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A Design Guide
Part and Mold Design
Engineering Polymers
THERMOPLASTICS
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INTRODUCTION
The manual focuses primarily onplastic part and mold design, but alsoincludes chapters on the design process;designing for assembly; machining andfinishing; and painting, plating, anddecorating. For the most part, it excludesinformation covered in the followingBayer companion publications:
Material Selection: Thermoplastics and
Polyurethanes : A comprehensive look atmaterial testing and the issues to consider
when selecting a plastic material.
Joining Techniques : Includes infor-mation and guidelines on the methodsfor joining plastics including mechanicalfasteners, welding techniques, inserts,snap fits, and solvent and adhesivebonding.
Snap-Fit Joints for Plastics : Containsthe engineering formulas and workedexamples showing how to design snap-fit joints for Bayer thermoplastic resins.
A product of the Bayer Design
Engineering Services Group, this manual
is primarily intended as a reference
source for part designers and molding
engineers working with Bayer thermo-
plastic resins. The table of contents and
index were carefully constructed to
guide you quickly to the information
you need either by topic or by keyword.
The content was also organized to allow
the manual to function as an educational
text for anyone just entering the field of
plastic-part manufacturing. Conceptsand terminology are introduced pro-
gressively for logical cover-to-cover
reading.
Contact your Bayer sales representativefor copies of these publications.
This publication was written specificallyto assist our customers in the design andmanufacture of products made from theBayer line of thermoplastic engineeringresins. These resins include:
Makrolon Polycarbonate
Apec High-Heat Polycarbonate
Bayblend Polycarbonate/ ABS Blend
Makroblend Polycarbonate Blend
Triax Polyamide/ABS Blend
Lustran and Novodur ABS
Lustran SAN
Cadon SMA
Centrex ASA, AES and ASA/AESWeatherable Polymers
Durethan Polyamide 6 and 66,and Amorphous Polyamide
Texin and Desmopan
Thermoplastic Polyurethane
Pocan PBT Polyester
1
This publication was written to assistBayer's customers in the design andmanufacture of products made fromthe Bayer line of thermoplasticengineering resins. These resinsinclude:
- Makrolon polycarbonate- Apec high-heat polycarbonate- Bayblend polycarbonate/ABSblend- Makroblend polycarbonate/ polyester blend- Texin and Desmopan
thermoplastic polyurethaneFor information on these materials,please call 1-800-662-2927 or visithttp://www.BayerMaterialScienceNAFTA.com.
The following additional productshighlighted in this publication are nowpart of LANXESS Corporation:
- Cadon SMA- Centrex ASA, AES and ASA/AESweatherable polymers- Durethan polyamide 6 and 66, andamorphous polyamide- Lustran and Novodur ABS- Lustran SAN- Pocan PBT polyester- Triax polyamide/ABS blend
For information on these products,please call LANXESS in NorthAmerica at 1-800-LANXESS or visit:
http://techcenter.lanxess.com/sty/ americas/en/home/index.jsp forstyrenic resinshttp://techcenter.lanxess.com/scp/ americas/en/home/index.jsp forpolyamide resins
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Bayer CAMPUS : Software containingsingle and multi-point data that wasgenerated according to uniform standards.Allows you to search grades of Bayerresins that meet a particular set ofperformance requirements.
www.bayer.com/polymers-usa: BayerWeb site containing product informationon-line.
This manual provides general information
and guidelines. Because each productapplication is different, always conducta thorough engineering analysis of yourdesign, and prototype test new designsunder actual in-use conditions. Applyappropriate safety factors, especiallyin applications in which failure couldcause harm or injury.
Most of the design principles covered inthis manual apply to all of these resins.When discussing guidelines or issuesfor a specific resin family, we referencethese materials either by their Bayertrade names or by their genericpolymer type.
The material data scattered throughoutthe chapters is included by way of example only and may not reflect themost current testing. In addition, much
of the data is generic and may differfrom the properties of specific resingrades. For up-to-date performance datafor specific Bayer resins, contact yourBayer sales representative or refer to thefollowing information sources:
Bayer Engineering Polymers Properties
Guide : Contains common single-pointproperties by resin family and grade.
Bayer Plastics Product Information
Bulletin : Lists information and propertiesfor a specific material grade.
In addition to design manuals, BayerCorporation provides design assistancein other forms such as seminars andtechnical publications. Bayer also offersa range of design engineering servicesto its qualified customers. Contact yourBayer sales representative for moreinformation on these other services.
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Chapter 1PART DESIGN PROCESS: CONCEPT TO FINISHED PART
7 Design Process
8 Defining Plastic Part Requirements
8 Mechanical Loading
8 Temperature
8 Chemical Exposure
8 Electrical Performance
8 Weather Resistance
8 Radiation
8 Appearance
9 Agency Approvals
9 Life Expectancy9 Dimensional Tolerances
9 Processing
9 Production Quantities
9 Cost Constraints
10 Assembly
10 Thermoplastic Processing Methods
10 Injection Molding
11 Extrusion
12 Thermoforming
12 Blow Molding
13 Rotomolding
13 Optimizing Product Function
14 Consolidation
14 Hardware
14 Finish
15 Markings and Logos
15 Miscellaneous
15 Reducing Manufacturing Costs
15 Materials
16 Overhead
17 Labor17 Scrap and Rework
17 Prototype Testing
Chapter 2GENERAL DESIGN
19 Wall Thickness
22 Flow Leaders and R estrictors
24 Ribs
24 Rib Design
24 Rib Thickness
26 Rib Size
27 Rib Location and Numbers
27 Bosses
30 G ussets
30 Sharp Corners
32 Draft33 Holes and Cores
34 Undercuts
34 Slides and Cores
36 Louvers and Vents
37 Molded-In Threads
40 Lettering
40 Tolerances
42 Bearings and Gears
TABLE OF CONTENTS
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Chapter 4DESIGN FOR ASSEMBLY
83 Part Consolidation
84 Mechanical Fasteners
85 Snap-Fit Joints
88 Welding and Bonding
89 Ultrasonic Welding
90 Vibration and Hot-Plate Welding
91 Spin Welding
91 Solvent and Adhesive Bonding
92 Retention Features
92 Alignment F eatures
94 Orientation94 Expansion Differences
94 Tolerances
Chapter 5MACHINING AND FINISHING
97 Drilling and Reaming
99 Tapping
99 S awing
100 Punching, Blanking, and Die Cutting
101 Milling
101 Turning and Boring
102 Laser Machining
103 Filing
103 Sanding
103 Polishing and Buffing
104 Trimming, Finishing, and Flash Removal
Chapter 3STRUCTURAL DESIGN
45 Structural Considerations In Plastics
46 Stiffness
46 Viscoelasticity
48 Stress-Strain Behavior
50 Molding Factors
51 Short-Term Mechanical Properties
51 Tensile Properties
52 Tensile Modulus
52 Tensile Stress at Yield
52 Tensile Stress at Break
53 Ultimate Strength53 Poisson's Ratio
53 Compressive Properties
53 Flexural Modulus
53 Coefficient of Friction
54 Long-Term Mechanical Properties
54 Creep Properties
56 Stress Relaxation
56 Fatigue Properties
58 Structural Design Formulas
58 Use of Moduli
59 Stress and Strain Limits
60 Uniaxial Tensile and Compressive Stress
61 Bending and Flexural Stress
65 Shear Stress
66 Torsion
67 Designing for Stiffness
67 Part Shape
70 Wall Thickness
71 Ribs
73 Long-Term Loading
76 Designing for Impact78 Fatigue Applications
80 Thermal Loading
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Chapter 7MOLD DESIGN
121 Mold Basics
121 Types of Molds
124 Mold Bases and Cavities
125 Molding Undercuts
128 Part Ejection
130 Mold Venting
130 Parting-Line Vents
131 Vent Placement
133 Sprues, Runners, and Gates
133 Sprues
134 Runners137 Runners for Multicavity Molds
140 Gates
144 Other Gate Designs
145 Gate Optimization
147 Gate Position
149 Hot-Runner Systems
149 Hot-Runner Designs
149 Hot-Runner Gates
151 Valve Gates
151 Thermal Expansion and Isolation
152 Flow Channel Size
153 Mold Cooling
154 Mold-Cooling Considerations
155 Cooling-Channel Placement
158 Cooling-Line Configuration
159 Coolant Flow Rate
160 Mold Shrinkage
162 Mold Metals
163 Surface Treatments
164 Mold Cost and Quality
APPENDICES
165 Index
169 Part Design Checklist
Chapter 6PAINTING, PLATING, AND DECORATING
105 Painting
105 Types of Paints
106 Paint Curing
106 Paint-Selection Considerations
107 Spray Painting
108 Other Painting Methods
108 Masking
109 Other Design Considerations for Painting
109 In-Mold Decorating
110 Film-Insert Molding
111 Metallic Coatings111 Electroplating
112 Design Considerations for Electroplating
113 Molding Considerations for Electroplating
114 Vacuum Metallization
115 Design Considerations for Vacuum Metallization
115 EMI/RFI Shielding
115 Design Considerations for EMI/RFI Shielding
116 Printing
118 Labels and Decals
119 Texture
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Chapter 1
PART DESIGN PROCESS:CONCEPT TO FINISHED PART
DESIGN PROCESS
Like a successful play in football,successful plastic product design andproduction requires team effort and awell-developed strategy. When designingplastic parts, your team should consistof diverse players, including conceptualdesigners, stylists, design engineers,materials suppliers, mold makers,manufacturing personnel, processors,finishers, and decorators. Your chance
of producing a product that successfullycompetes in the marketplace increaseswhen your strategy takes full advantageof team strengths, accounts for memberslimitations, and avoids overburdeningany one person. As the designer, youmust consider these factors early instrategy development and makeadjustments based upon input from thevarious people on the design team.
