design for manufacture

1423

CHAPTER 46DESIGN FOR MANUFACTURE ANDASSEMBLY WITH PLASTICS

James A. HarveyUnder the Bridge ConsultingCorvallis, Oregon

1 INTRODUCTION 1423

2 PLASTIC MATERIALSSELECTION 14242.1 Polymers 14242.2 Plastics 14292.3 Reinforced Plastics 1430

3 PLASTIC MATERIALSSELECTION TECHNIQUES 1430

4 PLASTIC JOINING TECHNIQUES 1431

5 PLASTIC PART DESIGN 1431

6 PLASTIC PART MATERIALSELECTION STRATEGY 1431

7 CONCLUSION 1435

SUGGESTED READING LIST 1435

REFERENCES 1436

1 INTRODUCTION

To write a chapter in a book such as the Handbook of Materials Selection youneed a good introduction or an opening that sets the pace of the information onewishes to present.

From the title we will only be dealing with plastic materials used for designedparts; however, some of the hints, tips, criteria, and suggestions may apply toother materials such as metals and ceramics.

I would have liked to start this chapter with an excellent illustration of aplastic part design. But the best I could come up with is a metal example andhopefully throughout this chapter I will refer to some plastic designs.

The example I would have liked to use is the long swords used by KingArthur and the Round Table. From a distance one can hardly imagine the breadthof one who uses such a long sword in battle and who swings this enormoussword at an enemy to chop off various body parts. The sword was hollow andpartially filled with mercury. The sword was relatively easy to manipulate in theso-called rest or nonbattle position. When the knight raised the sword, the mer-cury being a liquid flowed to the handle. As the sword was swung, the mercuryflowed to the tip of the sword, thus giving it extra force at the tip. In my opinion,this is a brilliant example of materials selection in the development of a product.Imagine having to lift the sword if it were solid.

Handbook of Materials Selection, Edited by Myer KutzISBN 0-471-35924-6 2002 John Wiley & Sons, Inc., New York

1424 DESIGN FOR MANUFACTURE AND ASSEMBLY WITH PLASTICS

This chapter will consider three main topics: plastic materials selection, plas-tics joining techniques, and plastic part design. The major focus will be onplastic materials selection. The information in this chapter will be based uponboth the lectures I gave and the information I received in short courses taughtto practicing engineers and scientists that were involved in all the aspects ofcommercial plastics part designs and in graduate school courses to budding newmaterials scientists and engineers.

If you search the open literature for material selection, you will find articleswith titles similar to The Science of Material Selection or the Art of MaterialSelection. Hopefully this chapter will eliminate some of the mystery or con-fusion of material selection.

2 PLASTIC MATERIALS SELECTION

2.1 Polymers

In the selection of plastic materials for a commercial part design the first stepis as in all technology development; that is, you must learn the basic definitions,concepts, and principles of that technology. Following will be a series of termsthis writer thinks are significant for assisting one in the selection of materialsfor plastics part design: thermoplastics, thermosets, elastomers, polymerizationreactions, characterization techniques, molecular weights and distribution, mo-lecular structure of polymers, five viscoelastic regions of polymers, Carothersequation, and additives.

The starting point is with the definition of a polymer. A polymer is a com-pound consisting of repeating structural units. A simple example of a repeat unitis theCH2chemical moiety. Two repeated units are equivalent to the organiccompound ethane. Ethane is a gas at room temperature with a total molecularweight of 30 atomic mass units (amu). A polymer family with hundreds ofthousands of these repeatCH2units represents the polyethylenes with mo-lecular weights in the millions.

A thermoplastic polymer is a polymer that consists of linear polymer chains.Whenever you use a thermoplastic, it is usually in its final molecular weightform. The major thermal event that one does is to process it into the final partform. There are three types of thermoplastics: amorphous, semicrystalline, andliquid-crystal polymers.

A thermosetting resin is one that contains a highly cross-linked polymer net-work when processed. One has to cook or cure the resin before it can beformed into its final shape.

An elastomer is a very lightly cross-linked polymer with the ability to beextend to a high elongation and snap back to its original dimensions when theforces has been removed.

Polymerization reactions play a usual role in the process of material selection.From the name of the polymer and its polymerization reaction, one can make areasonable first attempt to select a plastic material. But the reader must be cau-tious with statements as the last one. These are general rules. Let us look atpolyethylene as one example. Polyethylene is named from the monomer fromwhich it is made. These monomers are polymerized through an addition reaction.Being formed through an addition reaction, the resultant polymers are water

2 PLASTIC MATERIALS SELECTION 1425

hating, or hydrophobic. For the first approximation this would be a good materialto be put into a waterlike environment.

