General design principles for dupont engineering polymers.pdf

Engineering Polymers

General Design Principles for

DuPont Engineering Polymers

Module I

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Engineering Polymers

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General Design Principles for DuPont Engineering Polymers

Table of Contents

1 General Page

Defining the End-Use Requirements . . . . . . . . . . . . . . 3

Design Check List . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

Prototyping the Design . . . . . . . . . . . . . . . . . . . . . . . . . 5

Testing the Design . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

Writing Meaningful Specifications . . . . . . . . . . . . . . . . 6

2 Injection Moulding

The Process and Equipment . . . . . . . . . . . . . . . . . . . . . 7

Trouble Shooting guide for Moulding Problems . . . . . 8

3 Moulding Considerations

Uniform Walls . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

Configurations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

Draft and Knock-Out Pins . . . . . . . . . . . . . . . . . . . . . . 12

Fillets and Radii . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

Bosses . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

Ribbing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

Holes and Coring . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

Threads . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

Undercuts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

Moulded-in Inserts . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17

Tolerances . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18

4 Structural Design Formulae

Short Term Loads . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19

Structural Design Formulae . . . . . . . . . . . . . . . . . . . . . 21

Other Loads . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33

Long Term Loads . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34

5 Design Examples

Redesigning the Wheel . . . . . . . . . . . . . . . . . . . . . . . . . 43

Chair Seats Reevaluated . . . . . . . . . . . . . . . . . . . . . . . . 46

Wheelbarrow Frame – a Potential Design . . . . . . . . . . 46

6 Springs

9 Assembly Techniques – Category I

Mechanical Fasteners . . . . . . . . . . . . . . . . . . . . . . . . . 71

Screwed Joints . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 74

Press Fittings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 77

Snap-Fits . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 79

Hub Joints . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 83

10 Assembly Techniques - Category II

SPIN WELDING . . . . . . . . . . . . . . . . . . . . . . . . . . . . 87

Practical Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . 87

Pivot Welding . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 87

Inertia Welding . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 90

Machines for Inertia Welding . . . . . . . . . . . . . . . . . . . 92

Jigs (Holding Devices) . . . . . . . . . . . . . . . . . . . . . . . . 94

Joint Profiles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 97

Calculations for Tools and Machines . . . . . . . . . . . . . 98

Graphical Determination of Parameters . . . . . . . . . . . 99

Quality Control of Welded Parts . . . . . . . . . . . . . . . . . 100

Welding Double Joints . . . . . . . . . . . . . . . . . . . . . . . . 102

Welding Reinforced and Dissimilar Plastics . . . . . . . . 103

Spin Welding Soft Plastics and Elastomers . . . . . . . . . 103

ULTRASONIC WELDING . . . . . . . . . . . . . . . . . . . . 107

Ultrasonic Welding Process . . . . . . . . . . . . . . . . . . . . 107

Welding Equipment . . . . . . . . . . . . . . . . . . . . . . . . . . 108

Part Design Considerations . . . . . . . . . . . . . . . . . . . . . 111

Ultrasonic Welding Variables . . . . . . . . . . . . . . . . . . . 115

Guide to Equipment Operation . . . . . . . . . . . . . . . . . . 116

Welding Performance . . . . . . . . . . . . . . . . . . . . . . . . . 117

Other Ultrasonic Joining Techniques . . . . . . . . . . . . . 119

Safety . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 121

VIBRATION WELDING . . . . . . . . . . . . . . . . . . . . . . 122

Definition of Motion Centre . . . . . . . . . . . . . . . . . . . . 122

Arrangements for Producing Vibrations . . . . . . . . . . . 123

Welding Conditions . . . . . . . . . . . . . . . . . . . . . . . . . . 124

Joint Design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 125

Test Results on Angular Welded Butt Joints . . . . . . . . 126

Joint Strength versus Welded Surface . . . . . . . . . . . . . 126

Joint Strength versus Specific Welded Pressure . . . . . 127

Design Examples . . . . . . . . . . . . . . . . . . . . . . . . . . . . 127

Comparison with other Welding Techniques . . . . . . . 128

Design for Vibration Welded Parts . . . . . . . . . . . . . . . 129

HOT PLATE WELDING . . . . . . . . . . . . . . . . . . . . . . 131

RIVETING . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 134

11 Machining, Cutting, Finishing

Machining H YTREL ® . . . . . . . . . . . . . . . . . . . . . . . . . . 137

Machining and Cutting of D ELRIN ® . . . . . . . . . . . . . . . 139

Finishing of D ELRIN ® . . . . . . . . . . . . . . . . . . . . . . . . . 140

Annealing of D ELRIN ® . . . . . . . . . . . . . . . . . . . . . . . . . 140

Machining and Cutting of Z YTEL ® . . . . . . . . . . . . . . . . 141

Finishing of Z YTEL ® . . . . . . . . . . . . . . . . . . . . . . . . . . 143

Annealing of Z YTEL ® . . . . . . . . . . . . . . . . . . . . . . . . . . 144

7 Bearings

Shaft Hardness and Finish . . . . . . . . . . . . . . . . . . . . . . 49

