Conductive Polymers and Plastics in Industrial Applications Part 14 pdf

telecommunications industry. These housings were dropped a total of 18 times from a height of 1.5 meters onto a concrete surface at room temperature. The 21.5 gram housings were weighted with a 160 gram internal steel plate secured on 6 bosses, to simulate the influence of internal components on impact. Any crack in the housing was considered to be a failure. All housings were preconditioned for a minimum of 2 weeks at 50% relative humidity. Results of this testing are summarized in Table 1. The data generally show that: • The unfilled and 15% glass-filled nylon 6 materials consistently passed this test; • Nylon with 30% or higher glass-filler levels passed this test about half the time; • The two grades of PC/ABS did not pass this test. Failures generally occurred at cracks along the weldline on the bottom of the part. These results are consistent with tests performed in our laboratory on commercial thin-wall hous- ings. PROCESSING COMPARISON OF SIMULATION WITH EXPERIMENT Filling of the 1-mm-thick housing shown in Figure 2 was simulated for the nylon 6 IM0 mate- rial and the PC/ABS-1 HF material. This simulation was performed using the Multilaminate Filling Analysis from Moldflow of Australia, Ltd. As shown in Figure 3, the predictions (lines) are in reasonable agreement with injection pressure data (solid symbols) obtained us- ing a data acquisition system to measure pressure at the sprue. 270 Conductive Polymers and Plastics Figure 2. Cellular phone test housing with 1-mm-thick walls. Table 1. Summary of results for drop impact testing Days Nylon 6 IM0 pass pass 14 Nylon 6 IM2 20%glass pass 27 Nylon 6 IM2 15% glass pass pass 19,21 Nylon 6 IM1 15% glass pass pass 15,16 Nylon 6 IM2 33% glass pass fail 27 Nylon 6 IM2 30% glass pass fail 26 Nylon 6 IM1 30% glass fail pass 14,15 PC/ABS-1 HF fail fail 14+ PC/ABS-2 fail 27 SIMULATION OVER A RANGE OF OPERATING CONDITIONS A series of simulations was performed to obtain predictions for injection pressure and clamp force vs. fill time for the nylon 6 IM0 compound using recommended combinations of mold temperatures of 60 o C and 82 o C and melt temperatures of 271 o C and 293 o C. The injection pressure vs. fill-time predictions in Figure 4 show the expected U-shaped curve resulting from: (a) Increase of injection pressure with injection rate at fill times that are too short for cooling to occur, and (b) Increase of injection pressure with fill time at longer fill times due to cooling in the mold. 1 The melt temperature has a much greater influence on injection pressure than does the mold temperature. Only at the longer fill times where cooling is more significant does mold temperature begin to influence the injection pressure. Similar curves are shown for PC/ABS-1 HF at its recommended processing temperature. The predictions show that this material requires anywhere from 25-35% greater injection pressure to fill than nylon 6 IM0 for the same temperatures. The processing window for nylon 6 as indicated by the simulation is fairly wide. For the highest mold/melt temperature combination 82 o C/293 o C, the injection pressure increases by no more than 10% from the minimum over injection times ranging from about 0.4 to 2.8 sec. The corresponding process window for PC/ABS- 1 HF ranges from about 0.3 to 1.2 sec. Clamp force vs. fill-time predictions are shown in Figure 5. For nylon 6, the minimum in the clamp force profile occurs at fill times of about 0.35 sec, whereas the minimum in the pressure profile occurs at fill times greater than 1. 1 sec. An examination of pressure profiles (not shown) indicates that as fill time increases, a relatively larger portion of the pressure drop occurs within the cavity relative to the runners and gate. Consequently, clamp force starts to Thin-wall Housings 271 Figure 3. Comparison of simulation vs. data for injection pressure. Figure 4. Simulation of injection pressure vs. fill time. increase with injection time at shorter fill times (0.35sec) than does injection pressure (> 1.1 sec). An examination of temper- ature profiles in the mold cavity (not shown) for the mold/melt temperatures of 60 o C/271 o C shows the highest predicted melt temperature is at the end of the cavity for fill times of 0.15 and 0.35 sec. For these short fill times, shear heating dominates cooling during mold filling. For fill times