Now let us look at another polymer, polyethylene terephthalate. It is formedfrom the reaction of ethylene glycol with terephthalic acid or terephthalic acidester. During the reaction in order for the polymer, polyethylene terephthalate,to build up molecular weights, it loses either water or alcohol as a by-product.The polymers formed from the type of reaction of adding two or more co-reactants under conditions of time, temperature, and other reaction conditionswith the formation of a by-product such as water and alcohol are said to beformed by condensation and are named by the new chemical functional groupformed. As a general rule of thumb these polymers are water loving, or hydro-philic.

Polyesters as mentioned are formed from the reaction of organic acids withorganic alcohols with water as a by-product. Nylons are formed from the con-densation reaction of organic acids with organic amines with water as a by-reaction. Polyimides are formed from the condensation reaction between acidanhydrides and organic amines with the release of water.

Knowledge of organic chemistry plays a very important role in the selectionof a polymer for a plastic part. This writer is only posing general rules to thereader, and to the first approximation one can made very reasonable selectionin the early stages of plastic part design using these general observations.

For example, if you were assigned to design a plastic part that had to existin a water environment, your first choice could be an addition polymer suchas polyethylene rather than a condensation polymer such as polyethyleneterephthalate. The polyethylene is water hating, or hydrophobic. Thus it shouldnot be affected by water.

This writer can already imagine the but what about this incident remarks.Yes, water bottles are made from polyesters. The bottles are dated for lifetimeand the companies that fill these bottles with their clear spring mountain freshwater want you to see how pretty their water is. However, over time the polyesterbottles will absorbed water. Water (sport) bottles that are used over and overand filled by the consumer are made from the addition polymers. They are alsoopaque. Transparency in these sport water plastic containers is not important.This polymer character of being transparent or opaque will be discussed later.

Molecular weight and molecular weight distributions are other important par-ameters for a polymer. The polymerization reaction is complicated. The polym-erization reaction does not make a simple molecule. The reaction yields manydifferent sizes of polymer chains. The molecular mass of each chain refers toits molecular weight. And as mentioned, since many different sizes of polymerchains are formed, there will be a distribution of the molecular weights.

Knowing the molecular weight and its distribution aids in the selection ofpolymers for a plastics part and in the lifetime of plastic part. The generaltechniques that can be used to determine molecular weights of polymers areachieved through viscosity measurements either in solution or using solid sam-ples. Solution viscosities consist of timing the flow of polymer solutions ofknown concentrations through a fixed volume. Melt index, or melt flow index,is a standardized test in which the solid is used instead of a solution. A givenamount of the polymer is heated to a certain temperature and a known force is


applied to the molten polymer and its flow is timed. If all things are equal, thelower molecular polymer will flow through the given volume, the fastest thusrevealing a low number. For the members of a given polymer family this is areasonable way to distinguish between low- and high-molecular weight versions.The final technique is gel permeation (size exclusion) chromatography. The pol-ymer is dissolved in a solvent. The solution is then passed through a series oftubes (columns) packed with different porous particles. As the solution passesthrough, the polymer chains with the highest molecular weights pass throughthe fastest. A detector measures the polymer chains as they exit the instrument.Thus one ends up with a chromatograph, which shows the distribution of thedifferent molecular weights of the polymer chain in the sample. Solution vis-cosities are usually used by the polymer manufacturer. Melt flow index is usedas an initial tool for material selection and as a tool to help determine themolding process. Gel permeation (size exclusion) chromatography was in thepast treated as a research tool, but lately it has gained a great deal of popularityas a quality control technique.

Another important parameter for the different polymers refers to their thermalbehavior. A typical thermoplastic is a solid at ambient temperatures. As thematerial is heated it starts to soften, then it flows and in some cases it melts.Then when it is cools down it solidifies. And depending of the container (mold)in which this is done, the thermoplastic will retain the shape of that mold. Thisprocess should be repeatable. Thus thermoplastics are recyclable. Another ther-mal property of the thermoplastics is creep. This property refers to the abilityof the material to flow under a load as a function of temperature.

