INDEX

INTRODUCTION

CLASSIFICATION OF DEFECTS

DEFECT REORGANIZATION AUTOMATING THE INSPECTION PROCESS

COMMON DIE CASTING DEFECT

ELEMENT AND PERFORMANCE CRITERIA

HYDROGEN GAS DEFECT IN IRON CASTING

DEFECT DIAGNOSIS AND CONTROL

CASTING DEFECT IN LOW PRESSURE

RESULT AND DISCUSSION

A SEMINAR REPORT

ON

Defect in casting

Guided By: - Prepared By:-

Prof. S.R.Patel Pathak Vimal R. Head of Production Engg. Dept. Roll No. 22 L.E. College, Morbi B.E. Production Engg. (Sem.VI)

Year: 2007 Production Engineering Department

Lukhdhirji Engineering College

Morbi - 363642

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Certificate

This is to certify that

Shree Pathak vimal R.. of Course Production Engineering Sem – VIth Roll No. 22 has satisfactorily completed the Term – work in

subject seminar

(Defect in Casting)

Date :

Place : Morbi

Guided By: Head of the Dept.:

Prof. S.R.Patel Prof. M.G.Bhatt

Head of Production Engg. Dept. Head of Production Engg. Dept. L.E. College, Morbi L.E. College, Morbi

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Introduction

Under normal conditions, like all metallurgical products, castings also contain

certain imperfections, which contribute to a normal quality variation.

Such imperfections are taken as defects or flaws only when they affect the

appearance or the satisfactory functioning of the castings and the castings in

turn do not come up to the quality and inspection standards beings applied.

Defective castings offer an ever-present problem to the foundry industry.

Defective castings account for the normally higher losses incurred by the

foundry industry.

Casting defects are usually not accidents; they occur because some step in the

manufacturing cycle does not get properly controlled and some where goes

wrong.

A defect may be the result of a single clearly defined cause or of a combination

of factors in which case necessary preventive measures are more obscure

Close control and standardization of all aspects of manufacturing techniques

offer the occurrence of defects in casting

Defects found in casting may be divided into three classes

I. defects which can be noticed on visual examination or measurement of the

casting

II. defects which exist under the surface and are revealed by machining sectioning

or radiography

III. material defects discovered by mechanical testing of the casting

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Classification of defects

Logical classification of casting defects present great difficulties because of the wide

rang of contributing cause however a rough classification may be made by grouping the

defects under certain broad types of origin such as

a) Defects caused by patterns and molding box equipment

b) Defects due to improper molding and core making materials.

c) Defects due to improper sand making and distraction.

d) Defects caused by molding core-making getting etc.

e) Defects due to improper mold drying and core banking.

f) Defects occurring while closing and pouring the molds.

g) Defects caused by molten metal.

h) Defects occurring during fettling etc.

i) Defects due to faulty heat treatment.

j) Defects due to cast metal.

k) Warpage.

Only more important defects have been discussed below in detail

A. Defects caused by patterns and molding box equipment

1. Mismatch or mold shift

It produces a casting which does not match at the parting line

There is mismatch of top and bottom parts of the mold joint.

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Causes:

Worn or loose dowels in patterns made in halves.

Faulty registering of top and bottom halves of patterns mounted on plates.

Worn out, loose, bent or ill-fitting molding box clamping pins

Remedies:

Remedies involve removing the causes listed above.

2. Variation in wall Thickness of the Casting

Causes:

Worn core boxes giving oversize core dimensions.

Worn core prints allowing a core to float or move,

Inadequate core print area, permitting lifting of cores due to buoyancy of molten

metal.

3. Fins, Flash and Strain.

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Fins,flash and strain usually occur at the parting line and result in excess metal

which has to be ground off. Flashes or fins commonly appear along the mold

joint at the places where the mold halves do not fit together properly because of

much wear or warping of flask halves or improper fastening of the cope to the

drag.

Straining or movement of the mold makes a casting appreciably thicker than the

pattern.

Causes:

1. Fins or flash at the mold joint may occur when,

Bottom boards are too flexible,

Patter plates are not sufficient rigid to keep straight during ramming.

2. Patterns having insufficient taper and thus requiring excessive rapping for their

withdrawal from the sand result in fins at the joint.

