A Design Guide

Material SelectionEngineering Polymers

THERMOPLASTICS ANDPOLYURETHANES

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Because of improved quality and costcompetitiveness, plastic materials aredisplacing traditional materials in amyriad of diverse and demanding indus-tries. Today, engineering plastics canbe found in virtually every aspect of ourlives. From food containers to automo-biles, appliances, toys, office equip-ment, and life-saving medical devices,plastics affect each and every one of us.Product designers and consumers alikeacknowledge that todays advancedplastics, in tandem with proper design,add to product value and versatility.

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The growing number of thermoplasticsand thermosets with their combinationsof physical and mechanical properties makes the proper material selection diffi-cult. A resin is judged by any number ofcriteria strength, toughness, aesthetics,etc. depending upon a parts final use.Any particular plastics performanceacross these criteria can vary widely.

This manual is designed to help you the design engineer, product engineer,process engineer, and others who workwith plastic materials select materialsfor your specific application. It beginswith a basic overview of the nature ofplastics, then explains the specific testsused to compare and evaluate engineeringplastics. We hope this information helpsyou develop parameters to consider whenselecting a group of plastics for furtherinvestigation. Many rules of thumb appearin the text. Naturally, there may be someexceptions to these rules of thumb ortimes when one conflicts with another. If this happens, talk with your moldmaker/designer and Bayer Corporationpersonnel for appropriate action.

Specific resin data and typical propertyinformation have not been included inthis manual except as examples for gen-eral information. All values that appearin this manual are approximate and arenot part of the product specifications. Do not use this data for product specifi-cation. For more specific information ona particular resin, please read the appro-priate Bayer Product InformationBulletin (PIB) as a preliminary step formaterial selection. Ultimately, materialselection must be based upon your prototype testing under actual, end-use

conditions. This brochure does not coverpart design. While design and materialselection are interrelated, we have chosen to discuss part and mold designin separate manuals, EngineeringThermoplastics: Part and Mold DesignGuide and Engineering Polymers: RIM Part and Mold Design Guide.Throughout this manual, relevant testsfrom the American Society for Testingand Measurement (ASTM), theInternational Standards Organization(ISO), Underwriters Laboratories (UL),German Standards Institute (DIN), andthe International Electro-Technical Com-mission (IEC) are given where possible.Efforts were made to include the perti-nent tests specified in ISO 10350 theemerging international standard for poly-mer properties and test procedures.

While providing a good overview of thetopics you should consider when selectinga plastic, this manual does not provide all the information youll need to make afinal resin choice. Final material selectionmust be based upon prototype testinginformation and final part testing in actu-al, in-use settings prior to commercializa-tion. Published data should be used onlyto screen potential candidate materials.

Bayer Corporation and our parent com-pany, Bayer AG (Germany), offer a widerange of engineering thermoplastics, aswell as many polyurethane systems, forengineering end uses. As a service to ourcustomers, we also have technical ser-vice engineers ready to help you withpart design and production. Please feelfree to contact us with specific questions.

INTRODUCTION

Chapter 1 UNDERSTANDING ENGINEERING PLASTICS

5 Plastics: Origins and Definitions

6 Thermoplastics and Thermosets

7 RIM Polyurethane Systems8 Crystalline and Amorphous Polymers

9 Blends

10 Copolymers and Terpolymers

10 Elastomers

10 Molecular Weight

10 Fillers and Reinforcements

11 Shrinkage

12 Additives

12 Combustion Modifiers12 Release Agents12 Blowing Agents12 Catalysts

Chapter 2 MECHANICAL BEHAVIOR OF PLASTICS

13 Viscoelasticity

14 Creep

15 Stress Relaxation

15 Recovery

16 Loading Rate

16 Factors Affecting Mechanical Properties

17 Processing17 Thermoplastic Regrind17 Polyurethane Recycling18 Weld Lines19 Residual Stress20 Orientation21 Water Absorption22 Chemical Exposure22 Weathering

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Chapter 3 MECHANICAL PROPERTIES

23 Short-Term Mechanical Properties

23 Tensile Properties25 Tensile Modulus25 Tensile Stress at Yield26 Elongation at Yield26 Tensile Stress at Break26 Elongation at Break26 Ultimate Strength26 Ultimate Elongation27 Poissons Ratio27 Flexural Properties27 Flexural Modulus27 Ultimate Flexural Stress28 Cut-Growth Resistance28 Compressive Properties28 Compressive Strength29 Compressive Set29 Shear Strength29 Tear Strength29 Impact Properties32 Hardness Properties34 Miscellaneous Mechanical Properties34 Coefficient of Friction34 Abrasion and Scratch Resistance35 Long-Term Mechanical Properties

36 Creep Properties37 Stress Relaxation38 Fatigue Properties

TABLE OF CONTENTS

Chapter 6 ENVIRONMENTAL PROPERTIES

51 Water Absorption

51 Hydrolytic Degradation

52 Thermal and Humid Aging

53 Chemical Resistance

54 Weatherability

55 Gas Permeability

Chapter 7 OTHER PROPERTIES

56 Density

56 Specific Gravity

56 Specific Volume

57 Haze and Luminous Transmittance

57 Refractive Index

57 Oxygen Index

57 Flammability Class

59 Flash Point

Chapter 4 THERMAL PROPERTIES

40 Deflection Temperature Under Load (DTUL)

41 Coefficient of Linear Thermal Expansion (CLTE)

41 Thermal Conductivity

42 Specific Heat

42 Relative Temperature Index (RTI)

42 Vicat Softening Temperature

43 Torsional Pendulum

44 Thermal Transmission Properties

44 Open-Cell Content: Foamed Polyurethane Materials

45 Heat (High-Temperature) Sag

Chapter 5 ELECTRICAL PROPERTIES

46 Volume Resistivity

46 Surface Resistivity

47 Dielectric Strength

48 Dielectric Constant

48 Dissipation Factor

48 Arc Resistance

49 Comparative Tracking Index (CTI)

50 Hot-Wire Ignition (HWI)

50 High-Current Arc Ignition (HAI)

50 High-Voltage Arc-Tracking Rate (HVTR)

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Chapter 9 MATERIAL SELECTION: THINGS TO CONSIDER

69 Cost Considerations

70 Environmental Considerations

70 Load70 Temperature70 Chemical Resistance71 Weather Resistance71 Material Properties

72 Processing

73 Appearance

73 Agency Approvals

74 Actual Requirements

74 Prototype Testing

74 Resin Suppliers

74 Systems Approach

Chapter 10 TECHNICAL SUPPORT

75 Health and Safety Information

75 Design and Engineering Expertise

76 Technical Support

76 Design Review Assistance76 Application Development Assistance76 Product Support Assistance76 Regulatory Compliance

77 Regrind Usage

77 For More Information

APPENDICES

78 List of Figures and Tables

80 Index

83 Related ISO-ASTM-IEC Test Methods

BACK POCKET

Bayer Materials Properties Guide

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Chapter 8 PROPERTIES USED IN PROCESSING

60 General Processing Parameters

60 Mold Shrinkage60 Viscosity60 Solution Viscosity61 Viscosity Versus Shear Rate Curves62 Polyol and Isocyanate Viscosity62 Rotary Viscosity (Brookfield Viscosity)62 Thermoplastics

62 Melt Strength63 Spiral Flow64 Polyurethanes

64 Hydroxyl Number65 Percentage NCO and Amine Equivalent65 Acidity67 Free-Rise Density67 Cream Time67 Gel Time67 Tack-Free Time67 Water (Weight Percent)

Although plastics appear in nearlyevery industry and market, few peoplehave training in polymer chemistry andstructure. Understanding this basicinformation will help you select theright resin. This section explains theconcepts of polymer chemistry andstructure, and shows how theseelements affect material properties.

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PLASTICS: ORIGINS AND DEFINITIONS

To understand plastic materials, youshould have some insight intopolymers, the building blocks of plas-tics. Polymers, derived from the Greekterm for many parts, are large mole-cules comprised of many repeat unitsthat have been chemically bonded intolong chains. Silk, cotton, and wool areexamples of natural polymers. In thelast 40 years, the chemical industry hasdeveloped a plethora of synthetic poly-mers to satisfy the materials needs for adiversity of products: paints, coatings,fibers, films, elastomers, and structuralplastics are examples. Literally thou-sands of materials can be grouped asplastics, although the term today istypically reserved for polymeric materi-als, excluding fibers, that can be moldedor formed into solid or semi-solid objects.

Polymerization, the process of chemi-cally bonding monomer building blocksto form large molecules, can occur byone of several methods. In additionpolymerization, a chain reaction addsnew monomer units to the growingpolymer molecule one at a time. Eachnew unit added creates an active site forthe next attachment (see figure 1-1). Incondensation polymerization, thereaction between monomer units orchain-end groups releases a small mole-cule, often water (see figure 1-2). Thisreversible reaction will reach equilibri-um and halt unless this small molecularby-product is removed. Commercialpolymer molecules are usuallythousands of repeat units long.

Chapter 1UNDERSTANDING ENGINEERING PLASTICS

Addition polymeri- zation of ethylene into polyethylene. The growing molecules become commercial-quality polyethylene when the number of repeat units (n) reaches approxi- mately 100,000.

Figure 1-1

R + C = C R C C + C = C

C C C = C + C C C C R

H H

H H

H H

H H

H H

H H

H H H H

H H H H

H H

H H n

H H

H H

6Condensation polymerization of polycarbonate (PC) via condensation of water. Although not a common commercial process, the reverse of this reaction is the mechanism by which PC can degrade in the presence of water and high heat.

