1

1. Comparison of materials

Plastic is not metalComparison of materials – Many plastic designs still continue to be derived from“metal parts”. In the series commencing here the authors set out to describe the pointsthat require attention when designing in plastics rather than traditional materials.

Different basic material characteristicsThe properties of plastics materials can vary over a far wider range than all other engineeringmaterials. Through the addition of fillers/reinforcing materials and modifiers the property profile ofvirtually any base polymer can be radically altered. Most basic properties of plastics, however, forma marked contrast to those of metals. For example, in a direct comparison, metals have higher

– density– maximum service temperature– rigidity/strength– thermal conductivity and– electrical conductivity,

while the

– mechanical damping– thermal expansion– elongation at break and– toughness

of engineering thermoplastics are greater by orders of magnitude (see Fig. 1). To produce functionalparts in plastic and at the same time save costs, radical design modification is generally necessary ifthe plastic is being used to replace metal. This process affords an opportunity for complete redesignof the component with possible integration of functions and geometric simplification.

TOP TEN DESIGN TIPS – A SERIES OF 10 ARTICLESBy Jürgen Hasenauer, Dieter Küper, Jost E. Laumeyer and Ian Welsh

Strength/rigidity characteristics of differentmaterials (average values)

Tensile modulus (1000 N/mm2)1000100

100Yie

ld s

tres

s/te

nsi

le s

tren

gth

(N

/mm

2 )

200

300

400

101

PP

PA6

PA66 PPGR30

PA6GR30

PBTGR30

PA66GR30

ABSPS

POMPC

PETGR30

Magnesium

Aluminium

Zinc

Structural steel

Unreinforced thermoplastics

Reinforced thermoplastics

Metals

PBT

1. Comparison of materials2. Material selection3. Wall thickness4. Ribbing5. Gate positioning

6. Cost-saving designs7. General assembly technology8. Welding technology9. Tolerances

10. Check list

Fig. 1

2

Different material behaviourPlastics sometimes exhibit completely different behaviour to that of metals under the same serviceconditions. For this reason, a functionally efficient, economic design in cast metal can easily fail ifrepeated in plastic with excessive haste. Plastics designers must therefore be familiar with theproperties of this group of materials.

Temperature and time dependence of deformation characteristicsThe nearer the service temperature of a material is to its melting point, the more the material’sdeformation behaviour will be temperature- and time-dependent. Most plastics exhibit a change intheir basic mechanical properties at room temperature or on exposure to short-term stress. Metals,on the other hand, usually display largely unchanged mechanical behaviour right up to the vicinityof their recrystallization temperature (> 300 °C).If the service temperature or deformation rate is varied sufficiently, the deformation behaviour ofengineering thermoplastics can change from hard and brittle to rubbery-elastic. An airbag cover, for example, in its particular application involving explosive opening, exhibits completely differentdeformation behaviour from that of a slowly assembled snap-fit element made of the same material(Fig. 2). Similarly, this snap-fit element has to be assembled in a different way according to whetherthe temperature conditions are hot or cold. The effect of temperature here is significantly greaterthan the effect of loading rate.

TOP TEN DESIGN TIPSBy Jürgen Hasenauer, Dieter Küper, Jost E. Laumeyer and Ian Welsh

Effect of type of stress on deformation behaviour

kg

Static short-term stress

Stress duration 1 sec < x < 10 min

kg

kg

kg

kg

Static long-term stress(constant stress)

Static long-term stress(constant strain)

Type of stress Application example Calculation characteristicsEffects on

deformation behaviour

Loadability to basicstrength

Stress-strain graphUse of secant modulus

Increase in initialstrain over time(Creep)

Decrease in initialstress over time (Relaxation)

Creep strength graphUse of creep modulus

Creep strength graphUse of relaxation modulus

Wöhler curveAttention to stressing range(e.g. alternating tensile-compressive stress range/fluctuating tensile stress range)

