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DESIGN AND MANUFACTURE OF USABLE CONSUMER PRODUCTS:PART I - REVIEW OF THE LITERATURE

Majorkumar Govindaraju and Anil Mital

Industrial Engineering

University of Cincinnati

Cincinnati, OH 45221-0116

ABSTRACT

Survival of a company in these times of

increased global competition depends upon

developing high quality products at

affordable cost. It calls for a strategic

approach to developing usable and needed

products by integrating product planning, its

design, and manufacturing. A hard to use

product, even one with many functions, will

fall by the way side. Usability of a product is

generally determined by how easily and

completely it meets the users’ needs. The

criteria for usability have, however, been

gradually changing. Recent trends, such as

increased customer demand to satisfy

personal needs, are forcing the

manufacturers to design a variety of usable

products customized to individual needs.

Also, as a result of an increased

environmental awareness, customers are

seeking products that are environment

friendly, energy efficient, and recyclable.

Thus, the attributes of a product that make it

usable are changing to encompass its entire

life cycle. These changes in usability need to

be reflected in the design of a product and

the selection of processes to achieve its

manufacture. This article, part I of a two-

part paper, defines product usability in the

context of the global market and reviews

tools and guidelines available in the

published literature to produce usable

products.

1.- INTRODUCTIONProduct design is the process of creating

new and improved products for people to

use. Consumer products are products

designed for use by the general public

whereas commercial products are products

used to produce goods and services.

Consumer products are different from

commercial products in several respects as

far as the user is concerned: (a) the user is

generally untrained, (b) the user often works

unsupervised, and (c) he/she is part of a

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diverse population (Cushman, 1991). The

process of designing and manufacturing

consumer products is greatly influenced by

the needs and demands of the customers.

In the early twentieth century,

consumer products were primarily designed

to provide functionality. Later, the form and

appearance began to be emphasized. Though

this resulted in nice looking products with

an array of features, such products were

often difficult to use (Ulrich, 1995). During

the 1980s, designers started emphasizing

user friendliness of consumer products.

Requirements such as product-user interface

design and safety were incorporated into the

design. Concern for the environment and

resource utilization in recent years has

stimulated new awareness among users to

seek products that pose minimal risk of

environmental pollution, consume less

energy, have very little toxic emissions

during use, and are recyclable when

disposed. For making products usable by

making them environmentally friendly,

designers need to emphasize energy

efficiency, recyclability, and disposability.

This calls for considering all life-cycle

phases of a product, i.e., design, production,

distribution, usage, maintenance, and

disposal/recycling, simultaneously in

determining its usability. Figure 1 shows the

various phases in the life cycle of a product.

Figure 1: Life Cycle Analysis of a Product

Recently, designers are emphasizingcustomizing the products to meet thedemands from the users to satisfy theirindividual tastes and preferences.

The following may be regarded as

the criteria for designing and manufacturing

usable consumer products:

1. Functionality

2. Ease of operation

3. Aesthetics

4. Reliability

5. Maintainability/Serviceability

6. Environment friendliness

7. Recyclability/Disposability

8. Safety and

9. Customizability

The needs and wants of customers listed

above are linked to the product design and

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manufacture. To fulfill these needs and

wants, consumer products need to be

designed to incorporate those features that

meet the user requirements and then

manufactured by appropriate selection of

materials, processes, and tools (Figure 2).

Figure2: Usability Criteria and Design /Manufacturing Factors

The purpose of this paper (part I of a two-

part paper) is to review published literature

pertaining to design tools, methodologies,

and guidelines that are available to design

and manufacture usable consumer products.

How the usability criteria may be linked to

manufacturing attributes is shown in Part II.

2.- Criteria for designing and

manufacturing usable consumer

products:

2a. FUNCTIONALITY

The design activity is usually preceded by

obtaining information about the needs and

wants of the users through market research

(McClelland, 1990). Figure 3 shows a

structured approach to obtaining information

pertaining to user needs in the design

process for developing usable consumer

products (Mital, 1992). Conceptual design

deals with the activities that happen early in

the product development (Dika, 1988). It

involves creation of synthesized solutions in

the form of products that satisfy users’

perceived needs through the mapping

between the functional requirements in the

functional domain and the design parameters

in the physical domain, through proper

selection of design parameters that satisfy

the functional requirements. This mapping is

not unique and the outcome depends on the

creative process of individual designer.

Many techniques have been advanced to

enhance the creative process, including: (1)

trigger-word technique, (2) checklist

technique, (3) morphological technique, (4)

attribute-seeking technique, (5) Gordon

technique, and (6) brainstorming technique

(Suh, 1990).

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Figure 3: Design flow for Product Usability

In the trigger word technique, the

verb word in the problem definition

statement is analyzed recursively to create

different set of connotations and ideas to

solve the problem. The checklist method

consist of a series of standard set of

question; each question can have more

related questions. The checklist serves to

focus at various ways of looking at the

problem and to stimulate the imagination to

explore less obvious concepts surrounding

the problem. The morphological chart

technique involves analyzing the problem to

determine the independent parameters that

are involved. Each of these parameters is

then considered separately for possible

alterative methods. All methods are

tabulated in a matrix which can be cross

correlated to produce possible solutions to

the problem. In the attribute-seeking

technique, all essential characteristics (i.e.,

attributes), that comprise a possible solution

to the problem, are singled out and analyzed

individually using either trigger-word or

checklist approach. The Gordon technique

deals with the basic underlying concepts

involved in the situation, instead of

considering the obvious aspects of the given

problem. This approach compels the

designer to take a much broader view by

analyzing the reasons why the problems

exist in the first place. For example, when

designing home-disposal appliance, one may

seek to eliminate the cause of trash rather

than dealing with the disposal of trash.

