4 Causes of Molded-Part Variation: Material

This isotropic shrinkage is defined as equal shrink in both the flow direction in-flow and the direction transverse to flow cross-flow.. Articles molded from semicrystalline plastic res-

When screening materials for a particular

appli-cation with specific tolerances, it is important to

con-sider the shrinkage tendencies of the candidate resins

Amorphous and semicrystalline resins have unique

shrinkage characteristics, and both may be altered by

the addition of fillers or reinforcements As discussed

in Chs 3 and 5, design elements such as gate location

can significantly affect a part’s shrinkage and its

dif-ferential shrinkage, leading to warping of the finished

part The amount of shrinkage in a finished part is

pri-marily controlled by the temperature and pressure used

in injection molding to fill the tool cavity volume Due

to some compressibility of the resins during the

pack-ing phase of processpack-ing, the overall shrinkage may be

controlled to some degree by the process conditions

This chapter examines these effects, presents results,

and explains the differences in the behavior of

amor-phous and semicrystalline resins This chapter also

looks at the effects of additives used to modify the

per-formance of each class of polymer resin A method for

estimating final part shrinkage is presented that

uti-lizes pressure-volume-temperature (PVT) data

gener-ally available from resin suppliers Some examples of

PVT curves and data may be found in the data section

a molder may expect to hold for a particular resin.Another type of tolerance chart that is commonlyused is shown in Fig 4.2 This type of chart suggests

an acceptable range of tolerances for various types offeatures in parts molded from a polycarbonate resin.Similar charts are available from the Society of Plas-tics Industries (SPI) for each type of plastic resin SPIalso provides a bulletin that outlines the Standards andPractices of Plastics Custom Molders.[53] (A relatedstandard is the German Standard DIN 16901.) Thesedata, along with a well-grounded understanding ofshrinkage, are the basis for selecting the optimum resinfor a tight tolerance application

Figure 4.1 Fine and commercial tolerances for nylon.[9](Courtesy of DuPont.)

Figure 4.2 Recommended tolerances for a polycarbonate.[10]

Plus or Minus in Thousands of an Inch Drawing Code Dimension

0.000 1.000 2.000 3.000 4.000 5.000

Fillets, Ribs, Corners

Reference Notes

1 These tolerances do not include allowancefor aging characteristics of material

2 Tolerances based on 1/8" wall section

3 Parting line must be taken into ation

consider-4 Part design should maintain a wall ness as nearly constant as possible Com-plete uniformity in this dimension is im-possible to achieve

thick-5 Care must be taken that the ratio of thedepth of a cored hole to its diameter doesnot reach a point that will result in ex-cessive pin damage

6 These values should be increased ever compatible with desired design andgood molding technique

when-7 Customer-Molder understanding sary prior to tooling

neces-4.1.1 Amorphous Polymers

Amorphous polymers with rapid relaxation rates

generally produce parts with isotropic shrinkage This

isotropic shrinkage is defined as equal shrink in both

the flow direction (in-flow) and the direction transverse

to flow (cross-flow) Amorphous resins exhibit a broad

softening range when heated through their glass

tran-sition temperature (T g) With additional heating above

T g, the polymer viscosity gradually decreases until the

desired processing flow is achieved The process of

adding energy (heat) to the molecular mass increases

the molecular motion, driving the polymer chains to

occupy more local volume, and increasing the specific

volume of the resin The more energetic (hotter) resin

flows more easily, but must be cooled again to T g for

solidification The time required for cooling allows for

local molecular relaxations, thereby resulting in the

more isotropic shrinkage Examples of amorphous

res-ins with isotropic shrinkage include ABS,

polycarbon-ate, and polystyrene

Table 4.1 provides a brief list of flow-direction

shrinkage values for typical amorphous resins and

dem-onstrates the effects of incorporated fillers on resultant

shrinkage A more complete list appears in the “Data”

appendix at the end of this book Shrinkage is

gener-ally reported as a dimensionless value or as a

percent-age The shrink value is determined by measuring the

amount of shrinkage along a given dimension, and

nor-malizing it by the length of that dimension Units may

also be reported as inches/inch or mm/mm, both units

being dimensionless Confusion may result from

inter-pretation of the data when reported as a percent in one

table and a dimensionless unit in another Table 4.1

shows both types of units for comparison

Processing conditions play an important role in theresultant shrinkage of an amorphous resin Following

is a summary of key processing effects:

