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Reusing Zinc Alloy Die Cast Scrap
Process Background
SCS Co-Sines
manufactures zinc alloy die cast signs and embossed stainless
steel and brass identification plates. Zinc alloy castings are
produced in a hot chamber die-casting process in which molten
metal is injected into a die chamber, partially cooled in the
chamber, and released into a cooling bath and quenched (cooled)
to room temperature. The resulting castings are machined, polished,
cleaned, and electroplated and/or painted.
The
die casting operation at SCS runs at an efficiency rate of
about 65 percent with one out of every three castings rejected.
This efficiency rate is affected by the wide variety of products
made, which requires tooling changes up to 25 times daily.
Tooling changes cause temperature fluctuations that can affect
product quality and increase the number of rejected parts.
Although
the efficiency rate is 65 percent, 88 percent of the virgin
alloy used at SCS becomes scrap. During the casting process,
each pound of casting produces approximately five to ten pounds
of scrap waste in the form of flashings and sprue waste. For
every 1,000 pounds of new ingot material added, 120 pounds
becomes acceptable parts, 20 pounds becomes dross waste (a
slag-like mixture of zinc oxide and impurities), and 860 pounds
becomes scrap waste.
Incentive
for Change
SCS wanted to reduce its scrap metal waste because of
cost; each pound of scrap resulted in a net loss of 23 cents.
The primary areas to change were to: improve the efficiency
of the zinc die-casting operation in order to reduce the amount
of unacceptable parts; and reuse the waste material (sprue
waste) produced during normal operations. SCS speculated that
understanding the factors that produced acceptable castings
would help in finding ways to reduce both scrap generation
and production costs in the die cast operation.
Intern
Activities
The MnTAP intern identified the factors that affect the
efficiency and the quality of die casting; determined the
differences between optimal die casting procedures and those
typical
at SCS Co-Sines; and made suggestions for improving operating
practices. The intern also researched the potential for reusing
metal scrap. Scrap zinc alloy was remelted and processed in
normal die casting operations, and the alloy was analyzed
to determine the depletion of the required components and
the accumulation of impurities. Based on this information,
the intern developed a method for reusing the scrap metal,
which involved a combination of improving the die-casting
process, designing equipment modifications and developing
a reuse process and schedule.
Results
After evaluating the die casting operation at SCS and
researching into die casting and zinc alloys, the intern developed
suggestions for improving the process control to reduce scrap.
In addition, a study conducted by the intern showed that up
to 50 percent of the scrap metal alloy could be put back into
SCS' production. (Note: these suggestions were not implemented
by SCS during the intern project; however, since that time,
SCS has implemented some of the suggestions outlined below.)
Improving
Die-casting Process Control
The intern evaluated the feasibility of controlling two
areas of the die-casting production process: 1) reducing temperature
fluctuations to maintain an optimal operating temperature;
and 2) improving equipment maintenance and operation.
Temperature.
Understanding the influence of temperature on die casting
was found to be an important element in reducing scrap waste.
A stable operating temperature, with little or no fluctuation,
is needed to continuously produce acceptable parts. By preheating
the tools and using a die heater, fewer "shots"
(when molten metal is forced into the die) would be needed
for the die-casting process to reach its optimal operating
temperature. This, in turn, would reduce the amount of scrap
generated. In addition, adding metal to the furnace pot in
the proper amounts and times also would reduce pot temperature
fluctuations and temperature-induced casting defects.
Maintenance.
Properly maintained die-casting machinery has less down time,
which allows the casting process to sustain its optimal operating
temperature for a longer time than when equipment is shut
down for repairs and/or adjustments. Disrupting the system
results in producing unacceptable parts that must be discarded
due to defects such as porosity, bent studs, cold shot and
other part deformities. Understanding the equipment maintenance
factors that influence part defects can help operators adjust
machine settings and maintenance schedules to reduce defects,
equipment downtime and scrap volume.
