Simulation Case Histories: Solving Problems while Optimizing Processes.
Casting simulation software utilizes the laws of physics to mathematically describe the filling and solidification of metal in a mold cavity. Originally, this tool was used to design gating and feeding systems. Today's software has become a problem solver by recognizing and helping to eliminate casting defects as well as assisting in the up-front component design for validation.
The following case histories give evidence as to how the computer simulations have moved beyond filling and feeding to optimize the casting process. These case histories detail how computer simulation identified and resolved problems, whether actual or anticipated, and reduced overall costs in casting low carbon steel, ductile iron, aluminum and magnesium components.
Casting: A 20-lb low carbon steel adapter was being cast for use in mining operations to hold digger teeth for buckets. Two castings were poured per green and sand mold at 2900F (1593C). The gating system consisted of a downsprue with two separate ingates, one for each casting, fed by a single 3 x 6-in. exothermic feeding system.
Objective: Using computer simulation, a foundry molding with resin-bonded sand wanted to improve the casting quality and optimize the yield (45%) of the adapter component. Inclusion generation also needed to be minimized.
Simulation Process: The simulation data evaluated the conventional gating system. Simulations indicated whether the gating system was turbulent or laminar, and whether the casting filled evenly and uniformly.
To solve the inclusion problem and improve quality, the software was used to evaluate the gating and risering concepts and then analyzed the mold filling and solidification. One modified gating design used a single 3 x 6-in. feeder and basin method. Positioned upon the original feeder basin, the unit replaced the downsprue and gating. The system also included a ceramic filter to reduce turbulence and metal reoxidation within the mold during filling.
The simulation indicated that pouring directly through the riser and eliminating the gating would increase yield. Figure 1 compares the temperature/solidification variation of the two designs.
Result: Implementing the modified gating method resulted in enhanced casting appearance and quality, eliminated sand inclusions, improved feed margin and increased yield by 11%. Since the new pouring method also required a filter, the filtered metal and reduced turbulence aided in casting better quality parts. Furthermore, the total pour weight for each mold dropped from 89 to 71 lb, which added to anticipated cost savings.
Ductile Iron Ring
Casting: Using nobake molding, a foundry was casting a 5600-lb retaining ring more than 6 ft in diameter. The ductile iron was poured at 2462F (1350C) through a gating system to a single mold cavity with eight 10-in. insulated risers.
Objective: The computer simulation's goal was to increase the casting's yield (77%) and decrease the scrap rate caused by porosity (50%).
Simulation Process: The first step in the engineering project was to simulate the filling and solidification of the casting as it was currently being produced. Doing this allowed the foundry engineer to better understand the causes of the problem, and therefore determine a course of corrective action. The initial simulation pinpointed the porosity areas underneath the riser sleeves (Fig. 2). It was decided to investigate the possibility of pouring the casting with no risers. The prediction of porosity in iron castings requires the use of software capable of accounting for graphite expansion. The subsequent simulation showed that porosity inside of the casting had been eliminated. Porosity was still indicated close to the surface, but this area was to be removed in machining and would not cause a problem to the final customer.
Result: The foundry has been producing these castings with no risers. The elimination of the risers not only resulted in the production of sound castings but in saving the cost of eight riser sleeves per casting. The revised casting method lowered the scrap rate from 50% to 4% and improved yield from 77% to 83%.
Although the retaining ring casting was a low-volume job, the first year cost savings was estimated at $83,680. With a production rate of 40 castings/yr, the foundry experienced a yield saving of 480 lb/casting. By eliminating the eight risers, the added cost of riser sleeves and riser removal was also eliminated.
Aluminum Piston (4-in.)
Casting: Aluminum was being poured at 1250F (676C) into a multi-cavity diecasting for a 4-in, aluminum transmission piston that weighed 0.5 lb. The gating system directly fed five castings, each with three overflows.
Objective: The foundry wanted to try different designs to eliminate porosity defects caused by trapped air (air entrainment) without re-tooling the die each time. The 3% rejection rate of these automotive components needed to be lowered for cost savings.
Simulation Process: To accomplish this, computer simulation modeled the placement and design of gates and overflows, which proved critical in reducing part defects. The simulation traced the defect to the gating design and subsequent filling pattern. The flow imbalance produced laminations in the part and air entrainment formed porosity. The simulation identified what was causing the defect was that the last section to fill was inside the part, opposite the ingate.
The gating redesign then was simulated instead of re-tooling the die in conventional trial-and-error fashion. The simulation's first computerized cycle studied the effect of removing the small "delta" from the ingate. Even without the delta, analysis showed very little change in the fluid pattern.