• The hotter a part is on removal from thetool, the longer the post-mold cooling timewithout the constraint of the cavity This

“free shrinkage” is generally higher thanshrinkage in a constrained tool becausethe cold tool surfaces tend to freeze thepart in a more constrained volume How-ever, the rapid constrained cooling gen-erally results in higher residual stresses

in the finished part Annealing a quenched amorphous part by heating it

fast-to near its T g will result in some stressrelief, but may actually increase the finalshrinkage of the part

• Increasing a part’s wall thickness willincrease its cooling time and also increasethe time for shrinkage Thicker wall sec-tions also exhibit greater temperature dif-ferentials between the rapidly frozen skinand the slower cooling core at the center

of the cavity thickness This condition willresult in residual stresses through the partthickness When the part wall thicknessexceeds recommended dimensions, thecooling stresses can result in void forma-tion at the core as the cooling melt nearthe walls causes the core to develop suf-ficiently high isotropic tensile stresses thatfracture the melt

• Injection hold time must be sufficientlylong to allow for gate freeze When the

hold time is too short, material can leak

from the cavity prior to solidification,

thereby decreasing hold pressure and

in-creasing shrinkage The optimum hold

time can be readily determined by

weigh-ing a series of parts formed usweigh-ing

increas-ing hold times Startincreas-ing with a short hold

time, the part weight will continue to

in-crease proportionally to increasing hold

time When the part weight stabilizes, the

gate is properly frozen prior to the

re-lease of injection hold pressure

• Hold pressure is used to compress the

melt in the tool during solidification A

constant hold pressure is used to

main-tain a constant volume of resin in the tool

cavity As this resin cools, the specific

volume decreases at constant pressure,

and additional melt may be squeezed into

the tool prior to gate freeze The

addi-tional melt volume added prior to gate

freeze will decrease the overall

shrink-age of the final molded part However,

excess hold pressure will overpack that

cavity and make part ejection difficult

To prevent overpacking, good practice

demands a switch from injection pressure

to hold pressure slightly before the

cav-ity is completely filled

• Increasing the melt temperature will

re-sult in a hotter melt in the cavity when

the gate freezes This hotter melt will

in-crease the overall cooling time and have

the same result on shrinkage as described

in the discussion above on part

tempera-ture at ejection

Post-mold shrinkage is both time and temperature

dependent Accurate post-mold shrinkage should be

measured at least twenty-four hours after part

ejec-tion During this time, stress relaxation in the freshly

formed part can lead to additional changes in the part

dimensions Increasing the temperature will decrease

the time to stabilize shrinkage Post-mold shrinkage

can account for up to one percent of the part’s final

dimensions

Articles molded from semicrystalline plastic

res-ins generally display anisotropic shrinkage, meaning

that there will be a different amount of shrinkage in theflow direction and the transverse flow direction Asopposed to amorphous polymers, semicrystalline poly-

mers exhibit a sharp melting transition (T m) associated

with melting the crystals themselves Below T m, the

polymer is a rubbery solid, while above T m thepolymer’s crystal structure is dissolved and the poly-mer flows readily Common examples of semicrystal-line polymers include polypropylene, polyethylene,nylon, and acetal

Polymer crystallization involves the local ing of short lengths of adjacent chains that, once nucle-ated, grow through drawing on the available polymerchains in the local melt This process may involve chainfolding as molecules are reeled from the melt onto agrowing crystal face On cooling, nucleation takes placethroughout the melt, and the crystal structure growsradially from each nucleation point during primary crys-tallization The resulting structure is spherical around

order-the nucleation point and is referred to as a spherulite.

Within the spherulite are layers of crystalline lamellaeseparated by non-crystallized (amorphous) regions

Secondary crystallization is the process of

incorpo-rating additional available molecular segments onto theestablished crystals This slower secondary crystalli-zation is responsible for additional shrinkage in moldedparts heated above their glass transition temperature.Crystallization can be viewed as both a kinetic andthermodynamic process Kinetically, the degree of un-dercooling (melt temperature minus crystallization tem-perature) drives both the nucleation and crystalliza-tion processes Thermodynamically, the crystal is a low-energy state that forms through exothermic collapse ofthe energetic melt into a stable solid regular lattice (thecrystal lattice characteristic of each semicrystallineresin) The addition of a nucleating agent will decreasethe degree of undercooling necessary to initiate crys-tallization as well as produce a solid consisting ofsmaller spherulites The absolute degree of crystallin-ity is dependent on the rate of crystallization and thecooling rate In injection molding, many semicrystal-line polymers do not achieve their full potential crys-tallization because of rapid quenching of the melt in acold tool

