57071148 samsung design guide for assembly of injection molded plastic parts

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



Press Fit AssemblyPress Fit Assembly

Simplest means of assembling plastic parts - use Elastic or spring like characteristics of plastics

hub

shaft



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.



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.



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)



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



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)




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




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.



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



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



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



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




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.




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





Torque

Start Drive torque

Maximumdrive torque

(clamping starts)


torque:stripping...

Wide separation

(reduces riskof stripping)

Stripping torqueMaximum drive torque

= 4:1

Torque


Start Drive torque

Maximumdrive torque

(clamping starts)


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).




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.




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.



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



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




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




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)




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




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




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



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)



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




Lower part fixed

Butt joint

Tongue andgroove joint

T & G jointwith traps Scarf joint Shear joint

Basic joint configuration for the spin welding process




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



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



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



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



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



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

57071148 samsung design guide for assembly of injection molded plastic parts

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