In the next design change, the central overflow was rotated so that the inlet to the overflow was directly across from the ingate. Also, a second ingate on the overflow was added on the opposite side of that overflow, attempting to provide a vent for the trapped air. This design showed that it would not eliminate the defect. Placing an ingate in the cover half of the die also proved ineffective according to the simulation.
Result: A new approach to the gating issue then became apparent. By tapering the ingate direction, the flow pattern moved around the part, forcing the trapped gas outward toward a newly added overflow as shown in Fig. 3. The design had the added benefit of removing two overflows and a vent along with a significant reduction in gate volume.
The dies were re-tooled once as a result of the simulations. Scrap caused by porosity decreased. The rejection rate dropped from 3% to 0.5%, which led to cost savings. An unexpected benefit was yield improvement.
This "radical" gating modification would not have been attempted using actual tooling methods due to cost and time demands. The simulation ran its cycles on a PC from 6-10 hr overnight rather than having the die re-tooled during business hours. If re-tooling was made in-house, die modifications could take 2-3 days. If the die was sent out, re-tooling would take up to a week.
Aluminum Piston (6-in.)
Casting: A 6-in. transmission piston that weighed 1.5 lb was being diecast. The aluminum was being poured at 1238F (670C) into a gating system that fed three castings per mold.
Objective: The foundry wanted to eliminate porosity problems without the time consuming, and thereby, costly method of re-tooling a die through trial-and-error. The scrap rate of 15% needed to be decreased to reduce costs.
Simulation Process: The simulation immediately revealed that the flow into the mold cavity was uneven. Flow imbalance was due to uneven runnering and runner distances from the biscuit to the respective ingates of three castings. The ingate and geometry in front of the ingate produced turbulence and air entrainment, which led to the porosity defect (Fig. 4).
A different runner system with different ratios of cross-sectional areas solved the flow imbalance. The metal approached the ingates at a transition time from slow to fast shot. An overflow was added opposite the ingate to attempt to control the trapped gases. The overflow gate thickness resulted in volumes of liquid metal being enclosed inside the part, leading to shrink porosity. Analysis of the simulations determined that overflows caused more problems and needed to be reduced.
Result: A tangential ingate was introduced to push the initial cavity air around the part to a controlled region with an overflow. The single stream of metal was much easier to control along with the number and placement of overflows. By introducing these design modifications, scrap rates dropped from 15% to less than 1%, again providing cost savings.
Like the other aluminum casting, simulations were run overnight from 6-10 hr on a PC instead of repeatedly re-tooling the die.
Magnesium Valve Cover Casting
Casting: The Ford Co. ordered a 24 x 18.5 in., 4.85-lb magnesium valve cover for its 5.4 liter Triton engine.
Objective: The foundry wanted to reduce the overall design cycle time before the first casting was poured. Computer simulation was to be used during the part design cycle of the valve cover. Even though the final part design was not complete, the base geometry was developed, allowing for much of the gating design to be formulated and tested.
For this casting, the heat stake bosses on the underside of the cover were an added concern. These bosses were inserted through the holes in the baffle plate. During the casting process, the end of the boss was to melt slightly and press down, thereby locking the baffle plate in place with the valve cover.
Simulation Process: The software simulated magnesium being poured at 1250F (676C) fed to a single cavity through a seven-gated system. As is typical in cold-chamber diecasting, process concerns focused on timing (slow to fast shot transition) and trapped gas in the cavity.
The simulation showed that the initial casting design did not provide an adequate amount of metal to a section opposite the timing chain end of the cover. Further analysis indicated that the bosses would be difficult to fill due to air entrapment. By not having the proper amount of metal in the end of the boss, not enough metal would melt to keep the baffle plate in place (Fig. 5).
To solve the first problem, a kicker gate was added on the left to feed the section opposite the timing chain. With the appropriate runnering, the slow to fast shot time was kept intact. To solve the baffle/boss problem, the bosses and sections leading to the bosses were made thinner, thus reducing the metal volume required for these regions and allowing for a complete filling of the mold.
Result: No retooling of a die was required. The part modification was done concurrently with the part and casting design phases to allow for interaction between design and manufacturability. As a result, the production could begin sooner because potential delays and excess costs were anticipated and eliminated.
Since the valve cover was a complex design, 510 fluid thermal cycles were run to reach a steady state (because that many cycles would be needed to run the dies at the correct temperatures). The simulations took 2-3 days to run on a PC.