8 Controlling Mold and Post-MoldShrinkage and Warpage

Until the gate freezes, the holding pressure addsmaterial to make up for any shrinkage during cooling.Even after the gate freezes, the part continues to shrink.The extent of plastic part

Shrinkage and Warpage

In practice, plastic-part dimensions and potential

for warpage and internal-stress levels will be influenced

by a variety of parameters such as material, tooling,

and processing-related factors discussed in earlier

chap-ters Some of the factors associated with dimensional

control are further discussed in this chapter,

empha-sizing a systematic and practical approach Generally,

the best approach is done in this order:

Find the cause of the problem This is the most

important step Making changes to the processing

pa-rameters or to the mold without understanding the cause

of the problem could make things worse

Revise the processing parameters Often a

modi-fication of the molding parameters can reduce the

shrinkage and warpage enough to make satisfactory

parts This is the first and least expensive change to

make, unless a significantly longer cycle-time is

nec-essary If the cycle time causes a significant part price

increase, it may be more economical to consider one or

more of the following

Try a different material Sometimes a change of

material or reinforcing filler can improve shrink and

warp

Modify the tooling Tooling changes of any kind

are much more expensive than process changes, unless

high quantities of parts and longer cycle-times offset

the costs of tooling modifications

Redesign the part Part redesign is the most

ex-pensive and time-consuming modification Part

modi-fication implies tooling modimodi-fications as well Much

of the material in the previous chapters of this book

address the design of parts to minimize shrinkage and

warpage If the guidelines mentioned earlier are

fol-lowed, this step should never be necessary

8.1 Finding the Cause

What has changed? The part may not have changed

at all, but the inspector or the inspection criteria may

have changed It is possible that the part was never

fully specified in writing and “signed off,” but was

nev-ertheless approved by someone in authority If the

au-thorizing person has withdrawn and will not accept

responsibility for the approval, and the mold builder

takes the position that “you approved it, you bought it,

its yours;” a messy lawsuit may ensue

Is the customer using incoming inspection to trol inventory? Maybe the product is not selling as well

con-as expected and he does not want to buy any moreparts right now That is why a clear and documentedunderstanding of what is acceptable must be on hand,and the customer must be obligated to accept good parts

if they have been ordered In other words, you musthave documents that allow you to reject his reject

On the other hand, if the part did at one time meetall inspection criteria and does not now, then some-thing truly has changed

The following checklist is a general guide for ing the cause of shrinkage and warpage problems:

find-1 Is the mold running on the same moldingmachine? A different machine will prob-ably have a different-sized heating cylin-der, so the residence time will be differ-ent for the material The actual pressure

on the plastic during injection may bedifferent, even though the hydraulic pres-sure is the same Each molding machinehas a step-up ratio between the hydraulicpressure and the actual pressure at thenozzle; the most common step-up ratio is

10 to 1, or the plastic has ten times thepressure of the hydraulic pressure in theinjection cylinder The actual temperatureinside the heating cylinder may be differ-ent due to thermocouple location, heater-band location, or the thermal conductiv-ity of the heating cylinder

2 Has the mold been damaged in somemanner that causes an unacceptable part?For example, minor flash problems, if notstopped, usually lead to major flash prob-lems The flash, being thinner than themolded part, shrinks less in the mold thandoes the part As the part cools, the cav-ity pressure is reduced until the full ton-nage of the machine is applied to the thinflash between the parting lines This of-ten results in progressively more defor-mation of the steel at the flash point andprogressively more and larger flash

If neither of the above apply, then the problem isprobably related to the process or material:

3 Examine the processing conditions Is the

plastic being molded at the proper

tem-perature and pressure? Is the holding time

adequate? Is the cure time adequate? Is

the plastic dry enough as it enters the

molding machine? Are there variations in

cycle time or ambient temperature?

4 Is the mold temperature correct? Are the

cooling hoses and fittings of adequate

size? Are they the same size or

configu-ration as when acceptable parts were

made? Are there adequate coolant

feed-lines to separately feed each cooling zone?

Is the temperature of the cooling water

constant? Is the flow of the cooling

wa-ter constant?

5 Is the flow pattern, combined with

mo-lecular or fiber orientation, contributing

to shrink or warp? Can a material change

improve the orientation problem? Can a

change in the number or location of gates

improve the flow pattern?

6 Are there thickness variations or ribs that

are causing uneven shrinkage? Are there

bosses attached to sidewalls that

contrib-ute to thickness variations? Is the part

constrained in one area and not another,

causing uneven shrinkage?

