Transcript MFGT 124 Solid Design in Manufacturing Product Evaluation for Cost, Manufacture, Assembly, and Other Measures Professor Joe Greene CSU, CHICO Reference: The Mechanical Process, 3rd Edition,

MFGT 124
Solid Design in Manufacturing
Product Evaluation for Cost, Manufacture,
Assembly, and Other Measures
Professor Joe Greene
CSU, CHICO
Reference: The Mechanical Process, 3rd Edition, David Ullman,
McGrall Hill New York (2003)
Reference: Design for Manufacturability Handbook, J. Bralla,
McGraw Hill (1999)
MFGT 124
Copyright 2003 Joseph Greene All Rights Reserved
1
Chap 12: Product Evaluation, Cost, DFM
• Topics
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–
–
–
–
–
Introduction
Cost Estimating in Design
Value Engineering
Design for Manufacture
Design for Assembly Evaluation
Design for Reliability
Copyright 2003 Joseph Greene All Rights Reserved
2
Cost Estimating in Design
• Cost for manufactured parts
– Direct costs
•
•
•
•
Material
Labor
Tooling
Purchased parts
– Indirect Costs
• Overhead- building, utilities, land, capital equipment, etc.
• Sales expense- advertising, sale promotions, rebates, low% financing
• Profit: typically 5-50%
Copyright 2003 Joseph Greene All Rights Reserved
3
Injection Molding Costs
• Many methods are used to determine the cost of injection molded part
– rough estimates based upon rules of thumb or experience
– extremely detailed analysis based on costs for numerous plant functions
– spreadsheet based analysis, IBIS and Associates
• Pocket knife example
– MFGT 142: Cost estimating form for injection molding
– Appendix: Blank Form
• Part 1: Introduction Section
– Part name, customer, molder,
– Tool source, date, estimator, approver
Copyright 2003 Joseph Greene All Rights Reserved
4
Injection Molding Costs
• Part 2: Resins and Additives Costs
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Type and grade of resin, e.g., pocket knife is nylon 6,6 (Dupont Zytel)
Cost of resin depending upon quantity, e.g., boxcar, gaylord, bag
Additives cost, e.g., colorants, fillers, stabilizers, etc. (black is $3/lb)
Total Material Cost = (resin cost)*(resin fraction) + (additives cost)*
(additives fraction)
• Example, Total Cost = $1.36 * 0.99 + $3.00 * 0.01 = $1.38 nylon and color
• Part 3: Part Costs
– Part costs = material costs plus factory costs
– Material cost is materials plus scrap from runners, sprues, and part rejects.
• Example: Knife weighs 3.8 grams. The runners, sprues, and scrap is 5%. Total
material uses us 3.8 + 0.05*3.8 = 4 grams.
• Cost = $1.38 * 4/454 lbs = $0.0122 per part
– Factory costs represent convenient price figure and is a factor*material costs
• Example: Factor = 1000. Then, factory costs = 1000 * $0.0122 = $12.00
Copyright 2003 Joseph Greene All Rights Reserved
5
Injection Molding Costs
• Part 4: Tooling Costs
– Type of tooling material, e.g. steel, aluminum, kirksite.
– Number of cavities in tool, e.g., single cavity has 1 part, dual has 2, multicavity tool
can have 4, 6, 8 pieces, or more.
– Number of slides or lifters in a tool to mold parts that have an internal flange or
under cut.
– Number of years the tool is amortized for tool life, e.g., payoff tool in 1 year, 3
years, or five years. (Every company has different accounting practices.
– Internal (In-House) tool construction versus External (Outside) tool construction.
Internal is usually less expensive per tool but has more overhead, thus need many
jobs to reduce overhead costs.
– Example, knife handle
• $20,000 per tool (dual cavity, $5K internal, $15K external)
• Cost per part is $20,000/ 4million parts * 1000 (pieces) = $5 per 1000 pieces
Copyright 2003 Joseph Greene All Rights Reserved
6
Injection Molding Costs
• Part 5: Machine Costs
– Cost of machine is dependent upon the time the machine is in use to make parts and
whether the machine has an operator or not.
– Each machine will have two rates, e.g., automatic or manual.
– Rates are determined from
•
•
•
•
•
Original cost of machine
Ongoing operations costs
Special equipment costs for particular jobs, e.g., special controllers or chillers
Cycle time
Example, Knife
– Cycle time is 30 seconds yields 240 parts per hour (120 per cavity)
– Hourly rate is $25 for manual with operator, and $15 on automatic
– Knife example needs operator to cut runner and sprue off part
– Cost = $25 per hour / 240 parts per hour * 1000 = $104.17
Copyright 2003 Joseph Greene All Rights Reserved
7
Injection Molding Costs
• Part 6: Secondary Operations Costs
– Many parts are subject to other operations costs after molding
• placed or glued into an assembly, drilling of holes or attachments
– Rates are determined from some rate and cycle time
• Rate costs are dependent upon the type of machine used, $ per hour
• Cycle time, parts per hour
• Runner and sprue removal are not considered secondary operations since they are
removed at the press after molding.
• Example, Knife
– Securing the blade to the handle with screws after injection molding
– Cycle time is 10 seconds yields 60 parts per hour
– Cost = $7 per hour / 60 parts per hour * 1000 = $19.44 per 1000 pieces
• Part 7: Purchase Items Costs
– Many items are purchased and included in assembly
• Example, Knife use Costs of blade and screw purchases
– Cost of blades = $1250 per 1000 pieces
– Cost of screw = $2.00 per 1000 pieces
Copyright 2003 Joseph Greene All Rights Reserved
8
Injection Molding Costs
• Part 8: Packaging and Shipping Costs
– Costs for shipping cartons, bags, blister packages, foam materials
– Costs for transportation can be included
• Example, Knife
– Costs for blister packages = $50 per 1000 pieces
– Cost of carboard box = $0.70 per box that holds 1000 pieces
•
•
•
•
Total Factory Costs per 1000 pieces = $1443.51
General Administration Costs = 10% = $144.35
Marketing and Profit = 20% = $288.70
Total Cost per 1000 parts =$1,876.16,or $1.88 per knife
Copyright 2003 Joseph Greene All Rights Reserved
9
Injection Molding Costs_ Example
• Spreadsheet: IBIS and Associates
INJECTION MOLDING TECHNICAL COST MODEL
INJECTION MOLDING TCM: COST SUMMARY
IBIS Associates, Inc.
