Design for Assembly
Cheil Industries Inc.
Design Guide For Assembly of Injection Molded Plastic Parts
R&D, Application Development Center Last Updated SEP 15 1999Cheil Industries Inc. 2
Assembly techniquesAssembly techniques
Improving both product quality and reducing manufacturing cost
¡ Press fit assembly
¡ Self fit assembly
¡ Mechanical Fastening techniques
¡ Welding techniques
¡ Adhesive bonding
¡ Solvent bonding
Design Guide For Assembly of Injection Molded Plastic Parts
R&D, Application Development Center Last Updated SEP 15 1999Cheil Industries Inc. 3
Press Fit AssemblyPress Fit Assembly
Simplest means of assembling plastic parts - use Elastic or spring like characteristics of plastics
hub
shaft
Design Guide For Assembly of Injection Molded Plastic Parts
R&D, Application Development Center Last Updated SEP 15 1999Cheil Industries Inc. 4
Press Fit AssemblyPress Fit AssemblyMaterials consideration
0.0 1.0 2.0 3.0 4.0 5.0
Tensile strain (%)
Tensilestress
X
Rigidbrittle
polymer
Designstrain
may be
Manufacturingrelated
tolerance
Ductile polymer
Large latitudefor interference
<
Rigid polymers are too brittle for press fit applications.Ductile polymers are more generous when considering the effects of manufacturing tolerance.
Design Guide For Assembly of Injection Molded Plastic Parts
R&D, Application Development Center Last Updated SEP 15 1999Cheil Industries Inc. 5
Press Fit AssemblyPress Fit AssemblyInterference for Press fit
0
0.002
0.004
0.006
0.008
0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0
Ratio of (Shaft diameter/Hub outside diameter)Ø s/Ø o
Interferencelimit - I
m/m of shaft Ø
Interference limits for nylon 66 hub
Nylon 66 shaft
Steel shaft
Room temp.50 % RH
I = [(σD•Ø s)/W]•[[(W + νh)/Eh] +[(1- νs)/Es]]
where: I = diametrical interference (inch) σD = design stress level (lbs/inch2) Ø o = hub outside diameter (inch) Ø s = shaft diameter (inch) Eh = hub modulus (lbs/inch2) Es = shaft modulus (lbs/inch2) νh = Poisson’s ratio for hub material νs = Poisson’s ratio for shaft material W = [1 + (Ø s / Ø o)2]/[1-(Ø s / Ø o)2]
The amount of interference for a press fit can be determined using design equations, orgraphs, indicating the maximum recommended interference for a particular material.
Design Guide For Assembly of Injection Molded Plastic Parts
R&D, Application Development Center Last Updated SEP 15 1999Cheil Industries Inc. 6
Press Fit AssemblyPress Fit AssemblyUse of undercut shaft for press fit hub/shaft assembly
Ø s1Ø s2
Ø i
I = Ø s2 - Ø i
Ø i < Ø s2 < Ø s1
Hub is pushed over high spoton shaft during assembly
(short time only)
Hub reachesfinal axial location
(long term)
Design Guide For Assembly of Injection Molded Plastic Parts
R&D, Application Development Center Last Updated SEP 15 1999Cheil Industries Inc. 7
Press Fit AssemblyPress Fit AssemblyShaft can be smooth, textured or even knurled
Time
Slippagetorque
Time
Slippagetorque
Press fitsmooth shaft
Press fitknurled or splined shaft
Ø sØ i
Ø i < Ø s
Rate of decrease determinedby design stress level andrelaxation characteristics ofthe hub material
Knurled, textured or splined surfaces can result in coldflow and some mechanicalinterlocking
Design Guide For Assembly of Injection Molded Plastic Parts
R&D, Application Development Center Last Updated SEP 15 1999Cheil Industries Inc. 8
Snap Fit AssemblySnap Fit AssemblySnap Joint assembly
satisfy both Design for assembly and Design for disassembly
Simple & Most versatile means of plastic product assembly
Insertion
Deflection
Elastic recovery
R
∆R
Lead-inangle (α)
Design Guide For Assembly of Injection Molded Plastic Parts
R&D, Application Development Center Last Updated SEP 15 1999Cheil Industries Inc. 9
Snap Fit AssemblySnap Fit AssemblySnap Joint assembly
LR d
y
Cantilever snap beamconstant rectangular cross section
d d/2
Tapered cantilever snap beamdecreasing depth from d to d/2
y = 0.67 ε L2/d
y = 1.1 ε L2/d
ε = maximum tensile strain
Permissible deflection for the tapered beam is about 60% greater than that of the constant cross section of rectangular beam
Design Guide For Assembly of Injection Molded Plastic Parts
R&D, Application Development Center Last Updated SEP 15 1999Cheil Industries Inc. 10
Snap Fit AssemblySnap Fit AssemblySnap Joint assembly
Stress concentrationat the corner
DeflectionNeutralaxis
+ σ
- σ
R
T
Applied Load
0 0.2 0.4 0.6 0.8 1.0 1.2 1.41.0
1.5
2.0
2.5
3.5
3.0
Radius/wall thickness (R/T)
Stre
ss c
once
ntra
tion
fact
or
Stress concentration factors show that larger radius values tend to reduce the stress concentration and mold filling orientation related problems, however, excessive radii can lead to complications due to sinks, voids, shrinkage stress.
