EASTERN ALLOYS, INC.P O B O X Q M A Y B R O O K , N E W Y O R K 1 2 5 4 3 - 0 3 1 6 ( 9 1 4 ) 4 2 7 - 2 1 5 1

TELEFAX (914) 427-5185

DIECASTING DEFECTS: IDENTIFICATION - - CAUSES & CURESby

Derek Cocks, Mgr. Customer Engineering Services

STEP 1:

USING PROCESS CONTROL TO PREVENT DEFECTS:

“Prevention is always better than cure”, therefore, process control techniques are important.Understanding the process variables is also the first step to both diagnosis and cure of defects.

Set-up sheets:

Have a set-up sheet available for each job - preferably covered in plastic and attached to themachine. If defects start to appear don’t try to diagnose at this stage, check the machine settingsagainst the set-up sheet.

The Design of the set-up sheet is important and should follow the following basic rules:

l Keep it simple - one side of an 8 l/2 x 11” page.

l Group the information for setters, quality inspector, time study, etc.

l Use term which are understood in your plant. e.g., “bottle pressure” or “line pressure” or “toppressure”, etc.

*A sample set-up sheet is attached.*p

Gauges:

Ensure that all gauges are present, are working and can be easily read. A 2 inch diameterpressure gauge with a dirty dial twelve feet up on top of the accumulator cannot be read.

Valves and switch positions should have marked scales so that they can be set without having tofind a tape measure or count turns of valves.

S P E C I A L I S T S I N T H E A L L O Y I N G A N D R E F I N I N G O F Z I N C A L L O Y S E A Z A L L ®

EASTERN ALLOYS, INC.

Training:

Don’t assume that because the data is on the set-up sheet the personnel know how to set thevalues. Have training sessions between engineering and setters, to ensure:

l Both parties know which control is being referred to.

l You know where the controls and gauges are on the machine.

l You know how to adjust it, and what its particular “quirks” are.

l Improvements to both set-up cards and machine controls can be made.

*A sample quiz sheet is attached.*

Measuring Equipment:

Every setter should have a small portable pyrometer with an immersion probe (for metaltemperature, etc.) and a surface contact thermocouple for measuring die temperature. One pershop is not enough.

Serious consideration should be given to the purchase of shot monitoring equipment. It isexpensive and requires skill to use, but the pay-back time is short.

Tie bar load gauges should be fitted and working, if the machining has provision for them.

QUIZ SHEET:

Identify each gauge or control and enter details from set-up sheet.

SET-UP CARD COMPT. NO.

EASTERN ALLOYS, INC.

STEP 2:

QUALITY CONTROL PROCEDURES TO SPECIFY AND IDENTIFY DEFECTS:

All castings contain defects -- it is essential to specifywhat is acceptable and what is not. Good quality controlprocedures will give early warning of problems.

Specifying Quality Levels:

Do not try to diagnose the cause of the defect at this stage.Concentrate on saying:

Where it is on the casting. (eg., surface near the gate)What it looks like. (eg., rough shiny marks)HOW bad it is - difficult to quantify.

It is almost impossible to quantify any defect. The best you cando is to have a sample available of the worst acceptable condition.Use photographs or Xerox prints or computer stored pictures ofsurfaces. Cut sections or reference radiographs may be useful forinternal defects.

Do not specify weight or density - this is frequently misleading.

Proof testing:

If possible devise a simple proof test which mimics the serviceconditions, eg.:

Break test using a torque wrench or weight.Pressure test for leakage.Polishing test for cosmetic surfaces, using polishingequipment, bright lights and a trained examiner.

Metallurgical Examination:

Radiography and machining tests require great technical skill andare more appropriate for defect diagnosis than as practical qualitycontrol techniques. At best they are subjective, since it is noteasy to put a numerate value on the results. At worst they can betotally misleading, for instance a machined surface may show nodefects in a very porous casting since the pores lie 0.002 inchesbelow the machined face in that particular casting. A radiographmay show only slight "shadows" which are in fact very tight crackswhich will seriously weaken the casting.

EASTERN ALLOYS, INC.

Process Control Parameters:

The Quality department should preferably concentrate on checkingprocess conditions and relating these to the type of defects found.

Defects are related to the "primary process variables", ie. whathappens in the die, which in turn are controlled by the processsettings. Relationships are complex and care must be used but thefollowing guide lines may prove useful.

Process Variable

Metal analysis

Metal temperature

Cavity fill time

Gate velocity during fill

Die face temperature

Lubricant-dilution quantityand spray pattern

Related Defects

All

All

Cold shuts, dimensions

Porosity, blisters,soldering, erosion

Cold shuts, soldering,cracks, dimensions

Porosity, blisters,soldering

Knowing the process variables is the first-stage of defectdiagnosis.

