A Case for Low Pressure Sand Casting of Aluminum
Less common than gravity sand casting and low pressure permanent mold casting, low pressure sand molding holds a distinct combination of advantages for large aluminum castings.
Franco Chiesa, Centre de Métallurgie du Québec, Trois-Rivières, Québec, Canada, and Jocelyn Baril, Technology Magnesium & Aluminium, Trois-Rivières, Québec, Canada
(Click here to see the story as it appears in the February issue of Modern Casting.)
A majority of aluminum castings are produced via sand or permanent mold casting, but for large precision components, another viable option for metalcasters to consider is low pressure sand casting, which uses principles from both low pressure permanent mold (LPPM) and gravity pour sand casting.
Low pressure sand casting marries the use of bottom pouring for tranquil filling of the mold (which avoids metal oxidation) with the flexibility to make larger parts. The capable process can be ideal when producing large “top quality” aluminum castings. The process also can be considered when walls are too thin (such as 0.1 in. [2.5 mm]) to be obtained by gravity casting.
LPPM produces high quality castings due to tranquil filling of the mold and the application of pressure to fill the mold efficiently and cleanly.
The two main characteristics of the LPPM process are:
- The filling from the bottom of the mold is perfectly controlled compared to the turbulent flow associated with gravity casting. Also, the liquid metal is drawn from under the melt surface, preventing dross entrainment into the mold cavity.
- Efficient feeding from the bottom injection pipe occurs through pressure applied to the melt during solidification, eliminating the need for risers. The resulting yield is high: typically 80-90% versus 50-60% for gravity permanent mold casting. However, not all casting geometries are amenable to the LPPM process.
In low pressure sand casting, the sand mold rests on top of a pressurized enclosure as shown in Figures 1 and 2. The similarity between LPPM and low pressure sand casting is in the controlled tranquil filling of the mold with a dross-free melt. Both processes also share the ability to produce thinner walls than gravity pouring would.
However, in contrast with the LPPM process, in low pressure sand casting, no excess pressure is applied at the end of filling. Feeding from the bottom is interrupted early and long before the casting is fully solidified, so risers are necessary, just as in gravity casting.
Low pressure sand casting eliminates liquid metal handling, so the process is also advantageous over gravity sand casting when pouring large parts.
Size, quality and wall thickness will be the primary considerations when deciding between LPSM and gravity sand casting.
Compared to gravity sand casting, the low pressure sand molding process simplifies the filling of the mold. A single operator can repeatedly fill the mold for a 600-lb. casting at the push of a button, compared to the manpower necessary to fill the mold by gravity through multiple sprues. The filling metal is also cleaner.
Solidification times are typically five times longer in sand casting than in permanent mold. This is why low pressure sand molding is no comparison to LPPM when castings are small enough to be produced on a LPPM press. Since the majority of aluminum castings are relatively small, the LPPM process is much more widely used than low pressure sand casting. But when the dimensions are too large for LPPM, low pressure sand casting is a viable option.
A good candidate is illustrated through the following case study of a cast A356 aluminum mold used to make plastic parts for the food container industry.
The overall dimensions of the casting are 32 x 18 x 66 in. (800 x 460 x 1700 mm). Its inner surface will be polished to a 60 grit finish, so the as-cast surface roughness must be less than 250 RMS. For the same reason, subsurface porosities greater than 150 µm are not acceptable.
In Figure 3, the quiescent filling is illustrated by showing the melt temperature at three seconds, 10 seconds, 20 seconds and 30 seconds after the start of filling. This rate of filling was obtained by applying a rise in pressure of 10 mB per second inside the crucible enclosure.
Figure 4 presents a map of the metal front temperature anywhere in the casting. It indicates no risk of cold shuts (seams in the casting) exists because the liquid metal front temperature never drops below 1,159 F (626 C). (Alloy A356 begins to solidify at 1,135 F [613 C].)
The molten aluminum is fed from the furnace to the runners by thin gage 1.5-in. (38-mm) diameter steel tubes. Given the great propensity of aluminum to dissolve iron, the composition of the A356 alloy after a run was measured in a runner and in the steel tube and then compared to that of the furnace melt. The results, shown in Table 1, indicate only a small amount of iron (up to ~0.02%) was picked up when the melt remained fully liquid and still inside the steel tube for several minutes. Because the transit time of the aluminum in the tube during filling is of the order of one second, the iron pick-up is negligible.
Thermal Analysis
In Figure 5, the three green dots indicate the locations of three thermocouples that were inserted into the mold cavity during molding.
The responses of the thermocouples are shown in Figure 6. The arrival time of the liquid metal and the start and finish of solidification are listed in Table 2.
