design for manufacture - project

DESIGN FOR MANUFACTURE AND ASSEMBLY (DFMA)

“DFM ASSIGNMENT – LOGITECH COMPUTER MOUSE”

PREPARED BY

MOHD NIZAM BIN ALI

DEPT. OF MANUFACTURING AND INDUSTRIAL ENGINEERING

FACULTY OF MECHANICAL ENGINEERING

UNIVERSITI TEKNOLOGI MALAYSIA

2010

I

LIST OF FIGURE

Page Figure 2.1 The product design phases and material selection

(Adapted from Mangonon 1999) 4 Figure 2.2 Factors influencing the material selection processes 5 Figure 2.3 Taxonomy of manufacturing processes (adapted from Groover, 1996) 8 Figure 2.4 Types of casting processes 10 Figure 2.5 Types of forging processes 10 Figure 2.6 Types of extrusion processes 11 Figure 2.7 Classification of various machining processes 12 Figure 3.1 PCB Lower Case (New Design) 13 Figure 3.2 PCB Upper Case (New Design) 13 Figure 3.3 PCB Upper Case Cover (New Design) 14 Figure 3.4 Clicker Wheel 14 Figure 3.5 Clicker Wheel Shaft Holders 15 Figure 5.1 Injection molding machine 55

II

LIST OF TABLES

Page Table 4.1 Compatibility between processes and materials 16 Table 4.2 Shapes attributes for PCB Lower Case (New Design) 17 Table 4.3 Process elimination based on 8 geometric attributes

PCB Lower Case (New Design) 18 Table 4.4 Final process selection based on process/materials combination of

PCB Lower Case (New Design) 19 Table 4.5 Compatibility between processes and materials 21 Table 4.6 Shapes attributes for PCB Upper Case (New Design) 22 Table 4.7 Process elimination based on 8 geometric attributes

PCB Upper Case (New Design) 23 Table 4.8 Final process selection based on process/materials combination of

PCB Lower Case (New Design) 24 Table 4.9 Compatibility between processes and materials 26 Table 4.10 Shapes attributes for PCB Lower Case Cover (New Design) 27 Table 4.11 Process elimination based on 8 geometric attributes

PCB Upper Case (New Design) 28 Table 4.12 Final process selection based on process/materials combination of

PCB Lower Case Cover (New Design) 29 Table 4.13 Compatibility between processes and materials 31 Table 4.14 Shapes attributes for Clicker Wheel 32 Table 4.15 Process elimination based on 8 geometric attributes

Clicker Wheel 33 Table 4.16 Final process selection based on process/materials combination of

Clicker Wheel 34 Table 4.17 Compatibility between processes and materials 36

III

Table 4.18 Shapes attributes for Clicker Wheel Shaft Holders 37 Table 4.19 Process elimination based on 8 geometric attributes

Clicker Wheel Shaft Holders 38 Table 4.20 Final process selection based on process/materials combination of

Clicker Wheel Shaft Holders 39 Table 5.1 Mould cavity pressure required depending on the product group 42 Table 5.2 PBT unreinforced and PBT reinforced process requirement 52 Table 7.1 Parts and materials requirement 60 Table 7.2 Parts attributes 61 Table 7.3 Final selection for material and processes 62

MMP 1663 DFM Assignment: Logitech Computer Mouse

1

1.0 OBJECTIVE

The objectives of the DFM assignment are:

Apply the principle of design for manufacturing for selected part in Logitech

computer mouse.

Suggested the suitable material to be used based on the part material requirement

and performance functions.

Examine the modern manufacturing operations capabilities and limitations

including machining, casting, forging, soldering, brazing, finishing, heat treating,

assembly, plastic materials processing, powder metallurgy and specialized

manufacturing processes.

Discuss the relation of the manufacturing process related to the part design and

cost.


2

2.0 INTRODUCTION

2.1 Manufacturing, Design and Design for Manufacturing (DFM)

Different uses of the word manufacturing create an unfortunate confusion.

Sometimes the word is used to refer to the entire product realization process, that is, to

the entire spectrum of product-related activities in a firm that makes products for sale,

including marketing (e.g., customer desires), design, production, sales, and so on. This

complete process is sometimes referred to as "big-M Manufacturing." But the word

manufacturing is also used as a synonym for production, that is, to refer only to the

portion of the product realization process that involves the actual physical processing of

materials and the assembly of parts. This is sometimes referred to as "little-m

manufacturing."

Design (as in a design process) is the series of activities by which the information

known and recorded about a designed object is added to, refined (i.e., made more

detailed), modified, or made more or less certain. In other words, the process of design

changes the state of information that exists about a designed object. During successful

design, the amount of information available about the designed object increases, and it

becomes less abstract. Thus, as design proceeds the information becomes more complete

and more detailed until finally there is sufficient information to perform manufacturing.

Design, therefore, is a process that modifies the information we have about an artifact or

designed object, whereas manufacturing (i.e., production) modifies its physical state.

Design for manufacturing (DFM) is a philosophy and mind-set in which

manufacturing input is used at the earliest stages of design in order to design parts and

products that can be produced more easily and more economically. Design for

manufacturing is any aspect of the design process in which the issues involved in

manufacturing the designed object are considered explicitly with a view to influencing

the design. Examples are considerations of tooling costs or time required, processing

costs or controllability, assembly time or costs, human concerns during manufacturing


3

(e.g., worker safety or quality of work required), availability of materials or equipment,

and so on. Design for manufacturing occurs or should occur throughout the design

process.

2.2 Consideration and Selection of Materials

After the conception of a product idea, the question that the research and

development (R&D) personnel must ask is, what would be the best material for the

product? More often this is closely followed by the question, is the material selected

easily manufacturable? In other words, what would be the best material and process

combination for developing a product that not only performs the indispensable functions

but is also economical to manufacture. A design criterion for the product based only on

either material or process has all the ingredients of a recipe for disaster. The choice of

material is a major determinant for the successful functioning and the feasible, low-cost

manufacture of any product.

Materials are at the core of all technological advances. Mastering the

development, synthesis, and processing of materials opens opportunities that were

scarcely dreamed of a few short decades ago. The truth of the statement is evident when

one considers the spectacular progress that has been made in such diverse fields as

energy, telecommunication, multimedia, computers, construction, and transportation.

It is widely accepted that the final cost of a manufactured product is determined largely at

the design stage. Designers tend to conceive parts in terms of processes and materials

with which they are familiar and, as a consequence, may not consider process and

material combinations that could prove more economical. Sometimes, the designers tend

to focus only on the cost aspect of materials and manufacturing and select a combination

of materials and processes that lead to products of substandard quality and reduced

operating life. In the long run, this not only leads to reduced brand loyalty for the product

but, in many cases, to huge financial losses as a result of litigations and product liability

lawsuits. The already difficult task of satisfying engineering and commercial

requirements imposed on the design of a product becomes even more difficult with the


4

addition of legislated environmental requirements. A vital cog in this product design

wheel is the materials engineer. The optimal selection of material used to construct or

make the product should lead to optimum properties and the least overall cost of

materials, ease of fabrication or manufacturability of the component or structure, and

environmentally friendly materials.

Figure 2.1 The Product Design Phases and Material Selection (adapted from

Mangonon, 1999).

Figure 2.1 shows the various stages of the design process with their associated

activities. The material selection process consists of the property, process, and

environmental profiles concurrently considered at each phase of design. What happens if

the material selection is not considered during each stage of the design decision process?