Design Guide For Assembly of Injection Molded Plastic Parts
R&D, Application Development Center Last Updated SEP 15 1999Cheil Industries Inc. 11
Mechanical Fastening TechniquesMechanical Fastening TechniquesMechanical fastener - Screw
Screws provide a simple, fast, and effective method of joining similar or dissimilar materials
¡Machine screws (I.e. nuts and bolts)
¡Machine screws with a threaded metal inserts or molded threads
¡Self threading screws
Design Guide For Assembly of Injection Molded Plastic Parts
R&D, Application Development Center Last Updated SEP 15 1999Cheil Industries Inc. 12
Mechanical Fastening TechniquesMechanical Fastening TechniquesMechanical fastener - Screw
Machine screw and bolt
£Esthetic interruption on both top and bottom surfaces£Many parts required for assembly£Access to both top and bottom of the part is required during assembly£Need locking hardware to avoid vibration loosening£Durable assembly
Machine screw and insert
£One smooth surface obtained£Fewer parts required for assembly£Internally threaded insert must be inserted into boss during or after molding£Requires special equipment/tooling for insert£Good overall durability£Suitable for repeated assembly
Self threading screw and plastic boss
£One smooth surface obtained£Minimum number of parts required for assembly£Mating plastic threads formed during assembly£Minimum fastener and equipment cost£Limited durability (mating thread is plastic)£Repeated assembly possible but limited
Design Guide For Assembly of Injection Molded Plastic Parts
R&D, Application Development Center Last Updated SEP 15 1999Cheil Industries Inc. 13
Mechanical Fastening TechniquesMechanical Fastening Techniques
Self threading screw
The screw type, size, and the design of the boss system are dependent upon
¡ Screw pull out resistance¡ Clamp load requirement and decay rate¡ Repeated assembly requirements¡ Torque retention and vibration resistance¡ Hoop stresses for the boss assembly¡ Assembly characteristics such as the strip to drive torque ratio
Throughclearance
boss
Blindpilotboss
Radialclearance
Minor screw Φ
Major screw ΦThread depth
Thread depth (%)Utilized
Pilot hole Φ
Design Guide For Assembly of Injection Molded Plastic Parts
R&D, Application Development Center Last Updated SEP 15 1999Cheil Industries Inc. 14
Mechanical Fastening TechniquesMechanical Fastening TechniquesSelf threading screw
Screw engagement (turns)
Torque
Start Drive torque
Maximumdrive torque
(clamping starts)
Failure(destruction)
torque:stripping...
Yield Recommendedtightening
torque
Prestress(clamping)
torque
Typical torque vs. turns of engagement behavior for a self threading screwbeing driven into a plastic boss
Design Guide For Assembly of Injection Molded Plastic Parts
R&D, Application Development Center Last Updated SEP 15 1999Cheil Industries Inc. 15
Mechanical Fastening TechniquesMechanical Fastening TechniquesSelf threading screw
LbLb
Sink mark Shrinkage voids
Blind boss
Throughboss Self
threadingscrew
The core pins that are used to form the holes in blind bosses should be extended as much as possible to core out excessive material.