Figure 1-2

Polycarbonate Repeating Unit

H O

H2O

C OCO

OH

CH3

CH3Bisphenol A Carbonic Acid

H +H

O

H O C OCH3

CH3

H2O

CO

OH + H O C OCH3

CH3

CO

OH

O C OCH3

CH3

OC O

n

Understanding the polymerizationprocess gives insight into the nature ofthe resulting plastic. For example, plas-tics made via condensation polymeriza-tion, in which water is released, candegrade when exposed to water andhigh temperatures. Under these condi-tions, depolymerization occurs, severingthe polymer chains.

THERMOPLASTICS AND THERMOSETS

How a polymer network responds toheat determines whether a plastic fallsinto one of two broad categories: ther-moplastics or thermosets. Thermoplas-tics soften and melt when heated andharden when cooled. Because of thisbehavior, these resins can be injectionmolded, extruded or formed via othermolding techniques. This behavior alsoallows production scrap runners andtrimmings, for instance to bereground and reused. Because somedegradation or loss of mechanical prop-erties can occur during subsequentremelting, you should limit the amountof recycled resin in the production resin

mix. This is particularly true if process-ing conditions are harsh. See specificBayer Product Information Bulletins forthe recommended maximum regrind fora given resin.

Unlike thermoplastics, thermosets formcross links, interconnections betweenneighboring polymer molecules thatlimit chain movement. This network ofpolymer chains tends to degrade, ratherthan soften, when exposed to excessiveheat. Until recently, thermosets couldnot be remelted and reused after initialcuring. Todays most-recent advancesin recycling have provided new methodsfor remelting and reusing thermosetmaterials.

Because they do not melt, thermosetsare processed differently than thermo-plastics. Heat will further polymerizesome thermosets, such as phenolic resin,which cure when injected into a hotmold. Other thermosets RIM poly-urethanes, for example rely upon acontrolled chemical reaction betweencomponents after they pass through amixing head into the mold. A third typeof thermoset, such as silicon, cures asvolatiles in the resin evaporate.

Although thermosets generally requirelonger cycle times and more secondaryoperations such as deflashing and trim-

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ming than thermoplastics, they usual-ly have less mold shrinkage and exhibitsuperior chemical and heat resistance.

RIM Polyurethane Systems

Bayer Polymers Division produces awide variety of Reaction InjectionMolding (RIM) polyurethanes whichuse two liquid components to chemicallyform plastic material in a mold. The liq-uids, an isocyanate (A component) anda polyol (B component), react to forma polyurethane resin with long polymerchains. These two components, coupled

with additives are generally referred to asa RIM system. Generally, RIM materi-als show excellent chemical resistance including resisting organic and inor-ganic acids, aliphatic hydrocarbons, andmany solvents and have good agingand weathering resistance. Usually, RIMprocessing uses less-expensive tooling,less energy, and lower-tonnage pressesthan thermoplastic processing.

Polyurethane resins can be classifiedinto two broad groups. Rigid poly-urethane materials generally havehigher flexural moduli and hardness.They offer good thermal resistance,electrical properties, chemical resis-tance and acoustical insulation.Elastomeric polyurethane systems

are generally found in applicationsrequiring superior impact strength.More flexible than typical rigid sys-tems, elastomeric polyurethane resinsexhibit good toughness and dimensionalstability throughout a wide range oftemperatures and have excellent corro-sion, abrasion, wear, and cut resistance.These broad categories are not absolute;they are ranges (see figure 1-3). Bothrigid and elastomeric materials alsohave a potential for high-quality, class A finishes with excellent paintand coating adherence.

Within these large classifications, thereare three types of polyurethane systems:

Foamed Polyurethane Systems,

non-isotropic materials, use a blow-ing agent to make parts with a ruggedskin and a lower-density, microcellu-

Chapter 1UNDERSTANDING ENGINEERING PLASTICS continued

Figure 1-3

TYPE

S O

F PO

LYUR

ETHA

NE M

ATER

IALS

FLEXURAL MODULUS (ksi)

0 00300 0600 0900 1200 1500

BAYDURRigidFoams

BAYDUR STRSolid

Composites (SRIM)

BAYDUR STRFFoamed

Composites

PRISMRigidSolids

Polyurethane Systems Classified by Flexural Modulus

BAYFLEXElastomericFoams

BAYFLEXElastomericSolids

RRIM

8lar-foam core in a sandwich-like com-position. The skin on elastomericfoamed systems is extremely tearresistant, making these materials agood choice for steering wheels, arm-rests, headrests, gearshift knobs andfurniture. Rigid systems have hard,solid skins and are found in businessmachine housings, automobile spoil-ers, skis and certain load-bearingapplications. Finally, cell size helpscategorize foamed polyurethane sys-tems. Large-celled foamed systemsfind use in seat cushions and beddingmaterials. Microcellular systems, thosewith cells as small as 0.0001 inch, finduse in shoe soles and furniture.

Solid Polyurethane Systems do notuse blowing agents, resulting in ahomogeneous, isotropic, rigid or elas-tomeric plastic. Solid elastomers arefound in many industries, includingautomotive, construction, agriculture,and recreational equipment. Commonparts include fenders, fascias, trimsand vertical panels. Additionally,fillers can be added to solid elas-tomers for improved stiffness. Solidrigid polyurethane systems havemany property values similar to thosefound in typical thermoplastics. They can make thin-walled parts and may be more economical thanthermoplastics.

Structural Composite Polyurethane

Systems are solid or foamed materi-als, molded in combination with fiberreinforcements, such as glass mat, in

the mold to improve the systemsmechanical characteristics. The matadds extremely high stiffness and highimpact strength to the part. Typicalapplications include door panels anddoors, automotive horizontal panels,and recreational equipment parts.

CRYSTALLINE AND AMORPHOUS POLYMERS

Thermoplastics are further classified bytheir crystallinity, or the degree oforder within the polymers overallstructure. As a crystalline resin coolsfrom the melt, polymer chains fold oralign into highly ordered crystallinestructures (see figure 1-4). Generally,polymer chains with bulky side groupscannot form crystalline configurations.

The degree of crystallinity dependsupon both the polymer and the process-ing technique. Because of molecularstructure, some polymers such aspolyethylene crystallize quickly andreach high levels of crystallinity.Others, such as PET polyester, requirelonger times in a hot mold to crystallize.If cooled quickly, PET polyesterremains amorphous in the final product,such as in beverage bottles. Becausemost crystalline polymers have bothamorphous and crystalline regions, theyexhibit both a glass transition temper-ature, the melting temperature range inthe non-crystalline region, and a crys-talline melt temperature, the typicallydistinct melting temperature in the crys-talline region. Crystalline thermoplas-tics must be heated above the resinscrystalline-melt temperature forextrusion and injection molding.

In crystalline resins, a percentage of the polymer chains orient into ordered, crystalline structures.

Figure 1-4

Crystalline Structures

Amorphous Regions

Amorphous polymers, ones with littleor no crystallinity, have random chainentanglements and lack a discrete melt-ing point. As they are exposed to heat,these polymers soften and become morefluid-like, allowing the polymer chainsto slide past one another. As the poly-mer cools, chain movement diminishes,and the polymers viscosity increases.Generally, the higher a polymers glasstransition temperature, the better it willperform at elevated temperatures. As arule, transparent plastics those usedin headlight lenses and lighting fixtures,for example are amorphous ratherthan crystalline. The most commontransparent thermoplastics includepolycarbonate, polystyrene, andpoly(methyl) methacrylate.

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Crystalline and amorphous plastics haveseveral characteristic differences. Theforce to generate flow in amorphousmaterials diminishes slowly as the tem-perature rises above the glass transitiontemperature. In crystalline resins, theforce requirements diminish quickly asthe material is heated above its crys-talline melt temperature (see figure 1-5). Because of these easier flow characteristics, crystalline resins havean advantage in filling thin-walled sections, as in electrical connectors.Additionally, these resins generallyhave superior chemical resistance,greater stability at elevated tempera-tures and better creep resistance.Amorphous plastics typically exhibitgreater impact strength, less mold

shrinkage, and less final-part warpingthan crystalline counterparts. End-userequirements usually dictate whether anamorphous or crystalline resin is preferred.

BLENDS

Blending two or more polymers offersyet another method of tailoring resinsto your specific application. Becauseblends are only physical mixtures, theresulting polymer usually has physicaland mechanical properties that lie some-where between the values of its con-stituent materials. For instance, an auto-motive bumper made from a blend ofpolycarbonate resin and a thermoplasticpolyurethane elastomer gains rigidity

Chapter 1UNDERSTANDING ENGINEERING PLASTICS continued

INCR

EASI

NG IN

JECT

ION

FORC

E

INCREASING MELT TEMPERATURE

The force required to generate flow in a mold diminishes slowly above the glass transition temperature (Tg) in amorphous thermoplastics, but drops quickly above the crystalline melt temperature (Tc) in crystalline resins.

Figure 1-5

Amorphous Resin

Crystalline Resin

Tg

Tc

Injection Force vs. Temperature

10

from the polycarbonate resin and retainsmost of the flexibility and paintabilityof the polyurethane elastomer. Forbusiness machine housings, a blend ofpolycarbonate and ABS resins offers the enhanced performance of polycar-bonate flame retardance and UVstability at a lower cost.

Occasionally, blended polymers haveproperties that exceed those of the con-stituents. For instance, blends of poly-carbonate resin and PET polyester, orig-inally created to augment the chemicalresistance of polycarbonate, actuallyhave fatigue resistance and low-temperature impact resistance superiorto either of the individual polymers.