Thermosetting resin systems are quite different. When one processes a ther-moplastic into a particular plastic part, its molecular weight has already beenestablished at the manufacturer. With thermosetting resin systems, one starts withlow-molecular-weight reactants and to process these ingredients you cure orcook the reactants into the desired final shape. If the reactants have been fullyreacted, one ends up with one giant molecule. To process a thermosetting resinsystem into a part, the thermal events consist of heating the ingredients so theystart to soften; then some of the ingredients melt. As the temperature is raised,the system is totally liquid. As the temperature continues to rise, the onset ofcuring (cross-linking) occurs. As the reaction proceeds, the viscosity increasesand the part hardens. At the end of the curing reaction the part is solid; then itis cooled to ambient temperatures. Once formed the part cannot be reheated tochange its shape. If a thermosetting resin system has been properly cured, itshould not be affected by temperature or solvents.

The thermal behavior of elastomers is somewhat different that the thermo-plastics and thermosets. As a first approximation it behaves more like a ther-moplastic. We all know that car tires soften in the hot months of summer. Andmost elastomers will swell when placed into a solvent.

Thermal analyses are a set of techniques used to characterize the thermalbehavior of the different type of polymers. In addition to informing one as tothermal characteristics of the polymers, they can assist in determining a proc-essing cycle. Differential scanning calorimetry (DSC) yields the thermal eventsof a sample, i.e., melting points, onsets, maximums, and offsets of curing, de-composition temperatures, crystallization temperatures, and glass transition tem-peratures (this term will be discussed later). Thermogravimetric analyses (TGA)


give the changes in mass of a sample as a function of temperature and environ-ment. Thermal mechanical analyses (TMA) reveal the changes in volume of asample (warpage and shrinkage) and glass transition temperatures. Dynamic me-chanical analyses (DMA) provide the modulus and changes in modulus and glasstransition temperature as a function of temperature, time, and oscillation (dy-namic load).

The internal structure of the polymer will determine if it is transparent oropaque. This internal structure is referred to as polymer morphology. Thermo-plastic polymers can be subdivided into amorphous, semicrystalline, crystalline,and liquid-crystal polymers. This classification is only reserved for thermoplas-tics. Morphology refers to how the polymer chains are arranged, in an orderedor disordered manner. Amorphous refers to total disorder. Crystalline refers tototal ordered. Semicrystalline is a combination of disorder with domains of orderwithin its structure. The liquid-crystal polymers refer to a special class of ther-moplastics that retains its order in the melt. Based upon chemical principles asa material goes from the solid state to the liquid state, it goes from a state oforder to one of disorder. The liquid-crystal polymers lack this transition, andthis unique characteristic has an enhanced effect of the processing of these ma-terials.

Let us now examine the internal structure of amorphous, semicrystalline, andcrystalline thermoplastics. We have the two extremestotally disordered in thearrangement of the polymer chains (amorphous) and at the other end total order(crystalline). Another way of looking at the arrangements involved is to viewthe polymer chains as spaghetti. We have cooked spaghetti (disordered) at oneextreme and uncooked (ordered) spaghetti at the other extreme. Except for theliquid-crystalline polymers, most thermoplastic polymers are either amorphousor semicrystalline (a combination of polymer chains in ordered crystalline do-mains). Due to the presence of the crystalline domains, the semicrystalline pol-ymers have a melting point, and light will be scattered as it hits these domains,thus giving the material an opaque appearance. Thus amorphous polymers donot have a melting point and are transparent.

The next important polymer definition or concept involves the five viscoelasticregions of polymers. If we plot the modulus of a thermoplastic material as afunction of temperature, we obtain a graph such as the one shown in Fig. 1.

Region 1 represents the behavior of the material at low temperatures. It is inits glassy state. The mobility of the polymer chains has slowed down. The ma-terial is hard. As the material is heated, it comes through a transition to region3. This region is known as the rubbery region and has lost in the range of threeorders of magnitude of strength. As the sample is heated to an even highertemperature, the polymer (region 4) starts to decompose and finally at region 5decomposition occurs with lose of strength.

A semicrystalline thermoplastic has the appearance of the dashed line in Fig.1. The drop in modulus from the glassy region to the rubbery region is not asdrastic with the semicrystalline polymers (region 2) as it is with the amorphouspolymers. As the semicrystalline thermoplastic reaches its melting point, itsstrength falls off greatly and, as you would expect, goes from a solid to a liquid.

The transition between the glassy region of a polymer to its rubbery regionis known as its glass transition and the temperature that it occurs at is its glasstransition temperature.


