DESIGN AND ANALYSIS OF A PLASTIC DOOR MODULE FOR CAR BODY APPLICATION

Abstract DESIGN AND ANALYSIS OF A PLASTIC DOOR MODULE FOR CAR BODY APPLICATION S. Vinay Seeba 1, S. Srikari 2, V. K. Banthia 3 1- (Engg.) Student, 2- Professor, 3- Professor and Head of Department Department of Automotive Design and Engineering M. S. Ramaiah School of Advanced Studies, Bangalore Door module is preassembly of various components on a carrier plate which can be directly mounted onto the door inner panel. These door modules, when directly shipped to the OEM s, save assembly time and cost in mass manufacturing. Long fiber reinforced thermoplastic polypropylene (PP-LFT) with glass filled fiber material is used for various automotive applications as it has improved structural and material properties over conventional materials like steel. Door modules made of PP-LFT material is manufactured using injection molding method. Although significant work was done on design and analysis of plastic door modules, there was no support information on how the final design of the door module was arrived at. Topology and shape optimisation were used in the present investigation to arrive at a design. 30 % glass filled STAMAX PP-LFT door modules were used to replace the existing steel door module plate. Stress analysis and dynamic door slam analysis were carried out on the door module of new design to assess its performance under various durability loading conditions. Finally Mold flow simulation was performed on the door module part to check its manufacturability in mass manufacturing. Final design of STAMAX PP-LFT based plastic door module is 33 % lighter than the existing steel door module plate and has improved strength, stiffness and manufacturability. Key Words: Plastic Door Module, Door Slam, Mold Flow Analysis, STAMAX Nomenclature E V ω ρ Young s Modulus, MPa Linear velocity, m/s Angular Velocity, rad/sec Density, tones/mm3 Abbreviations PP-LFT Long fiber reinforced thermoplastic polypropylene. 1. INTRODUCTION Doors are one of the major components in a car which provide easy access for passengers into the car. With the growing demand on car styling, comfort, safety and other systems integration (window regulator, latch, speaker, motor and electronics) in the door, designing this system is a great challenge to engineers. Door system mainly consists of window glass, window regulator assembly, door latch, sealing and structural components of the door assembly. Traditionally these parts were designed, manufactured and procured separately. A door module is an assembly of functional elements mounted onto a carrier plate. Unlike conventional door systems, where the window regulator assembly was directly attached to the door inner panel, the door module comprises of a carrier plate with window regulator assembly, glass motor and speaker. The window regulator consists of a motor assembly, one or two rails to guide the glass motion, cursor or glass clamps to support the glass, and mechanisms to move the glass up and down. The window regulator, speaker, and other wire harnesses are mounted on the carrier plate using bolts, rivets, and clips. Detailed figure of door module assembly is shown in Figure 1. The carrier plate is bolted to the inner panel. This module approach helps the car makers in reducing assembly time and hence cost. Thus, design and manufacture of door modules is very important. Fig. 1 Door module A door module should perform the following functions Window regulatory function Latch function Speaker function Sealing function A door module offers several advantages over conventional door systems. Some important advantages include [1]. Higher structural strength Better sealing against water vapour and hence door modules are called Sealed Carrier Systems Better noise insulation Dry side mounting of motor and door electronic components Weight savings and hence cost savings due to reduced sheet metal on the door inner panel SASTECH 1 Volume 9, Issue 1, April 2010

