6 Causes of Molded-Part Variation: Processing

Melt temperature, injection pressure and speed, holding pressures and time, mold-ing temperature, and demoldmold-ing temperature contrib-ute to successful molding conditions.. The higher

Process variables cause significant mold

shrink-age and warpshrink-age effects Melt temperature, injection

pressure and speed, holding pressures and time,

mold-ing temperature, and demoldmold-ing temperature

contrib-ute to successful molding conditions As is a recurrent

theme in this book, the effects of these process

vari-ables are interactive not only with each other but also

with materials, part design, and mold design variables

The processing of plastics takes place under specific

molding conditions set by the particular variables A

framework for understanding the molding conditions

and process variables that most affect shrinkage and

warpage are examined in depth in this chapter

Plastic materials have positive coefficients of

ther-mal expansion and are compressible in the molten state

As a result, the volume that a given mass of material

occupies will change with both temperature and

pres-sure Some general molding-condition considerations

are applicable to volume change:

1 The lowest possible plastic melt-temperature

that permits good molded parts will tend to

produce less shrinkage The smaller the

tem-perature range between the molten plastic as it

enters the mold and room temperature, the

smaller the amount of thermal contraction and

the less time available for crystallization

2 Because plastic is compressible, the amount

of holding pressure (after the cavity is filled)

affects the shrinkage of the plastic part Note

that the duration and effectiveness of packing

is dependent, to a great extent, on the size and

design of the gate and the runner system After

the gate or runner freezes, no further benefit

can be gained by continued application of

pack-ing pressure The higher the pressure in the

cavity when the gate freezes, the greater the

mass of plastic that is trapped in the mold and

the lower the total shrink of the molded part

Higher packing and holding pressures

gener-ally lead to a global reduction in mold

shrink-age, while lower pressures increase shrinkage

Unfortunately, pressures in the cavity vary

from a maximum at the gate to a minimum at

the end of the flow, due to melt compressibil-ity and viscoscompressibil-ity The pressure differential over the length of the cavity can be very signifi-cant, particularly for longer flow lengths or thinner-walled parts This pressure-history dif-ferential, which occurs over the course of the cycle, results in mold shrinkage values that tend

to be greater towards the end of the cavity com-pared to shrinkage closer to the gate area Dif-ferential mold-shrinkage due to cavity-pressure history differences can also lead to dimensional distortion or warpage of the molding

3 Longer holding times during the cooling por-tion of the cycle cause the plastic to stretch a little in the mold, thus reducing apparent shrinkage The core or other details of the mold restrain shrinkage as long as the part is trapped

in the mold This causes the plastic part to stretch and yield somewhat when the molding cycles are long The mold itself acts as a cool-ing fixture

4 Hot molds increase mold shrinkage but reduce post-mold shrinkage Cold molds have the op-posite effect Cold molds (cooling the plastic

as rapidly as possible) reduce shrinkage, es-pecially when molding crystalline materials However, they do freeze-in some stresses that may be relieved later with time and exposure

to elevated temperature Within some limits, a semicrystalline plastic will try to crystallize further, especially if it is exposed to elevated temperatures

5 Semicrystalline thermoplastics are particularly influenced by the cooling rate The polymer chains in the melt are in a disorganized state (from a crystallization standpoint), and

in solidification they form a dense structure With increasing crystallinity, the density and the shrinkage of the structure increases

6 Extra cooling is required on core pins and in-side corners of plastic parts to encourage the plastic to cool evenly Core pins and external corners of mold cores have more surface area exposed to heat per unit volume than other areas of the mold This causes greater heat loads on core pins and external corners of mold cores

7 Where accurate parts are necessary, a

mold-ing machine in top-notch shape with

well-cali-brated temperature and pressure controls will

give the most consistent parts

The effects of various changes in molding condi-tions on molded parts are organized into a series of

graphs labelled (a) through (o) in Fig 6.1.[31]

Figure 6.1 Injection-molding machine settings can affect properties of thermoplastics.[31] (Reprinted by permission of Hanser-Gardner.)

