1999-01-3177 Using CAE to Guide Material Selection ...

SAE TECHNICAL PAPER SERIES

1999-01-3177

Using CAE to Guide Material Selection Process in Automotive Interior Applications Kumar B. Kulkarni, Jack Chu and Ravi Thyagarajan Visteon Automotive Services

Reprinted From: 1999 IBEC Proceedings CD-ROM

International Body Engineering Conference and Exposition Detriot, Michigan September 28–30, 1999 400 Commonwealth Drive, Warrendale, PA 15096-0001 U.S.A.

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1999-01-3177

Using CAE to Guide Material Selection Process in Automotive Interior Applications Kumar B. Kulkarni, Jack Chu and Ravi Thyagarajan Visteon Automotive Services Copyright © 1999 Society of Automotive Engineers, Inc.

to soft IPs. At the same time polypropylene compounds with improved energy absorption characteristics, melt flow, scratch resistance, and dimensional stability are being increasingly considered for hard IPs. The biggest challenges faced by interior design engineers in this new era of material substitution is to achieve the same without sacrificing any functionality requirements.

ABSTRACT The increased focus on cost reduction remains one of the major interests of the global automotive industry in general and of interior systems suppliers in particular. This emphasis is heightened due to globalization and expansion of automotive OEMs in their product line, so that they may participate and compete in lower priced niche markets. The cost of plastic components in the automotive interior is about $500 per vehicle, of which a significant portion is material cost alone. Low cost materials hitherto not considered traditional autoplastics are making inroads due to the advancements in the interior component manufacturing technology.

IP FUNCTIONALITY REQUIREMENTS More often than not, most of the OEMs define their systems, their function and design requirements. Since IP system includes various subsystems such as driver information, control, safety, comfort, etc., the set of those definitions and functionality requirements are quite extensive and a detailed discussion of those is beyond the scope of this paper. However, a few of those for which CAE tools are widely used during their development are worth noting.

This paper describes the process of material selection for IPs using Computer Aided Engineering (CAE) tools to evaluate their functional requirements, such as noise vibration and harshness (NVH), sunload deformations, and safety performance. Various candidate materials were considered and CAE analyses were performed to evaluate a truck IP’s functionality requirements. Although materials with higher moduli are preferred for their stiffness, they tend to have diminished dimensional stability as well as scratch resistance. Further CAE analysis was performed to determine the optimum modulus to meet NVH and sunload requirements. A material with the resulting modulus was then evaluated for crash requirements and matched against the original design performance.

Some of the IP functionality requirements for which tools and methodologies are readily available for CAE can be broadly classified into two categories, namely: 1. FEDERAL REQUIREMENTS FMVSS 201 (Head Impact) – When an instrument panel that is within the head impact area is impacted by a 6.82 Kg, 0.165 m diameter head form the deceleration of the head form shall not exceed 80g continuously for more than 3 milliseconds at:

INTRODUCTION

a. A relative velocity of the head form of 19.2 Km/h for vehicles equipped with passenger side airbag.

The instrument panel (IP) is perhaps the most expensive and complex system in the automotive interior, both from appearance and functionality viewpoints. The plastic IP substrate which houses the structure underneath, climate comfort ductwork, modules such as radio, airbags, etc. is sometimes covered with foam and skin to provide “soft” look and feel. Such traditional soft skinned IPs are being replaced by either a laminate injection molded (LIM) construction or a mold-in-color “hard” IP construction. IP construction based on PC/ABS with a high degree of parts integration appear to offer cost savings compared

b. A relative velocity of the head form of 24.3 Km/h for all other vehicles. FMVSS 208 (Knee Impact) – Axial load through each upper leg of an unbelted hybrid III dummy must not exceed 10 KN in the event of a frontal collision with a fixed barrier perpendicular to the line of travel of the vehicle, or at any angle up to 30 degrees in either direction from the perpendicular to the line of travel of the vehicle. 1

Typical examples of "soft" IP include Thermoplastic Olefin (TPO) or Thermoplastic Urethane (TPU) retainers with vinyl and polyurethane or polypropylene foam for skins. Most common examples for retainer (substrate) materials are SMA, PC, ABS, PP, etc.

SYSTEM DESIGN REQUIREMENTS Static Stiffness and Strength – The IP structure, various IP mounted subsystems, the housing panel (substrate) including those finish panels which have the potential to come in contact with the customer, should be able to withstand various types of loads. Usually these loads and the allowable deflections are detailed in the OEM's system design requirements.

The materials used for hard IPs was dominated by PC, ABS or PC/ABS blend in North America in the past 30 years, mainly because those materials have excellent stiffness and impact balance to meet the IP functionality requirements. With the advancement of both materials and IP design itself, more polyolefins (I.e., polypropylene/ polyethylene copolymer) are used in interiors in recent years.

Sunload Performance – The allowable deformation of IP when exposed to high surface temperatures due to the sunload is another performance indicator which is usually set and agreed upon by the interior supplier and the OEM.

There are a number of benefits for using polyolefins for automotive interior applications. The unfilled polyolefin has a lower density than most other engineering plastics. This is especially important for OEMs to meet their weight reduction targets, and improve on their CAFÉ ratings. Polyolefins also has sufficient weatherablity with appropriate UV stabilizer so that it can be used moldedin-color instead of expensive and time consuming painting. It is apparent that this would reduce painting cost and also reduce Volatile Organic Compounds (VOC) emission. Polyolefins are also very resistant to most cleaning solvents, which would substantially help in reducing warranty and reliability concerns. Finally, polyolefins cost relatively lower compared to other engineering plastics. With these advantages, there is a strong push from OEM's to increased use of polyolefins for automotive interiors.

Noise Vibration and Harshness (NVH) – As noted before, IP is a major customer interface whose quality can significantly affect a customer's perception about the vehicle. Numerous studies have shown that squeaks and rattles are the primary IP quality concerns and about 30% of all squeak and rattle "Things Gone Wrong" (TGW) are IP related. Studies also have shown that IP quality problems consisting of squeaks and rattles are directly related to low frequency vibration excitation (