Conductive Polymers and Plastics in Industrial Applications Part 14 pdf

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 sp

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 matemate-rial 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

Figure 2 Cellular phone test housing with

1-mm-thick walls.

Table 1 Summary of results for drop impact testing

Days

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+

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 60oC and 82oC and melt temperatures of 271oC and 293oC 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 82oC/293oC, 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

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 60oC/271oC 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.2This 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;

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.

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 reducshield-ing 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 concon-stants 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 C1, C2 are material constants and I1, I2 are strain invariants, is a special case of the Ogden model,2,4

i i

m

=

∑αµ λα λα λα

1

whereλ λ λ1, 2, 3are the stretch ratios andα µi, i the material constants For a two-term Ogden model (m=2) with α1 =2,α2 = −2 and

µ1 =2C1, µ2 = −2C2, the Ogden and Moo-ney-Rivlin models become equivalent The above functions are in their original incom-pressible forms The actual functions employed

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

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)

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.

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

bipolaron 2 - 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