of 1.1 sec and above, the lowest predicted material temperatures occur at the last location to fill, a result which indi- cates that cooling dominates over shear heating. These observations on temperature profiles are consistent with and help to explain the observations on the injection pressure and clamp force profiles. CYCLE TIME The mold for the 1-mm housing was used to evaluate minimum cycle times for nylon 6 IM0 and PC/ABS-1 HF using surface defects as the limiting factor, the cycle times thus obtained were 11.5 sec for nylon and 15.4 sec for PC/ABS. This 25% reduction in cycle time correlates well with actual observation in molding trials across a variety of different applications. Shorter cycle times are expected for nylon 6 materials because of its favorable crystallization rate, which accelerates the increase of rigidity during cooling. 2 This results in a shorter hold- ing period with ejection from the mold able to occur much sooner than for amorphous materi- als. AESTHETICS The surface appearance of parts molded from nylon 6 compounds can be further enhanced by taking advantage of: (a) the wider processing window of nylon 6 compared to amorphous materials, especially in thin-wall housings; (b) the ability to vary the crystallization rate to create a resin-rich surface, particularly when glass-fiber reinforcement is present; 272 Conductive Polymers and Plastics Figure 5. Simulation of clamp force vs. fill time. (c) the ability to obtain a non-glossy and uniform surface, without flow lines or other imper- fections. EMI SHIELDING Nylon 6 compounds can be readily shielded by most of the common techniques currently in use by the electronics and telecommunications industry, as shown in Table 2. CONCLUSIONS In summary, nylon 6 compounds offer substantial processing and product-performance ad- vantages over amorphous materials across a wide variety of applications. These benefits be- come more pronounced with decreasing wall thickness, as is frequently the case in thin-wall housings for electronics and telecommunications. ACKNOWLEDGMENTS The authors wish to acknowledge technical discussions with: Kris Akkapeddi, Sudhir Bhakuni, Geoff Burgeson, Al Chambers, Randy Fleck, Mark Minnichelli, Bill McMaster, Clark Smith, Bruce Van Buskirk, and Robert Welgos. Molding and mechanical testing results reported in this work were performed by Rowena McPherson, Igor Palley, Juan Ruiz, Roberto Sanchez, and Robert Seville. Computational assistance was provided by Prasanna Godbole, Christopher Roth, and Craig Scott. REFERENCES 1 L. S. Turng, H. H. Chiang, J. F. Stevenson, Optimization Strategies for Injection Molding, SPE Technical Papers, 668, 41(1995). 2 R. H. Welgos, Nylon 6 and 6,6 aren't always the same, Machine Design, 55, Nov. 21, 1994. Thin-wall Housings 273 Finite Element Analysis Aided Engineering of Elastomeric EMI Shielding Gaskets Shu H. Peng and Kai Zhang Chomerics Division, Parker Hannifin Corporation, 77 Dragon Court, Woburn, MA INTRODUCTION Electrically conductive elastomeric gaskets traditionally play a very important role in shield- ing military or commercial electronic devices from EMI and reducing electromagnetic emis- sions from such devices. Ever since they were first developed some thirty-five years ago, conductive elastomeric gasket technology has been well known for its complexity. As more digital electronic devices, using higher power and faster switching speeds, are produced and deployed into the commercial world, a high performance but cost effective shielding design involving electrically conductive gaskets has become a sophisticated and challenging engi- neering task. A brief introduction to nonlinear FEA concepts and its application procedures is also presented. The Mooney-Rivlin model and the Ogden model are used to describe the highly filled electrically conductive silicone materials. FEA-assisted design examples are presented, which include deformation of a formed-in-place conductive gasket, a composite plastic/con- ductive-elastomer gasket with improved load-deflection characteristics, and a modified hollow-”D” extruded conductive gasket with an enhanced installation/attachment feature. FINITE ELEMENT ANALYSIS FEA has become an important part in the product design and prototyping processes. It allows engineers to assess the product performance before a prototype is built. Using FEA, the de- sign can be modified quickly, with