nadca - overview of defets in die casting

5/28/2018 Nadca - Overview of defets in die casting

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EC-515 Die CastingDefects

Dr. Steve Midson


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BASICS FOR CONTROLLING DEFECTS

You cant correct and control defectswithout first measuring and reportingthem.

The scrap reporting system must be setup for those who have to make

improvements, not just for the frontoffice.

The scrap report should be available toeveryone in the plant. In fact, it shouldbe posted on the bulletin board so

everyone can see it.


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The daily scrap report must have the followingfeatures as a minimum:

1. It must be available first thing in themorning for the previous day

2. It must categorize scrap (as a minimum):

By defect type,By part number,

By die,

By shift,By operator,

By machine.


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The scrap reporting system should show longterm trends and be able to predict the customerrejects based on the current scrap activity -

Pareto charts are good ways to show theproblems

The report should include defects that are not

detected until the parts are downstream - and asystem developed so these defects can betracked to the shift and machine that produced

them All shots should be reported, even warm-up and

scrap that is returned to the furnace at the

machine (they cost in die life)



5/197

The process is complex, and a continuousreporting system must be set up to provide realtime feedback and effective process control if

defects are to be controlled.

The two major defects in die casting are surface

quality and porosity. Both of these requirejudgment decisions about severity.

This means a method of measuring the severityof defects is a requirement and must bedevised for many situations.



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A rating system is intended to tell you if the defectproblems are getting better or worse, or whetherchanges made in the process are making a

difference.

What you are looking for is the ability to track anychanges or trends, and to know when corrections

are needed. This system also allows corrections tobe made before the defect level becomes a crisis.

The standards used for the rating system may notcoincide with the customer standards or the qualitydept. Ratings; they are for a different purpose anddo not need to coincide.



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For example, you may rank a porosity defectfrom the worst to the best with rankings from 1to 5.

A capability study could be done as follows:

Take 6 sets of samples of 5 sequential castings

at intervals of 1/2 hr to 2 hr.

Rate each casting and average the total. This

gives the average quality level; this should bechecked against similar studies to determine ifthe process is improving or degenerating



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One of the most difficult problems indeveloping a rating system is finding a methodof reporting and rating porosity

The most typical methods are x-ray, machining,or sawing.

A cheap and effective method is to use an oldlathe to approximately duplicate the customersmachining.

Always select examples for the rating systemand save them. They must not be used forany other purpose.



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Thus defect corrections must start with a goodscrap reporting system

Developing this system may start with definingthe names of defects, which means a

document or board with samples and names ofdefects

Bottom line: YOU CANT IMPROVE IT IF YOUCANT MEASURE IT!



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Die Casting Defects

1. Surface defects2. Laminations

3. Gas porosity

4. Blisters

5. Shrink porosity

6. Sinks7. Leakers

8. Cracks

9. Inclusions

10.Solder

11.Carbon12.Erosion/cavitation

13.Outgassing

14.Bending/warping

15.Flash

16.Stained castings17.Waves/lakes

18.Drags

19.Ejector pin defects

20.Cold flake

21.Excessive flux


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Surface Defects


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Causes Of Surface Defects

This part of the class is about those types ofdefects that appear on the surface of a diecasting

These defects are called by one of the followingnames:

Cold Flow Cold Shut Flow Lines Cold

Chill Non-fill

No-fill Poor-fill Laps Lines

Run Marks Misruns

Others:


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Pictures - Surface Defects


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Factors Controlling Surface Defects

A general list of the factors that control thiskind of defect is shown below. These arecontrolled by different people, maybe even

different companies The wall thickness

The casting shape

The fill time The flow pattern (gate design)

The die temperature

The metal temperature The type of alloy


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The first 2 factors on the list are controlled bythe part design Wall thickness and casting shape

Very important Regarding surface defect problems The wall thickness is the most critical,

Controls many casting parameters Required fill time

Required die temperature Distance the metal will flow Length of defect free surface that can be made

Good surface quality requires consistent wall

thickness, The designer and the die caster should focus on

reducing heavy sections

Try to make the wall thickness constant

Who Controls Surface Defects?


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The operating parameters for thin walls arevery different than for thick walls

A thin wall causes the flow to freeze and

develop cold flow quickly, For aluminum and magnesium, a typical

minimum wall thickness will be about 1.5mm

For a flow distance of about 150 mm or more For zinc, the minimum wall thickness would be

about 1.0 mm

Wall Thickness And Surface Defects


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For example, see the change in fill timerequired by change in wall thickness:

Change in Wall

Thickness Fill Time Change Required

2 mm to 1.5 mm 25% reduction in fill time

3.5 to 3.0 mm 12% reduction in fill tim2

Thus a small variation in plunger speed (filltime) on a thin wall (say 2.0 mm ) casting is

very noticeable for surface defects, while therewould not be much noticeable for the same filltime variation in a thicker wall (i.e. 5.0 mm)

aluminum casting

ll h k d f f


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The die temperature will also become muchmore critical and, in addition to becomingmuch more sensitive, the reduced mass of

the part will not provide enough heat for thedie

The basic requirement for making a thin wallcasting is a very fast fill time in a hot diewith a high gate velocity


W ll Thi k A d S f D f


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Summary of wall thickness issues for thin wallparts: Talk to designer early

Get wall thickness the same (consistent) Keep wall thickness variation down with narrow

range to toolmaker

Expect much a smaller process window, and set upmuch tighter process controls

Use low or very low fill times

Use direct feed from gate

Use high gate velocities (but within normal range) Use high die temperatures


Fill Ti A d S f D f t


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Fill Time And Surface Defects

Fill time has a major impact on surface quality

The fill time is defined as the time beginningwhen the metal first arrives at the gate andending when the cavity is full of metal (includingthe overflows and vacuum runners)

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The fill time can be calculated from the formulagiven in the NADCA gating course, which is:

Max fill time =

Where:

K = A constant

T = Average casting wall thickness

ti = Metal injection temperature

tf= Metal flow temp (solidus)td = Die temperature

S = Percent solids at cavity full

Z = Conversion for latent heat

K T t t S Z t t

i f

f d

* ( * )



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For some rough guidelines, the following aremaximum fill times based on calculations andexperience and will be reasonable for most

castings:Thin wall Average wall

2 mm

Al, approx. 2 kg .09 sec .1 secZn, approx. 1.4 kg .03 sec .05 sec

Mg, approx. 1 kg .02 sec .03 sec

Note: for high quality surface finish, reduce thefill times shown by about 50%




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Predicting the fill time is best done withthe PQ2 calculation

The PQ2

calculation predicts the change infill time and gate velocity from changingany of the following:

The gate areaThe plunger size

The machine hydraulic pressure

The plunger speed setting



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The PQ2 calculation provides the only way thatthe fill time can be predicted accurately

Without it you must guess, and this is expensive



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This list shows some of the things that canaffect the plunger speed, which in turn changesthe fill time and the casting surface finish.

