Designing Plastic Parts for Assembly Paul A. Tres ISBN 3-446-40321-3
Inhaltsverzeichnis
Weitere Informationen oder Bestellungen unter http://www.hanser.de/3-446-40321-3 sowie http://www.hanser.de/3-446-40321-3 sowie im Buchhandel
Contents
Introduction
1 1.1
........................ .................................... ........................ ........................ ........................ ........................ ........................ ........................ ........................ ........................ ............... ...
nderstanding Plasti stic Materials ........................ .................................... ........................ ........................ ........................ ................. ..... Underst
Basic Resins ........................ .................................... ........................ ........................ ........................ ........................ ........................ ........................ ........................ ..................... ......... 1.1.1 Thermoplastics ....................... ................................... ........................ ........................ ........................ ........................ ........................ ........................ ..................... ......... 1.1.2 Thermosets .......................................................................................................................... 1.2 Basic Structures ............................................................................................................................ 1.2.1 Crystalline............ lline ........................ ........................ ........................ ........................ ........................ ........................ ........................ ........................ ........................ ............... ... 2 2 r us A 1. . mo pho .......................................................................................................................... 1.2.3 Liquid Crystal Polymer ........................ .................................... ........................ ........................ ........................ ........................ ........................ ................... ....... 1.2.4 New Polymer Technologies ....................... ................................... ........................ ........................ ........................ ........................ ........................ .............. 1.2.4.1 Inherently Conductive Polymers ( ICP) ........ ............ ........ ........ ........ ........ ........ ........ ........ ........ ........ ........ ........ ........ ........ ........ ........ ........ .... 1.2.4.2 Electro-Optic Polymers (EOP)................................ )............................................ ........................ ........................ ........................ ..................... ......... 1.2.4.3 Biopolymers ........................ .................................... ........................ ........................ ........................ ........................ ........................ ........................ ..................... ......... 1.3 Homopolymer vs. Copolymer ...................................................................................................... 1.4 Reinforcements........................ .................................... ........................ ........................ ........................ ........................ ........................ ........................ ........................ ................. ..... 1.5 Fillers ........................ .................................... ........................ ........................ ........................ ........................ ........................ ........................ ........................ ........................ ............... ... 1.5.1 Glass Sphere....................... e................................... ........................ ........................ ........................ ........................ ........................ ........................ ........................ .............. 1.5.1.1 Microphere Properties ....................... ................................... ........................ ........................ ........................ ........................ ........................ ................... ....... 1.5.1.2 Compounding ........................ .................................... ........................ ........................ ........................ ........................ ........................ ........................ ................... ....... 1.5.1.3 Injection Molding............ olding ........................ ........................ ........................ ........................ ........................ ........................ ........................ ........................ .............. 1.5.1.4 Mechanical Properties in Injection Molded Thermoplastic Applications ...... ......... ...... ...... ...... ...... ...... ... 1.6 Additives ....................... ................................... ........................ ........................ ........................ ........................ ........................ ........................ ........................ ........................ ............... ... 1.7 Physical Properties ........................ .................................... ........................ ........................ ........................ ........................ ........................ ........................ ....................... ........... 1.7.1 Density and Specific Gravity ....................... ................................... ........................ ........................ ........................ ........................ ....................... ........... 1.7.2 Elasticity ........................ .................................... ........................ ........................ ........................ ........................ ........................ ........................ ........................ ................. ..... 1.7.3 Plasticity ....................... ................................... ........................ ........................ ........................ ........................ ........................ ........................ ........................ ................... ....... 1.7.4 Ductility ....................... ................................... ........................ ........................ ........................ ........................ ........................ ........................ ........................ ................... ....... 1.7.5 Toughness ........................ .................................... ........................ ........................ ........................ ........................ ........................ ........................ ........................ ............... ... 1.7.6 Brittleness ........................ .................................... ........................ ........................ ........................ ........................ ........................ ........................ ........................ ............... ... 1.7.7 Notch Sensitivity........................ .................................... ........................ ........................ ........................ ........................ ........................ ........................ ................. ..... 1.7.8 Isotropy ........................ .................................... ........................ ........................ ........................ ........................ ........................ ........................ ........................ ................... ....... 1.7.9 Anisotropy .......................................................................................................................... 1.7.10 Water Absorption ........................ .................................... ........................ ........................ ........................ ........................ ........................ ........................ .............. 1.7.11 Mold Shrink age ....................... ................................... ........................ ........................ ........................ ........................ ........................ ........................ ................. ..... r rt s 1.8 Mechanical P ope ie .................................................................................................................. 1.8.1 Normal Stress ...................................................................................................................... 1.8.2 Normal Strain................................. in............................................. ........................ ........................ ........................ ........................ ........................ ........................ .............. 1.8.3 Stress-Strain Curve ........................ .................................... ........................ ........................ ........................ ........................ ........................ ........................ .............. 1.9 Creep ........................ .................................... ........................ ........................ ........................ ........................ ........................ ........................ ........................ ........................ ............... ... 1.9.1 Introduction................................ ion............................................ ........................ ........................ ........................ ........................ ........................ ........................ ................. ..... 1.9.2 Creep Experiments .............................................................................................................. 1.9.3 Creep Curves........................ .................................... ........................ ........................ ........................ ........................ ........................ ........................ ....................... ........... 1.9.4 Stress-Relaxation ....................... ................................... ........................ ........................ ........................ ........................ ........................ ........................ ................. ..... 1.10 Impact Properties..........................................................................................................................
xvii
1 1 1 2 2 2 3 3 4 4 5 6 6 7 7 8 8 9 1 0 10 11 1 2 1 2 1 3 14 14 14 15 15 20 20 20 21 23 23 23 24 26 26 26 27 29 29
xii
C ont ents
1.11 Thermal Properties ....................................................................................................................... 1.11.1 Melting Point .................................................................................................................... 1.11.2 Glass Transition Temperature .......................................................................................... 1.11.3 Heat Deflection Temperature ........................................................................................... 1.11.4 Coefficient of Thermal Expansion ................................................................................... 1.11.5 Thermal Conductivity ...................................................................................................... 1.11.6 Thermal Influence on Mechanical Properties .................................................................. 1.11.7 Case History: Planetary Gear Life Durability ..................................................................