Graph (a) illustrates the effect on molecular

orien-tation in response to changes in several other variables

As mold temperature or cavity thickness increases,

more time is available for stretched and oriented

mol-ecules to relax and reorient before the melt solidifies

and freezes the molecular orientation As injection

pres-sure and packing time increase, more stress and

stretch-ing of the molecules occur and are maintained closer

to the freezing time This increases orientation in two

ways The higher stress causes higher orientation in

the first place The longer packing time maintains a

low level of flow into the mold for a longer time, which

maintains more orientation Initially, as melt

tempera-ture increases, the individual molecules have more

free-dom to align themselves with the flow of the material

However, as the melt temperature rises further, the time

the part remains molten increases after the part is filled

out This allows more time for stress relief and

mo-lecular disorganization

Graph (b) shows how pressure loss through the

gate decreases as melt temperature increases because

the plastic becomes less viscous as it is heated and is

easier to push into the mold At low temperatures, it is

hard to supply enough pressure to fill the cavity As

stated elsewhere in this book, low shrinkage is

associ-ated with low injection temperatures As temperatures

increase, so does shrinkage; so while the cavity is easier

to fill at higher temperatures and filling and packing

pressures are more effective at higher temperatures,

eventually, the higher shrinkage associated with high

melt-temperatures tends to overcome the filling and

packing pressures, leading to higher shrink

In graph (c), the falling weight (f.w.) impact

strength of a part is increased as melt temperature

in-creases The higher temperature allows for filling with

lower stress and longer time for stress relief, both of

which yield lower molded-in stresses and higher

im-pact strength

In graph (d), the effect of mold temperature on flow

and cross-flow shrinkage is shown Higher

tempera-tures allow more time for disorganization of the

mol-ecules and thus more stress relief In semicrystalline

materials, higher mold temperature allows more time

for crystallization and more shrinkage

Graph (e) shows that part-weight increases with

increasing packing or holding time, up to the point

where the gate freezes After that time, more holding

or packing time does not affect the part weight

Higher melt and mold temperatures, as shown in

graph (f ), allow more time for the material to conform

perfectly to the mold surface, and to achieve a higher

level of gloss

Higher melt temperature, graph (g), can affect the

IZOD impact-strength in two ways First, longer ex-posure to higher melt-temperatures increases the heat history of the plastic and typically causes material deg-radation and a reduction of property values Under cer-tain circumstances, higher melt-temperature causes in-creased molecular orientation (and molded-in stress) This means there is less give in the molecular structure before the molecules reach their breaking point There-fore the part is more brittle

High IZOD impact-strength in polypropylene cor-relates well with successful application of the living hinge If a part has a low IZOD impact-strength, that implies low elongation before rupture It should be ob-vious that a great deal of elongation is necessary for a living or integral hinge application to be successful

As the cavity thickness increases, graph (h), there

is more time for the plastic to relax internal stresses A greater percentage of the thickness of the part will be

in tension More time is available for crystallization of semicrystalline materials Thus there is more shrink

Graph (i) shows that increasing packing time and

pressure increases cooling time More material is forced into the cavity by increasing the packing time and pres-sure Since there is more material in the cavity, there is

a slight increase in cooling time due to the increased mass of material to be cooled

As mold temperature increases, graph (j), there

is more time for crystallization to take place That means that, all other things being equal, the plastic will be denser because of the greater degree of crys-tallization

Higher melt temperature, graph (k), reduces the

stress on the material as it flows into the mold be-cause the viscosity of the material is lower Also, the higher melt-temperature allows more time for any stresses to relax as the material cools Lower

molded-in stress levels mean the material can withstand higher temperatures before it distorts In other words, higher melt-temperature equates to higher heat-distor-tion temperature

The more material that is packed into the cavity before the gate freezes, the less the shrinkage will be

Graph (l ) shows that as long as the gate is fluid,

in-creasing packing time reduces shrink Once the gate freezes, more packing time is of no value The graph shows these effects for a restricted gate and for an open gate It also shows the effects of small (restricted) gates versus larger (open) gates The larger the gate, the easier

it is for more material to flow into the cavity Thus, larger gates lead to less shrinkage even with very short packing time In addition, the larger gates will stay fluid