Solicit simultaneous input from the var-ious players early in product develop-ment, before many aspects of the designhave been determined and cannot bechanged. Accommodate suggestions forenhancing product performance, or forsimplifying and improving the variousmanufacturing steps such as moldconstruction, processing, assembly,and finishing. Too often designs pass
sequentially from concept developmentto manufacturing steps with featuresthat needlessly complicate productionand add cost.
Many factors affect plastic-part design.
Among these factors are: functional
requirements, such as mechanical
loading and ultraviolet stability;
aesthetic needs, such as color, level of
transparency, and tactile response; and
economic concerns, such as cost of
materials, labor, and capital equipment.
These factors, coupled with other
design concerns such as agency
approval, processing parameters,
and part consolidation are discussed
in this chapter.
Early input from various design andmanufacturing groups also helps tofocus attention on total product costrather than just the costs of individualitems or processes. Often adding aprocessing step and related cost in onearea produces a greater reduction intotal product cost. For example, addingsnap latches and nesting features mayincrease part and mold costs, and at thesame time, produce greater savings inassembly operations and related costs.
Likewise, specifying a more-expensiveresin with molded-in color and UVresistance may increase your raw-material cost, while eliminatingpainting costs.
When designing and developing parts,focus on defining and maximizing partfunction and appearance, specifyingactual part requirements, evaluatingprocess options, selecting an appropri-ate material, reducing manufacturingcosts, and conducting prototype testing.For the reasons stated above, theseefforts should proceed simultaneously.
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Chemical Exposure
Plastic parts encounter a wide variety of chemicals both during manufacturingand in the end-use environment,including mold releases, cutting oils,degreasers, lubricants, cleaning sol-vents, printing dyes, paints, adhesives,cooking greases, and automotive fluids.Make sure that these chemicals arecompatible with your selected materialand final part.
Electrical Performance
Note required electrical property valuesand nature of electrical loading. Forreference, list materials that are knownto have sufficient electrical performancein your application. Determine ifyour part requires EMI shielding orUL testing.
Weather Resistance
Temperature, moisture, and UV sunexposure affect plastic parts propertiesand appearance. The end-use of a productdetermines the type of weather resistancerequired. For instance, external automo-tive parts such as mirror housings must
withstand continuous outdoor exposureand perform in the full range of weatherconditions. Additionally, heat gain fromsun on dark surfaces may raise the uppertemperature requirement considerablyhigher than maximum expected temper-atures. Conversely, your requirements
DEFINING PLASTIC PARTREQUIREMENTS
Thoroughly ascertain and evaluate yourpart and material requirements, whichwill influence both part design andmaterial selection. When evaluatingthese requirements, consider more than
just the intended, end-use conditionsand loads: Plastic parts are often sub-
jected to harsher conditions duringmanufacturing and shipping than in
actual use. Look at all aspects of partand material performance includingthe following.
Mechanical Loading
Carefully evaluate all types of mechanicalloading including short-term staticloads, impacts, and vibrational orcyclic loads that could lead to fatigue.Ascertain long-term loads that couldcause creep or stress relaxation. Clearlyidentify impact requirements.
Temperature
Many material properties in plastics impact strength, modulus, tensilestrength, and creep resistance to name a
few vary with temperature. Considerthe full range of end-use temperatures,as well as temperatures to which the partwill be exposed during manufacturing,finishing, and shipping. Remember thatimpact resistance generally diminishesat lower temperatures.
may be less severe if your part isexposed to weather elements onlyoccasionally. For example, outdoorChristmas decorations and other season-al products may only have to satisfy therequirements for their specific, limitedexposure.
Radiation
A variety of artificial sources such
as fluorescent lights, high-intensity dis-charge lamps, and gamma sterilizationunits emit radiation that can yellowand/or degrade many plastics. If yourpart will be exposed to a radiationsource, consider painting it, or specifyinga UV-stabilized resin.
Appearance
Aesthetic requirements can entail manymaterial and part-design issues. Forexample, a need for transparency greatlyreduces the number of potential plastics,especially if the part needs high clarity.Color may also play an important role.Plastics must often match the color of other materials used in parts of anassembly. Some applications require theplastic part to weather at the same rate
as other materials in an assembly.
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PART DESIGN PROCESS:CONCEPT TO FINISHED PART continued
contact, United States Department of Agriculture (USDA) for plastics inmeat and poultry equipment, andNational Sanitation Foundation TestingLaboratory, Inc. (NSF) for plastics infood-processing and potable-waterapplications. Always check forcompliance and approval fromappropriate agencies. Determine ifyour part requires flame resistance inaccordance with UL 94. If so, noterating and thickness.
Life Expectancy
Many functional parts need to meetcertain life-cycle expectations. Lifeexpectancy may involve a time duration as in years of outdoor exposure time at a specific set of conditions such as hours in boiling water orrepetitions of an applied load orcondition as in number of gammasterilization cycles or snap-armdeflections. Determine a reasonablelife expectancy for your part.
Dimensional Tolerances
Many applications have featuresrequiring tight tolerances for proper fit
and function. Some mating parts requireonly that mating features have the samedimensions. Others must have absolutesize and tolerance. Consider the effectof load, temperature, and creep ondimensions. Over-specification oftolerance can increase productcost significantly.
In resins, custom colors generally costmore than standard colors, particularlyfor small-order quantities. For certaincolors and effects, some parts may needto be painted or decorated in the mold.Depending upon the application, partswith metallic finishes may requirepainting, in-mold decorating or vacuummetallization. Surface finishes rangefrom high-gloss to heavy-matte.Photoetching the mold steel can impartspecial surface textures for parts.
Styling concerns may dictate the prod-uct shape, look, and feel, especially if the product is part of a component sys-tem or existing product family. Note allcosmetic and non-cosmetic surfaces.Among other things, these areas mayinfluence gate, runner, and ejector-pinpositioning.
Many part designs must include mark-ings or designs such as logos, warnings,instructions, and control labels.Determine if these features can bemolded directly onto the part surfaceor if they must be added using one of the decorating methods discussed inChapter 6.
Agency Approvals
Government and private agencies havespecifications and approval cycles formany plastic parts. These agenciesinclude Underwriters Laboratories(UL) for electrical devices, Military(MIL) for military applications, Foodand Drug Administration (FDA) forapplications with food and bodily-fluid
Processing
Determine if your part design placesspecial demands on processing. Forexample, will the part need a moldgeometry that is particularly difficultto fill, or would be prone to warpageand bow. Address all part-ejection andregrind issues.
Production Quantities
The number of parts needed mayinfluence decisions, including processingmethods, mold design, material choice,assembly techniques, and finishingmethods. Generally for greater productionquantities, you should spend money tostreamline the process and optimizeproductivity early in the design process.
Cost Constraints
Plastic-part cost can be particularlyimportant, if your molded part comprisesall or most of the cost of the final product.Be careful to consider total system cost,not just part and material cost.
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THERMOPLASTICPROCESSING METHODS
A variety of commercial methods areused to produce thermoplastic products.Each has its specific design require-ments, as well as limitations. Usuallypart design, size, and shape clearlydetermine the best process.Occasionally, the part concept lendsitself to more than one process. Becauseproduct development differs depending
upon the process, your design teammust decide which process to pursueearly in product development. Thissection briefly explains the commonprocesses used for thermoplastics fromBayer Corporation.
Assembly
Address assembly requirements, such asthe number of times the product will bedisassembled or if assembly will beautomated. List likely or proposedassembly methods: screws, welds,adhesives, snap-latches, etc. Note matingmaterials and potential problem areassuch as attachments to materialswith different values of coefficient of linear thermal expansion. State any
recycling requirements.
The Part Requirements and DesignChecklist in the back of this manualserves as a guide when developing newproducts. Be sure not to overlook anyrequirements relevant to your specificapplication. Also do not over-specifyyour requirements. Because partsperform as intended, the costs of over-specification normally go uncorrected,needlessly increasing part cost andreducing part competitiveness.
Injection Molding
The most common processing methodfor Bayer thermoplastics, injectionmolding, involves forcing moltenplastic into molds at high pressure. Theplastic then forms to the shape of themold as it cools and solidifies (seefigure 1-1). Usually a quick-cycleprocess, injection molding can producelarge quantities of parts, accommodatea wide variety of part sizes, offer
excellent part-to-part repeatability,and make parts with relatively tighttolerances. Molds can produce intricatefeatures and textures, as well as structuraland assembly elements such as ribs andbosses. Undercuts and threads usually
10
The injection molding process can quickly produce large quantities of parts inmulti-cavity molds.
