Lightweight Design worldwide

, Volume 11, Issue 5, pp 54–57 | Cite as

Plastics with Unidirectional Reinforcement for the Series

  • Yoshito Kuroda

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.

OEMs and their suppliers have long pinned their hopes on thermoplastics. Offering substantial benefits when used to make mounting components within the vehicle such as front-end brackets or tanks, they are lighter and enable faster cycle times than conventional steel or aluminum parts, or even thermosetting plastics, Figure 1. Indeed, they may one day replace those materials almost totally. This is particularly true of Continuous Fiber(CF)-reinforced thermoplastics - a polymer matrix with an embedded continuous fiber, which may be glass or carbon. In production, thermosetting plastic parts can take up to 10 min to mold. Fiber-reinforced thermoplastics, on the other hand, can go through the heating, handling and cooling process in as little as 3 min. With roughly similar investment costs for both production methods, thermoplastics can be a very attractive option, especially for large batch numbers.

Figure 1 Unidirectionally reinforced, duroplastic plastics have considerable weight benefits (© Toray)

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.

Unlike prepregs and resin matrix systems, UD tapes are easily integrated in automated manufacturing processes. The tapes are supplied in rolls that look like big rolls of Scotch tape, Figure 2. First, the right length of tape is measured, trimmed off the roll and laid in a preform. For larger components it is possible to lay different lengths of tape side by side or even laterally across one another. The preform and the tape are placed in a furnace for preheating before being molded to make the tape assume the approximate shape of the preform. Once preformed, the tape is inserted in a second mold where it is welded with a thermoplastic material - and the hybrid component is ready.

Figure 2 Arriving in large rolls, the UD tapes can easily and accurately be portioned for each component (© Toray)

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

Though shorter cycle times and lower costs are important for manufacturers, what really matters in the end is whether or not the component displays significantly better mechanical properties than the same component without UD tape. Here, too, the hybrid process offers a convincing performance, doubling the part's flexural strength and increasing the flexural modulus as much as fivefold. For the tests, a polyamide component with 30 % glass fibers (PA6/GF30) was compared with a component in the same material onto which Toray's UD tape had been added on both sides, Figure 3 (left). The difference is even starker when the component is exposed to heat and moisture. In wet conditions at room temperature or in dry conditions at 80 °C, the flexural modulus is reduced by 50 % in the standard component, whereas in the UD tape-reinforced component it goes down to no less than 80 %, making the values eight times better overall, Figure 3 (right).

Figure 3 Hybrid structures show improved flexural strength (left) and flexural modulus (right) and are less affected by moisture and high temperatures (© Toray)

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

Users will wonder whether the improved mechanical properties might come at the cost of higher weight for the tape - after all, the aim of using fiber-reinforced plastics is to reduce weight. Tests on a complex shaped beam have shown that there is no such trade-off. When a beam weighing 1350 g is loaded with 100 kg on the cantilevered edge, it displaces by 4.5 mm. If you want it to be stiffer and only bend 3.8 mm, the beam needs to be thicker, weighing 1500 g. However, replacing 20 g of plastic with 20 g of UD tape will give you the same level of displacement, while the beam still weighs 1350 g. That is a weight saving of 10 %, Figure 4. What does all of this mean for manufacturers? Running through the example of a complex component such as the back frame of a car seat can illustrate the consequences quite well. Built the conventional way, a car seat back frame consists of seven steel parts. Built with the use of UD tape, on the other hand, there is only one part to the component, and it is also 20 % lighter. So when it comes to weight saving and functionality, fiber-reinforced plastics are clearly a step ahead. However, using the conventional molding process with separate steps for the preform and the welding stage, costs are a good 40 % higher. This is due to both the higher material costs and the higher process costs. In such a price-squeezed industry as the automotive sector, this price premium is so significant that an OEM may decide against the UD tape version in spite of the weight advantage it offers, or they may decide to use it only in their premium models. The rule of thumb in the automotive industry is that a cost of up to about ten euros is acceptable for each kilogram of weight shaved off the total.

Figure 4 Replacing 20 g of plastic with UD tapes increases stiffness by 20 % (© Toray)

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.

But when all of the process steps are performed in one go, the three programs can also be integrated in a single system. And that is exactly what Toray Engineering has achieved with the development of 3D Timon. The proprietary software encompasses the injection molding stage, the prediction of the material's properties and the anisotropic structural data in one. Combining fiber analysis results and test data on tensile strength, Figure 5, it can, for example, calculate the anisotropic properties of the component. It additionally features an input/output interface for different structural analysis systems, such as LS-Dyna or Abaqus, and can model all of the physical aspects that come up in the engineering process - from flow analysis to fiber orientation and distribution.

Figure 5 Toray uses drop towers like this to test energy absorption and to validate simulated data (© Toray)

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.

The Upshot

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. |

Copyright information

© Springer Fachmedien Wiesbaden GmbH, part of Springer Nature 2018

Authors and Affiliations

  • Yoshito Kuroda
    • 1
  1. 1.TorayMünchenGermany

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