Fig. 1 Five viscoelastic regions of a linear amorphous polymer. The dashed line represents thebehavior of a semicrystalline polymer (Ref. 2).

Fig. 2 Comparison of the glass transition temperature (a) to a melting point (b) of a thermo-plastic polymer (Ref. 2).

The glass transition is defined as the reversible change in an amorphousmaterial or in amorphous regions of a partially crystalline material, from (or to)a viscous or rubbery condition to (or from) a hard and relatively brittle one.1

Some individuals use the term glass transition temperature while discussingcured thermosetting resin systems. To this writer, if the thermosetting resin iscompletely cured, it should not have a glass transition temperature. If it is com-pletely cured as you heat the material over a temperature range, it should beunaffected by temperature until it reaches its decomposition temperature. If youperform an analysis and observe a glass transition temperature, it is either be-cause the thermoset is not being completely cured or because of the thermo-plastic nature of the cross-linked network. If you perform a thermal techniqueto determine glass transition temperature, cool the sample to ambient tempera-ture and repeat the analysis on the same sample. If the material is not fullycured, the repeat run should indicate a higher apparent glass transition temper-ature and a lower drop in modulus.

Figure 2 shows a comparison between the glass transition temperature andthe melting point of a thermoplastic polymer. The key feature of the graph shows


that as the material reaches its melting point that is a discontinuous in its volume.In the case of the glass transition temperature its rate of volume changes withtemperature. As one passes through the polymers glass transition, its thermo-dynamic properties change. Thus, in selection of a thermoplastic for a plasticpart, it is always best to use the material above its glass transition temperature.There are exceptions to this rule. For example, polymers like the polyethylenesare used in their rubbery region due to their subambient temperature glass tran-sition temperatures.

This writer is including the Carothers equation in the discussion of the selec-tion of polymeric materials for plastic molded parts. The equation is relativelysimple, but the impact of it on a thermoplastic polymerization is very critical:

2X n 2 pr

The term refers to number-average degree of polymerization, p is the extentXnof reaction, and r either indicates the ratio of reactants or purity of the reactants.Basically, what this equation tells us is that one needs a high conversion of purereactants and the correct stiochiometry to obtain the proper molecular weight.Small changes in purity, incomplete reaction, and incorrect ratio of reactants canhave a drastic effect on the moldability of your part or its performance behavior.Thus consistency and repeatability of the molded part you shipped to your cus-tomer is highly dependent upon the consistency and repeatability of the materialsfrom your polymer manufacturer, your compounder, and your molder.

2.2 Plastics

Plastics are simply polymers with additives. These additives perform many dif-ferent functions. Some refer to these materials as foo-foo dust. The additionof additives to the polymer enhances the process of the making of the part, theproduct performance and lifetime of the part, and the appearance of the part. Apartial list of these additives include antioxidants, light stabilizers, acid scav-engers, lubricants, polymer processing aids, antiblocking additives, slip addi-tives, antifogging additives, antistatic additives, antimicrobials, flame retardants,chemical blowing agents, cross-linking and controlled degradation of polyole-fins, colorants, fluorescent whitening agents, fillers, nucleating agents, and plas-ticizers.

From the name of the additive one can figure out the function of the additive.There are exceptions to this, and in most cases we do not know what the additivepackage compounders put into a polymer. This information is treated as confi-dential. Thus one must be careful when one switches material from one supplierto another. Even though the starting polymer may be same, different additivepackages will affect the performance behavior and lifetime of the designed plas-tic part. Thus once a material has been selected, this writer highly recommendsthat one completely characterize that material in case your supplier changes thematerial or its consistency changes or you are involved in determining productfailures. As a consultant, this writer has been involved in several failure analysesprojects and the worst thing that happens is that there is no baseline materialdata. These problems are difficult and expensive to solve without baseline data.


Two types of additives will be defined in this section due to their importance.These are fillers and plasticizers. Fillers are added to polymers to affect the colorand smoothness properties of the final molded part, to assist in the molding ofthe part by changing the flow behavior of the plastics, and lastly to reduce thecost of the molded part.

Plasticizers are unique materials. They are added to a polymer to reduce thehardness of the polymer or make it more flexible. A good example of a plasti-cizer is the smell we experience when we purchase a new car. The new carsmells comes from the plasticizer and the seats are nice and soft. Over time thesmell is gone, the seat becomes hard and brittle, and we have to clean the insideof the windshield. The reverse of this can occur with hydrophilic polymers inthe presence of water. The polymer absorbs water, its glass transition temperaturedrops, and the material becomes softer. In the case of hydrophobic thermoplasticmaterials oils will have a similar effect.

