Lightweight design remains one of the automotive industry's most important topics. Trinseo has gained a great reputation for expertise in replacing metal parts with innovative lightweight plastic solutions. One of our latest lightweight material developments is the new mineral-fiber-filled PULSE™ XT PC/ABS series designed for large off-line painted exterior components. Mineral-filled PULSE™ XT grades offer the chance to reduce the weight of vehicles in order to improve emissions and handling.
Trinseo's expert team has recently created a technical case study on the mineral-fiber-filled PULSE™ XT material development and first applications. Read the report and learn how your products can benefit from our innovative material solution.
Mineral-fiber-filled PC/ABS blend designed for large off-line painted exterior components
A newly developed mineral-fiber-reinforced PC/ABS satisfies all OEM requirements for large, painted, exterior components. It delivers a very low coefficient of linear thermal expansion value (CLTE), which ensures high dimensional stability as needed for low gap designs. The low CLTE value is combined with a low density of only 1.25 g/ccm, which facilitates the manufacturing of lightweight components and potential cost optimization.
In addition to low density, the material offers very good flow properties that not only permit thin walls but also guarantee a high degree of design freedom, even with large, long moldings. The good flow also decreases cycle times and contributes to a cost-efficient production process.
The new substrate material has proven to provide a smoth, high-quality class A surface of the final component after painting. The good paint adhesion meets the most stringent OEM requirements, such as climate aging. Overall, the new mineral-fiber-reinforced PC/ABS formulation has proven its technical suitability and is a cost-efficient alternative to metal for exterior automotive applications. These improved properties of the material will help accelerate the trend to plastic exterior panels in connected and autonomous vehicles, since they allow much better pass-through of various communication and lighting signals compared to metal body panels.
The exterior components of the car are usually coated with a body paint. This means the carrier material has to meet a number of basic requirements that are laid down by the OEM and tier 1 supplier in the technical specifications for the material producer. For example, plastics used to produce visible exterior components must be able to provide a high-gloss, class A surface after the painting step. These surface properties must be guaranteed to withstand constant weathering effects and stresses, as they are in service for ten years or more. At the same time, high impact resistance, strength, and stiffness are further important quality criteria that the carrier material needs to satisfy.
The molded part must be produced to tight tolerances and have the greatest possible dimensional accuracy and stability to ensure a high-precision fit and small clearances when installing the component in its intended location in the vehicle. Notwithstanding the above criteria, the weight of the molded part must also be reduced to achieve goals such as fuel efficiency and low CO2 emission values. The carrier material must therefore offer a high degree of design freedom for the component so that large and complex shapes can be produced economically. Finally, in view of the globalized material streams and production lines in the automotive sector, it is important to ensure worldwide security of supply for the carrier material over a number of years.
New material development
CLTE and density
In the material selection process, physical properties are clearly not the only factor. Other than the standard physical properties, real-life part performance tests need to be satisfied. These tests consider the influence of weathering and real-life use. The demanding requirements in terms of precision fit and processing quality, which together determine appearance, functionality and durability, can be achieved with a low CLTE of less than 4x10-5cm/cm/°C. For plastic automotive components, an amorphous polymer blend such as PC/ABS is often used. However, these materials do not provide the necessary dimensional stability for all applications without the addition of a filler. By choosing a suitable filler, the CLTE value and stiffness can be adjusted and adapted without jeopardizing the required ductility.
Figure 1 shows the influence of the new mineral fiber filler and other fillers such as talc or glass fibers on the strength and toughness of the PC/ABS used.
The choice of the right filler is the most important lever for adjusting the ductility of a filled PC/ABS polymer blend. The additional objective is to achieve the required high dimensional stability and stiffness with the lowest possible material density. Since large-sized exterior components are targeted, the achievable weight savings is considerable. The reduction of weight high up in the vehicle will also benefit vehicle handling. Low material density was therefore given a high priority in the PC/ABS blend development. The targeted CLTE value has a direct influence on the density of the material through the filler type, filler level and polycarbonate content.
Figure 2 explains how the CLTE and density correlate and how optimized performance is achieved with the new filler technology as well as through optimization of the PC content. The new mineral-fiber-reinforced PC/ABS grade, called PULSE™ XT7215, contains only 15% filler and provides a lower density and CLTE value than existing grades with 20% filler.
Fillers will – to a certain extent – cause anisotropic CLTE results for the final component. This fact can be used to advantage when designing large, long exterior applications such as roof rails or spoilers. Figure 3 shows the dependence of the CLTE results on the size of the injection molded test specimen: the superior low CLTE performance inflow on a plaque in comparison to an unfilled PC/ABS and a competitive PC/SAN with 20% filler level. It is also clear from figure 3 that the lowest CLTE values are obtained on ISO tensile bars.
Class A surface quality and durability
Figure 2 shows that a low material density at a targeted CLTE level can also be provided by a glass-fiber-reinforced PC/ABS. However, in figure 1 it was shown that such a carrier material has a greater tendency to brittleness and also lacks the desired smoothness for a high-gloss class A surface. A mineral-fiber-reinforced PC/ABS behaves differently and has the added advantage over the glass-filled variant of producing an injection-molded component with a smooth, high-gloss surface. A smooth, pitting-free surface is an essential requirement if components are to be painted and ultimately achieve a very high-quality class A surface. In addition, applied paints must adhere long-term to the component, as verified in aging and weathering tests as well as intensive abrasion and stone-chip tests. The newly developed 15% mineral-fiber-reinforced PC/ABS meets the most stringent OEM paint adhesion requirements.