3. Top part boxes inadequately weighted, permit the top box (i.e., cope) to lift slightly,

when poured, thereby causing flash along the mold joint.

4. Crush:

It is the displacement of sand while closing a mold, thereby deforming mold

surfaces.

A crush shows itself as an irregular sandy depression in the casting.

Causes:

Excessive weighting of the green sand mold (cope portion).

Core print too small for the core.

Core too large for the core print.

Careless assembly of molding boxes and cores.

B. Defects Due to Improper Molding and Core-making Materials

(i.e., Improper Sand Conditions)

1. Blowholes:

Blowholes are smooth, round holes.

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Blowholes visible on the surface of a casting are called open blows whereas

those occurring below the surface of castings and not visible from outside are

termed as blowholes.

Blowholes may occur in cluster or there may be one large smooth depression.

Blowholes are entrapped bubbles of gas with smooth walls.

Causes

1. Excess moisture in the molding sand.

2. Low permeability and excessive fine grain sands.

3. Rusted and amp chills, chaplets and inserts.

4. Cores, neither properly baked not adequately vented.

5. Presence of gas producing ingredients in the mold or core

6. Extra hard rammed sand

7. Mold being not adequately vented

Remedies

1. Involve removing the causes the promoting a defects .

2. Drop .

A drop occurs when cope surface cracks and breaks thus the pieces

of sand fall into the molten metal .

i) Low green strength {owing to less mulling, time moisture or clay content }.

ii) Low mold hardness i.e, soft ramming.

iii) Insufficient reinforcement of sand projection in the cope.

3.scab

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It occurs when a portion of the face of a mold lifts and metal flows underneath in

thin layer. In other word ,liquid metal penetrates behind the surface layer of

sand.

Causes:

1. Too fine a sand.

2. Sand having low permeability.

3. High moisture content of sand.

4. Uneven mold ramming.

5. Intermittent or slow running of molten metal over the sand surface thereby

producing intense local heating.

4. Pin-holes

Pin-holes are numerous very small holes revealed on the surface of a casting

after the surface has been cleaned by shot blasting.

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Causes:

i) Sand with high moisture content.

ii) Sand containing gas generating ingredients.

iii) Faulty metal.

iv) Gas dissolved in the alloy not being properly degassed.

v) Metal mold reaction (results pin holing in steel castings).

5. Metal penetration and rough surface

Molten metal enters into the space between the sand grains and results in metal

penetration and rough casting surface.

Causes:

1. High permeability.

2. Large grain sized sands.

3. Low dry strength of sand.

4. Soft ramming.

6. Hot tears (Pulls) refer fig)

They are internal or external cracks having ragged edges.

Immediately after solidification, metals have low strength; if at this stage, solid

shrinkage of the casting develops sufficiently high stresses, the metal fails with a

resulting hot tear.

Causes:

1. Very hard ramming and therefore excessive mold hardness.

2. High dry and hot strength of the sand mold.

3. Insufficient collapsibility of core or of a portion of mold.

4. Too much shrinkage of metal while solidifying.

5. Faulty design causing some portions of casting to be restrained while cooling.

6. Slow running of molten metal due to small gates or metal lacking in fluidity.

7. High sulphur content (Promotes hot tearing).

8. Too low pouring temperature.

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(c) Defects due to improper sand mixing and distribution

Improper sand mixing and distribution give rise to faulty sand conditions and

various defects which results, have been discussed earlier under section(b).

(d) Defects caused by molding, core-making, gating, etc.

1. Hot tears [discussed earlier under (b)-6].

2. Shigys[discussed earlier under (a)-1]

3. Fins and flash [discussed earlier under (a)-3]

4. Crush [discussed earlier under(a)-4].

5. Cold taps (shuts) and misrun.

If molten metal is too cold or casting section is too thin, entire mold cavity may

not be fi lled during pouring before the metal starts solidifying and the result is

Misrun.

If molten metal enters mold cavity through two or more ingates or otherwise if

two streams of metal which are too cold, physically meet in the mold cavity but

do not fuse together, they develop cold shut defect.

Causes.

1. too cold molten metal.

2. too thin casting section.

3. too small gates.

4. too many restrictions in the gating system.

5. metal lacking in fluidity.