Bearing Surface . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49

Accuracy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50

Bearing Clearance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51

Lubrication . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51

Protection Against Dirt Penetration . . . . . . . . . . . . . . . 51

Thermal Conditions . . . . . . . . . . . . . . . . . . . . . . . . . . . 52

Calculation of Bearings . . . . . . . . . . . . . . . . . . . . . . . . 52

Design Examples . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 54

Testing Guidelines . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55

8 Gears

Gears Design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 57

Gear Proportions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 59

Backlash and Centre Distances . . . . . . . . . . . . . . . . . . . 61

Mating Material . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 62

Lubrication . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 62

Testing Machined Prototypes . . . . . . . . . . . . . . . . . . . . 63

Prototype Testing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 63

Helical Gear Design . . . . . . . . . . . . . . . . . . . . . . . . . . . 64

Worm Gear Design . . . . . . . . . . . . . . . . . . . . . . . . . . . . 64

Mating Material . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 67

Fillet Radius . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 67

Methods of Fastening . . . . . . . . . . . . . . . . . . . . . . . . . . 67

Combined Functions – Design Examples . . . . . . . . . . . 68

When to Use D ELRIN ® or Z YTEL ®

. . . . . . . . . . . . . . . . . 70

1 – General

Introduction

This section is to be used in conjunction with the product

data for specific DuPont Engineering Thermoplastic

resins – D ELRIN ® acetal resins, Z YTEL ® nylon resins inclu-

ding glass reinforced, M INLON ® engineering thermoplastic

resins and C RASTIN ® (PBT) and R YNITE ® (PET) thermo-

plastic polyester resins. Designers new to plastics design

must consider carefully the aspects of plastic properties

which differ from those of metals: specifically, the effect

of environment on properties, and the effect of long term

Defining the End-Use Requirements

The most important first step in designing a plastic part

is to define properly and completely the environment in

which the part will operate. Properties of plastic materials

are substantially altered by temperature changes, chemi-

cals and applied stress. These environmental effects must

be defined on the basis of both short and long term,

depending of course on the application. Time under stress

and environment is all-important in determining the

extent to which properties, and thus the performance

of the part will be affected. If a part is to be subject

to temperature changes in the end-use, it is not enough

to define the maximum temperature to which the part will

be exposed. The total time the part will be at that temper-

ature during the design life of the device must also be cal-

culated. The same applies to stress resulting from the

applied load. If the stress is applied intermittently, the

time it is applied and the frequency of occurrence is very

important. Plastic materials are subject to creep under

applied stress and the creep rate is accelerated with

increasing temperature. If loading is intermittent, the

plastic part will recover to some extent, depending upon

the stress level, the duration of time the stress is applied,

the length of time the stress is removed or reduced,

and the temperature during each time period. The effect

of chemicals, lubricants, etc, is likewise time and stress

dependent. Some materials may not be affected in

the unstressed state, but will stress crack when stressed

and exposed to the same reagent over a period of time.

DuPont engineering thermoplastic resins are particularly

resistant to this phenomena.

Property data for plastics are obtained from physical tests

run under laboratory conditions, and are presented in a

similar manner as for metals. Test samples are moulded in

a highly polished mould cavity under optimum moulding

conditions. Tests are run under ASTM and / or ISO condi-

tions at prescribed tensile rates, moisture levels, tempera-

tures, etc. The values shown are representative, and, it

should be recognized that the plastic part being designed

will not be moulded or stressed exactly as the test samples.

The following aspects affect, for instance, the strength and

toughness of a plastic part:

• Part thickness and shape

• Rate and duration of load

• Direction of fibre orientation

• Weld lines

• Surface defects

• Moulding parameters

The designer must also have information regarding the

effect of heat, moisture, sunlight, chemicals and stress.

The following checklist can be used as a guide.

In plastic design, therefore, it is important to understand

the application thoroughly, use reference information

which most closely parallels the application, prototype

the part and test it in the end-use application.

The purpose of the DuPont Handbook is to provide the

designer with the information necessary to create good

designs with the best materials in terms of factors, such

as: environment, process, design and end use effects.

The objective is to obtain a cost effective and functional

part design that can be achieved in the shortest possible

time.

This information allows parts to be designed with a mini-

mum weight and, at the same time, with a maximum

of possibilities for disassembly and recycling, so that the

impact on the environment can be reduced.