31 31 31 31 31 33 33 34
2
Understanding Safety Factors ...................................................................................
40
2.1 2.2
What Is a Safety Factor ................................................................................................................ Using the Safety Factors .............................................................................................................. 2.2.1 Design Safety Factors ......................................................................................................... 2.2.1.1 Design Static Safety Factor .............................................................................................. 2.2.1.2 Design Dynamic Safety Factor ........................................................................................ 2.2.1.3 Design Time-related Safety Factor .................................................................................. 2.2.2 Material Properties Safety Factor ....................................................................................... 2.2.3 Processing Safety Factors ................................................................................................... 2.2.4 Operating Condition Safety Factor .....................................................................................
4 0 4 0 4 0 41 41 41 4 2 4 3 43
3
Strength of Material for Plastics .............................................................................
44
3.1
Tensile Strength............................................................................................................................ 3.1.1 Proportional Limit............................................................................................................... 3.1.2 Elastic Stress Limit ............................................................................................................. 3.1.3 Yield Stress ......................................................................................................................... 3.1.4 Ultimate Stress .................................................................................................................... 3.2 Compressive Stress ...................................................................................................................... 3.3 Shear Stress .................................................................................................................................. 3.4 Torsion Stress ............................................................................................................................... 3.5 Elongations ................................................................................................................................... 3.5.1 Tensile Strain ...................................................................................................................... 3.5.2 Compressive Strain ............................................................................................................. 3.5.3 Shear Strain......................................................................................................................... 3.6 True Stress and Strain vs. Engineering Stress and Strain ............................................................ 3.7 Poisson’s Ratio............................................................................................................................. 3.8 Modulus of Elasticity ................................................................................................................... 3.8.1 Young’s Modulus................................................................................................................ 3.8.2 Tangent Modulus ................................................................................................................ 3.8.3 Secant Modulus................................................................................................................... 3.8.4 Creep (Apparent) Modulus ................................................................................................. 3.8.5 Shear Modulus .................................................................................................................... 3.8.6 Flexural Modulus ................................................................................................................ 3.8.7 The Use of Various Moduli ................................................................................................ 3.9 Stress Relations ............................................................................................................................ 3.9.1 Introduction......................................................................................................................... 3.9.2 Experiment .......................................................................................................................... 3.9.3 Equivalent Stress ................................................................................................................. 3.9.4 Maximum Normal Stress .................................................................................................... 3.9.5 Maximum Normal Strain .................................................................................................... 3.9.6 Maximum Shear Stress ....................................................................................................... 3.9.7 Maximum Deformation Energy .......................................................................................... 3.10 Conclusions ..................................................................................................................................
44 45 45 45 4 6 4 6 4 7 48 49 49 51 51 51 5 2 54 54 55 5 6 5 6 5 7 5 7 58 58 58 59 59 59 60 60 61 62
C ont ents
xiii
4
Nonlinear Considerations ............................................................................................
63
4.1
4.4
Material Considerations ............................................................................................................... 4.1.1 Linear Material.................................................................................................................... 4.1.2 Nonlinear Material .............................................................................................................. Geometry ...................................................................................................................................... 4.2.1 Linear Geometry ................................................................................................................. 4.2.2 Nonlinear Geometry............................................................................................................ Finite Element Analysis (FEA) .................................................................................................... 4.3.1 FEA Method Application.................................................................................................... 4.3.2 Using FEA Method ............................................................................................................. 4.3.3 Most Common FEA Codes ................................................................................................. Conclusions ..................................................................................................................................
63 63 63 64 64 64 65 65 65 66 66
5
Assembly Techniques for Plastics .........................................................................