Besides, misrun is often the result of interrupted flow of metal from ladle into the

mold.

6. slag holes

They are smooth depressions on the surfaces of castings.

They usually occur near the ingates.

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Cause and remedy

Slag holes result when slag enters the mold cavity,hence they cane be obviated by

inserting slag traps in the gating systems.

7. shrinkage defects

metals shrink as they solidify; if this shrinkage is not compensated by providing

risers, etc. voids will occur on the surface (i.e surface shrinkage) or inside (i.r.,

internal shrinkage ) the casting.

(e) Defects Due to Improper Mold Drying And Core Baking

1. A layer of moisture collected under the impervious layer of mold paint(on the

surface of the mold) will cause sand and paint scab and peel, with the result that

casting shows,

Sand Washes

Scabs

Blowholes.

2. Oil sand cores, if over-baked, are not strong enough to resist the flow of molten

metal and cause

Sand washes

Rough surface

Metal penetration, and

Undersized holes.

3. Oil sand cores, if under-baked, absorb moistures from atmosphere or green

sand mold and cause defects like

Core sand wash,

Core blow, and

Blow holes.

(f) Defects occurring while Closing and Pouring the molds

1. Shift or mismatch of the cope and drag at the mold joint [discussed earlier under

(a)-1].

2. Misrun[discussed earlier under (d)-5]

3. Cold laps or cold shuts [ discussed earlier under (d)-5]

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4. Crush [discussed earlier under (a)-4]

5. Run out.

A run-out occurs when molten metal leaks out of the mold during pouring and

results in an incomplete casting.

Causes:

1. Faulty molding box equipment

2. faulty molding

6. Inclusions

Any separate undesirable foreign material present in the metal of a casting is

known as inclusion. An inclusion may be

i) Oxides, slag, dirt etc. which enter the mold cavity along with the molten metal

during pouring.

Such inclusions should be skimmed off before pouring molten metal into the

mold cavity.

ii) Sand cracked and broken from gating system, mold cavity, cores, etc.

Sand sinks in molten light metals and causes sand cavities in the drag whereas

in heavier metals (e.g. Steel, etc) sand either floats to the cope surface of the

casting or becomes entrapped within the casting itself.

Remedy:

i) Proper molding.

ii) Molding sands should possess adequate hot strength.

iii) Skimming off or screening of molten metal before pouring.

(g) Defects Caused by Molten metal

1. Misruns [discussed earlier under (d) 5]

2. Cold shuts [discussed earlier under(d)-5]

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In pouring aluminum bronze, drops of metal that become separated from the main

stream during pouring get surrounded by a fi lm of alumina that causes a cold shut.

3. Excessive penetration [refer to (b)5 also]

In heavy steel castings, penetration of metal into mold or core (sand) is the

result of high casting temperature.

Incropper base alloys and cast iron, prenetration occurs because of excessive

fluidity of molten, due to high phosphorous content or high casting temperature.

4. Tin and lead sweat

It occurs in high leaded copper-base alloys as spots and lumps.

The presence of an excessive amount of hydrogen dissolved in the molten metal

may force lead to the surface to cause inverse segregation.

Silicon in copper alloys and a low tin content cause tin and lead sweat.

When the surface of copper alloy is covered with a discontinuous thin layer of

metal containing a higher content of tin than the parent alloy, this is known as tin

sweat.

5. Hot tears [discussed earlier under (b)6].

6. Sand cuts and washes

Molten metal as it flows over the mold and core surfaces, crodes the same and

results in defects known as cuts and washes.

The place from where the sand has been cut or washed is occupied by molten

metal and thus an excess metal appears on the casting surface in the form of

rough jumps or ragged spots.

Causes:

1. Soft ramming.

2. Weak sand.

3. Insufficient draft on patterns.

4. insufficiently bounded or overbaked cores.

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5. improper gating system.

7. Fusion

Sandy may fuse and stick to the casting surface with a resultant rough glossy

appearance.

Causes:

1. Lack of refractoriness of sand.

2. Too high molten metal temperature.

3. Faulty gating system.

8. Gas porosity, Gas-holes, sponginess

Gas porosity differs from blow holes which result due to the molding sand having

low permeability, excessive moisture or having been rammed too hard.