Copyright (c) 1997
IBIS Associates, Inc.
Copyright (c) 1997
---------------------------------------------------- ---------------------- ---------------------- ------------- ---------------------- ---------------------------------------------------Updated: 2/4/98
PRODUCT SPECIFICATIONS
VARIABLE COST ELEMENTS
Part Name
Toothbrush
NAME
Material Cost
Weight
30 grams
WGT
Direct Labor Cost
Maximum Wall Thickness
12 mm
THKM
Utility Cost
Average Wall Thickness
12 mm
THKA
External Surface Area
300 sq cm
SAREA
FIXED COST ELEMENTS
Projected Area
300 sq cm
PAREA
Equipment Cost
Tooling Cost
Number of Cavities
8
CAV
Building Cost
Number of Actions in Tool
1
ACT
Maintenance Cost
Surface Finish [3=best]
2 [1,2 or 3]
FIN
Overhead Labor Cost
Cost of Capital
Annual Production Volume
200 (000/yr)
NUM
Length of Production Run
5 yrs
PLIFE
TOTAL OPERATION COST
MATERIAL SPECIFICATIONS
Material Type
Material Price
Scrap Credit Value
Density
Thermal Conductivity
Heat Capacity
Melt Temp
Tool Temp
Eject Temp
PROCESS RELATED FACTORS
Dedicated Investment
Operation Rejection Rate
Material Scrap Rate
Average Equipment Downtime
Direct Laborers Per Station
---------------------------------------------------Nylon
$3.30
$0.00
1.2
0.24
1675
260
60
80
$/kg
$/kg
g/cm^3
W/mK
J/kgK
C
C
C
0 [1=Y 0=N]
0.1%
0.5%
20.0%
0.5
MAT
PRICE
SCPRI
DENS
TCOND
HTCAP
MTEMP
TTEMP
ETEMP
EXOGENOUS COST FACTORS
Direct Wages
Indirect Salary
Indirect:Direct Labor Ratio
Benefits on Wage and Salary
Working Days per Year
Working Hours per Day
Capital Recovery Rate
Equipment Recovery Life
Building Recovery Life
200 sec/cycle
$150 (000)
$100 (000)
10
$50,000
0.4
30.0%
260
24
15.0%
8
20
/hr
/yr
yrs
yrs
Adjusted Material Scrap
Cumulative Rejection Rate
Effective Production Volume
Tool Complexity Factor
Energy Adjustment Factor
Clamping Force
Cooling Time
DED
REJ
SCR
DOWN
NLAB
OPTIONAL INPUTS
Cycle Time
Equipment Cost per Station
Tool Cost per Set
INTERMEDIATE CALCULATIONS
Part Name
Material Designation
Product Weight
Raw Material Price
Material Scrap Price
Material Density
OCYCLE
OEQUIP
OTOOL
EXOG
WAGE
SALARY
ILAB
BENI
DAYS
HRS
CRR
ELIFE
BLIFE
CALCULATED
200.0
$150,000
$100,000
Cycle Time
Piece Cost of Flying Disk
$4.50
$4.00
$3.50
$3.00
$2.50
$2.00
$1.50
$1.00
$0.50
$0.00
Series1
0
100,00 200,00 300,00 400,00
0
0
0
0
Part Volume
Runtime for One Station
Number of Parallel Stations
Productive Tool Life
Tool Sets/Station
Equipment Investment/Station
Tooling Investment/Set
Power Consumption/Station
Building Space/Station
Equipment Annuity
Tooling Annuity
Copyright 2003 Joseph Greene All Rights Reserved
10
Design for Manufacturing
Reference: Design for Manufacturability Handbook, J. Bralla, McGraw Hill (1999)
• Cost for manufacturing item is dependent upon
–
–
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–
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Type of machining operation: use general purpose
Material selection: use common materials
Production quantities: higher quantities = lower cost
Design changes: keep the number small
Dimensional accuracy: keep tolerances generous where allowable
Sand Mold cating versus Die Casting
Process
Gray-Iron Casting
Cost of Item Unit Cost
$5,000
$0.10
$0.20
$1.20
0.30 h at $8/h
$0.00
Tooling
Material
Casting Setup
Casting Direct
Labor
0.80 h at $8/h
Machining:
setup
$50 for 5 ops
Machining:
direct labor
0.05h at $8/h
Total unit cost
Al Die Casting
Cost of Item Unit Cost
$35,000
$0.70
$0.70/lb
$1.40
0.4h at $8/h
$0.32
$0.64
0.04h at $8/h
$0.32
$0.02
$25 for 3 ops
$0.01
$0.40
--------------$2.36
0.03h at $8/h
$0.24
--------------$2.99
Copyright 2003 Joseph Greene All Rights Reserved
11
Design for Manufacturing
• CNC Factors versus manual methods
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Lead time is reduced
Complex parts routinely produced
Optimize process conditions for feed rates and speeds
Math data taken right from computer to cutter
Lathe versus screw machines
Process
Turret Lathe
Auto Single
Cost of Item Unit Cost
Cost of Item Unit Cost
Tooling
$350
$0.35
$680
$0.68
Setup
1h per 500 pieces
$0.03 2h per 500 pieces
$0.06
Direct labor
2 min
$0.67
.6 min
$0.05
----------------------------Total unit cost
$1.05
$0.79
Copyright 2003 Joseph Greene All Rights Reserved
Auto Multispindle
Cost of Item
Unit Cost
$680
$1.00
3h per 500 pieces
$0.08
.2 min
0.03
--------------$1.11
12
DFM Principles
•
•
•
•
•
•
•
•
Use of standards
Use of common components
Design to specifications and tolerances
Use of manufacturing guidelines in the early stages of
design that maximize quality of manufactured part
Minimize the use of materials
Minimize the use of floor space in plant
Locate all necessary components near functional operation
Use of automated machining for minimal errors
Copyright 2003 Joseph Greene All Rights Reserved
13
DFM Design Rules
• Simplify the design & reduce the number of parts required.