longer, allowing more material to be packed into the

mold before the gate freezes, which also reduces

shrinkage

Graph (m) shows that thick parts or short

flow-paths fill easier and at lower pressure than thin parts

or long flow-paths The lower the pressure required to

fill the cavity, the less the clamp pressure required to

hold the mold shut Conversely, long flow-paths or thin

parts, requiring high injection-pressures, demand higher

clamping pressure to prevent flashing at the parting

line

Graph (n) shows that as the distance from the gate

increases, the density of the plastic decreases This is

caused by two phenomena First, the plastic furthest

from the gate is cooler than that at the gate; therefore it

does not have as much time to crystallize Secondly,

the pressure is higher at the gate than it is anywhere

else Lower pressure away from the gate leads to lower

density also

Weld tensile strength increases with increasing

in-jection pressure, as shown in graph (o), because the

strength of the weld line is proportional to the force

with which the two flow fronts are forced together Also,

higher injection-pressure implies more shear at the gate

and a higher melt-temperature when the flow fronts

meet, allowing more time and better (higher

tempera-ture) conditions for some molecular migration across

the front

There are upper and lower limits for mold tem-perature, melt temtem-perature, and injection pressure that can produce a fully filled part with no flash Figure 6.2 represents a typical upper and lower limit of injec-tion pressure and mold temperature for a given melt temperature.[31] The area inside the curve is called the

molding window Higher temperatures or pressures

cause flash around the part Lower temperatures or

pressures result in a short shot, that is, an incomplete

part

A more accurate representation of the molding win-dow is a three-dimensional graph of conditions that permit the mold to fill without flashing Figure 6.3 shows such a three-dimensional window.[31] The size and shape of the window will vary with the design of the part, the mold, and the plastic being molded The rate at which the cavity is filled has some influence on the size and shape of this window, however the influ-ence is relatively small It behooves the mold designer and the part designer to maximize the size of the mold-ing window The larger the moldmold-ing window, the more flexibility the molder has to take action to control shrinkage and warpage

Figure 6.2 A molding “window” at a given melt

temperature [31] (Reprinted by permission of

Hanser-Gardner.)

Figure 6.3 A three-dimensional representation of the

“molding window.” [31] (Reprinted by permission of Hanser-Gardner.)

6.2 Injection Melt Temperature

The U-shaped curve in Fig 6.4 shows that

shrink-age is higher at both high and low melt temperatures

At low melt temperatures, the plastic barely fills the

cavity before the gate freezes The pressure gradient

from gate to end of flow is high and there is no

signifi-cant time to pack-out the cavity The pressure at the

end of flow is low, so the shrinkage is high

At high melt temperatures, a lot of shrinkage is

inherent as a result of temperature change The melt

core is hotter when the gate freezes than it is at lower

melt temperatures (unless the gate is the same

thick-ness as the part) At very high melt temperatures, the

holding time may end before the gate freezes This can

happen when melt temperature is raised without

in-creasing the holding-pressure time Both the high

melt-core temperatures and the likelihood that the gate stays

open past the holding time cause increased shrinkage

At some midpoint, the melt viscosity is such that a good

balance of pressure exists across the cavity with good

cavity-packing when the gate freezes At this point,

the shrinkage due to melt temperature is at a minimum

Melt temperatures range from a low of about 350°F

up to 700°F or more, depending on the plastic being

molded

The injection rate and the injection pressure are

interrelated in the injection-molding process On older

molding machines, a flow-control valve controlled the

maximum rate of injection, but the minimum rate was

determined by the injection-pressure setting The

in-jection rate has a twofold effect:

First, a slow rate of fill allows a thicker wall to build up as the material flows into the mold, thereby raising the pressure requirements to fill the mold A slow fill-rate results in cooler material at the end of the fill cycle A thicker, cooled wall causes a smaller flow-channel in which makeup resin flows, and a greater pressure-drop across the part during the holding phase

of the molding cycle Cooler plastic can cause prema-ture freezing at the gate, less effective packing, greater orientation, and more shrinkage

Secondly, there is a certain amount of friction heat-ing that occurs at the gate caused by the pressure drop across the gate Higher fill-rates raise the melt tem-perature in the cavity Higher melt temtem-perature allows the injection pressure to be more effective in filling the cavity and all fine details within it Excessively high fill rates can cause plastic degradation and flash The density of crystalline polymers is inversely pro-portional to the distance from the gate, because pres-sures and temperatures near the gate are higher than they are at locations remote from the gate Higher pres-sure and temperature near the gate allow more time for crystallization and more packing time

High injection-pressure forces the two advancing plastic flow-fronts, downstream of a core where the flow

is divided around the core, into more intimate contact, which helps create a better bond at the weld line Inadequate cavity pressure can fail to hold the plas-tic against the walls of the cavity tightly enough to form a smooth skin Surface wrinkles are more com-monly associated with low injection-pressure and low injection-rate The low rates allow a thin skin to form along the cavity walls This thin skin sometimes moves slightly with the flowing plastic causing a wrinkle The lower the cavity pressure and injection speed, the more pronounced the appearance of the wrinkle

Injection rate, or speed, influences secondary heat-ing of the plastic as it moves through the gate and into the mold Frictional heat is generated at the gate re-striction, and between the flowing material in the cen-ter of the part and the macen-terial against the walls of the part that have already solidified There can be a sig-nificant temperature rise, primarily at the gate, depend-ing on pressure and injection speed One study shows

a peak temperature at the interface between the frozen plastic against the mold wall and the moving molten plastic in the center of the thickness of a plastic part.[4]

Figure 6.4 The relationship between melt temperature and

shrinkage.

