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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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 [email protected] 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.
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