COPOLYMERS AND TERPOLYMERS

Unlike blends, or physical mixtures ofdifferent polymers, copolymers containrepeat units from two polymers withintheir molecular chain structure, such asacetyl resin, styrene acrylonitrile(SAN), and styrene butadiene. In ter-polymers, polymers with three differentrepeat units, individual components canalso be tailored to offer a wide range ofproperties. An example is ABS, a ter-polymer containing repeat units of acry-lonitrile, butadiene, and styrene.

ELASTOMERS

Elastomers are a class of polymericmaterials that can be repeatedlystretched to over twice the originallength with little or no permanent defor-mation. Elastomers can be made ofeither thermoplastic or polyurethanematerials and generally are tested andcategorized differently than rigid mate-rials. Commonly selected according totheir hardness and energy absorption characteristics rarely considered in rigidthermoplastics elastomers are foundin numerous applications, such as auto-motive bumpers and industrial hoses.

MOLECULAR WEIGHT

A polymers molecular weight, the sumof the weights of individual atoms thatcomprise a molecule, indicates the aver-age length of the bulk resins polymerchains. Low-molecular-weight polyeth-ylene chains have backbones as small as1,000 carbon atoms long. Ultrahigh-molecular-weight polyethylene chainscan have 500,000 carbon atoms alongtheir length. Many plastics polycar-bonate, for instance are available in avariety of chain lengths, or differentmolecular-weight grades. These resinscan also be classified by an indirect viscosity value, rather than molecularweight. Within a resin family, higher-molecular-weight grades have higherviscosities. For example, in the viscosi-ty test for polycarbonate, the melt flow rate ranges from approximately

4 g/10 min for the highest-molecular-weight, standard grades to more than 60 g/min for lowest-molecular-weight,high-flow, specialty grades.

Selecting the correct molecular weightfor your injection-molding applicationgenerally involves a balance betweenfilling ease and material performance. If your application has thin-walled sections, a lower-molecular-weight/lower-viscosity grade offers better flow.For normal wall thicknesses, these resinsalso offer faster mold-cycle times andfewer molded-in stresses. The stiffer-flowing, high-molecular-weight resinsoffer the ultimate material performance,being tougher and more resistant tochemical and environmental attack.

FILLERS AND REINFORCEMENTS

Often, fibrous materials, such as glass orcarbon fibers, are added to resins to cre-ate reinforced grades with enhancedproperties. For example, adding 30%short glass fibers by weight to nylon 6improves creep resistance and increasesstiffness by 300%. These glass-rein-forced plastics usually suffer some lossof impact strength and ultimate elonga-tion, and are more prone to warpingbecause of the relatively large differencein mold shrinkage between the flow andcross-flow directions.

Plastics with non-fibrous fillers suchas spheres or powders generallyexhibit higher stiffness characteristicsthan unfilled resins, but not as high asglass-reinforced grades. Resins withparticulate fillers are less likely to warpand show a decrease in mold shrinkage.Particulate fillers typically reduceshrinkage by a percentage roughly equalto the volume percentage of filler in thepolymer, an advantage in tight-tolerancemolding. When considering plasticswith different amounts of filler or rein-forcement, you should compare the costper volume, rather than the cost perpound. Most fillers increase the materialdensity; therefore, increasing filler con-tent usually reduces the number of partsthat can be molded per pound.

SHRINKAGE

As a molded part cools and solidifies, itusually becomes smaller than its moldcavity. Shrinkage characteristics affectmolding costs and determine a partsdimensional tolerance limit. Materials

11

with low levels of isotropic shrinkagetypically provide greater dimensionalcontrol, an important consideration intight-tolerance parts. The exact amountof this mold shrinkage depends primar-ily upon the particular resin or systemused. For instance, semicrystallinethermoplastics generally show higherlevels of shrinkage than amorphousthermoplastics because of the volumereduction during crystallization.

Other factors including part geome-try, wall thickness, processing, use andtype of fillers, and gate location alsoaffect shrinkage. For instance:

Holes, ribs and similar part featuresrestrain shrinking while the part is inthe mold and tend to lower overallshrinkage.

Shrinkage generally increases withwall thickness and decreases withhigher filling and packing pressures.

Areas near the filling gate tend toshrink less than areas further away.

Particulate fillers, such as mineralsand glass spheres, tend to reduceshrinkage uniformly in all directions.

Fibrous fillers, such as glass or carbon fibers, decrease shrinkage primarily in the direction of flow.Fiber-filled parts often shrink two tothree times more in the cross-flowversus the flow direction.

Post-mold shrinkage, additionalshrinking that may appear long aftermolding, occurs often in parts that wereprocessed to reduce initial shrinkageand later are exposed to elevated tem-peratures. Over time, molded-in stresseswill relax, resulting in a size reduction.Elevated temperatures can also lead tosolid-state crystallization and additionalshrinkage in some semi-crystallinematerials.

Chapter 1UNDERSTANDING ENGINEERING PLASTICS continued

Photo courtesy of Xerox Corporation.

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ADDITIVES

Additives encompass a wide range ofsubstances that aid processing or addvalue to the final product, includingantioxidants, viscosity modifiers, pro-cessing aids, flame retardants, dyes andpigments, and UV stabilizers. Found invirtually all plastics, most additives areincorporated into a resin family by thesupplier as part of a proprietary pack-age. You can select some additives byspecifying optional grades to maximizeperformance for your specific applica-tion. For example, you can choose stan-dard polycarbonate resin grades withadditives for improved internal moldrelease, UV stabilization, and flameretardance; or nylon grades with addi-tives to improve impact performance.

Additives often determine the successor failure of a resin or system in a par-ticular application. In RIM polyurethanesystems, additives are usually part ofthe B component. Four common addi-tives are discussed below. Before mak-ing your final material selection, youshould discuss your part and its require-ments with your Bayer representative.

Combustion Modifiers

Combustion modifiers are added topolymers to help retard the resultingparts from burning. Generally requiredfor electrical and medical-housing appli-cations, combustion modifiers and theiramounts vary with the inherent flamma-bility of the base polymer. Polyurethanesystems are more flammable than mostthermoplastic resins. Flammabilityresults are based upon small-scale labo-ratory tests. Use these ratings for com-parison purposes only, as they may notaccurately represent the hazard presentunder actual fire conditions.

Release Agents

External release agents are lubricants,liquids or powders, that coat a moldcavity to facilitate part removal. Internalrelease agents, usually proprietary tothe system producer, find use in manyplastic materials.

Blowing Agents

Used in foamed thermoplastic andpolyurethane materials, blowing agentsproduce gas by chemical or thermalaction, or both. When heated to a spe-cific temperature, these ingredientsvolatilize to yield a large volume of gasthat creates cells in foamed plastics.

The nations growing concern for ozonedepletion in the upper atmosphere aswell as other environmental issues hasled Bayer to minimize use of chloro-fluorocarbons (CFCs/HCFCs) and develop new blowing agents. Theplastics-producing industry as a wholecontinues to search for safer, environmen-tally friendly solutions to these issues.

Catalysts

Catalysts, substances that initiate orchange the rate of a chemical reaction,do not undergo a permanent change incomposition or become part of the mol-ecular structure of the final product.Occasionally used to describe a settingagent, hardener, curing agent, promoter,etc., they are added in minute quantities(typically less than 1%) compared tothe amounts of primary reactants inpolyurethane systems. A catalyst pack-age is custom-tailored for a specificpolyurethane system to yield therequired foam characteristics within thetime and processing parameters.

Plastics offer a wide range of mechani-cal properties, as well as some unusualmechanical behaviors. Changes in thepolymer repeat units, chain length,crystallinity, or level of cross-linkingcan yield materials with propertiesranging from strong to weak, brittle totough, or stiff to elastic. Under certainconditions such as elevated tempera-tures and/or long-term loading plas-tics behave quite differently from otherengineering materials. This section dis-cusses the unusual mechanical behaviorof plastics and how to address theseissues when designing parts for yourapplication.

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VISCOELASTICITY

Plastics have a dual nature, displayingcharacteristics of both a viscous liquidand a spring-like elastomer, or traitsknown as viscoelasticity. This dualityaccounts for many of the peculiarmechanical properties found in plastics.Under mild loading conditions suchas short-term loading with low deflec-tions and small loads at room tempera-tures plastics usually respond like aspring, returning to their original shapeafter the load is removed. No energy islost or dissipated during this purelyelastic behavior: Stress versus strainremains a linear function (see figure 2-1). Increasing the applied load adds aproportional increase to the partsdeflection.

Many plastics exhibit a viscous behav-ior under long-term heavy loads or ele-vated temperatures. While still solid,

plastics will deform and flow similarlyto a very high-viscosity liquid. Tounderstand this viscous behavior, youmust understand two terms: strain ()and stress (). Strain is measured inpercent elongation; stress is measured inload per area. Typical viscous behaviorfor tensile loading shows that strainresulting from a constant applied stressincreases with time as a non-linearresponse to these conditions (see figure2-2). This time-and-temperature-depen-dent behavior occurs because the poly-mer chains in the part slip and do notreturn to their original position whenthe load is removed.

The Voight-Maxwell model ofsprings and dashpots illustrates theseviscoelastic characteristics (see figure2-3). The spring in the Maxwell modelrepresents the instantaneous response toloading and linear recovery when theload is removed. The dashpot connected

Chapter 2MECHANICAL BEHAVIOR OF PLASTICS

Loading and Unloading Follow the Same Path

STRE

SS (

) INC

REAS

ING

STRAIN ( ) INCREASING

Figure 2-1

Linear relationship of stress and strain idealized by elastic spring.