Gas porosity is caused by the gases absorbed by the molten metal. The main

gases dissolved by practically all metals are, oxygen, nitrogen and hydrogen.

Hydrogen is responsible for gas porosity.

Molten metals (especially aluminum and copper-base alloys) may absorb

hydrogen from, unburnt fuel gas, moisture in the air, dampness in the furnace

and green sand mold.

As molten metal solidifies, many small voids distributed quite uniformly

throughout the metal are found and it is known as Pin hole porosity.

In non-ferrous melting, hydrogen and sulphur dioxide in the molten metal cause

gas holes just below the surface of the castings.

Causes:

1. Hydrogen or sulphur dioxide dissolved in molten metal.

2. Excessively high pouring temperature.

3. Damp ladles.

4. Low permeability of sand.

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5. High moisture content of the mold

6. Slow solidification rate.

In ferrous alloys, hydrogen, if present, forms gas holes in heavier sections rather

than in thinner more quickly cooled sections.

Oxygen is more often the cause of gas holes in ferrous castings.

Remedy:

1. Remove the dissolved gases from the melt.

2. Avoid the conditions (as discussed above) promoting pick up of gases by the

molten metal.

9. Shot metal

If the molten metal is at relatively lower temperature and during pourig into the

mold, it splashes, a few small particles separate from the main stream, they

solidify and form shots. These shots, if do not fuse with the rest of the molten

metal in the mold, get embedded in the casting and are revealed on the

fractured surface, thus causing a defect known as shot metal.

Causes:

1. As explained above.

2. Excess sulphur content in the molten metal.

3. Higher moisture content of the molding sand.

4. Faulty gatting system.

5. Improper pouring of molten metal.

10. Rattals and buckles

If molten metal having very high temperature is poured in to the mold cavity, a

then outer sand layer of mold cavity expands, bulges, gets separated from the

sand behind it and remains on the surface of the casting. The casting surface

shows either a step or a shallow indentation along the path of incipient mold

failure, often with a short metal fin representing the original crack. This, surface

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fissure or line defect is known as rattail and it appears as an irregular line across

the surface of a casting.

A rattail is the result of slight compression failure of the thin layer of molding

sand.

Another defect known as Buckles is more severe compression failure of the

sand surface.

Buckles and scabs usually appear in cope surfaces of the castings

They are alike in appearance.

Buckles shown extensive overlapping of metal whereas scabs are relatively

small (than buckles).

Causes:

1. Soft ramming.

2. Insufficient weighting of the molding boxes during casting.

3. Low strength of mold.

4. Mold being not adequately supported.

(h) Defects Occuring During Fettling, etc.

Defective castings may result from carelessness during the fettling operation,

e.g.,

1. Sand and scale not properly removed from casting surface to be machined later

on.

2. Sand not properly removed from cavities where oil is to be circulated.

3. Distorted castings not properly straightened.

4. Cracks caused in brittle castings by too heavy grinding.

5. Chisel marks left on the castings.

6. Heads burned off too low or too high, thereby requiring building up by metal

deposition or removal by machining operations.

(i) Defects due to Faulty Heat-Treatment

1. Uncontrolled initial heating operation may cause cracking of brittle castings.

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2. Careless stacking of castings while they are in a semi plastic state at the heat-

treatment temperature, will tend tem(i.e., castings) to distort.

3. Fast cooling rates may develop cracks in the castings.

4. Improper heat-treatment furnace atmosphere may prove detrimental to the

surface appearance of the casting.

(j) Defects due to Cast Metal

Hard spots

Hard spots occur in gray iron castings having insufficient silicon content.

Such castings get hardened by the chilling action of molding sand.

Hard spots make machining of the castings difficult.

Causes:

1. Faulty metal composition.

2. Faulty casting design, leading to rapid cooling of some parts of the casting as

compared to other parts.

(k) Warpage

Castings warp(i.e.. misalign) or deform because of the stresses set up in them

internally, due to differential solidification rates experienced by different sections

of large, long and wide flat castings.

Causes:

1. Faulty casting design

2. Absence of directional solidification.

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Defect Recognition – Automating The Inspection Process

I. Introduction

X-ray inspection is a well-established NDE technique in the automotive casting

industry. In recent years,

There is a growing trend towards automating the X-ray inspection process due

to several factors.