• Design for low labor cost operations, e.g., punch hole
rather than drill.
• Make specific notes on drawings and avoid generalized
statements, e.g., polish this surface.
• Dimensions should be made from specific surfaces and not
points in space. (Don’t dimension off a center of a circle)
• Minimize part weight whenever possible.
• Avoid sharp corners, use generous fillets and radii.
• Dimensions should be from one datum point rather than
from a variety of points.
• Design part so that as many operations can be used without
repositioning part
Copyright 2003 Joseph Greene All Rights Reserved
14
DFM Design Rules
• Cast, molded, or stamped parts should be made with no
stepped parting lines.
• Keep uniform wall thickness.
• Space holes so that they can be made in one operation
without tooling weakness
• Follow minimum draft requirements for cast or molded
parts
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15
DFM Quick References
• Surface finish from Various Processes
• Normal maximum surface roughness of common
machined parts
• Dimensional tolerances from machining
• Processes for flat surfaces
• Processes for 2D contoured surfaces
• Processes for hollow shapes
• Commonly used materials and metal working processes
• Formed metal parts
Copyright 2003 Joseph Greene All Rights Reserved
16
Material Selection
• Proper material selection is a major factor for a successful
designed and manufactured product.
• Common engineering materials
• Common commercial forms of selected raw materials
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–
–
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–
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Ultimate tensile strength of selected materials
Specific gravity (density) of selected materials
Melting point of selected materials
Thermal conductivity
CLTE
Relative Cost per unit weight and per unit volume
Copyright 2003 Joseph Greene All Rights Reserved
17
Ferrous Metals
• Hot-rolled steel
– Produced in a variety of cross sections and sizes
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•
•
•
•
•
•
Round bars from 6 to 250 mm in D
Square bars from 6 to 150mm per side
Rounded corners squares 10 to 200 mm per side
Flat bars from 5 mm in thickness to 200 mm in width
Angles, channels, tees, zees and other sections
Ovals, half rounds
Sheets 1.5 mm (16guage) or thicker and plates
– Common cross-sectional shapes for hot rolled steel
Copyright 2003 Joseph Greene All Rights Reserved
18
Ferrous Metals
• Hot rolled steel
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Process and Characteristics
Economic Quantities
Design Recommendations
Dimensional Factors and Tolerances
• Cold-Finish steel
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Definition and Applications
Available Shapes and Sizes
Design Recommendations
Standard Tolerances
• Stainless Steel
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Definition and Applications
Available Shapes and Sizes
Design Recommendations
Standard Tolerances
Copyright 2003 Joseph Greene All Rights Reserved
19
Hot-Rolled Steel
• Produced by passing a heated billet, bloom, or ingot of steel through a set of
shaped rollers.
– Repeated passes, the rollers increase the length of the billet and change it to a cross
section of specified shape and size.
– After rolling the shape is pickled (immersion in water, dilute sulfuric acid) to remove
scale and then is oiled
• Characteristics
– Produced in a variety of cross sections and sizes, which the following are:
• Round bars from 6 (0.25in) to 250 mm in diameter
• Square bars from 6 to 150 mm per side
• Round-cornered squares, 10 to 200 mm per side
• Flat bars from 5 mm in thickness and up to 200 mm in width (< 80 cm2 area)
• Angles, channels, tees with largest cross section dimension of 75 mm
• Ovals, half rounds and other special cross sections
• Sheets, 1.5 mm or thicker
– Hot rolled steel is about 30% lower in price than cold-finish steel.
– HRS has more dimensional variation, rougher surface, mill scale, less
straightness, less strength, and poorer machinability.
Copyright 2003 Joseph Greene All Rights Reserved
20
Hot Rolled Steel Characteristics
• Common shapes
Plates
Sheets
Strips
Squares
Rounds
I-Beams
Angles
(unequal length)
Half
rounds
Half
Ovals
Channels
Tees
Angles
(equal length)
Zees
Seamless
tubing
• Hot rolled steel is employed in applications for which only a small
amount of machining is required for which a smooth finish is not
necessary.
• Examples are tie rods, welded frames, lightly machined shafts,
cover plates, riveted and bolted racks, railroad cars, ships, bridges,
buildings
• Has low carbon content
(< 0.25%)
21
Copyright 2003 Joseph Greene All Rights Reserved
Economic Quantities
• Standard cross sections are suitable for all levels of production.
– Purchased from steel distribution warehouse or directly from mills
• Special cross section (Fig 2.2.2) hot-rolled steel are available in large quantities
form high production levels of 100 tons minimum
– Jack channels, studded T bars, U-harrow bars, hexagons, Diamonds, Ovals
• Grades for Further Processing
– Machining: moderately low-carbon grades (> 0.15%C) are best, otherwise it should be
hardened and tempered.
– Carbon content between 0.15 to 0.30%, machinability is good.
– Machinability is good for Carbon content between 0.30 and 0.50% and better if prior
annealed and partially speriodized.