Reduced original equipment assembly operations as the module is directly bolted to the door inner panel 2. PROBLEM DEFINITION The present day cars use steel door modules for door structure. Steel door modules are heavier, costlier for assembling and require more time during assembly of doors due to non-integration of components. Literature review presented a great amount of research on door module design and analysis, application of plastic material for automotive structures like door modules. From literature study it was found that there were no references on how the final design of plastic door module was arrived. For example Won- Jong Noh et al. [2] required 8 design iterations to arrive at the final design of plastic door module to replace existing steel door module. The disadvantages with the steel door module and design methodology of door module development provided an opportunity to explore alternate design process methodology and application of plastic material for door module in car body application. 2.1 Definition of the Problem To design and analyze a plastic door module for car body application to achieve a weight reduction of 20% compared to existing weight of steel door module. 2.2 Objectives 1. To review literature on door design, development of door modules, durability assessment of door designs 2. To benchmark steel door module for strength and durability requirements 3. To design a plastic door module to meet the application requirements 4. To assess the plastic door module with respect to durability and manufacturability. Achieve at least 20% weight saving over existing steel door module 2.3 Methodology 1. Design of door module was understood using literature, journals, books and other reference media. 2. Surface model of existing front door assembly with steel door module plate was created using CATIA-v5-r17 using reverse engineering. 3. Benchmark values for existing door assembly under durability loads were arrived by performing stress analysis using Altair HyperMesh, Ansys, and LS-Dyna. 4. Plastic door module to replace existing steel door module was designed with help of optimisation using Altair Optistruct v9.0 and CATIA-v5-R17. 5. The plastic door module was assessed for durability using Altair HyperMesh, Ansys and LS- Dyna to demonstrate its performance enhancements over existing steel door. 6. Mold flow analysis was performed using Autodesk mold flow analysis tool to assess mass manufacturing capabilities and to check the warping/distortion on plastic door module. 3. BENCHMARKING OF STEEL DOOR MODULE 3.1 Geometric Model of Front Door Assembly The geometric model of front door assembly was generated by reverse engineering. Existing steel front door module plate with window regulator mechanism was measured in the benchmarking lab. Using the measured data and available CAD drawings for other parts of front door the geometric model of the door assembly was created. 3.2 Finite Element Modelling of Front Door Assembly The geometric model of the front door assembly was descritised into small finite elements to analyse the structure. All the components were modelled using shell elements at mid plane. Finite element model of door module, glass rails and front door assembly is shown in Figure 2. Since door module pull and glass motor stall analyses are local phenomenon only door module and glass rails were included for the study [3, 5] Entire front door assembly was included for door slam analysis [3] to understand the effect of assembly components on door module plate and glass rails under dynamic loads. Door seals and latch striker interaction were modelled as load versus displacement curves using non-linear discrete spring in LS-Dyna [4]. Curve plot for door seals and latch striker interaction are shown in Figure 3 Fig. 2 Finite Element model of front door Based on the component geometry, the thickness properties were applied to the corresponding shell elements. All the components of the door module were assigned with high strength low alloy SAE J2340 300 X grade steel materials [5, 6]. Detailed material property is shown in Figure 4. SASTECH 2 Volume 9, Issue 1, April 2010

(a) Fig. 5 Boundary condition for door module pull and glass motor stall analysis (b) Fig. 3 Curves used for (a) door seal and (b) latch striker behavior For the door slam analysis body side hinges, ends of body side seal springs and striker were constrained in all translational and rotational degrees of freedom. The ends of seal springs on the door side were constrained in vertical translation (Uz) and fore-aft translation (Ux) degrees of freedom [3]. 3.4 Model Set up for Solution Door module pull loads are experienced on the door module plate when the passenger pulls the door handle to close the door. The door module plate was subjected to load of 200 N in the lateral direction (Y-axis) of the vehicle co-ordinate system as shown in Figure 6 [1, 2]. Fig. 4 HR SAE J2340 300X material property Various components of the front door assembly were connected using rigid elements at bolt and weld locations. Contact between mating parts were represented using Automatic Single Surface contact in LS-Dyna. 3.3 Boundary Conditions Since door module pull and glass motor stall analyses are local phenomenon only door module plate and glass rails are included for the analysis. The location where the door module plate is bolted to the door inner panel was constrained in all translational and rotational degrees of freedom as shown in Figure 5. Fig. 6 Loading condition for door module pull analysis Fig. 7 Loading condition for glass motor stall load analysis SASTECH 3 Volume 9, Issue 1, April 2010

Fig. 8 Loading condition for door slam analysis When the window glass reaches full up or full down position the motor is under maximum torque due to winding of the drum. At this position due to maximum torque the motor stalls. During stall, rails and door module on which the rail is mounted experience maximum loads [3]. Loading condition for glass motor stall load analysis is shown in Figure 7. Slam load is experienced on the door module when the door is closed with a bang. In the present study, door assembly was subjected to a slam speed of 1.7 m/s (1.9 rad/s). Loading condition for door slam analysis is shown in Figure 8 [3] Table 1. Summary of benchmark reference parameters for steel door module 4. DESIGN OF PLASTIC DOOR MODULE USING OPTIMISATION Plastic door module plate was designed using optimisation method in Altair OptiStruct software [7]. Topology and shape optimisation were used to arrive at the final design of the door module plate. 3.5 Results and Discussion For the door module pull analysis maximum displacement was observed in the pull handle region. For the glass motor stall analysis maximum displacement was observed for front rail and rear rail bottom stall loads cases. Maximum von-mises stress was observed for front rail and rear rail bottom stall load cases. For the door slam analysis maximum von-mises stress was observed near rear rail lower mount when the door strikes the latch. The results of door module pull, glass motor stall and door slam analysis for existing steel door module are summarised in Table 1. These values are the benchmark reference parameters for plastic door module design. Fig. 9 STAMAX 30YM240 material property 4.1 Material Selection for Plastic Door Module Long fiber reinforced thermoplastic materials like STAMAX have excellent material properties with improved stiffness, strength and manufacturing process advantage [1, 2, 8, 9]. STAMAX 30YM240 with 30% glass filled 12 mm long fiber is used for the study. The material has Young s Modulus of 7000 MPa and break strength of 109 MPa at 23 C [10]. Detailed material property values for STAMAX 30YM240 are shown in Figure 9. SASTECH 4 Volume 9, Issue 1, April 2010

