Mohawk College - Introduction to Metal Casting
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MATL MTB70 Module 7 - Course Notes...
This
the last in the series of notes from the "Introduction to Metal
Casting" course. In closing I'm satisfied that I've learned enough from
this course that it was worth the Effort, Money and time to complete. It is as
the name implies an Introduction.
I
found the global perspective of the Casting Industry to be interesting with a
broader view than I'd considered before. Although some of the processes and
equipment would never scale to the Small foundry operation such I'm trying to
develope, enough can be adopted beyond what I'd considered.
Cleaning and
Inspection
This is the last
stage of the casting process before the finished unit is ready for the Finishing
process (Not covered in this Module).
Cleaning
This stage
actually starts with the shake-out process. Beyond simply dumping the mold and
pulling the casting, there are vibrating tables, rolling drums this active
impacting or hammers to break away the molding material.
Once the casting
is free from the mold, the surface can be cleaned with wire brushes manually or
mechanically (for softer alloys) or a series of blasting techniques available
for harder alloys. These processes also include removing any residual Core
material.
Blast
Cleaning Types - Air, Water & Mechanical are used to clean any remaining
sand or scale from the surface.
Air Blast
cleaning is an efficient method that can utilize a broad range of Media that are
selected by the degree of abrasion required to clean the casting and the
softness of the alloy.
Media include but
are not limited to the following;
-
Angular Sand
or Grit (beware Silicosis!!!)
-
Shot -
typically a round media including a variety of ceramics and metals
-
Soft Media -
such as walnut shell, corn husk, etc...
All methods of air
blast should be conducted within proper enclosures to eliminate (not reduce)
operator health & safety hazards.
Water Blast cleaning
eliminates many of the respiratory hazards associated with the Air Blast
methods, but introduces the requirement of adequate cleaning of the effluent or
discharge water after use before releasing to a "Grey Water"
destination.
Mechanical
Blasting poses similar respiratory hazards as air blasting but the delivery
mechanism of the media is not through a blast of air. The media is discharged by
a paddle-wheel of sorts with adequate velocity that it impacts the casting with
an abrasive action.
Cutting
& Fettling the Casting
The casting is
inspected visually every step by every person that touches the unit. I believe
(perhaps naively) that ever employ on the payroll has the right and the
responsibility to scrap a piece at any stage of processing if they know the end
product will not meet the highest standard.
The casting is
removed from the feeder network, there are a number of methods ranging from a
manual hack saw through to a plasma cutter. The manual method is a tremendous
amount of work and has a tangible expense associated with it, in terms of
quality blades (that can only last up to 10 Castings with 3 to 6 gates in my
experience).
Currently I use a
Sawz-All reciprocating saw and have shaved the per casting cutting time by
2/3rds to 15 min typically, I believe that this time could be further reduced by
2/3rds again to 5 min with a Plasma Cutter with the appropriate nozzle
selection... Any Company that would like to send such a
unit would never regret the gesture as I'd sing your praise from the highest
hills.
Depending on the
alloy there are numerous power tools that can be used including Band Saws,
Circular Saws (of various orientations including Hand, Table and Chop) ensuring
that the appropriate blade is matched to the alloy... Note do-not cut
non-ferrous alloys with abrasive disks (just don't...) as well as shears and
cutting torches.
The Fettling
process is the removal of fins or minor deformations with some mix of manual and
machinery assisted labour... I use a hammer and a 4 1/2" angle grinder with
a Zircon Flap-disk.
Castings that are
obvious defects should not even be cut away unless necessary for re-melt, though
cleaning is required regardless. The lecture notes specify the option of repairs
to castings at this point as well, though I personally have never repaired a
casting, either it is kept or re-melted.
Destructive
& Non-Destructive Inspection
Destructive
inspection, though not a component of this module is worth talking about briefly.
These would include cutting castings through cross-sectional areas in pursuit of
Sub-surface porosity defects, measuring tensile and shearing strength of
castings as well as impacting various surface areas to determine hardness.
Obviously these
tests are carried out on sample(s) that would be representative of a casting
run, and typically where the casting is being used in an application that
requires very strict quality control such as vehicular applications... Where I
would not cast 6 plaques and destroy one randomly for fear that it requires
1250lbs of force to break it and it broke at 1247lbs...
Non-Destructive
Inspection is a process that can be performed on a casting numerous
times and leaves the casting in such a state that it can proceed to the
finishing stage if found to be within specifications. This is becoming more
common place as technological advances are made, the methods that are discussed
are;
Visual
Inspection - Obviously is Surface Quality comparison to a reference or
based on the experience of the person performing the inspection. This method is
has a labour cost associated with it, though is comparatively low. The speed and
easy of inspection lacks any record keeping and may be subjective or
inconsistent if performed by personnel of varying experience. Machine Vision is
a promising technology that incorporates high resolution cameras that feed image
data to a comparative processor that can actual gates or arms to discharge
castings into a scrap pile or pass them down a conveyor for further processing.