Nowadays, it is common to find 100% of manufactured parts required to be X-

ray inspected. The

Necessities of production and shipping schedules translate to X-ray inspection

machines being operated

24x7. There is an increasing focus on quality of inspection, with an emphasis on

more quantitative and

Uniform product evaluation. Frequently, as well, X-ray inspection is perceived to

be the bottleneck in the

Overall production process, resulting in a constant requirement for faster

inspection rates. Shortage of

Experienced X-ray inspection operators, operator fatigue, training and

motivational issues, and

Inspection cost reduction are other major drivers to this increasing need for

automation at every stage of

The inspection process.

The objective of this paper is to provide an overview of automation components

and architecture for X-ray

Inspection in the automotive casting industry. In particular, the paper examines

automation aspects

Applicable to inspecting the X-ray image, storing and analyzing inspection

records, and real-time

Feedback of inspection information to casting stations for process control.

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II. Automation Components

A typical X-ray inspection machine for castings consists of a lead-shielded

cabinet enclosing the X-ray Source and detector, a part conveyor, automated or

manual part loading and unloading, and an image

Acquisition and display computer for manual inspection of the X-ray image. The

imaging software on the computer acquires the image from the detector and

displays the image cycling through different views (a view is a pre-programmed

setting for Look-Up-Tables (LUTs), zoom, and pan).

The operator Makes a decision to accept or reject the part after inspecting each

image view.

Automating the X-ray inspection process can be looked at in terms of combining

automation Components into different architectures. The following automation

components are considered here:

• Automated Defect Enhancement (ADE)

• Automated Defect Recognition (ADR)

• Inspection Records Database

• Real-Time Feedback for Process Control

III. Defect Enhancement and Recognition

Automated Defect Enhancement (ADE)

The Automated Defect Enhancement (ADE) component consists of image

enhancement tools to augment human inspection.

These tools enhance the acquired X-ray image so that defects in different

Thickness sections are displayed in a single image. This drastically reduces, and

in most cases eliminates,

The need for setting up and cycling through separate image views for inspecting

each part thickness Region. The benefits are a significantly reduced inspection

time and improved quality of inspection.

Automated Defect Recognition (ADR)

With the Automated Defect Recognition component, inspection of the X-ray image is

fully automated and done by a computer. Depending on who makes the final inspection

decision to accept or reject the part, three categories of ADR can be distinguished:

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(i) human operator makes decision based on output of ADR; or,

(ii) decision is a logical combination of independently made human and ADR decisions;

or,

(iii) inspection decision is done only by ADR.

In general, the ADR process can be split into three phases: defect detection, defect

classification, and defect evaluation. In defect detection, the goal is to identify the

physical extent and location of the defect in the X-ray image. This step is usually the

most difficult to perform reliably from an image processing standpoint, since defects

can occur in any region without any restriction on size (area and depth) or shape.

Additional factors affecting this step are part movement, detector noise, and variations

in X-ray source and detector calibration. After defect detection, the next step is defect

classification: in this the defect is classified into, for example, shrink cavity, shrink

sponge, gas holes, gas porosity, or foreign material. After defect classification, the

defect is evaluated and graded using predefined standards (for instance, quantitative

equivalent of ASTM E155) and the inspection decision made on the basis of the

evaluation.

IV. Storage, Analysis and Feedback

Inspection Records Database

In this automation component, inspection records for each part are stored in a

centralized, plant-wide database. An inspection record consists of defect-related data

such as defect location, size and type along with the production data for the part such

as mold number, cavity number and casting station. Optionally, a compressed ADE

image of the part can also be stored as part of the inspection record. The stored data

can then be utilized to improve the overall casting process, utilizing appropriate

analysis software to analyze the stored defect data. Furthermore, the availability of the

stored image makes it possible to monitor and improve the quality of the inspection

process itself, since an independent review of the stored image can be made a nd

compared with the ADR or operator’s decision.

Real-Time Feedback

The real-time feedback automation component pipes information back to the casting

station in real-time for process control. An ADE image of every rejected part is

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displayed on a monitor at the appropriate casting station with the defect location and

area highlighted, and defect-related data overlaid on the image. This enables casting

station personnel to control the process quality and reduce the scrap rate.