For example, paint adhesion tests after severe climate aging conditions against the BMW norm GS94007 had positive results. As such is PULSE™ XT7215 released in BMW’s approval list GS93016. In addition to the BMW approval it has been fully validated and approved to FCA Paint Performance Standard PF-10677 when used with the approved paint materials listed on MS-PZ-2-1 and applied in accordance with the appropriate process standard PS-7051. Toyota has validated the material to Material Standard TSM5526G-2 and it passed all Toyota Paint Performance Requirements for exterior application.
A main contributor for the performance of the newly developed PC/ABS grade is the use of MAGNUM™ ABS. The MAGNUM™ ABS grades are produced with a continuous mass polymerization process that provides high-impact performance and also ensures excellent durability of the product after severe climate aging conditions. Another key aspect is the reduced PC level, which helps to decrease density and increase the material’s practical flow. The selected PC level assures that sufficient heat and impact resistance are maintained.
The reduction of the stiffness over a certain temperature range can be compared in a dynamic mechanical analysis (DMA). The elastic modulus obtained from this analysis is plotted in figure 4. A comparison is made to PULSE™ XT9215 with higher PC content, the unfilled PULSE™ 2000EZ, and a plain high-heat ABS MAGNUM™ 3416. The elastic modulus of the new mineral-filled PC/ABS resin shows higher values until 120˚C in comparison to PULSE™ 2000EZ. This result reinforces the confidence level that the heat resistance of the new resin meets the final part requirements since PULSE™ 2000EZ is a proven solution for large exterior components (e.g. spoilers). A higher level of PC content than in the PULSE™ XT9215 formulation is not necessary to provide the required heat resistance for the targeted exterior applications.
In addition to the DMA storage modulus, the flexural modulus is also measured for temperatures up to 120˚C. The results in figure 5 show that a high stiffness level of more than 2000 MPa is still achieved at 100˚C.
PC/PET grades have been used successfully for many years for large dimensionally stable exterior painted components. However, mineral-filled and impact-modified PC/SAN grades have gained popularity due to better dimensional stability and lower component weight. Improved dimensional stability is obtained through the use of 100% amorphous polymers. The final weight saving obtained with the newly developed PC/ABS resin is 8% vs. the 20% mineral-filled PC/PET grades and 4% vs. the 20% mineral-filled PC/SAN grade.
In addition to the weight saving, the new resin also provides very good practical flow properties, which is reflected in a significant reduction of injection pressure. This facilitates the production of large parts on machines with lower clamping force. In practice, this also means a reduced molded-in stress component and faster cycle times.
As listed in table 1, the stiffness, impact and CLTE of the new grade is equivalent or better in comparison with the benchmark materials.
Mold design and flow simulation
For the production of components with class A surfaces, it is particularly important to pay close attention to the tool design. Location of the gates and the optimal filling sequences will determine surface quality and part dimensions. Trinseo’s Application Development and Design Center (AEDC) team supports customers by carrying out mold-filling simulations and FEA analysis. A common objective is to ensure a homogeneous filling with low molded-in-stress surfaces. Figure 6 shows the influence of different processing parameters on the dimensional performance including effects of shrinkage and flow orientation.
The new mineral-fiber-filled PC/ABS resin has gone through multiple large exterior part qualification efforts with very positive results. These parts are painted in the exterior body's colors or an accent color such as black or silver. Although a broad processing window in the molding process is obtained at injection pressures up to 25% lower than the current material solutions, care is needed to ensure proper drying of the resin. Positive results are obtained when drying the resin for 5 hours at 120˚C in a desiccant dryer with dew point of -40˚C.
Increasing demand is expected for plastic outer panels in future car designs offering smart lighting options and connection signal transparency. The case studies below describe some actual examples of the use of the PULSE™ XT7215 resin.
A full validation took place on the Wrangler JL cowl vent, and FCA approval was obtained against material standard MS-DB-300 CPN5155. All vehicle color combinations were tested and validated to paint performance standard PF-10677. For this specific application, the request from the OEM's Material Engineering Group was a high tensile modulus material for metal replacement, and our material met the requirement. In addition to the high tensile modulus, the material met other requirements such as low CLTE and good paintability.
The roof rail is a typical application where the use of the new mineral-fiber-filled PC/ABS shows strong performance. The material is approved by several OEMs for this application, and commercial use will start soon on a high-volume J-OEM vehicle model.
Compared to the current mineral-filled PC/PET material, PULSE™ XT7215 allows for substantially reduced injection pressures and more accurate dimensional results.
Roof trim and bow
PULSE™ XT7215 is implemented on the new BMW 5 Series roof trim seal. The material is approved in BMW’s material list GS93016 and met all requirements against the paint performance specification GS94007.
A new mineral-fiber-filled PC/ABS grade was developed to fulfill the part performance needs of large painted exterior parts. The main requirements for the plastic substrate of such parts are stable dimensions with low CLTE and a smooth surface finish with high compatibility to the exterior paint systems. The class A surface finish needs to be maintained during the lifetime of the vehicle. The required paint adhesion performance is simulated in stringent OEM test requirements which include severe climate aging conditions. The new developed substrate material has proven to withstand those stringent paint adhesion requirements.
The additional benefits provided are easy flow and low density compared to other PC compounds used in such applications. The selection of a mineral-fiber filler allowed a reduction of the reinforcing agent content by 25% compared to competitive mineral filler solutions, while maintaining equivalent dimensional performance. In combination with an optimized PC content, both a reduced density level and improved practical flow level are also achieved. This facilitates the production on machines with lower clamping force and also results in reduced molded-in stress levels and faster cycle times.
For more details and further information on our mineral-filled PULSE™ XT PC/ABS or our portfolio for automotive lightweight design, please contact Frank Schumann, Global Marketing Manager, Automotive at email@example.com