Design Guide For Assembly of Injection Molded Plastic Parts
R&D, Application Development Center Last Updated SEP 15 1999Cheil Industries Inc. 16
Mechanical Fastening TechniquesMechanical Fastening TechniquesSelf threading screw
Lb
Lt
Lb
Lt
Lb=length of hole in the blind boss Lt= length of hole in the through boss
The blind boss core pin length is reduced to minimize the molding problems associated with long, cantilever core pins
Design Guide For Assembly of Injection Molded Plastic Parts
R&D, Application Development Center Last Updated SEP 15 1999Cheil Industries Inc. 17
Mechanical Fastening TechniquesMechanical Fastening TechniquesSelf threading screw
Screw engagement (turns)
Torque
Start Drive torque
Maximumdrive torque
(clamping starts)
Failure(destruction)
torque:stripping...
Wide separation
(reduces riskof stripping)
Stripping torqueMaximum drive torque
= 4:1
Torque
Screw engagement (turns)
Start Drive torque
Maximumdrive torque
(clamping starts)
Failure(destruction)
torque:stripping...
Limitedseparation
Stripping torqueMaximum drive torque
= 2:1
Low strip/drive ratio
Torque-turn curve for a self threading screw showing poor strip/drive torque ratio (left) and high strip/drive torque ratio (right).
Design Guide For Assembly of Injection Molded Plastic Parts
R&D, Application Development Center Last Updated SEP 15 1999Cheil Industries Inc. 18
Mechanical Fastening TechniquesMechanical Fastening TechniquesSelf threading screw
Torque
Turns of engagement
Correct pilot hole diameter Oversized pilot hole diameterUndersized pilot hole diameter
High strip / drive ratioand
high strip torque
High strip / drive ratiobut
low strip torque
High strip torquebut
low strip / drive ratio
The boss pilot hole diameter has a significant influence on both maximum drive torque and strip / drive torque ratio. Low drive torque and high strip /drive torque ratios are most desirable as this limits the potential for failure during the initial or subsequent assembly procedures.
Design Guide For Assembly of Injection Molded Plastic Parts
R&D, Application Development Center Last Updated SEP 15 1999Cheil Industries Inc. 19
Mechanical Fastening TechniquesMechanical Fastening TechniquesSelf threading screw
0
100
200
300
400
500
0.05 0.06 0.07 0.08 0.09 0.10 0.11 0.12 0.13 0.14
Boss wall thickness (inches)
Failureload intension
(lbs)
Screwpulls out
Screwpulls out
Screwpulls out
Bosstensilefailure
Bosstensilefailure
1.8 turnsengagement
2.7 turnsengagement
3.6 turnsengagement
•Polypropylene homopolymer•#6 BT thread cutting screw•0.114 inch pilot hole diameter (60% thread depth)
Tensile screw pull out results for a type BT screw and a polypropylene boss.