Because of the close packing of chains in a crystallattice, the density of the semicrystalline solid will beproportional to the degree of crystallinity Mechani-cally, a semicrystalline polymer exhibits an increasedstiffness because the crystals themselves act to physi-cally lock the polymer structure together Also, becausecrystallization is a volume-reduction process, a crys-

tallized polymer will exhibit higher shrinkage than

would be predicted without crystallization

A slow rate of crystallization or a low degree of

total crystallinity has the effect of reducing shrinkage

and thereby reducing warpage in semicrystalline

poly-mers By contrast, nucleated resin grades result in

higher amounts of shrinkage, and proportionally higher

degrees of warp This is true for copolymers as well as

the homopolymers discussed so far

Molecular weight can also influence the degree of

shrinkage Higher molecular-weight resins exhibit a

higher viscosity on filling, and a higher pressure drop

in the tool cavity during filling Higher packing

sure must be used to compensate for the cavity

pres-sure drop or else the lower prespres-sure melt will result in

higher shrinkage in the final part

Branched polymers crystallize differently from

lin-ear polymers The presence of side chains on the

mo-lecular backbone inhibits the ability of a molecule to

fit into a developing crystal structure The longer the

side chains, the lower the resulting crystallinity Highly

branched polymers also have a higher degree of chain

entanglements that may also inhibit rapid

crystalliza-tion For example, polyethylene may be produced by

different processes that each result in a different

de-gree of branching High-density polyethylene (HDPE)

is produced with a low degree of branching and

crys-tallizes easily The degree of crystallinity for HDPE

can range from 60% to 80% crystal structure with

as-sociated densities of 0.940 to 0.965 g/cc By contrast,the more branched medium-density polyethylene(MDPE) attains only about 50% crystallinity at a den-sity of 0.930 g/cc

Table 4.2 provides shrinkage values of varioussemicrystalline polymers The mold shrinkage valueslisted are those found on most typical property datasheets and are generated using test specimens of 1/8-inch thickness The reported values are measured inthe fill or in-flow direction

Shrinkage also depends on processing factors andtool design As shown in Fig 4.3,[11] a series of poly-ethylene grades increases shrinkage as the wall sectionincreases The melt remains hot for a longer time inthick wall sections, thereby increasing the time for ki-netically driven crystallization For very thin wall sec-tions, premature gate freeze can diminish the effect ofhold pressure, resulting in additional shrinkage In ad-dition, design factors such as the number of gates andtheir locations can change the filling dynamics of apart and result in different amounts of shrinkage Todetermine shrinkage accurately, complex computerizedmodels are used that strive to take into effect the localpressures and cooling kinetics of a polymer melt dur-ing solidification

Table 4.3 shows some of the shrinkage changesthat one can expect in polyethylene from part and pro-cess changes

Figure 4.3 Differences in shrinkage based on section thickness for a variety of polyethylene injection-molding resins.[11](Courtesy

4.2 Effects of Fillers,

Reinforcements, Pigments,

Time, and Stress

A common misunderstanding is that the shrinkage

values listed on data sheets are a direct indication of

potential part warpage A more reliable indication of

warp would be the differential shrinkage obtained by

subtracting the shrinkage in the flow direction from

that in the transverse direction, as illustrated in Fig

4.4.[7] This is equally valid for semicrystalline and

amorphous resins, but greater attention to differential

shrinkage is required with semicrystalline plastics

Fillers also influence the shrinkage by offsetting

some volume of polymer with a low-shrinking filler

particle The shrinkage of resins containing isotropic

fillers, such as glass beads or powders, will be more

isotropic than resins containing high-aspect-ratio

fill-ers like fibfill-ers or platelets This results from

orienta-tion of the fillers in the flow path during filling, and

the restricted shrink along the long axis of the filler

particles Fibers are known to create excessive warp

as the restricted shrink in the flow direction is

compen-sated by an increased shrink of the polymer in the

trans-verse direction

Although the topic of thermoplastic shrinkage and

warpage is extremely complex, a number of general

characteristics can be established For example, while

the molecular chains of both amorphous and

semi-crystalline resins pack together differently upon

cool-ing, the molecules in semicrystalline resins pack

to-gether more tightly, resulting in higher shrinkage forsemicrystalline materials In addition, the shrinkage ofparts molded from any filled resin is governed by thetype and level of fillers and reinforcements added tothe plastic as discussed in this section