7 Are the tolerances unrealistic? Will the

part fulfill its fit and function

require-ments even though it does not meet the

print? One possible part-design solution

is to loosen tolerances

And finally:

8 If good parts were never produced on the

mold, then there may be a tooling

prob-lem that must be addressed

8.2 Processing Considerations

The injection-molding process is a semicontinuous,

sequential process with a number of phases as described

elsewhere (see Ch 6) The packing phase of the

pro-cess begins once the melt flow-fronts have reached the

extremities of the cavity Since plastics are

compress-ible to a fair degree, the magnitude of the packing

pres-sure determines the weight of material ultimately

in-jected into the fixed-mold cavity volume Holding

sure is applied to the plastic melt in the cavity via

pres-sure on the molding-machine screw through the sprue,runner, and gate until the gate freezes The frozen gatekeeps any plastic from leaking out of the cavity there-after Until the gate freezes, the holding pressure addsmaterial to make up for any shrinkage during cooling.Even after the gate freezes, the part continues to shrink.The extent of plastic part shrinkage and potentialwarpage is a direct result of the pressure transmitted

to each section of the part via the gate and runner tem Areas experiencing the highest pressures will ex-hibit the lowest amounts of shrinkage Those sectionsnearest the gate will shrink the least The level of shrink-age will increase towards the periphery of the part.Since this situation is always present, warpage willresult if the part is exposed to elevated temperaturesthat are high enough to allow stress relaxation to oc-cur If the part has been designed with a uniform wallthickness, and if great care is taken in designing thegating system, wall thickness warpage still can result

sys-It may, at times, be advantageous to deviate from some

of the guidelines presented in this book in order to tain the desired result For example, it may be desir-able to gradually diminish the wall thickness from thegate area to the outer edges of the part to compensatefor the pressure gradient throughout the part Thethicker sections will tend to shrink more and help toadjust for any imbalances created by pressure differ-ences in the molding process

ob-8.2.1 Melt Temperatures and Uniformity

One of the many factors that affect the ity of the molding process is with the uniformity of themelt Several factors contribute to the melt uniformity

repeatabil-In the old days before screw injection units, it was siderably more challenging to make a uniform melt.The screw mechanism within the molding machine isdesigned to encourage uniformity due to its tendency

con-to assist in mixing the melt as it conveys the plasticforward along the screw Additional mixing and heat-ing is added as the backpressure on the screw is in-creased Backpressure is hydraulic pressure applied tothe injection side of the hydraulic cylinder that movesthe screw during injection Higher backpressure addsfriction heat to the melt and increases the mixing action.The following are some of the more common sources

of problems with melt temperature and uniformity

• Fast cycles with the molding machine at ornear its maximum plasticizing capacitycan lead to unmelted plastic pellets in the

melt stream and, obviously, to nonuniform

melt temperature and viscosity Under

these conditions, it is even possible for a

gate to be plugged by an inadequately

melted pellet of plastic before the mold

cavity is filled or adequately packed This

causes short shots or erratic shrinkage

• The molding machine itself may be the

source of a problem For example, if the

non-return valve in the injection unit is

leaking, the machine may not be able to

maintain injection or holding pressure

(“lose the cushion”), causing greater

shrinkage Nonuniform heating from

in-adequate backpressure or burned-out

heating bands can cause problems

• Inadequate mixing can cause uneven

shrinkage when colorant is added to the

melt Since colorants can act as

nucleat-ing agents, if the color is unevenly

dis-persed throughout the melt, the

crystal-linity ratio will be uneven, causing more

shrinkage where the colorant

concentra-tion is highest

8.2.2 Mold Temperatures and Uniformity

If mold temperature varies for any reason

through-out a product run, there is going to be some variation

in the shrinkage of the molded part As stated

else-where (see Ch 6), higher mold temperatures lead to

higher post-mold shrinkage, but more stable parts in

the long term However, if the mold temperature rises

without a corresponding increase in holding-pressure

time, there can be backflow out of the cavity into the

runner causing erratic shrinkage

Changes in the environmental temperature or

hu-midity can cause fluctuations in mold temperature

dur-ing the production run If a central cooldur-ing tower is

used, the ambient temperature of the cooling tower will

vary depending on the number of molding machines

running at any given time and on environmental

condi-tions Depending on a cooling tower without auxiliary

temperature-control devices is unwise

Many molding shops operate in an ambient air

con-dition That is, they do not have temperature and

hu-midity controls in the molding department Therefore,

ambient air temperature can influence the temperature

of the molding machine and its clamping system Air

temperature can affect the efficiency of the

molding-machine cooling system as well as the temperature

con-trols for the mold Radiation cooling of the mold andthe heating section of the molding machine influencetheir temperatures The temperature of the plastic pel-lets, as they are added to the molding machine hopper,can affect the heat load required to melt and processthe plastic And if there are openings to the outside ofthe building, such as overhead doors or windows,breezes through these openings can influence the mold-ing machine and end product