Brainstorming is a group-ideation technique

usually consisting of 6 to 8 individuals who

are conversant with the field. A moderator

defines the situation and provides

interpretation of the problem. The success of

this technique depends on the compounding

effect of each person in the group

responding to the ideas expressed by others.

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Umeda et al. (1997) proposed

Function-Behavior-State (FBS) modeling

and a conceptual design-support tool called

FBS Modeler based on it. The FBS modeler

has knowledge bases for function

prototypes, physical features, and physical

phenomena. With these knowledge bases,

the FBS Modeler supports conceptual design

as follows:

1. The designer selects required

functions from the function prototype’s

knowledge base.

2. Aided by decomposition knowledge

of function prototypes, a designer

decomposes the required function and

sub-functions.

3. The designer chooses physical

features that can embody each

subfunction. After instantiating physical

features, the designer might discover that

some features cannot occur. In such a

case, a subsystem, Qualitative Process

Abduction System (QPAS), reasons out

candidates for the missing physical

features to satisfy the physical

conditions.

4. Next, the designer connects the

instantiated physical features to

complete the functional hierarchy. This

process constructs the behavioral-level

network structure.

5. Then, a qualitative reasoning

subsystem simulates behavior. As a

result of the simulation, the system

might discover inconsistencies between

the FBS model constructed by the

designer and the result obtained. The

system will then indicate phenomena

that will not occur even though the

designer specifies it in the initial FBS

module.

The main deficiency of the FBS method is

that it does not explicitly deal with the

geometry and kinematics of the product

which are essential concepts in mechanical

design. The approach by Chakrabarti (1996)

relates functions to the relative motions of

parts, unlike most approaches where

functions are related to the components.

These tools help designers develop the

physical design of a product given the

functional requirements specified by the

user. Any such conceptual design needs to

be further evaluated to determine whether it

is easy to use by consumers.

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2b. EASE OF OPERATION

A product is considered user friendly if the

functions allocated to humans are within the

limitations of their abilities and constraints,

and the product-user interface is physically

comfortable and mentally not stressful

(Haubner, 1990, Nielsen, 1993a). The

system should be easy to learn, easy to

remember and relatively error free (Nielsen,

1992; Nielsen, 1993b).

Lee et al. (1997) have devised a

formal and systematic approach for

integrating the user needs and demands in

product design through a process called

High Touch. Once the customer needs are

obtained by appropriate tools such as focus

group methodology (Caplan, 1990), High

Touch process can be used for consumer’s

implicit needs and potential demands on a

product into design details. It includes

hierarchical structure of design variables,

relationship matrix among design variables,

and systematic evaluation of potential

product functions. The High Touch process

consists of a series of ergonomic analysis of

the product. A group of expert ergonomists

systematically evaluates the product through

focused group interviews, task analysis, and

field test from the consumer’s viewpoint.

Based on the results, ergonomic analyses

including design variables, such as human

characteristics, product functions, and

human-product interface variables, are

performed using a checking procedure. By

systematically analyzing the evaluation

results, many High Touch solutions such as

new product ideas, new product functions,

and design improvement are generated.

As the user-product interaction is

becoming less physical and more cognitive,

it is essential to understand the cognition of

the product semantics, i.e., the symbolic

interaction between users and products. Lin

et al.(1996), using multidimensional scaling

(MDS), present an approach that can be used

to study product semantics in product

design. MDS is a process whereby a matrix

of distance, either psychological or physical,

among a set of objects can be translated into

a representation of those objects in space.

The results from MDS analysis provide

designers with an idea of how to concentrate

their efforts in using product semantics for

consumer product design.

The consumer electronic products

are becoming more graphical user interface

intensive in recent years (Shneiderman,

1998) that is made possible by incorporation

of growing size of embedded software

(Tervonen, 1996). Product quality in such

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products is greatly influenced by the

software quality (Kitchenham, 1996). Zero-

defect software can be obtained only by

emphasizing quality during all the phases of

software development cycle involving

requirement analysis, protype-software

development, architecture and component

design, realization, and testing (Rooijmans,

1996). The consumer product can be finally

tested for its ease of use by usability testing

procedures

such as thinking aloud method where users

work on a prototype (Jorgensen, 1990).

2c. AESTHETICS

A customer’s perception of a product’s

value is, in part, based upon its aesthetic

appeal (Logan, 1994). An attractive product

may create an aesthetic appeal and a sense

of high fashion, image, and pride of

ownership (Akita, 1991). The design of

products should induce a positive sensual

feeling (Hofmeester, 1996).

Kansei Engineering is a technology

that translates consumers’ feelings and

image for a product into design elements

(Nagamachi, 1995). Kansei Engineering

(KE) technology is classified into three

types: KE Type I, II, and III. KE Type I

deals with design elements of new products.