An injection rate versus shrinkage curve is as

shown in Fig 6.5

Figure 6.6 illustrates that if the fill rate is too slow,

the material begins to cool before the cavity is filled

and the pressure required to fill the cavity goes up

Too low an injection rate inhibits the packing of the

cavity because the material cools during the filling

phase and the gate will freeze very quickly after the

mold is filled This leads to higher shrinkage

At the other extreme, if the cavity is filled too

quickly, the pressure drop at the gate becomes

exces-sive and the pressure required to fill the part goes up

Shrinkage can increase because the temperature of the

plastic in the cavity is likely to be significantly higher

than the temperature at optimum fill rate The

opti-mum fill rate is found near the miniopti-mum filling

pres-sure The optimum fill rate for a part depends on its

geometry, the size and location of the gate, the mold

temperature, and the melt temperature Rapid fill rates

often create a better surface finish; although rapid fill

also can cause jetting and/or gate smear Warpage can

sometimes be improved or eliminated by careful

ad-justment of injection speed and mold temperature When

molding parts with thick sections and a relatively small

gate, it is sometimes helpful to raise mold

tempera-tures and reduce injection rates to delay gate freeze

These changes usually sacrifice some surface gloss or

finish, but yield an improvement in molded part

shrink-age

Injection pressures must be high enough to fill the

cavity, forcing material into the furthest reaches

In-jection pressures commonly range between 70 and 112

MPa (10–16 kpsi) Higher pressures tend to minimize

average mold shrink The maximum injection pressure

may be limited by the clamping capacity of the mold-ing machine because the effective cavity pressure (less than the nozzle pressure) times the projected area of the part must not exceed the clamping pressure Ex-cessive pressure can cause the mold to open and allow parting-line flash or even damage a structurally un-sound mold

There usually is some variation in cavity pressure with the highest pressure near the gate and the lowest

at the last point to fill The variation in pressure de-creases with the increase in material thickness, the in-crease in injection or holding pressure, the dein-crease in material viscosity, and the increase of injection speed (possibly due to the lower viscosity caused by higher frictional heating at the gate at high injection-speeds) Usually the material furthest from the gate solidifies first The frictional heat generated at the gate usually keeps the material closest to the gate somewhat warmer, causing that area to solidify last A warmer gate area often results in less shrinkage near the gate, which is caused by longer sustained pressure near the gate Dif-ferential shrinkage (warpage) is partly the effect of dif-ferential cavity pressure

Note that molds have been bent by excessive in-jection pressure and area, combined with inadequate

or concave platens on molding machines Molding ma-chines can be damaged by improper setup of small molds on large platens For example, if the platens are deflected into a concave shape by excessive clamping pressure on small molds, the bending stress may be beyond the yield strength of the platens and they can

be permanently bent If a large mold is then mounted

on these bent platens, the mold can be forced open in

Figure 6.5 Relationship of injection rate to shrinkage Figure 6.6 The relationship between the wall thickness, the

injection pressure required for filling the part, and the fill

time for the part, where t = wall thickness.

the center under injection pressure The mold plates

then conform to the shape of the platens, bending the

mold plates and causing flash in the center of the mold

The molder, thinking that he needs more clamping

pres-sure to hold the mold shut, may increase the clamping

pressure This does not hold the mold shut because the

platens are bent The molder may increase the

clamp-ing pressure to the point that the corners of the mold

are compressed beyond the yield strength of the mold,

hobbing (distorting) the corners of the mold This kind

of damage causes the molder and mold builder a great

deal of expense and grief

One way to monitor the cavity pressure is to place

a flow tab in the mold A flow tab is sort of a heavy

vent off a runner (or even a part) See Fig 6.7.[26] The

flow length that is expected from the material being

molded determines the depth of the flow tab Typically

the flow tab is about 0.5 mm (0.020 inch) thick at any

convenient width and is marked at regular intervals It

is cut from the runner to the edge of the mold (to the

atmosphere) A flow tab is an excellent runner vent

The flow tab must be thick enough to easily see a

varia-tion in flow length for any significant variavaria-tion in

nor-mal molding conditions The length of the tab should

be such that the plastic will normally flow about half

the length of the tab Variations in injection pressure

or speed, mold temperature, or melt temperature will affect the length of the plastic flow into the flow tab A flow tab will not give any indication of holding pres-sure or holding time because it is likely that the flow tab will have frozen before the cavities fill Consistent part-weight is a better indication of holding pressure and time