67
Ultrasonic Welding ...................................................................................................................... 5.1.1 Ultrasonic Equipment ......................................................................................................... 5.1.2 Horn Design ........................................................................................................................ 5.1.3 Ultrasonic Welding Techniques .......................................................................................... 5.1.4 Control Methods ................................................................................................................. 5.1.5 Common Issues with Welding ............................................................................................ 5.1.6 Joint Design ........................................................................................................................ 5.1.6.1 Butt Joint Design.............................................................................................................. 5.1.6.2 Shear Joint Design............................................................................................................ 5.2 Ultrasonic (Heat) Staking............................................................................................................. 5.2.1 Standard Stake Design ........................................................................................................ 5.2.2 Flush Stake Design ............................................................................................................. 5.2.3 Spherical Stake Design ....................................................................................................... 5.2.4 Hollow (Boss) Stake Design............................................................................................... 5.2.5 Knurled Stake Design ......................................................................................................... 5.3 Ultrasonic Spot Welding .............................................................................................................. 5.4 Ultrasonic Swaging ...................................................................................................................... 5.5 Ultrasonic Stud Welding .............................................................................................................. 5.6 Spin Welding................................................................................................................................ 5.6.1 Process ................................................................................................................................ 5.6.2 Equipment ........................................................................................................................... 5.6.3 Welding Parameters ............................................................................................................ 5.6.4 Joint Design ........................................................................................................................ 5.7 Hot Plate Welding ........................................................................................................................ 5.7.1 Process ................................................................................................................................ 5.7.2 Joint Design ........................................................................................................................ 5.8 Vibration Welding........................................................................................................................ 5.8.1 Process ................................................................................................................................ 5.8.2 Equipment ........................................................................................................................... 5.8.3 Joint Design ........................................................................................................................ 5.8.4 Common Issues with Vibration Welding............................................................................ 5.9 Electromagnetic Welding............................................................................................................. 5.9.1 Equipment ........................................................................................................................... 5.9.2 Process ................................................................................................................................ 5.9.3 Joint Design ........................................................................................................................ 5.10 Solvent and Adhesive Bonding .................................................................................................... 5.10.1 Types of Adhesives .......................................................................................................... 5.10.2 Advantages and Limitations of Adhesives ....................................................................... 5.10.3 Stress Cracking in Bonded Joints ..................................................................................... 5.10.4 Joint Design......................................................................................................................
67 67 70 72 75 78 81 8 2 8 3 8 6 8 6 8 7 88 89 9 0 91 9 2 9 2 9 3 9 3 9 6 9 6 9 7 1 00 1 03 1 04 1 06 1 08 11 0 111 11 3 114 115 115 11 6 118 119 1 20 1 20 1 21
4.2
4.3
5.1
xiv
C ont ents
5.11 Radio Frequency (RF) Welding ................................................................................................... 5.11.1 Equipment ........................................................................................................................ 5.11.2 Process .............................................................................................................................. 5.12 Laser Welding .............................................................................................................................. 5.12.1 Equipment ........................................................................................................................ 5.12.2 Process .............................................................................................................................. 5.12.2.1 Surf ace Heating ............................................................................................................. 5.12.2.2 Through Transmission................................................................................................... 5.12.2.3 Staking ........................................................................................................................... 5.12.3 Techniques for Laser Welding ......................................................................................... 5.12.4 Polymers ........................................................................................................................... 5.12.5 Applications ..................................................................................................................... 5.13 Conclusion....................................................................................................................................
6 6.1 6.2 6.3 6.4 6.5 6.6 6.7 6.8 6.9
Press Fitting .........................................................................................................................
Introduction .................................................................................................................................. Definitions and Notations............................................................................................................. Geometric Definitions .................................................................................................................. Safety Factors ............................................................................................................................... ....................................................................................................................................... Creep Loads ....................................................................................................................................... Press Fit Theory ........................................................................................................................... Design Algorithm......................................................................................................................... Case History: Plastic Shaf t – Plastic Hub .................................................................................... 6.9.1 Shaf t and Hub Made of Different Polymers ....................................................................... 6.9.2 Safety Factor Selection ....................................................................................................... 6.9.3 Material Properties .............................................................................................................. 6.9.3.1 Shaf t – Material Properties at 23 ° C ................................................................................ 6.9.3.2 Shaf t – Material Properties at 93 ° C ................................................................................ 6.9.3.3 Creep Curves at 23 ° C ...................................................................................................... 6.9.3.4 Creep at 93 ° C .................................................................................................................. 6.9.3.5 Pulley at 23 ° C ................................................................................................................. 6.9.3.6 Pulley at 93 ° C ................................................................................................................. 6.9.3.7 Creep, Pulley at 23 °C ...................................................................................................... 6.9.3.8 Creep, Pulley at 93 °C ...................................................................................................... 6.10 Solutions: Plastic Shaf t – Plastic Hub .......................................................................................... 6.10.1 Case A .............................................................................................................................. 6.10.2 Case B .............................................................................................................................. 6.10.3 Case C .............................................................................................................................. 6.10.4 Case D .............................................................................................................................. 6.11 Case History: Metal Ball Bearing – Plastic Hub .......................................................................... 6.11.1 Fusible Core Injection Molding ....................................................................................... 6.11.2 Upper Intake Manifold Background ................................................................................ 6.11.3 Design Algorithm............................................................................................................. 6.11.4 Material Properties ........................................................................................................... 6.11.4.1 CAMPUS ....................................................................................................................... 6.11.5 Solution ............................................................................................................................ 6.11.5.1 Necessary IF at Ambient ............................................................................................... 6.11.5.2 IF Available at 118 °C ................................................................................................... 6.11.5.3 IF Verification at –40 °C ............................................................................................... 6.11.5.4 Stress Check at –40 ° C, Time = 0.................................................................................. 6.11.5.5 Stress Level at –40 °C, Time = 5,000 h......................................................................... 6.11.5.6 Stress Level at 23 °C, Time = 5,000 h........................................................................... 6.11.5.7 Stress Level at 118 °C, Time = 5,000 h......................................................................... 6.11.6 Conclusion........................................................................................................................
1 1 1 1 1 1 1 1 1 1 1 1 1
23 23 24 25 25 27 28 28 29 30 32 34 37
1 38 1 38 1 38 1 39 1 39 14 0 14 0 141 14 3 144 144 144 145 145 149 15 0 151 15 2 155 15 6 15 7 158 158 159 1 60 1 61 1 63 1 63 1 64 1 67 1 68 1 69 1 70 1 74 1 75 1 75 1 76 1 76 1 76 1 77 1 77
7
C ont ents
xv
Living Hinges .....................................................................................................................