The customers’ feeling about a product are

broken down into a tree structure to get the

details about the design of the product.

Type II utilizes current computer

technologies, such as expert systems, neural

network models (Ishihara, 1995), and

genetic algorithms (Tsuchiya, 1996), and is

called computer assisted Kansei Engineering

System (KES). The KES architecture

basically has four databases: Kansei

Database, image database, knowledge base,

and design and color database. A consumer

inputs his image words concerning the

desired product in KES. The KES receives

these words firstly through Kansei word

database and tries to recognize them. The

inference engine in this stage works by

matching the rule-base and the image

database. Then, the inference engine

determines the design details and the KES

controller displays the part and color details

of the product on the screen. Type III is the

mathematical logic model (Nagamachi,

1995).

The Hybrid KES, a new framework

of KES, supports both the consumers and

designers. It consists of Forward Kansei

Engineering and Backward Kansei

Engineering. In the Forward KES, the

designer obtains the desired design through

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an input of the Kansei words and outputs the

product design details. In the Backward

KES, the designer draws a rough sketch on

the computer screen and the computer

system recognizes the pattern of the design

input by the designer and shows the

estimated level of Kansei about the design

(Matsubara, 1997).

Functionality and user-friendliness,

designed into the product as indicated

above, implies that the product is able to

perform the desired functions without

posing excessive demands on the user at any

given time; the ability of the product to

function satisfactorily over a period of time

is indicated by its reliability.

2d. RELIABILITY

Reliability of a product is the probability

that it will perform satisfactorily for a

specified period of time under a stated set of

conditions (Anderson, 1991). Mean time to

failure (MTTF) is used as a measure of

reliability. MTTF is the average or mean

lifetime for a population of products.

Failures per billion operating hours (FITS),

a reciprocal of MTTF, is also used as a

measure of reliability.

Thayer (Thayer, 1986) describes a

three step process to improve the reliability

of a product during the concept design,

design verification and production

verification phases. First step is an

estimation of the reliability of each

subsystem of the existing design. The

product is depicted in the form of a chart

showing the structural hierarchy of

subassemblies. The subassemblies are

further divided into field replaceable units

(FRUs) identified by a certain numbering

scheme. Failure data are collected by part

number during the field diagnosis along with

an estimate of operating time to failure. The

failure analysis during the first step

identifies those low reliability subsystems

that should be designed out of the next

generation. During the second step involving

design verification a prototype is tested to

evaluate the design concepts. Engineering

changes are made to replace weak parts and

subassemblies and any reliability

improvement is verified through reliability

growth tests. Duane plots are a plot of the

log of cumulative test time vs the log of

either mean time between failure or the

failure rate. They are very effective in

predicting the ultimate reliability of the

product at the completion of the engineering

design phase. The design is found

satisfactory if the engineering changes are

effective in improving the reliability by

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increasing the time between successive

failures to an acceptable level. The third step

is to release the finalized design to

production and test the reliability of units

built with the production tooling and labor.

The reliability testing is performed by

Weibull plotting of failure data. The Weibull

plots are usefull in predicting the mean time

between failure of the final product and in

identification of failures due to

manufacturing errors, wearout, or chance.

Reliability improvement is usually

achieved through continuous improvement

in materials, product design, manufacturing

processes and use environment (Alonso,

1990, Comizolli, 1990). Reliability growth

test management is a critical component of

the product assurance function (Bieda,

1992). Computer application such as

knowledge based decision support system

(DSS) are often used to assist in

quantification and monitoring of reliability

growth during the product development

phase (Nasser, 1989). The following choices

are available to a design engineer to

optimize reliability:

6. Simplify the design as much as

possible. The design with the least

complexity and fewer parts will

generally exhibit higher reliability

during operation. The reliability of the

individual components comprising the

product should be improved. Use

standard parts and materials with

verified reliability ratings (Priest, 1988).

7. Design products with redundant,

duplicate or backup systems to enable

them to continue operation should a

primary device fail. Use component

derating to improve the ratio of load to

capacity of the components. The

operation of a part at less severe stresses

than those for which it is rated is known

as derating (Alexander, 1992).

8. Give priority to improving weak

components than other parts. Design to

avoid fatigue failures such as corrosion

fatigue (Rao, 1992). Stress concentration

points are most prone to fatigue failures.

Designers should eliminate sharp

internal corners as they act as stress

concentrators. The prime cause of

reduced service life of electronic

products is overheating. Adequate means

such as ventilation or heat sinks must be

provided to prevent overheating.

9. Reduce the adverse effects of the

environment in which the product must

operate by: (a) providing insulation from

sources of heat, (b) providing seals

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against moisture, (c) using shock

absorbing mounts, ribs, and stiffeners to

make the product rugged against shock,

and (d) providing shield against

electromagnetic and electrostatic

radiation (Bralla, 1996).

It is either technically difficult or

prohibitively expensive to produce fail proof

products. Every consumer is aware of the

fact that during the life span of the product,

repair or maintenance service will be

needed. However, when a product fails it

should fail safely and the down time should

be as short as possible. A product that can be

repaired or serviced easily and quickly has a

high maintainability. Serviceability and

maintainability can be considered as

equivalent terms.