ElasticSpring

Stress-Strain Behavior

14

to the spring simulates the permanentdeformation that occurs over time. TheVoight model shows the slow deforma-tion recovery after the load is removed.While it is not a practical model forstructural design use, the Voight-Maxwell model offers a unique way tovisualize viscoelastic characteristics.

CREEP

One of the most important conse-quences of plastics viscoelastic behavior, creep, is the deformation thatoccurs over time when a material is subjected to constant stress at a constanttemperature. Under these conditions,the polymer chains slowly slip past oneanother. Because some of this slippageis permanent, only a portion of thecreep deformation can be recoveredwhen the load is removed.

The tensile test in figure 2-4 clearlydemonstrates creep. A weight hungfrom a plastic tensile bar will causeinitial deformation d increasing thebars length. Over an extended period of time, the weight causes moreelongation, or creep c.

If you are designing parts for long-termloading, particularly for elevated-tem-perature service, you must account forcreep characteristics. See Bayers manuals, Engineering Thermoplastics:Part and Mold Design Guide orEngineering Polymers: RIM Part andMold Design Guide, for using long-termcreep data in designing plastic parts.

Viscous behavior of plastics with varying stress levels over time.

STRA

IN (

)

LOAD DURATION ( t )

3

2

= Stress Level

Figure 2-2

Voight-Maxwell model simulating viscoelastic characteristics.

Figure 2-3

Spring A

Dashpot A

Dashpot BSpring B

Maxwell

Voight

STRESS RELAXATION

Another viscoelastic phenomenon,stress relaxation, is defined as a grad-ual decrease in stress at constant strainand temperature. Because of the samepolymer-chain slippage found in creep,stress relaxation occurs in simple ten-sion, as well as in parts subjected tomultiaxial tension, bending, shear, andcompression. The degree of stress relax-ation depends upon a variety of factors,including load duration, temperature,and types of stress and strain.

Figure 2-5 shows that a large weightinitially produces elongation d and astrain, d/L (L = original length). Tomaintain the same elongation and strainin the test bar over time, less weight isneeded because of stress relaxation.Simply stated:

In the creep example, elongation con-tinues as the weight remains constant;

15

In the stress relaxation example, theweight is reduced to maintain theelongation.

If you are designing parts that will besubjected to a constant strain, youshould account for stress relaxation. A typical press fit, such as a metal insert

in a plastic boss, relies upon stressesfrom the imposed strain of an interfer-ence fit to hold the insert in place.However, polymer-chain slippage canrelax these stresses and reduce the insertretention strength over time. A methodfor calculating the degree of stress relax-ation for simple shapes is explained inBayers Engineering Thermoplastics:Part and Mold Design Guide.

RECOVERY

The degree to which a plastic materialreturns to its original shape after a loadis removed is defined as its recovery.Involving many factors, most of whichare shape- and application-specific,recovery characteristics are extremelydifficult to predict. By way of example,refer to figure 2-6. In this example,strain is plotted versus time. The strain(deformation) from a load applied to a

Chapter 2MECHANICAL BEHAVIOR OF PLASTICS continued

Under a constant load, deformation increases over time.

Figure 2-4 Creep Phenomenon

L

L+d

(d) In

itial

Def

orm

atio

n

L+d+

C

Time (t0)

Creep (C)

Time (t1)

ConstantForce

Tota

l Def

orm

atio

na

t Tim

e (t 1

)

L

L+d

L+d

ConstantStrain

Reduced

Force

Over time a smaller load is required to maintain constant deformation.

Figure 2-5

Time (t0)Time (t1)

Stress Relaxation

d

16

plastic part produces an initial strain(point A). Over time, creep causes thestrain to increase (point B). When theload is removed, the strain immediatelydrops (point C). From this point, if fullrecovery were possible, the part mightreturn to original size (point E). Morecommonly, the part retains somepermanent deformation (point D).

LOADING RATE

The rate at which a part is stressed, the loading rate, greatly affects themechanical behavior of plastics. Partsare exposed to a variety of loading ratesthroughout their life cycle: from verylow, static loading to high-speed impactloading. In general, thermoplasticsbecome stiffer and fail at smaller strainlevels as the strain rate increases (seefigure 2-7). Increasing the plastics tem-perature usually has the opposite effect:At higher temperatures, plastics losetheir stiffness, becoming more ductile.When selecting materials, you willnormally have to compromise betweenhaving acceptable impact strength at thelower end of the applications tempera-ture range, and maintaining the properstiffness and creep resistance at theupper end of the temperature range.

FACTORS AFFECTING MECHANICAL PROPERTIES

Most of this manual defines andexplains material property data found inmaterial-specific data sheets. These

PermanentDeformation

STRA

IN (

)

TIME ( t )

Figure 2-6Load and Recovery Behavior

LoadRemoved

A

B

C

D

E

Figure 2-7

Effects of strain rate and temperature on material behavior.

Higher Strain Rate

Lower Temperature Higher Temperature

Lower Strain Rate

Ductile

Brittle

STRA

IN (

)

STRESS ( )

Brittle and Ductile Behavior

Product Information Bulletins (PIBs),which describe the general properties ofthe materials, are useful for screeningmaterials, and provide data for estimat-ing finished-part performance. Youshould remember that these data aregenerated in a laboratory under a nar-row set of conditions and cannot coverall production environments. Many fac-tors encountered in actual productionand final use can alter material perfor-mance, in particular the mechanicalproperties. This section discusses themajor factors that affect the mechanicalproperties of plastic parts.

Processing

Published property data is derived fromtesting standardized test plaques, mold-ed under optimum processing condi-tions. Improper processing can degradeplastics, changing certain mechanical-property performance, such as impactstrength and elongation at break. Ifmaterial is improperly processed, theresulting mechanical performance maydiffer significantly from published values.

Common thermoplastic molding errorsthat can affect mechanical propertiesinclude excessive melt temperature,inadequate resin drying prior to mold-ing, excessive residence time in thepress barrel, and inadequate gate size.Keep injection speeds, as well as moldand melt temperatures, within publishedparameters. Insufficient injection speedor cold-melt temperature causes cold-flow fronts that can lead to weak weld

17

lines and high levels of molded-instress. Additionally, crystalline resinsusually require higher mold tempera-tures to fully crystallize. Using lowermold temperatures can decrease crys-tallinity, as well as reduce stiffness andchemical resistance, while increasingductility and impact strength.

Processing problems, such as incorrectmix ratio and improper mixing, candiminish the mechanical performance ofRIM polyurethane materials. Additionally,cold mold temperatures can lead to brit-tle skins in foamed polyurethane materi-als, greatly reducing impact strength.Hot mold temperatures, on the otherhand, reduce skin thickness and lowerpart stiffness. Published data applies tomaterial processed within recommendedparameters. If you have questions, callyour Bayer representative.

Thermoplastic Regrind

Scrap thermoplastic produced during themolding process sprue and runner sys-tems, partially filled parts, rejected parts,etc. can be reused. Typically, this scrapis chopped up into small pellet-sizedpieces, called regrind, and mixed withvirgin material to produce more parts.

When regrind has been remelted severaltimes, as can happen when scrap and run-ners are repeatedly fed back into thepress, it can become badly degraded.Regrind is also vulnerable to contamina-tion and/or abusive processing, which can

adversely affect the mechanical and cos-metic properties. For these reasons, youshould limit the ratio of regrind to virginmaterial and completely avoid using it incritical applications or when resin proper-ties must be equivalent to virgin-materialproperties. Closely monitor part qualitywhen using regrind in the mix to assureadequate material and end-use properties.

Polyurethane Recycling

Because of recent advances, severalmethods can be used to recyclepolyurethane materials, depending uponthe type of material. All polyurethaneresins can be granulated or ground intopowder for use as a filler in new parts.Granulated material can also be com-pression molded under high pressureand temperature to produce new parts.Parts made this way may retain theiroriginal elongation and over 50% oftheir tensile strength. Adhesive bondingrecycles thermoformable polyurethanefoam. In this process, granulated materi-al is coated with a binder and curedunder heat and pressure. New injection-molding techniques are also being usedto recycle polyurethane materials.

Chapter 2MECHANICAL BEHAVIOR OF PLASTICS continued

18

Polyurethane materials can be convert-ed into energy: One pound of RIMpolyurethane materials containsbetween 12,000 and 15,000 BTUs,approximately the same as oil or coal.Finally, glycolysis, a new way to con-vert polyurethane materials back to theiroriginal raw materials, is showing greatpromise. Talk to your Bayer representa-tive for the latest information onpolyurethane recycling.

Weld Lines

The hairline grooves on the surface of amolded part where flow fronts join dur-ing filling, called weld lines or knitlines, cause potential cosmetic flawsand reduced mechanical performance(see figure 2-8). Because few polymerchains cross the boundary when theflow fronts butt, the tensile and impactstrength in the weld-line area isreduced. The resulting notches on theweld line also act as stress concentra-tors, further reducing impact strength.

Additionally, if the flow fronts arecovered with a film from additives or a layer of impurities, they may not bind properly, which again can reduceimpact and tensile strength.

Weld-line strength in thermoplasticsvaries with specific resin and processingparameters, such as flow-front tempera-ture, distance from the gate, filling pres-sure, and level of packing. For instance,Makrolon polycarbonate resins usuallyhave exceptional weld-line tensilestrength, typically over 95% of thestrength without weld lines. Otherresins can suffer over 50% loss of tensile strength at the weld line.