Figure 1-1Injection Molding
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Chapter 1
PART DESIGN PROCESS:CONCEPT TO FINISHED PART continued
of each part. The same mold producing500,000 parts would contribute only$0.10 to part cost. Additionally, mold
modifications for product designchanges can be very expensive. Verylarge parts, such as automotive bumpersand fenders, require large and expensivemolds and presses.
require mold mechanisms that addto mold cost.
The injection molding process generallyrequires large order quantities to offsethigh mold costs. For example, a$50,000 mold producing only 1,000parts would contribute $50 to the cost
Extrusion
In extrusion forming, molten materialcontinuously passes through a die thatforms a profile which is sized, cooled,and solidified. It produces continuous,straight profiles, which are cut tolength. Most commonly used for sheet,film, and pipe production, extrusion alsoproduces profiles used in applicationssuch as road markers, automotive trim,store-shelf price holders, and window
frames (see figure 1-2). Productionrates, measured in linear units, such asfeet/minute, ordinarily are reasonablyhigh. Typically inexpensive for simpleprofiles, extrusion dies usuallycontribute little to product cost. Partfeatures such as holes or notchesrequire secondary operations thatadd to final cost.
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Extrusion
The extrusion process produces profile shapes used in the manufacture of window frames.
Figure 1-2
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Thermoforming
Thermoforming creates shapes from athermoplastic sheet that has been heatedto its softening point. Applied vacuumor pressure draws or pushes the softenedsheet over an open mold or form whereit is then cooled to the conformingshape. The process of stretching thesheet over the form or mold causesthinning of the wall, especially alongthe sides of deep-drawn features. Mold
or form costs for this low-pressureprocess are much lower than for injectionmolds of comparable size.
Thermoforming can produce large parts(see figure 1-3) on relatively inexpensivemolds and equipment. Because theplastic is purchased as sheet stock,materials tend to be costly. Material
selection is limited to extrusion grades.Secondary operations can play a largerole in part cost. Thermoformed partsusually need to be trimmed to removeexcess sheet at the part periphery. Thisprocess cannot produce features thatproject from the part surface such asribs and bosses. Cutouts and holesrequire secondary machining operations.
Blow Molding
Blow molding efficiently produceshollow items such as bottles (seefigure 1-4), containers, and light globes.
12
The automobile industry has taken advantage of the production efficiency, appearance, lightweight, and performance of thermoformed engineering thermoplastics for many OEM andafter-market products like this tonneau cover.
Figure 1-3Thermoforming
Blow Molding
This large water bottle was blow molded inMakrolon polycarbonate resin.
Figure 1-4
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Rotomolding
In rotomolding , a measured quantity of thermoplastic resin, usually powdered,is placed inside a mold, which is thenexternally heated. As the mold rotateson two perpendicular axes, the resincoats the heated mold surface. Thiscontinues until all the plastic melts toform the walls of the hollow, molded
shape. While still rotating, the mold iscooled to solidify the shape.
Design permitting, the process mayalso produce hollow shapes such asautomotive air ducts and gas tanks.Wall thickness can vary throughout thepart and may change with processing.Blow molding cannot produce featuresthat project from the surface such asribs and bosses. Part geometrydetermines mold and equipment costs,which can range as high as those forinjection molding.
The two most-common types of blowmolding are extrusion and injection. Inextrusion blow molding , mold halvespinch the end of a hanging extrudedtube called a parison until itseals. Air pressure applied into the tubeexpands the tube and forces it againstthe walls of the hollow mold. Theblown shape then cools as a thin-walledhollow shape. A secondary step removesthe vestige at the pinch-off area.
Injection blow molding substitutes amolded shape in place of the extrudedparison. Air pressure applied frominside the still-soft molded shapeexpands the shape into the form of thehollow mold. This process eliminatespinch-off vestige and facilitates moldedfeatures on the open end such as screwthreads for lids.
This process is used for hollow shapeswith large open volumes that promoteuniform material distribution, includingdecorative streetlight globes (see figure1-5) or hollow yard toys. Mold andequipment costs are typically low, andthe process is suited to low-productionquantities and large parts. Cycle timesrun very long. Large production runsmay require multiple sets of molds.
OPTIMIZINGPRODUCT FUNCTION
The molding process affords manyopportunities to enhance part function-ality and reduce product cost. For exam-ple, the per-part mold costs associatedwith adding functional details to thepart design are usually insignificant.Molds reproduce many features practi-cally for free. Carefully review allaspects of your design with an eyetoward optimization, including partand hardware consolidation, finishingconsiderations, and needed markingsand logos, which are discussed inthis section.
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Rotomolding
Rotomolding can produce large hollowparts such as this polycarbonate streetlight globe.
Figure 1-5
Chapter 1
PART DESIGN PROCESS:CONCEPT TO FINISHED PART continued
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Consolidation
Within the constraints of good moldingpractice and practical mold construction,look for opportunities to reduce thenumber of parts in an assembly throughpart consolidation. A single molded partcan often combine the functionality of two or more parts.
Hardware
Clever part design can often eliminateor reduce the need for hardware fastenerssuch as screws, nuts, washers, andspacers. Molded-in hinges can replacemetal ones in many applications (seefigure 1-6). Molded-in cable guidesperform the same function as metal onesat virtually no added cost. Reducinghardware lessens material and assemblycosts, and simplifies dismantling forrecycling.
Finish
Consider specifying a molded-in colorinstead of paint. The cost savings couldmore than justify any increase in mater-ial cost for a colored material with therequired exposure performance. If you
must paint, select a plastic that paintseasily, preferably one that does notrequire surface etching and/or primer.
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Molded-in hinge features can eliminate the need for hinge hardware.
Figure 1-6 Hinges
SlightUndercut
PL
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Chapter 1
PART DESIGN PROCESS:CONCEPT TO FINISHED PART continued
REDUCINGMANUFACTURING COSTS
Although many factors contribute tocosts of producing plastic parts, mostcosts fall into one of four basic categories:materials, overhead, labor, and scrap/ rework. This section highlights potentialmethods for reducing these manufacturingcosts. Carefully evaluate the effect eachcost-reduction step may have on yourproducts performance and overall cost.
Materials
To reduce material costs, you mustreduce material usage and obtain thebest material value. Within the limits
Markings and Logos
Secondary methods of adding direc-tions, markings, and logos includinglabels, decals, printing, stamping, etc. add cost and labor. Molded-in tech-niques, when applied properly, producepermanent lettering and designs at avery low cost (see figure 1-7). Mixturesof gloss and texture can increase contrastfor improved visibility.
Miscellaneous
Look for opportunities to add easily-molded features to simplify assemblyand enhance product function such asaligning posts, nesting ribs, finger grips,guides, stops, stand-offs, hooks, clips,and access holes.
of good design and molding practice,consider some of the following:
Core out unneeded thickness andwall stock;
Use ribs, stiffening features, andsupports to provide equivalentstiffness with less wall thickness;
Optimize runner systems tominimize waste;
Use standard colors, which are lessexpensive than custom colors;
Compare the price of materials thatmeet your product requirements, butavoid making your selection basedupon price alone; and
Consider other issues such as materialquality, lot-to-lot consistency, on-timedelivery, and services offered bythe supplier.
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Molded-In Illustrations
This molded in schematic is a cost effective alternative to labels or printing.
Figure 1-7
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This last option requires carefulevaluation to determine if machine-cost-per-part savings compensate forthe added mold cost.
Mold costs, usually amortized over aspecified number of parts or years, canalso make up a significant portion of part cost. This is particularly true ifthe production quantities are low. Thecomplex relationship between moldcost, mold quality, and molding
efficiency is covered in Chapter 7.
Overhead
Hourly press rates comprise a significantportion of part cost. The rate varies byregion and increases with press size.Some options to consider whenevaluating overhead costs include:
Maximizing the number of partsproduced per hour to reduce themachine overhead cost per part;
Avoiding thick sections in your partand runner system that can increasecooling time;
Designing your mold with goodcooling and plenty of draft for easyejection; and
Increasing the number of cavities ina mold to increase hourly production.
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Chapter 1
PART DESIGN PROCESS:CONCEPT TO FINISHED PART continued
Scrap and Rework
Part and mold design can contribute toquality problems and scrap. To avoidrework and minimize scrap generation,consider the following:
Follow the part design recommenda-tions and guidelines outlined inChapter 2;
Avoid specifying tighter tolerances
than actually needed; and
Adjust the mold steel to produceparts in the middle of the tolerancerange, when molding parts withtight tolerances.
In the long run, this last suggestionis usually less expensive than tryingto produce parts at the edge of thetolerance range by molding in a narrowprocessing window.
Do not select your mold maker basedon price alone. Cheap molds oftenrequire costly rework and frequentmold maintenance, and are prone topart quality problems.
