Case Studies

PULSE™ GX50 BMWi3 case study

BMW i3 door sill PULSE™ GX50 unpainted interior

Insights developed through close collaboration with BMW

Abstract

With PULSE™ GX50, Trinseo Automotive has introduced an optimized PC/ABS product. This new product is suitable for most interior applications due to its high impact strength at low temperatures combined with good resistance to heat distortion. Furthermore, the PC content was reduced, thus making PC/ABS components not only lighter in weight but also more economical to produce. Along with its low density, the product’s easy processability also contributes to creating low-weight parts with optimized wall thickness.    

More importantly, a unique feature for these PC/ABS blends is the integrated low-gloss surface characteristics, which means the usual painting step for PC/ABS components can be eliminated. This approach has been successfully demonstrated for the first time for numerous interior components in the new BMW i3 Series. BMW chose unpainted PC/ABS from Trinseo for the following visible components in their i3 series: A-pillar trim, trunk sill, clove box, inner panel, side panels, front panel, rear panel, trim part, arm rest, doorsill, rear seat trim.

BMW i3 door sill PULSE™ GX50 unpainted interior

Introduction

Trinseo Automotive already has a strong position in the automotive market with its PULSE™ PC/ABS resins. The main attributes of these high-performance resins are their high heat resistance and low temperature ductility. PC/ABS is typically selected as the material of choice for interior components in premium segment cars. Being an amorphous polymer, it facilitates a high-quality interior design through excellent dimensional stability and good post-operation performance. In addition, the high strength and ductility over a broad temperature range guarantees that it meets the most stringent part performance requirements, including airbag deployment and crash exposure. Typical applications include instrument panel retainers, knee bolsters, glove boxes and center- and mid-consoles in premium segment cars. These injection-molded parts are often covered with paint, foam and skin, decorative cloth or laminates.

The continuous drive to reduce costs has resulted in consumers demanding easier flowing, lower-density and lower-cost resins. Such lower-viscosity resins provide the opportunity for manufacturing cost optimization. Since most PC/ABS interior parts are painted because of the typical high-gloss appearance associated with the use of PC/ABS, it is clear that opportunities to eliminate the paint step will also be of interest.

The abovementioned market demands were taken into consideration for developing a next-generation PC/ABS formulation for automotive interior components.

Polymer science in PC/ABS as a basis for PULSE™ GX50

The immiscible PC/ABS polymer blend is a very successful thermoplastic material for engineering because of its high heat resistance and synergistic low-temperature toughness. The heat resistance and toughness attributes are primarily a result of the polycarbonate (PC) phase. However, the PC itself has a relatively high viscosity, which can be improved to some extent by reducing the molecular weight. As shown in Figure 1, ABS is an effective flow promoter for the PC/ABS blend due to its shear thinning behavior at the high shear levels typical for an injection-molding process. The rubber in the ABS is the essential component for providing outstanding high-impact performance at low temperatures. This unique characteristic toughness performs better than PC and ABS components separately. The rubber morphology of ABS resins produced via mass polymerization in particular has been proven to provide best-in-class ductility at low temperatures in PC/ABS blends.

PULSE™ GX50 BMW i3 viscosity vs. shear rate

Figure 1: Shear thinning for ABS and PC

Because of ABS's strong shear thinning effect, a minimum PC level is beneficial when targeting easy-flowing PC/ABS grades. For this reason, the first step in flow optimization is to minimize the PC content to the level needed to meet the practical heat requirements for parts. For European OEM material specifications, this minimum PC level was typically around 60% for standard PC grades and 75% for high-heat grades - although this high level is over-designed for most interior parts.

Overview of PULSE™ GX50 development criteria 

The end result of the development of PULSE™ GX50 is a resin that provides a ductile performance at -30°C in the falling dart test and improved practical flow of 15% over the well-know PULSE™ A35-105 grade material. In addition, the density of the new product is reduced by ~3%. The heat resistance performance is also reduced with the optimization of the PC content, but it is still able to meet practical heat exposure requirements of 100º to 110ºC. In fact, this temperature range covers requirements for almost all interior parts.

Figure 2 shows the fit of PULSE™ GX50 as a high-ductility material in between HHABS grades and the prevalent PC/ABS grades. These PC/ABS grades traditionally contain over 60% PC, providing heat-resistance properties much greater than the practical needs for most applications. Yet the high PC levels were needed to assure low-temperature ductility.

PULSE™ GX50 BMW i3 Filling the gap with good low-temperature ductility performance

Figure 2: Filling the gap with good low-temperature ductility performance

Figure 3 shows the outstanding retention of low-temperature ductility at the high practical flow levels typical for PC/ABS blends.

PULSE™ GX50 BMW i3 Ductility vs. flow

Figure 4 shows the additional density benefit of the blend with decreasing PC level.