The designer would be unaware of any problems about the availability of the final

material, the costs associated with the manufacturing processes, or the processability of

the product to be manufactured. Consider a designer who needs to design a product but

has no idea of the material from which to make it. Suppose the designer designs the


5

product considering it to be a metallic, but management decides to make it of ceramics at

a later stage. The processing of a ceramic product is entirely different from that of a

metallic product. Ceramic and metallic products vary in structure, strength properties,

manufacturability, and so on. Therefore, it is critical that decisions regarding materials to

be used for manufacturing a product be made in a timely fashion (Mangonon, 1999). The

selection of an appropriate material and its conversion into a useful product with the

desired shape and properties is a complex process. The first step in the material selection

process is the definition of the needs of the product. Figure 2.2 shows the factors

affecting the material selection process:

Figure 2.2 Factors Influencing the Material Selection Processes.

a. Physical factors: The factors in this group are the size, shape, and weight of the

material needed and the space available for the component. Shape considerations

greatly influence selection of the method of manufacture. Some typical questions

considered by a materials designer are. What is the relative size of the


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component? How complex is its shape? Does it need to be one piece or can it be

made by assembling various smaller pieces? How many dimensions need to be

specified, and what are the tolerances on these dimensions? What are the surface

characteristic requirements for the product? All the factors in this category

interrelate to the processing of the material. For example, shape and size might

constrain the heat-treatment of the material. The shape of the product determines

whether casting could be used. Material consideration, to a large extent, also is

determined by the space available for the component.

b. Mechanical factors: The ability to withstand stress and strain is determined by

these factors. Strength, ductility, modulus, fatigue strength, and creep, are some

mechanical properties that influence what material needs to be used. The

mechanical properties also are affected by the environment to which the materials

are exposed. Some typical questions that designers consider while narrowing

down the material to be used are. What are the static strength needs of the

product? What is the most common type of loading to which the product would be

subjected during its use (tensile, compressive, bending, and cyclic)? Is the loading

static or dynamic? Would the product be subjected to impact loading? Does the

product require wear resistance? What temperature range must the mechanical

properties possess?

c. Processing and fabrication factors: The ability to form or shape a material falls

under the processing and fabrication factors. Casting and deformation processing

are commonly used. Typical questions that arise out of consideration of these

factors are Has the design addressed the requirements that facilitate ease of

manufacture? Machineability? Weldability? Formability? Hardenability?

Castability? How many components are to be made? What must be the production

rate? What are the maximum and minimum cross-sectional dimensions? What is

the desired level of quality for the finished product? Small objects more

commonly are investment casted, while intricate shapes are produced as casting.

Powder metallurgy, or a sintering process, is commonly used for the brittle

materials like ceramics.


7

d. Life of component factors: These factors relate to the life of the materials to

which they perform the intended function. The properties of this group are the

external surface properties like oxidation, corrosion, and wear resistance and

some internal properties like fatigue and creep. The performance of materials

based on these properties is the hardest to predict during the design stages.

e. Cost and availability: With reduced lead times from design to market, there is a

tendency to jump to the first material that fits the selection profile. It is important

to note that additional effort determining the correct material helps optimize the

manufacturing costs. Also, standardization of parts and materials is related to the

cost of the final product. Special processing requirements or rare materials with

limited availability increase the final cost and affect the timely manufacture of the

product.

2.3 Economic of Material Selection

After developing a comprehensive list of requisite properties in a material,

categorize these properties according to their level of criticality. Some property

requirements may be absolute, while others may be relative. The absolute ones cannot be

compromised and should be used as a filter to eliminate the materials that cannot be used.

It is apparent that no one material would emerge as the obvious choice. Here, the

knowledge of a materials engineer and the handbook-type data need to be utilized. Also,

the cost factor of materials needs to be closely analyzed here. Cost is not a service

requirement, but it plays an important part in the selection process, both the material cost

and the cost of fabricating the selected material. The final decision involves a

compromise between the cost, producibility, and service performance.

Current market and economic trends force companies to produce low-cost, high-

quality products to maintain their competitiveness at the highest possible level. There is

no doubt that reducing the cost of a product is more effective at the design stage than at

the manufacturing stage. Therefore, if the product manufacturing cost can be estimated


8

during the early design stage, designers can modify the design to achieve proper

performance as well as a reasonable cost at this stage, and designers are encouraged to

design to cost.

2.4 Selection of Manufacturing Processes

The manufacturing process is the science and technology by which a material is

converted into its final shape with the necessary structure and properties for its intended

use. Formation of the desired shape is a major portion of processing. The product

processing could be a simple, one step operation or a combination of various processes,

depending on the processability of the material used and the specifications for the

finished part, which includes surface finish, dimensional tolerances, and so forth. The

method of selecting the appropriate process is closely tied to the selection of material.

What leads to a successful manufacturing process? The performance of any

manufacturing process depends on:

a. Rate: material flow through the system.

b. Cost: material, labor, tooling, equipment.

c. Time: lead time to procure materials, processing time, setup time.

d. Quality: deviation from the target.

All these factors result from decisions made in selecting the process-material-part

combination. As designers and engineers developing a new product, at this juncture, we

already have the basic part drawing and a selection of various material-process

combinations feasible for the part. The next stage is arriving at the material-

manufacturing process combination that is technically and economically feasible. Figure

2.3 shows the taxonomy of manufacturing processes. The processes are arranged by

similarity of function. Manufacturing processes can be broadly classified into three

categories. Based on the desired outcome, they are primary, secondary, or tertiary

processes.


9

Figure 2.3 Taxonomy of Manufacturing Processes (adapted from Groover, 1996)

2.5 Primary Processes

The primary process generates the main shape of the final product. The primary

process is selected to produce as many required shape attributes of the part as possible.

Such processes appear at the top of the sequence of operation for a part and include

processes such as casting, forging, molding, rolling, and extrusion.

a. Casting: Casting is the fastest way to attain simple or complex shapes for the part

from its raw material. The casting process basically is accomplished by pouring a

liquid material into a mold cavity of the shape of the desired part and allowing it

to cool. The different types of casting methods (for both metals and nonmetals)

are shown in Figure 2.4.


10

Figure 2.4 Types of Casting Processes

b. Forging: Forging is a deformation process in which the work is compressed

between two dies using either impact or gradual pressure to form the part. The

different types of forging processes are shown in Figure 2.5.

Figure 2.5 Types of Forging Processes

c. Extrusion: Extrusion is a compression forming process in which they worked

metal is forced to flow through a die opening to produce the desired cross-

sectional shape. Extrusion usually is followed by a secondary process, cold

drawing, which tends to refine the molecular structure of the material and permits

sharper corners and thinner walls in the extruded section. The different extrusion

processes can be classified as shown in Figure 2.6.


11

Figure 2.6 Types of Extrusion Processes

2.6 Secondary Processes

The secondary processes, in addition to generating the primary shape, form and

refine features of the part. These processes may appear at the start or later in a sequence

of processes. These include all the material removal processes and processes such as

machining, grinding, and broaching.

Machining is the process of removing material from a workpiece in the form of

chips. The term metal cutting is used when the material is metallic. Most machining has a

very low setup cost compared to the forming, molding, and casting processes. However,

machining is much more expensive for high volumes. Machining is necessary where tight

tolerances on dimensions and finishes are required.

The different machining processes are shown in Figure 2.7. It is commonly

divided into the following categories:

a. Cutting generally involves single-point or multipoint cutting tools, each with a

clearly defined geometry.

b. Nontraditional machining processes utilize electrical, chemical, and optimal

sources of energy.


12

c. Abrasive machining processes are categorized under surface treatment and, hence,

are discussed as tertiary processes.

Figure 2.7 Classifications of Various Machining Processes

2.7 Tertiary Processes

The tertiary processes do not affect the geometry or shape of the component and

always appear after one or more primary and secondary processes. This category consists

of finishing processes, such as surface treatments and heat treatments. Selection of a

tertiary process is simplified because many tertiary processes affect only a single attribute

of the part.