Only very limited possibilityfor calculated estimation(practical trials necessary)

Significant reductionin endurable strainsand stresses

Rubbery elastic materialsdisplay tough to brittledeformation behaviour

Dynamic long-term stress

Sudden shock stress

Stress duration > 10 min

Stress duration > 10 min

Stress duration < 1 sec

Repeated increasing anddecreasing stress

Snap-fit hooks

Encapsulationof metal inserts

Pipes underinternal pressure

Bellows

Airbag cover

Fig. 2

3

Factors influencing component propertiesThe characteristics of plastics are not purely material properties. The basic property level of a plasticcomponent can be changed by various factors (e.g. UV radiation, see Fig. 3) right up to the point ofunserviceability. A well designed moulded part can easily fail if the material is processed underinappropriate conditions. Similarly, processors cannot generally eliminate moulding design faultsduring processing. Only through a process of optimization that takes into account all influencingfactors (Fig. 4) can a good plastics component be guaranteed.Since plastics are less tolerant of faulty design than metals, greater attention must be devoted tocorrect material design in designing plastics components. Every design process must therefore startwith a thorough and precise analysis of all requirements and boundary conditions.

TOP TEN DESIGN TIPSBy Jürgen Hasenauer, Dieter Küper, Jost E. Laumeyer and Ian Welsh

Material degradation as a resultof excessive UV radiation

Factors influencing component properties

Moulding geometrySharp edgesWall thicknesses

Serviceenvironment

TemperatureChemicals

UV radiationMoisture

StressDeformation rate

Stress durationType of stress

Force application

ProductionOrientation(molecules, fillers and reinforcingmaterials)Internal stressesDegree of crystallizationWeld linesAir occlusionsProcessing conditions (material degradation)

Fig. 3

Fig. 4

Publisher: Evelyne Schütz, DuPont Engineering Polymers,2, ch. du PavillonCH-1218 Le Grand-SaconnexGenevaSwitzerlandTel: +41 22 717 51 11Fax: +41 22 717 52 00

Editor: Andrew Wilkins

Layout & Johan Hedqvist, production: Geneva

Engineering Design is published in English,French, German, Italian, Spanish and Russian byDu Pont de Nemours International S.A., P.O.Box 50, CH-1218 Le Grand-Saconnex, Geneva,Switzerland.

The information set forth herein is furnishedfree of charge and is based on technical datathat DuPont believes to be reliable. It isintended for use by persons having technicalskill at their own discretion and risk. DuPontmakes no warranties, express or implied, andassumes no liability in connection with anyuse of this information.

©2007 E.I. du Pont de Nemours and Company

Printed in Switzerland

L-13906

www.plastics.dupont.com

EUROPE/MIDDLE EAST/AFRICA

Belgique / BelgiëDu Pont de Nemours (Belgium)Antoon Spinoystraat 6B-2800 MechelenTel. +32 15 44 14 11Telefax +32 15 44 14 09

BulgariaServiced byBiesterfeld Interowa GmbH & Co. KG.See under Österreich.

Ceská Republika aSlovenská RepublikaDu Pont CZ, s.r.o.Pekarska 14/268CZ-155 00 Praha 5 – JinoniceTel. +42 257 41 41 11Telefax +42 257 41 41 50-51

DanmarkDu Pont Danmark ApSSkjøtevej 26P.O. Box 3000DK-2770 KastrupTelefax +45 32 47 98 05Telefax +45 32 47 98 05

DeutschlandDu Pont de Nemours(Deutschland) GmbHDuPont Straße 1D-61343 Bad HomburgTel. +49 6172 87 0Telefax +49 6172 87 27 01

EgyptDu Pont Products S.A.Bldg no. 6, Land #7, Block 1New MaadiET-CairoTel. +202 754 65 80Telefax +202 516 87 81

EspañaDu Pont Ibérica S.A.Edificio L’IllaAvda. Diagonal 561E-08029 BarcelonaTel. +34 227 60 00Telefax +34 227 62 00