Labor
When looking to maintain or lower yourlabor costs, consider the following:
Simplify or eliminate manual tasksas much as possible;
Design parts and molds for automaticdegating or place gates in areas thatdont require careful trimming;
Keep parting lines and mold kiss-off areas in good condition to avoidflash removal;
Design parting lines and kiss-off points to orient flash in a less criticaldirection; and
Streamline and/or automatetime-consuming assembly steps.
PROTOTYPE TESTING
Prototype testing allows you to testand optimize part design and materialselection before investing in expensiveproduction tooling. Good prototypetesting duplicates molding, processing,and assembly conditions as closely aspossible. Molded prototype parts canalso be tested under the same range of mechanical, chemical, and environmen-tal conditions that the production parts
must endure.
Simplifying or eliminating prototypetesting increases the chance of problemsthat could lead to delays and expensivemodifications in production tooling.You should thoroughly prototype testall new designs.
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Chapter 2
GENERAL DESIGN
WALL THICKNESS
Wall thickness strongly influencesmany key part characteristics, includingmechanical performance and feel,cosmetic appearance, moldability, andeconomy. The optimum thicknessis often a balance between opposingtendencies, such as strength versusweight reduction or durability versuscost. Give wall thickness carefulconsideration in the design stage toavoid expensive mold modificationsand molding problems in production.
In simple, flat-wall sections, each10% increase in wall thickness providesapproximately a 33% increase in
While engineering resins are used
in many diverse and demanding
applications, there are design elements
that are common to most plastic parts,
such as ribs, wall thickness, bosses,
gussets, and draft. This chapter
covers these general design issues,
as well as others you should consider
when designing parts made of
thermoplastic resins.
stiffness. Increasing wall thickness alsoadds to part weight, cycle times, andmaterial cost. Consider using geometricfeatures such as ribs, curves, andcorrugations to stiffen parts. Thesefeatures can add sufficient strength,with very little increase in weight, cycletime, or cost. For more informationon designing for part stiffness, seeChapter 3.
Both geometric and material factorsdetermine the effect of wall thicknesson impact performance. Generally,increasing wall thickness reducesdeflection during impact and increasesthe energy required to produce failure.In some cases, increasing wall thickness
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Figure 2-1
I Z O D I M P A C T S T R E N G T H
( f t l b / i n )
THICKNESS (in)
Izod impactstrength of Makrolonpolycarbonate vs.thickness at varioustemperatures.
140 F (60 C)
20
18
16
14
12
10
8
6
4
2
0
0.100 0.140 0.180 0.220 0.260 0.300 0.340
73 F (23 C)
-4F (-20 C)
Critical Thickness
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Avoid designs with thin areassurrounded by thick perimetersections as they are prone to gasentrapment problems (see figure 2-2);
Maintain uniform nominal wallthickness; and
Avoid wall thickness variationsthat result in filling from thin tothick sections.
Thin-walled parts those with mainwalls that are less than 1.5 mm thick may require special high-performancemolding equipment to achieve therequired filling speeds and injection
can stiffen the part to the point that thegeometry cannot flex and absorb theimpact energy. The result can be adecrease in impact performance. Somematerials, polycarbonate for example,lose impact strength if the thicknessexceeds a limit known as the criticalthickness . Above the critical thicknessparts made of polycarbonate can show amarked decrease in impact performance.Walls with thickness greater than thecritical thickness may undergo brittle,
rather than ductile, failure duringimpact. The critical thickness reduceswith lowering temperature and molecularweight. The critical thickness formedium-viscosity polycarbonate atroom temperature is approximately3/16 inch (see figure 2-1).
Consider moldability when selectingthe wall thicknesses for your part. Flowlength the distance from the gate tothe last area fill must be withinacceptable limits for the plastic resinchosen. Excessively thin walls maydevelop high molding stresses, cosmeticproblems, and filling problems thatcould restrict the processing window.Conversely, overly thick walls canextend cycle times and create packingproblems. Other points to considerwhen addressing wall thickness include:
pressures. This can drive up themolding costs and offset any materialsavings. Thin-wall molding is generallymore suited for size or weight reductionthan for cost savings. Parts with wallthicknesses greater than 2 mm can alsobe considered as thin-walled parts if their flow-length-to-thickness ratios aretoo high for conventional molding.
Usually, low-shrinkage materials,such as most amorphous or filled resins,
can tolerate nominal wall thicknessvariations up to about 25% without sig-nificant filling, warpage, or appearanceproblems. Unfilled crystalline resins,because of their high molding shrinkage,
20
Non-uniform wall thickness can lead to air traps.
Figure 2-2 Racetracking
ConsistentWall
Thickness
Correct
ThickThin
Air Trap
Incorrect
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Chapter 2
GENERAL DESIGN continued
Many designs, especially those convertedfrom cast metal to plastic, have thick sections that could cause sinks or voids.When adapting these designs to plastic
parts, consider the following:
Core or redesign thick areas tocreate a more uniform wall thickness(see figure 2-3);
can only tolerate about half as muchthickness variation. These guidelinespertain to the parts main walls. Ribsand other protrusions from the wall
must be thinner to avoid sink. For moreinformation about designing ribs andother protrusions, see the section onribs in this chapter.
Make the outside radius one wall-thickness larger than the inside radiusto maintain constant wall thicknessthrough corners (see figure 2-4); and
Round or taper thickness transitionsto minimize read-through andpossible blush or gloss differences(see figure 2-5). Blending alsoreduces the molded-in stresses andstress concentration associated withabrupt changes in thickness.
In some cases, thickness-dependentproperties such as flame retardency,electrical resistance, and sound deaden-ing determine the minimum requiredthickness. If your part requiresthese properties, be sure the materialprovides the needed performance at thethicknesses chosen. UL flammabilityratings, for example, are listed with theminimum wall thickness for whichthe rating applies.
21
Core out thick sections as shown on right to maintain a more uniform wall thickness.
Figure 2-3Coring
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should extend from the gate withoutrestrictions.
To avoid possible warpage and shrink-age problems, limit the added thicknessto no more than 25% of the nominalwall for low-shrinkage, amorphous orfilled materials and to 15% for unfilledcrystalline resins. Carefully transitionthe flow leader into the wall to minimizeread-through and gloss differences onthe other side of the wall.
FLOW LEADERS ANDRESTRICTORS
Occasionally designers incorporatethicker channels, called flow leaders orinternal runners , into the part design.These flow leaders help mold fillingor packing in areas far from the gate.Additionally, flow leaders can balancefilling in non-symmetrical parts, alterthe filling pattern, and reduce sink inthick sections (see figure 2-6). For
best results, the flow-leader thickness
Flow restrictors , areas of reducedthickness intended to modify the fillingpattern, can alleviate air-entrapmentproblems (see figure 2-7) or moveknitlines. When restricting thick flowchannels as in figure 2-7, use thefollowing rules of thumb in your design:
Extend the restrictor across theentire channel profile to effectivelyredirect flow;
22
Internal and external corner radii should originate from the same point.
Figure 2-4 Corner Design
Too Thin
Too Thick
t
R2 = R1 + t
R1R2
Blend transitions to minimize read-through.
Figure 2-5 Thickness Transitions
Incorrect
Correct
Correct
Correct
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Chapter 2
GENERAL DESIGN continued
Flow leader and restrictor placementwere traditionally determined by trialand error after the mold was sampled.
Reduce the thickness by no more than33% in high-shrinkage resins or 50%for low-shrinkage materials; and
Lengthen the restrictor todecrease flow.
Today, computerized flow simulationenables designers to calculate thecorrect size and placement beforemold construction.
23
Corners typically fill late in box-shaped parts. Adding flow leadersbalances flow to the part perimeter.
Figure 2-6Flow Leaders
Flow Leader
Flow restrictors can change the filling pattern to correct problemssuch as gas traps.
Figure 2-7Flow Restrictors
Gate
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This section deals with general guide-lines for ribs and part design; structuralconsiderations are covered in Chapter 3.
Rib Design
Proper rib design involves five mainissues: thickness, height, location,quantity, and moldability. Considerthese issues carefully when designingribs.
RIBS
Ribs provide a means to economicallyaugment stiffness and strength in moldedparts without increasing overall wallthickness. Other uses for ribs include:
Locating and captivating componentsof an assembly;
Providing alignment in matingparts; and
Acting as stops or guides formechanisms.
Rib Thickness
Many factors go into determining theappropriate rib thickness . Thick ribsoften cause sink and cosmetic problemson the opposite surface of the wall towhich they are attached (see figure 2-8).The material, rib thickness, surfacetexture, color, proximity to a gate,and a variety of processing conditionsdetermine the severity of sink. Table 2-1gives common guidelines for rib thick-
ness for a variety of materials. Theseguidelines are based upon subjectiveobservations under common conditions
24
Sink opposite thick rib.
Figure 2-8 Sink
Offset rib to reduce read-through and sink.
Figure 2-9 Offset Rib
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Chapter 2
GENERAL DESIGN continued
often tolerate ribs that are thicker thanthe percentages in these guidelines. Onparts with wall thicknesses that are 1.0mm or less, the rib thickness should beequal to the wall thickness. Rib thicknessalso directly affects moldability. Verythin ribs can be difficult to fill. Because
and pertain to the thickness at the baseof the rib. Highly glossy, critical sur-faces may require thinner ribs. Placingribs opposite character marks or stepscan hide rib read-through (see figure2-9). Thin-walled parts those withwalls that are less than 1.5 mm can
of flow hesitation , thin ribs near the gatecan sometimes be more difficult to fillthan those further away. Flow enteringthe thin ribs hesitates and freezes whilethe thicker wall sections fill.