much more ease and much less cost than building another prototype for testing. The effective use of FEA serves to accelerate the design process, saving engineering time and cost. In recent years, the use of FEA for the design of elastomeric products has increased sub- stantially. Most applications of elastomeric gaskets, including EMI gaskets, involve large compressive deformation. Achieving an adequate simulation accuracy requires use of an ad- vanced FEA program capable of tackling nonlinear problems, such as kinematic nonlinearity due to large deformation, material nonlinearity and changing boundary conditions. At the same time, software should also be user-friendly and efficient, which means that a FEA pro- gram should have a graphic user interface, efficient pre- and post-processors and an automatic mesh generator. A nonlinear finite element program, MARC K6, 1 was used for the static analysis re- ported in this paper. Four-node plane strain Hermann elements were used to model the gasket cross-section. The compression of the gasket was simulated using the contact elements. The plastic spacer was considered as a rigid body since it is much stiffer than the elastomer mate- rial. The Mooney-Rivlin strain energy function is used to model the gasket material. The Mooney-Rivlin model in MARC does not allow the input of bulk modulus. In order to take into account the near incompressibility of elastomeric materials, the Mooney-Rivlin con- stants are converted to the constants of the two-term Ogden strain energy function. MARC supplements the Ogden model by using the bulk modulus to account for the near incompressibility of elastomers. The Mooney-Rivlin strain energy function, 2,3 W=C 1 (I 1 -1)+C 2 (I 2 -1) [1] where C 1 ,C 2 are material constants and I 1 ,I 2 are strain invariants, is a special case of the Ogden model, 2,4 () W i i i m iii = ++− = ∑ µ α λλλ ααα 1 123 3 [2] where λλλ 123 ,, are the stretch ratios and αµ ii , the material constants. For a two-term Ogden model (m=2) with αα 12 22==−, and µµ 12 22==−CC 12 ,, the Ogden and Moo- ney-Rivlin models become equivalent. The above functions are in their original incom- pressible forms. The actual functions employed 276 Conductive Polymers and Plastics Table 1. Material constants of Cho-Seal 1310 Bulk modulus K = 1380 MPa Ogden constants µ 1 = -1.19 MPa α 1 =2 µ 2 = -3.6 MPa α 2 =-2 in FEA programs usually include an addi- tional bulk term to account for the near incompressibility. 1-2 Chomerics Cho-Seal 1310, a sili- cone-based compound filled with fine silver plated glass powder, is used as the gasket material. Table 1 lists the two-term Ogden constants and the bulk modulus of this com- pound, as obtained from material testing. FEA AIDED DESIGN OF PLASTIC SPACER GASKET The product designed using FEA in this work is a composite plastic/elastomer EMI spacer gasket. A spacer gasket features a thin plastic retainer frame onto which a conduc- tive elastomer is molded. The elastomer can be located inside or outside the retainer frame, as well as on its top and bottom surfaces. The gasket is used as a grounding device between the EMI shielded housing of a cellular phone handset and its interior printed circuit board. It is a new approach to designing EMI gaskets into handheld electronics. A sketch of the gasket is shown in Figure 1 and its cross-section profiles in Figure 2. One of the design requirements in this type of application is that the gas- ket needs to deflect under a low compressive closure force. Using FEA, a sophisticated spacer gasket design was optimized to provide satisfactory deflection under low closure force, while also ensuring proper electrical/mechanical contact area to guarantee EMI performance. EMI Shielding Gaskets 277 Figure 1. A schematic diagram of a plastic spacer gasket. Figure 2. Typical spacer gasket cross-section profiles. Figure 3. FEA simulated gasket shapes after the gaskets are installed. (a) Existing design (b) An improved design (c) The optimized design. An existing design (a) was analyzed using FEA. The deformed shape after the compres- sive installation is shown as exhibit (a) of Figure 3. The straight horizontal lines at the top represent the flat mating surfaces, at positions before and after installation. The original gas- ket profiles before installation are indicated by solid curvature lines. The stress in the vertical direction is shown in colored contour