Kutz M. Handbook of materials selection

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1422 COMPOSITES IN CONSTRUCTION

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1423

CHAPTER 46

DESIGN FOR MANUFACTURE AND

ASSEMBLY WITH PLASTICS

James A. Harvey

Under the Bridge Consulting

Corvallis, Oregon

1 INTRODUCTION 1423

2 PLASTIC MATERIALS

SELECTION 1424

2.1 Polymers 1424

2.2 Plastics 1429

2.3 Reinforced Plastics 1430

3 PLASTIC MATERIALS

SELECTION TECHNIQUES 1430

4 PLASTIC JOINING TECHNIQUES 1431

5 PLASTIC PART DESIGN 1431

6 PLASTIC PART MATERIAL

SELECTION 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 you

need a good introduction or an opening that sets the pace of the information one

wishes to present.

From the title we will only be dealing with plastic materials used for designed

parts; however, some of the hints, tips, criteria, and suggestions may apply to

other materials such as metals and ceramics.

I would have liked to start this chapter with an excellent illustration of a

plastic part design. But the best I could come up with is a metal example and

hopefully throughout this chapter I will refer to some plastic designs.

The example I would have liked to use is the long swords used by King

Arthur and the Round Table. From a distance one can hardly imagine the breadth

of one who uses such a long sword in battle and who swings this enormous

sword at an enemy to chop off various body parts. The sword was hollow and

partially ﬁlled with mercury. The sword was relatively easy to manipulate in the

so-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 mercury

ﬂowed 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.

HandbookofMaterialsSelection.EditedbyMyerKutz

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 on

plastic materials selection. The information in this chapter will be based upon

both the lectures I gave and the information I received in short courses taught

to practicing engineers and scientists that were involved in all the aspects of

commercial plastics part designs and in graduate school courses to budding new

materials scientists and engineers.

If you search the open literature for material selection, you will ﬁnd articles

with titles similar to ‘‘The Science of Material Selection’’ or the ‘‘Art of Material

Selection.’’ 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 step

is 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 terms

this writer thinks are signiﬁcant for assisting one in the selection of materials

for plastics part design: thermoplastics, thermosets, elastomers, polymerization

reactions, characterization techniques, molecular weights and distribution, mo-

lecular structure of polymers, ﬁve viscoelastic regions of polymers, Carothers

equation, 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 unit

is the—CH

—chemical moiety. Two repeated units are equivalent to the organic

compound ethane. Ethane is a gas at room temperature with a total molecular

weight of 30 atomic mass units (amu). A polymer family with hundreds of

thousands of these repeat—CH

—units 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 weight

form. The major thermal event that one does is to process it into the ﬁnal part

form. There are three types of thermoplastics: amorphous, semicrystalline, and

liquid-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 be

formed into its ﬁnal shape.

An elastomer is a very lightly cross-linked polymer with the ability to be

extend to a high elongation and snap back to its original dimensions when the

forces 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 a

reasonable 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 at

polyethylene as one example. Polyethylene is named from the monomer from

which 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 material

to be put into a waterlike environment.

Now let us look at another polymer, polyethylene terephthalate. It is formed

from the reaction of ethylene glycol with terephthalic acid or terephthalic acid

ester. 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 conditions

with the formation of a by-product such as water and alcohol are said to be

formed by condensation and are named by the new chemical functional group

formed. 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 with

organic 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 acid

anhydrides and organic amines with the release of water.

Knowledge of organic chemistry plays a very important role in the selection

of a polymer for a plastic part. This writer is only posing general rules to the

reader, and to the ﬁrst approximation one can made very reasonable selection

in 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 exist

in a water environment, your ﬁrst choice could be an addition polymer such

as polyethylene rather than a condensation polymer such as polyethylene

terephthalate. The polyethylene is water hating, or hydrophobic. Thus it should

not 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 lifetime

and the companies that ﬁll these bottles with their clear spring mountain fresh

water want you to see how pretty their water is. However, over time the polyester

bottles will absorbed water. Water (sport) bottles that are used over and over

and ﬁlled by the consumer are made from the addition polymers. They are also

opaque. 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 many

different sizes of polymer chains. The molecular mass of each chain refers to

its molecular weight. And as mentioned, since many different sizes of polymer

chains are formed, there will be a distribution of the molecular weights.

Knowing the molecular weight and its distribution aids in the selection of

polymers for a plastics part and in the lifetime of plastic part. The general

techniques that can be used to determine molecular weights of polymers are

achieved through viscosity measurements either in solution or using solid sam-

ples. Solution viscosities consist of timing the ﬂow of polymer solutions of

known 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 given

amount of the polymer is heated to a certain temperature and a known force is

1426 DESIGN FOR MANUFACTURE AND ASSEMBLY WITH PLASTICS

applied to the molten polymer and its ﬂow is timed. If all things are equal, the

lower molecular polymer will ﬂow through the given volume, the fastest thus

revealing a low number. For the members of a given polymer family this is a

reasonable 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 of

tubes (columns) packed with different porous particles. As the solution passes

through, the polymer chains with the highest molecular weights pass through

the fastest. A detector measures the polymer chains as they exit the instrument.