13

3.0 SELECTED PARTS

Part that are selected to be analysed in DFM as shown below:

A: PCB Lower Case (New Design)

Figure 3.1 PCB Lower Case (New Design)

B: PCB Upper Case (New Design)

Figure 3.2 PCB Upper Case (New Design)


14

C: PCB Upper Case Cover (New Design)

Figure 3.3 PCB Upper Case Cover (New Design)

D: Clicker Wheel

Figure 3.4 Clicker Wheel


15

E: Clicker Wheel Shaft Holder

Figure 3.5 Clicker Wheel Shaft Holders


16

4.0 SUITABLE MATERIALS AND PROCESSES FOR EACH SELECTED PART.

4.1 PCB Lower Case (New Design)

Table 4.1 Compatibility between process and materials

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Sand Casting Investment Casting Die Casting Injection Moulding Structural Form Moulding Blow Molding (Ext.) Blow Molding (Inj.) Rotational Molding Impact Extrusion Cold Heading Closed Die Forging Powder Metal Parts Hot Extrusion Rotary Swaging Machining (From Stock) ECM EDM WEDM Sheet Metal (Stamp/bend) Themoforming Metal Spinning

Compatible between process and materials Not applicable Normal practice Less common


17

Table 4.2 Shapes attributes for PCB Lower Case (new design)

No. Shapes Attributes Yes/No

1. Depression Yes

2. Uniform Wall Thickness Yes

3. Uniform Cross-Section No

4. Axis of Rotation No

5. Regular Cross-Section No

6. Captured Cavity No

7. Enclosed Cavity No

8. No Draft No

Material Requirement

A Used snap fit features (flexibility)

B Excellent of Electrical Resistivity (> 1015) µΩ.cm

C Good impact resistance

D Light weights


18

Table 4.3 Process elimination based on 8 geometric attributes of PCB Lower Case

(new design).

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Sand Casting Investment Casting Die Casting Injection Moulding 6,2 Structural Form Moulding 6,2 Blow Molding (Ext.) 7,2 Blow Molding (Inj.)

Rotational Molding

Impact Extrusion Cold Heading Closed Die Forging Powder Metal Parts

3 Hot Extrusion 4,1 Rotary Swaging

Machining (From Stock) ECM EDM WEDM

2 Sheet Metal (Stamp/bend) 2 Themoforming

4,2 Metal Spinning Compatible between process and materials Not applicable Normal practice Less common


19

Table 4.4 Final process selection based on process/materials combinations of PCB Lower

Case (new design)

A A A A A A A A A A A B B B B B B B B B B B C C C C C C C C C C D D D D D D D D D D

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Rotational Molding


3 Hot Extrusion 4,1 Rotary Swaging

Machining (From Stock) ECM EDM WEDM

2 Sheet Metal (Stamp/bend) 2 Themoforming

4,2 Metal Spinning Compatible between process and materials Not applicable Normal Practices Less Common


20

Based on the final process selection shown above, INJECTION MOLDING is suitable

processes to be selected. Meanwhile, THERMOPLASTICS material is selected to be used for

PCB lower case (new design). The characteristic of part chosen, such as below:

a. Part:

• Size: 115 mm x 60 mm x 25 mm

• Weight: 50 gram

b. Tolerances:

• General: ± 0.15 mm

c. Surface Finish:

• 0. 2- 0.3 µm

d. Product Life Volume

• More than 5 years

e. Process Recommended:

• Injection Molding

f. Materials:

• Thermoplastic

g. Production Volume:

• 10, 000 units


21

4.2 PCB Upper Case (New Design)


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Table 4.6 Shapes attributes for PCB upper case (new design)


1. Depression Yes

2. Uniform Wall Thickness No






8. No Draft No





D Light weights


23

Table 4.7 Process elimination based on 8 geometric attributes of PCB Upper Case

(new design).

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Sand Casting Investment Casting Die Casting Injection Moulding

6 Structural Form Moulding 6 Blow Molding (Ext.) 7 Blow Molding (Inj.) Rotational Molding


3 Hot Extrusion 4,2,1 Rotary Swaging

Machining (From Stock) ECM EDM WEDM Sheet Metal (Stamp/bend) Themoforming



24

Table 4.8 Final process selection based on process/materials combinations of PCB Upper

Case (new design)


Cas

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PCB upeper case (new design). The characteristic of part chosen, such as below:

a. Part:


• Weight: 40 gram

b. Tolerances:


c. Surface Finish:

• 0. 2- 0.3 µm





f. Materials:

• Thermoplastic

g. Production volume

• 10, 000 units


26

4.3 PCB Upper Case Cover (New Design)


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Table 4.10 Shapes attributes for PCB Upper Case Cover (new design)


1. Depression Yes

2. Uniform Wall Thickness Yes






8. No Draft No





D Light weights


28

Table 4.11 Process elimination based on 8 geometric attributes of PCB Upper Case Cover

(new design).

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Table 4.12 Final process selection based on process/materials combinations of PCB Upper

Case Cover (new design)


Cas

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PCB upper case cover (new design). The characteristic of part chosen, such as below:

a. Part:


• Weight: 30 gram

b. Tolerances:


c. Surface Finish:

• 0. 2- 0.3 µm





f. Materials:

• Thermoplastic


• 10, 000 units


31

4.4 Clicker Wheel


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Table 4.14 Shapes attributes for Clicker Wheel


1. Depression Yes



4. Axis of Rotation Yes




8. No Draft No


A Good shock absorption.



D Light weights


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Table 4.15 Process elimination based on 8 geometric attributes of Clicker Wheel

Cas

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Rotational Molding


3 Hot Extrusion 1 Rotary Swaging Machining (From Stock) ECM EDM WEDM

2 Sheet Metal (Stamp/bend) 2 Themoforming 1 Metal Spinning



34

Table 4.16 Final process selection based on process/materials combinations of Clicker Wheel


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ys

Nic

kel a

nd A

lloys

Ref

ract

ion

Met

als

Ther

mop

last

ics

Ther

mos

ets








35

Based on the final process selection shown above, INJECTION MOLDING processes is

suitable processes to be selected. Meanwhile, THERMOPLASTICS material is selected to be

used for Clicker Wheel. The characteristic of part chosen, such as below:

a. Part:

• Size: Ø30 mm

• Weight: 20 gram

b. Tolerances:


c. Surface Finish:

• 0. 2 - 0.3 µm





f. Materials:

• Thermoplastic


• 10, 000 units


36

4.5 Clicker Wheel Shaft Holder


Cas

t Iro

n

Car

bon

Stee

l

Allo

y St

eel

Stai

nles

s Ste

el

Alu

min

ium

and

Allo

ys

Cop

per a

nd A

lloys

Zinc

and

Allo

ys

Mag

nesi

um a

nd A

lloys

Tita

nium

and

Allo

ys

Nic

kel a

nd A

lloys

Ref

ract

ion

Met

als

Ther

mop

last

ics

Ther

mos

ets




37

Table 4.18 Shapes attributes for Clicker Wheel


1. Depression Yes







8. No Draft No


A Good shock absorption.



