About AFS and Metalcasting

Fab to Aluminum Casting Conversion for Computer Processor

While it was prototyping the product, Pixellexis opted for a fluid, intricate design for the front plate of the unit, mimicking the company’s logo. The design was produced through water jet cutting and machining, and while the final cost to manufacture each front plate was substantial, the company was pleased with the look. 

However, before Pixellexis was able to introduce its product to the market, the market changed. Potential buyers wanted something less pricey, and the expensive front plate was blowing the budget. The company didn’t want to sacrifice its look, but an alternative, less costly manufacturing method was needed to fill the role. 

In the end, Pixellexis went with an unlikely choice. Although it needed only 25 units for its first batch and expected just 250-500 units ordered per year, the company found that the low pressure permanent mold  (LPPM) casting process furnished the same aesthetic quality at one quarter the price of the fabricated part. The cost of the permanent mold tooling—which usually dictates a higher volume to be economically feasible—was fully paid back after the first 30 castings.

faceplate1.jpg

When Pixellexis started the LexiGrid project, it worked with a metal-sheet subcontractor with an in-house designer that offered a turnkey cabinet to house the processor.

“The goal here was to get prototypes fast,” said Stefany Allaire, CEO of Pixellexis. “Since our product was a niche one at the time, the cost associated with manufacture was not so much a problem.”

The in-house designer worked with Pixellexis to come up with a grid design for the front panel of the unit. Originally, the panel grid of the video image processor housing was cut from an aluminum plate by the water jet process and machined to give it contour. In the water jet process, water is sprayed with abrasives at ultra high pressure (more than 30,000 psi). Each Lexigrid front panel required several hours of water jet cutting and machining to achieve the desired look, contributing to the steep price tag.

When it was determined the market required a less expensive option, Pixellexis didn’t want to lose its initial investment on the project and began looking for a new process to create the front panels. But keeping the edgy appearance was a sticking point. First, it considered a different material, and Pixellexis’ initial subcontractor suggested a molded plastic version manufactured in Asia.

“Honestly, our mission was to create a high-end look, and to sell a high-end product with a plastic finish was not the best idea,” Allaire said. “We began looking in our own backyard to see if we could find a specialty manufacturer that could get us what we wanted at a decent cost.”

Allaire was drawn to Technologies du Magnesium et de l’Aluminium (TMA), Trois Rivières, Quebec, Canada, because it was nearby, produced parts in magnesium and aluminum and used high-precision sand molding.

Casting LPPM

While Allaire was originally looking for a sand caster, TMA realized that the LPPM capabilities achieved through a machine run by the Centre Intégré de Fonderie et de Métallurgie (CIFM), Trois-Rivières, Québec, Canada, better fit the requirements of the front panel. LPPM is a process in which a steel or iron die is filled from the bottom via a transfer tube immersed into a furnace located under the mold. By controlling the pressure applied on the surface of the melt, the filling of the mold can be controlled, eliminating the turbulence created when a mold is filled from the top, as in gravity-fed permanent mold casting. Also, the pressure applied on the melt during solidification is equivalent to several yards of pressure head, resulting in excellent feeding of the casting when designed for directional solidification, which is when metal solidifies progressively toward the liquid metal ingate. TMA collaborated with CIFM on the casting of the processor’s front panel.

In order to avoid premature freezing of the liquid aluminum during LPPM filling, the mold is dressed with a coating sprayed as a water-based suspension of minerals with sodium silicate added as a binder. A wide range of coatings are available, and their selection is the result of a compromise between high insulation, smooth surface finish, lubrication and durability. The mold coating is generally applied at a relatively low temperature (typically 392-446F [200-230C]) before the mold is mounted on the machine. Retouching the coating during the casting run is kept to a minimum to avoid scale build up and loss of control of the process; retouches during a run are made while the mold is typically between 662 and 752F (350 and 400C), where the temperature is too high to obtain good coating adherence.

A properly dressed mold can produce several hundred castings before it has to be removed from the machine, sand blasted and re-coated.

The coatings can be roughly separated into two families: “black” coatings, which provide lubrication for easy release of the casting on ejection, and “white” coatings, which provide insulation that prevents premature solidification during filling. A surface finish coating can result in 150-200 RMS finish.

In order to amortize tooling costs, the LPPM process is normally considered for production runs in the thousands or more. However, in particular instances when machining is the only alternative, such as the front panel of the Lexigrid, the net-shape potential of the process can make it economical for much shorter series of intricately shaped parts.

The cosmetic requirement of the grid face (RMS less than 200) caused the CIFM-TMA team to choose a surface finish white coating for one face of the mold. The ejector side of the mold was sprayed with a rougher, more insulating coating with a graphitic overcoat to ensure proper release on ejection. The coatings were applied carefully on the fully accessible mold faces before the assembly was mounted on the LPPM machine so that no retouch would be necessary over the expected short cycle casting run.

Given the intricate design of the grid, which featured several holes, thin walls and numerous cut-outs, it was clear the challenge in casting the part would be to fill the mold cavity before the metal stopped flowing because the small section channels of the grid would cool quickly. CIFM had previous research and development experience casting walls as thin as 0.079 in. (2 mm) with its LPPM process.

According to the law of physics applied to pressure, the equilibrium position of the liquid metal front inside the mold cavity relates directly to the gas pressure applied on the melt contained in the crucible underneath the mold. Thus, a 10 mB increase in pressure in the crucible will result in a 1.57-in. (40-mm) rise of the liquid metal front inside the mold cavity. However, this is only true if the filling is not too fast, as a back pressure can build up if the air inside the mold cavity is not freely expelled; this will take place even with good venting. Also, the viscosity of the liquid metal flowing in the narrow channels can cause the actual filling time to be up to twice as long as the theoretical filling time, based on the above 10 mB/1.57 in. relationship. 

The gating, pouring temperature and mold temperature distribution were selected on the basis of previous thin-wall casting experiences.

Filling simulations were performed in order to determine the  slowest pressure ramp that would ensure complete filling of the mold cavity. For complete filling, the tips of the liquid streams flowing inside the grid rectangular channels should not reach a temperature less than 1,135F (613C), the temperature at which aluminum A356 starts to solidify.

After a three-hour casting campaign, 37 good castings were produced with a reject rate of 4%. The cost of the finished part (including the final machining and painting) was four times less than the original panel. After the first casting order, the mold was fully paid back.

While LPPM is often reserved for high volume applications to accomodate tooling costs that are more expensive than that of sand casting, in some narrow applications, the near net shape potential and the filling capability of the low pressure permanent mold process results in considerable savings. Numerous similar conversions of machined parts could be considered in markets for intricate parts, even when only a few hundred are needed. METAL