Humidity affects the efficiency of heat exchangersand the moisture content of plastic pellets As the mois-ture content of the pellets rises, the effort required toremove or boil off the moisture before and during themolding process increases This can influence the tem-perature and condition of the melt as it enters the mold.The percentage of regrind and its pellet size and mois-ture condition contribute to the temperature and uni-formity of the plastic melt Physical properties changewith each cycle through the machine and the grinder,and there may be some mechanical rupturing of themolecular chains Regrinding may also change thelengths of any fibrous reinforcements These variationsaffect the shrink rate, the strength, and the rigidity ofthe molded part

Inadequate coolant flow or too long a flow pathcan cause variations in mold temperature from start-

up until an equilibrium condition is reached Then, anyhesitation or inconsistency in cycle time will cause tem-perature fluctuations

The cooling load, due to gate proximity or sectionthickness variations in the molded part, may requirethat certain areas of the mold be cooled more aggres-sively in order to approximate the ideal condition ofcooling all areas of the molded part at the same rate.One of the more common problems in moldingshops is inadequate mold cooling The supply line to themolding machine from the cooling tower may be toosmall The pressure differential between the tower supplyand return lines may be too low There may not be a suf-ficientnumber of outlets to separately control each zone

of the mold Many molding shops have about four supplyand return lines available for the mold, while the moldhas eight or more cooling zones The usual (unsatis-factory) practice is to plumb several zones in series.For optimum performance, the water flow ratethrough the mold should be high enough that the flow

is turbulent Turbulent flow continually mixes the ter in the cooling channels so that the water against thewall of the cooling channel is the same temperature asthe water in the center of the channel If there is a no-ticeable difference in the inlet temperature and the out-let temperature, the flow is not adequate

wa-Are the feed lines to the mold large enough? If a

mold has cooling channels that are larger than the

in-side diameter of the feed lines or fittings, the cooling

flow is being choked and the mold cooling is inadequate

In critical applications, thermostatically controlled

water may be required on each cooling zone

8.2.3 Filling, Packing, and Holding Pressures

Both higher melt temperatures and higher mold

temperatures cause higher shrinkage; the influence of

mold temperature is generally the greater of the two,

since it usually may be varied over a greater range

But injection and holding pressures and time also have

a significant influence on shrinkage If injection or

hold-ing time and/or pressure are increased within limits

imposed by machine pressure and clamping

capabili-ties, the shrinkage decreases

Any of the following will tend to lower shrinkage

in polypropylene (and most other plastics as well) and

may be used in combination with other options:

• A plastic with a high melt flow index

• A plastic with controlled rheology

• An unnucleated plastic

• Increase the injection pressure

• Raise the holding pressure

• Extend the injection (hold) time

• Decrease the mold temperature

• Lower the melt temperature

Effective pressure in the cavity will vary with melt

uniformity, melt temperature, and mold temperature

Uniform cavity pressure from cycle to cycle is required

for constant shrinkage Molding-machine injection

pres-sures may vary because of machine wear or

molding-machine hydraulic-oil temperature variation caused by

inadequate cooling

Figure 8.1 shows a typical cavity-pressure trace

that indicates the pressure in the cavity during a

typi-cal molding cycle.[6] Initially, there is no pressure in

the cavity until the plastic flow-front passes the

pres-sure-measuring transducer Then the pressure increases

as the flow front moves past the transducer, and more

pressure is required to move the flow front as it moves

away from the transducer

When the cavity is full, there is a rapid rise in

pres-sure as the plastic in the cavity is compressed during

the packing phase At the end of the packing phase, the

pressure on the plastic is reduced for the duration of

the holding phase The rapid drop in pressure early in

the holding phase is a result of the programmed chine-pressure drop Then, as the plastic cools and be-comes more viscous, the pressure at the transducerdrops gradually because the holding pressure is notadequate to overcome viscous friction and maintain aconstant pressure throughout the cavity The position

ma-of the transducer relative to the gate affects the slope

of the pressure gradient in this phase The nearer to thegate the transducer is, the more constant the cavity pres-sure will appear to be If the transducer is remote fromthe gate, the cavity pressure will drop more rapidly.When the gate freezes, no more plastic can enter thecavity and the pressure drop is more rapid When theshrinkage exceeds the compression on the plastic, thecavity pressure drops to zero After this point, the in-moldshrinkage causes the part to become smaller than thecavity As long as there was positive pressure in thecavity, the part was potentially larger than the cavity.Finally, when the part has cooled enough to be struc-turally sound, the mold is opened and the part is removed.Process variables such as the magnitude of thepacking and holding pressures have a very significanteffect on the shrinkage and final dimensions of a moldedpart If appropriate packing and holding pressures arenot used, the volumetric shrinkage of a plastic mate-rial can reach as much as 25% Holding pressures must

be high enough to compensate for shrinkage, yet lowenough to avoid overpacking, which can lead to highlevels of residual stress and ejection difficulties