Ribs usually project from the main wallin the mold-opening direction and areformed in blind holes in the mold steel.To facilitate part ejection from themold, ribs generally require at leastone-half degree of draft per side (see
figure 2-10). More than one degree ofdraft per side can lead to excessive ribthickness reduction and filling problemsin tall ribs.
Thick ribs form thickened flow channelswhere they intersect the base wall.These channels can enhance flow in therib direction and alter the filling pattern.The base of thick ribs is often a goodlocation for gas channels in gas-assistmolding applications. The gas-assistprocess takes advantage of these channelsfor filling, and hollows the channelswith injected gas to avoid problemswith sink, voids, or excessive shrinkage.
Rib thickness also determines thecooling rate and degree of shrinkage inribs, which in turn affects overall partwarpage. In materials with nearly
uniform shrinkage in the flow andcross-flow directions, thinner ribs tendto solidify earlier and shrink less thanthe base wall. In this situation, the endsof ribbed surfaces may warp toward the
25
Rib Thickness as a Percentage of Wall Thickness
Resin Minimal Sink Slight Sink
PC 50% (40% if high gloss) 66%
ABS 40% 60%
PC/ABS 50% 66%
Polyamide (Unfilled) 30% 40%
Polyamide (Glass-Filled) 33% 50%
PBT Polyester (Unfilled) 30% 40%
PBT Polyester (Filled) 33% 50%
Table 2-1
Rib design guidelines.
Figure 2-10Rib Design Guidelines
T
Draft*
Radius = 0.125T
*Minimum 0.5 Per Side
0.5T
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direction becomes more aligned alongthe length of the ribs, this effectdiminishes. Warpage can reverse asthe ribs become thicker than the wall.
opposing wall (see figure 2-11). As ribthickness approaches the wall thickness,this type of warpage generally decreases.However, ribs that are the same thick-ness as the wall may develop ends thatwarp toward the ribbed side. To preventthis warpage, design extra mold coolingon the ribbed side to compensate for theadded heat load from the ribs.
For glass-filled materials with highershrinkage in the cross-flow versus flowdirection, the effect of rib thickness onwarpage can be quite different (seefigure 2-12). Because thin ribs tend to
fill from the base up, rather than alongtheir length, high cross-flow shrinkageover the length of the rib can cause theends to warp toward the ribs. As ribthickness increases and the flow
Rib Size
Generally, taller ribs provide greatersupport. To avoid mold filling, venting,and ejection problems, standard rules of thumb limit rib height to approximatelythree times the rib-base thickness.Because of the required draft for ejection,the tops of tall ribs may become too thinto fill easily. Additionally, very tall ribsare prone to buckling under load. Ifyou encounter one of these conditions,
consider designing two or more shorter,thinner ribs to provide the same supportwith improved moldability (see figure2-13). Maintain enough space betweenribs for adequate mold cooling: forshort ribs allow at least two times thewall thickness.
26
Warpage vs. rib thickness in unfilled resins.
Figure 2-11Warpage vs. Rib Thickness
Thin Rib Thick Rib
Warpage vs. rib thickness in glass-filled resins.
Figure 2-12 Warpage vs. Rib Thickness
Thin Rib Thick Rib
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Chapter 2
GENERAL DESIGN continued
to mold-cooling difficulties and warpage.Typically much easier to add thanremove, ribs should be applied sparing-ly in the original design and added asneeded to fine tune performance.
BOSSES
Bosses find use in many part designsas points for attachment and assembly.The most common variety consists of
Rib Location and Numbers
Carefully consider the location andquantity of ribs to avoid worseningproblems the ribs were intended tocorrect. For example, ribs added toincrease part strength and preventbreakage might actually reduce theability of the part to absorb impactswithout failure. Likewise, a grid of ribsadded to ensure part flatness may lead
cylindrical projections with holesdesigned to receive screws, threadedinserts, or other types of fasteninghardware. As a rule of thumb, the outsidediameter of bosses should remain within2.0 to 2.4 times the outside diameter of the screw or insert (see figure 2-14).
27
Replace large problematic ribs with multiple shorter ribs.
Figure 2-13Multiple Ribs
t
2t
0.5t
t
2.0 to2.4D
D
0.060 in(1.5 mm)
0.3t max.
t
d
Typical boss design.
Figure 2-14Boss Design
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To limit sink on the surface opposite theboss, keep the ratio of boss-wall thick-ness to nominal-wall thickness the sameas the guidelines for rib thickness (seetable 2-1). To reduce stress concentra-tion and potential breakage, bossesshould have a blended radius, ratherthan a sharp edge, at their base. Largerradii minimize stress concentration butincrease the chance of sink or voids.
For most applications, a 0.015-inch blend (fillet) radius provides agood compromise between strengthand appearance.
Specifying smaller screws or insertsoften prevents overly thick bosses.Small screws attain surprisinglyhigh retention forces (see the BayerJoining Techniques manual). If theboss-wall thickness must exceed therecommended ratio, consider adding arecess around the base of the boss(as shown in figure 2-15) to reduce theseverity of sink.
Avoid bosses that merge into sidewallsbecause they can form thick sectionsthat lead to sink. Instead, position thebosses away from the sidewall, and if needed, use connecting ribs for support(see figure 2-16). Consider using open-boss designs for bosses near a standingwall (see figure 2-17).
28
A recess around the base of a thick boss reduces sink.
Figure 2-15Boss Sink Recess
30
0.3tt
Connecting bosses to walls.
Figure 2-16Bosses
Incorrect
Correct
Normally, the boss hole should extendto the base-wall level, even if the fulldepth is not needed for assembly.Shallower holes can leave thick sections,resulting in sink or voids. Deeper holesreduce the base wall thickness, leadingto filling problems, knitlines, or surfaceblemishes. The goal is to maintain auniform thickness in the attachmentwall (see figure 2-18).
Because of the required draft, tallbosses those greater than five timestheir outside diameter can create afilling problem at their top or a thick section at their base. Additionally, the
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Chapter 2
GENERAL DESIGN continued
cores in tall bosses can be difficult tocool and support. Consider coring a tallboss from two sides or extending tall
gussets to the standoff height ratherthan the whole boss (see figure 2-19).
29
Open bosses maintain uniform thickness in the attachment wall.
Figure 2-17Boss in Attachment Wall
A
A
Section A-A
Figure 2-18Boss Core Depth
Core Too Short
Correct
Core Too Long
Radius Too Large
Boss holes should extend to the base-wall level.
Incorrect Correct
Options to reduce the length of excessively long core pins.
Figure 2-19Long-Core Alternatives
CoreTooLong
Correct Correct
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is a concern. Because of their shape andthe EDM process for burning gussetsinto the mold, gussets are prone toejection problems. Specify properdraft and draw polishing to help withmold release.
The location of gussets in the moldsteel generally prevents practical directventing. Avoid designing gussets thatcould trap gasses and cause filling andpacking problems. Adjust the shape
or thickness to push gasses out of thegussets and to areas that are more easilyvented (see figure 2-21).
Other alternatives include splitting along boss into two shorter mating bosses(see figure 2-20) or repositioning theboss to a location where it can be shorter.
GUSSETS
Gussets are rib-like features that addsupport to structures such as bosses,ribs, and walls (see figure 2-21). Aswith ribs, limit gusset thickness to one-
half to two-thirds the thickness of thewalls to which they are attached if sink
SHARP CORNERS
Avoid sharp corners in your design.Sharp inside corners concentrate stressesfrom mechanical loading, substantiallyreducing mechanical performance.Figure 2-22 shows the effect of rootradius on stress concentration in asimple, cantilevered snap arm. Thestress concentration factor climbssharply as the radius-to-thicknessratio drops below approximately 0.2.
Conversely, large ratios cause thicksections, leading to sinks or voids.
30
Excessively long bosses can often be replaced by two shorter bosses.
Figure 2-20 Mating Bosses
Contour lines show flow front position at incremental time intervals.Squared gussets can trap air in the corners.
Figure 2-21 Gussets
Incorrect Correct
Incorrect Correct
Air Trap Positionof flowfront atregular timeintervals
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Chapter 2
GENERAL DESIGN continued
A radius-to-thickness ratio of approximately 0.15 provides a goodcompromise between performanceand appearance for most applicationssubjected to light to moderateimpact loads.
Initially use a minimal corner radiuswhen designing parts made of high-shrinkage materials with low-notchsensitivity, such as Durethan polyamide,to prevent sink and read-through. Inside
corner radii can then be increased asneeded based upon prototype testing.
In critical areas, corner radii shouldappear as a range, rather than a maximumallowable value, on the product drawings.A maximum value allows the mold makerto leave corners sharp as machinedwith less than a 0.005-inch radius.Avoid universal radius specificationsthat round edges needlessly andincrease mold cost (see figure 2-23).