D Light weights


38

Table 4.19 Process elimination based on 8 geometric attributes of Clicker Wheel Shaft Holder

Cas

t Iro

n

Car

bon

Stee

l

Allo

y St

eel

Stai

nles

s Ste

el

Alu

min

ium

and

Allo

ys

Cop

per a

nd A

lloys

Zinc

and

Allo

ys

Mag

nesi

um a

nd A

lloys

Tita

nium

and

Allo

ys

Nic

kel a

nd A

lloys

Ref

ract

ion

Met

als

Ther

mop

last

ics

Ther

mos

ets








39

Table 4.20 Final process selection based on process/materials combinations of Clicker Wheel

Shaft Holder


Cas

t Iro

n

Car

bon

Stee

l

Allo

y St

eel

Stai

nles

s Ste

el

Alu

min

ium

and

Allo

ys

Cop

per a

nd A

lloys

Zinc

and

Allo

ys

Mag

nesi

um a

nd A

lloys

Tita

nium

and

Allo

ys

Nic

kel a

nd A

lloys

Ref

ract

ion

Met

als

Ther

mop

last

ics

Ther

mos

ets








40

Based on the final process selection shown above, INJECTION MOLDING processes is

suitable processes to be selected. Meanwhile, THERMOPLASTICS material is selected to be

used for Clicker Wheel. The characteristic of part chosen, such as below:

a. Part:


• Weight: 20 gram

b. Tolerances:


c. Surface Finish:

• 0. 2 - 0.3 µm





f. Materials:

• Thermoplastic


• 10, 000 units


41

5.0 REASONS FOR THE MATERIALS SELECTIONS

5.1 Reasons for Material Selections – Thermoplastic

Thermoplastics are resins that repeatedly soften when heated and harden when cooled.

Most thermoplastics are soluble in specific solvents and can burn to some degree. Softening

temperatures vary with polymer type and grade. Because of thermoplastics’ heat sensitivity, care

must be taken to avoid degrading, decomposing, or igniting the material. Nylon, acrylic, acetal,

polystyrene, polyvinyl chloride, polyethylene, and cellulose acetate are just a few examples of the

many rigid thermoplastic resins currently available. Because of this behavior, thermoplastic also

allow production scrap such as runners and trimming to be ground and reused. Furthermore,

thermoplastic can be injected molded, extruded or formed via other molding techniques.

Unlike thermoplastics, thermosets form cross links, inter-connections between

neighboring polymer molecules that limit chain movement. This network of polymer chains tends

to degrade, rather than soften, when exposed to excessive heat. Until recently, thermosets could

not be remelted and reused after initial curing. Today’s most-recent advances in recycling have

provided new methods for remelting and reusing thermoset materials.

Based on above explanation, thermoplastic materials are chosen as material to be used in

this project. The next stage is to select the specific material among the thermoplastic materials.


42

5.2 Material Selected for Thermoplastics

Table 5.1 shows some of the material applications of the thermoplastics. Selection of the

specific material to be used in producing the mouse parts is based on Table 5.1:

Table 5.1 Mould cavity pressure required depending on the product group

Application Group Examples Typical

Materials

Flow Path To

Wall Thickness

Ratio

Quality Requirement

Required Mould Cavity Mould

Consumer goods

Parts with low surface quality and dimensional requirement, simple toys

PS, PP, PE, ABS 100 - 150 Low 200 - 500

Closures

Screw caps/closures, lids and closures for pharmaceutical and cosmetics

PE, PP 30 - 100 Medium 350 – 400

Covers

Vacuum cleaner housings, bumpers, instrument panels, lawn movers, power drills

PS, PP, ABS/PC 100 -150 Medium 350 – 400

Engineering Packaging

3.5” floppy disks, CD jewel cases, video cassettes, slide frame

PS, ABS 100 - 150 Medium 350 - 400

Housings Telephone, TV, radio and computer housings

ABS, PC, PA 100 - 150 Medium 400 - 500

Optical Parts

Automotives rear lenses, mirrors, optical lenses, head lights, glasses, emergency triangles

PMMA, PC 30 - 100 High 600 – 500

Engineered Function Parts

Gear wheels, connectors, terminal strips, camera and camcorder housings

PC, POM, PBT 50 - 200 High 600 – 800

Thin wall Containers

Drinking cups, planters, yogurt cups, ice cream, containers, pails, containers

PS, PE, PP 200- 350 Medium -


43

By referring to the table 5.1, one (1) application groups are suitable to be used for the

chosen Logitech computer mouse; COVERS & ENGINEERED FUNCTION PARTS. This

selected application groups of materials, there are:

a. PS b. PP c. ABS d. PC e. POM f. PBT

Polystyrene (PS)

This is probably the best known of the thermoplastics materials. It has an excellent

resistance to corrosion by most common chemicals and is unaffected by foodstuffs. It is tough

and flexible, has a high electrical resistivity, has a low density, and is easily moulded and

machined. Since it is also comparatively cheap to produce, it is not surprising that polythene

finds such a wide range of applications. Polythene is available in several different modifications

with the following properties such as:

• High hardness and dimensional stability. • Thermal dimensional stability up to 80°C. • Glossy finish. • Very good electrical insulating properties. • PS is susceptible to environment stress cracking even in air, especially with odourants.

Polystyrene is widely easy to process by means of extrusion, injection molding, extrusion

blow molding, injection blow molding and thermoforming. The good flow properties of the melt

ensure easy processing. Objects that are injection molded show orientation in the flow direction.

This can cause a decrease in mechanical strength perpendicular to the direction of flow. The

brittleness of PS can be reduced by stretching during processing and PS fibres are used for

braiding in cables. Molded items and semi finished stock can be welded and glued. Application

of the polystyrene are:

• Packaging: For cosmetics, pharmaceuticals, appliances, clocks, electronic parts, etc.


44

• Household articles: Bowls, plates, beakers, plant holders, disposable cups, tumblers and

party cutlery.

• Electronics industry: Reels for film, covers for relays, insulating foils, tape cassettes

and refrigerator parts.

• Foamed and expanded components: Heat insulating sheets for the building and

automobile industry, disposable heat conserving tumblers and packaging for impact-

sensitive objects.

Polyproplene (PP)

This is similar in structure and properties to polyethylene but it has a higher temperature

tolerance. PP is polymerised from the gas propylene. PP is strong and is used for a wide variety

of mouldings where greater strength and rigidity. The higher melting point of PP as compared

make it suitable for fibre manufacture, whilst large amounts are also produced as clear film for

wrapping cigarettes and crisps. Typical properties of polypropylene are:

• Low density.

• High stiffness, hardness and strength.

• Temperature resistance up to 110°C.

• Embrittlement temperature at 0°C for homopolymers (copolymers have a lower

embrittlement temperature).

• Milky translucency.

• Electrical properties are comparable to that of PE.

• Resistant to weak inorganic acids and alkalis, alcohol and some oils and washing soda up

to 100°C.

• Susceptible to attack by strong oxidising agents and halogenated hydrocarbons. PP swells

in aliphatic and aromatic hydrocarbons such as benzene or benzine (especially at high

temperatures).


45

Meanwhile, application of polypropylene consists of following criteria:

• Machine and automobile industry: Pipes for heating, ventilation fans, bellows, air

supply filters and pump bodies.

• Household articles: Components for washing machines, dishwashers and vacuum

cleaners, and cooking foils.

• Electrical applications: Cable couplings, antenna components, and cable covering.

• Transport industry: Crates, toolboxes, boxes, strapping tape, bags, rope, binder twine

and packaging films.

• Building industry: Piping, fittings and under-floor heating.

• Construction of apparatus: Piping and reaction vessels.

• General: Woven bags, carpet backing, ^-artificial grass, toys, medical syringes and

footwear parts.

Acrylonitrile/butadiene/styrene erpolymer[ABS]

Acrylonitrile/butadiene/styrene terpolymer is made up of three monomers. The most

important commercially used polymer is a graft terpolymer polymerised from polybu-tadiene and

styrene/acrylonitrile (SAN). To obtain good grafts between the SAN-matrix and the

polybutadiene particles the elastomer particles are coated with a graft coating of SAN. ABS has the

following:

• Good impact and notched impact resistance, even at low temperatures (to -40°C) • Good scratch resistance and hardness • Good toughness - can be used with metal inserts • Good shock absorption, due to the presence of butadiene which acts as a mechanical

shock absorber • Good thermal dimensional stability and cyclic temperature resistance (up to 100°C) • High surface gloss when graft polymerised. Blends have a slightly matt surface • Low water absorption • Good environmental stress cracking resistance • Good chemical resistance.