FranceDuPont de Nemours (France) SASDéfense Plaza 23/25 rue Delarivière Le Foullon Défense 9 92 064 La Défense Cedex Phone: +33 (0)1 41 97 44 00 Telefax +33 1 47 53 09 67

HellasBiesterfeld Hellas Intralink S.A.Trading Establishment149, AG. Triados Menidi AcharnesGR-13671 AthensTel. +30 210 24 02 900Telefax+30 210 24 02 141

IsraëlGadot Chemical Terminals (1985) Ltd.16 Habonim StreetNetanya – South Ind. ZoneIL-42504 NetanyaTel. +972 3 526 42 41Telefax +972 3 528 27 17

ItaliaDu Pont de Nemours Italiana S.r.L.Centro Direzionale “Villa Fiorita” Via Piero Gobetti, 2/A 20063 Cernusco s/N (MI) Tel. +39 02 92629.1 (switchboard) Fax +39 02 36049379

MagyarországDuPont Magyarország Kft.HU - 2040 BudaörsNeuman J.u. 1Tel. +36 23 509 400Telefax: +36 23 509 432

MarocDeborel Maroc S.A.40, boulevard d’Anfa – 10°MA-CasablancaTel. +212 227 48 75Telefax +212 226 54 34

Norway / NorgeDistrupol NordicOstenssjoveien 36N-0677 OlsoTel. +47 23 16 80 62Telefax +47 23 16 80 62

ÖsterreichBiesterfeld Interowa GmbH & Co. KGBräuhausgasse 3-5P.O. Box 19AT-1051 WienTel. +43 1 512 35 71-0Fax +43 1 512 35 71-31e-mail: info@interowa.atinternet: www.interowa.at

PolskaDu Pont Poland Sp. z o.o.ul. Powazkowska 44CPL-01-797 WarsawTel. +48 22 320 0900Telefax +48 22 320 0910

PortugalBiesterfeld Iberica S.L.Rua das MatasP-4445-135 AlfenaTel. +351 229 698 760Telefax +351 229 698 769

RomaniaServiced byBiesterfeld Interowa GmbH & Co. KG.See under Österreich.

RussiaDuPont Russia LLC. ul. Krylatskaya 17/3 121614 Moscow Tel. +7 495 797 22 00 Fax. + 7 495 797 22 01

Schweiz / Suisse / SvizzeraBiesterfeld Plastic Suisse GmbHDufourstrasse 21Postfach 14695CH-4010 BaselTel. +41 61 201 31 50Telefax +41 61 201 31 69

SlovenijaServiced byBiesterfeld Interowa GmbH & Co. KG.See under Österreich.

Suomi / FinlandDu Pont Suomi Oy PO Box 54 (Keilaranta 12)FI-02150 ESPOOTel: +358 207 890500 Fax: +358 207 890501

SverigeServiced byDu Pont Danmark ApS.See under Danmark.

TürkiyeDu Pont Products S.A.Buyukdere Caddesi No. 122Ozsezen Ismerkezi, A block, Kat: 3Esentepe, 34394 IstanbulTel. +90 212 340 0400Telefax +90 212 340 0430

UkraineDu Pont de NemoursInternational S.A.Representative Office3, Glazunova StreetKyiv 252042Tel. +380 44 294 96 33 / 269 13 02Telefax +380 44 269 11 81

United KingdomDu Pont (UK) Limited Wedgwood Way Stevenage Hertfordshire SG1 4QN Tel. +44 1438 734000Telefax ++44 1438 734109

South AfricaDuPont de Nemours Societe AnonymeSouth African Branch Office4th Floor Outspan House 1006 Lenchen Avenue North Centurion Pretoria 0046 Tel. +27 0 12 683 5600 Telefax +27 0 12 683 5661