2e. SERVICEABILITY /

MAINTAINABILITY

Maintainability / serviceability is the

element of product design concerned with

assuring the ability of the product to perform

satisfactorily throughout its intended useful

life span with minimum expenditure of

effort and money. Maintenance can either be

preventive maintenance (regular or routine

service required for preventing operating

failures) or breakdown maintenance (repair

service after some failure or decline of

function has occurred). Designing for good

serviceability means providing for ease of

both these kinds of maintenance (Blanchard,

1995).

There is a strong overlap between the

objective of achieving high product

serviceability and other desirable design

objectives such as reliability and ease of

assembly/disassembly. Easy serviceability

can often compensate for lower reliability. If

a component is prone to failure but can be

easily replaced or repaired, the

consequences of failure are less severe. The

availability of product for use depends both

on the reliability and serviceability. High

availability means that the product is ready

for full use a high percentage of the time

because failure of components is rare or

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because replacement of failed components is

rapid or both (Smith, 1993).

Berzak (1991) has developed a

methodology to rate a product design for its

serviceability based on the calculation of the

total cost to service a product. The three

major independent contributors to the

service cost are cost per part, the failure

distribution and the labor associated with the

repair. The first two contributors are

interrelated. The failure distribution can be

reduced by raising the quality of the product

through selecting better materials, choosing

higher factor of safety in the design, or

applying more rigorous quality assurance

methods. However, all these measures will

increase the cost per part. The labor

associated with repair can be reduced by

easing the accessibility to those parts which

have to be serviced often, selection of

appropriate method of assembly and

sequence of assembly. Calculation of cost

and frequency associated with any given

service enables a designer to identify

problematic areas and to correct them before

the product is produced, rather than devising

methods of dealing with them after the

product is already in the market. A designer

has many options available to facilitate

effective and economical service.

10. Design the product so that

components prone to wear or failure are

easily visible and accessible for

inspection, testing, and easy replacement

(Mital, 1995). The covers, panels and

housings should be easy to be removed

and replaced. The product must be

designed so that the parts with high

reliability are assembled first and in a

lower, less accessible position and those

with less reliability are assembled last so

that they are closer to the cover and in an

accessible position when the cover is

removed. High-mortality components

should be located such that they can be

replaced without removing or changing

the settings of the other parts.The

product should be repairable by the user

rather than demanding attention of a

specialist. For easy field replacement

and repair the design should require

commonly available standard types of

tools (Bralla, 1996).

11. Use quick disconnect attachments

and snap fits to join the high-mortality

parts, or those that may need frequent

replacement or removal for service.

Funnel openings and tapered ends and

plug-in or slip fits facilitate easy

disassembly. Avoid press fits, adhesive

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bonding, riveting, welding, brazing, or

soldering of such parts.

12. Consider the use of modules which

are easily replaced when necessary and

easily tested to verify their operability.

As a module is a self-contained unit

comprising a group of components and

subassemblies serving a particular

function, they all can be easily installed

or replaced as one unit at the same time

(Moss, 1985). Testing and other

maintenance is also facilitated especially

when it is advantageous to do this when

the module is removed from the basic

product. Modular design makes it easier

to isolate faults. If spare modules are

available, the defective one can be

removed and repaired while it is

replaced with a spare, thus putting the

product back in service much more

quickly. The use of modules, however, is

not always preferable. Modules are

effective when testing and replacement

are rapid and when the accompanying

parts in the module are not expensive

(Karmarkar, 1987).

13. Design the product for easy

testability. Some testability principles

are: (a) as much as possible, design the

product and its components so that these

tests can be made with standard

instruments, (b) incorporate built-in test

capability and, if possible, built-in self-

testing devices in the product, (c) make

the tests themselves easy and

standardized, capable of being

performed in the field, (d) provide

accessibility for test probes: for example,

make test points prominent and provide

access parts or tool holes, and (d) make

modules testable while still assembled to

the product (Anderson, 1991).

2f. ENVIRONMENTAL

FRIENDLYNESS The accelerated flow of

waste and emission due to explosion in

industrial activities spurred by rising

demand for consumer products is causing an

increase in the pollution of the eco-system.

The consumers are demanding ‘green’

products as a result of a new environmental

awareness and the responsibility of the

manufacturers is gradually expanding over

the entire product life cycle (Tipnis, 1993).

A design that has minimal or no harmful

effects during manufacture, use and disposal

is considered environment friendly (Kaila,

1996). Life cycle assessments (LCA) tools

have been developed to analyze and

compare the environmental impact of

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various product designs (Hoffman, 1997).

LCAs review a product by summing up the

influence of all the processes during the life

of a product on various envrionmental

impact classes such as ozone depletion,

global warming, smog, acidification,

eutrophication, heavy metals, pesticides and

carcinogenics. The disadvantage with life

cycle analysis is that in order to evaluate the

environmentally responsible product rating

every LCA tool needs substantial database

for process information of all stages of the

life cycle and for various impact classes

with weighting factors for all materials,

emissions and other reaction products during

the product design stage itself (Nissen,

1997).

Some simplified procedures use

mass and energy used in a product or

processes as indicators instead of looking at

the often diffuse environmental impact

properties of a design. This is based on the

assumption that an ecologically undesirable

product will consume large material and

energy resources during its manufacture and

usage, and will need additional resources to

suppress the side effects during product

disposal.