8.2.4 Filling, Packing, and Holding Times

Packing and holding times are discussed in detail

in Ch 6 The filling and packing time must be cient to allow the plastic to reach the furthest extremi-ties of the cavity and pressurize those areas to ensureminimum shrink there The holding time must exceedthe time required for the gate to freeze to avoid losingcavity pressure through the gate The holding pressure

suffi-Figure 8.1 A typical cavity-pressure trace.

is usually lower than the packing pressure to reduce the

pressure gradient across the cavity, that is, to allow

the region near the gate to have a cavity pressure more

nearly the same as the pressure remote from the gate

8.2.5 Part Temperature at Ejection

The part temperature at ejection must be low

enough that the part will not remelt or deform as it

continues to cool out of the mold On thick parts, it

may be necessary to provide a cooling bath to keep the

part from deforming See Sec 6.6

8.2.6 Clamp Tonnage

The molding machine must be able to hold the faces

of the mold together with sufficient pressure to

over-come the actual pressure in the projected area of the

cavity perpendicular to the parting line For example,

if the projected area of the cavity and runner system

was 10 square inches and the actual cavity pressure

was 4,000 psi, then there would be a separating force

at the parting line of 40,000 pounds or 20 tons The

clamping force of the machine must exceed this

sepa-rating force or the mold will open, the parting line will

be damaged, and there will be flash on the part Once

flashing occurs, it will get worse and parting-line

dam-age will increase

A common rule-of-thumb is to select a machine

that can develop at least 2½ tons (5,000 pounds) of

clamping force per square inch of the projected cavity

and runner area

8.2.7 Post-Mold Fixturing and Annealing

The use of cooling fixtures is a last resort option

It involves extra expense to build the fixtures and

ex-tra labor to use them It resists automation It is more

art than science Parts must be restrained in such a

manner that when cooled and released at room

tem-perature, they are the desired size and shape

Usually, the parts have to be stressed using a weight

or clamp during cooling so that they are held in a shape

opposite to the undesired warpage Thus when they

are released they relax some of the frozen stress and

assume the desired shape However, if they are cooled

in a fixture without annealing, they contain stresses

that will eventually show themselves, after time and

exposure to elevated temperature, by assuming some

or all of the original undesired warp

The elevator gib discussed in Ch 10.15 is an ample of a part requiring fixturing The relativelyskinny core could not be cooled fast enough to main-tain a temperature below that of the mold base aroundthe outside of the part The only way the warpage prob-lem could be solved other than fixturing was to rebuildthe mold, allowing for the inevitable warp The in-usetemperature was not excessive so post-mold stress re-laxation was not a factor A rail was built (based ontrial and error) to spread the center opening enough tomake the side walls of the part parallel after the partwas removed from the fixture rail The thick walls re-quired a long cycle so only a few parts were on thefixture at any one time

ex-8.2.8 Special Problems With Thick Walls and

Sink Marks

Parts with thick wall sections are the most cult to cool and pack Thicker sections take longer tocool and require additional packing When parts haveboth thick and thin sections, gating into the thick sec-tion is preferred because it enables packing of the thicksection (provided the gates and runners are largeenough), even if the thinner sections have solidified.The different cooling and packing requirements of thethick and thin sections lead to shrinkage-related inter-nal stresses in the wall-thickness transition regions

diffi-In practice, it is essentially impossible to maintaincompletely uniform part-wall thickness due to the com-plexity of part designs As illustrated in Fig 8.2, de-sign features such as bosses, flow leaders, or ribs re-sult in local wall-thickness changes and, as a result,represent areas where cooling stresses can develop.[6]

Figure 8.2 Diagram showing good and bad wall-thicknesses

and radius/fillets [6] (A) Proper rib thickness and radius (B) Excessively large radius (C) Excessively thick rib with proper radius (D) Thick corner section due to square outside corner (E) Uniform wall thickness at corner because outside radius matches inside radius plus wall thickness (F) Potential areas for sink marks on the outside surface or voids in the center of the inscribed circles Arrows ( ← → ) show varying thicknesses and diameters of inscribed circles.

