Monthly Archives: August 2020

Draft

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Tall fin parts like to stick in the mold

True to my college days, I would rather write about beer when the topic of draft comes up. In the emerging electric vehicle marketplace, die casters will be charged with the task of creating parts with tall cooling fins. As you might suspect the taller fins are more likely to stick in the mold. This is due to a lack of space to add draft. Certain historic die cast parts like fan clutches also required tall cooling fins.

Only a small subset of die casters have produced fan clutches due to their difficulty. They are supported by premier die shop that have the skill to make the required details. Draw polishing the cavity making the pictured parts costs US$5000. As a part of my task to return a casting shop to profitablity, I found it necessary to teach my selected die shop how to create a mold without using draw polish. They took some convincing, so we agreed to draw polish two diagonal cavites on a 4 cavity die. As I expected the draw polished cavities stuck in the mold on the first shot. I was not surprised. Normal EDM sinking creates ribs with negative draft. It is called underburn. It takes a lot of draw polishing to restore posiitve draft

Heat Check

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Toothed machining locators help mitigate heat check

I learned die casting from a fanatic. He honed his skill to the point where he was able to achieve unbelievabe die life. In the five years that I worked with him, he never welded a casting die surface (introducing heat check) that contacted molten aluminum. I have to take responsibility for the one time that it happened. The OEM engineering group forced us to implement a panic shape change. Welding the die was the only option to meet timing. Because of his fanaticism we regulary achieved die lives up to 500,000 shots on heat check sensitive parts like transmission stators.

Torque converter performance require heats check free blades

The die casting industry is learning how to economically make the emerging structural castings. These castings are made from alloys that have a hotter casting temperature. Higher casting metal temperatures result in more heat check. Heat check begins in as little as 5000 shots. In most cases it is a good practice to use toothed machining locators The teeth ignore some heat check and improve located consistancy. When the volume dictates multiple machining nests, removing a tooth makes it possible to mark the processed casting to indentify which nest was used.

Corundum

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Ruby Chrystals are a Corumdum Gem

Corundum is nothing more than aluminum oxide. Adding the correct impurities, man has learned to synthetically grow beautiful gems like star saphires. The melt furnaces that are used to create aluminum castings also have the correct conditions to synthetically grow corundum crystals on their walls. Poor cleaning practices dislodge these crystals into the melted aluminum. They remain suspended because the density of aluminum oxide is very close to that of molten aluminum.
Many prospectors still search the globe looking for corundum gems like rubies and saphires. People who machine aluminum castings have automated the process of discovering corundum crystals. A discovery event occurs when the machining carbide is chipped. Carbide is only 8.5 on the Mohs scale which is less than the 9 Moh number for Corundum. Most machining processors that I know are very unhappy with discovery events.


Since aluminum die castings are usually made of material that is 95% recycled, they contain corundum crystals within the allowed 5% of non-metallic impurities. Most likely these crystals were a part of the material that was recycled and were not introduced by poor practice by the current casting producer.

The real question is what should be done to reduce the frequency of chipping carbide during machining. The simple answer is to use primary aluminum as the casting material. Usually not attractive because it costs twice as much as secondary (recycled) aluminum. More effective is to design a better match between the casting and the machined part. For example the .030 (.75mm) skin is free of oxide impurities. Accomplishing a machining reduction is always an effective way to reduce carbide chipping but this requires a higher skill. Certain parts like fan clutches, that are mostly machined, including deep grooves made using expensive cutters, are in another category. Purification processes such as Wedron that separate out the imprurities are needed. This increases material cost because someone must pay for buying the material that was removed and scrapped.