Powders, flakes, and fibers are generally rated into plastic resins to selectively modify mechani-cal properties of the original resin For example, highmodulus fillers are added to increase the stiffness andcreep-resistance of a polymeric system for applicationsrequiring a high-modulus material A secondary effect

incorpo-of using such filler systems is that the composite incorpo-offiller and resin will have a different shrinkage from theparent resin Use of fiber reinforcements will also pro-duce differential shrinkage between the molding axes

of the part, resulting in warpage

Most fillers and reinforcements are inorganic andhave relatively low coefficients of thermal expansion.When an injection-molded composite is cooled duringprocessing, the fillers and reinforcements tend to shrinksignificantly less than the polymeric matrix to whichthey are added Particulate and flake fillers both tend

to reduce the overall shrinkage when added to phous or semicrystalline polymers The reduction inshrink is approximately proportional to their concen-tration Powders, beads, and flakes are geometricallymore uniform than fiber fillers The addition of low-aspect ratio fillers (e.g., powders, beads, or flakes) doesnot create problems with anisotropic shrinkage Withthese fillers, the shrinkage in all directions is reducedproportionally to the filler content Particulate fillershave the ability to reduce shrinkage in all directionsand also improve dimensional control Particulate fill-ers are approximately the same size in all directionsand, therefore, do not become oriented in a flow field,yet by taking up space they reduce shrinkage

amor-Fibers are geometrically defined by their aspect

ratio, determined as the ratio of the fiber length to its

diameter Inorganic fibers, produced from materialssuch as glass or graphite, are commonly used as rein-forcing agents in polymers When chemically coupled

to the resin matrix, fibers offer a number of tages in terms of end-use performance, however theiruse can also create several processing-related prob-lems For example, compared to particulate- or flake-filled polymers, the differential shrinkage between thein-flow and cross-flow directions of fiber-reinforcedpolymers can be significantly different, as shown inFig 4.5.[6] This anisotropic shrinkage can make it moredifficult to determine the appropriate cavity dimensionsunless the anisotropic shrinkage behavior is properlyunderstood and taken into account in tool design Dif-

advan-Figure 4.4 Differential shrinkage equals transverse

shrinkage minus flow shrinkage [7] (Courtesy of GE

Plastics.)

ferential shrinkage can also lead to warpage in a molded

plastic part

Anisotropic shrinkage of fiber-reinforced polymers

can be attributed to the fact that the fibers become

ori-ented in the flow-shear field during injection molding

Unlike polymer molecules that can orient and relax

dur-ing filldur-ing and cooldur-ing, fibers have no tendency to

re-orient in the cooling melt Flow-induced fiber re-

orienta-tion is maintained during polymer cooling Both shear

and elongational flow will influence the orientation of

fiber reinforcements Processing variables such as fill

rate, cavity thickness, melt viscosity, and gating scheme

are all significant factors affecting fiber orientation

As a result, flow-related design decisions, such as gate

location, are more critical when molding with

fiber-reinforced polymers

Anisotropic shrinkage can result from molecular

orientation and relaxation during filling and cooling

an unreinforced resin These resins tend to orient in the

flow direction during part filling, and will relax during

cooling This relaxation of orientation tends to

pro-duce more shrinkage in the flow direction than the

cross-flow direction For reinforced resins, the trend is

re-versed: fibers that become oriented in the flow

direc-tion during filling are frozen into that orientadirec-tion

dur-ing cooldur-ing Because the fiber shrinks less than the resin,

shrinkage is reduced in the flow direction Because

volume of the part must be conserved during cooling,

the polymer will tend to shrink even more in the

cross-flow direction Cross-cross-flow shrinkage for a

fiber-rein-forced resin can exceed the cross-flow shrink of the

base polymer

Figure 4.6 shows micrographs of sections takenthrough a glass-fiber–filled polypropylene molding.[2]The upper view shows the section parallel to the flowdirection Near the part surface (at the top and bottom

of the micrograph) a skin layer is found where the bers are frozen into a random pattern This skin layer

fi-is formed from melt that fountains from the core of themolding and freezes immediately on contact with thetool surface Just inside the skin layer is a region ofhighly-oriented fibers This layer forms as fibers areoriented along the edges of the flowing melt front be-cause of the shear profile established by the advancingmelt front This oriented layer is seen to extend towardthe center of the part, with more random orientationresulting at further distances from the wall Finally, inthe center of the part is a randomized area of fiberorientation In the core of the part, the melt being pushedforward develops a flattened profile and fibers withinthis region do not orient without a well-developed shearflow