PULSE™ GX50 BMW i3 Low density with optimized PC content

Figure 4: Low density with optimized PC content

Another unique feature obtained from the mass ABS-rich PC/ABS formulation is its low-gloss performance. Both the semi-continuous morphology of the blend and the mass ABS rubber morphology provide a unique low-gloss appearance for molded surfaces.

The low viscosity of the ABS-rich formulation will further help improve reproducing the micro-roughness as present on the tool surface. This means that in certain cases the expensive painting step is no longer necessary. The low-gloss level obtained is an improvement on the current benchmark low-gloss PULSE™ 920MG PC/ABS grade. Figure 5 shows how the PULSE™ GX50 gloss performance compares to the prevalent grades.

PULSE™ GX50 BMW i3 Low gloss performance

Figure 5: Low-gloss performance

A critical performance characteristic for any new product is its durability in practical use. This can include color stability with UV exposure and resistance to scratching or marring, not to mention being able to retain critical physical properties after long-term aging in severe environmental conditions. PC/ABS can be sensitive to PC molecular-weight breakdown under heat exposure in the molding process and during long heat aging, especially in humid conditions. Developing a new UV-stabilization package in combination with the inherent high stability of the mass ABS used in PULSE™ GX50 UVB has resulted in a very stable product that retains its high impact performance after the environmental cycle tests.

Tooling design support for BMWi3 unpainted visible parts

Non-uniform surface esthetics for an unpainted part are caused by variations in the surface's micro-texture. Assuming the polymer composition is fixed for the application, these non-uniform esthetics can be caused by differential reproduction or damage to the micro-texture due to the part's design, the tool construction, the manufacturing process or a combination of the above. Thus special attention must be paid to all three aspects in order to achieve satisfactory unpainted surface esthetics (i.e. smaller yellow area).

Part-Design, Processing and Tool Construction

Part design

The process of constructing a part's geometry has a decisive impact on the possibility of achieving an outstanding unpainted part. Therefore it is critical that part and tool designers acknowledge that the part will be unpainted and simultaneously understand the part's visible surfaces. As a result, they can avoid surfaces that angle too steeply, can place ribbing on non-visible surfaces and apply a styling feature to an opening in order to "camouflage" resulting weld lines as several examples.

Calculated volumetric shrinkage

Figure 6: The above images show the calculated volumetric shrinkage for two types of rib structures. The honeycomb design shows lower shrinkage, thereby suggesting that it is less sensitive to sink marks.

Tool construction

Carefully constructing a mold is the second most important step towards achieving an attractive unpainted injection-molded part. Awareness of tool design requirements for unpainted parts is essential to avoid expensive tooling modifications at a later stage of development. For instance, such characteristics are used to place ejectors, date dials, graphics, etc. on invisible surfaces. In the case of ejectors, using non-circular shapes allows them to be placed on the part's edges. On the core side of the part, the surface should be flush across ejectors, sliders, etc., while moving cores need to be well embedded to avoid movement. Gate positioning and/or flow leaders define a weld line's quality and length. A weld line will be visible if formed by flow fronts meeting in a frontal manner (as can be seen in Figure 7). In case these flow fronts meet at an angle (see Figure 8), the resulting weld line will be much shorter or possibly disappear entirely.

 Weld-line formed by two flow-fronts meeting frontally.

Figure 7 (left): Weld-line formed by two flow-fronts meeting frontally. Figure 8 (right): Applying a flow promoter into the BMW i3 tool helped to improve its aesthetic quality and reduced its length.

Processing

Proper part and tool design will guarantee the molding of an attractive, low gloss part while allowing a broad processing window. The molding process offers few means to overcome defects caused by incorrect part and/or tool design. Better, more uniform surface esthetics are achieved by using profiled injection, while over-packing is moderately applied to those areas closer to the gates. Weld lines on larger parts can be avoided by using the sequential gating technique. In order to avoid visible switchover marks, valve gate controllers are offered that allow these gates to be opened gradually.

Calculated filling pattern for the long BMW i3 doorsill trim part

Figure 9: Above images show the calculated filling pattern for the long BMW i3 doorsill trim part using the sequential gating approach.

Summary

Producing an excellent unpainted plastic part requires efforts from all contributing parties. Ideally, all parties - OEM, tier-1 supplier, mold maker and material supplier - work together from the very start of the design process. This collaborative approach increases the chances of creating an attractive, unpainted car interior at an approximately 40% lower cost as compared to the equivalent painted part. The collaboration between Trinseo and BMW on the new i3 is an example of how collaboration can result in true innovation for automotive interior components. The newly developed PULSE™ GX50 provides the desired combination of good low-temperature ductility - one that is maintained after the environmental testing cycles - good UV resistance and low gloss, making it the preferred fit for multiple unpainted components in the BMW i3 interior.

For more informations, contact the authors of this case study:

Norwin van Riel, Technology Leader Automotive TS&D,  nvanriel@trinseo.com

Berend Hoek, Senior Development Specialist, bhoek@trinseo.com

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