46

In addition to the basic ABS types (DIN 16772) special types of ABS are

available, for instance the types which are easily electroplateable. It’s also to the

terpolymers the ABS range includes numerous polymer mixtures. Those terpolymers that

are not modified with butadiene rubber are described as AXS products. The X is the

symbol for the elastomer components. The properties of compounds of ABS with other

polymers depend on the mix ratio of the individual components. The following are

available in the trade:

• ABS/PVC alloys - ABS acts as an impact modifier and processing aid for PVC. These

alloys are self-extinguishing.

• ABS/PC blends - these blends have high impact resistance (to -50°C) and high

dimensional heat stability (up to 115°C).

• ABS/PUR blends - these materials have very high cold impact resistance. The PUR

components improve the abrasion properties. The flow properties of these blends are

excellent, rendering them suitable for the production of thin-walled injection-moulded

articles. A disadvantage of ABS is its low weathering resistance and lower transparency be-

cause of the butadiene content. The butadiene is replaced by other elastomer components

for exterior applications.

Because of its good physical properties ABS, lends itself to following technical applications:

• Electrical and audio industry: Telephones, computer housings, lamps, watch cases,

office machines, copiers, video cassettes, projectors, portable TV housings, chassis and fronts

for Hi-Fi apparatus and video camera and film apparatus.

• Automobile industry: Body parts, fan blades, air ducts, light housings, spoilers, hub caps

and instrument panels.

• Plumbing: Pipes, fittings, couplings and WC Flush tanks.

• Sport and recreation: Surfboards, chairs, technical toys, aeroplane and railway models and

chair shells.

• General: Suitcases, briefcases, hair dryer parts, hard hats for industry and mining,

Christmas tree light holders and aircraft parts.


47

• Foamed articles: Portrait frames, umbrella handles, chairs, tool handles, decorative

articles and table place mats.

Polycarbonate became available in 1956 from Bayer and two years later from General

Electric. Polycarbonate is an amorphous thermoplastic with a degree of crystallisation of up to 5%

only. PC is manufactured by polycondensation from bisphenol A, which in turn is manufactured

from phenol, acetone, and phosgene. About 120 basic units make up one polymer chain. The

presence of aromatic rings in the molecular chain reduces the movement of the macromolecules

and gives the polymer rigidity and temperature resistance. PC has the following properties:

Polycarbonate (PC)

• High strength, hardness and toughness

• Temperature resistance from -150°C to 135°C and to 145°C in glassfibre reinforced grades

• Glass clear

• Good electrical insulation

• Low water absorption

• Resistant to petrol, oils, greases and saturated aliphatic hydrocarbons. It can also be

immersed in boiling water for a short period and can be sterilised at 120°C

• Not resistant to strong acids and alkalis, aromatic hydrocarbons and chlorinated

hydrocarbons nor to lengthy immersion in hot water

• Prone to stress cracking (tempering at 120°C helps to release internal stresses).

• Good weathering properties

• PC burns but is extinguished if the ignition source is removed. PC can be made fire

retardant by condensation with bisphenol A with chlorine or bromine atoms in the aromatic

ring.

The stiffness of PC can be enhanced with the addition of glass fibres up to a glass

content of 30%. The stress/strain relationship of PC is characterised by a wide strain flow

area. When reinforced with glass fibres, this disappears from improving stiffness, tensile

strength, heat deformation resistance, and compressive strength, the addition of glass fibres


48

also improves the dimensional stability and reduces the burning rate of PC. The light

transmission of glass clear PC is between 80% and 90% depending on thickness.

Injection moulding, extrusion and blow moulding PC has to be dried to a moisture content

of 0,02% before processing. This is achieved by placing it in a 2cm deep layer in an oven at 120°C

for four hours. Even when the polymer is packed in an airtight container this pre-heating is

recommended. The use of a heated hopper is recommended because the polymer takes up moisture

upon cooling. Conversion temperatures are high when compared to other moulding compounds.

Heating and control elements must be capable of temperatures of at least 350°C. Follow-on

pressure in injection moulding must be as low as possible to reduce residual stresses.

Machining of articles is easily done, but care should be taken with the cooling methods

used. No oil emulsions but only air or clean water should be used. Welding is best done by

means of heated elements. Other welding methods can also be used. Tempering is required after

welding to remove stresses. Gluing is by means of solvents, solvent-based glues and reactive

adhesives. When using solvent glues it is necessary to heat the parts to higher temperatures. For

thermoforming of sheet the material must be preheated at 110°C for several hours depending

on wall thickness. For stretch forming (i.e. drape and vacuum form etc), whether by compressed

air or vacuum, the stock temperature should be 180°C to 210°C. The moulds must also be

heated to about 100°C.

Due to its excellent properties, PC is used as an engineering plastic as well as in do-1

mastic applications.

• Machines and appliances: Housings, typewriter and sewing-machine components,

computer housings, medical equipment, sight glasses, filter cases, ventilation fans and

covers, electric razors and cam discs.

• Electronics: Plugs, wall plugs, coil cores, couplings, switchgear housings, fluorescent, light

mountings and switches.

• Photographic and audio apparatus: Radio and television equipment, telephones,

compact discs, binoculars and instrument housings.

• Automotive industry: Indicators, rear lights, air ducts, ventilation and cooling grills.


49

• Lighting and building industry: Lighting strips, domes, glazing, protective glazing,

balcony and bridge railings, bus and telephone shelters, greenhouse walling and ar-J

moured "glass" panels.

• Household articles: Utensils, vacuum cleaner components, bottles, canisters and

crockery.

Polyacetal [POM]

Polyacetal, also known as polyoxymethylene, has been available since 1958. It is available as

homo- and copolymers, the main difference being that the copolymers have greater thermal and

chemical stability. POM is a semi-crystalline thermoplast. The crystallinity of homopolymers is

greater than that of the copolymers and is somewhere in the region of 80%. The monomer for the

manufacture of this polymer is formaldehyde i.e. trioxane -a cyclic trimer of formaldehyde

obtained when the formaldehyde molucules form a ring structure.

In copolymers, the chains are divided by means of other monomers such as cyclic ethers, which

retard chain decomposition of the melt.

POM is characterised by high strength, hardness and rigidity over a wide temperature range.

The materials show little creep over a wide stress range even during a relatively long loading period.

Further characteristic properties of POM are as follows:

• High toughness down to -40°C

• High abrasion resistance

• Low coefficient of friction

• High heat distortion resistance

• Good electrical and dielectric properties


• Resistant to organic solvents such as alcohols, esters, ketones, as well as oils, fats, petrol,

watery bases and acids not resistant to strong acids and oxidising agents

• Resistance to weathering is poor. Sunlight has an embrittling effect on POM, but it can

be retarded by the addition of correct stabilisers. To delay the degradation of the polymer


50

chain one of the best methods of protection against UV light is the addition of carbon

black.

POM is mainly processed by injection moulding, extrusion and blow moulding. With glass

reinforced POM a higher injection pressure has to be used; it is also necessary to increase the gate

cross sectional area by about 20%. Extrusion is normally done by means of single screw

machines without breaker plates and screens. In case of any machine stoppage, great care should

be exercised.

Above a temperature of 240°C and during long residence periods in machines, the

polymer decomposes to form formaldehyde which can easily be identified by its pungent smell. A

build-up of formaldehyde gas in the cylinder can cause an explosion through the feed hopper and

nozzle. Protective clothing and glasses should be worn and processing temperatures reduced.