– High carbon grades (>0.55%C) annealing must provide a completely spheriodized
structure
– For heavy machining, free machining grades require sulfur or lead
• Forming: Low carbon grades are best. Lower the yield strength and higher
ductility yields easier forming
• Welding: Low carbon grades are the best. Materials with 0.15%C or less are
easier to weld. Weld-ability decreases with increasing carbon content or alloy
content
Copyright 2003 Joseph Greene All Rights Reserved
22
Design Recommendations
• Grade selection design criterion is to design for minimum strength
– Grades with higher carbon content or low alloy content will provide lower cost
parts than that can be made from plain low-carbon grades due to lighter sections
can be used.
• Bending hot finished steel to help avoid fracturing material at bend:
– Bend line should be at right angles to grain direction from the rolling operation.
– Bend radius should be as generous as possible.
• To achieve a true surface from machining,
– Remove sufficient stock to get below the surface defects and irregularities.
• Include seams, scale, deviations from straightness or flatness
• AISI (American Iron and Steel Institute)
– Machine allowance
• 1.5mm per side for finished diameters or thicknesses from 40 to 75mm
• 3.0mm per side for diameters or thicknesses over 75mm.
Copyright 2003 Joseph Greene All Rights Reserved
23
Dimensional Factors and Tolerances
• Dimensional variations are considerably wider than with coldfinished material due to lack of secondary operations
• Standard Tolerances
– Angles for Hot-Rolled steel: Tolerances for Thickness, Length of Leg, and Outof-Square
• Note: Longer leg of unequal angle determines the size for tolerance.
• Note: Out-of-Square tolerance in either direction is 1.5°
Specified length
of leg, in
< 1 in
1 - 2 in
2 - 3in
Tolerance for
length of leg
Thickness tolerance for thicknesses given (over/under), in (Over/under), in
To 3/16
Over 3/16 to 3/8 Over 3/8
0.008
0.01
1/32
0.01
0.01
0.012
3/64
0.012
0.015
0.015
1/16
– Straightness Tolerance for Hot-Rolled Steel Bars
• Normal straightness Tolerance: ¼” in any 5-ft length or ¼ x length(ft) /4
• Special:1/8” in any 5-ft length or 1/8 x length (ft) /5
Copyright 2003 Joseph Greene All Rights Reserved
24
Cold-Finish Steel
• Definition
– Cold finished steel is more physically refined product than hot-rolled steel and:
• Higher surface finish, dimensional accuracy, and superior grain structure
• Higher tensile and yield strength
– Products include bars (round, square, hexagonal, flat, spherical), flat products
(sheets, strips, or plates), tubular products, wire or various cross sections
• Typical Applications
– Cold finished steel is best for applications where the following are required
• Greater accuracy and smoother surface finish
• Added mechanical properties, yield and tensile strength. 12% reduction in cross
section yields 20% increase in tensile strength and 60% increase in yield strength
• Improved machinability, formability, and freedom from surface scale
• Mill Processes used individually or in combination
– Cold drawing, cold rolling and machining.
– Can be subjected to heat treatments, e.g., stress relieving, annealing,
normalizing, carbon restoration
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25
Grades For Further Processing
• Machining
– Improved due to increased hardness from drawing operation
– Sulfur, lead, and tellurium are added to improve machining
• Cold Finished Steel Bar Formulations with Good Machinabilty
• Stamping
Common Designation Machiability rating, %
1214
158
1215
137
1213
137
1212
100
1119
100
1211
94
1116
92
1109
80
– Cold-rolled sheet steel is better for stamping than hot finished
• Absence of scale, greater uniformity of stock thickness, better formability
– Surface finish is superior
– Grade of steel required depends upon the severity of stamping
• Deep drawn parts may require Al kiln or drawing quality
– Lower carbon content (<0.10% C) is better
Copyright 2003 Joseph Greene All Rights Reserved
26
Grades For Further Processing
• Welding
– Material is fully weldable, especially low C, low-alloy steel
– Distortion is inherent to arc-welding, might be better with hot
finish steel
– Resistance weldament is great for cold-rolled steel
– Preferred material for arc and resistance welding is C < 0.35%
• Brazing
– Best accomplished with steels of lower C and alloy content.
• Ideal materials have C in range of 0.13 to 0.2% and Mn in 0.3 to 0.6%
• Plating
– All cold finish bars are suitable for plating
– Additional polishing is required
Copyright 2003 Joseph Greene All Rights Reserved
27
Grades For Further Processing
• Heat Treating
– Cold working materials are heat treatable and widely used.
– Grades for heat treatment processes
•
•
•
•
•
•
Carburizing: 8620, 4620, 1020, 1024, 9310
Nitriding: 4140, 4340,8640
Flame hardening: Medium carbon steels (0.35 – 0.70 %C)
Cyaniding/carbonitriding: 1020, 1022, 1010
Induction hardening: 1045,1038, 1144
Other through-hardening: 4140, 4130
• Painting
– All cold finish bars are practical for painting
– Extensive cleaning is not required
Copyright 2003 Joseph Greene All Rights Reserved
28
Shapes and Sizes
• Some common shapes of Cold-finished Steel
Min thick or D
Max thick or D
Normal Length
in
in
ft
Round bars
0.125
12
10 to 24 ft
Square bars
0.125
6
10-12 ft
Hex bars
0.125
4
10-12 ft
Flats
0.125 x .25 wide
3 in x 8 in wide
10-12 ft
Sheet
0.015 x 13 wide
0.179 in x 48 wide 4 to 10 ft or coil
Tubing, round seamless .125 OD x .049 wall 12 OD x .5 wall
17-24 ft
Tubing, round welded
.25 OD x .035 wall
6 OD x .25 wall
17-24 ft
Copyright 2003 Joseph Greene All Rights Reserved
29
Design Recommendations
• Design approach is to specify a size and shape of material
that minimize subsequent machining
– Use as-drawn or as-rolled surfaces and dimensions
• Other Rules
– Use simplest cross-sectional shape possible; avoid holes & grooves
• With special shapes, undercuts and reentrant angles can be produced but $$
–
–
–
–
Use standard rather than special shapes.
