Reusing Zinc Alloy Die Cast Scrap
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.
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.
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.
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.
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.
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.
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.