Sink marks or voids are also common problems

for parts containing reinforcing ribs on one side of the

molding Thick ribs provide improved structural

ben-efits and are easier to fill; however, the magnitude of

sink associated with thick ribs can be excessive The

sink problem is magnified if large radii are used at the

intersecting walls to reduce stress-concentration

fac-tors and improve flow In practice, rib-wall thicknesses

are typically 40% to 80% as great as the wall from

which they extend, with base radius values from 25%

to 40% of the wall thickness The specific rib designs

are material dependent, and are influenced primarily

by the shrinkage characteristics of the material

When proper guidelines are followed, the size of

the sink associated with a feature such as a rib is

mini-mized, but some degree of sink will generally be

no-ticeable Localized mold cooling in the area of the sink

mark can be beneficial in reducing the severity of the

sink

Various methods can be used to disguise the sink

mark, as illustrated in Fig 8.3.[6] One of the most

com-mon reasons that surface textures are used with

injec-tion-molded plastic parts is to disguise aesthetic

de-fects such as sink marks or weld lines As a last resort

in the fight against sink marks, molders will sometimes

add small quantities of a blowing agent to the base

resin, and produce a conventional injection-molded part

with structural foam-like regions in the thicker section

of the molding (the sink is eliminated due to the

inter-nal foaming action) However, the blowing agent can

create surface defects such as streaks or splay as the

blowing agent creates bubbles on the surface of the

molded part Maintaining a high air pressure in themold during the filling phase can minimize the forma-tion of surface bubbles

8.2.9 Nozzles

One often neglected topic in controlling shrinkageand warpage is the selection and use of nozzles at theinterface between the mold and the heating cylinder.General-purpose (standard) nozzles, shown in Fig 8.4,are the most commonly used They are effectively full-bore until near the tip

A continuous-taper nozzle is shown in Fig 8.5.These encourage even flow without holdup Whenmaterials tend toward drool, continuous-taper nozzlescan help

The reverse-taper nozzle, as shown in Fig 8.6, ismore commonly used with highly fluid materials likenylon It has its minimum diameter near the center ofthe nozzle The minimum diameter of the nozzle must

be large enough to allow adequate flow to fill the moldwithout undue shear-stress in the nozzle orifice Theheaters and thermocouple for the nozzle must be placed

so that the temperature is as uniform as possiblethroughout the length of the nozzle The controller forthe nozzle should be proportional, as opposed to an off

or on device, to maintain as constant a temperature aspossible in the nozzle

Of utmost importance, the same nozzle size andtype with the same size heaters in the same locationand the same thermocouple location must be used each

Figure 8.3 Methods of disguising sinks near heavy sections.

Figure 8.4 A general-purpose nozzle.

Figure 8.5 A continuous-taper nozzle.

time the mold is run All too often mold setup

per-sonnel do not change to the appropriate nozzle unless

forced to The end result is that a mold may be run

with different nozzles from time to time As a result,

the molding conditions are different Instead of

chang-ing the nozzle, operators too often blame the material

When troubleshooting molding problems, nozzles with

very small diameters are often found feeding sprue

bushings with diameters two or three times the nozzle

diameter This type of situation causes high shear

heat-ing, slow fill, and lower mold-cavity pressure relative

to the machine injection-pressure setting

8.2.10 Excessive or Insufficient Shrinkage

Excessive shrinkage occurs in molded parts when

the material is inadequately packed into the mold or

when the melt temperature is too high Inadequate

pack-ing, creating greater shrinkage, can result from low

injection-pressures, low injection-speeds, short

plunger-forward times, or short clamp-time Sometimes,

how-ever, high injection-pressures can cause excessive

shrinkage by increasing the melt temperature due to

the frictional heat generated High melt-temperatures

cause the plastic to experience large temperature

changes between the injection temperature and the

tem-perature at which the parts can be ejected from the

mold, and the resulting large thermal contraction causes

excessive shrinkage However, under some

combina-tions of condicombina-tions, an increase in melt temperature will

increase the effective cavity-pressure, which will

in-crease packing and result in a dein-crease in shrinkage

Insufficient shrinkage will result if the injection

pressure is too high, plunger-forward time is too long,

clamp time is too long, injection speed is too fast, or

melt temperature is too low Injection pressure, tion speed, and cylinder temperature are interrelatedand have a combined effect on cavity pressure andshrinkage Again, as previously mentioned (see Ch 6),high injection-pressures and/or injection-speeds gen-erate frictional heat, which increases melt temperaturesand sometimes increases the shrinkage of the moldeditem.[3]

injec-In plastics in general, and polyethylene in lar, shrinkage can be reduced by many means All toooften, customers strive for a less expensive part by using

particu-a lower quparticu-ality or lower strength plparticu-astic or too low particu-amold temperature, which, in the long run, causes end-user dissatisfaction and a bad name (again) for plastic.The cheapest price is not always the best bargain

8.2.11 Secondary Machining

If a part that is essentially flat is machined over asignificant portion of its flat surface, the machiningoperation removes some of the surface material that is

in compression The surface compression is a naturalresult of the surface of a molded part cooling soonerthan the core of the part When the material in com-pression is removed, the center of the part, which is intension, is moved closer to the finished surface Thiscauses a tendency for the part to bow concave towardthe machined surface Figure 8.7 shows how the com-pressive stress in the surface of a part is machined away,and the distribution of stresses is changed

8.2.12 Quality Control

There are many factors that are under the control

of the molder Some of these are the injection sures at various times during the cycle, the time thatthe pressures are applied, the injection rates, the plas-tic material, and the mold temperature Figure 8.8shows a schematic of a system that monitors some of

pres-Figure 8.6 A reverse-taper type nozzle for use with nylons,

polyamides, acrylics, and similar expansive and

heat-sensitive materials The sprue breaks inside the nozzle,

providing expansion area and reducing drool.