plot. The mesh lines represent the finite elements used. The predicted closure load as a function of deflection is illustrated in Figure 4. The clo- sure force for this existing design was too high and needed to be decreased significantly to meet the application requirement. After many design trials using FEA, only a limited number of design ideas proved to be effective in reducing the closure force. The best approach seemed to be modifying the gasket shape in such a way that the top portion of the gasket bends during installation. Exhibit (b) of Figure 3 shows the original and the deformed shapes of an improved design (b). A bending mechanism clearly existed after the design modification from the existing design. This bend- ing mechanism led to a much reduced closure force, as indicated in Figure 4. Design (b) met the design requirement in terms of the closure force. However, the gasket top tilts away from the plastic spacer and into the interior and may interfere with the circuit board, which should be avoided. Further efforts led to the final design as shown as Exhibit (c) of Figure 3. A detailed com- parison of design (b) and design (c) is illustrated in Figure 5. The shape of the gasket top was modified to reverse the direction it tilts when compressed. The plastic spacer was also rede- signed. The corner was cropped to increase the thickness of the elastomeric part on the gasket at that location, thereby further reducing the closure force, as indicated in Figure 4. Design (c) 278 Conductive Polymers and Plastics Figure 4. FEA predicted load-deflection curves for the initial design (a) an improved design (b) and the optimized design (c). Figure 5. Comparison of the improved design (b) and the optimized design (c). also feature some other advantages over design (b): more stable interface contact, larger con- tact area and less tearing of the elastomeric part. Following the FEA-assisted design, prototype spacer gaskets were produced. Those parts met the application requirements of closure force and EMI shielding during perfor- mance trials. The design was approved for production without additional prototyping. This example clearly demonstrates the value of advanced simulation technologies, such as FEA, in designing better products with reduced prototyping time and cost. Using finite ele- ment analysis, one can accurately predict and simulate a gasket in use, and reduce the possibility of a poor gasket design even before prototyping, which may cause poor EMI shielding performance. REFERENCES 1 MARC K6.2, User Information, MARC Analysis Research Corp., Palo Alto, California, 1996. 2 S. H. Peng and W. V. Chang, A Compressible Approach in Finite Element Analysis of Rubber- Elastic Materials, J. Comput. & Struct., 62(3), 1997. 3 R. G. Treloar, The Physics of Rubber Elasticity, Third Edition, Clarendon Press, Oxford, 1975. 4. Ogden, Nonlinear Elastic Deformation , Ellis Howard Limited, New York, 1984. EMI Shielding Gaskets 279 A abrasion 211 accelerator 36 actuators 115 adhesion 195 adhesives 123 AFM 202, 206 aging 96 alcohols 19 alloy 198 aluminum 195 amorphous 11, 28 Anderson localization 4 anisotropy 65 anticorrosion 1 anti-electrostatic 231 antistatic 209 association 22 Avrami exponent 154 Avrami rate constant 157 B batch mixing 77 batteries 201 bipolaron2-3 black bag 241 blending 88 blends 43, 51, 77, 181, 193, 219, 268 C capacitance 203 capacitors 167 capillary rheometry 236 carbon black 43, 51, 57, 77, 87, 184, 210, 219, 227 carboxylic acids 19 cellular phones 268 cesium sputtering 36 chain conformation 111 mobility 154 chaotic mixing 78, 86 charge transfer 203 chromophores 189 clamp force 271 clamshell container 239 coatings 201 compatibilizer 182 composites 43, 62, 95, 147, 153 compression molding 36 conducting pathways 77 conductive blends 1 coatings 259 conductivity 1, 40, 45 conjugated double bonds 69 conjugated polymers 135 contact angle 39 contamination 211 conversion 105 copper 4, 146, 195, 259 core 231 core thickness 235 corrosion 7, 201 Coulombic repulsion 31 counter ion 31 creep 268 creep compliance 59 critical loading 212 critical volume fraction 60 Index [...]