Avoid sharp corners & use the largest filets & radii (min 0.08mm)
Grooves width should be less deep than 1.5 times the width.
Keep section thickness as constant as possible, avoid abrupt
changes should be avoided to reduce local stress concentrations
– Specify the most easily formed materials and lowest cost
– With tubular sections, welded rather than seamless types are more
economical, especially if drawing after welding without mandrel
Copyright 2003 Joseph Greene All Rights Reserved
30
Design Recommendations
• Other Rules
– Avoid undercuts and reentrant angles
• Poor
Better
– Avoid sharp corners
• Poor
Better
Copyright 2003 Joseph Greene All Rights Reserved
31
Stainless Steel
• Definition and Applications
– Alloys that posses unusual resistance to attack by
corrosive media
– Applications include aircraft, railway cars, trucks, trailers,...
• AISI developed a 3digit numbering system for stainless steels
– 200 series: Austenitic- Iron-Cr-Ni-Mn
• Hardenable only by cold working and nonmagnetic
– 300 series: Austenitic- Iron-Cr-Ni
• Hardenable only by cold working and nonmagnetic
• General purpose alloy is type 304 (S30400)
– 400 series:
• Ferritic- Iron-Cr alloy are not hardenable by heat treatment or cold working
– Type 430 (S43000) is a general purpose alloy
• Martensitic- Iron-Cr alloys are hardenable by heat treatment and magnetic
– Type 410 (S41000) is a general purpose alloy
Copyright 2003 Joseph Greene All Rights Reserved
32
Stainless Steel
• Corrosion of steels can be slowed with addition of Cr and Ni.
• Stainless steels have chromium (up to 12%) and Ni (optional)
– ferritic stainless: 12% to 25% Cr and 0.1% to 0.35% Carbon
• ferritic up to melting temp and thus can not form the hard martensitic steel.
• can be strengthened by work hardening
• very formable makes it good for jewelry, decorations, utensils, trim
– austenitic stainless: 16% to 26% Cr, 6% to 23% Ni, <0.15% Carbon
• nonmagnetic and low strength % to 25% Cr and 0.1% to 0.35% Carbon
• machinable and weldable, but not heat-treatable
• used for chemical processing equipment, food utensils, architectural items
– martensitic stainless: 6% to 18% Cr, up to 2% Ni, and 0.1% to 1.5% C
• hardened by rapid cooling (quenching) from austenitic range.
• Corrosion resistance, low machinability/weldability used for knives, cutlery.
– Marging (high strength) steels: 18% to 25% Ni, 7% Co, with others
• heated and air cooled cycle with cold rolled
• Machinable used
for large structures, e.g., buildings, bridges, aircraft
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33
Stainless Steel
• Cold forming
– 200 and 300 series:
• Excellent bending characteristics
– Withstand a free bend of 180° with a radius equal to ½ material
thickness.
– As hardness increases, the bending becomes more restrictive
• Can be stretched more than carbon steel
– Excellent stretch-forming characteristics, preferred 301 or
201 due to cold working induced high strength
– 305 series exhibit excellent deep drawability
– 400 series:
• Good bending characteristics
– Less ductility than the 300 series with minimum radius equal to
thickness.
• Cannot be stretched severely without thinning and fracturing
• Can be processed for deep drawability
Copyright 2003 Joseph Greene All Rights Reserved
34
Stainless Steel
• Hot forming
– Stainless steels are readily formed by hot operations, I.e.,
rolling, extrusion, and forging
• Machining
– Machining characteristics are substantially different than
for carbon or alloy steels
• Stainless steel types are difficult to machine and are tough and gummy and
tend to seize and gall.
• 400 series- Easiest to machine, although produce a stringy chip
• 200 and 300 series- Most difficult to machine due to gumminess and workharden at a rapid rate.
• Ways to improve machinability:
– Specify that the bar for machining be in a slightly hardened condition.
– Order a Stainless steel that is chemically altered for machining
– Order a free-machining Stainless steel that has sulfur, selenium, Pb, Cu
• Types- 303, 303Se, 430F, 430F Se, 416, 416Se, 402F
Copyright 2003 Joseph Greene All Rights Reserved
35
Stainless Steel
• Machinability
Machinability of Stainless Steel
304
303
430
430F
410
416
420
420F
Ratings, %
60
75
60
92
60
100
52
65
120
100
Ratings, %
Type
80
60
40
20
0
304
303
430 430F 410
416
420 420F
SS type
Copyright 2003 Joseph Greene All Rights Reserved
36
Stainless Steel
• Welding
– Weld rod selection is important because the filler metal should have
composition equivalent to or more highly alloyed than the base
material
– Cr carbides can precipitate in the grain boundaries when stainless
steels re heated and cooled during welding through a temperature
range of 430°C to 900°C (800°F to 1650°F)
• This lessons the corrosion resistance
• Prevention
– Low carbon stainless steels are used, e.g., 304J, 316L, 317L
– Stabilized with niobium or titanium to prevent precipitation
– Austenitic types are 321 and 347
– Ferritic types 409 and 439
• Soldering and Brazing
– Stainless steel can be soldered readily as long as proper heat
treating techniques are employed to avoid Cr Carbide precipitation
Copyright 2003 Joseph Greene All Rights Reserved
37
Stainless Steel
• Design Recommendations
– Use least expensive stainless
– Use rolled finishes
– Use thinnest gauge required
– Use thinner gauge continuously backed
– Use standard roll-formed sections
– Use simple sections for economy of forming
– Use concealed welds to eliminate refinishing
– Use stainless steel types that are especially suited to
manufacturing processes, e.g., free machining
• Dimensional Factors Standard Tolerances
– Provided in the Steel Products Manual for Stainless Steels
– Comparable to to carbon and alloy steels
Copyright 2003 Joseph Greene All Rights Reserved
38
Dimensional Factors and Tolerances
• Dimensional variations are considerably wider than with coldfinished material due to lack of secondary operations
• Standard Tolerances
– Angles for Hot-Rolled steel: Tolerances for Thickness, Length of Leg, and Outof-Square
• Note: Longer leg of unequal angle determines the size for tolerance.