Figure 8.7 The molded-in stresses are affected by

machining away the surface of a molded part.

these variables.[42] This type of system can be a

closed-loop system to change machine settings if the system

detects unauthorized changes

This type of closed-loop system improves the

qual-ity and consistency of molded parts, but does not

guar-antee the quality of the finished product Since molded

parts continue to shrink over time, and the majority of

that shrinkage occurs over the first forty-eight hours

after molding, one cannot reliably determine that a part

is satisfactory until the part has been examined at least

two days after it is molded Since it is possible to mold

thousands of parts in some cases over a 48-hour

pe-riod, some immediate indication of quality must be used

Some of the indirectly controlled measurements are

the weight of the finished part, the maximum cavity

pressure measured at a particular point in the cavity,

the cavity pressure at the end of the holding cycle, the

time required for the pressure in the cavity to reach the

maximum, and the time at which the cavity pressure

reaches zero Several directly controlled parameters

affect each of these indirectly controlled variables

Some of these indirectly controlled measurements

are more closely correlated to the quality of the

fin-ished part A study by B H Min[42] among others has

determined that the highest correlation between

shrink-age and the quality of the finished part is the weight of

the finished part In other words, if two parts weigh

the same and one part is known to be good, the

likeli-hood that the other part is good is greater than 91%

The next highest correlation between two

accept-able parts is in the maximum cavity pressure measured

during the molding cycle for the two parts If two parts

are molded with the same peak cavity pressure and

one of the two parts is known to be good, then the

likelihood that both are good is better than 84% Since

both of these variables can be measured at the time a

part is molded, they provide the quality-assurance

per-sonnel a method to immediately determine if a molded

part is satisfactory

If both weight and maximum cavity pressure arewithin limits for a given part, it is virtually certain thatthe parts are acceptable

For maximum quality assurance, mold sample parts

at a variety of weights and maximum cavity pressuresand after forty-eight hours determine which of theseparts meet quality requirements Then any parts thatare molded that fall within the established limits aregood Figure 8.9 shows the relationship between al-lowable tolerance limits and the range of indirectly con-trolled parameters.[42]

8.3 Material Considerations

The suitability of a particular plastic (there are ahundred or so commercial generic plastics and morethan 41,000 grades) for an application as far asstrength, chemical resistance, lubricity, etc., are not inthe purview of this book However, all other thingsbeing equal, it is more difficult to control shrinkageand warpage, and consequently the dimensions, of apart made of a semicrystalline plastic than one made

of an amorphous plastic Amorphous plastics havelower and more uniform shrink rates than do semi-crystalline plastics If tight tolerances and minimumwarpage are of primary concern, and if an amorphousplastic with the necessary physical properties can befound, then it should be the preferred choice

The injection-molding process is generally used toproduce parts that require fairly tight dimensional tol-erances In some cases very tight tolerances are re-quired For example, molded plastic parts that must

Figure 8.8 Schematic of a quality monitoring system.[42]

(Courtesy of SPE.)

Figure 8.9 Quality-control relationship between tolerances

and indirectly controlled parameters [42] (Courtesy of SPE.)

mate with other parts to produce an assembly must be

molded to accurate dimensional specifications Many

plastic materials exhibit relatively large

mold-shrink-age values, and unfortunately, mold shrinkmold-shrink-age is not

always isotropic in nature If a plastic material

exhib-its anisotropic mold-shrinkage behavior, establishing

cavity dimensions is no longer a simple “scale up”

pro-cedure In addition, anisotropic shrinkage will lead to

a degree of warpage (out-of-plane distortion) or

inter-nal stress

Where close tolerance and stability are a concern,

the coefficient of thermal expansion must be

consid-ered Some applications depend on different coefficients

of thermal expansion in order to perform their

func-tion, even with metal materials A common example is

the bimetallic spring in home thermostats As

tempera-tures change, the thermostat spring coils tighter or

un-coils to open or close a mercury switch to start the

heating or cooling cycle as appropriate When parts

with tight tolerances must operate over a wide range of

temperatures, the materials used must have

compat-ible coefficients of thermal expansion If not, parts can

come apart or break as a result of temperature-induced

size change and stress As mentioned in Ch 4, the

plas-tic chosen for an application must be compatible with

the end-use temperature range for the expected stress

loads

In some respects, mold shrinkage can be compared

to linear thermal contraction or expansion A mass of

molten polymer cooling in a mold contracts as the

tem-perature drops Holding pressure is used to minimize

shrinkage, but is only effective as long as the gate(s)

remains open If the polymer is homogeneous, all parts

should shrink essentially the same amount even after

the pressure is removed or the gates freeze This

gen-erally is the case with amorphous polymers such as

polystyrene, polycarbonate, ABS, etc Published

val-ues for mold shrinkage of these materials are very low

and do not exhibit a broad range Generally they are in

the order of less than 0.010 units/unit Why are

polypro-pylene, polyethylene, nylon, acetal, etc., different?