For tons of detail on this exciting technology (and man is it ever EXCITING!!!) http://vsd.pennnet.com/home.cfm
Liquid (Dye)
Penetration Inspection (LPI) is a process that is able to detect narrow
seams and cracks at or just below the surface of a casting by applying a liquid
dye or phosphorescent dye, the penetrant is wiped away and the casting is
inspected for residue that has properties that make for easy inspection or
detection.
The chemical
composition of the effective dye(s) requires specific handling procedures that
will be available off their respective MSDS. Personally I think that this
process also would lend itself well to a Vision Systems solution, as the range
or spectrum of detection of a sensor can be tailored to significantly extend
beyond the range of the human eye.
Also of
consideration is the ability to records results for archival purposes and defect
analysis. This is definitely not for smaller foundries, but a shop that makes
under 1000 Aero-space grade parts that require "Mission-Critical"
tolerances and verifiable inspection documentation, I'd think about implementing
some sort of PACS or Picture Archive Communications System.
Magnetic Particle
Inspection (MPI) - This process is limited to Ferrous castings and
requires less overhead than the name implies, the fundamental process is the
application of a magnetic field to the casting, coating the surface with a
magnetic particle compound and examining the retained particles for markers that
would denote fractures or inclusions in the casting by the distribution of the
adhering particle matter.
MPI offers
advantages over LPI, in that it can be performed faster, requires fewer material
specific handling considerations, offers less clean-up, and can detect defects
up to 0.25" below the casting surface.
The noted
drawbacks are the limitation of testing to Ferro-Magnetic castings, potential
for residual magnetism in the casting, a higher intellectual overhead in terms
of training and competency of staff as well as a measure of casting cleaning
(though still less than LPI).
Ultrasonic
Inspection - This inspection system works on the same principles of
Medical Ultra Sonography... An energy or acoustic wave is propagated through a
medium via a transducer, the wave is reflected and the reflection is examined on
a display.
In this case the
medium the wave propagates through would be the casting. This inspection system
offers superior sub-surface analysis than other processes listed so far. The
reflected wave can indicate a properly formed casting or identify internal
fractures or structures that could not be otherwise found.
This process
offers very rapid analysis of casting integrity, as well as lends it's self to
automated and archival processing. The cost and intellectual overhead of
implementing such a system is significant, and this process does not work well
with coarse grained castings due to reflection by the grain structure. USI does
require reasonably flat and or smooth surfaces for the transducer to propagate
and receive the reflected waves.
Eddy Current
Inspection (ECI) - This is an emerging technology that is similar to
both US and MPI. An EMF (Electro Motive Force) is induced in the casting
(Ferrous or Non-Ferrous) and the induced EMF is read by an inductor that is
sensitive enough to supply detailed information that is analyzed and displayed.
ECI can
diagnose and identify grain structure and offer a level of elemental composition
that USI can-not at this time. My guess is that this won't be cheap...
Radiography
- Much Like USI is based on the medical practice of Diagnostic Imaging via
X-Ray and the imaging of the x-rays that passed through the medium on to film.
The X or Gamma rays are absorbed or modified by the thickness, density and/or
atomic weight of the cross section being examined.
This area has
undergone Radical change over the last 10 years, Film is being replaced with
Crystal cartridges that can be digitized and erased for re-use, any radiology
system being install today MUST BE digital... The savings are always touted to
be on the cost of the film, chemicals and processing... True there are savings,
but the labour associated with processing, handling and filing (for archival
purposes) far out weighs the other savings.
Similarly digital
radiology techniques have developed to the point that through digital filtering
of image data information is made available that could never have been detected
with traditional film techniques.
If you have to ask
what the cost is, you can't afford it, though systems are spec'd based on qty of
images processed and the number of image acquisition points (X-Ray station) and
the number of view stations for radiographic interpretation. Although the
temptation exists to develop an archival data structure in house to save on
costs, an open architecture approach should be adopted to facilitate data
sharing between the foundry and customers... If you are creating castings that
have a requirement for Radiographic Inspection, you'll likely be called upon to
produce sample images to the customer's QC dept.
These notes are
based on AFS Text Chapter 18, lecture audio, text and 5 video segments.
...this
is the end of Course notes for Module 7 and MATL MTB70, thank you for taking the
time to visit theworkshop.ca, watch for other courses that are part of this
same program.