• Note: Out-of-Square tolerance in either direction is 1.5°
Specified length
of leg, in
< 1 in
1 - 2 in
2 - 3in
Tolerance for
length of leg
Thickness tolerance for thicknesses given (over/under), in (Over/under), in
To 3/16
Over 3/16 to 3/8 Over 3/8
0.008
0.01
1/32
0.01
0.01
0.012
3/64
0.012
0.015
0.015
1/16
– Straightness Tolerance for Hot-Rolled Steel Bars
• Normal straightness Tolerance: ¼” in any 5-ft length or ¼ x length(ft) /4
• Special:1/8” in any 5-ft length or 1/8 x length (ft) /5
Copyright 2003 Joseph Greene All Rights Reserved
39
Non-metallic Parts
Plastics and Thermosets
Professor Joe Greene
CSU, CHICO
Copyright 2003 Joseph Greene All Rights Reserved
40
DFM for Injection Molding
• Typical characteristics of injection molding
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41
DFM for Injection Molding
• Effects of shrinkage
– Parts are designed with shrinkage included early in the design and
before tool build
• Shrink rates for common materials
Material
Max Shrinkage
– Acetal
2.5%
– Acrylic
0.8%
– ABS
0.8%
– Nylon
1.5%
– PC
0.7%
– PE
5.0%
– PP
2.5%
– PS
0.6%
– PVC rigid
0.5%
– PVC flexible
5.0%
Copyright 2003 Joseph Greene All Rights Reserved
42
DFM for Injection Molding
• Design Guidelines
– Gate and Ejector pin locations
•
•
•
•
•
•
Edge, Fan, Submarine, Flash, Tunnel, Ring, Diaphragm, disk, or sprue gate
Not on a show surface
Not near a structural member or hole or fastener
Minimize flow length
Minimize the number of weld lines
Gate thick to thin
– Suggested wall thickness
• Have constant wall thickness in part
• Have transitions from thick to thin regions if have different thickness. Have
gentle no sharp transitions
Copyright 2003 Joseph Greene All Rights Reserved
43
DFM for Injection Molding
• Design Guidelines
– Holes
–
–
–
–
–
–
–
–
–
Holes are possible with slides but can cause weld lines
Min spacing between two holes or a hole and a sidewall should be 1D
Should be located 3D or more from the edge of a part to min stresses
Through hole is preferred to a blind hole because core pin that produces
hole can be supported at both ends and is less likely to bend
Holes in bottom of part are better than holes in side, which requires
retractable core pins
Blind holes should not be more than 2D deep.
Use steps to increase the depth of a deep blind hole
For through holes, cutout sections in the part can shorten the length of a
small-diameter pin.
Use overlapping and offset mold cavity projections instead of core pins
to produce holes parallel to the die parting line (Perpendicular to the
mold-movement direction)
Copyright 2003 Joseph Greene All Rights Reserved
44
Sprue Guidelines
• The sprue must not freeze before any other cross section.
This is necessary to permit sufficient transmission of
holding pressure.
• The sprue must de-mold easily and reliably.
Dco  tmax + 1.5 mm
Ds  Dn + 1.0 mm
  1º - 2º
tan  = Dco - Ds / 2L
Copyright 2003 Joseph Greene All Rights Reserved
45
Runner Guidelines
• Common runners
– Full-round runner
– Trapezoidal runner
– Modified trapezoidal runner (a combination of round and
trapezoidal runner)
– Half-round runner
– Rectangular runner
Copyright 2003 Joseph Greene All Rights Reserved
46
Gate Design
• Gate Design Overview
– Single vs. multiple gates
• Single gate is usually desirable because multiple gates have weld lines
– Gate dimension
• The gate thickness is usually two-thirds the part thickness.
• The gate thickness controls packing time
• Chose a larger gate if you're aiming for appearance, low residual stress, and
better dimensional stability.
– Gate location
• Position the gate away from load-bearing areas.
• Position the gate away from the thin section areas, or regions of sudden
thickness change to avoid hesitation and sink marks
Copyright 2003 Joseph Greene All Rights Reserved
47
Gate Design
• Gate Design Overview
– Gate Types
• Manually trimmed
– Requires an operator to separate parts from runners during a secondary
operation
– Types include sprue, tab, edge, overlap, fan, disk, ring, film, diaphragm,
spider
• Automatically trimmed gates
– Automatically trimmed gates incorporate features in the tool to break or
shear the gate
– Should be used to
» Avoid gate removal as a secondary operation
» Maintain consistent cycle times for all shots
» Minimize gate scars
– Types include Pin, Submarine, hot-runner, and valve
Copyright 2003 Joseph Greene All Rights Reserved
48
Gate Design
• Design Rules
– Gate location
• Should be at the thickest area of the part, preferably at a spot where the function and
appearance of the part are not impaired
• Should be central so that flow lengths are equal to each extremity of the part
• Gate symmetrically to avoid warpage
• Vent properly to prevent air traps
• Enlarge the gate to avoid jetting
• Position weld and meld lines carefully
– Gate Length
• Gate length should be as short as possible to reduce an excessive pressure drop
across the gate. Ranges from 1 to 1.5 mm (0.04 to 0.06 inches)
• The gate thickness is normally 50 to 80 percent of the gated wall section thickness.