1 78
7.1 7. 2 7. 3 7.4 7.5
Introduction .................................................................................................................................. Basic Design for PP, PE ............................................................................................................... Common Living Hinge Design .................................................................................................... Basic Design for Engineering Plastics ......................................................................................... Living Hinge Design Analysis ..................................................................................................... 7.5.1 Elastic Strain Due to Bending............................................................................................. 7.5.1.1 Assumptions ..................................................................................................................... 7.5.1.2 Geometric Conditions ...................................................................................................... 7.5.1.3 Strain Due to Bending ...................................................................................................... 7.5.1.4 Stress Due to Bending ...................................................................................................... 7.5.1.5 Closing Angle of the Hinge.............................................................................................. 7.5.1.6 Bending Radius of the Hinge ........................................................................................... 7.5.2 Plastic Strain Due to Pure Bending..................................................................................... 7.5.2.1 Assumptions ..................................................................................................................... 7.5.2.2 Strain Due to Bending ...................................................................................................... 7.5.3 Plastic Strain Due to a Mixture of Bending and Tension ................................................... 7.5.3.1 Tension Strain .................................................................................................................. 7.5.3.2 Bending Strain.................................................................................................................. 7.5.3.3 Neutral Axis Position ....................................................................................................... 7.5.3.4 Hinge Length.................................................................................................................... 7.5.3.5 Elastic Portion of the Hinge Thickness ............................................................................ 7.6 Computer Flow Chart ................................................................................................................... 7.6.1 Computer Notations ............................................................................................................ 7.7 Computer Flow Chart Equations .................................................................................................. 7.8 Example: Case History ................................................................................................................. 7.8.1 World Class Connector ....................................................................................................... 7.8.1.1 Calculations for the ‘ Right Way’ Assembly .................................................................... 7.8.1.2 Calculations for the ‘ Wrong Way’ Assembly .................................................................. 7.8.2 Comparison Material .......................................................................................................... 7.8.2.1 ‘ Right Way’ Assembly ..................................................................................................... 7.8.2.2 ‘ Wrong Way’ Assembly................................................................................................... 7.8.3 Ignition Cable Bracket ........................................................................................................ 7.8.3.1 Initial Design .................................................................................................................... 7.8.3.2 Improved Design .............................................................................................................. 7.9 Processing Errors for Living Hinges ............................................................................................ 7.10 Coined Hinges .............................................................................................................................. 7.11 Conclusion.................................................................................................................................... 7.12 Exercise .......................................................................................................................................
1 78 1 78 18 0 18 0 181 181 181 18 2 18 2 18 3 184 184 184 184 184 18 6 18 6 189 19 0 19 0 19 3 194 194 19 6 198 198 199 201 203 203 204 205 206 206 208 209 212 212
8
Snap Fitting ..........................................................................................................................
218
8.1 8.2 8.3
Introduction .................................................................................................................................. Material Considerations ............................................................................................................... Design Considerations.................................................................................................................. 8.3.1 Safety Factors...................................................................................................................... Snap Fit Theory ............................................................................................................................ 8.4.1 Notations ............................................................................................................................. 8.4.2 Geometric Conditions ......................................................................................................... 8.4.3 Stress Strain Curve and Formulae....................................................................................... 8.4.4 Instantaneous Moment of Inertia ........................................................................................ 8.4.5 Angle of Deflection............................................................................................................. 8.4.6 Integral Solution.................................................................................................................. 8.4.7 Equation of Deflection........................................................................................................
218 219 221 223 224 224 225 226 229 229 229 231
8.4
xvi
C ont ents
8.4.8 Integral Solution.................................................................................................................. 8.4.9 Maximum Deflection .......................................................................................................... 8.4.10 Self-locking Angle ........................................................................................................... Case History: One-way Continuous Beam with Rectangular Cross Section ............................... 8.5.1 Geometrical Model ............................................................................................................. Annular Snap Fits ......................................................................................................................... 8.6.1 Case History: Annular Snap Fit – Rigid Beam with Sof t Mating Part ............................... 8.6.2 Notations ............................................................................................................................. 8.6.3 Geometric Definitions ......................................................................................................... 8.6.4 Material Selections and Properties...................................................................................... 8.6.5 Basic Formulas.................................................................................................................... 8.6.6 Angle of Assembly ............................................................................................................. Torsional Snap Fits....................................................................................................................... 8.7.1 Notations ............................................................................................................................. 8.7.2 Basic Formulae ................................................................................................................... 8.7.3 Material Properties .............................................................................................................. 8.7.4 Solution ............................................................................................................................... Case History: Injection Blow Molded Bottle Assembly .............................................................. Tooling ....................................................................................................................................... Assembly Procedures ................................................................................................................... Issues with Snap Fitting ............................................................................................................... Serviceability ................................................................................................................................ Conclusions ..................................................................................................................................
232 232 235 235 237 240 241 241 242 242 243 244 245 245 247 248 248 250 251 252 254 255 255
Appendix A: Enforced Displacement .................................................................................................... Appendix B: Point Force ....................................................................................................................... References ............................................................................................................................................. World Wide Web References Related to Plastic Part Design ............................................................... Index ......................................................................................................................................................