4.2 Topology Optimisation Topology optimisation is carried out to find the load path for material distribution of plastic door module design. For the door module plate available packaging space was measured to be 48 mm. Design and non design space was defined in the available packaging space. Base thickness was assumed to be 2 mm [11,12, 13]. Detailed plot of packaging space, design space and non design space is shown in Figure 10. Fig. 11 Critical load path plot from topology optimisation Fig. 10 Topology optimisation parameters Design constraints for optimisation parameters like displacement, mass fraction, and stiffness were defined using responses to achieve the benchmark reference parameters tabulated in Table 1 for plastic door module plate. Objective for the topology optimisation study was to maximize stiffness of the plastic door module part. Topology optimisation was carried out for door module pull loads, glass motor stall loads, motor and speaker self weight loads. Fig. 12 Geometry output from topology optimisation 4.2 Results for Topology Optimisation The results from topology optimisation are shown in Figure 11 and 12. Figure 11 shows the element density plot which identifies the material distribution in the critical load path for the design which satisfies the design objective for the design constraints specified. Figure 12 shows the geometry output from topology optimisation processed using OSS smooth in Altair OptiStruct. Fig. 13 Shape optimisation parameters SASTECH 5 Volume 9, Issue 1, April 2010

4.3 Shape Optimisation Topology optimisation provided geometry layout for the plastic door module plate and location for reinforcements on the plastic door module plate to achieve the performance parameters defined in design constraints. Shape optimisation was necessary to define proper shape of the rib topology. Final shape (height) of the reinforcement ribs on the plastic door module carrier plate was fixed by carrying out shape optimisation in Altair OptiStruct software. Shape optimisation was carried out by defining rib geometry shape as domain and handles as shown in Figure 13. Design variable for shape optimisation was defined by varying the rib height of domains at handle locations. Design variable for the rib shape was set as (-1, +1). Design constraints, loads and boundary conditions were similar to the design constraints used for topology optimisation with respect to displacement values. Design objective for the shape optimisation problem was to minimize the mass. 4.4 Results for Shape Optimisation The results from shape optimisation are shown in Figure 14. It shows the final rib pattern with minimized mass of ribs overlaid on original rib pattern. Final weight of the ribs after shape optimisation was 43.7 grams compared to initial weight of 61.1 grams, 28% reduction in rib weight due to shape optimisation. 4.5 Plastic Door Module Plate Final design of plastic door module plate based on topology and shape optimisation results was generated using CATIA V5R17. Plastic door module plate has thickness of 2mm for the plate base and ribs. Pull handle support bracket was integrated to the door module plate. Rear rail lower mount was provided with double screws since the mount had least fatigue life as per benchmarking Fig. 14 Final rib shape obtained from shape optimisation reference parameters listed in Table 4.1. Final design of the plastic door module plate is shown in Figure 15. Final weight of the plastic door module plate is 1.2 kg. Table 2. Fatigue life data for STAMAX 30YM240 STAMAX 30YM240 estimated failure stress [MPA] at 23 C Cycles, n Failure Stress, MPa 1 85.0 3,000 55.0 20,000 46.3 40,000 44.8 100,000 42.4 Fig. 15 Plastic door module 5. VALIDATION AND RESULTS Plastic door module was assessed for door module pull, glass motor stall and door slam loads to verify its performance so that it can replace the existing steel door module plate. Fatigue life data for STAMAX 30YM240 is shown in Table 4.2. SASTECH 6 Volume 9, Issue 1, April 2010