Cavity pressure at the moment the gate freezes is roughly inversely proportional to shrinkage Because plastic is compressible, the greater the cavity pressure, the less the shrinkage As a general rule, the higher the holding pressure, the less the shrink However, if ex-cessively high holding pressures are held long enough (generally this requires a rather large gate), some ma-terials will seem to grow If there is a sufficiently high pressure compressing the plastic in the cavity at the moment of gate freeze, the compression can exceed the shrink When molding conditions reach this point, it is usually difficult to remove the part from the mold be-cause the part is larger than the cavity

One study determined that holding pressure has a greater effect on shrinkage than any one other vari-able, when molding polypropylene homopolymers (EXON Escorene® PP 1105) Figure 6.8 illustrates this relationship

Figure 6.7 An example of a flow tab.[26] (Courtesy of

DuPont.)

Figure 6.8 The relationship between cavity pressure

(holding pressure) and shrinkage.

According to this study, increasing packing

pres-sure decreases shrinkage The effect of injection rate is

small compared to packing pressure Increases in

pack-ing time decreased the amount of shrinkage whereas

increases in mold temperature did not have any

appre-ciable effect on shrinkage

Shrinkage after 168 hours was greater than in-mold

shrinkage The shrinkage variation in direction of flow

immediately after molding was approximately 1%, and

after 168 hours it was approximately 1.3% As seen

from Fig 6.9,[32] lower injection velocities (rate)

pro-duced less shrinkage immediately after molding, but

more shrinkage 168 hours later Shrinkage in the

di-rection of flow was significantly greater than in the

transverse direction

For Delrin® and other semicrystalline materials,

unlike polypropylene, both the packing time and

pres-sure have a great effect on the degree of

crystalliza-tion, along with other factors The larger the gate and

the hotter the mold, the longer the packing pressure

can be applied Higher mold temperatures allow more

time for crystallization, which causes more in-mold

shrink However, high mold temperatures increase

long-term stability because there is less post-mold shrink in

a part molded in a warmer mold The longer the

hold-ing time and the higher the holdhold-ing pressure, the less

apparent the shrink The holding phase is very

impor-tant for dimensional stability since it helps maintain a

uniform and gradual crystallization Figure 6.10 shows

the effect on shrinkage of holding pressure at three mold

temperatures.[33] There is further discussion of the

ef-fect of mold temperature in Sec 6.4

Figure 6.9 Mold shrinkage in the direction of flow

immediately after molding and after 168 hours, vs packing

(or holding) pressure for two injection rates [32] (Courtesy

of SPE.)

Figure 6.10 The effect of holding pressure on mold

shrinkage at three different mold temperatures, for Delrin ®

500 HPT: holding-pressure time [33] (Courtesy of DuPont.)

Note that, as discussed in Ch 5, the minimum gate dimension must be at least one-half of the part thick-ness Thin parts may require gates that are thicker in proportion to the wall thickness An inadequate gate size will cause higher mold shrinkage The holding-pressure time (HPT) effects are shown in Fig 6.13 in Sec 6.4.2,[33] and must be sufficient to hold pressure

on the cavity until the gate has frozen

The shape of the molded part determines the amount

of resistance to shrinkage that the part will experience The greater the restraint, the less the apparent shrink-age Post-mold exposure to time and higher tempera-tures will encourage post-mold shrinkage Table 6.1 shows shrinkage for some Delrin® grades.[14]

Flow patterns and distance from the gate also af-fect shrinkage Shrinkage far from the gate is typically 0.1% to 0.3% higher than the shrink near the gate Shrinkage in the flow direction is typically about 0.1% higher than the cross-flow shrinkage for Delrin® Hold-ing pressure can be used for small adjustments of part dimensions It has very little effect on post-molding shrinkage