Some of these are: Dragging tip Plunger lubrication Poor sleeve condition

Poor plunger condition Poor cooling water flow to the plunger Sleeve deflection Gooseneck and plunger ring conditions Hydraulic pressure Low (or high) nitrogen charge Gate size



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Summary of fill time adjustments:Set fill time maximum values with

calculations and experience, then use adisciplined process

Use PQ2 to predict adjustments to get the

right values and eliminate costly trial anderror

Measure and control process variables with a

monitor systemMaintain control of sleeve and gooseneck

conditions to keep the fill time within limits

Flow Pattern And Surface Defects


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Flow Pattern And Surface Defects

The metal flow pattern is the criteria in gatedesign

The gate design is a function of design rules as

taught in the NADCA gating classes Flow the short way across the casting

Avoid mixing flows if possible

Flow pattern can be simulated with computerprograms and reviewed with short shots

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Flow Patterns And Surface Defects


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These rules include:

Use PQ2 to size the gate and the plunger, using theappropriate gate velocity, fill time, and cavity

pressure criteria Divide the casting into zones

Proportion gates so as to fill each zone at the same

time Flow the short way across the casting

Avoid mixing flows if possible (unless close to gate)

Gate into heavy section porosity and/or importantsurface finish areas




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Flow pattern rules (continued): Avoid jet flows at all times and distribute flow as

much as possible

If possible, set gate location so venting can be usedopposite the gate

Avoid flow directly on cores if possible (but do it ifneeded for best flow pattern

Keep runner lengths equal (avoid tree typerunners)

Eddies in the flow path (from cores or openings) will

cause swirls, gate to avoid this Gate to allow for high momentum (cores can be

bypassed)


Die Temperature And Surface Defects


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The die temperatures effect on surface defectsand the die temperature operating window willbe discussed next

A low die temperature affects surface defects bycooling the fluid metal stream and increasing thepercent of solidified metal in the metal stream

If the percent of solidified metal is high, then it

becomes stiff and solid and does not knittogether well; And the flow forms "wrinkles", orcold flow

A cold die can be compensated for by having ashorter fill time - this means a higher plungerspeed

In other words, we can exchange plunger speedfor die temperature.



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Measuring die temperature should be done onevery job - most dont do it enough, but it isrequired to really minimize surface defects

In general, measuring can be done three ways: Hand held probe Thermocouple in the die Infra red device

Each has advantages and disadvantages: Hand held probe: accurate, but must stop the

machine Thermocouple in die: continuous, but does not

measure surface temperature Infra red: easy, but not as accurate




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Temperature ranges for good surface finish:(Measured with a hand held surface probe

just after the casting ejects)

Metal Good Finish Average Finish

Al 250 315oC 190 - 315oC

Zn 230 - 290oC 190 - 290oC

Mg 220 - 290oC 200 - 290oC



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The operator can control die temperature withthe following controls:

Die spray

Water/oil flow ratesCycle time

The engineer can change the design and affect

how much difference some of these make, butthe operator often controls the actual use of allof them. Thus the operating temperature of the

die is often controlled directly by the operatorand this die temperature control is probably themost important activity of the operator on thefloor. We will review these in sequence


Die Temperature and Surface Defects


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Die spray is lubricant mixed with water It is mostly water The water controls the temperature, not the die

spray itself The spray cools the die quickly because of the

large amount of heat quickly pulled out of the

die as the water in the spray boils away


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Some techniques include: spray in 2-3 secondincrements with careful adjustment of thespray pattern - spray the areas that need

cooling, not just where the casting might stick- automatic sprayers with a set manifold foreach job are a good way to keep spray to aminimum

Keep spray equipment in good shape, the mostimportant factor is consistency

Document pressures, nozzles sizes, flowadjustments, and spray times in detail -undocumented changes should not be allowed(changes should not be stopped, just

documented)




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Factors in the spray actions that are veryimportant in controlling die temperature:

Length of time of spray

Spray nozzle adjustment, or spray pattern

Distance from nozzle to the die

Balance between air pressure and lube pressure

(drop size and velocity) Minimize over spraying

Document everything - spray time, pressures, spray

location, mixtures, etc... Be consistent

e e pe atu e d Su ace e ects



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The second die temperature control factor is theadjustments made to the water or the hot oilsystems for internal die cooling- this can be

done by the operator or technician

e e pe atu e a d Su ace e ects

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With water, flow rate is usually more important thanthe temperature of the fluid. A small difference inflow rate will do more than a large difference in

temperature

Measuring flow rate is a very desirable way to

improve process control Using flow meters is more and more common

Keeping the pressure constant is another way

A constant flow rate can help to eliminate the coldflow that seems to come and go without explanation

p



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The flow rate is determined by the smallestopening in the supply line - this is usually thequick connect fitting

Water (or oil) manifolds affect the flow rate ifthey are not carefully designed - there often ismore output area than input area

Then which line gets the most flow? Adjustment valves should be easy to see, easy

to use, and have large openings for good flow

The water pressure at each machine should beconsistent, and not vary Many plants should have new piping installed

p



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Hot oil systems will often make a significantdifference in the surface defect rate for tworeasons:

It keeps the die hot during stoppages - thestart up scrap is often a high percentage ofthe total scrap, (this is where hot oil caneasily pay for itself)

It can add heat to the die as needed to getbetter surface conditions

Hot oil units cool about half as effectively aswater, so the thermal design must accountfor this to get the cycle times desired Use higher flow rates, move lines closer, make

larger, etc..

p



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Adding overflows are another method ofincreasing die heat

p

41



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Some things to consider: Do not place water lines around the outside edge of

the cavities (cool the cold areas)

Give priority to cooling/heating lines, even if thismeans moving ejector pins or other changes

Do not use the same line to control temperatures in

both a hot area and a cold area Depth of the line is critical, set depths carefully

Size of line must match the flow rate

Treat water because deposits of only .005 cut heattransfer by about 40%



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Another important control for the dietemperature is the cycle time.