There should be adequate ventilation. All methods of welding except HF-welding can be used. A

low rotational speed should be used in friction welding, because the POM has a low melt

viscosity and molten material might be thrown away from the work face by centrifugal force.

Gluing - Bonding by means of adhesives and solvent adhesives is feasible, but two component

adhesives require the pre-treatment of surfaces with chromic acid.

POM has very low creep combined with low frictional properties and good abrasion

resistance. This makes this material an excellent engineering plastic. Because of its high

toughness with good resilience it is used for the manufacture of snap and press fittings in machine

and appliance construction. POM is used as follows:

• Machines and appliances: Gear wheels, bushes, bearings, housings, springs, chains,

rollers, screws, nuts, pump parts, valve and control elements, snap & press fittings,

ventilation fans and slide and guide elements.

• Electronics: Isolators, small motor parts, relay components, telephone components,

radios, projectors, spools and plugs.

• Automotive: Eevers for direction indicators and covers for universal joints.

• Furniture: Edging, closing devices, hinges, handles and rollers for sliding doors and

curtains.


51

• Packaging: Aerosol cans, cigarette lighters and gas cartridges.

Poly(butylene terephthalate) [PBT]

The manufacture of poly(butylene terephthalate) is very much the same as that of

poly(ethylene terephthalate). PBT is a semi-crystalline thermoplastic which has similar properties

to PET. The slightly poorer mechanical properties compared to PET are often cancelled out by the

better processing properties. The technical and physicals properties of PBT are:

• For a thermoplastic PBT has good hardness, stiffness, and strength

• High toughness, even under cold conditions

• Good friction and abrasion properties

• Good creep resistance

• Good thermal dimensional stability

• Service temperature between -60°C and 110°C (short term up to 170°C); filled

grades up to 200°C

• Naturally translucent and has a high surface gloss

• Good electrical properties including tracking resistance and dielectric performance


• Physiologically acceptable

• Similar resistance properties to PET

• Not resistant to aromatic and aliphatic hydrocarbons

• Resistant to stress cracking and weathering

• Burns with a yellow-orange sooty flame without the formation of droplets.

Due to the fact that PBT burns easily, most grades are modified with flame retar-dants. To

improve certain mechanical properties, fibre reinforcement is commonly used. This leads to

shorter cycle times, wider melt temperature ranges, faster recrystallis-ation and lower melting

points when compared to PET. The partial replacement of glass fibres with glass microspheres, or

mica, results in moulding compounds with better resistance to warpage.


52

Modification by means of blends or graft polymerisation increases the notched impact

resistance of the material. For blends, PC, PA, PTFE, and TPE with a styrene butadiene base are

used. PBT compounds are described in DIN 16779.

Process - The guidelines given in regarding moisture content are also applicable to PBT. It is

preferable to pre-dry PBT granules that have become wet:

Granulate layer depth - 2 to 3 cm

Drying temperature - about 120°C

Table 5.2: PBT unreinforced and PBT reinforced process requirement

Process

PBT unreinforced

PBT reinforced

Injection moulding

Melt temperature, (°C)

Mould temperature, (°C)

Injection pressure (bar)

Shrinkage , (%)

230 to 260 60

800 to 1200 1,

0 to 2,0

250 to 270 up to 120

800 to 1200 0,4 to 1,3

Extrusion Melt temperature, (°C)

Die temperature, (°C)

250 to 280

250 to 270

5.3 Selected Materials

Base on 6 types of thermoplastic material above, the suitable material for CLICKER

WHEEL and CLICKER WHEEL SHAFT HOLDER is Poly (butylenes terephthalate)

[PBT]. This selection base on the properties PBT is suitable with part requirement and application.

The advantages of material properties compare to others is:

• high thermal stability, great stiffness, and hardness

• temperature resistance – 40° c to 100° c

• low water absorption,

• good resistance to stress cracking

• excellent anti friction


53

• good dimensional stability

• low cost of material

Meanwhile, for PCB Lower Case (new design), PCB Upper Case (new design) and

PCB Upper Case Cover (new design), ABS is material to be used on producing particular parts.

This selection base on the properties ABS is suitable with part requirement and application. The

advantages of material properties compare to others is:

• Good impact and notched impact resistance, even at low temperatures (to -40°C) • Good scratch resistance and hardness • Good toughness - can be used with metal inserts • Good shock absorption, due to the presence of butadiene which acts as a mechanical

shock absorber • Good thermal dimensional stability and cyclic temperature resistance (up to 100°C) • High surface gloss when graft polymerised. Blends have a slightly matt surface • Low water absorption • Good environmental stress cracking resistance • Good chemical resistance.


54

6.0 REASONS FOR THE PROCESSES SELECTIONS

6.1 Reasons for Process Selections – Injection Molding

One of the most common methods of converting plastics from the raw material form to an

article of use is the process of injection molding. This process is most typically used for

thermoplastic materials which may be successively melted, reshaped and cooled. Injection

molded components are a feature of almost every functional manufactured article in the modern

world, from automotive products through to food packaging. This versatile process allows us to

produce high quality, simple or complex components on a fully automated basis at high speed

with materials that have changed the face of manufacturing technology over the last 50 years or

so. Below is the reason for the injection processes selections for producing about 10, 000 units of

each selected parts for Logitech computer mouse product as mention before:

• Parts can be produced at high production rates.

• large volume production is possible (for this project – 10, 000 units volume of

production)

• Relatively low labor cost per unit is obtainable.

• Process is highly susceptible to automation.

• Parts require little or no finishing (good surface finish).

• Many different surfaces, color and finishes are available.

• For many shapes this process is the most economical way to fabricate.

6.2 The Injection Molding Process

Figure 5.1 shows the equipment necessary for injection molding. It consists of two main

elements, the injection molding machine and the injection mold. An injection molding machine

can be broken down into the following components:-

a. Plasticizing/injection unit,

b. Clamping unit,

c. Control system and


55

d. Tempering devices for the mold.

Figure 5.1 Injection molding machine

6.3 Injection Molding Cycle

The modern day process has developed and matured significantly to the level where fully

automated, closed loop, microprocessor controlled machines are the 'norm', although in principle

injection molding is still a relatively simple process. Thermoplastic injection molding requires

the transfer of the polymeric material in powder or granule form from a feed hopper to a heated

barrel. In the barrel, the thermoplastic is melted and then injected into a mould with some form of

plunger arrangement. The mould is clamped shut under pressure within a platen arrangement and

is held at a temperature well below the thermoplastic melt point. The molten thermoplastic

solidifies quickly within the mould, allowing ejection of the component after a pre determined

period of cooling time. The basic injection molding process steps with a reciprocating screw

machine are as follows.

Mould Close and Clamping

The mould is closed within the platen arrangement and clamped using necessary force to

hold the mould shut during the plastic injection cycle, thus preventing plastic leakage over the

face of the mould. Present day molding machines range from around 15 to 4,000 metric tons

available clamping force (150 to 4000 kN).


56

Many systems are available for opening/closing and clamping of mould tools, although

usually they are of two general types. Direct Hydraulic Lock is a system where the moving

machine platen is driven by a hydraulic piston arrangement which also generates the required

force to keep the mould shut during the injection operation. Alternatively, smaller auxiliary

pistons may be used to carry out the main movement of the platen and a mechanical blocking

arrangement is used to transfer locking pressure from a pressure intensifier at the rear of the

machine, which moves only by a few millimeters, through to the platen and tool.

The second type of general clamping arrangement is referred to as the Toggle Lock. In

this case a mechanical toggle device, which is connected to the rear of the moving platen, is

actuated by a relatively small hydraulic cylinder; this provides platen movement and also

clamping force when the toggle joint is finally locked over rather like a knuckle arrangement.