RIM polyurethanes form weld linesmore readily in areas filled after gellinghas begun. Areas filled at the end oflong flow paths are particularly prone toweld-line problems. Severe weld linesdramatically reduce mechanical perfor-mance. Choose gate configurations thatavoid weld lines in critical, structuralareas.

Merging melt fronts (cross-sectional view).

Figure 2-8

Weld LineWeld Line

Melt Front Melt Front

Weld Line

Use published tensile and impactstrength data cautiously, because most isbased upon test samples molded withoutweld lines. Contact your Bayer represen-tative for this data or if you have anyquestions regarding weld line strengthfor a specific resin and application.

19

Residual Stress

Molding factors such as uneven partcooling, differential material shrinkage,or frozen-in flow stresses causeundesirable residual stresses in moldedthermoplastics (see figure 2-9). Highlevels of residual stress can adverselyaffect certain mechanical properties, aswell as chemical resistance and dimen-sional stability. Based upon simpleinjection-molded samples, publishedproperty data reflects relatively low levels of residual stress.

When molded-in tensile stresses on aparts surface are exceptionally high, asin parts where the geometry has extreme-ly thin walls or dramatic thickness varia-tions, impact and tensile strength can bereduced. Avoid high-stress features,because the molded-in stresses and theirultimate effect on mechanical perfor-mance can be difficult to predict. Certainstress-analysis techniques, such as sol-vent-stress testing, locate areas of highresidual stress, but only after the moldhas been built and mechanical problemsmay have developed.

Chapter 2MECHANICAL BEHAVIOR OF PLASTICS continued

Filling-analysis results showing areas of high-flow stress.

Figure 2-9

Elevated Stresses inLast Areas to Fill

Elevated Flow StressesNear Gates

Runner System

Flow Stresses

20

Orientation

As a molten thermoplastic fills a mold,its polymer chains tend to align with thedirection of the flow (see figure 2-10).Part thickness and a variety of process-ing variables injection speed, moldtemperature, melt temperature, and holdpressure determine how much of thisflow orientation remains in the solidi-fied part. Most molded parts retainenough orientation to show small but

noticeable differences in material prop-erties between the flow and cross-flowdirections at any location. Generally,mechanical properties in the cross-flowdirection are lower than those in theflow direction. Although usuallyunnoticed in the aggregate, directionaldifferences can affect mechanicalperformance in parts where polymerchains align uniformly along or acrossstructural features.

The glass fibers in outer layers of glass-reinforced plastics tend to align in thedirection of flow, resulting in highertensile strength and stiffness in thisdirection. They also exhibit greaterresistance to shear forces acting acrossthe fibers. Generally, fiber-filled materi-als have much higher shrinkage in thecross-flow than in the flow direction.Cross-flow shrinkage can be as much astwo to three times greater. Address

Polymer chains and fibrous fillers in the outer layers of molded parts tend to align in the direction of flow during molding.

Figure 2-10 Fiber Orientation

these orientation effects in both moldand part design. In many cases, carefulprocessing and optimum gate placementcan reduce or eliminate mechanicalproblems associated with orientation ininjection-molded parts.

Water Absorption

Many plastics are hygroscopic: Over timethey absorb water. Too much moisture in a thermoplastic resin during moldingcan degrade the plastic and diminishmechanical performance. Follow yourresin suppliers drying procedures toprevent this problem. Absorbed water inRIM polyurethane components can causeunwanted foaming and change the reac-tion-process chemistry, dramaticallyaffecting the mechanical performance ofthe end product. Because post drying isnot feasible, take precautions to preventmoisture from entering liquid-RIMcomponents.

Additionally, water absorbed aftermolding can harm mechanical proper-ties in certain resins under specificconditions. Through a process calledhydrolysis, water in the resin severs thepolymer chains, reduces molecularweight, and decreases mechanical prop-erties. Longer exposure times at elevat-ed temperatures and/or loads worsenhydrolytic attack. Polyester-based RIM

21

polyurethane resins are particularlyprone to hydrolytic attack at elevatedtemperatures. When designing parts forprolonged exposure to water or highhumidity, check available data onhydrolytic degradation.

Water absorption can also change thephysical properties of polyamide resins(nylons) without degrading them. Somepolyamides absorb relatively largeamounts of moisture, causing them toswell. Volumetric and linear increasesof 0.9% and 0.3% respectively, for each

1% of absorbed water are common. Atthe same time, these materials showincreased toughness and reduced stiff-ness (see figure 2-11). Other mechani-cal and electrical properties may alsochange significantly with increasedmoisture content. These changes arereversible: The mechanical propertieswill revert to their original values whenthe part is dried. For more information,read the technical data sheet for your Durethan polyamide resin for property data on both dry and moisture-conditioned samples.

Chapter 2MECHANICAL BEHAVIOR OF PLASTICS continued

Figure 2-11

FLEX

URAL

STR

ESS

AT A

GIV

EN S

TRAI

N (M

Pa)

TEMPERATURE (C)Flexural stress vs. temperature at a given strain based upon the flexural test (DIN 53452) for unfilled PA 6 with varying water contents.

-50

200

160

120

80

40

Durethan B 40 SKas molded 0.6%water content 1.3%water content 2.0%water content 2.9%water content 3.5%water content 8.3%

-20 -0+ 20 50 90

22

Chemical Exposure

The effects of chemical exposure on aspecific resin can range from minormechanical property changes to imme-diate catastrophic failure. The degree ofchemical attack depends upon a numberof factors: the type of resin, the chemi-cal in contact, chemical concentration,temperature, exposure time, and stresslevel in the molded part, to name a fewof the more common. Some plastics canbe vulnerable to attack from families ofchemicals, such as strong acids ororganic solvents. In other instances, aresin may be vulnerable to a specific orseemingly harmless chemical. Verify amaterials resistance to all the chemicalsto which it will be exposed during processing, assembling, and final use.

Weathering

The effects of outdoor weather partic-ularly ultraviolet (UV) radiation on aplastics appearance and properties canrange from a simple color shift to severematerial embrittlement. After severalyears in direct sun, most plastics showreduced impact resistance, lower overallmechanical performance, and a changein appearance. Bayer has weatheringdata for aesthetic properties. Data formechanical degradation is less common.

If you are designing a structural partthat will be exposed to sunlight, contact your Bayer representative forweathering data.

SHORT-TERM MECHANICAL PROPERTIES

Short-term mechanical data, basedupon testing done over a short period oftime, does not account for long-termphenomena, such as creep or stressrelaxation. This information should beused only when loading or other stressis applied for such a short period oftime that the long-term effects areinsignificant. All mechanical propertiesare tested at room temperature (73F or23C) unless otherwise stated.

23

Tensile Properties

Tensile properties, important in struc-tural design, are used to compare therelative strength and stiffness of plas-tics. The standard tensile tests for rigidthermoplastics (ASTM D 638 and ISO527) or soft plastics and elastomericmaterials (ASTM D 412) involveclamping a standard molded tensile barinto the test device (see figure 3-1). Thedevices jaw then moves at a constantrate of separation between the clamps,typically 5 mm/min for glass-filled mate-

Chapter 3MECHANICAL PROPERTIES

Testing device and typical dogbone specimen used to test the tensile properties of most plastics.

Figure 3-1 Tensile Tester

LoadMeasurement

MovableHead

FixedHead

TestSpecimen

GrippingJaws

Ove

rall

Leng

thTestingRegion

Head Moves atConstant Rate

Mechanical properties stiffness,hardness, toughness, impact strength,and ability to support loads areimportant in most plastic applications.Mechanical property data is used regu-larly to preselect materials, estimatepart performance, and predict deforma-tions and stresses from applied loads.Examples of these and other calcula-tions showing the use of this data can befound in Bayers Engineering Thermo-plastics: Part and Mold Design Guide.

As previously mentioned, test resultsfound in most technical data sheetshave been derived from laboratory testsand may not directly apply to yourspecific part or application. This datashould be used for comparison purposesonly, because real-world applicationfactors such as environment, tempera-ture, and loading rate also affectmaterial performance.

Bayers material property values andlimits are given at face value no safety factors or margins for error havebeen built-in. Use these data conserva-tively with appropriate safety marginsto account for:

Differences between testing and end-use conditions;

Material and processing variability;

Unforeseen environmental or loadingstresses.

See Bayers part and mold designguides for further discussions of designand application safety factors.

24

rials and 50 mm/min for unfilled plastics.The result usually expressed as acurve illustrating the relationship betweenstress, or the force per original cross-sectional area, and the strain, defined aspercentage change in length yields awealth of information about a resinsbehavior under tensile load (see figure 3-2).

The standard test for determining tensileproperties in microcellular polyurethanematerials (ASTM D 3489) uses a 1/8-inch or 1/4-inch-thick test specimenwith molded skins. These test proce-dures are the same as for rubber (ASTMD 412). In these tests, a specimen ispulled while equipment records theforce and displacement until failure.

For rigid polyurethane foams, ASTM D 1623 covers both tensile propertiesand tensile adhesion properties of afoamed plastic to its skin. In the test aspecimen is placed in grips on acrosshead movement testing apparatus(see figure 3-3). The specimen is sub-jected to a tensile load, with a measure-ment taken at the rupture point.

Dividing the breaking load by the origi-nal minimum cross-sectional area givestensile strength. For rigid structuralpolyurethane foams, use ASTM D 638(ISO 527).

For flexible foams, test results (ASTM D 3574 or ASTM D 5308 (ISO 1789),depending upon the final application)show the effect of tensile forces, measur-ing tensile stress, tensile strength and ulti-mate elongation. The stress is recorded asthe part stretches and finally ruptures.