Thus one ends up with a chromatograph, which shows the distribution of the

different molecular weights of the polymer chain in the sample. Solution vis-

cosities are usually used by the polymer manufacturer. Melt ﬂow index is used

as an initial tool for material selection and as a tool to help determine the

molding process. Gel permeation (size exclusion) chromatography was in the

past treated as a research tool, but lately it has gained a great deal of popularity

as a quality control technique.

Another important parameter for the different polymers refers to their thermal

behavior. A typical thermoplastic is a solid at ambient temperatures. As the

material 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. This

process should be repeatable. Thus thermoplastics are recyclable. Another ther-

mal property of the thermoplastics is creep. This property refers to the ability

of 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 been

established at the manufacturer. With thermosetting resin systems, one starts with

low-molecular-weight reactants and to process these ingredients you cure or

‘‘cook’’ the reactants into the desired ﬁnal shape. If the reactants have been fully

reacted, one ends up with one giant molecule. To process a thermosetting resin

system into a part, the thermal events consist of heating the ingredients so they

start 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 of

curing (cross-linking) occurs. As the reaction proceeds, the viscosity increases

and the part hardens. At the end of the curing reaction the part is solid; then it

is cooled to ambient temperatures. Once formed the part cannot be reheated to

change its shape. If a thermosetting resin system has been properly cured, it

should 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. And

most elastomers will swell when placed into a solvent.

Thermal analyses are a set of techniques used to characterize the thermal

behavior of the different type of polymers. In addition to informing one as to

thermal characteristics of the polymers, they can assist in determining a proc-

essing cycle. Differential scanning calorimetry (DSC) yields the thermal events

of 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)

2 PLASTIC MATERIALS SELECTION 1427

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 a

sample (warpage and shrinkage) and glass transition temperatures. Dynamic me-

chanical analyses (DMA) provide the modulus and changes in modulus and glass

transition temperature as a function of temperature, time, and oscillation (dy-

namic load).

The internal structure of the polymer will determine if it is transparent or

opaque. 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 ordered

or disordered manner. Amorphous refers to total disorder. Crystalline refers to

total ordered. Semicrystalline is a combination of disorder with domains of order

within 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 as

a material goes from the solid state to the liquid state, it goes from a state of

order to one of disorder. The liquid-crystal polymers lack this transition, and

this unique characteristic has an enhanced effect of the processing of these ma-

terials.

Let us now examine the internal structure of amorphous, semicrystalline, and

crystalline thermoplastics. We have the two extremes—totally disordered in the

arrangement of the polymer chains (amorphous) and at the other end total order

(crystalline). Another way of looking at the arrangements involved is to view

the polymer chains as spaghetti. We have cooked spaghetti (disordered) at one

extreme and uncooked (ordered) spaghetti at the other extreme. Except for the

liquid-crystalline polymers, most thermoplastic polymers are either amorphous

or 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 do

not have a melting point and are transparent.

The next important polymer deﬁnition or concept involves the ﬁve viscoelastic

regions of polymers. If we plot the modulus of a thermoplastic material as a

function 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 in

its 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 region

3. This region is known as the rubbery region and has lost in the range of three

orders of magnitude of strength. As the sample is heated to an even higher

temperature, the polymer (region 4) starts to decompose and ﬁnally at region 5

decomposition 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 as

drastic with the semicrystalline polymers (region 2) as it is with the amorphous

polymers. As the semicrystalline thermoplastic reaches its melting point, its

strength 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 region

is known as its glass transition and the temperature that it occurs at is its glass

transition temperature.

1428 DESIGN FOR MANUFACTURE AND ASSEMBLY WITH PLASTICS

Fig. 1 Five viscoelastic regions of a linear amorphous polymer. The dashed line represents the

behavior 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 amorphous

material 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.’’

Some individuals use the term glass transition temperature while discussing

cured thermosetting resin systems. To this writer, if the thermosetting resin is

completely 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 be

unaffected by temperature until it reaches its decomposition temperature. If you

perform 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 technique

to determine glass transition temperature, cool the sample to ambient tempera-

ture and repeat the analysis on the same sample. If the material is not fully

cured, 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 and

the melting point of a thermoplastic polymer. The key feature of the graph shows

2 PLASTIC MATERIALS SELECTION 1429

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 with

temperature. As one passes through the polymer’s glass transition, its thermo-

dynamic properties change. Thus, in selection of a thermoplastic for a plastic

part, it is always best to use the material above its glass transition temperature.

There are exceptions to this rule. For example, polymers like the polyethylenes

are 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 relatively

simple, but the impact of it on a thermoplastic polymerization is very critical:

X ⫽

2 ⫺ pr

The term refers to number-average degree of polymerization, p is the extent

of 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 pure

reactants and the correct stiochiometry to obtain the proper molecular weight.