Injection

At this stage in the machine cycle the helical form injection screw is in a 'screwed back'

position with a charge of molten thermoplastic material in front of the screw tip roughly

equivalent to or slightly larger than that amount of molten material required to fill the mould

cavity. Injection molding screws are generally designed with length to diameter ratios in the

region of 15:1 to 20:1, and compression ratios from rear to front of around 2 : 1 to 4 : 1 in order

to allow for the gradual densification of the thermoplastic material as it melts. A check valve is

fitted to the front of the screw such as to let material pass through in front of the screw tip on

metering (material dosing), but not allow material to flow back over the screw flights on

injection. The screw is contained within a barrel which has a hardened abrasion resistant inner

surface.

Normally, ceramic resistance heaters are fitted around the barrel wall, these are used to

primarily heat the thermoplastic material in the barrel to the required processing temperature and

make up for heat loss through the barrel wall, and due to the fact that, during processing most of

the heat required for processing is generated through shear imparted by the screw. Thermocouple

pockets are machined deep into the barrel wall so as to provide a reasonable indication of melt


57

temperature. Heat input can therefore be closed loop controlled with a Proportional Integral and

Derivative (PID) system. The screw (non-rotating) is driven forward under hydraulic pressure to

discharge the thermoplastic material out of the injection barrel through the injection nozzle,

which forms an interface between barrel and mould, and into the molding tool itself.

Holding Pressure and Cooling

The screw is held in the forward position for a set period of time, usually with a molten

'cushion' of thermoplastic material in front of the screw tip such that a 'holding' pressure may be

maintained on the solidifying material within the mould, thus allowing compensating material to

enter the mould as the molded part solidifies and shrinks. Holding pressure may be initiated by

one of three methods: by a set time in seconds from the start of the injection fill phase; by the

position of the screw in millimeters from the end of injection stroke; or by the rise in hydraulic

pressure as measured by a pressure transducer in the mould itself or in the injection hydraulic

system.

As the material solidifies to a point where hold pressure no longer has an effect on the

mould packing, the hold pressure may be decayed to zero; this will help minimize residual

stresses in the resultant molding. Once the hold pressure phase has been terminated the mould

must be held shut for a set period of cooling time. This time allows the heat in the molding to

dissipate into the mould tool such that the molding temperature falls to a level where the molding

can be ejected from the mould without excessive distortion or shrinkage. This usually requires

the molding to fall to a temperature below the rubbery transition temperature of the thermoplastic

or Tg (glass transition temperature). Depending on the type of plastic this can be within a few

degrees or over a temperature range. Mould temperature control is incorporated into the tool

usually via channels for pressurized water flow. The mould may be connected to a cooling unit or

water heater depending on the material being processed, type of component and production rate

required.


58

Material Dosing or Metering

During the cooling phase, the barrel is recharged with material for the next molding cycle.

The injection screw rotates and, due to its helical nature, material in granule or powder form is

drawn into the rear end of the barrel from a hopper feed. The throat connecting the hopper to the

injection barrel is usually water cooled to prevent early melting and subsequent material bridging

giving a disruption of feed. The screw rotation speed is usually set in rpm which is measured

using a proximity switch at the rear of the screw. Screw rotation may be set as one constant speed

throughout metering or as several speed stages.

The material is gradually transferred forward over the screw flights and progressively

melted such that when it arrives in front of the screw tip it should be fully molten and

homogenized. The molten material transferred in front of the tip progressively pushes the screw

back until the required shot size is reached. Increased shear is imparted to the material by

restricting the backward movement of the screw; this is done by restricting the flow of hydraulic

fluid leaving the injection cylinder. This is referred to as `back pressure' and it helps to

homogenize the material and reduce the possibility of unmelted material transferring to the front

of the screw.

Mould Open and Part Ejection

When the cooling phase is complete the mould is opened and the molding is ejected. This

is usually carried out with ejector pins in the tool which are coupled via an ejector plate to a

hydraulic actuator, or by an air operated ejector valve on the face of the mould tool. The molding

may free fall into a collection box or onto a transfer conveyer, or may be removed by an

automatic robot. In this latter case the molding cycle is fully automatic. In semi-automatic mode,

the operator may intervene at this point in the cycle to remove the molding manually. Once the

molding is clear from the mould tool, the complete molding cycle can be repeated.


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6.4 Other Advantages of Injection Molding Processes

Injection molding is particularly advantageous when intricate parts must be produced in

large quantities. Although there are limitations, generally the more irregular and intricate the

parts, the more likely it is that injection molding will be economical. In fact, one major advantage

of the injection-molding method is that one molded part can replace what would otherwise be an

assembly of components. In addition, color and surface finish often can be molded directly onto

the part, so that secondary finishing operations are not necessary. Injection-molded parts are

generally thin-walled. Heavy sections and variable wall thicknesses are possible, though they are

normally not recommended.

Because thermoplastics are generally less strong than metals, they are more apt to be

found in less highly stressed applications. Housings and covers are common uses rather than, for

example, frames and connecting rods. However, thermoplastic materials are gradually being

developed with better and better strength characteristics and are increasingly finding themselves

used for moving parts and in more structural applications. The “engineering plastics,” nylon,

polycarbonate, acetal, phenylene oxide, polysulfone, thermoplastic polyesters, and others,

particularly when reinforced with glass or other fibers, are functionally competitive with zinc,

aluminum, and even steel.


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7.0 DISCUSSION

In producing a product starting from zero until a finish product is produced in the market,

a series of processes is being gone through step by step consist of DfA and DfM which runs

concurrently. The main objectives is to produce a product with the less part number, less

manufacturing processes, less lead time and for most less overall cost.

As for Logitech computer mouse product, 5 parts had to be evaluated and analysis in the

DfM methodology introduced by Boothroyd-Dewhurst. The parts are as below:

• Part 1 – PCB Lower Case (new design)

• Part 2 – PCB Upper Case (new design)

• Part 3 – PCB Upper Case Cover (new design)

• Part 4 – Clicker Wheel

• Part 5 – Clicker Wheel Shaft Holder

7.1 Material Requirement

Table 7.1 Parts and Material Requirement

Part

Part Name


1. PCB Lower Case (new design) 1. Plastic materials 2. Used snap fit features

(flexibility). 3. Excellent of Electrical

Resistivity (> 1015

4. Good impact resistance ) µΩ.cm

5. Light weight

2. PCB Upper Case (new design) 3. PCB Upper Case Cover (new

design)

4. Clicker Wheel 1. Plastic materials 2. Good shock absorption 3. Excellent of Electrical

Resistivity (> 1015

4. Good impact resistance ) µΩ.cm

5. Light weight

5. Clicker Wheel Shaft Holder


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The material requirement will be used as guide in selecting the best material which suite

the part‘s design.

7.2 Parts Attributes

Table 7.2 Parts attributes

Part 1 Part 2 Part 3 Part 4 Part 5

Depression

Yes Yes Yes Yes Yes

Uniform Wall

Yes No Yes No No

Uniform Cross-Section

No No No No No

Axis of Rotation

No No No Yes No

Regular Cross-Section

No No No No No

Capture Cavities

No No No No No

Enclosed

No No No No No

No Draft

No No No No No


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7.3 Final Selection for Material and Processes

Table 7.3 Final selection for material and processes

Part 1 Part 2 Part 3 Part 4 Part 5

MATERIAL

ABS Poly (butylenes

terephthalate) [PBT]

PROCESSES

Injection Molding

After using the elimination method, above results was finalised. The final

selections also consider the part’s specifications and the design guideline which in the end

will produce a product with minimum overall manufacturing cost.