Tensile stress-strain curves graphicallyillustrate transitional points in a resinsstress-strain behavior (see figure 3-4).Point A, the proportional limit for thematerial, shows the end of the region inwhich the resin exhibits linear stress-strain behavior. Point B is the materialselastic limit, or the point after whichthe part will be permanently deformedeven after the load is removed.Applications that cannot tolerate anypermanent deformation must stay belowthe elastic limit. Point C, the yieldpoint, marks the beginning of theregion in which ductile plastics continueto deform without a correspondingincrease in stress. Elongation at yieldgives the upper limit for applicationsthat can tolerate the small permanentdeformations that occur between theelastic limit and yield point, but not thelarger deformations occurring duringyield. Point D, the break point, showsthe strain value at which the test barbreaks. These five transitional points,important in plastics part design, are thebasis for several common tensile properties.

TENS

ILE S

TRES

S (

) (MP

a)

ELONGATION () (%)

Figure 3-2

These curves illustrate the characteristic differences in the stress-strain behavior of various plastics.

Cast PolyesterNon-Reinforced(rigid, brittle)

PC (ductile)

PU Elastomer(rubber-like)(95 Shore A)

ABS(ductile)

0 10 20 20 200 400 600 800 1,000

100

80

60

40

20

0 //

Tensile Modulus

Used commonly to compare variousmaterials and make structural calcula-tions, the tensile modulus measures aresins stiffness. Higher modulus valuesindicate greater stiffness. Because ofplastics viscoelastic tensile behavior,determining tensile modulus is moresubjective and less precise for plasticsthan it is for metals or other materials.Mathematically, you can determine thetensile modulus by taking the ratio ofthe stress to strain as measured belowthe proportional limit on the stress-

25

strain curves. When dealing with mate-rials with no clear linear region, you cancalculate the modulus at some specifiedstrain value, typically at 0.1% (secantmodulus). For some applications, buck-ling analysis for example, it may bemore appropriate to derive a tensilemodulus from the slope of a straightline drawn tangent to the curve at apoint on the stress-strain diagram (tangent modulus).

Tensile Stress at Yield

The tensile stress at yield, the stresslevel corresponding to the point of zeroslope on the stress-strain curve, general-ly establishes the upper limit for appli-cations that can only tolerate small per-manent deformations (see point C infigure 3-4). Tensile-stress-at-yield val-ues can only be measured for materialsthat yield under testing conditions.

Chapter 3MECHANICAL PROPERTIES continued

Tensile test for rigid polyurethane foam.

Figure 3-3

Specimen BeforeTesting

Specimen After

Testing

Foam Tensile Tester

Yield Point

Proportional LimitBreak Point

Ultimate Strength

B

A

E

D

C

Elastic Limit

Typical stress-strain behavior of unreinforced plastics.

STRE

SS

STRAIN

Figure 3-4

26

Elongation at Yield

Elongation at yield, the strain value atthe yield point, is a more convenientlimit than stress at yield if you know theparts strain levels. Much like stress atyield, elongation at yield determines theupper limit for applications that can tol-erate the small permanent deformationsthat occur before yield.

Tensile Stress at Break

Defined as the stress applied to thetensile bar at the time of fracture duringthe steady-deflection-rate tensile test,data for tensile stress at break estab-lishes upper limits for two types ofapplications: one-time-use applications,which normally fail because of frac-tures; and those parts that can stillfunction with the large deformationsthat occur beyond the elastic limit.

Elongation at Break

Most useful for one-time-use applica-tions that fail by fracture rather than bydeformation, elongation at break mea-sures the strain at fracture as a percent-age of elongation. Brittle materialsbreak at low strain levels; ductile andelastic materials attain high strain levelsbefore breaking.

Ultimate Strength

Ultimate strength measures the higheststress value during the tensile test. Thisvalue should be used in general strengthcomparisons, rather than in actual cal-culations. Ultimate strength is usuallythe stress level at the breaking point inbrittle materials. For ductile materials, it is often the value at yield or a valueslightly before the breaking point (seepoint E in figure 3-4).

Ultimate Elongation

Listed in place of elongation at break,ultimate elongation is often shown for highly elastic resins, such as elasto-meric polyurethanes, some of which can stretch over 500% before failing.The test for ultimate elongation usesnarrower test bars and faster deflectionspeeds, typically 500 mm/min, than theelongation-at-break test.

Flexural test set-up and stress distribution in specimen under load.

Figure 3-5

Neutral Axis

Compressive

TensileOuter Fiber Stress

h

h

Test SpanF

Poissons Ratio

Parts subjected to tensile or compres-sive stresses deform in two directions:with the load and perpendicular to it.This physical characteristic is easy tovisualize with a rubber band. As youstretch the band, its cross sectionbecomes narrower. Poissons ratiomeasures the ratio of lateral tolongitudinal strains.

Poissons ratio usually falls between0.35 and 0.42 for engineering resins.Some rubbery materials have ratiosapproaching the constant-volume valueof 0.50. For many structural analysisequations, Poissons ratio is a requiredconstant. A Poissons ratio of 0.38 for polycarbonate and polycarbonateblends, or 0.40 for nylons and rigid polyurethanes, generally givessatisfactory results.

Flexural Properties

Flexural properties relate to a plasticsability to bend or resist bending underload. In the tests for most flexural prop-erties (ASTM D 790 and ISO 178), atest bar placed across two supports isdeflected in the middle at a constantrate, usually 2 mm/min for glass-rein-forced materials and 20 mm/min forunfilled plastics (see figure 3-5). Youcan use standard beam equations to con-vert the force-versus-deflection datainto an outer-fiber, stress-versus-straincurve.

27

Flexural Modulus

Defined as the ratio of stress to strain in the elastic region of a stress-straincurve, flexural modulus measures a

resins stiffness during bending. A testbar subjected to the bending loads dis-tributes tensile and compressive stressesthrough its thickness. Because stressvaries through the cross section, theflexural modulus is based upon theouter fiber stress, whereas tensile modulus is based upon a stress which is constant through the cross section.

Test values for tensile modulus typical-ly correlate well with those of the flex-ural modulus in solid plastics, but differgreatly for foamed plastics that formsolid skins. Foamed materials gain stiff-ness because of their sandwich structureof a foamed core between plastic skins.

Although flexural modulus is moreapplicable for simple bending calcula-tions, tensile modulus usually can besubstituted when flexural data isunavailable.

Ultimate Flexural Stress

The ultimate flexural stress, takendirectly from the stress-strain curve,measures the level after which severedeformation or failure will occur. Forbrittle materials, it is usually the stressvalue at break. In ductile materials, theultimate flexural stress value usuallycorresponds to the yield point, or thepoint at which additional deflectiondoes not cause increasing stress.Because this stress level is beyond theresins elastic limit, some permanentdeformation is likely.

Chapter 3MECHANICAL PROPERTIES continued

The Ross Flexing Machine tests a pierced specimen bending freely through a 90 angle.

Figure 3-6

90

PiercedSection

28

A resins resistance to bending, or ulti-mate flexural strength, cannot always bedetermined using the flexural test,because many resins do not yield orbreak in bending. For these materials,Bayers data sheets list flexural stress ata stated strain, often 5%.

Cut-Growth Resistance

Used in the shoe-sole industry, cut-growth resistance, a cold flex test, deter-mines hole-propagation characteristics inpolyurethane materials. In the standardtest (ASTM D 1052), a 1/4-inch (6.4 mm)- or 1/2-inch (12.7 mm)-thick specimen with a small hole in its centeris placed in a Ross Flexing Machine (seefigure 3-6). The specimen is flexed untilthe hole develops cracks that split thesample. To test at temperatures otherthan room temperature, the specimen is

conditioned for a minimum of 30 min-utes after reaching the specified tempera-ture and before starting the test.

Compressive Properties

How a resin responds to compressionmay also be important in some applica-tions. Compressive properties includemodulus of elasticity, yield stress,deformation beyond yield point andcompressive strength: important consid-erations to part designers, particularlythose planning to use RIM polyurethanematerials in structural applications.Typically, these tests help to determinea materials hardness and load capabili-ties. Specific compressive properties arediscussed in this section, along withstandard testing procedures to deter-mine compressive property values.

In the standard tests for compressiveproperties (ASTM D 695 or ISO 604),a specimen is compressed at a constantstrain rate between two parallel platensuntil it ruptures or deforms by a certainpercentage (see figure 3-7). Becausethermoplastic parts rarely fail in com-pression, this data is of limited use inpart design for thermoplastics.

Compressive properties for rigidfoamed materials used in non-structuralapplications are tested to ASTM D1621 (ISO 2799). In this test, specimensizes range from 4 square inches to 36 square inches with a minimum thick-ness of 1 inch (25.4 mm) (see figure3-8). The entire loading surface receives

a uniform load from a crosshead motionat a rate of 0.1 in/min for every inch ofspecimen thickness. This test measuresthe force at yield point and at predeter-mined deflections (e.g. 10%).

For flexible foamed material, ASTM D 3574 (ISO 3386-1) measures theforce necessary to deflect a specimen to25% of its original thickness. After thespecimen is deflected for one minute,the load is recorded. Then, deflectioncontinues to 65% of the specimensoriginal thickness and holds for oneminute for a second load reading. Ifusing a semi-flexible foam, use ASTMD 5308, which measures the force nec-essary to compress a specimen 50%.Compression tests for elastomeric mate-rial are covered under ASTM D 575.

Compressive Strength

Useful for load-bearing applications,compressive strength testing measuresthe maximum compressive stressrecorded during testing. Data fromASTM D 695 or ISO 604 also can be used to calculate compressive modulus, the ratio of stress to strainbelow the proportional limit.