257 266 276 283 287
8.5 8.6
8.7
8.8 8.9 8.10 8.11 8.12 8.13
Designing Plastic Parts for Assembly Paul A. Tres ISBN 3-446-40321-3
Leseprobe
Weitere Informationen oder Bestellungen unter http://www.hanser.de/3-446-40321-3 sowie im Buchhandel
3
Strength
of Material for Plastics
3.1
Tensile Strength
a material’s ability to withstand an axial load. In an ASTM test of tensile strength, a specimen bar (Figure 3-1) is placed in a tensile testing machine. Both ends of the specimen are clamped into the machine’s jaws, which pull b oth ends of the ba r. S tress is automatically plotted a gainst the strain. T he a xial load is applied to the specimen when the machine pulls the ends of the specimen bar in opposite directions at a slow and constant rate of speed. Two different speeds are used: 0.2 in. per minute (5 mm / min) to a pproximate the material’s behavior in a hand assembly operation; and 2.0 in. per minute (50 mm / min) to simulate semiautomatic or automatic assembly procedures. T he ba r is marked with gauge mark s on either side of the mid point of the n arrow, middle portion of the bar. As the pulling progresses, the specimen bar elongates at a uniform rate that is proportionate to the rate at which the load or pulling force increases. The load, divided by the cross-sectional area of the specimen within the gage mark s, represents the unit str ess r esist ance of the plastic material to the pulling or tensile force. T ensile str engt h is
Figure 3-1
Test specimen bar
3 .2
45
C ompr essive S tr ess
The stress (σ-sigma) is expressed in pounds per square inch (psi) or in Mega Pascals (MPa). 1 MPa equals 1 N ewton per square millimeter (N / mm2). T o c onvert psi into MPa, multiply by 0.0069169. To convert MPa into psi, multiply by 144.573.
σ
=
F A
=
TENSILE LOAD
(3.1)
AREA
) a s P s e M r ( t i
Yield po int
Ultimate st r ess
S s
p
Elastic limit
Propor tional limit
Figure
3-2 Typical stress/strain diagram for plastic materials
Str ain
%
3.1.1
Proportional Limit
The proportional relationship of force to elongation, or of stress to str ain, continues until the elongation no longer complies with the Hooke’s law of proportionality. The greatest stress that a plastic material can sustain without any deviation f rom the law of proportionality is called pr oport ional str ess limit ( Figure 3-2).
3.1.2
Elastic Stress Limit
Beyond the proportional stress limit the plastic material exhibits an increase in elong ation at a f aster rate. E last ic str ess limit is the greatest stress a material can withstand without sustaining any permanent strain af ter the load is released ( Figure 3-2).
3.1.3
Yield Stress
Beyond the elastic stress limit, f urther movement of the test machine jaws in opposite directions causes a permanent elongation or deformation of the specimen. There is a point beyond which the plastic material stretches briefly without a noticeable increase in load. This point is known as the yield point . Most unreinforced materials have a distinct yield point. Reinforced plastic materials exhibit a yield r egion.
46
S tr engt h
of
Mat er ial
for Plast ics
It is important to note that the results of this test will vary between individual specimens of the same material. If ten specimens made out of a reinforced plastic material were given this test, it is unlikely that two specimens would have the same yield point. This variance is induced by the bond between the reinforcement and the matrix material.
3.1.4
Ultimate Stress
the m aximum stress a material takes before f ailure. Beyond the plastic m aterial’s elastic limit, continued pulling causes the specimen to neck down across its width. T his is accompanied b y a f urther acceleration of the a xial elongation (deformation), which is now largely confined within the short necked-down section. The pulling force eventually reaches a maximum value and then f alls rapidly, with little additional elongation of the specimen before f ailure occurs. In f ailing, the specimen test bar break s in two within the necking-down portion. The maximum pulling load, expressed as stress in psi or in N / mm2 of the original cross-sectional area, is the plastic material’s ultimate tensile strength (σULTIMATE). The two halves of the specimen are then placed back together, and the distance between the two mark s is measured. The increase in length gives the elongation, expressed in percentage. The cross-section at the point of f ailure is measured to obtain the reduction in area, which is also expressed as percentage. Both the elongation percentage and the reduction in area percentage suggest the m aterial ductility. In structural plastic part design it is essential to ensure that the stresses that would result f rom loading will be within the elastic range. If the elastic limit is exceeded, permanent deformation takes place due to plastic flow or slippage along mole cular slip planes. This will result in permanent plastic deformations. U lt imat e str ess is
3 .2
Compressive Stress
C ompr essive str ess is
the c ompressive force divided b y cross-sectional a rea, me asured in
psi or MPa. It is general practice in pl astic p art design to a ssume that the c ompressive strength of a plastic material is equal to its tensile strength. This can also apply to some structural design calculations, where Young’s modulus (modulus of elasticity) in tension is used, even though the loading is compressive. The ultimate compressive strength of thermoplastic materials is of ten greater than the ultimate tensile strength. In other words, most plastics can withstand more compressive surf ace pressure than tensile load. The compressive test is similar to that of tensile properties. A test specimen is compressed to rupture between two parallel platens. The test specimen has a cylindrical shape, measuring 1 in. (25.4 mm) in length and 0.5 in. (12.7 mm) in di ameter. The load is applied to the specimen f rom two directions in axial opposition. The ultimate compressive strength is measured when the specimen f ails by crushing.