The die temperature at any given time is

the direct result of the number of pounds ofmetal that went through the die in the lastone to two hours.

A consistent cycle time is one of the mostimportant factors in good defect control

The cycle time should be measured and

displayed to the operator or technician insome way to get good control



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Die temperature changes slowly, which can causesome delayed effects, and perhaps some confusion

Example: A change that adds nozzles to the spray

manifold, and shortens the spray time with evenmore cooling than before, and also shortens thecycle time

There will be two effects, short and long term. The

short term from the increase in spray cooling, thelong term from the change in cycle time

Increasing the cooling from spray will reduce the

die temperature quickly (short term) Reducing the cycle time will increase the die

temperature (long term)



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Summary of correcting surface defects with dietemperature: Measure die temperature to know where to

change and how much Establish temperature goals for minimum defects Increase die temperature in the defect area by:

Reducing spray Reducing water flow rate Adding overflows

Increase overall die temperature by:

Reducing cycle time Increasing hot oil temperature and flow rate



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Summary (cont.): Use good spray practices and keep consistent

Use good computer aided thermal analysis for

cooling/heating line design Measure flow rates and control

Use good engineering to develop good quality

water at consistent pressures Minimize start up scrap and marginal production

situations with die pre-heating; Keep die hot

during short stops

Metal Temperature And Surface Defects


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The metal temperature can make a significant differencein the surface finish

In general, the most desirable situation is to keep themetal temperature at a high range, but not high enough

to cause a lot of other problems Keeping the zinc at 430oC max,

and the aluminum at about 690oC

magnesium should be kept at about 677oC

Control metal temperature on furnaces to within +/- 5oC

Use consistent times between ladling and shot

If possible, control temperatures in shot sleeve

Use consistent set point, do not use metal temperature as avariable unless absolutely necessary


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Laminations

Laminations


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Laminations have several sources

Usually they are the result of metal flowconditions where one flow lays on the top of

another, and the flows were too cold to mix asthey came to rest

This is shown in the next overhead

Laminations


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Flow paths 1 and 2 meet, and are relativelythin layers at this point - they can be peeledaway from the layer underneath (flow 3) by

grit blasting, machining or similar activity

FLOW 1FLOW 2

FLOW 3

DIE SURFACE

COLD FLOW LINE

Laminations


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The correction of these kinds oflaminations is to correct the processconditions, which include:

Flow pattern (gating design)

Gate location

Gate velocity Fill time

Die temperature

Metal temperature

Laminations


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These layers can come from the way the metalflows inside the casting, which may be due tothe geometry of the casting

In this case, the flow pattern can be difficult tochange with gate modifications, so theimportant corrections would be:

Decrease fill time Increase die temperature

Increase metal temperature if possible

Changing gate velocity (either up or down) may also

affect these kinds of laminations

Laminations


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Generally changing fill time is the best It is very common that laminations are due

to some metal being splashed into the cavity

while the plunger is at slow speed; In thiscase the correction would be increase thelength of fast shot (move the switch towards

the pour hole) Laminations are also possible from a flexing

die - when the intensifier comes in, the die

may flex and another layer of metal could beadded outside the initial casting skin - thecorrection is add support to the die

Laminations


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Laminations can also come from oxide skins.These skins come from the holding furnace, ormay be formed in the cold chamber duringinjection

These will be random in location, and usuallyare fairly small, perhaps .08 (2mm) in size.When dislodged by machining or sanding, they

can be mistaken for porosity The corrections are good metal handling,

including:

Skimming the holding pot properly Keep the time in the cold chamber to a minimum Filtering Fluxing and degassing properly

Laminations


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Another cause of laminations is flash capturedin the casting

This happens when the die is not cleaned

properly, and the flash left on the die drops intothe cavity as the die closes

The incoming metal will not remelt the flash

and cause it to mix with the rest of the casting;In fact, the molten metal may barely adhere tothe flash

This flash can make a very weak spot in thecasting, causing cracks in addition to layers onor near the surface (laminations)

Laminations


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The corrections include those activitiesthat reduce flash, such as:Cleaning the die between shots

Not postponing die repairs

Using good process design to selectappropriate metal pressures

Proper adjustment of intensifier settings

Engineering the die cooling to keep dieexpansion as even as possible


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Porosity

Porosity


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Porosity is the biggestproblem in die casting.

The two basic types of

porosity in die castingsare:

Shrinkage

Gas

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It is critical that those who are responsible forsolving defects determine the kind of porosity

before trying to correct it Each kind takes a completely different

corrective action, but they can look alike

Porosity


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It is important that some time be taken toreview porosity before starting to makecorrections

A quick examination can be misleading

Generally, a porosity defect should be

examined under 5 to 10 powermagnification

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Gas Porosity

Gas Porosity


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Gas porosity is the biggest singleproblem in die casting

The high gas content prevents heattreating or welding and makes thestrength unpredictable

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Gas Porosity


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There are three major sources of gasporosity for die castings:

Trapped air

Steam

Gas from lubricant

Gas porosity is round and generally smooth,

although it can be flattened to some extentby pressure

The actions to reduce gas porosity, ingeneral, are not the same as the actions forreducing shrink porosity

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Gas Porosity


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Gas Porosity Trapped Air


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Any turbulence in the metal movement thatallows some air bubbles to be trapped in themetal These bubbles will remain trapped when the casting

solidifies

Air can be trapped in: Shot sleeve

Gating system

Die cavity

Starting with the shot sleeve, we will reviewpotential sources of trapped air and possiblecorrections

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Gas PorosityTh fi t t i t i t i th t


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The first step is to maintain the same pour rateand shot delay time Especially important if the fill % is below about 50%.

When the fill is less than 50%, a wave isgenerated by the pouring action This wave travels back and forth from the parting

line to the shot tip

The time at which the plunger tip starts tomove and its speed and acceleration aredesigned so air is not trapped

A surfing wave traps air

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Gas Porosity


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If the wave is met by the tip as it movesforward, then extra splashing and sloshing isgenerated, and this captures some bubbles.