Figure 4.5 The mold shrinkage for 30%-glass-fiber

reinforced PBT varies with direction (in-flow vs cross-flow)

and with part thickness [6](Reproduced by permission of

Hanser-Gardner.)

Figure 4.6 Glass-filled polypropylene sections parallel and

perpendicular to the flow [2] (Reprinted by permission of Oxford Science Publications.)

The lower micrograph shows a section of the same

part taken perpendicular to the direction of flow In

this section, fibers are found to show little orientation

as the view in the flow direction exposes the fibers in

cross section, consistent with their alignment in the flow

direction At the core of the part, there is a tendency

for fibers to be aligned across the flow direction, which

is the width direction of the part In this micrograph,

no fiber alignment is seen through the thickness of the

part These two perpendicular sections of an

injection-molded part give a good representation of the

com-plexity of fiber orientation found in any

injection-molded composite

Figure 4.7 shows the mold shrinkage behavior of

a glass-fiber–reinforced semicrystalline polymer such

as acetal.[6] For the semicrystalline polymer, unfilled,

(glass-fiber content = 0), both the flow and cross-flow

shrinkage are relatively high (e.g., 1.5% to 2.0%), with

the in-flow shrinkage somewhat higher As the fiber

content increases, the in-flow–direction shrinkage drops

dramatically, while the cross-flow–direction shrinkage

drops only slightly The large difference between these

behaviors is of primary importance

The difference between in-flow and cross-flow

molded part shrinkage increases as the fiber content

increases While the differential shrinkage between the

in-flow and cross-flow directions is found for all

fiber-reinforced polymers, it tends to be more pronounced in

semicrystalline polymer composites because of the

ex-cess shrinkage in the resin itself during crystallization

Designers should always consider the differentialshrinkage and the resulting potential for warpage whenfiber-reinforced polymers are used If part flatness is

of primary importance, the designer may be forced toselect a composite with a lower fiber concentration tominimize differential shrinkage In addition, the designermust balance the differential shrinkage, caused by theaddition of fibers, against the stiffening effects the samefibers impart to the composite Higher modulus fibers,such as carbon, may actually counteract the effects ofwarp caused by differential shrinkage in some designs

As discussed in Ch 3, wall thickness plays an portant role in part shrinkage This is especially truefor semicrystalline polymers where thicker walls lead

im-to longer cooling times With the increased cooling time,the crystalline microstructure becomes more developedand the polymer reaches a higher degree of crystallin-ity Because crystallization reduces volume within thepolymer, longer cooling times found in thicker sectionshave higher shrinkage This same effect is found inboth in-flow and cross-flow directions (Fig 4.5)

Regrind or recycled fiber-reinforced polymers will

exhibit different mold-shrinkage characteristics thanthose of the virgin resin The process of regrindingmolded parts for remolding produces a distribution ofshorter fibers than were present in the first-generationpolymer composite The shorter fibers produce a dif-ferent orientation distribution in the molded part, andcreate different shrinkage characteristics compared tothe first generation material

Figure 4.7 Warpage can occur as a result of anisotropic shrinkage in a relatively simple part like this glass-fiber reinforced acetal

disc The differential shrinkage tends to cause the part to warp (cup/diameter) like a round potato chip [6] (Reproduced by permission of Hanser-Gardner.)