Small changes in purity, incomplete reaction, and incorrect ratio of reactants can

have 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 materials

from 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 addition

of additives to the polymer enhances the process of the making of the part, the

product performance and lifetime of the part, and the appearance of the part. A

partial 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 additive

package compounders put into a polymer. This information is treated as conﬁ-

dential. Thus one must be careful when one switches material from one supplier

to another. Even though the starting polymer may be same, different additive

packages will affect the performance behavior and lifetime of the designed plas-

tic part. Thus once a material has been selected, this writer highly recommends

that one completely characterize that material in case your supplier changes the

material or its consistency changes or you are involved in determining product

failures. As a consultant, this writer has been involved in several failure analyses

projects and the worst thing that happens is that there is no baseline material

data. These problems are difﬁcult and expensive to solve without baseline data.

1430 DESIGN FOR MANUFACTURE AND ASSEMBLY WITH PLASTICS

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 color

and smoothness properties of the ﬁnal molded part, to assist in the molding of

the part by changing the ﬂow behavior of the plastics, and lastly to reduce the

cost of the molded part.

Plasticizers are unique materials. They are added to a polymer to reduce the

hardness 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 car

smells comes from the plasticizer and the seats are nice and soft. Over time the

smell is gone, the seat becomes hard and brittle, and we have to clean the inside

of the windshield. The reverse of this can occur with hydrophilic polymers in

the presence of water. The polymer absorbs water, its glass transition temperature

drops, and the material becomes softer. In the case of hydrophobic thermoplastic

materials oils will have a similar effect.

Thus, it should be noted that these plastic systems are dynamic and they are

in a constant change. If you design a plastic part for a lifetime of 5 years, it

would be ‘‘nice’’ to test the part for 5 years under the operating conditions of

the plastic part assembly. However, based upon ‘‘time to market’’ that is not an

option. One can retain samples of production.

2.3 Reinforced Plastics

Reinforced plastics are plastics containing reinforcing elements within a plastic

matrix. There are many different types and forms of reinforced plastics. And

whenever one refers to reinforced plastics, they are referring to thermoplastics

reinforced with either long or short discontinued (chopped) ﬁbers, and the parts

are manufactured by injection molding. Reinforced plastics are in essence forms

of composite materials. A composite is a heterogeneous mixture of matrix resin,

reinforcement, and other components that act in concert with each other. The

matrix resin protects the reinforcement from itself due to wear and it ‘‘glues’’

the reinforcement in place. The reinforcement provides strength to the plastic

part and enhances the properties of the matrix resin. Also the reinforcement

helps dissipate the energy throughout the structure when impacted.

Properties of reinforced plastics should be obtained from the supplier. There

are 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 particular

design. At ﬁrst the tenacity is to hope that you doing a second version of a

previous designed plastic part. This is wishful thinking. Second one hope that

one is exposed to an individual in their organization whom has experience in

plastic material selection. Or, if you are really fortunate, you have a materials

engineer in your organization that knows the material science of plastic mate-

rials.

This writer’s ﬁrst experience in selecting materials was relatively simple. The

criteria were basically set by the equipment available for the project. Then cri-

teria such as the temperature, chemical requirements, and the number of parts

needed are used to pick the material. An eight-step criteria was then developed.

The criteria were functionality, chemical resistance, external processing (your

supplier), 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 criteria

were developed and added to the list and is represented by Table 1.

The above scheme works reasonably well. But it does not include the most

important factor in a material selection. That factor can be represented with such

words 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 plastic

parts together. Each technique has its own unique advantages and limitations.

This writer has more experience with adhesive bonding. In the selection of

adhesives one can follow most of the criteria and subcriteria as listed in

Table 1.

In the adhesive bonding of plastic parts together there are several other issues

that you should take into account. One refers to the failure mechanism that you

want to strive for in your design. There are three failure mechanisms for an

adhesive joint. They are adhesive, cohesive, and substrate failures. An adhesive

failure is failure of adhesive joint at the interface. Cohesive failure is failure

within the adhesive and ﬁnally substrate failure of the plastics. This is failure

within the plastic parts being joined together. Failure within the plastic parts

would be the best case. And with surface preparation techniques, especially with

treatments involving silane coupling agents, this failure mode is not unrealistic

as observed by this writer.

The golden rule of design using adhesives was (and is) to design to at least

cohesive failure.

One chemical principle that you should take into account in the selection of

adhesives is ‘‘like dissolves like.’’ It translates into the more similar the adhesive

is 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 design

plastic parts. I cannot draw a straight line with a ruler. I rely on the suppliers’

design 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 only

a phone call away. However, I do perform failure analyses on molded plastic

parts, and there are tricks I employed during an analysis. One technique is to

perform an ashing of a plastic molded part. One can obtain information as to

the ﬂow of the plastic and ﬁber orientation if the part is reinforced.

One can also section the molded reinforced plastics into smaller specimens

that can be analyzed by TGA to determine percent resin and reinforcement

contents; 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 polymer

chains can separate. The smaller polymer chains travel faster than the larger

chains. To verify the consistency of the polymer molecular weight throughout

the molded plastic part, one can section the part and subject the specimen to gel

permeation chromatography.

6 PLASTIC PART MATERIAL SELECTION STRATEGY

After reading the ﬁrst part of this chapter one may be either totally confused or

have a headache. This writer understands perfectly. Materials selection is not an