Pin and submarine gates range from 0.25- 2.0 mm (0.01”- 0.08”)
• The freeze-off time at the gate is the max effective cavity packing time.
• Fiber-filled materials require larger gates to minimize breakage of the fibers
Copyright 2003 Joseph Greene All Rights Reserved
49
Boosting structural integrity with ribs
• Structural integrity: the goal of every design
– The major component of designing for structural integrity, in many cases, is to
design the structure to be stiff enough to withstand expected loads. Increasing the
thickness to achieve this is self-defeating, since it will:
• Increase part weight and cost proportional to the increase in thickness.
• Increase molding cycle time required to cool the larger mass of material.
• Increase the probability of sink marks.
– Well-designed ribs can overcome these disadvantages with only a marginal increase in part weight.
•
Typical uses for ribs
– Covers, cabinets and body components with long, wide surfaces that must have good appearance with
low weight.
– Rollers and guides for paper handling, where the surface must be cylindrical.
– Gear bodies, where the design calls for wide bearing surfaces on the center shaft and on the gear teeth.
– Frames and supports.
Copyright 2003 Joseph Greene All Rights Reserved
50
Ribs Design Rules
• Keep part thickness as thin and uniform as possible.
– This will shorten the cycle time, improve dimensional stability, and
eliminate surface defects. .
– If greater stiffness is required, reduce the spacing between ribs, which
enables you to add more ribs.
• Rib geometry
– Rib thickness, height, and draft angle are related: excessive thickness will
produce sinks on the opposite surface whereas small thickness and too great
a draft will thin the rib tip too much for acceptable filling.
– Ribs should be tapered (drafted) at one degree per side.
• Less draft can be used, to one-half degree per side, if the steel that forms the sides of
the rib is carefully polished.
• The draft will increase the rib thickness from the tip to the root, by about 0.175 mm
per centimeter of rib height, for each degree of draft angle.
• The maximum recommended rib thickness, at the root, is 0.8 times the thickness of
the base to which it is attached.
• The typical root thickness ranges from 0.5 to 0.8 times the base thickness.
Copyright 2003 Joseph Greene All Rights Reserved
51
Recommended Rib Design Parameters.
• See Figure 1 for recommended design parameters.
Copyright 2003 Joseph Greene All Rights Reserved
52
Ribs Design Rules
• Location of ribs, bosses, and gussets
– Ribs aligned in the direction of the mold opening are the least expensive design option to
tool.
– As illustrated in Figure 1, a boss should not be placed next to a parallel wall; instead,
offset the boss and use gussets to strengthen it.
– Gussets can be used to support bosses that are away from the walls. The same design
rules that apply for ribs also apply for gussets.
• Alternative configurations
– As shown in Below, ribs can take the shape of corrugations.
– The advantage is that the wall thickness will be uniform and the draft angle can be
placed on the opposite side of the mold, thereby avoiding the problem of the thinning rib
tip.
– Honeycomb ribbing attached to a flat surface provides excellent resistance to bending in
all directions.
– A hexagonal array of interconnected ribs will be more effective than a square array,with
the same volume of material in the ribs.
Copyright 2003 Joseph Greene All Rights Reserved
53
• Bosses
Design Rules
– Bosses are protruding pads that are used to provide mounting surface or reinforcements
around holes.
– Use same guidelines as for ribs
• Undercuts
– Require sliding cores, split molds, or stripping plate
– Shallow undercuts may be strippable from mold without need for core pulls.
– Allowable undercut for common materials.
Strippable
Material undercut, mm
Acrylic
1.5
ABS
1.8
Nylon
1.5
PC
1
PE
2
PP
1.5
PS
1
Psfone
1
Vinyl, flex
2.5
Copyright 2003 Joseph Greene All Rights Reserved
54
Screw Threads
• Try to Avoid screw threads
– Use a core that is rotated after molding is complete to unscrew part
– Put axis of the screw at the parting line of the mold
– Make threads few, shallow, and of rounded form
• Inserts
– Useful and practical to provide reinforcement where stresses exceed the strength
of the plastic
– Sharp corners should be avoided
– Recommended designs
• Lettering and surface decorations
– Lettering in the part should be raised
– Lettering should be perpendicular to the parting line of the mold, otherwise there
will be an undercut.
Copyright 2003 Joseph Greene All Rights Reserved
55
Design Rules
• Draft
– Draft is needed to assist in demolding
• Common draft angles for materials
Material
Acetal
Acrylic
Nylon
PE
PS
Min Draft angledegrees
0 to 1/4
1/4
0 to 1/8
1/4
1/2
• Corners: Radii and Fillets
–
–
–
–
Sharp corners should be avoided. They add stress to polymer
Fillets and radii should be generous as possible.
Desirable minimum is 0.5 mm
Preferable min is 1.0 mm
Copyright 2003 Joseph Greene All Rights Reserved
56
Design Rules
• Surface Finish
– Surface polish and textures are possible with plastic parts.
– High gloss are feasible but can accentuate sinks and blemishes
– Dull, matte, or textured surfaces are preferred
• Flat Surfaces
– Feasible but prone to show irregularities than gently curved surfaces which are
preferred.
• Molding parting line
– The line created when the two mold halves come together
– Should be straight as the two mold halves come together in one plane. Offset
dies may help avoid appearance defects.
– For non-straight parting lines, add a bead or raised surface to clean flash on
demold.