Design Guide For Assembly of Injection Molded Plastic Parts
R&D, Application Development Center Last Updated SEP 15 1999Cheil Industries Inc. 20
Welding TechniquesWelding TechniquesMolecular diffusion and entanglement
Upper part
Lower part
Weldinterface
Welded thermoplastic parts Entangled polymer chains at theweld interface
Molecular diffusion and entanglement must occur during welding. The molecular mobilitycan be the result of solvent swelling or thermal energy
£Ultrasonic welding - High frequency vibration , intermolecular friction£Vibration welding - surface friction and/or viscous dissipation£Spin(rotational) welding - interfacial friction£Hot tool welding - heat conduction£Induction welding - heating by high frequency electromagnetic field£Resistance welding - heat conduction by conductive wire£Hot gas welding - use hot air or nitrogen£Extrusion welding - use hot air or nitrogen
Design Guide For Assembly of Injection Molded Plastic Parts
R&D, Application Development Center Last Updated SEP 15 1999Cheil Industries Inc. 21
Welding TechniquesWelding TechniquesUltrasonic Welding
Pneumaticcylinder
Air pressure
Electricalpowersupply
High frequencyelectrical out
Low frequencyelectrical in
Converterhigh frequency electrical signal in -
high frequency (ultrasonic) mechanicalvibration out
Amplitude transformer(booster horn)
Plastic partsto be welded
Ultrasonic sinusoidalaxial vibration
Welding horn
Horn face amplitudeof vibration
Support andalignment fixture
Base plateand frame
Ultrasonic welding process is commonly used for the assembly of small to medium size thermoplastic parts
weld process variables include:weld timehorn positionweld pressure
Design Guide For Assembly of Injection Molded Plastic Parts
R&D, Application Development Center Last Updated SEP 15 1999Cheil Industries Inc. 22
Welding TechniquesWelding TechniquesUltrasonic Welding
Buttjoint
Butt jointwith energy
director
Flashtrap
added
Stepjoint
u Poor but joint designu Excessive weld timeu Excessive weld energyu Exuding melt results in a visual defect
u Improved butt joint designu Reduced weld timeu Reduced weld energyu Exuding flash (visible)
u Flash trap addedu Reduction in weld areau Exuding melt does not result in a visual defect
u Step joint designu Improved shear resistanceu Exuding melt does not result in a visual defectu Assists in locating parts
Design Guide For Assembly of Injection Molded Plastic Parts
R&D, Application Development Center Last Updated SEP 15 1999Cheil Industries Inc. 23
Welding TechniquesWelding TechniquesUltrasonic Welding
H
θ
hDim. Small part Large part Small part Large part
h 0.3 - 0.4 0.5 - 0.6 0.5 - 0.7 0.1 - 1.0
θ 60o to 90o 90o
Amorphous polymer Semi-crystalline polymer
Typical energy director dimensions(millimeters)
Design Guide For Assembly of Injection Molded Plastic Parts
R&D, Application Development Center Last Updated SEP 15 1999Cheil Industries Inc. 24
Welding TechniquesWelding TechniquesUltrasonic Welding
Lower part Lower part Lower part Lower part
Plastic partbefore welding
Insufficient spreadingof energy director
Excessive flow of melt
Optimumweld
Energydirector
Texturedsurface
Surface textures opposite the energy director can improve weld quality for joint such as the butt joint
Design Guide For Assembly of Injection Molded Plastic Parts
R&D, Application Development Center Last Updated SEP 15 1999Cheil Industries Inc. 25
Welding TechniquesWelding TechniquesUltrasonic Welding
Butt joint energy director:corner consideration
Incorrect energy director design:concentration of weld material at corner
Optional design:surrounding energy director
interrupted at corner
Improved design:surrounding energy director
follows corner radius
Design Guide For Assembly of Injection Molded Plastic Parts
R&D, Application Development Center Last Updated SEP 15 1999Cheil Industries Inc. 26
Welding TechniquesWelding TechniquesUltrasonic Welding
Material Notation Complete compatibility Partial compatibility
ABSPC/ABS alloy
AcetalAcrylic
Acrylic MultipolymerCellulosics
FluoropolymerNylon
Polyphenylene oxidePolyamide-imide
PCPolyester
PolyethylenePolymethylpentane
Polyphenylene sulfidePPPS
PolysulfonePVCSAN
ABCDEFGHIJKLMNOPQRST
A,B,DA,B,K
CA,DEFGH
I,QJ
B,KLMNOP
I,QRST
TD---
B,E,J,K,TA,D,Q,T
---------
D,K,T---
D,I,R---------------
E,TK---
A,D,E,I,Q
General Guidelines on the Compatibility of Various Thermoplastics for Ultrasonic Assembly
Design Guide For Assembly of Injection Molded Plastic Parts