The measured solidification times are reasonably close to the predicted values shown in Figure 7. The solidification progresses from the mid-height of the casting, down to the feeding gates maintained under pressure until their complete solidification (10 minutes), and from the mid-height up toward the top risers in the other direction. This ensures a directionally solidified, shrink-free casting.
Because the casting will be submitted to service temperatures up to 482 F (250 C), any hardening via heat treatment would be lost after a few hours of operation. Consequently, the mold will be used in the as-cast condition. The inner surface was polished to a pit-free finish shown in Figure 8.
To ensure the lowest porosity level, the melt was degassed to a Reduce Pressure Test sample density of 2.63. The porosity level was related to the local solidification time and temperature gradient. Because these thermal parameters were readily available from solidification modeling, it was possible to predict the distribution of the porosity (Fig. 9).
Samples were cut out at two locations where thermocouples had been inserted, i.e. in the feeder tube and in a gate. The measured porosity in the gate was 0.8%, in reasonable agreement with the predicted results. The actual porosity level of the aluminum solidified inside the steel tube was 0.4%, much lower than predicted. The long solidification time (19.2 minutes) of the quiescent liquid melt inside the steel tube is believed to have allowed natural degassing to take place. Figure 10 shows the metallographic aspect of both samples at low magnification.
Due to the longer solidification time inside the tube, the secondary dendrite arm spacing was larger than in the gate (90µm vs 71µm).
While low pressure sand casting is not as common as gravity sand casting or LPPM, it holds a distinct combination of advantages when pouring large castings, including tranquil filling, obtaining metal from underneath the oxidized surface of molten aluminum for higher metallurgical quality, thin wall capability, easy metal handling, and cost efficiency.
ncountering a scenario in which you are forced to suddenly and immediately suspend melting operations for an extended period can be a death sentence for many metalcasting facilities. Small to mid-size businesses are the backbone of the industry, but many do not survive when forced into extended downtime. One disaster-stricken metalcaster, however, found resilience through its own perseverance and a circle of support from peers, friends, suppliers, teams from installation and repair providers, an original equipment manufacturer and even competitors.
Tonkawa Foundry, a third-generation, family-owned operation in Tonkawa, Okla., was entering its 65th year of operation this year when a significant technical failure ravaged the power supply and melting furnaces on January 17. Thanks to the textbook evacuation directed by Operations Manager Carrie Haley, no one was physically harmed during the incident, but the extent of emotional and financial damage, and just how long the event would take Tonkawa offline, was unclear.
Tonkawa’s power supply and two steel-shell furnaces would have to be rebuilt. No part of the reconstruction process could begin until the insurance company approved removal of the equipment from the site. The potential loss of Tonkawa’s employees and customers to competing metalcasters seemed inevitable.
Within two days of the incident, repair, installation and equipment representatives were on site at Tonkawa to survey the damage. Once the insurance company issued approval to begin work, the installation team mobilized within 24 hours to remove the equipment and disassemble the melt deck.
Since the damaged equipment was installed in the 1980s and 1990s, Tonkawa and an equipment services and repair company quickly strategized a plan and identified ways to enhance the safety, efficiency and overall productivity of Tonkawa’s melt deck.
“The most critical issue was for our team to organize a response plan,” said Steve Otto, executive vice president for EMSCO’s New Jersey Installation Division. “We needed to arrive at Tonkawa ready to work as soon as possible and deliver quickly and thoroughly so they could get back to the business of melting and producing castings, and minimize their risk of closing.”
Several years after Tonkawa’s melt deck was originally installed, an elevation change was required to accommodate the use of a larger capacity ladle under the spout of the furnaces. Rather than raising the entire melt deck, only the area supporting the furnaces was elevated. As a result, the power supply and workstation were two steps down from the furnaces, creating a number of inconveniences and challenges that impacted overall work flow in the melt area. Additionally, the proximity of the power supply to the furnaces not only contributed to the limited workspace, but also increased the odds of the power supply facing damage.
The damage to the melt deck required it to be reconstructed. It was determined to be the ideal opportunity to raise the entire deck to the same elevation and arrange the power supply, workstation and furnaces onto one level. The furnace installation company provided the layout concepts, and with the aid of Rajesh Krishnamurthy, applications engineer, Oklahoma State Univ., Tonkawa used the concepts to generate blueprints for the new deck construction. The results yielded a modernized melt system with an even elevation, strategically placed power supply, enhanced worker safety and increased operator productivity.
“Eliminating the steps and relocating the power supply farther from the furnaces was a significant improvement to our melt deck,” Tonkawa Co-Owner Jim Salisbury said.
Within four days of insurance company approval, all damaged equipment had been removed and shipped for repair.