In addition to reducing mechanicalperformance, sharp corners can causehigh, localized shear rates, resulting inmaterial damage, high molding stresses,and possible cosmetic defects.
31
Effects of a fillet radius on stress concentration.
Figure 2-22Fillet Radius and Stress Concentration
3.0
2.5
2.0
1.5
1.00.2 0.4 0.6 0.8 1.0 1.2 1.4
R/h
S t r e s s
C o n c e n
t r a
t i o n
F a c
t o r
R
P
h
Round Edges
Easy
Easy Difficult
Difficult
Avoid universal radius specifications that round edges needlessly and increase mold cost.
Figure 2-23
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DRAFT
Draft providing angles or tapers onproduct features such as walls, ribs,posts, and bosses that lie parallel to thedirection of release from the mold eases part ejection. Figure 2-24 showscommon draft guidelines.
How a specific feature is formed in themold determines the type of draft needed.Features formed by blind holes or
pockets such as most bosses, ribs,and posts should taper thinner as theyextend into the mold. Surfaces formedby slides may not need draft if the steelseparates from the surface before ejection.Other rules of thumb for designingdraft include:
Draft all surfaces parallel to thedirection of steel separation;
Angle walls and other features thatare formed in both mold halves tofacilitate ejection and maintainuniform wall thickness;
Use the standard one degree of draftplus one additional degree of draft forevery 0.001 inch of texture depth asa rule of thumb; and
Use a draft angle of at least one-half degree for most materials. Designpermitting, use one degree of draftfor easy part ejection. SAN resinstypically require one to two degreesof draft.
32
Common draft guidelines.
Figure 2-24 Draft
Correct
Parts with Draft
Correct
Correct
0.5 Min.
0.5 Min.
0.5 Min.
Draw Polish
Incorrect
Parts with No Draft
Incorrect
Incorrect
Less draft increases the chance ofdamaging the part during ejection.
Additionally, molders may have toapply mold release or special mold sur-face coatings or treatments, ultimatelyleading to longer cycle times and higherpart costs.
The mold finish, resin, part geometry,and mold ejection system determine
the amount of draft needed. Generally,polished mold surfaces require less draftthan surfaces with machined finishes.An exception is thermoplastic poly-urethane resin, which tends to eject easierfrom frosted mold surfaces. Parts withmany cores may need a higher amountof draft.
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Chapter 2
GENERAL DESIGN continued
add slides or hydraulic moving coresthat can increase the cost of moldconstruction and maintenance (seesection on undercuts).
During mold filling, the advancingplastic flow can exert very high sideforces on tall cores forming deep orlong holes. These forces can push orbend the cores out of position, alteringthe molded part. Under severe conditions,this bending can fatigue the mold steeland break the core.
Generally, the depth-to-diameter ratiofor blind holes should not exceed 3:1.Ratios up to 5:1 are feasible if fillingprogresses symmetrically around theunsupported hole core or if the core isin an area of slow-moving flow.Consider alternative part designs that
Some part designs leave little room forejector pins. Parts with little ejector-pincontact area often need extra draft toprevent distortion during ejection. Inaddition to a generous draft, some deep
closed-bottomed shapes may need airvalves at the top of the core to relievethe vacuum that forms during ejection(See figure 7-13 in Chapter 7).
HOLES AND CORES
Cores are the protruding parts of themold that form the inside surfaces of features such as holes, pockets, andrecesses. Cores also remove plasticfrom thick areas to maintain a uniformwall thickness. Whenever possible,design parts so that the cores can separatefrom the part in the mold-openingdirection. Otherwise, you may have to
avoid the need for long delicate cores,such as the alternative boss designs infigures 2-19 and 2-20.
If the core is supported on both ends,
the guidelines for length-to-diameterratio double: typically 6:1 but up to10:1 if the filling around the core issymmetrical. The level of support onthe core ends determines the maximumsuggested ratio (see figure 2-25).Properly interlocked cores typicallyresist deflection better than cores thatsimply kiss off. Single cores for through-holes can interlock into the oppositemold half for support.
Mismatch can reduce the size of theopening in holes formed by mating cores.Design permitting, make one coreslightly larger (see figure 2-26). Even
33
The ends of the long cores should interlock into mating surfacesfor support.
Interlocking CoresFigure 2-25
Reduced Hole Correct Through-Hole
When feasible, make one core larger to accommodate mismatch inthe mold.
Core MismatchFigure 2-26
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part can flex enough to strip from themold during ejection, depending uponthe undercuts depth and shape and theresins flexibility. Undercuts can onlybe stripped if they are located awayfrom stiffening features such as cornersand ribs. In addition, the part must haveroom to flex and deform. Generally,guidelines for stripping undercuts fromround features limit the maximumamount of the undercut to a percentagedefined as follows and illustrated in
figure 2-28 as:
Generally, avoid stripping undercutsin parts made of stiff resins such aspolycarbonate, polycarbonate blends,
% Undercut =D d
x 100D
with some mismatch, the required holediameter can be maintained. Tight-tolerance holes that cannot be steppedmay require interlocking features on thecores to correct for minor misalignment.These features add to mold constructionand maintenance costs. On shortthrough-holes that can be molded withone core, round the edge on just oneside of hole to eliminate a mating coreand avoid mismatch (see figure 2-27).
UNDERCUTS
Some design features, because of theirorientation, place portions of the moldin the way of the ejecting plastic part.Called undercuts, these elements canbe difficult to redesign. Sometimes, the
and reinforced grades of polyamide 6.Undercuts up to 2% are possible in partsmade of these resins, if the walls areflexible and the leading edges arerounded or angled for easy ejection.Typically, parts made of flexible resins,such as unfilled polyamide 6 or thermo-plastic polyurethane elastomer, cantolerate 5% undercuts. Under idealconditions, they may tolerate up to10% undercuts.
Slides and Cores
Most undercuts cannot strip from themold, needing an additional mechanismin the mold to move certain componentsprior to ejection (see Chapter 7). Thetypes of mechanisms include slides,
34
Rounding both edges of the hole creates a potential for mismatch.
Mismatch No Mismatch
Mismatch Figure 2-27
Undercut features can often successfully strip from the mold duringejection if the undercut percentage is within the guidelines for thematerial type.
Figure 2-28Stripping Undercut Guidelines
d
D
30 45 Lead Angle
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Chapter 2
GENERAL DESIGN continued
split cores, collapsible cores, split cavi-ties, and core pulls. Cams, cam pins,lifters, or springs activate most of theseas the mold opens. Others use externaldevices such as hydraulic or pneumaticcylinders to generate movement. All of these mechanisms add to mold cost andcomplexity, as well as maintenance.They also add hidden costs in the formof increased production scrap, qualityproblems, flash removal, and increasedmold downtime.
Clever part design or minor designconcessions often can eliminate complexmechanisms for undercuts. Variousdesign solutions for this problem areillustrated in figures 2-29 through 2-31.Get input from your mold designerearly in product design to help identifyoptions and reduce mold complexity.
35
Figure 2-29Sidewall Windows
Bypass steel can form windows in sidewalls without moving slides. Snap-fit hook molded through hole to form undercut.
Figure 2-30Snap Fit
Snap Fit Draw
Simple wire guides can be molded with bypass steel in the mold.
Figure 2-31Wire Guides
Draw
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Carefully consider the molding processduring part design to simplify the moldand lower molding costs. Extendingvents over the top of a corner edge canfacilitate straight draw of the vent coringand eliminate a side action in the mold(see figure 2-32). Angling the louversurface can also allow vent slots to bemolded without side actions in the mold(see figure 2-33).
LOUVERS AND VENTS
Minor variations in cooling-vent designcan have a major impact on the moldingcosts. For instance, molds designed withnumerous, angled kiss-offs of bypasscores are expensive to construct andmaintain. Additionally, these moldsare susceptible to damage and flashproblems. Using moving slides or coresto form vents adds to mold cost andcomplexity.
Consult all pertinent agency specifica-tions for cooling vents in electricaldevices. Vent designs respond different-ly to the flame and safety tests requiredby many electrical devices. Fully test allcooling-vent designs for compliance.
36
Extending vent slots over the corner edge eliminates the need for a
side action in the mold.
Figure 2-32 Vent Slots
Louvers on sloping walls can be molded in the direction of draw.
Figure 2-33 Louvers on Sloping Wall
Direction of Draw
MoldCore
MoldCavity
Mold
Mold
Part
Direction of Draw
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Chapter 2
GENERAL DESIGN continued
cost. Typically, threads that do not lieon the parting line require slides or sideactions that could add to molding costs.All threads molded in two halves areprone to parting line flash or mismatch.
Thread designs requiring unscrewingdevices add the most cost to the mold.Most of the mechanisms for moldinginternal threads such as collapsibleand unscrewing cores significantlyincrease the molds cost and complexity.
MOLDED-IN THREADS
The molding process accommodatesthread forming directly in a part, avoidingthe expense of secondary, thread-cutting
steps. The cost and complexity of thetooling usually determines the feasibilityof molding threads. Always comparethis cost to the cost of alternative attach-ment options, such as self-tapping screws.