V. Automation Architectures

Automation components can be combined into automation architectures that are easily

extensible depending on requirements. One can start with a base architecture and

then progress up the automation ladder by adding components. Two sample

automation architectures are shown below in Figure 1 and 2

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- 24 -

Demonstrate knowledge of common die casting defects

Level 2

Credits 5

Purpose People credited with this unit standard are able to: identify and describe

common die casting defects; check castings for defects during manufacture; and scrap

defective castings.

Subfield Mechanical Engineering

Domain Metal Casting

Status Registered

Status date 19 May 2006

Date version published 19 May 2006

Planned review date 31 December 2011

Entry information Open.

Accreditation Evaluation of documentation by NZQA.

Standard setting body (SSB) Competenz

Accreditation and Moderation Action Plan (AMAP) reference 0013

This AMAP can be accessed at http://www.nzqa.govt.nz/framework/search/index.do.

Special notes

1 This unit standard is for operators of a die casting manufacturing process. Operators

need to be able to recognise common defects to ensure product quality at all stages of

the manufacturing process. Product defects caused by the die casting process may

become apparent at different stages of manufacture as a result of other processes, eg

chemical pre-treatment, or machining. At this level operators are not required to identify

the specific cause of the defect or know how to rectify faults in machinery or procedure.

2 Defects in die-casting depend on the procedure. Defects common to all procedures

may include the following: surface defects; laminations; cold skin; explosions; flashing;

bubbles; cracks; solder or carbon build up; pin push; drags; porosity; fill; and

stained, bent or warped castings.

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3 The stages of a typical process for the manufacture of die-cast products may include;

die casting, heat treatment, non destructive testing (x-ray/dye penetrant), machining,

shot blasting, linishing, vibro, polishing, pre-treatment, powder coating, anodising or

electroplating, and assembly.

4 The material used in die casting is non-ferrous metal.

5 Worksite procedures refer to documents that include: worksite rules, codes of

practice,

Equipment operating instructions, maintenance schedules, quality management

systems, health and safety procedures, and emergency procedures.

6 Legislation and guidelines relevant to this unit standard include:

Health and Safety in Employment Act 1992;

Resource Management Act 1991;

Hazardous Substances and New Organisms Act 1996;

Health and Safety Guidelines on the Management of Hazards in the Metal Casting

Industry. New Zealand: Casting Technology NZ Inc and Occupational Safety and

Health (OSH), 1997.

Note: the above editions were current at the time of registration of this unit standard.

It is recommended to use the latest editions if different from above editions.

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Elements and performance criteria

Element 1

Identify and describe common die casting defects.

Performance criteria

1.1 Common defects which may occur in the die casting process are identified.

Range at least five defects.

1.2 Defects are described using accepted industry terms that are in accordance

with worksite procedures.

1.3 Indicators of acceptable product quality range are described for selected die

cast products in accordance with worksite procedures.

Range for the identified five defects;

indicators may include but are not limited to – colour, shape,

surface; and type, size, and location on product of any defects.

New Zealand Qualifications Authority 2006

Element 2

Check castings for defects during manufacture.

Performance criteria

2.1 Inspection processes are performed in accordance with worksite procedures.

Range may include but is not limited to – visual check of first-off castings,

periodic sample checks, checks against sample boards, 100%

checks.

2.2 Any defects are identified and reported to machine operator and/or supervisor in

accordance with worksite procedures.

Range type and quantity of defects.

Element 3

Scrap defective castings.

Performance criteria

3.1 Defective castings are scrapped in accordance with worksite procedures.

3.2 Documentation and/or electronic data input for reporting scrap is completed in

accordance with worksite procedures.

Please note

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Providers must be accredited by the Qualifications Authority, or an inter-institutional

body with delegated authority for quality assurance, before they can report credits from

assessment against unit standards or deliver courses of study leading to that

assessment.Industry Training Organisations must be accredited by the Qualifications

Authority before they can register credits from assessment against unit

standards.Accredited providers and Industry Training Organisations assessing against

unit standards must engage with the moderation system that applies to those

standards.