Un-like amorphous polymers, these semicrystalline resins

are not homogeneous; they have a structure containing

both amorphous and crystalline components (see Fig

1.1) As these resins cool, a multitude of crystals form

that are surrounded by amorphous regions The

crys-talline regions shrink much more than the amorphous

regions This imbalance in shrinkage causes a net

in-crease in shrinkage and introduces sensitivity to other

molding parameters, which have additional effects on

the shrinkage

Another factor influencing shrinkage is the coelastic characteristic of high molecular-weight poly-mer melts The long molecular-weight chains are liter-ally stretched, and placed under tensile stress, as theyfill the mold As the stresses are relieved during cool-ing, the chains try to relax, analogous to stretching arubber band and slowly letting it return to its originalsize This relaxation also influences the shrinkage, es-pecially in different flow directions Both the averagemolecular weight and the molecular weight distribu-tion are key material factors that influence this facet ofmold shrinkage

vis-The relative proportion of crystalline to amorphouscomponents changes shrinkage This is a very criticalvariable with polyethylene, but is not as significant withpolypropylene, as evidenced by the much narrowerrange of specific gravity, another property affected bythe degree of crystallinity

There are many properties listed in standard datasheets for each of the hundreds of plastics currentlyavailable Which of those properties are of importance

in a particular application must be determined by aknowledgeable engineer or designer

Strength may be an important factor If so, eration must be given to creep characteristics Will theplastic support the proposed load over long periods oftime or will it gradually give way? Will the proposedpart distort under load in such a manner that the prod-uct will become unsatisfactory over time? See Ch 4.2.4.Closely related to strength is the heat-deflection tem-perature This property gives an indication of the ef-fect of heat on the plastic’s strength

consid-Chemical resistance is frequently important Willthe chemicals in the environment cause swelling orcracking? Remember that water is a chemical and manyplastics, especially nylon, absorb significant amounts

of water If the size of the plastic part changes cantly due to chemical absorption, the part may fail orbecome unusable Aromatic hydrocarbons, for ex-ample, attack many plastics such as polycarbonate.Coefficient of friction can be important in gears orbearings where there is sliding contact Acetal and ny-lon have low coefficients of friction while others in asimilar environment will wear quickly

signifi-Toughness is indicated by various types of impacttests When impact loads are expected, the impact rat-ings give an indication of toughness for comparisonpurposes between various plastics Environmental vari-ables can affect toughness For example, nylon is typi-cally much tougher after it has absorbed some waterthan it is dry Typically, increasing toughness is ac-companied by a reduction in rigidity