However, if the tip is started forward justafter the wave has been reflected from it,then the tip chases the wave, and this will

give the best chance for minimizing airentrapment

The timer that sets the time delay between

the end of pour and the start of shot willdetermine when the tip starts forward inrelationship to this wave

Gas Porosity


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The next part of the sequence that can addtrapped air (bubbles) and porosity is theacceleration of the plunger to the slow shot

speed This acceleration rate should be slow enough

to keep the metal from tumbling over

(surfing), and fast enough to preventtrapped air between the generated wave andwaves reflected from the die

This acceleration rate will vary with thepercent fill and the length of the sleeve, butthe usual range will be between 2 and 2.8

inches per second per inch of travel

Gas Porosity


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The optimum acceleration profile can be closelyapproximated by a straight line (linearacceleration) when the sleeve fill is below about50% (which is where most of the problems occur)

Above about 50% fill, the optimum accelerationtrace will be more of a curve

Using these methods, the acceleration will

normally cause the plunger to reach the criticalslow shot speed 1 or 2 inches before sleeve full

This is very close to the start of fast shot, so

there is little time to spend at the slow shotspeed The trace shown on the next page is for a fill

percentage less than 50%

Gas Porosity


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0

5

1 0

1 5

2 0

2 5

3 0

0 1 2 3 4 5 6 7 8 9 1 0 1 1 1 2 1 3 1 4P l u n g e r P o s i t i o n ( i n )

PlungerVelocity(in

/sec)

Optimum acceleration profile with a 32% full sleeve, (3 in.sleeve, 20 in. length, 400 ton cold chamber machine)

Note that the straight line closely approximates the

optimum profile

OPTIMUM

Gas Porosity


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Typical overall shot profile at 32% fill, using linear

acceleration. 400 ton cold chamber machine

Gas Porosity


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The next phase of the shot profile will be thecritical slow shot speed - this will be the speedthat minimizes the trapped air during the slow

shot phase. This speed is calculated from theformula:

Css = k x [(100 - %fill)/100 ] x tip dia

Where k = 22.8 for ips (inch system)

This speed will minimize the air trapped in thisportion of the shot

Wave Formation in Sleeve

Sl h t l it t l Sl h t l it t f t


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Slow shot velocity too slow

Ideal slow shot velocity

Slow shot velocity too fast

The following settings should be considered

Gas Porosity


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The following settings should be consideredimportant when trying to reduce air trapped inthe shot sleeve. While one of these settings maynot seem to be important by itself, there are

interactions and its recommended they berepeated as close as possible once a good settingis foundPour rateDelay time before shotPour hole speedChange over point from pour hole speed to

slow shot speedSlow shot accelerationSlow shot speed

Fast shot start point73

Gas Porosity


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The next location of trapped air is likely to be inthe runners. Any sharp corners or small to largearea changes in the metal flow path in therunner system will cause air entrapment

The main rule is that the runner has smooth,rounded corners, that it has ever decreasingarea from the plunger to the gate

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VERY POOR RUNNER

DESIGN, SWIRLS TRAPAIR AND GENERATE

GAS POROSITY

CASTING

Gas Porosity


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Effect of short ejector pins

TRAPPEDAIR

BUBBLES

(POROSITY)

SHORT EJECTOR PIN

RUNNER

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Gas Porosity


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Once the metal starts to enter the cavity, it willnormally flow at a high velocity, very turbulentflow condition, and will trap some of the air

present as gas porosity The flow pattern design should be such that themetal tends to push the air through the cavityto the vents

Much of the air in the shot sleeve and the cavitycan be pushed out the vents or the vacuumsystem

Vents must be sized correctly and go to theedge of the die if they are to be effective

Vents must be kept clean of flash & lubricant

buildup76

Gas Porosity To summarize, the control of trapped air porosity


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, pp p ywill involve a check list like the following:Plunger control settings

Pour rate

Delay before shooting Pour hole speed Start slow shot point Slow shot speed

Fast shot start pointRunner area No square corners No low or high ejector pins

Decreasing runner area in the metal flow pathCavity

Vent location at last place to fill Vents sized right and go to edge of die

Vents to be kept clean77

Gas Porosity


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Steam is the second source of gas porosity Steam comes from water on the cavity surface

when the metal arrives

This gas is mostly trapped in the metalbecause there is little chance to push the gasout the vents - the gas is not present until themetal arrives, and so is mixed with theturbulent metal flow as soon as it is generated

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Gas Porosity


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The water is mostly from the die spray but itcan also be from other sources

Some of the water will evaporate from a hot

die, but you cannot count on this happening Therefore, it is critical that the die be dry

when it is closed

Other sources of water on the die:Leaking water lines

Dripping overhead sprayers

Leaking hydraulic cylinders

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Gas Porosity


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Steam porosity tends to be either a few largelarge bubbles or a group of smaller bubbles

If it is from a water line leak, the bubbles may

always congregate in about the same location

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Gas Porosity


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Checklist to reduce gas porosity fromsteam:

To much water based die lubricants on the die

(the die must be dry as it closes)Leaking water lines

Leaking water pipe connections

Crack in the die into a water line

Sprayer dripping on the die as it closes

Water glycol hydraulic fluid getting on the die

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Gas Porosity The third source of gas porosity is lubricant


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g p y

The lubricant used on the dies or on theplunger can generate gas when heated by the

incoming metal This gas (like the steam) is only formed when

the metal arrives, and so it is not possible to

force most of the gas out the vents ahead of themetal flow

All lubricants give off some gas when heated to

the temperature of the molten metal - theamount and type of gas will vary from lubricantto lubricant

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Gas Porosity


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The biggest single lubricant source of gasporosity is the plunger lubricant The usual problem is that the lubricant is

applied ahead of the plunger much heavierthan is needed This is especially true when a dragging or

worn tip is nursed along with extra lubricant

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Gas Porosity


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Check list of actions for gas porosity fromlubricants:Check the amount of plunger lubricant

Reduce the die spray lubricantLook for pockets where the lubricant can

accumulate on a cold die

84


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Blisters

Blisters

Blisters are another version of gas porosity, the


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gas just happens to be near the surface of thecasting.

The trapped gas is under high pressure at the

end of fill and the metal may shrink andsqueeze it more.

When the casting is taken out of the die, and

the die surface is no longer there to hold thecasting shape, pressure from the trapped gas isable to push up a blister.

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Blisters

h d f


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The same corrections used for gas porosityapply to blisters:

Reduce trapped air

Reduce spray and plunger lubricant

Eliminate water on the die

Correct venting and vacuum problems

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Blisters

Bli t h ld b li i t d b ti


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Blisters should be eliminated by correctingthe gas porosity problem. However, youcan:

Cool the die in the immediate area wherethe blisters occur Cool the blister area with die spray Cool the blister area by adjusting water lines

Cool the whole die by slowing the cycle time

Cool the casting immediately after ejectionby quenching in water (this will keep the

skin strong and resist blister formation)

Reduce metal temperature (but watchfor other problems)

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Shrinkage Porosity

Shrink Porosity Shrinkage develops because the metal

i l h lid th it did


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occupies less space when solid than it didwhen it was liquid.