3 .2
C ompr essive S tr ess
47
A stress/strain diagram is developed during the test, and values are obtained for the four distinct regions: the proportional region, the elastic region, the yield region, and the ultimate (or break age) region. The structural analysis of thermoplastic parts is more complex when the material is in compression. Fa ilure develops under the influence of a bending moment that increases as the deflection increases. A plastic part’s geometric shape is a significant f actor in its capacity to withstand compressive lo ads.
σ
Figure 3-3
=
P A
=
COMPRESSIVE FORCE AREA
(3.2)
Compressive test specimen
The stress/strain curve in compression is similar to the tensile stress/strain diagram, except the values of stresses in the compression test are greater for the corresponding elongation levels. This is because it takes much more compressive stress than tensile stress to deform a plastic.
3 .3
Shear Stress
str ess is the shear load divided by the area resisting shear. Tangential to the area, shear stress is measured in psi or MPa. There is no recognized standard method of testing for shear strength (τ-tau) of a thermoplastic or thermoset material. Pure shear loads are seldom encountered in structural part design. Usually, shear stresses develop as a by-product of principal stresses, or where transverse forces are present. S hear
48
S tr engt h
of
Mat er ial
for Plast ics
The ultimate shear strength is commonly observed by actually shearing a plastic plaque in a punch-and-die setup. A ram applies varying pressures to the specimen. The ram’s speed is kept constant so only the pressures vary. The minimum axial load that produces a punch-through is recorded. This is used to calculate the ultimate shear stress. Exact ultimate shear stress is difficult to assess, but it can be successf ully approximated as 0.75 of the ultimate tensile stress of the m aterial.
τ =
Q A
=
SHEAR LOAD AREA
(3.3)
Q
Q
3.4
Figure
3-4 Shear stress sample specimen example: (a) before; (b) af ter
Torsion Stress
loading is the application of a force that tends to cause the member to twist about its axis (Figure 3-5). Torsion is referred to in terms of torsional moment or torque, which is the product of the externally applied load and the moment arm. The moment arm represents the distance f rom the centerline of rotation to the line of force and perpendicular to it. The principal deflection caused by torsion is measured by the angle of twist or by the vertical movement of one side. T orsional
Figure 3-5
Torsion stress
3.4
T orsion S tr ess
49
When a shaf t is sub jected to a torsional moment or torque, the resulting shear stress is:
τ
=
Mt J
=
Mt r I
(3.4)
Mt is the torsional moment and it is: Mt
=
FR
(3.5)
The following notation has been used: J polar moment of inertia I moment of inertia R moment arm r radius of gyration (distance f rom the center of section to the outer fiber) F load applied
3.5
Elongations
is the deformation of a thermoplastic or thermoset material when a load is applied at the ends of the specimen test bar in opposite axial direction. The recorded deformation, depending upon the nature of the applied load (axial, shear or torsional), can be measured in variation of length or in variation of angle. Strain is a r atio of the increase in elong ation by initial dimension of a m aterial. Again, strain is dimensionless. Depending on the nature of the applied load, strains can be tensile, compressive, or shear.
E longat ion
ε
3.5.1
=
∆L
L
(%)
(3.6)
Tensile Strain
A test specimen bar similar to that described in Section 3.1 is used to determine the t ensile str ain. The ultimate tensile strain is determined when the test specimen, being pulled apart by its ends, elongates. Just before the specimen break s, the ultimate tensile strain is recorded. The elongation of the specimen represents the strain ( ε-epsilon) induced in the material, and is expressed in inches per inch of length (in / i n) or in millimeters per millimeter (mm / mm). This is an adimensional measure. Percent notations such as ε = 3% can also be used. Figure 3-2 shows stress and strain plotted in a simplified graph.
50
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of
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Figure
3-6 Tensile specimen loaded showing dimensional change in length. The difference between the original length (L) and the elongated length is ∆L
Figure 3-7
Compressive specimen s howing dimensional change in length. L is the original length; P is the compressive fo rce. ∆ L is the dimensional change in length
3.5
3.5.2
51
E longat ions
Compressive Strain
The compr essive str ain test employs a set-up similar to the one described in Section 3.2. The ultimate compressive strain is measured at the instant just before the test specimen f ails by crushing.
3.5.3
Shear Strain
str ain is a measure of the angle of deformation γ – g amma. As is the case with shear stress, there is also no recognized standard test for shear strain. S hear
Q Q
Q Q
γ
Figure 3-8
3 .6
Shear strain: (a) before; (b) af ter
True Stress and Strain vs. Engineering Stress and Strain
E ngineer ing str ain is
the ratio of the total deformation over initial length. E ngineer ing str ess is the ratio of the force applied at the end of the test specimen by initial constant area. T rue str ess is the ratio of the instantaneous force over instantaneous area. Formula 3.7 shows that the true stress is a f unction of engineering stress multiplied b y a f actor based on engineering strain. = σ (1+ ε )
σ TRUE
(3.7)
T rue str ain is
the ratio of instantaneous deformation over instantaneous length. Formula 3.8 shows that the true strain is a logarithmic f unction of engineering strain. ε
= ln(1 + ε )
TRUE
(3.8)
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By using the ultimate strain and stress values, we can easily determine the true ultimate stress as: σ
= σ ULTIMATE (1+ ε ULTMATE )
ULTIMATETRUE
(3.9)
Similarly, by replacing engineering strain for a given point in Equation 3.8 with engineering ultimate strain, we can easily find the value of the true ultimate strain as: ε ULTIMATE
TRUE
= ln(1 + ε ULTIMATE )
(3.10)
Both true stress and true strain are required input as material data in a v ariety of finite element analyses, where non-linear material analysis is needed.
