Additive adds efficiency, precision to EDM

John Bradford laughs as he tells the story now, but for his customer, the sticky situation surrounding one part’s release was anything but funny. For molders, unduly extended cycles and high scrap rates rarely are.

Bradford, a technical sales specialist with Makino’s Die/Mold Technologies group, was working with a client on a speaker grille application. Created using EDM, the grille’s tool, a P-20 mold with a hardness of 40 Rockwell C and a cavity perimeter of 12 by 18 inches, was molding parts that were reluctant to release despite a Teflon-nickel coating, lapping compound and wire-brush treatment, and a mold release application prior to every shot.

In addition, the final step in the part’s 72-second cycle involved a worker gingerly extracting it from the mold by hand while trying not to cause any distortion or warpage. The customer only had one description of this step for Bradford, which he admittedly found humorous.

“[The customer] said it was like peeling the skin off an orange,” Bradford says with a laugh. Peeling an orange can be a tedious, difficult process. Peeling a part proved equally time-consuming, and costly. Scrap rates ranged from 30 to 40 percent, according to Bradford.

It’s the Little Things

So what was the holdup? Bradford says tests conducted by the company found that a finish of 6 µm on the tool’s surface was necessary for a speaker grille application of this kind to release automatically. His client was able to achieve a finish of only 12 to 14 µm after 150 hours of finishing work using conventional EDM technology.

“After 150 hours, they still could not achieve any better surface finish than that,” Bradford explains. “They had met a physical impasse with the EDM machine, and that relates directly to the amount of surface area vs. the surface-finish requirement for a particular detail.”

The detail in this case was the speaker grille’s 12,000 standing pins and the 400 sq in of surface area on the mold. The client’s technology had hit the wall. Another 150 hours of finishing work wouldn’t make a difference. A change was needed. Enter Makino and its High Quality Surface Finish (HQSF) technology.

Tweaking the Dielectric Cocktail

Makino originally introduced its HQSF additive technology in 1997 as an answer to problems posed by traditional additives found in dielectric fluids and their effects on the EDM process. Bradford said there were three main issues Makino hoped to resolve.

First, when using traditional additives, moldmakers could not perform roughing and finishing in the same additive oil because of the high levels of energy needed for roughing. This amount of power destroyed the additive particles and the surface agents used to keep the additive from clumping together. Consequently, workpieces needed to be roughed in one bath and finished in another, or the roughing tank had to be completely drained and cleaned before finishing could begin.

The second problem was that as the surface agent broke down and the additive clung together to form clumps, the distance between the workpiece and the electrode was shortened in certain spots where energy would refocus with obvious consequences.

“If you have several particles sticking together,” Bradford explains, “that creates a short distance. The energy will always tend to focus in that area for the discharge to take place, and that’s a distinct no-no in EDM.”

Bradford likens it to a lightning rod. Electricity always finds the shortest distance between two points, and the consequence in EDM is uneven distribution of energy, and one surface of the tool potentially degrading under the stress.

The final problem experienced with traditional EDM revolved around day-to-day management of the dielectric bath—a complex recipe of additive, dielectric oil, and surfacing agents that had to be kept within certain ratios. With traditional additives like chrome or silicon, proper suspension must be ensured, and this takes levels of 20g of additive per 1 liter of dielectric oil.

Makino’s patent-pending additive has a specific gravity that lets it maintain proper suspension without such a heavy dilution. The additive, µSC (micro semiconductor), only requires a mix of 1g of additive per 1 liter of dielectric oil. The other advantage of the additive is that it requires no surface agents. This means that particles won’t clump together and cause uneven distribution of current, which allows finer details to be achieved. It also means that roughing and finishing can be done in the same tank, saving time and money in the EDM process.

Attention to Detail

Ready to save time and money on their costly and time-consuming process for molding a speaker grille, Bradford’s client turned to Makino, its HQSF technology, and the µSC additive. Makino produced a surface finish of 5 µm on the existing cavities after only 60 hours of refinishing them.

Skipping the previous steps of hand lapping, applying a nickel-Teflon coating, and spraying mold release on the tool prior to injection, Bradford’s client ran the tool directly out of the EDM machine and achieved automatic part release in a 52-second cycle, instead of 71 seconds.

Was Bradford’s client pleased with the result? Their next request of Makino would indicate, yes, they were. “In addition to these cavities,” Bradford says, “[the client] asked us to remachine 10 additional cavities that had the same problems.”