For die casting alloys, the difference involume will be about 6% to 8%

This extra space will be concentrated atthe last point to solidify, which is thehottest spot in that section of the casting

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Shrink Porosity

Si th l ti f h i k it il h h i h i f


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Since the location of shrink porosity isalso the hottest spot in that section ofthe casting, it is usually in the center of a

heavy section This hot spot location can be controlled to

some extent by die temperature

The hot spot can often be moved bychanging the die temperature, therefore

the shrinkage porosity can be moved

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Shrink Porosity An even casting temperature will cause the

porosity to spread out and to be roughly on


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porosity to spread out, and to be roughly onthe center line (the last point to solidify)

WARM WARM

centerline

porosity

92

Shrink Porosity

If there is a large temperature difference


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If there is a large temperature difference,then there will be large porosity at the lastpoint to solidify

The spray and the water line keep one end

of the casting cold, the gate keeps the otherend hot, and the shrink porosity will tend tobe at the hot end

HOT

COLD

93

Shrink Porosity

If the hot and cold spot can be reversed then


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If the hot and cold spot can be reversed, thenthe shrink porosity will follow the hot spot.

Shrink porosity will always be in a hot spot in

the casting

HOTCOLD

94

Shrink Porosity

Note that the temperatures that effect thel ti f h i k it i id th ti


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location of shrink porosity are inside the castingitself

The die temperatures will influence the internalcasting temperature, but they are not alwayseffective in controlling it completely

Shrink porosity in a heavy section will be harderto move, shrink porosity in a thin section will berelatively easy to move

95

Shrink Porosity Shrink porosity is rough and irregular in shape

and this characteristic is the quickest and


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and this characteristic is the quickest andeasiest way to identify shrink porosity

The shape and appearance of the porosity

comes from the way the casting solidifies

96

Shrink Porosity

The first metal to contact the die surface freezes


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The first metal to contact the die surface freezesquickly and forms the skin

The skin is a very strong, dense, and fine grained

surface with very low porosity, due to the rapidsolidification/freeze rate

Once the skin has formed, the rate slows down

and a dendritic structure starts to appear

SKIN FORMS

97

Shrink Porosity

Dendrites are tree like structures that form inthe solidifying liquid


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the solidifying liquid

The dendrites grow slowly, and by the time the

last metal solidifies, there will be a lot ofdendrites in this area

98

Shrink Porosity It will be difficult to identify shrink porosity in

some castings and without identifying it you


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some castings, and without identifying it youmay very well take the wrong action forcorrection thus the identification is a key factor.

You should know that the walls of shrinkporosity have a characteristically roughstructure

The shrink porosity is usually crack like inappearance, and is jagged, rough and irregularin general

Occasionally it can have a rounded and smoothappearance, when this happens it is sometimescalled worm holes

99

Shrink Porosity


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Shrink Porosity To summarize on the effect of temperature - it

will move the porosity or spread it out - not


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will move the porosity or spread it out notnecessarily reduce it - controlling porosity withdie temperature is possible only on some

castings The shrink porosity location is determined by the

temperature difference between areas in the

casting - the hot and the cold spots determinethe location of the porosity

Watch temperature difference between die

halves Remember, you can heat up the cold spot as wellas cool the hot spot

101

Shrink Porosity

Shrink porosity can be reduced with metalpressure


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pressure

Die casting is a high pressure process and the

only reason die casting machines use highpressure is to reduce shrink porosity

The pressure can fill some of the voids as they

develop, but timing and temperature arecritical and very hard to control

102

Shrink Porosity The intensifier systems used with die casting

machines are there only to add pressure during


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machines are there only to add pressure duringsolidification and thus to reduce shrink porosity

Timing is critical because: the porosity is not

there when the metal is liquid, so pressure atthat time doesnt help (it will only add toflashing)

After the casting is solid, the pressureobviously will not help

Therefore, the only time pressure can be used

to feed more metal into the developing porosityholes is during solidification

103

Shrink Porosity The most common die casting alloys have a

freezing range - which means they go through


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g g y g ga mushy stage as they solidify

It is during this mushy stage that we can addpressure and reduce shrinkage porosity byfilling some voids as they are forming

Some typical freezing ranges for the mostcommon die casting alloys would be as follows:

380 aluminum 45oC

384 aluminum 65oC

413 aluminum 8oC

104

Shrink Porosity Using pressure to fill the emerging shrinkage

voids during this mushy stage is a key factor in


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g y g ycontrolling shrinkage porosity. Several keycontrol elements important in the use of

pressure are: The casting configuration, especially between the gate

and the point of interest (most important factor) The amount of metal pressure available at the end of

the plunger stroke - the packing pressure The intensified metal pressure The die temperature The gate freeze time The injection temperature

Solid

Liquid

Mushy Zone

105

Shrink PorosityTypical values for minimum pressures would be:

Static Intensified


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ALUMINUM 3,000 psi 8,000 psi

20 MPa 55 MPa

These should be regarded as minimum for goodcasting quality internally - lower pressures are

sometimes used when the internal quality is notthat critical

For heavier section castings, some die casters use10,000 psi (70 MPa) as the minimum intensifierpressure

106

Another operational factor is biscuit size

Cavity pressure decreases very rapidly once the

Shrink Porosity


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Cavity pressure decreases very rapidly once thebiscuit reaches a certain minimum size

This minimum varies with the size of the shotsleeve, the metal temperature, the fill time andother factors

Below this minimum, the internal soundness ofthe casting will deteriorate very rapidly

107

Shrink Porosity


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D E N S I T Y V S B I S C U I T T H I C K N E S S

00 . 2

0 . 4

0 . 6

0 . 8

1

0.

1

0.

3

0.

5

0.

7

0.

9

1.

1

1.