R&D, Application Development Center Last Updated SEP 15 1999Cheil Industries Inc. 27
Welding TechniquesWelding TechniquesVibration Welding
Typical joint design for vibration welding: the butt joint, and a butt joint modified with flash traps
Standard butt jointbefore welding
Standard butt joint after welding
Butt joint modifiedwith flash traps
Pressure
Welding flash
Joint withflash traps
Groovesfor welding
tools
Singleplane
parting line
Relativehorizontal
motion
Frequency: 120 to 240 Hz.Amplitude: 0.030 - 0.160 inch
(0.8 - 4.0 mm)
Design Guide For Assembly of Injection Molded Plastic Parts
R&D, Application Development Center Last Updated SEP 15 1999Cheil Industries Inc. 28
Welding TechniquesWelding TechniquesSpin(rotational) Welding
Clutchand
flywheel
Pneumaticcylinder
Drive head
Drivenplastic part
Fixedplastic part
Holdingfixture
Variablespeeddrive
and brake
Controls:�pressure�speed�weld time�hold time
Design Guide For Assembly of Injection Molded Plastic Parts
R&D, Application Development Center Last Updated SEP 15 1999Cheil Industries Inc. 29
Welding TechniquesWelding TechniquesSpin(rotational) Welding
Lower part fixed
Butt joint
Tongue andgroove joint
T & G jointwith traps Scarf joint Shear joint
Basic joint configuration for the spin welding process
Design Guide For Assembly of Injection Molded Plastic Parts
R&D, Application Development Center Last Updated SEP 15 1999Cheil Industries Inc. 30
Welding TechniquesWelding TechniquesSpin(rotational) Welding
Axial force
Rotating part
Fixedpart
Temperature
Time
Displacement
Temperature
Axialdisplacement
Softeningtemperature
Ambienttemperature
Start
Phase 1abrasion and
external friction
Phase 2initial melt
layer formed
Phase 3steady state
shearing
Phase 4end rotationand cooling
Typical temperature and axial displacement over the course of the spin welding cycle
Design Guide For Assembly of Injection Molded Plastic Parts
R&D, Application Development Center Last Updated SEP 15 1999Cheil Industries Inc. 31
Welding TechniquesWelding TechniquesInduction welding
Before
Before
Before
Before
After After
After After
Flat to groove joint Tongue to groove joint
Shear joint Step joint
Typical joint configuration for the induction welding
Design Guide For Assembly of Injection Molded Plastic Parts
R&D, Application Development Center Last Updated SEP 15 1999Cheil Industries Inc. 32
Welding TechniquesWelding TechniquesHot tool welding
Upper holding fixture
Lower holding fixture
Plastic part
Plastic part
Step1. Parts placed in upper(moving) and lower(fixed)holding /alignment fixtures
Step3. Fixtures close and bring parts in contact with the heated platen
Step5. The fixtures close and pressure is applied to weld parts
Step2. Electrically heatedwelding platen is insertedbetween parts
Step4. The fixtures openand the heated platenis removed
Step6. The fixtures openand the welded assemblyis removed
Design Guide For Assembly of Injection Molded Plastic Parts
R&D, Application Development Center Last Updated SEP 15 1999Cheil Industries Inc. 33
Welding TechniquesWelding TechniquesHot gas welding
Hot dry, cleanair or inert gas
Force
Plastic weldingrod(same material)
Plastic partsto be welded
Beveledgroove
Welding tool
Force
Finished weld
Traveldirection
Design Guide For Assembly of Injection Molded Plastic Parts
R&D, Application Development Center Last Updated SEP 15 1999Cheil Industries Inc. 34
Adhesive BondingAdhesive BondingAdvantages
¡ Esthetics and design flexibility
¡ Uniform stress distribution
¡ Can join dissimilar materials
¡ Can provide weather /water / gas tight seal
¡ Flexible adhesives, compliant in shear, can compensate for thermal expansion mismatches between components
¡ Flexible adhesives can dampen vibration
¡ Can be used with thin, flexible substrates
¡ Provide electrical and thermal insulation
Design Guide For Assembly of Injection Molded Plastic Parts
R&D, Application Development Center Last Updated SEP 15 1999Cheil Industries Inc. 35
Adhesive BondingAdhesive BondingAdhesive theory
Adhesive isapplied (liquid)
Adhesive wetsthe surface
Temporaryfixturing
Finishedpart (solid)
The adhesive must:
•Wet the surfaces to be bonded (spreading and capillary action)
•Attract surfaces effectively (adhesive strength)
•Develop into a strong solid (cohesive strength)
Water droplet Time
Non-polar surface
Polar liquid on non-polar surface:poor wetting, eater tends to bead up
Time
Polar surface
Polar liquid on polar surface:good wetting, water spreads over surface