Copyright 2003 Joseph Greene All Rights Reserved
57
DFM for Thermosets
• Thermosets are processed with
–
–
–
–
–
–
–
–
Compression molding- Medium pressure- 1 to 2 minute cycle time
Injection molding- High pressure - 0.2 to1 minute cycle time
Extrusion- Low pressure - continuous
SRIM - Low pressure - 1 to 3 minutes cycle time
RIM - Low pressure - 1 to 3 minutes cycle time
RTM - Low pressure - 5 to 10 minutes cycle time
Hand Lay-up - Low pressure - 1 to 8 hours cycle time
Filament Winding - Low pressure - 1 to 2 hours cycle time
• Tooling is very important in the manufacturing operations
• Tool materials are P-20 steel, aluminum, bronze, epoxy,
polyester, and rubber
Copyright 2003 Joseph Greene All Rights Reserved
58
Thermosetting Materials
•
•
•
•
•
•
•
•
Epoxy
Polyester and vinyl ester
Polyurethane
Phenolic compounds
Urea compounds
Melamine compounds
Polyimides
Bismaleimides
Copyright 2003 Joseph Greene All Rights Reserved
59
Design Recommendations
• Effects of Shrinkage
–
–
–
–
–
–
–
–
–
Material
Phenolic
Urea
Melamine
Diallyl phthalate
Alkyd
Polyester
Epoxy
Silicones
Shrinkage during molding
0.1-0.9
0.6-1.4
0.8-12.
0.3-0.7
0.5-1.0
0-0.7
0.1-1.0
0-0.5
Copyright 2003 Joseph Greene All Rights Reserved
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Design Recommendations
• Wall thickness
–
–
–
–
–
–
–
–
–
Material
Phenolic
Urea
Melamine
Diallyl phthalate
Alkyd
Polyester
Epoxy
Silicones
Suggested thickness, mm
1.5 -3
1.5 -3
1.5-3
1.1 -2.4
2 -3
1.1 -2.4
1 -2
1-2
Copyright 2003 Joseph Greene All Rights Reserved
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• Tooling
Design Recommendations
– Undercuts
• Parts must be designed so that they can be easily removed from the mold.
• If external undercuts are essential, straight draw is not possible, a side activated slide
is required or split the mold with removable sections
• Internal Undercuts are difficult and should be avoided
– Mold Parting line
• Two mating mold surfaces must be sealed off to mold a flash free part
• Contour and step partings lines are difficult and should be avoided
– Sharp corners
• All corners should have a radius or fillet at set-in sections of the mold or at the
parting line
• Avoid sharp corners
• Specify fillets and corner radii of 0.8mm to 1.1 mm
– Holes or openings
• Steel pins or section are needed in the mold to incorporate holes or irregularly shaped
openings
• Spacing between holes and next to side walls should be as large as possible
Copyright 2003 Joseph Greene All Rights Reserved
62
Value Engineering
• Value Engineering
– Developed in the late ‘40s by General Electric and evolved in ‘80s.
– Customer-oriented approach to the entire design process.
– Value = (Worth of a feature, component, or assembly) / (Cost of it)
• Worth- functionality it provides to customer
• Value- function provided per dollar of cost
– Value formula
• Step 1: Determine what the feature does.
• Step 2: Identify the life-cycle cost of the feature.
– Includes manufacturing, energy, use costs, disposal costs, etc.
• Step 3: Identify the worth of the function to customer.
– Can use QFD technique (pg 134) Quality function deployment
• Step 4. Compare worth to cost ratio of features and identify features that
have low worth to cost ratio. Consider removing low ratio features and
keeping high value features.
Copyright 2003 Joseph Greene All Rights Reserved
63
Quality Engineering
• Quality Engineering
– Reliability
• Potential failures of a product can be identified and then the MTBF
• MTBF: Mean time between failures
– Average elapsed time between failures.
Mechanical failures per Million hrs
Bearing
Ball
13
Roller
200
Sleeve
23
Brake
13
Clutch
2
Compressor
65
Differential
15
Fan
6
Heat Exchanger
4
Gear
0.2
Pump
12
Shock Absorbver
3
Spring
5
Valve
14
Electrical failures per Million hrs
Meter
26
Battery
Lead acid
0.5
Mercury
0.7
Circuit board
0.3
Connector
0.1
Generator
AC
2
DC
40
Heater
4
Lamp
Incandescent
10
Neon
0.5
Motor
Small HP
8
Large
4
Solenoid
1
Switch
6
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64
Quality Engineering
• Quality Engineering
– Defects: Automotive Defects per 100 vehicles.
– JD Power 2003 results
• Based upon buyers’ reports of problems during the
first 90 days of ownership
http://www.jdpower.com/cc/auto/auto.jsp
Car Company
Defects per
Toyota
Porsche
BMW
Honda
Industry Ave
GM
Nissan
Ford
DaimelerChrysler
Volkswagen
Hyundai
Suzuki
Subaru
Mitsubishi
Kia
100
115
117
124
126
133
134
135
136
139
141
143
144
143
148
168
Top Rated Trucks
• Compact: Mazda B-Series
• Full size: Ford F Series
• Entry SUV: Honda CR-V
• Midsize SUV: Toyota Highlander
• Fullsize SUV: Chevy Suburban
• Luxury SUV: Lexus RX300
•Compact van: Olds Silhouette
Copyright 2003 Joseph Greene All Rights Reserved
Car Brands Defects per
Lexus
Cadillac
Infiniti
Acura
Buick
Mercury
Porsche
BMW
Toyota
Jaguar
Honda
Volvo
Chevrolet
Audi
Mercedes-Benz
Industry Ave
Olds
Chrylser
Ford
Dodge
Lincoln
Nissan
Pontiac
Hyundai
Volkswagen
GMC
Suzuki
Jeep
Subaru
Saturn
Saab
Mini
Kia
Land Rover
Hummer
100
76
103
110
111
112
113
117
118
121
122
128
128
130
132
132
133
134
136
136
137
139
139
142
143
143
144
146
146
148
158
160
166
168
190
65
225
Patents
• Patent office http://www.uspto.gov/
•
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