The following values should be measured (since equipment is notexpensive):

Metal analysisMetal temperatureDie face temperatureLubricant - dilution and quantity

Cavity fill timeGate velocityFinal metal pressure

DETERMINING CAVITY FILL CONDITIONS

BY MEASUREMENT: If shot monitoring equipment is available the averageplunger speed during fill can be used to calculate gate velocity 'V'and cavity fill time 't' as shown below:

Plunger speed during fill = 40 ipsPlunger diameterGate area

= 3.0 in. diam. = 7.07 sq.in.

Casting weight= 0.02 x 10 in. = 0.20 sq.in.= 1.25 lb. = 5.43 cu.in.

Gate Velocity ‘V’ = 40 x 7.070 . 2 = 1404 i.p.S. or 117 f.P.S.

Cavity fill time 't' = 5.437.07 x 40

= 0.0192 secs. or 19.2 millisecs.

BY PQ2 - NADCA "FLOW PREDICTOR":

Use either manual or computer systems to predict values of 'V' and 't'.Machine performance data may either be measured as above ormanufacturer's data. Accuracy is usually ± 10%.

l Step 1

Graph the machine shot-end capability by putting two valueson the graph supplied.

F l o w R a t e Q

l Step 2Choose a transparent resistance graph for the metal being cast.

Gate Area

F l o w R a t e Q

l Steps 3-6

Put the two together and take readings, then make threesimple calculations.

Note: This is analogous to putting the die on the machine.

• Steps 7-6

Fill in readings on the worksheet table and compare values.This gives fill conditions for a range of gate areas and isanalogous lo carrying out several die trials.

GATE AREA (A) SQ. IN. G A T E V E L O C I T Y ( V ) F P S F I L L0 . 1 0 0 1210 . 1 5 0 1 2 0

I - - - - - 0 . 2 0 0 - - - - - - - - - - - - - - - - 1 1 7 - - - - - - - - - - - - - - - - - - - -

I - - - - - 0 . 2 6 0 - - - - - - - - - - - - - - - - 1 1 4 - - - - - - - - - - - - - - - - - - -I - - - - - 0 . 3 0 0 - - - - - - - - - - - - - - - - 1 1 1 - - - - - - - - - - - - - - - - - -

I - - - - - 0 . 3 5 0 - - - - - - - - - - - - - - - - 1 0 7 - - - - - - - - - - - - - - - - -

I - - - - - 0 . 4 0 0 - - - - - - - - - - - - - - - - 1 0 4 - - - - - - - - - - - - - - - -

I - - - - - 0 . 4 5 0 - - - - - - - - - - - - - - - - - 1 0 0 - - - - - - - - - - - - - - - -

0 . 5 0 0 9 6

PLUNGER DIAMETER = 3.00 IN.HYDRAULIC PRESSURE = 1200 PSISHOT SPEED CONTROL = 100 %

T I M E ( T ) M S3 7 . 32 5 . 31 9 . 3 - - - - I1 5 . 9 - - - - I1 3 . 6 - - - - I1 2 . 0 - - - - I1 0 . 9 - - - - I1 0 . 1 - - - - I

9 . 4

EASTERN ALLOYS, INC.

Process Control "Windows":

Usually there is a range of values, within which good castings areproduced. Often several process variables are related, thus a"control window" can he specified.

The attached graph shows the relationship between minimum die facetemperature and maximum cavity fill time for various wallthicknesses.

MINIMUM DIE TEMP. v MAXIMUM FILL TIMEFOR VARIOUS WALL THICKNESSES

STICKING NORMAL RANGEI

SOLDERING

80

70

60

50

40

30

20

10

0 I I I I I I I I240 280 320 360 400 440 480 620 660

DIE FACE TEMP DEG F.

EASTERN ALLOYS, INC.

STEP 3:

EXTERNAL DEFECTS -- DIAGNOSIS AND CURE:

Diagnosis requires care and skill. An incorrect diagnosis is worsethan none at all, since you may easily make matters worse.

Defects do not normally appear separately, but several are presenttogether, often in the same area of the casting.

You will need:

• Bright lights - good fluorescent lights plus an adjustablespot lamp.

• A magnifying lens or binocular microscope.• Polishing buff or steel wool.• Photo copier or camera.• Sharp knife.

Make a Careful Examination:

Examine all the evidence carefully step-by-step, take your time andbe thorough.

Obtain details of all the process variables in Step #2. Carryout calculations and predictions as necessary.