Thus, it should be noted that these plastic systems are dynamic and they arein a constant change. If you design a plastic part for a lifetime of 5 years, itwould be nice to test the part for 5 years under the operating conditions ofthe plastic part assembly. However, based upon time to market that is not anoption. One can retain samples of production.

2.3 Reinforced Plastics

Reinforced plastics are plastics containing reinforcing elements within a plasticmatrix. There are many different types and forms of reinforced plastics. Andwhenever one refers to reinforced plastics, they are referring to thermoplasticsreinforced with either long or short discontinued (chopped) fibers, and the partsare manufactured by injection molding. Reinforced plastics are in essence formsof composite materials. A composite is a heterogeneous mixture of matrix resin,reinforcement, and other components that act in concert with each other. Thematrix resin protects the reinforcement from itself due to wear and it gluesthe reinforcement in place. The reinforcement provides strength to the plasticpart and enhances the properties of the matrix resin. Also the reinforcementhelps dissipate the energy throughout the structure when impacted.

Properties of reinforced plastics should be obtained from the supplier. Thereare many variations of reinforcement forms and sizes.

3 PLASTIC MATERIALS SELECTION TECHNIQUES

It is difficult to discuss how one starts to select plastic materials for a particulardesign. At first the tenacity is to hope that you doing a second version of aprevious designed plastic part. This is wishful thinking. Second one hope thatone is exposed to an individual in their organization whom has experience inplastic material selection. Or, if you are really fortunate, you have a materialsengineer in your organization that knows the material science of plastic mate-rials.

This writers first experience in selecting materials was relatively simple. Thecriteria were basically set by the equipment available for the project. Then cri-teria such as the temperature, chemical requirements, and the number of partsneeded are used to pick the material. An eight-step criteria was then developed.The criteria were functionality, chemical resistance, external processing (yoursupplier), internal processing, lifetime, design margins, cost, and greenness of

6 PLASTIC PART MATERIAL SELECTION STRATEGY 1431

the part. As the demands on the materials increased, subcriteria to the criteriawere developed and added to the list and is represented by Table 1.

The above scheme works reasonably well. But it does not include the mostimportant factor in a material selection. That factor can be represented with suchwords or phrases like timing, schedule, and time to market.

4 PLASTIC JOINING TECHNIQUES

Table 2 contains a list of most of the different techniques used to join two plasticparts together. Each technique has its own unique advantages and limitations.This writer has more experience with adhesive bonding. In the selection ofadhesives one can follow most of the criteria and subcriteria as listed inTable 1.

In the adhesive bonding of plastic parts together there are several other issuesthat you should take into account. One refers to the failure mechanism that youwant to strive for in your design. There are three failure mechanisms for anadhesive joint. They are adhesive, cohesive, and substrate failures. An adhesivefailure is failure of adhesive joint at the interface. Cohesive failure is failurewithin the adhesive and finally substrate failure of the plastics. This is failurewithin the plastic parts being joined together. Failure within the plastic partswould be the best case. And with surface preparation techniques, especially withtreatments involving silane coupling agents, this failure mode is not unrealisticas observed by this writer.

The golden rule of design using adhesives was (and is) to design to at leastcohesive failure.

One chemical principle that you should take into account in the selection ofadhesives is like dissolves like. It translates into the more similar the adhesiveis to the plastic you are trying to bond together the stronger the bond.

5 PLASTIC PART DESIGN

In this section, I have to be honest and admit that I do not know how to designplastic parts. I cannot draw a straight line with a ruler. I rely on the suppliersdesign guides and the two design books (Malloy and Dym) listed in the Sug-gested Readings. And I am also fortunate to have a good network that is onlya phone call away. However, I do perform failure analyses on molded plasticparts, and there are tricks I employed during an analysis. One technique is toperform an ashing of a plastic molded part. One can obtain information as tothe flow of the plastic and fiber orientation if the part is reinforced.

One can also section the molded reinforced plastics into smaller specimensthat can be analyzed by TGA to determine percent resin and reinforcementcontents; this in turn will show you the consistency of the molded part.