Since only the design of door module plate was changed in the front door assembly modelling details, section properties, contact and connection definitions, loads and boundary condition remain same for door module pull, glass motor stall and door slam analysis. Mold flow analysis was performed on the plastic door module plate to check for its manufacturability by defining the mold material inlet gating locations as shown in Figure 16. 5.1 Results and Discussion For the door module pull analysis maximum displacement of 3.8 mm was observed in the pull handle region. This value is less than 3.9 mm observed for steel door module pull analysis. For the glass motor stall analysis maximum displacement was observed for front rail and rear rail top stall loads cases. Maximum von-mises stress was observed for front rail top stall and rear rail bottom stall load cases. Maximum von-mises stress on the glass rails with plastic door module plate was less than the stress observed on glass rails with steel door module plate. Maximum displacement on glass rails for stall analysis was 4 mm with plastic door module plate compared to 2.4 mm for steel door module plate. However there was no interference of the glass rails with any other assembly part of the door due to increased Fig. 16 Gating location for material injection into mold Table 3. Summary of results for plastic door module displacement. Hence the design was assumed to be acceptable. For the door slam analysis maximum von-mises stress of 36.1 MPa was observed near rear rail lower mount during the door rebound after the door slam. Maximum von-mises stress of 22.91 MPA was observed when door hits the striker. Maximum von-mises on door module plate for door slam analysis was 36.1 MPa. Comparing the maximum von-mises stress value observed for door slam analysis with the fatigue life data for STAMAX 30YM240 material in Table 2 it was found that the material has fatigue life greater than 100,000 cycles, meeting the slam life standard to be met for door module designs [3, 11]. From mold flow analysis results it was observed that the time required to fill the door module part with STAMAX material was 3.4 s. Maximum clamp force was 750 tonnes. Maximum out of plane distortion(warpage) on the finished part was 4.9 mm. The maximum distortion observed was less than benchmark reference value of 5 mm [11]. Hence the plastic door module design was assumed to be acceptable for mass manufacturing. The results of door module pull, glass motor stall and door slam analysis for plastic door module design are summarised in Table 3 below. Sl No Load Case Max. displacement mm Max. von-mises stress MPa Fatigue life cycles 1 Door module pull analysis 3.8 20.0 N.A. 2 Glass motor 4.0 256.1 N.A. stall load analysis 3 Door slam analysis N.A. 36.1 >100,000 4 Mold Flow analysis Out of plane distortion/warpage = 4.9 mm 5 Weight of plastic door module plate 1.2 kg SASTECH 7 Volume 9, Issue 1, April 2010

6. CONCLUSION 1. The study helped in designing plastic door module plate for car body application using 30% glass fiber PP-LFT (STAMAX ) material, topology and shape optimisation techniques. 2. Final design of the plastic door module is 33% lesser in weight with improved structural, properties compared to existing steel door module plate. 3. The design and analysis methodology adopted here to design plastic door module plate to replace existing steel door module plate can be adopted to design and analyse plastic parts to replace semi structural automotive parts currently made from steel. 7. REFERENCES [1] Maria Ciliberti, Warden Schijve, Developments in Thermoplastic Door Modules SAE Technical paper series, 2003-01-0793, 2003. [2] Won-Jong Noh, Min Ho Choi, Chi Hoon Choi and Tae Won Hwang, Development of Door Module Plate with Long-fiber-reinforced Thermoplastic Polypropylene, SAE Technical Paper, 2006-01-0330, 2006. [3] Devadas Kumbla, Pan Shi, Joseph Saxon, Simulation methods for door module design, SAE Technical Paper, 2005-01-0883, 2005. [4] LS-Dyna User Manual, Version 971, 2006, Livermore Software Technology Corporation. [5] Kalpak Shah, Hong Tae Kang and Upendra Deshmukh, Design of Dual Sliding Door for a Small Size Car and Its Validation Using CAE Tools, SAE Technical Paper, 2007-01-0889, 2007. [6] HR SAE J2340 300X HSLA steel material properties, www.asp.org/database/custom/hss_stampingdesignmanu al.pdf, 24 th June, 2009. [7] HyperWorks User Manual, Version 9.0, 2009, Altair Incorporation, www.altair.com [8] Takahiro Tochioka, Development of integrated Functions Module Carriers by Injection Molding with Long Glass Fiber Reinforced Polypropylene, SAE Technical Paper series, 2003-01-2810, 2003. [9] Manish Chaturvedi, Warden Schijve, Matthew Marks, Advanced Thermoplastic Composites for Automotive Semi Structural Application, SAE Technical Paper, 2009-26-086, 2009. [10] Unanimous, http://plastics.sabic.eu/_scripts/gradeselector.pl? product=196c0e98-0f0e-4fd4-a476-51f83d2b8888&template=product, 15 th August, 2009. [11] Scott E. Zilincik, Wm. Jeffrey DeFrank, and Scott G. Miller, Slam Life Assessment Method for Closures Durability SAE Technical Paper, 982307, 1998. [12] A paper entitled Application of topology, sizing and shape optimisation methods to optimal design of Aircraft components, presented at Altair engineering conference, 2003. [13] A paper entitled A new approach for sizing, shape and topology optimisation, SAE International congress and exposition, 2002, http://www.fedesign.de/fileadmin/publikationen/publikationen19 96/sae_org.pdf, 24 th June, 2009. SASTECH 8 Volume 9, Issue 1, April 2010