Compression Tester Figure 3-7

HardenedBall

HardenedBlock

Testing Machine Head

Testing Machine Head

TestSpecimen

Compressive Set

Both thermoplastic and polyurethaneelastomers subjected to long-term com-pressive loads may deform permanent-ly, a condition called compressive set.Because compressive set increases dra-matically as part temperature rises, testdata cannot be extrapolated to highertemperatures. Materials with lowercompressive-set values have less per-manent deformation when exposed tocompressive loads. Compressive-setdata, intended for comparison purposesonly, should not be used in structuralcalculations.

The most common test for compressiveset is ASTM D 395 method B (ISO815). In the test, a 1/2-inch-thick stackof 1-inch-diameter samples are com-pressed to a thickness of 1/8 inch for aspecified period of time at a predeter-mined temperature. Thirty minutes afterthe sample stack is released, its thick-ness is re-measured. The percentage ofcompression remaining is the compres-sive set. Mathematically, compressiveset is defined as the difference betweenthe beginning and ending thicknessesdivided by 1/8 inch, recorded as a per-centage. For all flexible foamed materi-als, ASTM D 3574 (ISO 1856) is used.

29

Shear Strength

Shear strength measures the shearingforce required to make holes or tears invarious specimens. Also useful in struc-tural calculations for parts that may failin shear, this data should be used cau-tiously, as testing does not account forstress concentrators and molded-in stresses.

In the shear strength test (ASTM D 732), a punch tool is pressed at afixed speed into a standard-sized discmounted on the testing device. Shearstrength, the force needed to make thehole, divided by the sheared area ismeasured in units of force per area.

Tear Strength

Tear resistance, the force needed to ripa specimen divided by the specimenthickness, provides good data for comparing the relative tear strength of elastomers.

A test procedure (ASTM D 624) mea-sures the tear resistance of thermoplas-tics. In one test, a V-shaped nick 0.50mm deep is made in a die-cut specimenof specific shape and size. The tabs ofthe specimen are then clamped into thetesting device, which separates at a rateof 500 mm/min until the specimen tears.There are also several variations forsample preparation.

For microcellular polyurethane materi-als, the standard test (ASTM D 3489)uses a 1/8-inch specimen with moldedskin. For the split tear strength, thedirection of tear must include skin on top and bottom surfaces (ASTM D 1938).

For flexible foamed materials, a block-shaped specimen is clamped betweenjaws, which move apart at a speed of0.75 to 0.9 mm/sec (ASTM 3574).Semiflexible foam is tested to ASTM D 5308, and elastomeric material is testedto either ASTM D 624 or ASTM D 1938.

Impact Properties

Important in a variety of applications,impact properties, particularly impactstrength, will help you select the propermaterial. Impact strength, a plasticparts ability to absorb and dissipateenergy, varies with its shape, thicknessand temperature. While impact proper-ties can be critical in some applications,test results are among the most difficultto relate to actual part performance.Variables such as part geometry, tem-perature, stress concentration points,molding stresses, and impact speedreduce the accuracy of general impactdata for quantitative calculations. Thecomplex and dynamic nature of resinperformance during impact has led tothe development of a variety of teststhat more closely represent different in-use conditions. The most common ofthese tests are described in this section.

Chapter 3MECHANICAL PROPERTIES continued

30

In one of the most widely used tests, the Izod impact test (ASTM D 256, D 4812, or ISO 180), a pendulum armswings from a specified height and hitsa cantilevered piece of test material,causing the piece to break (see figure 3-8). The arm then continues travelingat a lower speed, because of the energy

lost on impact. This loss of energy, cal-culated from the difference in beginningand ending heights, determines the Izodimpact strength, measured in ft-lb/in, orJ/m. Samples may be notched on thenarrow face, with the notch facing theimpact side as specified in the test.Results should note whether the samplewas notched and list sample thicknessand test temperature.

A second, less common method of mea-suring impact strength, the Charpyimpact test (ISO 179), differs fromIzod impact in the way a specimen issupported and oriented in the test device(see figure 3-8). Instead of being can-tilevered, Charpy samples are supportedat both ends, with the notch facing awayfrom the impact side. Charpy testingmeasures impact strength in kJ/m2.Charpy and Izod test results generallycorrelate well with the behavior of solidplastics; however, unnotched Charpytest results are typically more useful forfoamed plastics with solid skins.

Sample thickness and notch radiusaffect the results of both tests. In fact,beyond a certain thickness, known asthe critical thickness, further thicknessincreases can reduce impact strength insome materials. This phenomenon isapparent in impact-strength-versus-thickness curves at various temperaturesin polycarbonate resins (see figure 3-9).A sharp notch radius also reducesimpact strength. For example, testsshow that a polycarbonate resin speci-men with a 0.005-inch notch radius hasless than one-quarter of the Izod impactstrength as compared to a specimenwith a notch radius of 0.010 inch (seefigure 3-10). Avoid sharp corners in allapplications regardless of polymer,especially those involving high loads.

Beam Simply Supported

Charpy

Izod and Charpy impact tests.

Figure 3-8

Beam Cantilevered

Impact

Izod

Test Bar

Pendulum Impact Tester

Impact Point

Impact

Clamp

While neither of these tests providesimpact performance data for a particularpart or geometry, both are valuable forgeneral material preselection and comparison, as well as providing goodindications of a given plastics notchsensitivity. Additionally, impactstrength and tensile modulus proper-ties provide insight into the plasticsbasic mechanical nature.

Generally, high impact strengthcoupled with large tensile modulussuggests a tough material;

31

High impact strength and smalltensile modulus indicate a ductile,flexible material;

Low impact and large tensilemodulus typify a brittle material.

Tensile impact tests (ASTM D 1822 orISO 8256) measure a plastics ability toabsorb impact energy when notcheffects are not a concern. This test iswell-suited for evaluating impactperformance of thin sheets, films, softmaterials, and other plastics which can-not be easily tested via other methods(see figure 3-11). In the test, a sample ismounted on a pendulum at one end anda crosshead clamp at the other. At thebottom of the pendulum swing, theclamp impacts fixed anvils, transferringlarge tensile stresses to the test bar,causing it to fracture. The results arerecorded as the energy required to breakthe test piece, divided by the cross-sec-tional area of the necked-down region.

Chapter 3MECHANICAL PROPERTIES continued

Figure 3-9

IZO

D IM

PACT

STR

ENG

TH (ft

-lb/in

)

THICKNESS (in)

Izod impact strength of Makrolon polycarbonate vs. thickness at various temperatures.

140F (60C)

20

18

16

14

12

10

8

6

4

2

0.100 .140 .180 .220 .260 .300 .340

73F (23C)

-4F (-20C)

Critical Thickness

R = 0.010 in R = 0.005 in

16 to 18ft-lb/in

2 to 4ft-lb/in

Figure 3-10 Effect of Notch Radius on the Izod Impact Strength of Polycarbonate

32

Two other impact tests help to deter-mine relative puncture-impact strength.In the falling dart impact test, alsoknown as Gardner impact (ASTM D 3029), a weighted puncturing devicewith a standard tip diameter usually5/8 inch drops onto a supported sam-ple disc from increasing heights untilthe impact causes a rupture or cracking.Typically measured in foot-pounds, thefalling dart impact strength is the dropenergy of the average height causingrupture. The instrumented impact test(ASTM D 3763) gives more detailedinformation than the falling dart test. Inthis test, a high-speed dart with a round-ed tip usually 0.5 inch in diameter impacts a sample disc. Unlike thefalling dart impact test, the dart velocityremains constant throughout impact. At impact, a device measures the maxi-mum force transmitted, the energytransmitted, the deflection at maximumforce, and the type of fracture. Dartvelocity, test temperature, sample thick-ness, and clamp distance are usuallylisted with test results.

If your application has stress concentra-tors in anticipated impact areas, do notuse either of the test values describedabove for material comparisons. Mostsuitable for comparing a plastics rela-tive puncture-impact strength in appli-cations without sharp corners, notches,or other stress concentrators, these testvalues vary greatly with temperature,impact speed, and dart shape. Extremelyvaluable in applications that cannot tol-erate brittle failure, these tests help todetermine whether specific materialsfail in brittle or ductile mode.

Hardness Properties

The hardness properties of plastics,mainly used to compare indentationresistance, may not correlate to thematerials actual abrasion, scratch, orwear resistance. The two most commontests for comparing relative hardness aredescribed in this section.

The Rockwell hardness test (ASTM D 785 or ISO 2039-2) applies loads toan indentor, which presses against astandard-sized plastic specimen (seefigure 3-12). After the minimum loadrequired to indent the sample has beenestablished, the load is increased to ahigher value for a short time and thenreturned to the starting value. Theincrease in impression depth determinesthe Rockwell hardness. Smaller impres-sion depths correspond to greater hard-ness and higher Rockwell values.Hardness values are always listedaccording to the appropriate Rockwellhardness scale. For most engineeredplastics, either a Rockwell R or moresevere M scale is used.

Better suited for testing hardness insofter materials such as polyurethaneelastomers, the indentation hardness orDurometer test (ASTM D 2240 or ISO868) uses a pointed indentor projectingfrom a pressure foot to measure hard-ness. A specially calibrated dial indica-tor registers hardness based upon theindentors depth of penetration whenpressed into the sample until the footbase rests upon the specimen surface.Recorded on a unitless scale from 0 to100, hardness values typically appearon a Shore A scale for soft plasticsand a Shore D scale for hard plastics,with higher values within a scalecorresponding to greater hardness.