Initial
Reduced
ar ea
ar ea
Figure 3-9
3 .7
True stress necking-down effect
Poisson’s Ratio
Provided the material deformation is within the elastic range, the ratio of l ateral to longitudinal strains is constant and the coefficient is called Poisson’s r at io.
ν
=
LATERAL STRAIN LONGITUDINAL STRAIN
(3.11)
In other words, stretching produces an elastic contraction in the two lateral directions. If an elastic str ain produces no change in volume, the two lateral str ains will be equal to half the tensile strain times –1.
53
3.7 Poisson’s Rat io b b
L
∆L
'
b
Figure
3-10 Dimensional change in only two of three directions
'
b
Under a tensile load, a test specimen increases (decreases for a compressive test) in length b y the a mount ∆L and decreases in width (increases for a compressive test) b y the amount ∆b. The related strains are: ε LONGITDINAL
ε LATERAL
=
=
∆L
L ∆b
(3.12)
b
Poisson’s ratio varies between 0, where no l ateral contraction is present, to 0.5 for which the contraction in width equals the elongation. In practice there are no materials with Poisson’s ratio 0 or 0.5. Table 3-1 Typical Poisson’s ratio values
for different materials
Material Type
Poisson’s Ratio at 0.2 in. / min (5 mm / min) Strain Rate
ABS Aluminum Brass Cast iron Copper High density polyethylene
0.4155 0.34 0.37 0.25 0.35 0.35
54
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(Continued)
Material Type
Poisson’s Ratio at 0.2 in. / min (5 mm / min) Strain Rate
Lead Polyamide Polycarbonate 13% gl ass reinforced polyamide Polypropylene Polysulfone Steel
0.45 0.38 0.38 0.347 0.431 0.37 0.29
The lateral variation in dimensions during the pull-down test is: ∆ b = b − b′
(3.13)
Therefore, the ratio of l ateral dimensional change by the longitudinal dimensional change is: ∆b ν
= b ∆L
(3.14)
L Or, by rewriting, the Poisson’s ratio is:
ν
=
ε LATERAL
(3.15)
ε LONGITUDINAL
3.8
Modulus
of Elasticity
3.8.1
Young’s Modulus
The Y oung’s mod ulus or elast ic mod ulus is typically defined as the slope of the stress/strain curve at the origin. The ratio between stress and strain is constant, obeying Hooke’s Law, within the elasticity r ange of any material. T his r atio is called Young’s modulus a nd is measured in MPa or psi.
E=
σ ε
=
STRESS STRAIN
= CONSTANT
(3.16)
Hooke’s Law is generally applicable for most metals, thermoplastics and thermosets, within the limit of proportionality.
Designing Plastic Parts for Assembly Paul A. Tres ISBN 3-446-40321-3
Vorwort
Weitere Informationen oder Bestellungen unter http://www.hanser.de/3-446-40321-3 sowie im Buchhandel
Pref ace to the Sixth Edition
As a design engineer in the automotive industry, I have experienced firsthand how powerf ul a resource Paul Tres’ Designing Plast ic Parts for Assembl y can be. Not only does the text cover the general properties of most plastics; it also utilizes contemporary, real-world examples to illustrate how these properties impact product design. The book is an excellent reference for experienced engineers and, at the same time, it does a superb job of educating the novice designer. A noteworthy addition in the sixth edition is a new cutting-edge materials section. The b ook’s early chapters start out by defining the differences between thermosets and thermoplastics, and go on to discuss crystalline versus a morphous pl astic properties. T his sounds simple enough, but I see the effects of poor material selection in my position all too f requently, and the end result is always w asted money. With the basics defined, Tres goes on to discuss the typical mechanical and thermal properties of common plastics. This, combined with the “Strength of Materials for Plastics” overview, leads the reader into the design chapters of the book equipped with the tools needed to maximize the messages of Tres’ real-world examples. It quickly becomes obvious that spending the appropriate amount of time in the design phase of any component, especially where tooling costs are high, is critical to the f unctional and e conomi c success of the p art. Tres also does a good jo b describing the joining techniques available to present-day plastic part designers. His inclusion of the surf ace preparation requirements associated with the v arious welding techniques is of particular importance to novice designers, as it details the sometimes hidden costs associated with each selection. From here, Tres guides the reader through the proper design of press fits, living hinges and snap fits. A gain, e ach design section is reinforced with modern examples and common pitf alls. I think it’s worth mentioning that I have personally coordinated two three-day seminars with Paul at the Mercedes-Benz Training Institute, to investigate injection molding p artdesign and tooling issues introduced in the book in greater detail. I have also recommended his services to other organizations inside and outside the auto industry. Since reading the book years ago and using Paul Tres as a consultant, Designing Plast ic Parts for Assembl y has become must-have literature for anyone joining the interiors design department at Mercedes-Benz U.S. International. I have no reservations about enthusiastically recommending this book to anyone involved in the field of pl astic parts
design. Don’t let the patchy beard Paul is sporting on the jacket of this book fool you – he is truly a gif ted professional in the plastics field. Tuscaloosa, Alabama
J eff L ubbers , P. E . Mercedes-Benz U.S. International
Foreword to First Edition
Knowing well the work and many special talents of Paul A. Tres, I take delight in the opportunity to introduce his new book , Designing Plast ic Parts for Assembl y, and recommend it to a broad r ange of readers. Material engineers, design and manuf acturing engineers, graduate and under-graduate students, and all others with an interest in design for assembly or plastic components development now have a clearly written, methodoriented resource. This practical book is an outgrowth of the like-named University of Wisconsin – Madison course which is being offered nationally and internationally. Just as his lectures in the course provide a detailed yet simplified discussion of material selection, manuf acturing techniques, and assembly procedures, this book will make his unique expertise and effective teaching method available to a much larger audience. Mr. Tres’s highly successf ul instructional approach is evident throughout the book. Combining f undamental f acts with practical techniques and a down-to-earth philosophy, he discusses in detail joint design and joint purpose, the geometry and nature of the component parts, the type of loads involved, and other vital information crucial to success in this dynamic field. Treatment of this material is at all times practice-oriented and focuses on everyday problems and situations. In addition to plastics, Mr. Tres has expert knowledge in computer sof tware, having directed the development of DuPont’s design sof tware. The course at the University of Wisconsin – Madison is indirectly an outgrowth of the sof tware he designed for living hinges and snap fits at DuPont. Mr. Tres holds numerous patents in the plastics field. He is known worldwide for his expertise in computer programming, manuf acturing processes, material selection and project management on both a national and international scale. Most recently, Mr. Tres’s accomplishments have earned him the DuPont Automotive Marketing Excellence Award as well as recognition in the 1994–1995 edition of Who’s Who Worldwide. Whether you are just entering the field, or are a seasoned plastic parts designer, Designing P last ic Parts for Assembl y is an excellent tool that will f acilitate c ost-effective design decisions, and help to ensure that the plastic parts and products you design stand
up under use.