4.2.2 Minimizing the Effects of Fiber

Reinforcements

Introducing non-fibrous reinforcements into a

com-posite may diminish differential shrinkage, but fiber

reinforcements tend to reduce mold shrinkage even

more In addition, the mold shrinkage of fiber-reinforced

thermoplastics may be lower in the direction of

mate-rial flow than in the cross-flow direction, causing

dif-ferential mold shrinkage and warpage

A number of techniques can minimize the

poten-tial for warpage in parts molded from fiber-reinforced

polymers One of the more common is to use a

poly-mer composite containing both fiber and flake

rein-forcements Flake-type reinforcements, like other

par-ticulate fillers, have a lower aspect ratio than long

fi-bers Hybrid composite materials, incorporating both

fiber and flake reinforcements, have mold shrinkage

values that tend to be more isotropic than conventional

fiber-reinforced polymers These hybrid composite

res-ins offer the mechanical performance of a

fiber-rein-forced composite, with a more isotropic shrinkage

These hybrid composites are widely used in

applica-tions requiring tighter tolerances on the finished parts

(see Fig 4.8).[6] For example, mixtures of mica flakes

with appropriate coupling agents and glass-fiber

rein-forcements can give consistently equal shrinkage in the

in-flow and cross-flow directions during molding This

reinforcement technique results in both lower warpage

and shrinkage in the final molded part

Studies on filler shape have shown that fibrous inforcements of non-circular cross sections can be use-ful in controlling warpage in fiber-reinforced polymers.One study[6] has shown a 30–40% reduction in warpfor semicrystalline polymers reinforced with glass fi-bers having a bi-lobe cross section (a fiber with someplate-like character) versus circular fibers of a smallercross sectional area This warp reduction was achievedwhile maintaining a mechanical performance similar

re-to the traditional fiber composite

Figure 4.9 shows the difference in in-flow versuscross-flow shrinkage for 30%-glass–reinforcedpolypropylene.[6] Differences in shrinkage betweencomposites reinforced with bead, flake, and fiber fill-ers are due to differences in aspect ratio among thefillers Glass beads do little other than occupy volume

in the composite; they reduce the shrinkage in all rections equally Flake-type reinforcements have alength and width that is significantly greater than theirthickness, so they impede shrinkage parallel to the plane

di-of the flake more than perpendicular to the plane di-of theflake In a flow field, flake-like reinforcements will tend

to align parallel to the cavity wall When frozen in thisorientation, flake reinforcements reduce shrinkage inthe plane of the wall section, and increase shrinkage inthe wall thickness direction

A test mold design that would typically be usedfor estimating mold shrinkage is also shown in Fig.4.9 Note how a fan gate is used to promote a uniformflow pattern into the part It is important to establish auniform flow field down the length of the part in order

Figure 4.8 An example of hybrid composite materials that include both flake and fibrous materials for reinforcement.[6]

to minimize cross-flow effects in the corners of the part.

In-flow–direction shrinkage can be measured at

sev-eral points along the part length Cross-flow shrinkage

can be determined at several points along the part Ribs

and walls are known by designers to restrict part

shrink-age In the test mold shown, ribs have been added along

several edges in order to determine their effects on

shrinkage for each resin being evaluated

Part design offers several techniques for

control-ling and minimizing shrinkage of a molded part

Fea-tures such as edge stiffeners and ribs can be helpful in

minimizing warpage in parts molded from

fiber-rein-forced polymers This same technique is widely used

for controlling shrinkage in structural foam moldings

Gating schemes are also used to minimize fiber

orien-tation A part with a large number of gates spread evenly

over the surface will have short flow lengths, will fill

primarily with radial flow patterns, and will pack

uni-formly Mold design is considered in more depth in

Ch 5 By reducing the degree of anisotropic

shrink-age, these design and molding factors can be used to

help reduce warpage in a finished part

When fiber-reinforced polymers need to be used in

the production of plastic parts with tight dimensional

or flatness requirements, computer mold-filling

simu-lations combined with shrinkage and warpage

analy-ses can be helpful These simulations help the designer

see how fibers will orient during mold filling, and give

some prediction of their impact on dimensions andwarpage However, it should be noted that CAE simu-lation is not yet an exact science The sophistication ofthe material models used, the accuracy and complete-ness in characterizing the polymeric materials, and theskill of the CAE operator all can affect the results.CAE models are useful today to predict trends in partperformance and show likely problems in tooling andmolding Molding conditions that vary in part produc-tion will generally cause the part to perform differ-ently than was predicted during CAE evaluations

In summary, shrinkage is controlled by both thetype of reinforcement and the concentration of the filler

in the composite By controlling the type, shape, andlevel of reinforcement, a composite can be producedwhich exhibits these characteristics:

• Less internal stress

• Greater mold shrinkage uniformity

• Lower warp tendenciesThe following five factors have the greatest influ-ence on final part shrinkage for reinforced thermoplas-tics:

• Base polymer shrinkage (amorphous orcrystalline polymer)

• Type of reinforcement (based on aspectratio)

• Level of reinforcement

Figure 4.9 The effects of glass bead, flake, and fiber on in-flow and cross-flow shrinkage.[6](Reproduced by permission of Hanser-Gardner.)