Several test methods are used forfoamed polyurethane materials. ASTMD 3489 (ISO 868) measures hardnesson a 1/4-inch (6.35 mm)-thick specimensimilarly to the Rockwell procedure

Tensile impact test (ASTM D 1822).

Figure 3-11

Test Specimen

Anvil

ImpactStop

PendulumArm

described above. Elastomeric and rigidmaterials are also tested under ASTMD 2240 (ISO 868). Test ASTM D 3574measures the force needed to produce25% and 65% indentations in foamproducts. This test uses specimens nolarger than 15 inches (380 mm) squarewith a thickness of 0.8 inches (20 mm).During the test, a flat, circular indentorfoot penetrates the specimen at a speedof 0.4 to 6.3 mm/sec (0.017 to 0.25in/sec), with results showing the forceneeded to produce the indentations.

33

If the part will be exposed to subnormaltemperatures, place the test specimenand equipment in a cold box at theexpected exposure temperatures.Testing procedures are the same as forother plastics (ASTM D 2440). The ini-tial (one-second) and five-second driftvalues the time delays after initialindentation are reported.

Figure 3-13 shows an approximate, relative comparison of hardness valuesfrom several common hardness testsand scales.

Chapter 3MECHANICAL PROPERTIES continued

Schematic of Rockwell hardness test.

Figure 3-12

0

50

75 25

SpecimenSteel Ball

Elevating Screw

Weight

Pivot

Approximate Correlation Between Various Hardness ScalesFigure 3-13

80

60

40

20

0ROCKWELL C

ROCKWELL B

110

100

80

6040200

12010080

60

40

20

140

ROCKWELL M

ROCKWELL R

90

80

604020

SHORE D

100

906030SHORE A160

10060

020

140

100

30

7050

90

BSHARDNESS BS

SOFTNESS

BARCOL

80

70

60

1,000

500

100

50

10

5

1BRINELL

HARDNESS NUMBER

120

100

80604020

34

Coefficient of Friction

The coefficient of friction is the ratio offriction force, the force needed to initi-ate sliding, to normal force, the forceperpendicular to the contact surfaces(see figure 3-14). Coefficients are com-monly listed for two types of friction:static friction, the forces acting on thesurfaces to resist initial movement, anddynamic or sliding friction, the forcesacting between surfaces moving relativeto each other.

Frictional property tests for plastics,such as ASTM D 1894 or ISO 8295,measure coefficients for combinationsof plastics and/or metals. Because of themultitude of combinations possible,finding data for specific types of plastics and/or metals can be difficult.Unless you are willing to test your specific material combination, you will

have to estimate frictional forces basedupon available data (see table 3-1).Frictional properties generally correlatewell with different grades of a particularplastic material. For applications inwhich the frictional force contributes asmall portion of the overall forces,approximate frictional data generallysuffices.

Published data on coefficients of fric-tion should be used for estimating pur-poses only. In addition to being verysensitive to speed, coefficient valuesdepend greatly upon the surface finishand the presence of lubricants andsurface contaminants. Because of thesefactors, generating a precise frictioncoefficient for design calculations canbe difficult.

Abrasion and Scratch Resistance

Important primarily for aesthetics anddurability, a variety of application-spe-cific tests typically measure abrasionand scratch resistance. The two most-common tests use a Taber abrader.Generally, a loss of volume or weightwhen a test piece is exposed to an abrasive surface under load determinesabrasion resistance.

An optical transmission/reflectance test,ASTM D 1044 measures the effect ofwear on a transparent thermoplastic resinto establish haze and luminous transmit-tance. Another standard test for scratchresistance moves a specimen under aloaded diamond point. The load dividedby the width of the resulting scratchgives the scratch-resistance value.

Table 3-1Coefficients of Friction (Static) Ranges for Various Materials

Material On Self On Steel

Frictional Force (FR)

The coefficient of friction is the ratio of the frictional force resisting sliding (FR) to force acting normal to the interface (FN).

Figure 3-14

Normal Force (FN)

Applied Force (P)

= FR FN

Miscellaneous Mechanical Properties

PTFE 0.10-0.25 0.10-0.25

PE rigid 0.40-0.50 0.20-0.25PP 0.35-0.45 0.25-0.35

POM 0.25-0.50 0.15-0.35PA 0.30-0.50 0.30-0.40

PBT 0.30-0.40 0.30-0.40

PS 0.45-0.60 0.40-0.50SAN 0.45-0.65 0.40-0.55PC 0.40-0.65 0.35-0.55PMMA 0.60-0.70 0.50-0.60

ABS 0.60-0.75 0.50-0.65PE flexible 0.65-0.75 0.55-0.60PVC 0.55-0.60 0.55-0.60

For polyurethane materials, ASTM D 3489 determines abrasion resistance.In this test, a technician abrades a speci-men using a 1000-gram load with a spe-cific grinding wheel. Results report themass loss in mg/1000 cycles.

LONG-TERM MECHANICAL PROPERTIES

Time and ambient temperature affectthe long-term mechanical properties ofplastics, because they affect polymer-chain mobility. Plastic parts under con-stant load tend to deform over time toredistribute and lower internal stresses.The mobility of polymer chains deter-mines the rate of this stress redistribu-tion. Higher temperatures increase thefree space between molecules, as wellas the molecular-vibration energies,resulting in a corresponding increase inpolymer-chain mobility. Even at moder-ate temperatures, polymer chains canreorient in response to applied loads, ifgiven enough time. Two long-termproperties creep, the added deforma-tion in a part that occurs over timeunder constant stress, and stress relax-ation, the reduction in stress in partssubjected to constant strain increasesignificantly with time and temperature.

35

Although their effects are similar, timeand temperature affect part performancedifferently. At different temperatures, agiven plastic shows immediate differ-ences in instantaneous or short-termmechanical properties. Time, however,does not significantly affect mechanicalproperties. Barring chemical or environ-mental attack, the material will have thesame strength and stiffness as it didbefore loading. Time affects the percep-tion of strength and stiffness: A partwhich has deformed after five years of

constant loading appears to have loststiffness, although, in fact, its stiffnesshas remained the same. Responding tothe load over time, individual polymerchains have moved to redistribute andlower stresses, causing the deformation.

Because long-term loading affects partperformance, most engineering plasticsare tested for long-term mechanicalproperties. This section discusses themost common of these tests.

Chapter 3MECHANICAL PROPERTIES continued

STRA

IN (

) (%)

TIME (hours)Creep and recovery of Makrolon polycarbonate at 73F (23C).

Figure 3-15

6,000 psi

5,000 psi

4,000 psi

3,000 psi

2,000 psi

Recovery

LoadRemoved

10-1 100 101 102 103 10-1 100 101 102 103 104

5

3

2

100

7

5

3

2

7

5

3

10-1

Creep

36

Creep Properties

Over time, parts subjected to a constantload often distort beyond their initialdeformation; they creep. Long-termcreep data helps designers estimate andadjust for this additional deformation. A common creep test involves hanginga weight axially on the end of a test barand monitoring increases in the barslength over time, as outlined in ASTMD 2990 or ISO 899. Flexural creep, amore common measure for structuralfoam materials, measures creep perfor-mance similarly to tensile creep, usingcantilevered test bars.

Presented graphically in a variety offorms, creep and recovery data is oftenplotted as strain versus time at variousstress levels throughout the creep andrecovery phases (see figure 3-15).Another popular form, the isochronousstress-strain curve, plots tensile stressversus resulting tensile strain at giventime increments (see figure 3-16).Occasionally creep data is presented asapparent modulus or creep modulus

versus time at various stress levels (seefigure 3-17). To determine the apparentmodulus, divide the stress by the actualstrain from an isochronous strain curveafter a specific load duration. For exam-ple, if we assume room-temperature con-ditions, a tensile stress of 2,800 psi (19MPa), and a load duration of 1,000 hoursusing a strain of 1.2%, we can calculatean apparent modulus of 220,000 psi(1,520 MPa) from the isochronousstress-strain curve in figure 3-16. Youcan also read the apparent modulusdirectly from the data in figure 3-17.

Figure 3-16

TEN

SILE

STR

ESS

()

STRAIN ( ) (%)Isochronous stress-strain curves at 73F (23C) for Makrolon polycarbonate.

2,800

10-11001011021031046x104

hour

s

Crazing

23C (73F)50% RH

7,000

6,000

5,000

4,000

3,000

2,000

1,000

50

40

30

20

10

1.20.5 1.0 1.5 2.0 2.5

MPapsi

3,750

5,200

0

MO

DULU

S (10

5 ps

i)

TIME (hours)Apparent modulus for unfilled Makrolon polycarbonate at various stress levels.

Figure 3-17

750 psi1,400 psi2,800 psi

4,200 psi

10 -2 10 -1 10 0 10 1 10 2 10 3 10 4

3.5

3.0

2.5

2.0

1.5

1.0

73F

Temperature affects creep properties.Compare figure 3-16, showing theisochronous stress-strain curve for aMakrolon PC resin at 73F (23C), andfigure 3-18, showing the same resin at176F (80C). In general, higher ambi-ent temperatures will cause more creepdeformation. See Bayers EngineeringThermoplastics: Part and Mold DesignGuide for more information on creep,test curves, apparent modulus, andeffects of temperature.

37

Stress Relaxation

Stress relaxation, the stress reductionthat occurs in parts subjected to constantstrain over time, is an important designconcern for parts that will be subjected tolong-term deflection. Because of stressrelaxation, press fits, spring fingers and similar parts can show a reducedretention or defl