The insurance company required an autopsy on the damaged furnace before any repair work could begin. The forensic analysis was hosted by EMSCO in Anniston, Ala., in the presence of insurance company personnel, as well as an assembly of industry representatives from the companies who had received notices of potential subrogation from the insurance company.
Tonkawa’s furnace was completely disassembled while the insurance company’s forensic inspector directed, photographed, cataloged and analyzed every turn of every bolt on the furnace over a nine-hour workday. The coil was dissected, and lining samples were retained for future reference.
While the furnace sustained extensive damage, it did not have to be replaced entirely.
Structural reconstruction was performed to address run-out damage in the bottom of the furnace, a new coil was fabricated and the hydraulic cylinders were repacked and resealed. Fortunately, the major components were salvageable, and ultimately, the furnace was rebuilt for half the cost of a new furnace.
“The furnace experienced a significant technical failure,” said Jimmy Horton, vice president and general manager of southern operations, EMSCO. “However, not only was the unit rebuilt, it was rebuilt using minimal replacement parts.”
Though work was underway on the furnaces, Tonkawa was challenged with a projected lead time of 14 weeks on the power supply.
When accounting for the three weeks lost to insurance company holds and the time required for installation, Tonkawa was looking at a total production loss of 18-20 weeks. From the perspective of sibling co-owners Sandy Salisbury Linton and Jim Salisbury, Tonkawa could not survive such a long period of lost productivity. After putting their heads together with their furnace supplier, it was determined the reason for the long turnaround on the power supply could be traced to the manufacturer of the steel cabinet that housed the power supply.
The solution? The existing cabinet would be completely refurbished and Tonkawa would do the work rather than the initial manufacturer. This reduced the 14-week lead time to just five weeks.
Tonkawa is the single source for a number of its customers. Although lead-time had been significantly reduced, the Tonkawa team still needed a strategy to keep the single source customers in business as well as a plan to retain their larger customers.
Tonkawa pours many wear-resistant, high-chrome alloys for the agriculture and shot blast industries. Kansas Castings, Belle Plaine, Kan., which is a friendly competitor, is located 50 miles north of Tonkawa. Kansas Castings offered Tonkawa two to three heats every Friday for as long as it needed.
“We made molds, put them on a flatbed trailer, prayed it wasn’t going to rain in Oklahoma, and drove the molds to Kansas Castings. We were molding, shot blasting, cleaning, grinding and shipping every Friday,” Salisbury Linton said.
Others joined the circle of support that was quickly surrounding the Tonkawa Foundry family.
Modern Investment Casting Corporation (MICC) is located 12 miles east of Tonkawa in Ponca City, Okla. Though MICC is an investment shop and Tonkawa is a sand casting facility, MICC’s relationship with Tonkawa dates back years to when Sandy and Jim’s father, Gene Salisbury, was at the helm.
“Gene was always willing to help you out,” said MICC owner, Dave Cashon. “His advice was invaluable for us over the years, so when the opportunity arose to support Sandy and Jim, we volunteered our help.”
MICC offered to pour anything Tonkawa needed every Friday in its furnace. Tonkawa brought its alloy, furnace hand and molds, while MICC provided its furnace and a furnace hand for three heats. Many of the specialty parts Tonkawa produces were completed with MICC’s support.
When Salisbury Linton approached Cashon and asked him to issue her an invoice to cover the overhead Tonkawa was consuming, Cashon told her if she brought in six-dozen donuts every Friday morning they’d call it even.
“We’re all kind of like family,” Cashon said. “We’re all part of the same industry and though we may be friendly competitors at times, you don’t want to see anybody go through what they’ve gone through and it could have just as easily been our furnace that failed. While we all take the appropriate measures and perform maintenance to prevent these scenarios from occurring, they unfortunately still occur from time to time in our industry.”
Tonkawa had recently added steel work to its menu of services and Central Machine & Tool, Enid, Okla., was able to take Tonkawa’s patterns and fulfill its steel orders so it would not fall behind with those customers, while CFM Corporation, Blackwell, Okla., took three of Tonkawa’s employees on a temporary basis and kept them working during the downtime. Additionally, a couple of Tonkawa’s major suppliers extended their payables terms.
Thanks to Tonkawa’s suppliers, friends and its personnel’s own passion, persistence and dedication, the business is up, running and recovering—placing it among the few shops of its size to overcome the odds and remain in business after facing calamity.
Nearly eight months after that devastating Saturday evening in January, Salisbury Linton reflected on the people and events that helped Tonkawa rise from the ashes. “We certainly would not have the opportunity to see what the future holds for Tonkawa if it weren’t for all the kind-hearted people who cared about what happened to us. Everyone still checks in on us.”