Reusing
Metal Scrap
Reusing zinc alloy scrap eventually depletes it of aluminum
and magnesium through the formation and removal of dross,
and the normal process burn-off. The intern consulted professional
and trade journals, smelting equipment manufacturers and a
zinc die-casting expert to identify component depletion and
composition limitations in the alloy. At the intern’s
request, a composition analysis of the virgin and scrap material
was also performed by a supplier. This information was used
to: 1) develop a virgin-to-waste metal input ratio, 2) determine
the scrap reuse limits, and 3) develop an operating schedule
for incorporating the scrap.
Scrap
Reuse Schedule. A schedule was developed in which a 30:70
mixture of virgin material to scrap metal was used to cast
production parts for five days. This was followed by recharging
the metal by adding only virgin alloy to the chamber's melting
pot for three days. Based on promising trial results of parts
made from the 30:70 mixture, the intern identified and developed
equipment modification designs to facilitate the scrap metal
reuse process.
Scrap
Reuse Equipment. The reuse equipment developed by the
intern and SCS would use three mechanical components: a drying
bin, a premelt furnace and a loading ramp. (Note: this equipment
was not installed by SCS during the project period). The drying
bin would dry the scrap and keep it dry during storage (storage
capacity would be between 500-800 pounds scrap). A premelt
furnace would be used for quality control by allowing the
dross formed by remelting scrap to be removed before the melted
scrap is added to the main furnace. This furnace also would
burn-off impurities or oils, and increase the temperature
of the metal added to the main furnace from the premelt furnace
to above 750°F, causing little or no disruption to the operation
of the existing die-casting furnace.
If wet
scrap or water from the cooling bath is introduced to the
molten bath, splattering can occur, which could cause serious
injury to people near the bath. The loading ramp, the third
component of the recycling process, was designed as a safety
factor to protect the operator from exposure to any accidental
splattering of molten metal when adding material to the premelt
furnace.
Problems
with Reusing Scrap
In addition to safety factors, another problem with reusing
scrap is an increase in smoke produced when remelting scrap
material. The increased smoke could be caused by the cooling
bath components, which could be removed from the scrap before
remelting. If the smoke is not reduced, SCS will need to test
air emissions quality and perhaps upgrade its ventilation
system to compensate for the increased smoke.
Summary
Monitoring
Product Quality
Once SCS has a scrap reuse
process installed, it will need to perform metal alloy component
tests in order to develop a production schedule that ensures
product quality. The tests should consist of tracking the
reused scrap (70 percent of the input metal) for several days
and taking samples from the casting pot for composition analysis.
Then, charge the die casting pot only with virgin material
and use the same tracking and testing procedure to determine
its composition.
With
the resulting information, SCS can plot the test results of
the scrap/virgin mixture added to the pot against the results
from adding only virgin material. This data should reveal
the amount and type of each component depleted in the mixture
so that it can be replenished. With this data, SCS can develop
an efficient and safe production schedule that incorporates
reused scrap. To increase the efficiency of this schedule,
SCS should use only the scrap with higher levels of aluminum
and magnesium.
Capital
Equipment Costs
SCS will need to invest in new equipment and make modifications
to existing equipment in order to reuse its scrap metal. A
premelt furnace will be required at a cost that varies from
$3,000 used to $17,000 new. Other equipment needed (not including
ventilation upgrades) will cost less than $2,000 for materials
and labor for construction and installation, with a payback
period of approximately six months.
Cost
Savings
Based on the intern's research, approximately 50 percent
(79,200 pounds) of the total 158,400 pounds of scrap can be
reused by SCS, saving approximately $18,200 annually in reduced
raw material purchase costs.
For
More Information
MnTAP has a variety of technical assistance services available
to help Minnesota companies manage and reduce their industrial
waste. If you would like assistance or more information about
MnTAP's Intern Program, call 612.624.1300 or 800.247.0015
in greater Minnesota.
This project was conducted in
1993 by MnTAP intern Todd Loushine.
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