NORTH AMERICA

USADuPont Engineering Polymers Barley Mill Plaza, Building 26 P. O. Box 800026 Wilmington, Delaware 19880 Tel. +1 302 992 4592 Telefax +1 302 992-6713

DuPont Automotive950 Stephenson HighwayP.O. Box 7013Troy, MI 48007-7013Tel. +1 248 583-8000

CanadaDuPont Engineering PolymersP.O. Box 2200Streetsville, MississaugaOntario, Canada L5M 2H3Tel. +1 905 821-5953

MexicoDuPont S.A. de C.V.Homero 206Col. Chapultepec Morales11570 Mexico D.F.Tel. + 525 557 221 000

SOUTH AMERICA

ArgentinaDu Pont Argentina S.A.Avda. Mitre y Calle 5(1884) Berazategui-Bs.As.Tel. +54 11 4239-3868Telefax +54 11 4239-3817

BrasilDuPont do Brasil S.A. Al. Itapecuru, 506 Alphaville 06454-080 Barueri-Sao Paulo Tel. + 5511 7266 8229

ASIA-PACIFIC

AustraliaDuPont (Australia) Ltd.168 Walker StreetNorth Sydney NSW 2060Tel: +612 9923-6111Fax: +612 9923 6011

Hong Kong/ChinaDuPont China Ltd.26/F, Tower 6, The Gateway,9 Canton RoadTsimshatsui, Kowloon, Hong KongTel: +852 2734 5345Fax: +852 2724 4458

Shanghai/ChinaDuPont China Holding Co. Ltd.15/F., Shui On Plaza333 Huai Hai Road (Central)Shanghai 200021Tel: +86 21 6386 6366Fax: +86 21 6386 6333

IndiaE.I. DuPont India Limited,"Arihant Nitco Park" Sixth floor,90, Dr. Radhakrishnan Salai,Mylapore,Chennai 600 004Tel: +91 44 28472800Fax: +91 44 28473800

JapanDuPont Kabushiki KaishaSanno Park Tower, 11-1Nagata-cho 2-chomeChiyoda-ku, Tokyo 100-6111Japan.Tel: +81 3 5521 8500Fax: +81 3 5521 2595

KoreaDuPont (Korea) Ltd.4/5 Floor, Asia Tower#726, Yeoksam-dong, Kangnam-KuSeoul 135-082Tel: +822 2222-5200Fax: +822 2222-5470

SingaporeDu Pont Company (S) Pte Ltd1 HarbourFront Place #11-01HarbourFront Tower OneSingapore 098633Tel: +65 6586 3688Fax: +65 6272 7494

TaiwanDuPont Taiwan Ltd.Hung Kuo Building, 13th floor#167 Tun Hwa North RoadTaipei 105Tel: +8862 2719-1999Fax: +8862 2719-0852

ThailandDuPont (Thailand) Limited6-7th Floor, M. Thai TowerAll Seasons Place87 Wireless RoadLumpini, PhatumwanBangkok 10330Tel : +66 2 659 4000Fax : +66 2 659 4001

To get Engineering Design in an e-format, go to

http://www.engineeringpolymers.com/engdesign/

The DuPont Oval Logo, DuPont™, Themiracles of science™, and Crastin®, Delrin®,DuPont™ ETPV, Hytrel®, Minlon®, Rynite®,Thermx®, Tynex®, Vespel®, Zytel®, Zenite® areregistered trademarks or trademarks ofDuPont or its affiliates.

Crastin® pbtthermoplastic polyester resin

Delrin®

acetal resin

Hytrel®

thermoplastic polyester elastomer

Minlon®

mineral reinforced nylon resin

Rynite® petthermoplastic polyester resin

Thermx® pcthigh performance polyester

Tynex®

nylon monofilament

Vespel®

parts and shapes

Zytel®

nylon resin

Zytel® htnhigh performance polyamide

Zenite® lcpliquid crystal polymer

DuPont™ etpvthermoplastic rubber that resists oil & heat