Easily molded in both mold halves,external threads centered on the moldparting line add little to the molding
Occasionally, threads in parts made of flexible plastics, such as unfilledpolyamide 6 or polyurethane elas-tomers, can be stripped from the moldwithout special mechanisms. Rarelysuited to filled resins or stiff plasticssuch as polycarbonate, this optionusually requires generously roundedthreads and a diameter-to-wall-thicknessratio greater than 20 to 1. Usually,molding threads on removable coresreduces mold cost and complexity
but adds substantially to the costs of molding and secondary operations.For this reason, limit this option tolow-production quantities or designsthat would be prohibitively complex tomold otherwise.
Thread profiles for metal screws oftenhave sharp edges and corners that canreduce the parts mechanical performanceand create molding problems in plasticdesigns. Rounding the threads crestsand roots lessens these effects. Figure2-34 shows common thread profilesused in plastics. Although less commonthan the American National (Unified)thread, Acme and Buttress threadsgenerally work better in plastic assemblies.Consider the following when specifyingmolded-in threads :
37
Common threadprofiles used inplastic parts.
Figure 2-34Thread Profiles
American National (Unified)
Acme
Buttress
60
P
29
P
0.371P
0.69P 50
50
5
P
0.125P
0.5P
R = 0.125P
0.5P
0.5P
0.125P
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Avoid tapered threads unless youcan provide a positive stop thatlimits hoop stresses to safe limitsfor the material.
Tapered pipe threads , common inplumbing for fluid-tight connections,are slightly conical and tapered and canplace excessive hoop stresses on theinternal threads of a plastic part. Whenmating plastic and metal tapered
Use the maximum allowable radiusat the threads crest and root;
Stop threads short of the end to avoidmaking thin, feathered threads that caneasily cross-thread (see figure 2-35);
Limit thread pitch to no more than32 threads per inch for ease ofmolding and protection fromcross threading; and
threads, design the external threads onthe plastic component to avoid hoopstress in plastic or use straight threadsand an O ring to produce the seal(see figure 2-36). Also, assure thatany thread dopes or thread lockers arecompatible with your selected plasticresin. Polycarbonate resins, in particular,are susceptible to chemical attack frommany of these compounds.
38
Design guidelines to avoid cross threading.
Figure 2-35 Threads
Incorrect Correct
Incorrect Correct
Standard NPT tapered pipe threads can cause excessive hoopstresses in the plastic fitting.
Figure 2-36 Pipe Threads
Plastic PipePlasticFitting
StraightThread O-Ring
CompressionSeal
Plastic Pipe Metal FittingNPT
Recommended
Metal or Plastic Pipe Plastic Fitting
Tapered threads createlarge hoop stress.
NPT L
T
t
F
Not Recommended
Bulge
Bulge
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Chapter 2
GENERAL DESIGN continued
For best performance, use threadsdesigned specifically for plastics. Partsthat do not have to mate with standardmetal threads can have unique threadsthat meet the specific application andmaterial requirements. The medicalindustry, for example, has developedspecial, plastic-thread designs forLuer-lock tubing connectors (see figure2-37). Thread designs can also besimplified for ease of molding asshown in figure 2-38.
39
Luer-lock thread used in medical applications.
Figure 2-37Medical Connectors
Examples of thread designs that were modified for ease of molding.
Figure 2-38 Molded Threads
Internal
External
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LETTERING
The molding process adapts easily formolding-in logos, labels, warnings,diagrams, and instructions, saving theexpense of stick-on or painted labels,and enhancing recyclability. Deep,sharp lettering is prone to cosmeticproblems, such as streaks and teardrops, particularly when near the gate(see figure 2-39). To address thesecosmetic issues, consider the following:
Limit the depth or height of letteringinto or out of the part surface toapproximately 0.010 inch; and
Angle or round the side walls of theletters as shown in figure 2-40.
TOLERANCES
Many variables contribute to thedimensional stability and achievabletolerances in molded parts, includingprocessing variability, mold construction,material characteristics, and partgeometry. To improve your abilityto maintain specified tolerancesin production:
40
Deep, sharp lettering can cause teardrop defects as shown ontop photo. The bottom shows the improvement with rounded,shallow lettering.
Figure 2-39 Lettering
W
W
dRR
30 W 2 d d = 0.010 in (Max.)
Design suggestions for the cross-sectional profile of lettering.
Figure 2-40 Lettering
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Chapter 2
GENERAL DESIGN continued
Use low-shrinkage materials in partswith tight tolerances;
Avoid tight tolerances in dimensionsaffected by the alignment of the moldhalves or moving mold componentssuch as slides;
Design parts and assemblies to avoidtight tolerances in areas prone towarpage or distortion; and
Adjust the mold to producedimensions in the middle of tolerancerange at optimum processingconditions for the material.
41
Unlike standardtolerancing,geometrictolerancing canallow a feature
position to varywith the size of the feature.
Figure 2-41Tolerances
0.7500.003
1.0000.003
0.500 Dia. 0.003
StandardTolerances
0.750
1.000
0.500 Dia. 0.003
GeometricTolerances
0.006 M
To avoid unnecessary molding costs,specify tight tolerances only when need-ed. Generally, the size and variability of other part features determine the actualtolerance required for any one componentor feature within an assembly. Ratherthan dividing the allowable variabilityequally over the various features thatgovern fit and function, allot a greaterportion of the total tolerance rangeto features that are difficult to control.Reserve tight tolerances for features
that can accommodate them reasonably.
Geometric tolerancing methods canexpand the effective molding toleranceby better defining the size and positionrequirements for the assembly. Ratherthan define the position and size of fea-tures separately, geometric tolerancingdefines a tolerance envelope in whichsize and position are consideredsimultaneously.
Figure 2-41 shows the size and positionof a hole specified in both standard and
geometric tolerances. The standard tol-erances hold the position and size of thehole to 0.003. The geometric toler-ances specify a hole size tolerance of
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0.003 but allow the position toleranceto vary within a 0.006 tolerance zonewhen the hole is at its smallest diameter(maximum material condition). Whenthe hole is larger than the minimumsize, the difference between the actualhole size and the minimum hole sizecan be added to the tolerance zone forthe position tolerance. At the maximumhole size, 0.503, the position tolerancezone for the center of the hole is 0.012or 0.006 from the stated vertical and
horizontal positions. As the holebecomes larger, the position can varymore without restricting the requiredthrough-hole for the post or screwthat passes through the hole (seefigure 2-42).
BEARINGS AND GEARS
Material friction and wear propertiesplay a key role in the performance of bearings and gears made of plastic. Forinstance, Durethan polyamide resinsexhibit properties suitable for many gearand bearing applications. Used frequentlyas over-molded, gear-tooth liners, Texinthermoplastic urethane elastomersdemonstrate excellent abrasion resistanceand shock-dampening properties.
Because plastic parts exhibit complexwear behavior, predicting gear andbearing performance can be difficult.However, certain trends prevail:
42
Tolerance Zonefor Largest Hole
Tolerance Zonefor Smallest Hole
Required Through-Hole Size
As the hole size increases, the position tolerance can increase without restricting thethrough-hole clearance.
Figure 2-42Tolerances
When the mating components of abearing or gear are made of the samematerial, the wear level is muchhigher, unless the load and temperatureare very low;
When both contacting plastics areunfilled, usually wear is greater onthe moving surface;
When plastic components willwear against steel, use glass fillers
to increase the life of plasticcomponents; and
When designing bearing parts forlongevity, keep frictional heatinglow and ensure that heat dissipatesquickly from the bearing surface.
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Chapter 2
GENERAL DESIGN continued
Avoid soft-metal shafts when theloads or rotational speeds are high;
Add holes or grooves to the insideof the bushing to capture debris andprevent premature wear;
Protect the bearings with seals orguards in dirty environments; and
Check the compatibility of lubricantswith your specific plastic.
The PV factor , a major factor in theformation of frictional heat, is the prod-uct of the pressure (P) exerted on theprojected area of the bushing and thesurface velocity (V) of the shaft. Testingshows that plastics exhibit a sharpincrease in wear at PV values above alimit characteristic of the specific resin(see table 2-2). The PV factor for thebushing must not exceed the PV limit(minus appropriate safety factor)established for the selected resin.
Many factors influence the effective PVlimit and actual bushing performance.For instance, bushings made of plasticlast longer when the shafts are hard andfinely polished. Other points to consider:
If chemically compatible, lubricantscan more than double the PV limitand greatly increase the life of gearsand bearings.
Differences in the coefficient of linearthermal expansion between the shaft andthe bushing can change the clearanceand affect part life. Calculate theclearance throughout the servicetemperature range, maintaining aminimum clearance of approximately
0.005 inch per inch of diameter. Alwaystest your specific shaft and bushingcombination under the full range of temperatures, speeds, loads, andenvironmental conditions beforespecifying a bushing material or design.
