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Silica sand is the most widely used molding
aggregate by the foundry industry. A common mistake by foundrymen,
and even researchers, is the concept that all silica sands possess the
same physical properties and will behave the same during the casting process.
Unfortunately, silica sands were not created equal. Because of slight
chemical alteration of silica sand, the performance and casting reliability
is tremendously affected from different sources. Part one of this
three-part series discusses the differences in silica sand and how they
can influence core production and defect prevention.
Two basic types of silica sand are commercially
available for foundries. The first type is the round grain silica
sand containing roughly 99% or higher of silica with minimal amounts of
trace materials. The second type is the lake and bank sands.
These sands contain approximately 94% silica with the balance containing
iron oxide, lime, magnesia, and alumina. Since impurities are removed,
round grain sands possess higher refractoriness than the lake sands.
However, because the higher refractoriness of the round grain sands, this
can result in a higher propensity for veining and metal penetration defects.
Lake and bank sands have lower refractoriness but also have a lower tendency
for casting defects.
Even within these groups of classifications,
slight variations depending on source location imparts slight measurable
difference in sand performance. It is difficult to say which sand
is best, round grain or lake sand. Sand selection is determined by
casting application, availability, and cost. The focus of this article
is to disseminate the differences between the variety of silica sand and
learn how impurities influence these properties. Though a foundry
might not consider a lake sand, understanding how the impurities affect
casting quality will assist foundries in selecting core sand additives
for round grain sands to reduce the risk of veining and metal penetration
defects.
Defects associated with expansion are veining
and, to a minor degree, penetration. Generally, round grain sand
users will combat penetration and veining problems by adding additives
in the core sand mix. Typical additions include iron oxide, Veinseal™,
and Macor™. However, these additions mimic the properties of lake
sand. The effect of natural impurities in silica sand is illustrated
below. Both the round grain and bank sand expands at the same rate
up to 1100oF, the point where quartz rapidly begins to change
crystalline shape. Beyond this point, noticeable expansion characteristics
can be observed. At approximately 1200oF, the linear expansion
levels off. The bank sand has an expansion of 1.3% where the silica
sand has an expansion of 1.6%, an appreciable difference. Also, notice
the difference in the expansion curve for the lake sand. Expansion
for the lake sand appears to occur earlier but at a lower total expansion
than the bank sand. These differences between the sand can be attributed
to the impurities in the lake and bank sand. Research work at the
Metal Casting Center exploring sand additives and blends have observed
this similar behavior when “different” materials are added to round grain
silica sand.
Casting trials have shown that the bank sand
has a lower propensity for veining defects compared to the round grain
silica sand. Comparing the expansion characteristics between the
two sands and the effect of veining, the rate of expansion and the total
amount of expansion have an influence on controlling veining defects.
The Metal Casting Center, along with sand suppliers, are developing a research
program to obtain a better fundamental understanding of the expansion characteristic
of sand. Key points, similar to that observed in cooling curve analysis,
can indicate the performance of the sand. This can lead to the exploration
of low cost sand additives to alter the expansion curve to prevent veining
defects. |
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This article was co-authored by Scott Giese and Jerry Thiel |
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Future articles will discuss the effect of commercial sand additives
and sand blends on the expansion characteristics. |
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Aluminum foundrymen are well aware
of hydrogen porosity problems that arise during the hot, humid weather
of summertime. Are you aware of the effect of slight moisture variations
with phenolic urethane cold box (PUCB) systems? These factors can
reduce casting quality and dimensional accuracy without you knowing it.
Some casting quality issues might be attributable to humidity in your foundry.
Iowa, as well as many other
northern states, enjoy cold but dry winters with relative humidity (R.H.)
as low as 15%. However, during the summer months the R.H. can reach
more than 70%. Even at equal humidity levels, more moisture is present
in the air during warm days than cold ones. This can be a serious
problem for users of PUCB cores. Research at the Metal Casting Center
has shown that PUCB cores produced at 70% R.H. have tensile strength properties
23% lower than cores produced in dry conditions even after one hour as
shown below.