A good design reduces the processing cost, assembly

cost, production waste in the form of rejects parts, sprues

and runners and end-use waste of the whole device pro-

duced, through avoidance of early failure of the device.

® DuPont registered trademark

Design Check List

Part Name

Company

Print No.

Job No.

A. PART FUNCTION

B. OPERATING CONDITIONS

Operating temperature

Service life (HRS)

Applied load (N, Torque, etc., – describe fully

on reverse side)

Time on

Duration of load

Time off

Other (Impact, Shock, Stall, etc.)

NORMAL

MAX.

MIN.

C. ENVIRONMENT

Chemical

Moisture

Ambient temp. while device not operating

Sunlight direct

Indirect

Waste disposal dispositions

Production waste

End-use waste

D. DESIGN REQUIREMENTS

Factor of safety

Max. deflection/Sag

Tolerances

Assembly method

Finish / Decorating

Agency / Code approvals

Disassembly after service life

Recyclability

E. PERFORMANCE TESTING – If there is an existing performance specification for the part and/or device, include

copy. If not, describe any known requirements not covered above

F. APPROVALS

Regulation Classification

Food, automotive, military, aerospace, electrical

G. OTHER

Describe here and on the reverse side, any additional information which will assist in understanding completely the

function of the part, the conditions under which it must operate and the mechanical and environmental stresses and

abuse the part must withstand. Also add any comments which will help to clarify the above information

Prototyping the Design

In order to move a part from the design stage to commer-

cial reality, it is usually necessary to build prototype parts

for testing and modification. The preferred method for

making prototypes is to simulate as closely as practical

the same process by which the parts will be made in com-

mercial production. Most engineering plastic parts are

made in commercial production via the injection mould-

ing process, thus, the prototypes should be made using a

single cavity prototype mould or a test cavity mounted in

the production mould base. The reasons for this are sound,

and it is important that they be clearly understood.

The discussion that follows will describe the various

methods used for making prototypes, together with their

advantages and disadvantages.

Die Casting Tool

If a die casting tool exists, it can usually be modified for

injection moulding of prototypes. Use of such a tool may

eliminate the need for a prototype tool and provide a num-

ber of parts for preliminary testing at low cost. However,

this method may be of limited value since the tool was

designed for die cast metal, not for plastics. Therefore,

the walls and ribbing will not be optimized; gates are usu-

ally oversized and poorly located for plastics moulding;

and finally the mould is not equipped for cooling plastic

parts. Commercialization should always be preceded by

testing of injection moulded parts designed around the

material of choice.

Prototype Tool

Prototype moulds made of easy-to-machine or cheap mate-

rials like aluminium, brass, kirksite, etc. can produce parts

useful for non-functional prototypes. As the right moulding

conditions demanded by the material and the part geometry

cannot be employed in most cases (mould temperature and

pressure especially), such low-cost moulds cannot produce

parts that could be evaluated under operational conditions.

Machining from Rod or Slab Stock

This method is commonly used where the design is very

tentative and a small number of prototypes are required,

and where relatively simple part geometry is involved.

Machining of complex shapes, particularly where more

than one prototype is required, can be very expensive.

Machined parts can be used to assist in developing a more

firm design, or even for limited testing, but should never

be used for final evaluation prior to commercialization.

The reasons are as follows:

– Properties such as strength, toughness and elongation

may be lower than that of the moulded part because

of machine tool marks on the sample part.

– Strength and stiffness properties may be higher than the

moulded part due to the higher degree of crystallinity

found in rod or slab stock.

– If fibre reinforced resin is required, the important effects

of fibre orientation can be totally misleading.

– Surface characteristics such as knockout pin marks, gate

marks and the amorphous surface structure found in

moulded parts will not be represented in the machined

part.

– The effect of weld and knit lines in moulded parts

can-not be studied.

– Dimensional stability may be misleading due to gross

differences in internal stresses.

– Voids commonly found in the centre of rod and slab

stock can reduce part strength. By the same token,

the effect of voids sometimes present in heavy sections

of a moulded part cannot be evaluated.

– There is a limited selection of resins available in rod

or slab stock.

Preproduction Tool

The best approach for design developments of precision

parts is the construction of a steel preproduction tool.

This can be a single cavity mould, or a single cavity in

a multi-cavity mould base. The cavity will have been ma-

chine finished but not hardened, and therefore some alter-

ations can still be made. It will have the same cooling as

the production tool so that any problems related to warp-

age and shrinkage can be studied. With the proper knock-

out pins, the mould can be cycled as though on a produc-

tion line so that cycle times can be established. And most

important, these parts can be tested for strength, impact,

abrasion and other physical properties, as well as in the

actual or simulated end-use environment.

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