7.4 Design Guidelines

Design for ease of fabrication and assembly. Select processes compatible with the design

intent, materials and production volumes. Select materials compatible with production processes

and that minimize processing time while meeting functional requirements. Avoid unnecessary

part features because they involve extra processing effort and/or more complex tooling.

Consider the following design guidelines:

• For higher volume parts, consider castings, extrusions or other volume manufacturing

processes to reduce machining and in-machine time

• Consult with manufacturing to determine and design for solid mounting or other fixture-

locating features on the component.

• Avoid thin walls, thin webs. or similar features that will result in distortions due to

manufacturing

• Avoid undercuts that will require special operations & tools


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• Design around standard cutters, drill bit sizes or other tools

• Avoid small holes and threaded features as tool breakage and part scrap increases

Threaded Holes

• Design for full thread depth. Usually 1.5 x major diameter provides adequate holding

strength

• Drilled hole depth (to the sharp point of the tool) is recommended to be at least equal to

the full thread plus ½ major diameter, but never less than .050"

• Material thickness as measured from the bottom of the drilled hole to next surface should

not be less than the major diameter of the thread or diameter of hole, and not less than

.050".

• When material thickness allows, thru holes are preferred

Fixture/tooling material selection

When designing steel fixtures or tooling where high accuracy flatness, perpendicularity,

parallelism or true position is required, specify the material as low carbon hot rolled. This

material is very stable and will retain form much better than CRS (Cold Rolled Steel).

Flatness

Flatness should be applied with reasonable overall form tolerance as well as on a per

unit basis as a means to prevent abrupt surface variation within a relatively small area of the

feature. Depending on material thickness and application, a note can be added to design

drawing: "FLATNESS MAY BE MEASURED WITH COMPONENT IN RESTRAINED

CONDITION". Where applicable, note should include specific retraining requirements


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Internal Radii

• Always specify largest radius possible. Small diameter tools add significant cost to

manufacturing process.

• When design requires metalized plating such as nickel, silver or other, specify a CR

"Controlled Radius" as applicable (CNC manufacturing). CAD model or design for non-

standard radii. CNC machining will create a "hard corner" in that the machine will race to

a radius corner and abruptly change onto the nelct direction. The CNC change of direction

often creates "tool chatter" resulting in rough sharp edges at the radius corner. Non-

standard or CR (Controlled Radius) will result in the CNC cutter to slow down and blend a

smooth radius at the corner feature. The smooth radius feature will facilitate good

metalized plating and avoid flaking common to small sharp edges.

• When depth exceeds 5 X the diameter of the pocket radii, consult manufacturing on

alternative fabrication methods. Depths of up to 10 X are possible when machining

aluminum but, not all manufacturing facilities have capability

• For deep sharp corner cutouts that require broaching or EDM, specify radii max at all

cutout corners i.e. (4X R .008 MAX)

Dimensional Tolerancing

For surface composite curves such as, internal pockets, or other profiles that for CNC

manufacturing a continuous cutting path will be established and manufactured. Design for and

specify unilateral tolerances (+/- .010). Reason: Often the machine tools used to manufacture

the components utilize a feature called "Cutter Compensation". This allows size control

variation of the features being machined without having to control the NC program (file) to an

exact match with the cutter diameter. For a continuous path, if "X" dimension has -+0, -.005 and

"Y" dimension has +.005, -0 tolerance specified, the cutter compensation cannot be used to

control size, because adding or subtracting from cutter path input automatically invokes an error

to the dimension of the other toleranced continuous path surface. Simply, a offset is input into


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the machine relative to the cutting tool to manufacture for mid tolerance of surface ''X' at -.0025

however, this path is not compatible with the "y" surface in that the nominal offset is .0025 out

of tolerance.

Design of tolerances should be within manufacturing capabilities.

Concurrently designing for manufacturing will greatly improve product quality and

reduce fabrication costs. Consult with manufacturing early in the design process. After

completion of preliminary drawings, meet with manufacturing and review design intent,

requirements and determine manufacturing process requirements. Manufacturing should review

tolerances and determine process capabilities to meet dimensional limits. Manufacturing should

identify tolerance challenges that require design and requirements review. In general, design

should avoid unnecessarily tight tolerances that are beyond the natural capability of the

manufacturing processes. Determine when new production process capabilities are needed early

to allow sufficient time to determine optimal process parameters and establish a controlled

process. Tolerance stack-ups should be considered on mating parts. Overall assembly tolerances

should be calculated, and interface as well as clearance requirements understood. Surface finish

requirements can be established based on actual manufacturing processes employed however,

surface finish requirements should be understood and design intent accurately defined.

Simplify design and assembly so that the assembly process is unambiguous.

Components should be designed so that they can only be assembled in one way; they cannot be

reversed. Roll pins, dowel pins or offset mounting holes can be employed.

Design for components orientation and handling to minimize non-value-added manual

effort, ambiguity or difficulty in orienting and merging parts. Basic principles to facilitate parts

handling and orienting are:

• Parts must be designed to consistently orient themselves. Examples are dowel pins.

• Product design must avoid parts that can become tangled, wedged or disoriented.

• Verify clearance for assembly tooling such as hand tools and fixtures.

• With hidden features that require a particular orientation, provide an external feature, guide


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surface or design alignment fixturing or tooling to correctly orient the part.

• Design in fasteners large enough that are easy to handle and install

Design for efficient joining and fastening.

Threaded fasteners (screws, bolts, nuts and washers) can be lime-consuming to

assemble. Consider design alternatives that will reduce fastener count. Use uniform screw sizes

here practical.

However, during the manufacturing processes in the plant, there might be some

changes in the detail process plan whenever the manufacturing personnel find out that

there are still improvement can be make in order to achieve the minimum manufacturing

cost such as combining the processes or parts as well.


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7.0 CONCLUSION

By using Boothroyd-Dewhurst method in design for manufacturing, whereby material for

each part is determined by eliminating the one which does not fulfill the basic requirements. The

overall activities in DFM are achieved with the goal of selection the materials and manufacturing

processes. From this Logitech computer mouse assembly project, it’s managed to:

i. Minimizes the cost of manufacturing.

ii. Minimizes the cost of the material.

iii. Minimizes the waste in manufacturing and in the product life cycle.

iv. Maximizes the performance of the part.

v. Maximizes the safety of the product.

It come to an end of the report regarding on the DFM method in determination of the

material and related manufacturing process. For the five selected parts namely; PCB Upper Case

(new design), PCB Lower Case (new design), PCB Upper Case Cover (new design), Click Wheel

and Click Wheel Shaft Holder, base upon the argument and rationale in the discussion section, it

is being decided that the most efficient manufacturing method is injection molding process. As

for the material selection, the PCB Upper Case (new design) , PCB Lower Case (new design),

PCB Upper Case Cover (new design) will be made of ABS materials whereby Click Wheel and

Click Wheel Shaft Holder will be made from Poly (butylenes terephthalate) [PBT].

MMP 1663 DFA Assignment: Logitech Computer Mouse

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REFERENCE:

[1] Boothroyd, G., “Product Design For Manufacture And Assembly”, 2nd

New York: Marcel Dekker Inc. 2001. 374p. Edition.

[2] Henry W. Stoll, “Product Design Methods and Practices”, New York: Marcel Dekker Inc. [3] Kjell B. Zandin, “Maynard’s Industrial Engineering Handbook”, 5th

edition, McGraw-Hill.

[4] Mark Curtis, “Boothroyd Dewhurst Design for Manufacture & Assembly’s”, Harland

Simon Automation Systems Ltd. [5] Hubert K. Rampersad, “The House of DFA”, Rotterdam School of Management,

Erasmus University.

design for manufacture - project

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