The design deficiencies identified

during the environmental assessment

procedure should be removed by (1)

eliminating environmentally unfriendly

materials from the product and

manufacturing process, (2) if elimination is

not possible, reducing the quantity of such

materials, (3) designing the product so that

components can be reused with or without

refurbishing, and (4) designing the product

so that such materials can be recycled

(Glantschig, 1990). Some of the options that

a designer has in enhancing environmental

friendliness are:

14. Use of toxic materials in the

product or in the production

processes should be avoided.

Eliminate use of substances such

as CFCs or HCFCs . Reduce

manufacturing residues such as

mold scrap, cutting scrap and

minimize the use of solvents, oils

and acids during the

manufacturing process. Minimize

equipment cleanouts that generate

liquid or solid residues (Billatos,

1997).

15. Avoid product materials which

are restricted in supply. Avoid

product materials which have

disposal problem. Use recycled

materials rather than virgin

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materials if possible (Ashley,

1993). Minimize the amount of

periodic disposal of solid

materials such as cartridges,

containers and batteries. Design

to utilize recycled consumables

from outside suppliers. Design

products to minimize liquid

replenishment such as coolants

and lubricants. Design products to

minimize gaseous emissions such

as carbon dioxide or tetraethyl

lead (Bralla, 1996).

16. Design products to consume less

energy. Also, choose the form of

energy alternative which has the

least harmful effect on the

environment. Design should

include features such as sleep

mode which conserves energy

during the time when the product

is not in use(Shiovitz, 1997).

17. Designs requiring spray-painted

finishes should be avoided

(Lankey, 1997). The need for

environmentally damaging

solvents can be avoided by using

powder coating, roll-coating or

dip-painting for surface finishing

of metals. Plastic parts which are

molded in color eliminates the

need for painting.

2g. RECYCLABILITY /

DISPOSABLITY Thousands of consumer

goods come to the end of their useful life

every day and joins the waste stream. It is

estimated that more than 10 million vehicles

reach the end of their useful lives every year

and an estimated 150 million discarded

personal computers will have been landfilled

by the year 2005 (Chen, 1993). To deal with

such a situation it is imperative that the

products are designed for recyclability.

Product recycling reduces adverse impact on

the environment by reducing the volume of

materials deposited in the landfills, and

conserves scarce natural resources (Tipnis,

1994, Pnueli, 1997). The steps involved in a

recycling program are (1) collection of

worn-out products, (2) disassembly of the

product and sorting of incompatible

materials, (3) cleaning, shredding, and

grinding of materials as necessary, and

separation of high value materials such as

steel for reclaiming, (4) conversion into

quality-consistent, usable material, and (5)

discarding the fluff to the waste stream or

landfill.

The considerations for design for

recyclabilty often overlap with the

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considerations for design for disassembly

(Zussman, 1994). Srinivasan has developed

a disassembly tool to support product design

for recyclability (Srinivasan, 1997). It

identifies the abstract design modules that

need to be developed to build a geometric

virtual disassembly tool. The modules are

software programs executed as part of four

step design process involving (1) product

analysis, (2) disassemblability analysis, (3)

optimal disassembly sequence generation,

and (4) design rating. The product analysis

step involves selection of components that

need to be disassembled and the appropriate

de-manufacturing application. The

information regarding the components to be

disassembled is obtained from the (1)

knowledge-base which consists of material,

environment and application domain

database, and (2) the user requirements. The

disassemblability analysis step consists of

determining the disassemblability

components and analyzing all possible

disassembly methods and the selection of an

appropriate disassembly that best fits the

user requirements. The third step consists of

generating an optimal disassembly sequence

and disassembly directions for the

components to be disassembled. The final

step involves disassembly evaluation,

disassembly rating and design

recommendations. A typical rating index

depends on the number of components

disassembled, ease of disassembly,

complexity of path, and time taken for

disassembly. The design recommendations

focuses on enhancing the product design by

minimizing the disassembly cost and time

involved in the overall product cycle.

Chen (1993) presents a cost benefit

analysis as another tool for assessing the

economics of designing for recyclability.

The cost of recycling includes cost of

disassembly, shredding, material recovery

and dumping. The total benefit from

recycling includes revenue from used parts,

revenue from used parts and recovered

material, and benefit of emission reduction

from energy saving. The guidelines that help

in reducing the cost and increase the revenue

due to recycling are:

18. The product and its components

should be designed such that they can be

reused. The major components should be

designed to be remanufactured or

refurbished rather than reclaimed only

for its materials.

19. Minimize the number of parts it

contains as fewer parts make sorting of

materials easy for recycling. Avoid the

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use of separate fasteners, if possible.

Snap-fit connections between parts are

preferable because they do not introduce

a dissimilar material and often easier to

disassemble with simple tools. The

number of screw head types and sizes

used in fasteners in one product should

be minimized so that changing of the

tools used to loosen and remove

fasteners is reduced during recycling

(Bralla, 1996). Use of fewer number of

fasteners reduces the disassembly time.

Modular design simplifies disassembly.