... Carlo 61 Mooney-Rivlin model 275 morphology 37, 43, 47, 51, 77, 164, 177, 183, 210 Mott’s equation 96 Mott’s model 4 multiphase structure 51 I immersion 204 impact strength 269 impedance 203 induction time 21 initiator 95 injection molding 147 , 216, 231, 271 injection velocity 233 insulating resist 109 insulators 1 interaction 20 interchain distance 111 interlocking structure 48 iodine 4, 6 ion beam... 59 resorcinol 20 response 175 RFI 143 rubber 57 rust 195 Index S scanners 268 scattering 27 intensity 16 peaks 15 SEM 52, 81, 94, 140 , 164 semiconductors 1, 69 semi-crystalline polymer 35 sensors 173, 177, 201 shear modulus 58 rate 77 stress 59 silane 174 silver 146 , 259 skin layer 231 soliton 2 solvent resistance 36, 38 specific gravity 35 spherulites 37, 153 spin-casting 70 stainless steel 146 static... d-spacings 15 dynamic mechanical properties 57 E ecological impact 243 elastomers 57 electrochemical synthesis 99 electrodeposition 117 electro-hydrodynamic reactor 99 electroless plating 260 electron charge 109 electronics packaging 239 electropolymerization 100, 115 electrostatic discharge 209, 225 electrostatic painting 181 Index emeraldine 1, 3, 6, 11, 14, 17, 19, 110, 128, 136 EMI 7, 61, 93, 96, 143 ,... leucoemeraldine 1, 3, 11 lithography 109 H hardness 39 HDPE 46 heat of fusion 22 heating time 71 humidity 248 hydrogel 117 hydrogen 37 bonding 22, 111, 128, 135 M mass transfer coefficient 104 matrix 93 mechanical properties 165, 213 melt 153 melt flow index 214 melting 49 point 35 membranes 201 metal particles 153 metal-coated substrates 146 metallic powders 77 metallocene 35 mica 234 microcircuits 121 microcracks...282 crosslinking 130 cryomicrotome 44 cryostat 29 crystalline packing 13 crystallinity 4, 17, 54 crystallization 21 kinetics 153 crystallography 25 cube blend 148 D degradation 127 dehydration 17 density 131 dielectric constant 6 diffraction spectra 12 diffractometer 29 diffusion 203 dispersions 100 displays 201, 253 dopant 2, 95, 110, 127 doping 2 - 3, 26 dose 35, 38 - 39 Drude’s... network 60, 156 284 nickel 146 , 153 nickel-coated graphite 62 Nielsen model 155 NIR 70, 194 NMR 110, 129 non-linearity 189 nucleation 153 O Ogden’s model 275 oligomerization 100 optical polymers 189 optoelectronic devices 189 orientation 26, 64, 133 oxidant 95, 160 oxidation potential 163 P packaging 122, 245 automation 242 packing fraction 62 pagers 268 painting 181 paracrystalline disorder 5 Pauli’s... EMI shielding 1 encapsulation 55, 121 energy level 35, 37 epoxy 124, 197 ESR 168 ethylene oxide 247 exciton 111 exotherm 130 explosions 245 extruder 155 extrusion 77, 147 F failure strength 132 fatty amines 246 Fermi’s level 4 - 5 fiber 127, 129, 147 fiber conductivity 62 filaments 147 film 203 finite element analysis 275 flakes 147 , 155 flexural modulus 268 flocculated structure 48 flow instability... poly(p-phenylene vinylene) 2 poly(p-phenylene) 2 polypropylene 43, 62, 153, 210, 219 polypyrrole 4, 69, 93, 106, 115, 159, 201 polystyrene 35, 77, 95, 106, 210 polythiophene 93, 115, 173, 201 polyvinyl alcohol 117, 190 polyvinyl chloride 69, 240, 259 powders 147 product design 277 protonic acid 19 PU 165 PVDF 46 pyrrole 70, 170 Q quinoid ring 11 R Raoult's law 163 reaction time 163 recycling 243 residual... foils 260 fouling 101 friction 39 - 40 fringe micelles 31 FTIR 20, 164 FT-Raman 70 functionalization 110 G gas emission 36 gas sensing systems 119 gaskets 275 GC-MS 168, 170 Index 283 gelation 26, 135 gel-inhibitors 127, 135 glass fibers 216, 267 goniometer 27 GPC 110, 138 graphite 181, 260 growth centers 153 L lamella 153 laminates 260 Langmuir-Blodgett film 253 lattice defect 4 leucoemeraldine 1, 3,... steel 202 steric constraints 109 steric stabilizer 99 - 100 stress 121 styrene 94 surface properties 35 resistivity 73, 209 roughness 193 tension 54 syndiotactic 35 synthesis 110 T tantalum 167 TEM 106 285 temperature 44 testing 144 Tg 190 TGA 70, 74, 131, 165 thermal conductivity 155 thermoformable films 259 thermoforming 241, 262 thermogravimetric analysis 27 thin-wall housing 268 transducer 173 transistors . 37 bonding 22, 111, 128, 135 I immersion 204 impact strength 269 impedance 203 induction time 21 initiator 95 injection molding 147 , 216, 231, 271 injection velocity 233 insulating resist 109 insulators. used as a grounding device between the EMI shielded housing of a cellular phone handset and its interior printed circuit board. It is a new approach to designing EMI gaskets into handheld electronics to increase the thickness of the elastomeric part on the gasket at that location, thereby further reducing the closure force, as indicated in Figure 4. Design (c) 278 Conductive Polymers and Plastics Figure