3

B IS C U IT T H IC K N ES S

AP

PROXIMATE

DENSITY

A P P R O X IM A T E

D E N S IT Y ,

P E R C E N T

Shrink Porosity Corrective actions for shrink porosity

Check the process design for appropriate


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p g pp pmetal pressure, both static pressure andintensified pressure (plunger size, pressure

settings)Check plunger tip and sleeve condition

Check biscuit thickness for consistency and

the appropriate value

109

Shrink Porosity Other considerations

If possible, move the gate close to the


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problem area to feed metal duringsolidification phase

Use squeeze pins to add pressure on thecasting after the gate is frozen

These pins are usually activated from 2 to 12seconds after the end of fill - the castingconditions must be the same from shot to shot tomake them effective, (something many die

casters cannot do because they dont control theprocess adequately)

The squeeze pins are also effective on leaker

problems, (leakers will be discussed later)110

Shrink Porosity

Hot chamber machines have similarproblems mostly they have metal leaking by


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problems, mostly they have metal leaking bythe plunger - this causes low pressure at theend of the shot and lack of pressure justwhen you need it

Trying to run too close to machine capacityin hot chamber machines can cause low

pressure at the end of the stroke Shrink porosity can occur at the gate

because this tends to be a local hot spot -

porosity in this location should respond tobetter pressure management (temperaturecontrol also)


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Sinks

Sinks Sinks (surface depressions) are a form of

shrink porosity


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A sink forms when the shrink porosity isclose to the surface, and as it cools it pullsthe thin skin on the die surface in towardsitself

The shrink porosity is close to the surfacebecause the surface of the casting is thehottest point in that area of the casting

113

Sinks


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114

Sinks The shrink porosity is formed in the location

shown when the casting is first made. As theti i l d l th di


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casting is run longer and longer, the dietemperature gets hotter and hotter in the areas

marked hotCOLD

HOT

POROSITY

115

Sinks It is typical to have a section that is heavy

enough so the gate is frozen before the areawith the shrink porosity has solidified (so the


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with the shrink porosity has solidified (so theshrink porosity cannot be fed from the gate as

it solidifies) The three hot areas shown in the last diagramcause the shrink porosity to gradually movedown towards the flat surface - the flat surfacewill likely get hotter than any other area

T H E H E A T F L U X

A R R O W S C R O W D

T O G E T H E R H E R E A N D

T H E H E A T C A N N O T

E S C A P E F A S T , S O T H E

C O R N E R G E T S V E R Y

H O T

T H E H E A T F L O W F L U X

A R R O W S H A V E L O T S

O F R O O M , A N D M O V E

H E A T O U T F A S T , S O

T H E C O R N E R S T A Y S

V E R Y C O L D

116

Sinks The flat surface gets very hot also, then the hot

spot gradually moves very close to the surface As the shrinkage porosity starts to contract it


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As the shrinkage porosity starts to contract, itpulls the skin away from the surface

As soon as the skin is pulled away from thesurface, heat flow is blocked to the die whichmakes the conditions worse

DIE SURFACE

SHRINK POROSITY

SINK FORMING

CASTING SKIN

117

Sinks

Eventually, the casting solidifies with the sink inthe surface


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The clue that the spot is getting hot and that a

sink might form is when the surface of thecasting starts to get rough (or frosty inappearance)

The operator should notice this condition, andstart to spray and/or adjust cooling line flow

What is needed is a change in temperature

balance between the hot and cold areas

118


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Leakers

Leakers Leakers are another form of shrinkage problems

There may not be any large voids, in fact theremay not be any visible porosity


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may not be any visible porosity

All that is needed is a continuous path and

enough space for gas or liquid to get through

120

Leakers This size of the space between dendrites

depends on the temperature differences thatexisted at the time of freezing and the ability to


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existed at the time of freezing and the ability tofeed new metal in during freezing

The center of the casting, or the last point tosolidify will have a loose dendritic structure thatis porous

The skin, however, is not porous - thus mostcasting would allow at least a little gas to passthrough if it were not for the dense non-porousskin

121

Leakers It takes a break in the skin (usually on both

sides of the casting) to generate a leak throughthe wall


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the wall

A typical situation is shown below

122

Leakers One way that a break occurs is when the last

point to solidify is on the surface. When thishappens, the surface generally has the rougher


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pp , g y gor frosty appearance, and the dendritic

structure is close to the surface Another way is to expose a break by machining

off some of the surface

123

Leakers The break in the surface often is from a

machining action on one side and a shapecondition on the other so that a hot spot is


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generated on the surface

GROOVEMACHINED HERE

HOT SPOTS DUE TOSHAPE OF THE

CASTING

LEAK PATH

124

Leakers The first correction should be to try to minimize

the temperature problem on the surface This can be done (sometimes) by running this


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This can be done (sometimes) by running thisarea cooler in the die; changing spray patterns,

changing or adding a water line; changing toone of the high heat transfer die materials

The temperature difference between die halves

is something to look out for (not over 100 deg fmax)

Remember that heating up a cold section can

also help restore thermal balance

125

Leakers Adding radius where possible around the leaker

area is a good idea, but can only help so much- adding more than about a .18 to .32 inch (4


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g (to 8 mm) radius does not help much

Adding pressure in that area can help -squeeze pins can work well; Moving a feedergate near the leaker area can help if the

pressure is managed correctly

126

Leakers

Also check the metal temperature - a lowerinjection temperature may make a littledifference but it may also cause other


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difference, but it may also cause otherproblems

It is best to give every opportunity for a goodskin to develop in the area where the leakerappears, be sure the die surface in this areais very smooth and clean.

127


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Cracks

Cracks Cracks, or tears, or hot cracks have many

causes, but usually are at least partially causedby shrinkage cracks on the surface


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Most often, the casting is stretched in the die as

it cools because the die doesnt changedimensions while the casting is cooling andcontracting. The stretching causes cracks at theweak point (the last point to solidify)

129

Cracks Shrinkage cracks on the surface occur during

solidification, and have a dark surface - cool thecorners or heat up the adjacent areas, add radiito the corners for this type of crack


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to the corners for this type of crack

For castings that crack while cooling in the die,the crack will also be at a hot spot increaseradii

130

Cracks Mechanical stress can cause cracks when the

die opens or the casting ejects (or during slideoperations)


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p )

Cracks at the base of long cores or fins,dragging or sticking of projections into one diehalf may indicate die shift when the diesseparate

The factors in die alignment should be checked,such as: die droop (no die carrier), wornguide pins, worn linkage, worn tie bar bushings,worn shoes under the moving platen, uneventie bar stress, etc.

131

Cracks The cracks at ejection are usually

accompanied by drag marks of some sort.