Sometimes if the plastic has a long distance to flow in a mold, the polymerchains can separate. The smaller polymer chains travel faster than the largerchains. To verify the consistency of the polymer molecular weight throughoutthe molded plastic part, one can section the part and subject the specimen to gelpermeation chromatography.

6 PLASTIC PART MATERIAL SELECTION STRATEGY

After reading the first part of this chapter one may be either totally confused orhave a headache. This writer understands perfectly. Materials selection is not an


Table 1 Eight-Step Criteria for the Selection of Materials (Thermoplastics, Thermosets,Elastomers, and Adhesives)

Main Criteria Subcriteria

Functionality Purpose of partType and magnitude of normal service stressesLoading pattern and time under loadFatigue resistanceOverloads and abuseImpact resistanceNormal range of operating temperaturesMaximum and minimum service temperaturesElectrical resistivityDielectric lossAntistatic propertiesTracking resistanceFlammabilitySurface finishColor matching and color retentionTolerances and dimensional stabilityWeight factorsSpace limitationsAllowable deflections

Materials acceptance Compatibility with chemicalsSolvent and vapor attackReactions with acids, bases, water, etc.Water absorption effectsUltraviolet light exposure and weatheringOxidationChemical erosion and /or corrosion (electrochemical effects)Attack by fungi, bacteria, or insectsLeaching of additives from the part material into its environmentAbsorption of components into the part from its environmentPermeability of vapors and gasesNormal range of operating temperaturesMaximum and minimum service temperatures

Environmental concerns Scrap ratesRecyclabilityChloro- and fluoro- polymers

Lifetime Product lifetimeReliabilityProduct specificationsAcceptance codes and specifications

Margin Safety design factorsInternal process Normal range of processing temperatures

Maximum and minimum processing temperaturesChoice of processesMethod of assemblySecondary processesFinishing and decoratingQuality control and inspectionContamination

External process (supplierof parts)

Normal range of processing temperaturesMaximum and minimum processing temperaturesChoice of processesMethod of assemblySecondary processesFinishing and decoratingQuality control and inspectionContaminationTiming for part design changesTiming for prototype moldsTiming for production molds

6 PLASTIC PART MATERIAL SELECTION STRATEGY 1433

Table 1 (Continued )

Main Criteria Subcriteria

External process (supplierof parts) (Continued )

Technical support from supplierContamination

Cost Materials costsMaterials availabilityAlternative material choicesSuppliers availabilityPart costsCost of capital plant: molds and processing machinesOperation costs of component including manufacturing and fuel con-

sumptionCapacity

Table 2 Plastic Joining Techniques

Adhesive bondingElectrofusion bondingFriction welding

LinearRotational

Heated tool weldingHot plateHot shoe

High-frequency weldingHot gas weldingInduction weldingInfrared weldingLaser welding

Mechanical fasteningBeadingHot stakesInterface fitsMolded-in and ultrasonic insertsMolded-in threads

RivetingSelf-threading screwsSnap fit

Solvent joiningThermal impulse weldingUltrasonic welding

easy task. Following represents a series of suggestions and hints that hopefullymake the task easier.

First, if you are fortunate to be part of a large organization, develop a teamof individuals that you think may help you. This writer has been in favor of atleast a four-person team. The team should consist of a design engineer, a ma-terials type (analytical chemistry people in ones organization may be a goodsubstitute if you have no true materials engineer), an internal experience engi-neer, and a representative from the procurement department or an internal com-pany buyer.

Some of the selections of the team are obvious. Each can handle part of thecriteria and subcriteria as listed in Table 1 or any other type of list of require-ments that you develop.

The first step is probably the most difficult. That is the selection of the pol-ymer families to evaluate. Initially, some consult the Modern Plastic Encyclo-pedia. I am partial to Domininghauss book on Plastics for Engineers forthe selection of a thermoplastic. The Modern Plastic Encyclopedia is an excel-lent and well-respected source book. However, it only contains one data pointwithin the total history of a particular thermoplastic. Domininghauss providespressurevolumetemperature (PVT) graphs on the various thermoplasticfamilies. That type of data is significant in the processing of materials. Thesepublications are listed in the Suggested Readings.


Now the next action is to obtain samples and technical information from thesuppliers. As previously mentioned, most suppliers have design guides for theirpolymers. These are an excellent source of information that can be helpful inyour efforts to design the plastic part. Also, obtain from the polymer supplierany analytical procedures as to how they characterize their materials. This willassist your internal analytical people to develop a material knowledge database.