Low shrinkage is usually desired for parts

requir-ing low warpage and tight tolerances, although low

shrinkage is often associated with plastics with high

long-term creep Electrical conductivity is important

where the plastic must isolate electrical charges In other

cases, some conductivity is necessary to avoid the

buildup of a static charge Tensile modulus is a

mea-sure of the stiffness of a plastic part Thermal

conduc-tivity may be important to help dissipate heat

These are usually the more important properties

to be considered in any given application, although

others may need to be considered as well See any

typi-cal plastic data sheet for a more complete listing

8.3.1 Filler or Reinforcement Content

Fibrous fillers cause amorphous plastics that are

essentially isotropic in their shrinkage behavior to

be-come anisotropic The cross-flow shrink rate bebe-comes

greater than the flow-direction shrink On the other

hand, the addition of small amounts of fibrous

rein-forcement to a semicrystalline plastic can make it

be-come more isotropic in its shrink behavior The

addi-tion of flake or particulate filler to semicrystalline

plas-tics reduces the overall shrink-rate and improves the

shrinkage predictability

Flake or particulate fillers that have lubricating

char-acteristicscan be added to amorphous materials to make

them more satisfactory for a wear or bearing

applica-tion without creating anisotropic shrinkage behavior

8.3.2 Degree of Liquid Absorption

Different plastics absorb different liquids See the

chemical-resistance data for a plastic to determine

which liquids (or gases) a particular plastic may

ab-sorb The amount of liquid that a plastic will absorb

and the effects of the liquid on the dimensions and the

physical characteristics of a plastic part must be

con-sidered If a part changes size considerably while

ab-sorbing a liquid, it can become unusable due to

inter-ference with an adjoining part If the molecular

struc-ture of a plastic is attacked by a fluid or gas, the

plas-tic may become brittle, crack, or even dissolve If a

plastic loses a fluid (such as a plasticizer that can leach

out as a fluid or vapor) during use, it may be come

unsatisfactory because it changes color, shrinks, or

becomes brittle and cracks

8.3.3 Regrind

Shrinkage is affected by the amount of regrind used.Each time the material passes through the moldingmachine, the material is degraded somewhat If thepercentage of regrind varies from time to time, theshrinkage and warpage will also vary This is espe-cially true of glass-fiber–reinforced plastics Some glassfibers are broken each time the material is processed,and they are broken more when the material is reground

in preparation for reuse

8.4 Tooling Considerations

Simply making a void in the mold that is the sizeand shape of the part to be molded plus the averagepredicted shrink is not adequate for making even asimple part A competent mold builder and designermust consider many different things to adequately de-sign a quality mold

8.4.1 Gate Locations

Gate location is one of the more critical aspects ofmold design First of all, if the part has thickness varia-tions, the gate must be placed to fill the thicker sectionfirst Then the mold designer must visualize the flowpatterns from the gate throughout the mold, and usethat visualization to predict any likely flow or shrink-age variations If thickness variations are such that athick area surrounds a thinner area, a void can form inthe molten plastic in the thin area, trapping air andpreventing the molding of a complete part Often thistrapped air is compressed and heated by the compres-sion to the point that the plastic around the void isburned, leaving a charred surface

Multiple gates may be required to fill the part equately with a minimum pressure drop across themolded part Where multiple gates are present, the flowpattern within the mold is more difficult to predict, butthe mold designer must consider the total flow pattern,especially for anisotropic materials

ad-The use of many gates often gets around the lems of differential shrinkage that leads to warpage.With multiple gates, the flow length is cut down, andcavity pressures tend to be more uniform (thereforemold shrinkage is more uniform) since all areas of thepart are then “near” the gate Alternatively, if the ap-propriate shrinkage data is available, the cavity dimen-sions can be cut to compensate for the different shrink-

prob-age values, but that is not a common practice That

data is more often used to design the multiple gates

layout

Shrinkage data generated on larger, plaque-type

test molds with well defined linear flow is preferred to

that generated using the oversimplified, standard

ASTM testing technique Using these larger parts,

materials suppliers can generate both inflow and

cross-flow shrinkage values close to and far away from the

gate region.[6]

8.4.2 Types and Sizes of Gates

Gate location may be influenced by the

appear-ance of the molded part Certain surfaces may be

cos-metically important and a gate mark on these surfaces

may be restricted or forbidden

Small gates are cosmetically desirable but usually

increase the shrink of the molded part Where control

of shrink is of paramount importance, larger gates must

be used

Where small gates direct the flow of plastic across

a flat surface, there is likely to be a tendency to jet a

thin stream of plastic across the surface Later, plastic

flow will fill in around the initial jet of material This

leaves an undesirable surface blemish showing the

pro-file of the initial jet of material To avoid jetting, the

gate should direct the flow of plastic against a core pin

or wall to cause the plastic to “puddle” immediately

Tab or fan gates discourage jetting and encourage

“pud-dling.” See an example of jetting in Fig 8.10

Figure 8.11 shows a method of causing immediate

puddling as plastic enters the mold cavity.[56] As the

cavity pressure builds, the core is pushed away from

the plastic and into its retracted position, providing awall in the retracted position for the completed part.Tunnel gates are preferred by many molders to au-tomatically separate the part from the runner Thisavoids secondary hand trimming and sorting of therunner system from the molded parts On the other hand,

if the molder is using robotic systems and is keepingeach cavity separated from all the others, it may bedesirable to select a gate that keeps the parts on therunner until the robot places the parts and they areseparated from the runner with some sort of die Goodcommunication between the mold designer and themolder is of utmost importance

Gate size must be adequate to control shrinkage.For semicrystalline materials, gate size should be be-tween 50% and 100% of the maximum part-thickness.The larger the gate, the better control the molder has

on the part shrinkage

8.4.3 Runner Systems

For minimum shrinkage in molded parts, any ner between the molded part and the molding machinenozzle must be greater in its minimum dimension thanthe maximum thickness of the part being molded Fur-thermore, the runner should increase in cross sectiontoward the sprue at any intersection or abrupt change

run-in direction The size of the runner must be largeenough that the runner remains fluid until after the parthas solidified If the runners are too small, then therunner solidifies before the part, causing higher shrinkrates On the other hand, if the runners are too large,then the cycle time must be extended far beyond what

Figure 8.10 An example of jetting in an injection mold Figure 8.11 A movable core that inhibits jetting.[56]