20. Minimize the amount of material in

the product. The less the amount of

material involved, the simpler the

eventual disposal problem when the

product has reached the end of its useful

life. Less material also means that,

eventually, the product will need less

landfill space. By designing for near-net-

shape manufacturing processes that

minimize material scrap, designers can

achieve benefits comparable to

designing smaller and lighter parts.

21. Reduce the number of different

materials in a product. Use of dissimilar

materials that can not be separated or are

difficult to separate from the basic

materials should be avoided (Berko-

Boateng, 1993). As thermoplastic

materials can be recycled by melting,

they are preferred to thermosetting

materials. For joining plastics solvent,

friction, or ultrasonic welding is

preferable to adhesive bonding and for

metals welded joints are preferable to

brazed or soldered joints (Dewhurst,

1993). If adhesive bonding can not be

avoided, an adhesive material that is

compatible need to be used when the

components are recycled. For labels

water-soluble adhesives facilitate

separation during recycling.

2h. SAFETY

The increasing number of injuries filed each

year in courts due to personal injuries while

using consumer products indicates that

safety may be the most basic consideration

in product design from human as well as

cost standpoint (Heideklang, 1990; Ryan,

1983). ‘Safety’ implies absence of hazards

or the minimal exposure to them during

entire life cycle of the product (Bass, 1984).

Schoone-Harmsen (1990) developed an

iterative three step method to detect and

solve safety problem during product design.

It consists of (1) analysis of the problem, (2)

identification of critical factors, and (3)

17

synthesis. The analysis step is used to

evaluate the product on their safety, by

gaining insight into possible accidents

connected with either the product, the

actions of the user, or environmental

conditions. The second step consists of

identifying the factors that are critical

among those found in the analysis. If a

critical factor is related to the product, the

hazard can be removed by changing the

corresponding characteristics of the product.

If actions or posture of the user or

environmental conditions cause the hazards,

the designer should change the product

features connected with such critical factors.

During the synthesis step a structured list of

solution strategies to the detected safety

problems is developed. Correcting a critical

product feature is done through selection of

a different working principle, deactivation

during use before injury or damage occurs,

separation of the user from the source of

danger, and limitation of the possibility of

the user to modify the product. Correcting a

critical action associated with the user or the

environment is achieved by influencing the

actions of the user through the product,

selection of the user by anthropometric or

cognitive characteristics, and influencing the

selection of place of use through the

product. The effectiveness of the design

solutions can be found by performing the

analysis step again. If the improvement is

not sufficient or new hazards have been

identified the whole process can be repeated

till all the hazards are either eliminated or

are found acceptable, and all the safety

standards are complied with (Wilson, 1984).

Standard techniques such as fault

tree analysis, failure mode analysis and

sneak circuit analysis can be used to design

safety into a product (Hammer, 1980).

Safety concerns often overlap with

reliability and ease of use. The following

considerations are intended to aid the

designer in creating a safe product:

22. The products should be fail-safe. As

users can occasionally make mistakes in

the operation of a product the design

should allow for human error. When

such human errors happen, or when

there is failure of mechanism, it should

not result in an accident. Products

should be designed to be user-friendly

and to operate with the human

capabilities to minimize the possibility

of human errors that can cause accidents

(Chow, 1978).

23. Parts that require service should be

freely accessible, easily repairable and

18

replaceable without causing interference

with other assemblies and without

posing hazards to the user. To avoid

shearing or crushing points in which

hands or other parts of an operator’s

body might be caught or injured

adequate clearances should be provided

between moving parts and other

elements. Design should replace sharp

corners with liberal radii as sharp

external corners are hazards during

operation and maintenance of the

product.

24. The design of the product should be

robust enough to withstand adverse

environment in which it will be used and

provide safeguards against those

environmental factors such as corrosion,

vibration, pressure changes, radiation,

and fire which could create safety

hazards (Witherall, 1985). Reduce the

level of noise (Lyon, 1994) and vibration

(Fraser, 1993) to avoid their harmful

effects on users. Provide adequate

ventilation and lighting.

25. Make the product from high-impact

or resilient materials so that it does not

break, when dropped accidentally, into

small fragments with sharp edges or

sharp points that are potentially

swallowable by children. Use of

flammable materials including

packaging materials should be

minimized. Avoid the use of materials

that are a hazard when burned, recycled,

or discarded (Bralla, 1996).

2i. CUSTOMIZABILITY

So far, the aim of product design and

development has been to create a product

that satisfies the needs of the average

customer. No consideration has been given

to differences in individual tastes and

preferences. Often, customers are willing to

pay more if their individual needs are better

satisfied. Design for mass customization

(DFMC) is a new approach to producing an

increasing variety of customer’s

requirements without a corresponding

increase in the cost and lead-time (Tseng,

1996). Providing products and services

which best serve the customers’ needs while

maintaining mass production efficiency is a

new paradigm for industries. The

recognition of each customer as an

individual and the subsequent production of

products with tailor-made features is the

basis of this new approach. The core of

DFMC is to develop a mass customization

oriented product family architecture (PFA)

19

with a meta level design process integration

as a unified product creation and delivery

process model. The inherent repetition in

product marketing, design, and

manufacturing can be recognized through

the establishment of patterns. Once patterns

are identified and formulated into a product

family architecture, scale of economy can be

applied for efficiency. The formulation of

PFA enables the optimization of

reusability/commonality in both product

design and process selection from the

product family perspective. It also provides

a basis to facilitate the front-end

configuration in order to fulfill the

individual requirements of the customers

(Tseng, 1997).