Next, you and your team should review all the available data on the polymerunder consideration. A good literature search through a technical library maysave you time, effort, and money. In addition, you may want to perform yourown tests to fill in missing information. The polymer supplier can be helpful inthis area. The supplier can provide molded, American Society for Testing andMaterials (ASTM) test coupons that you can use for your own testing. If youare designing a plastic part to be in a certain chemical environment, you maywish to test your selection in a chemical soak-type test. This can be achievedby soaking a test coupon in the chemical of concern. You may also want to soakthe test coupon at different temperatures within the operating range of the de-signed plastic part or within a linear range of behavior of the polymer. If youcan perform such a chemical soak at three different temperatures, you can predictthe lifetime of the polymer if it can be related to a chemical failure. However,data of this sort must be obtained using the principles of chemical kinetics.

An example of aging a part could be found within the different outcomes(failure mechanisms) of an egg. Take an egg and set it on a shelf and leave italone. After several months the egg becomes rotten. Take a similar egg andplace it under a hen and after a while we had a cute little chick. As a lastexample, take another egg and place it in boiling water and after about 10minutes you have a hard boiled egg. Never pick temperatures throughout theviscoelastic region of a thermoplastic; you will obtain three different responses(at its glassy region, at its glass transition temperature, and at its rubbery region)of the material.3

Now we can proceed with the plastic(s) of choice. This is the polymer withthe magical foo-foo dust that the compounder puts in it for various reasons.Chemical soak tests are extremely important in the cases where the plastic partis used in a chemical environment. We do not want anything from the plastic tobe extracted into the chemical environment thus either affecting the propertiesof the plastic or contaminating the chemical environment. And the reverse isalso true; we do not want the plastic to absorb chemicals from the chemicalenvironment. This could cause the properties of the plastics to be lower due toa plasticization effect.

Also the same type of chemical soak tests should be conducted on the finalmolded part with the chemical soaking to an exposed portion of the design. Inaddition to develop a material knowledge base for the particular part you aredesigning, you will need data in the event you or others have to perform failureanalysis on the molded part.

You may request some of the data mentioned in this chapter from your sup-pliers.

At this stage you should hopefully be dealing with the molded plastic partassembly. The next part is to design functional tests that reflect the functionality

SUGGESTED READING LIST 1435

of the plastic molded part assembly as it leaves your facility and as it performsin the field.

7 CONCLUSION

This chapter was written as hints, suggestions, and tricks to assist one in theselection of materials for plastic parts design. These hints, suggestions, and trickshave helped this writer in various industrial positions held and in consultingprojects completed.

Like everyone else I have experienced the phase we dont have time to doit right the first time, but we have time to redo it. It is always easier to dosome right the first thine.

I have participated in projects in which millions were spent to develop aplastic assembly and the project failed for the stubbornness of not spending acouple of thousands of dollars on an analytical test.

I have heard words like mechanical engineers can pick materials. This maybe true. But I have again experienced projects where looking in a polymerhandbook would have saved thousands of dollars and many months of work.Case in point, polyethylene terephthalate was selected as the material of choicefor a plastic part that had to withstand an internal processing step of beingadhesively bonded to another part for 2 min at 150C. The grade of polyethyleneterephthalate used was a recycled grade with a glass transition temperature inthe vicinity of 60C. Placing the final assembly in an oven at 60C to simulatean aging test failed all parts due to changes in dimensions of the part. Subjectinga polyethylene terephthalate coupon to 60C testing or checking the literaturewould have been helpful.

Another example was the use of polystyrene as a throw-away coffee cups.Several years ago a fast-food chain was sued for injuries a customer sufferedfor drinking coffee from one of these cups. Part of the injuries occurred becausethe cup was made from polystyrene. Polystyrene has a glass transition temper-ature in the vicinity of 100C. This temperature is the same as the boiling pointof water. If the coffee is extremely hot, it could reach temperatures close to itsglass transition temperature or in the vicinity when the material starts to transitfrom the glassy region to the rubbery region of the polymer. In one particularcase it did, thus losing its structural integrity, causing the coffee to spill out ofthe cup, burning the customer, and resulting in a legal action.

In the conclusion I would like to say that in writing this chapter I have beenfortunate to have the benefit of books, good teachers, a good network of sup-pliers, and good co-workers who were part of my team. Engineers who werewilling to learn, and management that had faith in my methods, and, most im-portant, my willingness to be taught by others. I wish you the same.