Flow patterns during the holding phase can be un-even There is a tendency toward a “river delta” effect: Any area that is slightly warmer than an adjacent area has less resistance to flow; therefore, it is more likely

to move and remain warmer than cooler areas nearby The warmer areas (the tributaries) will try to shrink more than the cooler areas This is one cause of high stress in the gate area

6.4.2 Holding-Pressure Time

Inadequate holding-pressure time (HPT) will

al-low material to expand out of the cavity into the

run-ner system, if the holding pressure is removed before

the gate has frozen Once the gate is solid, plastic can

no longer flow into or out of the cavity Additional

holding-pressure time after the gate freezes is

nonpro-ductive All it does is use energy, add wear-and-tear to

the molding machine, and add to the machine

oil-cool-ing load

Figure 6.11 shows the time required for a

semi-crystalline (nylon) material to crystallize from a

par-ticular melt temperature.[9] Note that the addition of

nucleation agents, reinforcement, or pigments decrease

the time required for crystallization Other

semicrys-talline materials have different crystallization times

The HPT must exceed the crystallization time to

mini-mize shrinkage If the gate crystallizes before the part

does, the effective HPT is reduced by virtue of the

fro-zen gate

Figure 6.12 shows that as holding-pressure time increases, there is an initial drastic reduction in the shrink rate As holding-pressure time increases further, the rate of reduction in shrinkage decreases until there

is no further reduction in shrink

Figure 6.13 shows the influence of holding pres-sure on Hytrel® 55 to 80 shore D materials.[34] These are softer, more rubbery materials than Delrin®, but

the influence of HPT (also called screw-forward time)

is readily apparent As usual, longer holding time re-sults in less shrinkage

Average Mold Shrinkage Delrin ® grade

In-flow (% ± 0.2%)

Transverse (% ± 0.2%)

511 P, 911 P 1.9 1.8

Colors* 1.8–2.1 1.7–2.0

*Depends on the pigments

Table 6.1 Average Mold Shrinkage for Various

Grades of Delrin ®

Figure 6.11 Crystallization time for several nylon grades.

The parts are 2 mm in thickness, molded at typical mold temperature with a hold pressure of 85 MPa The melt temperature is 290°C [9] (Courtesy of DuPont.)

Figure 6.12 The approximate relationship between

holding-pressure time (HPT) and shrinkage.

Figure 6.14 shows the HPT for three different wall

thicknesses of Delrin® 500 P.[33] The drastic

reduc-tion in shrinkage as the holding time increases is readily

apparent For any given part, and considering only the

change in holding pressure time, the minimum

shrink-age occurs when the HPT lasts until the gate freezes

Another conclusion that can be drawn from this figure

is that thicker walls cause slower cooling, which

in-creases crystallization Finally, the thicker walls

re-main melted longer than thin walls, allowing more time

for thorough packing of the cavity, provided the gate is

large enough, which results in less shrinkage Too short

a packing time can also cause porosity, voids, warpage,

sink marks, lower mechanical properties, and surface

pits or blemishes

Mold temperature affects the cooling rate The

faster the plastic part cools, the less time the individual

molecules have to order themselves and the less the

molded part shrinks Crystalline plastics require some

time to rearrange their molecules into the crystalline

Figure 6.14 The effect of holding-pressure time (HPT) on

mold shrinkage of Delrin ® 500 P for three different wall thicknesses [33] (Courtesy of DuPont.)

Figure 6.13 The influence of HPT (screw-forward time) on

the shrinkage of Hytrel ® 55 to 80 shore D materials for

3.2-mm ( 1 /8-inch) thick samples [34] (Courtesy of DuPont.)

structure The more time available, the larger and more numerous the structures and the more the material shrinks Likewise, amorphous plastics relax internal molecular stresses when cooled slowly, and the in-creased order and relaxed stresses result in greater material density and shrinkage Therefore, rapid cool-ing reduces shrinkage However, rapidly cooled parts are more prone to post-mold shrinkage and warpage with the passage of time and exposure to heat When parts are exposed to higher temperatures in their service life than the mold temperature at which they were manufactured, they may exhibit unusually high and possibly unacceptable post-mold shrink Higher mold temperatures increase cycle times and the time available for molecular stress relaxation in amorphous materials In semicrystalline materials, the longer cycle times also allow more time for crystalli-zation to occur In both cases, with rare exceptions (see the Zenite® LCP aromatic polyester resins), short-term shrinkage increases Post-mold shrinkage, however, decreases Within limits, higher mold temperatures improve long-term stability and minimize post mold shrink and creep