Check the ejector plate for worn bushings be


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Check the ejector plate for worn bushings, besure the ejector plate operates straight. Lookfor undercuts from erosion in the die thatcause the casting to hang up

132


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Inclusions

Inclusions

What are Inclusions/Nonmetallic Inclusions a) Particles of foreign material in a metalmatrix; b) any nonmetal material in the die


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matrix; b) any nonmetal material in the diecasting alloy.

Usually oxides, refractory particles, andsludge, but can be any material foreign to,

and essentially insoluble in, the metalmatrix.

134

Inclusions

Inclusions are mostly a problem inaluminum die casting, but there are issuesin zinc and magnesium also


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in zinc and magnesium also

Can cause quality issuesStrengthHard spots

Flow issues The most prevalent type ofinclusions are oxides

135

Oxide Inclusions The cast alloy is shiny, as evidenced by a

casting that has been machined or a furnacethat has been just skimmed


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j

The gray surface on castings or on the surfaceof the liquid metal is oxide.

This oxide layer on a casting can be very thin -

from a few microns thick to a few thousands

136

Oxide Inclusions Oxides in the furnace can not be totally

eliminated Aluminum we use is recycled, has had a lot of

i d h d id


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exposure to air and has generated oxide

Oxygen is picked-up during metal melting andhandling

Once formed in the furnace, the oxide

particles or skins remain

137

Oxide Inclusions Aluminum, zinc, and magnesium oxidize to

form dross When aluminum oxide is first formed


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Fairly soft

Less dense than the molten metal

Gamma Al2O3, dross

138

Oxide Inclusions Exposed to 1800oF (1000oC) or higher in the

presence of more oxygen gamma aluminumoxide transforms to very hard more dense phase

Al h Al O d


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Alpha Al2O3, corundum

Next to diamond on the Mohs scale Grinding wheel material

139

Oxide Inclusions Corundum can form in the melting or holding

furnaces in most plants The oxides stick to the wall and are scraped off

in the cleaning procedures


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in the cleaning procedures

Oxidesformathotcorners

1800 TO 2000 DEG

HOT CORNER

AL ALLOY - 1350

ALLOY

LEVEL

GOES UP

AND DOWN

140

Oxide Inclusions


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Casting which contained dross from dip-out well

141

Oxide Inclusions


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Cross-section of a casting containing dross

Oxide Inclusions


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Residual corundum particle from improper furnace cleaning

143

Oxide Inclusions


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Dispersion of corundum particles that can look like porosity

144

Refractory Particles Furnace refractory particles come off

the wall during furnace cleaning


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145

Refractory Particles Typical forms: brick, mortar,

castable, crucible

Some common refractory materials


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Some common refractory materials

alumina alumino-silicates

zircon

graphite

clay-graphite

silica silicon carbide

146

Refractory Particles


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Refractory particles, corundum, and flux

147

Intermetallic compounds whose formationis composition and temperature dependent

Factors contributing to sludging

Sludge


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Factors contributing to sludging

Metallic impurities

Some alloying elements

Low holding temperatures

Swings in temperature

Once formed almost impossible to dissolve

148

Fe (iron), Mn (manganese) and Cr (chromium)

can form sludge in aluminum alloys if theconcentration is high enough

Sludge


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149

Aluminum sludge factorFe + 2Mn + 3Cr3Fe + 2Cr + 3Mn

Sludge


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Hold to


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Sludge particles in aluminum alloy

151

Sludge


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152

X-ray of a hard spot (more dense inclusion)

Excess Flux Fluxes are used for various functions

Cover fluxes protect the melt from oxidation

Wall-cleaning fluxes react with wall build-up


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Degassing fluxes remove hydrogenDrossing or cleaning fluxes assist in partial

removal of oxides and reduction/recovery of

metal from drossRefining fluxes may modify, grain refine, or

remove specific metallic impurities

Too much flux gets entrained in the metal

153

Excess Flux


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Flux inclusion

154

Excess Flux


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Mixture of flux and oxide

155

Control of Inclusions

What we dont want!


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156

Inclusions To control dross

Minimize exposure to air and metal tempUse proper drossing procedures

Allow enough time after disturbing molten


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Allow enough time after disturbing moltenmetal bath

157

Inclusions To control corundum

Minimize formation of Al drossDont use higher than necessary metal

temperature


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p

Allow at least 30 minutes after furnacecleaning for settling of particles

158

Inclusions To control refractory particles and sludge

Use proper furnace cleaning procedures

Control furnace temperature


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Stay below sludge factor of 1.8

Allow at least 30 minutes after furnacecleaning for settling of particles

159

Inclusions Oxides can be removed by filtering, fluxing, or

by de-gassing the liquid metal

Refractory particles, sludge, intermetallics canbe removed by filtering


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y g

160


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Solder

Soldering phenomenon occurs when the molten

aluminum enters the die and contacts directly onsteel die cavity

Soldering


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The molten aluminum stream removes theapplied surface lubricant film and the iron oxidelayer or other coatings then erodes grain

boundaries and pits the die surface At a high enough temperature and pressure a

reaction takes place that causes the formation of

an aluminum-iron intermetallic and a directfusion between the die and the casting

162

Soldering Keep the gate velocity to a minimum, calculate

the gate velocity to stay below about 1600 ips(40 m/s) in aluminum, 2500 ips in zinc (60m/s), and about 3000 ips (75 m/s) in


163/197

magnesium - this is to avoid washing thecoating off the steel

Zinc alloys tend to solder in areas away from

the main metal flow The zinc alloys do not form a compound withthe die steel on the surface of the die (seebuild up), but build a layer on top of the steel

Die temperature is most important to keepthis type of zinc solder from forming

Soldering Surface roughness is also important,

a polished die surface will reduce thetendency to form the die solder in


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zinc (it is probably better called abuild up)

Keeping the die temperature is most

important to keep this type of zincsolder from forming

Draft angle is also important,especially if the die temperaturecannot be controlled

Soldering


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165

Soldering The best solution is keeping the die steel cool -

solder will not start then Cooling methods include die spray, adding

water channels, slowing the cycle speed, and


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reducing metal temperature. Also consider other die materials Anvilloy

Bi-metallic cores

Niobium

Other solutions Draft angle add to die

Gate velocity keep low

Die surface roughness keep smooth

166

Build Up A die build up usually comes from lubricant that

was not evaporated from the die If it is darker in color, it is often referred to as

carbon


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Review the die temperatures with the die spraysupplier to get the proper lubricant - keep ratiounder control