Accreditation requirements and an outline of the moderation system that applies to this

standard are outlined in the Accreditation and Moderation Action Plan (AMAP). The

AMAP also includes useful information about special requirements for organisations

wishing to develop education and training programmes, such as minimum qualifications

for tutors and assessors, and special resource requirements.

Comments on this unit standard

Please contact the Competenz qualifications@competenz.org.nz if you wish to suggest

changes to the content of this unit standard.

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System Approach to Casting Defect Analyses and Reduction:

Hydrogen Gas Defect in Iron castings

Casting defects can negatively impact the bottom line of a foundry. At the simplest

level, they manifest as rework costs or casting scrap costs. However, in many cases,

the casting defects may be discovered at the machining stage, at the assembly stage

or during use of the component. The resultant value added costs and warranty costs

may sometimes be passed on to the foundry by their customer. These charges may be

significantly more than the cost of the casting itself. Foundry personnel may not have

the time to conduct a detailed casting defect analyses, determine root causes and

implement effective corrective actions to prevent re-occurrence of these defects. The

purpose of this paper is to outline a systematic casting defect approach, which when

combined with various teams and headed by an appropriately trained Quality Engineer

can produce excellent results in reduction of casting defects. By applying these

principles, a foundry saved over $100,000 per month in casting scrap related to

hydrogen defects.

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PRACTICAL APPROACH TO DEFECT DIAGNOSIS AND CONTROL

THE TEAM APPROACH

The foundry observed increased blowholes on the top of a cylinder head casting. There

were three main challenges in eliminating or minimizing casting defects.

I. Casting defect identification

II. Cause /Variable Identification

III. Corrective action implementation

When the casting defect occurs, the responsibility of the corrective action is handed

over to a Quality or a Production Engineer. It is must be realized that a quality or a

production engineer does not have all the necessary information and knowledge to

carry out these steps. Constituting a team is very important to this process. When the

casting defect is discovered at the customer end, it is important to involve the

customer’s technical liaison or sales personnel. Multidisciplinary teams typically should

include a metallurgist, key production personnel and the engineering/tooling

department personnel. It is understood that casting defects may have many root

causes. It may be necessary to have separate teams to solve a specific problem – a

Melt team, a Core team and /or a Molding team. It is extremely important team/teams

all agree on what the root causes may be. The focus should be on finding consensus

and a solution rather than blame one individual or a group for the problem.

The team should decide set measurable goals for scrap reduction. The team decides

on the manner that the casting defect is tracked and measured. That could include

casting identification by date or hour of casting, location of the defect, nature of the

defect and determining the extent of the defect causing a scrap or rework. – tracking

defects after machining or at assembly level. Upfront planning by the project manager

(Quality engineer) is essential. He or she provides the focus for the team. The project

should be tracked; all key processes should be mapped. It may be necessary to clearly

mark various inputs and outputs at each stage – from the supplier to the end customer.

Short (30 minutes or less) multi-departmental meetings to update the status of various

action items are very helpful. These meetings are for communication only and can

often be substituted by an email to all team members. The actual work needs to be

done by the project manager and production personnel who may meet as often as

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necessary. Depending on the nature of the scrap, it may be advisable to involve

technical persons from key foundry suppliers.

BACKGROUND ON HYDROGEN BLOWHOLES

Hydrogen – pin holing and blowholes are created by pick-up of hydrogen from moisture

in the molding sand (Greenhill, 1971, Wallace 1989). Aluminum contamination (either

introduced through melting scrap or ferro-alloy addition) can exacerbate the problem.

Ladles and other pouring equipment should be dried well. Often hydrogen blowholes

can be observed in the first few castings poured from a ladle that has not been dried

properly. Moisture content of the mold should be kept to the minimum and cores should

be dried completely (Galante et al. 2001). Long runners systems should be avoided.

Pinholes in casting locations remote from the sprue can sometimes be caused due to

moisture pick-up by molten iron from long runner systems. Some hydrogen defects can

also appear as fissures.