43
Approximate PV Limitsat 100 Feet/Minute
Polycarbonate 500
Thermoplastic PU 1,500
Polyamide 6 2,000
Polyamide 6/6 2,500
Polyamide 6 30% GF 8,500
Table 2-2
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Chapter 3
STRUCTURAL DESIGN
STRUCTURALCONSIDERATIONSIN PLASTICS
When designing parts made of plastics,be sure to consider not only the magni-tude of mechanical loads but also theirtype and duration. More so than formost materials, plastics can exhibitdramatically different behavior dependingon whether the loading is instantaneous,long term, or vibratory in nature.
Temperature and other environmentalconditions can also dramatically affectthe mechanical performanceof the plastic material. Many aspectsof plastic behavior, including visco-elasticity and sensitivity to a variety of processing-related factors, make pre-dicting a given parts performance in aspecific environment very difficult. Usestructural calculations conservativelyand apply adequate safety factors. Westrongly suggest prototype testing for allapplications.
Plastic part design must also take intoaccount not only the structural require-ments anticipated in the end-useapplication, but also the less obviousmechanical loads and stresses thatcan occur during operations such asmanufacturing, assembly, and shipping.
These other sources of mechanicalloads can often place the higheststructural demands on the plastic part.Carefully evaluate all of the structuralloads the part must endure throughoutits entire life cycle.
This chapter assumes the reader has
a working knowledge of mechanical
engineering and part design, and
therefore focuses primarily upon those
aspects of structural design that are
unique or particularly relevant to
plastics. Two main goals of this chapter
are to show how to use published data
to address the unusual behavior of
plastics in part design, and to show how
to take advantage of the design freedom
afforded by molding processes to meet
your structural requirements.
The mechanical properties of plasticsdiffer from metals in several importantways:
Plastics exhibit much less strengthand stiffness;
Mechanical properties are time andtemperature dependent;
Plastics typically exhibit nonlinearmechanical behavior; and
Processing and flow orientation cangreatly affect properties.
The following sections briefly discussthe relevance of these differences whendesigning plastic parts. For more onthese topics, consult the BayerCorporation companion to this manual:
Material Selection: Thermoplastics
and Polyurethanes .
45
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Voight-Maxwellmodel simulatingviscoelasticcharacteristics.
Figure 3-1Voight-Maxwell Model
Spring A
Dashpot A
Dashpot BSpring B
Maxwell
Voight
Stiffness
Designing parts with adequate stiffnesscan be difficult, particularly if your partwas made of metal originally. If yourdesign needs the strength and/or stiffnessof a metal part, you must account forthe large disparity between plastic andmetal mechanical properties (see table3-1). Increasing wall thickness maycompensate for the lower stiffness of plastic resins. In practice, however, the
molding process limits wall thickness toapproximately 0.25 inch in solid, injection-molded parts. More typically, wallthickness ranges from 0.060 to 0.160inches. Generally, good part designsincorporate stiffening features and usepart geometry to help achieve requiredstiffness and strength. These designconsiderations are covered in greaterdetail in the section Designing for Stiffness on page 67.
46
* Conditioned
Steel 28.5 70 40 0.29Copper (Annealed) 15.6 32 5 0.36
Aluminum 10.0 56 34 0.33
SAN 0.47 4 5 0.35
Polycarbonate 0.35 10 9 0.38
ABS 0.34 6 0.39
PA* Unfilled 0.16 8 6 0.40
PA* 30% Glass 0.72 15 0.34
PC/ABS 0.35 7 8 0.38
Property Comparison of Metals and Plastics
Modulus of Tensile YieldElasticity Strength Strength Poissons
Material (10 6 psi) (1,000 psi) (1,000 psi) Ratio
Table 3-1
Viscoelasticity
Plastics exhibit viscoelastic behaviorsunder load: they show both plastic andelastic deformation. This dual behavioraccounts for the peculiar mechanicalproperties found in plastics. Under mildloading conditions, plastics usuallyreturn to their original shape when theload is removed, exhibiting an elasticresponse. Under long-term, heavy loadsor at elevated temperatures, this same
plastic will deform, behaving more likea high-viscosity liquid. This time- and
temperature-dependent behavior occursbecause the polymer chains in the partdo not return to their original positionwhen the load is removed. The Voight-Maxwell model of springs and dashpotsillustrates these characteristics (seefigure 3-1). Spring A in the Maxwellmodel represents the instantaneousresponse to load and the linear recoverywhen the load is removed. Dashpot Aconnected to the spring simulates thepermanent deformation that occurs
over time.
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Chapter 3
STRUCTURAL DESIGN continued
stress relaxation , the reduction in stress
over time in a part under constant strainor deformation. To account for thisbehavior, designers should use datathat reflect the correct temperature,load, and duration to which the part willbe exposed. These topics are discussedmore fully in the section Long-Term
Loading on page 73.
Viscoelasticity causes most plastics
to lose stiffness and strength as thetemperature increases (see figure 3-2).As a plastic part is exposed to highertemperatures, it becomes more ductile:yield strength decreases and the strain-at-break value increases. Plastic partsalso exhibit creep , the increase indeformation over time in parts undercontinuous load or stress, as well as
47
Figure 3-2
S T R E S S
STRAIN (%)
100
80
60
40
20
0
14,000
12,000
10,000
8,000
6,000
4,000
2,000
6,370
4,900
Makrolon 2658 PC
0 2 4 6 8 101.35 2.301.75
-20 C
0C
23 C
40
60 C
90 C
120 C
Stress-Strain vs. TemperatureMPa
psi
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Stress-Strain Behavior
A simple tensile test determines thestress-strain behavior of plastic materi-als. The results, usually expressed as acurve, show the relationship betweenstress , the force per original cross-sectional area, and strain , the percentageof change in length as a result of theforce. Nearly linear at very low stressand strain levels, the stress-strainbehavior of plastics tends to become
increasingly nonlinear as these loadsincrease. In this context, the term non-linear means that the resulting strainat any particular point does not varyproportionally with the applied stress.
48
Viscoelastic
(Voight-Maxwell)
Elastic (Hookean)
Metals
UnreinforcedPlastics
Metals usually function within the elastic (Hookean) range of mechanical behavior.Unreinforced plastics tend to exhibit nonlinear behavior represented here by thecombination of springs and dashpots.
Figure 3-3
S T R E S S
STRAIN
Viscoelasticity
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Chapter 3
STRUCTURAL DESIGN continued
Figure 3-3 shows typical stress-straincurves for steel and unreinforcedthermoplastic materials. While metalscan exhibit plastic behavior, they typicallyfunction within the elastic (Hookean)range of mechanical performance.Because of viscoelasticity, unreinforcedplastic materials tend to exhibit non-linear behavior through much of theiroperating range. Even at low strainvalues, plastics tend to exhibit somenonlinear behavior. As a result, using
the tensile modulus or Youngs modulus ,derived from stress over strain in thelinear region of the stress-strain curve,in structural calculations could lead toan error. You may need to calculate thesecant modulus , which represents thestiffness of a material at a specific strainor stress level (see figure 3-4). The useof secant modulus is discussed in theexample problems later in this chapter.
49
The Youngs modulus derived from the stress-strain behavior at very low strain can overstatethe material stiffness. A calculated secant modulus can better represent material stiffness ata specific stress or strain.
Figure 3-4
S T R E S S ( )
STRAIN ( )
Secant Modulus
ActualCurve
secant secant secant =
secant
secant
Youngs
secant
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50
Figure 3-5
S T R E S S ( N / m m
2 )
STRAIN (%)
This graph shows the stress-strain performance parallel to fiber orientation at various temperatures for a 30% glass-filled PA 6material after conditioning.
Stress-StrainParallel to Orientation
120
100
80
60
40
20
0
Durethan BKV 130
0 1 2 3 4
23 C
40 C
60 C
120 C90 C
150 C30
1.3
Figure 3-6
S T R E S S ( N / m m
2 )
STRAIN (%)
This graph shows the stress-strain performance perpendicular to fiber orientation at various temperatures for a 30% glass-filledPA 6 material after conditioning.
Stress-StrainPerpendicular to Orientation
80
70
60
50
40
30
20
10
0
Durethan BKV 130
0 1 2 3 4
23 C
40 C
60 C
120 C90 C
150 C
a given part can endure. Always addreasonable safety factors and testprototype parts before actual production.
In glass-filled resins, fiber orientation
also affects mechanical performance:fatigue strength for a given fiber-filledresin is often many times greater whenthe fibers are aligned lengthwise, ratherthan perpendicular to the fatigue load.Stress-strain performance in the directionof fiber orientation can also differ greatlyfrom the performance in the directionperpendicular to the fibers. Figures 3-5
Molding Factors
The injection-molding process introducesstresses and orientations that affectthe mechanical performance of plastic
parts. The standard test bars used todetermine most mechanical propertieshave low levels of molding stress. Thehigh molding stresses in an actual partmay reduce certain mechanical properties,such as the amount of applied stress
and 3-6 show stress versus strain for a30% glass-filled PA 6 in the parallel-to-fiber and perpendicular-to-fiber directions.
Unless otherwise stated, most mechanical
properties derive from end-gated testbars that exhibit a high degree of orien-tation in the direction of the applied testload. Mechanical calculations basedon this kind of data may over-predictmaterial stiffness and performance in
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Chapter 3
STRU
Recommended