While product customization enables

the design of products and processes to meet

individual customer needs, it is essential to

note that such needs change frequently,

forcing frequent modification in product

design. This calls for a dynamic

reconfiguration of manufacturing systems to

accommodate the swift changes in product

design. Development of Integrated

Manufacturing Systems (IMS) aimed at

multi-product, small-batch production, fast

and optimized design, speedy product

development, and just-in-time delivery made

possible by Strategic Information Systems

(SIS) has been in use as a strategy to achieve

this (Hitomi, 1991). Now, agile

manufacturing is an emerging concept in

industry that aims at achieving flexibility

and responsiveness to changing customer

needs. Agile manufacturing systems seek to

produce a large variety of products

efficiently and are recongfigurable to

accommodate changes in the product mix

and design changes (Kusiak, 1997).

3. DESIGN SUPPORT TOOLS /

METHODOLOGIESBesides the design approaches and

guidelines discussed so far, the following

design methodologies and tools are also

widely used: (1) Design for Producibility,

(2) Design for Assembly, (3) Robust Design,

(4) Group Technology, and (5) Quality

Function Deployment. Genetic algorithms

(Balakrishnan, 1996; Balakrishnan, 1995)

and Conjoint analysis (Kohli, 1987) methods

are mathematical tools associated with

product design.

3a. Design for Producibility: The design of

an individual component will have a strong

effect on the attributes of the product in

which it is used. Design for producibility

20

emphasizes that design of detailed parts

cannot be independent of the manufacturing

process (Burhanuddin, 1992). Design

principles and guidelines for a part that is

made with one process may not apply if

another process is used. For example, if a

part is to be die cast, the suitable materials,

the wall thickness, shape, complexity, size,

dimensional tolerances, and other

characteristics will be significantly different

from those applicable to a metal stamping or

a part made from metal powder. The

resultant part attributes, such as strength,

temperature resistance, and corrosion

resistance, may also be different. The

selection of part features and the processes

should occur simultaneously. There are

many guidelines for the design of individual

parts based on the manufacturing processes

used. Table 1 shows various processes and

the characteristics of parts made using them

and the variables that control the part quality

compiled from various manufacturing

handbooks (Bralla, 1986; Cubberly, 1989;

Dallas, 1976). The design principles given

below can however be applied to component

parts regardless of the process.

• Simplify the design of each part as

much as possible (Stoll, 1988). Use

simple shapes instead of complex

contours, undercuts, and elaborate

appendages. Parts of simple shape

have less opportunity to be defective.

Use the most liberal tolerance

possible, consistent with the quality

and functional requirements of the

part and with the capabilities of the

manufacturing processes involved.

Tolerances appropriate to the

primary operations eliminate the

need for costly secondary operations

to control dimensions and refine

surface finishes (Billatos, 1990).

• Select near-net-shape processes that

are capable of producing a part to or

near final dimensions with a limited

number of operations, particularly

minimum machining, such as

injection molding and powder

metallurgy. An injection molding

part can have all final dimensions,

identifying nomenclature, finish, and

color

provided in one operation. A powder metal

part can be complete with precision bearing

surfaces after only two or three high-

production operations. Standardize parts

21

features and minimize their number. Avoid

designs that require machining operations.

Often another process can be substituted for

one that primarily involves machining with

significant savings. For example, sheet

metal processes can be used to provide parts

with bearing surfaces, holes, reinforcing

ribs, etc. Extruding, precision casting, cold

rolling or the other near-net shape processes

may provide the precision needed for

elements and surfaces that otherwise would

require machining (Bralla, 1996). Use

materials formulated for easy manufacture.

For example, free-machining alloys for

machined parts, or high-ductility materials

for drawn part can be used.

3b. Design for Assembly: In this approach,

the overall assembly is analyzed primarily to

determine if components can be eliminated

or combined leading to a simplified product

assembly. Service and recycling are

facilitated when a product is simplified. A

product which is easy to assemble is

normally easier to disassemble when

maintenance, repair, or disassembly or

recycling take place (Eversheim, 1991).

Simpler assemblies can often be brought to

market sooner because of fewer parts to

design, procure, inspect, and stock with less

probability that a delay will occur. Products

with fewer parts will also have higher

reliability (Boothroyd, 1992; Boothroyd,

1994).

Processes such as injection molding

and die-casting permit very complex parts

that result when separate parts are combined

into one. By selecting flexible material and

making wall sections thin hinges and springs

can be incorporated in plastic parts. Integral

snap-fit elements, tabs, crimped sections or

catches, press fits and rivets can be used to

replace threaded fasteners (Joines, 1995).

With some manufacturing processes, it is

possible to incorporate elements such as

guides and bearings in the basic part by

selection of appropriate materials and

processes. Due to their natural lubricity

many plastic materials can be used in

applications involving bearing surfaces

when the velocity and pressure involved are

low. For more demanding bearing surfaces

powder metal part with sufficient precision

and porosity can be used as it can retain the

lubricating oil within itself (Bralla, 1996).