DEFECT IDENTIFICATION

Most foundries have good scrap tracking systems. Detailed analyses of the casting

defect, its location and frequency of occurrence needs to be performed (Greenhill,

1971). Defects samples should be sectioned, mounted and polished. Cataloging

defects will help foundries by reducing the time needed to identify future defects of the

same nature. In many cases, examining the casting defect under a metallurgical optical

microscope is sufficient to determine the nature of the casting defects. The casting

defects, in this case study, were sent to the metallurgical lab of a supplier-partner

company and to a University lab. Scanning Electron Microscope /EDAX analyses of

casting defects also can indicate the nature of the defect and possible sources of the

defects. Figure 1 shows an SEM photo of a typical hydrogen defect observed in

cylinder head castings. Variables that they think cause the problem (semi-quantitative

methods are available to sort out what the group feels are the rankings of these

variables. While this may be democracy at work, it must be realized that these

variables are just “based on past knowledge and experience. The strength of the

technique is that the collective knowledge of the group is used but the analyses must

not end at this stage. It is still unsupported by actual evidence at this “brainstorming”

stage.

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DATA GATHERING AND ACCURACY

While many foundries spend much time collecting data, little care may be taken to

verify its accuracy. Care must be taken to that the data is repeatable, accurate and

precise. It may be necessary to create new data gathering procedures to understand

the problem. In this case, the foundry had to find track moisture content in the cores.

This was achieved by measuring the weight of cores before and after drying to estimate

the volume of moisture that needed to be removed by the drying procedure.

Another issue is data access – how user-friendly is the format in which the data is

presented. In most cases, it is possible to export the data into a spreadsheet program.

Utilizing the collected data is the most important step. While most foundries keep track

of a variety of process variables such as air temperature, humidity and melt chemistry,

very little time is spent in using these data. Regression analyses carried out with the

casting defect as the dependent variable can help determine root causes of a problem.

In this case study, significant casting defects were observed in the third shift. Careful

analyses of the foundry data showed a strong correlation between the incidence of

hydrogen defect and the humidity of the foundry

Casting Defects in Low-Pressure

Die-Cast Aluminum Alloy Wheels

serve as one component of a pressurevessel in conjunction with the tire; highquality

surface finish, as wheels are one of the prominent cosmetic features on cars; and

geometric and rotational balance tolerances, which are becoming more stringent. In the

context of these requirements, the term defect constitutes any feature arising from the

manufacturing process that necessitates repair or rejection of the wheel. Many

castingrelated defects continue to challenge metallurgists and manufacturing

engineers, particularly since market pressures demand improvements in product quality

and expansion of the design envelope while at the same time reducing costs. The

formation and prediction of these defects, along with methods to remove them, are the

focus of this paper.

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WHEEL CASTING

PROCESSES

Owing to its ability to produce high quality wheels in a cost-effective manner, the

dominant process for casting aluminum alloy wheels is the low-pressure die-casting

process (LPDC). A typical LPDC casting machine comprises a die assembly containing

two die cavities located above an electrically heated holding furnace. The dies are

typically made from a combination of tool steel and cast iron. Each die cavity is

geometrically complex, with four sections as shown in Figure 1: a bottom die, two side

dies, and a top die. The casting process is cyclic and begins with the pressurization of

the furnace, which contains a reservoir of molten aluminum. The excess pressure in the

holding furnace forces liquid aluminum up into the die cavity, where it is cooled and

solidified by the transfer of heat from the aluminum to the die and then out to the

environment. After solidification is complete, the side dies open and the top die is

raised vertically. The wheel remains fixed to the top die (owing to thermal contraction)

for a short time and is then ejected onto a transfer tray rolled under the top die. The die

is then closed and the cycle restarted. Typical cycle times are 5 min. to 6 min.

Following the casting operation, the wheels are typically rough machined, heat treated,

finished,machined, and painted. During solidification, control of cooling rates is

important for product quality. In the bottom die, cooling is augmented by forcing air

through internal channels at various times during the casting cycle. In the top die,

cooling is controlled by air jets aimed at various sections of the exterior face. On the

side dies, cooling may be retarded by the addition of insulation to the cold face at

certain locations or augmented by air cooling, depending on the casting conditions. See

the sidebar for experimental procedures.

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RESULTS AND DISCUSSIONS

In any manufacturing process, defects are dependent on the tolerances that exist within

that process. In essence, a feature of the manufacturing process becomes a defect

when its presence results in the product failing to meet the prevailing

Figure 1. A die-tooling assembly for typical

LPDC process.