Use of hard water in the die lubricant cansometimes cause build up

167

Build Up Review the die temperatures with the die spray

supplier to get the proper lubricant Keep die spray dilution ratio under control

Dont spray too long


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Be consistent

168


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Erosion

Erosion and burn out refer to defects in the

casting that come from the effects of havingsome die steel eroded away

Erosion generally occurs in aluminum die

Erosion - Burn Out


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casting, usually where there are high metalvelocity and high steel temperatures

The biggest factor in erosion is usuallyuncontrolled and poorly managed gate velocities

170

The gate velocity should be kept about in the

following ranges:Aluminum, 1000 to 1600 ips (25-40 m/s)

Zinc, 1200 to 2000 ips (30-50 m/s)

( / )

Erosion - Burn Out


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Magnesium, 1200 to 3000 ips (30-75 m/s) Die temperature is next most important, and

should be kept as low as possible

Additional factors include the metaltemperature, the inclusions in the metal, thetype of alloy, proper gate design and flow

pattern

171

Burn out is a term usually applied to zinc

dies, and is mostly the result of cavitation Cavitation comes from gas bubbles in the

metal imploding when the go from a high

i th t l

Erosion - Burn Out


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pressure area in the runner to a lowpressure area in the cavity - if theyhappen to be in contact with the die steelwhen this happens, then cavitation occurs

Sometimes the gate location can bechanged so the metal doesnt impact onthe die steel just past the gate opening

The best correction for this is to minimize

the air bubbles - this includes factors suchas the following:

Using two speed plungers

Erosion - Burn Out


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Using runner sprue designs

Using proper and carefully designed

runners and gates

O t i


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Outgassing

Gate Porosity - Outgassing This defect occurs when the casting is heated,

usually for a secondary operation like painting,and gas comes out of the casting

This is due to the expansion of the trapped gasin the casting that escapes through a porous

ti f th ti


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section of the casting

It is not be possible to eliminate all the trappedgas, so most of the work should be focused onreducing the porous-ness of this section of thecasting

175

Gate Porosity - Outgassing To reduce the problem in the overflows,

minimize the number of overflows used andkeep the gate to the overflow as thin aspossible

For outgassing at the gate keep the gate as


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For outgassing at the gate, keep the gate asthin as possible, keep the gate area cool asmuch as possible, and increase metal pressure

Ensure there is enough pressure to feed theshrinkage

For all types of gate entrances to the casting,

do not have thin die sections that can build-upheat.

176

Gate Porosity - Outgassing

CASTING

GATE

HOT SPOT


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HOT SPOT INSTEEL

HOT SPOT,

POROUS AREA

This hot spot increases the amount of shrinkporosity formed at the gate which contributesto outgassing problems

B di W i


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Bending, Warping

Bending - Warping Bent and warped castings have many different

causes, starting with design, die build, machineconditions, and operating conditions

Once the design is set, the major operating

factor is that the casting and the die always be


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factor is that the casting and the die always beat the same temperature when the casting isejected

The use of a thermocouple to eject the castingat the same temperature every time is avaluable aid to precise dimensional control

179

The die and the machine must be maintained in

good condition to minimize bending and warping

Bending - Warping


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180

Flash


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Flash Flash occurs when liquid metal flows into an area

of the die where it is not expected, such as theparting line, under a slide, or along side anejector pin


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182

Flash Flash can be almost eliminated if proper attention

is applied to tool design and operational factors

A robust die is required, there can be no diedeflection

Good thermal balance is required so the die fitswell at operating temperature


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well at operating temperature

The machine locking conditions must be proper;such as equal load on the tie bars, proper die set

up, linkage not worn, platens flat, etc Metal pressures and impact force should be no

more than necessary

183

Flash Flashing can occur for one of three

reasons:1. The die seal-off is poor.

2. The die and machine do not work together

to seal off the mold cavity


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to seal off the mold cavity.3. The impact force at the end of the cavity

fill exceeds the clamping force of the

machine. To understand the reason why flashing

occurs, you must understand the

condition of the machine, the tool, andthe forces occurring within the machineat the end of cavity fill.

184

Flash Determining the root cause of a flashing

problem is sometimes quite complex. As an operator, you need to observe the die:

If the casting flashes in exactly the same way

every shot, then the tool or machine is likely


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every shot, then the tool or machine is likelythe problem.

If the flash pattern around the cavity changes

drastically, especially as a die heats up duringstartup, then it indicates two possible causes:

The impact force is exceeding the clamping

force of the machine. Thermal expansion within the tool is allowing

metal to escape the die cavity.

185

Flash Summary The two keys to minimizing flashing

are:Die design.

Machine maintenance.


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Machine maintenance. Die design.

Die squareness.Thermal design.

Die seal-off.

Centering the part within the die.

Wear plate and lock design.

186

Stained Castings


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Stained Castings

Stained Castings Dark gray or black discoloration

Almost always involves die or plunger lube

Typically, too much lube has been applied

Check:A t f l b b i li d


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Amount of lube being applied

Dilution ratio

Application procedure

Other sources dirt scrap

188

Waves and Lakes


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Waves and Lakes

Waves & Lakes Typically in zinc

Caused by metal flow problems Two flow fronts meet, one cooler than the

other, however, enough heat for re-melting

for the two fronts to attach this is thediff b t l i ti d &


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difference between laminations and waves &lakes.

Waves & Lakes Check:

Plunger acceleration needs to beaccelerated before metal hits the gate

Fast shot transition early enoughGate design


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y gGate design

Flow pattern

Die temperature

Metal temperature

Drags


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Drags

Drags Looks similar to solder, but is caused by the die

dragging on the part during ejection

Check:

Draft angles

Ejection problem, such as bendingErosion that may have caused an undercut


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Erosion that may have caused an undercut

Distorted, mushroomed cores

Flash on slides Die temperature

193

C ld l k


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Cold Flakes

Cold Flakes PSP - ESP Occurs in only cold chamber machines

Causes irregular breakout at the gate Most collects at the gate

Can cause flow problems

May be difficult to see without microstructural


195/197

examination

195

Cold Flakes PSP - ESP Corrections

Minimize shot delay time

Keep fill % in sleeve as high as possible

Keep sleeve temperature as high aspossible


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possible

Keep metal temperature high

196


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197

nadca - overview of defets in die casting

Documents

die lifebasics

die castingdefectsdr

rating system

scrap reporting system

continuousreporting

rating porosity

major defects

severityof defects