As modularity improves

serviceability and reliability, the design

should include modular subassemblies while

avoiding too many levels of subassembly at

the same time (Karmarkar, 1986). Adopt

layered and top-down assembly whereby

each successive part in the product can be

added to the assembly from above rather

than from the side or bottom. Design parts

22

such that they are self-aligning and that they

can not be inserted incorrectly. Design very

small or highly irregular parts that are

manually assembled for easy handling by

adding a grasping element to the parts.

3c. Robust Design: Robustness of a design

refers to the design that ensures that the

product will never fail to perform its

intended function during its useful life.

Robust design methodology, popularly

known as Taguchi Technique, provides a

way to develop specifications for robust

design by using the design of experiments

theory. The procedure attempts to find out

the settings of product design parameters

that make the product’s performance

insensitive to environmental variables,

product deterioration and manufacturing

irregularities. It is often more costly to

control causes of manufacturing variations

than to make a product or process

insensitive (or robust) to these variations

(Juran, 1974).

23

Taguchi separates off-line quality

planning and improvement activities into

three stages: system design, parameter

design, and tolerance design. System design

is the application of scientific and

engineering knowledge to produce a

functional prototype. This prototype model

defines the basic product/process design

characteristics (parameters) and their initial

settings. The goal of parameter design is the

identification of settings that minimize

variation in the performance characteristic

and adjust its mean to an ideal value.

Tolerance design is a method for

scientifically assigning tolerances so that

total product manufacturing and lifetime

costs are minimized (Nevins, 1989).

3d. Group Technology: Group Technology

procedure attempts to classify the system

into subsystems and subdivides them into

part families based on design attributes and

manufacturing similarities (Chang, 1991).

Group technology can be used for product

design and manufacturing system design.

For product design, components that have

similar shape are grouped into design

families and a new design can be created by

simply modifying an existing component

design from the same family. Using a coding

method, each part is given a numerical or

alphabetical code based on its geometrical

shape, complexity, dimension, accuracy and

raw material. By using this concept,

24

composite components can be identified.

Composite components are parts that

embody all the design features of a design

family or design subfamily (Farris, 1990).

For manufacturing purposes, parts with

similar processing requirements comprise a

production family. Since similar processes

are required for all family members, a

machine cell can be built to manufacture the

family. As a result the production planning

and control is made much easier and the

cycle time to manufacture a product is

greatly reduced even while maintaining

product variability. Thus, planning using

group technology method can be used in

production environment for manufacturing

goods for mass customization. (Shetty,

1993).

3e. Quality Function Deployment (QFD):

This is a methodology of translating the

requirements of the customers into product

and process design (Akao, 1990). The QFD

technique, using the house of quality, is used

to translate customer views systematically

into key engineering characteristics,

planning requirements, and, finally, into

production operations (Bergquist, 1996). It

is achieved through its four key documents

which are the product planning matrix, the

product deployment matrix, component

deployment matrix, and the operating

instruction sheet. The purpose of the product

planning matrix is to translate customer

requirements into important design features.

The individual customer needs are ranked

for importance and the cumulative effect on

each of the design features is obtained. A

product deployment matrix is then made for

each of the product features down to the

subsystem and component level. The

product deployment matrix shows to what

extent the relationship between component

and product characteristics are critical and

affordable. If a component is critical, then it

is further deployed and monitored in the

design, production planning, and control.

The component deployment matrix expands

the list of components or the exact

parameters required to design a complete

component. The operating instruction sheet

is the final key document that basically

defines the operator requirements as

determined by the actual process

requirements, the process checkpoints, and

the quality control points (Day, 1993). Thus

QFD tries to achieve quality products by

using the philosopy of concurrrent

engineering (Parsei, 1993) which integrates

product design, process design, and process

control (Maduri, 1993).

SUMMARY AND CONCLUSION

25

The various desirable usability objectives

and their realization through appropriate

product design and manufacture have been

reviewed in this article. The paper lists

various usability criteria and briefly reviews

the design and manufacturing issues for each

one of the them individually. As one may

note, in many cases, the design guidelines

serve and enhance more than one design

objective. For example simplifying the

design to incorporate a smaller number of

parts and using modular design improves

reliability as well as serviceability. Designs

that enhance safety often reduce the need for

physical exertion for the users, making such

designs easy to operate. Products which are

biodegradable are both recyclable and

environment friendly.

However, the design

recommendations are not always mutual and

are often seen to conflict with one another.

Using liberal tolerances reduces production

costs and eliminates expensive and time

consuming secondary operations. Tool

maintenance and quality inspection can be

reduced and higher speeds and feeds can be

employed. But liberal tolerances can lead to

more variations in components causing

variations in product performance, quality,

and reliability. Paints that enhance external

appearance often contain harmful heavy

metal elements and solvents which pose risk

to the environment. Hence the product and

process design recommendations should be

examined together for their effect on

enhancing or optimizing various usability

objectives. Such an examination calls for a

unified approach aimed at simultaneous

evaluation of various design options and

integrating the various phases of product

design, i.e., planning, concept design, and

process design. In Part II of this paper

(Govindaraju, 1998) we illustrate how QFD

matrices can be used for implementing an

holistic approach to product design.

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