• Molding conditions used in production

• Part design

When organic pigments are added to plastic, the

shrinkage anisotropy, defined as the difference between

the shrinkage parallel and perpendicular to the flow,

can increase by more than 300% In certain geometries,

some pigments also reverse the sign of this anisotropy

The warpage triggered in molded parts by these

pig-ments can be difficult to remove by adjusting

process-ing parameters

Some pigments, primarily the organic pigments,provide crystalline nuclei from which crystals grow.Earlier initiation of crystallization and more rapid crys-tallization result in a higher amount of crystallinity inpigmented resin when compared to natural resin.While molders may prefer the pigment supplier toreformulate the pigment to reduce shrinkage, it maynot be possible to cause all pigments to affect the resinshrinkage equally It is more likely that the molder willhave to adjust the molding conditions or fillers to com-pensate for variations from pigment to pigment.Table 4.4 shows the shrinkage for natural (uncol-ored) PBT and PBT with different pigments and con-centrations.[12] Note that all pigments caused an in-

Table 4.4 The Effect of a Variety of Pigments on the Linear Shrinkage of PBT

Colorant Type Color Index Concentration

(%)

Shrinkage (mm/mm)

Shrinkage vs Natural (%)

-Inorganic

Organic

crease in shrinkage The pigments usually promote

shrinkage by acting as a nucleating agent

The use of pigments tends to increase the

cross-flow shrinkage in semicrystalline materials For

ex-ample, polypropylene typically shrinks about 10% more

in the in-flow direction than in the cross-flow

direc-tion Some blue and red pigments can cause the

cross-flow shrink to increase to 40% more than the in-cross-flow

direction Especially notable are the following organic

pigments: phthalocyanine blue, quinacridone violet, and

indanthrone blue

Inorganic pigments such as ultramarines,

manga-nese violet, and carbazole violet cause the same type

of shrinkage change to a lesser degree.[13] The

pres-ence of foreign bodies like pigment particles or regrind

particles can effect the crystallization and, therefore,

the mold shrinkage Figure 4.10 shows the effect of

different pigments on Delrin® 500.[14] The results shown

here were obtained using standard bars The values

are not necessarily valid for all part configurations;

however, the effect on the test bars compared to the

natural material can indicate a trend in other molded

parts

Seemingly minor variations and irregularities

af-fect filling patterns, temperature, and shrinkage In

Ch 5, it is shown that seemingly balanced runner

sys-tems can cause variations in temperature and filling

patterns in multiple cavity molds

Minor variations in the temperature of one half of

the mold with respect to the other half encourage a

flow shift away from the center of the part toward the

warmer half of the mold because a thicker skin forms

on the cooler side of the flow path Assuming an lutely flat cavity, this flow shift results in an area ofgreater shrinkage that is slightly removed from the cen-ter or theoretically neutral axis of the part The off-center shrinkage creates a bending moment that tries

abso-to make the part concave abso-toward the warmer side Thisbending moment may be resisted by the stiffness of thepart until long after it is molded or until it is exposed

to elevated temperature If the moment is small enough,

it may not be noticed or ever cause problems; theless, it is there The temperature variations can becaused by uneven distribution of water lines or varia-tions in coolant flow rates, temperature, or patterns.When ribs are present, the flow is divided and theside branch is normally filled with cooler material whilethe warmer material tends to divert slightly toward therib or branch This tendency to move the warmer flowtoward the rib leads to off-center cooling, as above, aswell as the shrinkage normally associated with insidecorners of molded parts that is discussed in the MoldDesign chapter (Ch 5) In most cases, if temperaturesare relatively uniform, these variations will not sig-nificantly affect the end result Most mold-filling analy-ses operate on the assumption of symmetry Asym-metric analysis is more time consuming and costly andshould normally be used when there is significant tem-perature differential across the mold or where thereare numerous large ribs on one side of the part Evenunder these conditions, there may not be enough shrink/warp to significantly affect the function of the moldedpart

never-Figure 4.10 The effect of selected pigments on mold shrinkage of Delrin® 500 in a 2-mm thick part In some cases, different formulations of the same color are shown [14] (Courtesy of DuPont.)