How does this affect your operations? Foundries
using tensile strength for quality control could inadvertently collect
misleading data. Sand testing laboratories in environmentally controlled
rooms produce conditions that are not typical in the core room, skewing
the true core properties at the point of core making. Conversely,
testing laboratories in poorly controlled areas replicate core room conditions
but hide potential moisture (i.e. sand, binder, etc.) problems. This
can be a dilemma for the core room supervisor if corrective action needs
to be taken to rectify production problems.
Solutions to this dilemma start with understanding
how humidity can affect supporting core making systems and what the possible
solutions are. Considerations and recommendations include:
- Compressed air dryers work overtime during the summer months. Make
sure that the equipment is in good operating condition and that preventive
maintenance is done on a regular basis.
- Drain air tanks and receivers daily. One foundry complaining of low
air storage capacity reported draining over 50 gallons of water from their
accumulator tank. Look into automatic drains to lessen manpower.
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Design cores robustly to prevent core breakage due to minor variations
in core strength.
- Bulk storage or barrels used to store chemicals require vents to allow
the chemical to be pumped to the mixer. Use vents with built-in dessicators
or chemical dryers to prevent moisture contamination of the part II binder.
- Use cores as soon as possible after production. This not only prevents
moisture pickup, but lowers work in process inventory thereby saving dollars.
If cores need to be stored for an extended period of time, dry cores by
placing them in a 350oF oven for 15 minutes and allow the cores
to cool to room temperature before using.
Foundry suppliers provide the highest quality
materials to foundries. The goal for all foundries is to produce
the highest quality castings at the lowest cost. Tracking casting
defects with weather conditions could pinpoint processing conditions that
might have been overlooked during analysis. Adjusting binder levels
to obtain desirable core properties might not always be the correct action. |
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This article was co-authored by Scott Giese and Jerry Thiel |
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An improved direct pour gating system
with floating ceramic foam filter has been developed for aluminum and ductile
iron casting. This project investigated the design parameters
of the system, filter material requirements, filter flow rate, filter plugging
behavior and the system effectiveness to remove the inclusions. Trapped
nonmetallic inclusions have been identified by the Scanning Electron Microscope
(SEM) analysis.
Uses of new filtration systems will improve
the mechanical properties and quality of castings because it removes
non-metallic inclusions thus producing clean, inclusion-free ferrous and
non-ferrous castings. As a result of improved surface quality, machining
allowances can be reduced and, as machinability is improved, tool life
will be increased. This gating system could be incorporated into
a wide range of casting processes including conventional green sand molds,
no-bake molds, and permanent molds and can be used without an insert sleeve. |
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Article written by Dr. Yury Lerner |
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This project objective
is to develop practical data concerning usage of ultrasonic velocity and
sonic testing for quality control of cast irons over a wide range of graphite
morphologies and metallic matrices obtained either in as-cast condition
or after heat treatment. It was found that the ultrasonic velocity
values in ductile irons with the same level of nodularity decrease with
increasing the ferrite content. Minimum mechanical properties, required
by standard specification for ductile iron ASTM A-536, associated with
commercially acceptable level of nodularity greater than 85%, have been
achieved, when the ultrasonic velocity exceeded 0.2220 in/microsecond in
as-cast ductile iron containing 60 – 70% ferrite, and 0.2250 in/microsecond,
when the amount of ferrite decreases to 10—15%.
Dynamic elastic modulus
(DEM) depends on ferrite/pearlite ratio in the metallic matrix, and as
a result, DEM has different values for the different grades of ductile
iron with the same nodularity. With increasing ferrite content and
decreasing tensile strength, the dynamic elastic modulus decreases.
For the first time in permanent mold gray
iron practice ultrasonic velocity (USV) testing has been successfully applied
for mircrostructure/property evaluation and certification of industrially
produced castings. |
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Article Written by Dr. Yury Lerner |
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