Carefully examine several castings using every techniqueavailable - eg., hand lens or microscope before and afterpolishing. Cut open blisters with a craft knife. Use photocopymachine to record flat surface details.

Record relationship of defect position to gates, overflows, fillpattern, etc.

Above all take time. A diagnosis made in less than 30 minutesis likely to be incorrect.

The following table is a useful guide, but exceptions are common.

DIAGNOSING SURFACE DEFECTS

Appearance

Cold Shut

Splash & Spatter

at end of

Position

All Over

k /Lamination

at "windows"

Blisters

Causes

• Fill timetoo long

• Die temperaturetoo low

• Fill time anddie temperturemarginal

• Poorovermetal

controlfirstin

• Excessive swirlvortex stillpresent at timeof solidification

• Fill time anddie temperaturemarginal

• Dies separatingduring filling

• Trapped airor die lube

• Gases notwell distributed

Cures

• Improve fillconditions

• Increase dietemperature

• Improve filltime and/ordie temperature

• Improve feeddesign andfill pattern

• Improve filltime and/ordie temperature

• Improve fillpattern

• Flash overcut-outs

• Check die lock

• Check toolsupport

• Increase gatevelocity

• Reducelubrication

• Improve venting

• Increase freezetime

DIAGNOSING SURFACE DEFECTS

Appearance Position Causes Cures

Sink Marks • Liquid/solid • Improve designSink mark contraction to eliminate

• Lack of feed • Achieve optimumto local thick fill conditionssections

Lakes • • Optimize tooltemperature

extraction rateof tool halves

• Shrinkage due • Treat as shownto local hot in appropriate

• Fine cold shuts

• Gas porosity

• Zinc dissolves • Remove causeiron of die

• Take off atleast 0.002"

• High temperature from surface

• Oxide bloom • Oxidize dieremoved fromdie surface

Drag Marks • Under cuts • Increase draft

• Reduce "freeze"shrinking on to

• Eliminate• Oxide coat lostand soldering

EASTERN ALLOYS, INC.

STEP 4:

INTERNAL DEFECTS (POROSITY) DIAGNOSIS AND CURE:

There are many types of internal defects, some more dangerous orunacceptable than others, depending on the application.

The position and type of defect is often more important than thequantity. Quantitative assessment is difficult and usuallysubjective.

As with surface defects, it is usual to find several defects inassociation, for instance gas trapped in the inflowing molten metalwill usually migrate to the areas where shrinkage contractionoccurs as the metal solidifies. Removing the gas will change theform of the defect (probably from spherical to inter-dendritic),but probably not affect the position and quantity.

You will need:

• Saw, hammer vise• File, emery paper (or preferably) polishing equipment

Bright lights - adjustable spot lampMagnifying lens or binocular microscope

• Sharp scriber or needle

Optional extras:

• Crack detecting equipment (eg., dye penetrant spray cans)• X-ray equipment• Lathe or milling machine to cut consecutive sections

Method:

• Make surface examination first - use crack detection whereappropriate. Look for external evidence which indicatesinternal defects. eg., Sink marks may indicate internalshrinkage porosity.

• Examine process data for indications of defects likely to bepresent, Calculate fill time, gate velocity, etc.

• Examine internal structure by any appropriate means:

X-ray will show most of the internal structure, butneeds a skilled technician to interpret the results.

Sawn or machined sections will show internal. structureat the section chosen. Be careful that defects are notburred over.

Half-sawn and broken sections are preferred since itavoids burring.

The following table is a useful guide, but exceptions are common.

A p p e a r a n c e

Spherical shiny

DIAGNOSING INTERNAL DEFECTS

P o s i t i o n Causes

Uniformly dispersed Gased metal(Hydrogen)

Aluminum only

Cures

. Degas metal

. Increase metalpressure

Spherical dull Corners

Gate.

. Lubricant . Reduce plunger

. Air and die lube

. Water vapor (Zn .06 oz/lb)

. Use correct (Al .1 oz/lb)gate velocity diluted lube

. Reduce runnervolume

. Look for crackedtool

. Use correct gatevelocity

Swirls Close to surface . Cold shuts . Reduce f ill timefolded in . Increase die and

. Trapped air, metaletc. temperature

. Improvecirculationwithin the die

Irregular rough Centers of section . Shrinkage . Use lower metalporosity temperatures

. Thick . Design “out”sect ions thick sections

. Hot spots . Use high final

. High pressuretemperatures

Intergranular Center line and thick . Shrinkage . “Design out”

sections. Low metal and

Cracksnoncontinuous)

Sharp corners Shrinkage plus . Reduce die closestress time

. Eliminate hangups

. Blend corners

. Add buttresses