SUGGESTED READING LIST

As mentioned earlier, this chapter was written from the short courses and grad-uate lectures that this writer has given in the past. However, I feel obligated toprovide the reader with a reading list. The amount and nature of the informationis even overwhelming to the author.


Brostow, W. (ed.), Performance of Plastics, Hanser /Gardner, Cincinnati, OH, 1995.Brostow, W. and R. D. Corneliussen (eds.), Failure of Plastics Hanser /Gardner, Cincinnati, OH,

1986.Domininghaus, H., Plastics for EngineersMaterials, Properties, Applications, Hanser /Gardner,

Cincinnati, OH, 1993.Dym, J. B., Product Design with Plastics, A Practical Manual, Industrial Press, New York, 1983.Ezrin, M., Plastics Failure Guide Cause and Prevention, Hanser /Gardner, Cincinnati, OH, 1995,

1996.MacDermott, C. P., and A. V. Shenoy, Selecting Thermoplastics for Engineering Application, 2nd

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Arrhenius or Erroneous? Trends in Polym. Sci., 5(8), 250257, 1997 and the references therein.

Front MatterTable of ContentsPart I. Quantitative Methods of Materials SelectionPart II. Major Materials Part III. Finding and Managing Materials Information and Data Part IV. Testing and Inspection Part V. Failure Analysis Part VI. Manufacturing Part VII. Applications and Uses 36. Spacecraft Applications of Advanced Composite Materials 36.1 Introduction36.2 Use of Advanced Fiber- Reinforced Composites in Spacecraft36.3 Example Spacecraft ApplicationsReferences

37. Selection of Materials for Biomedical Applications 37.1 Introduction37.2 Orthopedic Biomaterials: Total Hip Arthroplasty37.3 Blood-Contacting Biomaterials: Vascular Prostheses37.4 Space-Filling Biomaterials: Breast Implants37.5 SummaryBibliography

38. Selecting Materials for Medical Products 38.1 Introduction38.2 Challenges of Medical Products38.3 Product Development Fundamental Factors38.4 Manufacturing Process on Materials Properties38.5 Product Performance38.6 ConclusionsReferences

39. Materials in Electronic Packaging 39.1 General39.2 Approach39.3 Dominant Considerations39.4 Overriding Considerations39.5 Typical Applications39.6 Candidate Materials 39.7 SummaryReferences

40. Advanced Materials in Sports Equipment 40.1 Introduction40.2 Characteristics of Materials of Importance in Sports Equipment Design40.3 The Impact of Advanced Materials on Sports Performance40.4 Ethical Considerations40.5 Concluding RemarksReferences

41. Materials Selection for Wear Resistance 41.1 Introduction41.2 Properties of Wear Materials41.3 Materials Selection Process41.4 Manufacturing Process Selection41.5 Basics of Wear Materials41.6 Substrate Selection41.7 Surface Modifications41.8 Film Thickness41.9 Applications and Examples of Wear Materials41.Bibliography

42. Diamond Films 42.1 Historical Background42.2 Properties of CVD Diamond42.3 Diamond Film Deposition42.4 Modifying CVD Diamond42.5 Diamond Film Roughness42.6 Diamond Film Thickness42.7 Diamond Film AdhesionBibliography

43. Advanced Materials in Telecommunications 43.1 Introduction to Communications43.2 Materials Selection for Select Components43.3 Communication System Components43.4 Synthetic Methods 43.5 Vision of Future Communication ComponentsReferences

44. Using Composites 44.1 Introduction44.2 Evaluating Potential Products44.3 Differences Between Composites and Metals44.4 Manufacturing44.5 The Design, Manufacturing, and Quality Control Interface44.6 Selection of Material and Manufacturing Concept44.7 Detail Design44.8 Producibility Checklist44.9 Eternal Quality Control Question44.10 Environmental Concerns

45. Composites in Construction 45.1 Introduction 45.2 Construction Applications of Composites45.3 Development of Codes and Standards45.4 New Strategy and RecommendationsBibliography

46. Design for Manufacture and Assembly With Plastics 46.1 Introduction46.2 Plastic Materials Selection 46.3 Plastic Materials Selection Techniques46.4 Plastic Joining Techniques46.5 Plastic Part Design46.6 Plastic Part Material Selection Strategy46.7 ConclusionSuggested Reading ListReferences

Index

design for manufacture

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