layers of tape are needed and in which direction they should be applied in order to withstand the calculated loads. As such, the final barrier to the full establishment of fiber-reinforced thermoplastics would appear to have been eliminated.
Hybrid injection molding has the potential to give fiber-reinforced plastics their breakthrough in lightweight construction. The benefits of the method are not hard to see in light of the improvement in thermoplastics and UD tapes combined with the shorter engineering process. And particularly in the automotive industry, where the EU's target for reduced carbon dioxide emissions is ramping up the pressure on OEMs, automakers will be forced to act. Even design engineers, who have been known to take a conservative approach to new materials, can be expected to want to capitalize on what the new technology offers. |