Madison, WI
Dr . D onald E . Ba x a University of Wisconsin-Madison
Pref ace to First Edition
It gives me great pleasure to write this pref ace for such an important contribution to engineering design. It is rather sad f act that while the c reative use of plastics has changed the very structure of consumer products over the past decade, m any engineering students graduate with very little knowledge of polymer engineering or plastic design principles. This book written by a recognized expert and practitioner in the field of plastic component design is both a valuable text for engineering courses and a resource for practicing design engineers. The f ull potential for the use of plastics in consumer products became recognized in the mid 1980s through the pioneering development of the IBM ProPrinter. The ProPrinter destroyed the myth, prevalent amongst product engineers at that time, that such design elements as plastic springs, plastic bearings, plastic securing elements, etc., lacked the structural integrity of their more common metal counterparts. In the ProPrinter, not only were these plastic design features shown to have the required reliability in regular use and abuse, they were combined into single parts to produce a new level of design elegance. For example, the injection molded side-f rames of the ProPrinter, which support the rollers a nd lead screw, in corporated b earings for all of these rotating members, springs to maintain the required paper pressure, and cantilever securing elements to allow the f rames to be snap fitted into the base. The result of such innovative design details produced a desk top printer which could be assembled in only 32 final assembly steps compared to the 185 steps required to assemble its main competitor in the m arketplace. Since the emergence of the ProPrinter, smart plastic design has become an essential tool in the competitive battle to produce products which have simpler structures with smaller numbers of discrete parts. Part count reduction, in particular, has been shown, through numerous case studies published over the past five years, to have a ripple effect on product manuf acture which improves the efficiency of the entire organization. Fewer parts means fewer manuf acturing and assembly steps, and fewer joints and interf aces, all of which h ave a positive effect on quality and reliability. Moreover, a reduction in the number of the parts results in a direct attack on the hidden or overhead cost of an organization. Thus, fewer parts also mean fewer vendors for purchasing to deal with, less documentation, smaller inventory levels, less inspection, simpler production scheduling and so on. Designing Plast ic Parts for Assembl y tackles all of the important issues to be f aced in designing multi-feature complex plastic parts. The book is thus much more than its title suggests. It deals with essential f undamentals for the development of competitive consumer products. Providence, Rhode Island
Dr . P et er Dewhurst Department of Industrial and Manuf acturing Engineering University of Rhode Island
Acknowledgments
A special thank you to Mr. Tom Matano, Director of Industrial Design at the Academy of Art University, San Francisco, CA, who made it possible to use the f uturistic car designs on the cover, which were created by his students John Liu and John Lazorack. The author gracef ully thank s Horia Blendea with Sc huk ra North A merica, a division of Leggett & Platt, and Dr. Andrew D’Souza with 3M for their contributions to the 6th edition of this book. A special thank you to Dr. Jessica Schroeder, Staff Research Scientist, Ms. Lisa Stanick , Leader – Brand Management, and Mr. Mike Meyerand, Communications, all three with General Motors Corporation which made it possible to use 2003 Chevrolet Corvette C5 see-through image on the cover of the 5th edition. The author gracef ully thank s David Grewell with the Iowa State University in Ames, IA, for his contribution entitled Laser Welding of Section 5.12. The author gratef ully acknowledges the a ssistance a nd encouragement of Professor Dr. Donald E. Baxa, Dino S. Tres, Dragos Catanescu, and Dr. Ranganath Shastri for their
helpf ul suggestions. I also thank BASF Corporation, Borg-Warner Automotive, Branson Ultrasonic Corporation, CAMPUS Consortium, DaimlerChrysler Corporation, Duk ane Corporation, ETS Inc., Forward Technology Industries, Inc., Hewlett-Pack ard Company – Corvallis Division, Leister Corporation, Miniature Precision Components, Inc., Molding Technology, Inc., Ohio State University, Sonics and Materials, Inc., Solvay Automotive, and TWI who supplied illustrations for this book.
