Peeking Inside A VW Gearbox Reveals Die Casting Truths

Recently, I was offered a 1997 Volkswagen Golf for the low, low price of free — assuming I could haul it away, as it suffered from a thoroughly borked automatic transmission. Being incapable of saying no to such an opportunity, I set about trailering the poor convertible home and immediately tore into the mechanicals to see what was wrong.

Alas, I have thus far failed to resurrect the beast from Wolfsburg, but while I was wrist deep in transmission fluid, I spotted something that caught my eye. Come along for a look at the nitty-gritty of transmission manufacturing!

With the transmission pan dropped and filter removed, I had a clear view of the internals of the transmission case. As a former casting engineer, certain elements of the finished part stood out to me. Various parts of the housing featured little bumps and protrusions — chunky pimples on what were otherwise flat surfaces or smooth rounded edges. These were matched by what appeared to be a series of fine crack-like patterns on the surface. These defects are evidence of imperfections in the surface of the die, suggesting the tooling is pitted and cracked where it should instead be smooth.

The area affected was not a machined section, merely acting as a fluid reservoir. Thus, the anomalies have no real effect on transmission performance. However, their presence does tell us a little bit about the state of the tooling used by Volkswagen to produce the castings. A brand new die fresh from the toolmaker does not typically produce parts with cracks, pits, and lumps evident on the surface. The rough nature of the 01M transmission housing in my 1997 Golf is evidence that Volkswagen was running its casting dies well into the tens of thousands of shots, perhaps even into the six figures. Given the nature of the features, which are due to the die's inherent physical condition, they’re not a one-off fault on a singular casting. Instead, line engineers and operators would be aware that the die is aging and becoming worn. Given the flaws occurred in a non-crucial location, a conscious decision was likely made to ignore the flaws and ship the parts, given they were unlikely to be noticed by the average driver who doesn't disassemble their transmission to kill some time on the weekend. Similar flaws in a visible or functional area might instead be "fettled", where the offending protrusions are ground off by a human operator or a machine.

Given the defects cause no functional impediment to the transmission, one can understand the decision to pass the parts in the interest of keeping costs low. However, to understand how the defects happened, let's take a little crash course in high-pressure aluminium die casting.

Modern transmission cases are often manufactured using the high-pressure aluminium die casting process. This process involves huge metal moulds, called dies, that come in two halves and are pressed together, creating a cavity between the two. Usually, vacuum is then applied to suck air out of the die cavity to reduce gas entrapment. Next, molten aluminium is poured into a cylinder, and a piston is used to inject the aluminium into the die cavity at very high pressure. Each time the piston fires molten aluminium into the die, this is called a shot.

By injecting molten metal under high pressure, it reduces the problems caused by the aluminium shrinking as it cools. This shrinkage can cause voids in the final product, referred to as shrinkage porosity. Keeping the pressure high ensures the dies are fully filled with as much aluminium as possible and reduces the amount of shrinkage of the final part. To resist the high pressure of the shot, the two halves of the die are held together with a special locking mechanism that is supposed to keep the die shut and stop metal leaking out around the seams where the two meet. Of course, if the dies aren't perfectly flat or aligned properly, sometimes metal escapes around the seams, called flash, which is where you may see a small parting line on a finished part. In extreme cases, some flash escapes the die entirely, and this is very scary the first time it happens on your shift. Typically, the line operators will chuckle heartily as you are startled by the stinging metal.

In the same way we track the miles travelled as an estimate of wear on an automobile, die casting machines track the number of shots made. Every cycle of filling the piston, injecting aluminium into the die, and removing the part puts wear on the machine and the dies themselves. Dies weigh many tons, and are constructed of high-strength tool steels. A die may cost many hundreds of thousands of dollars to produce, and thus must pay for itself by producing tens of thousands of parts. Engineers work hard to ensure dies last as long as possible in order to run a cost-effective casting operation.

A typical die may last over 100,000 shots with regular maintenance and repair. But over time, the wear and tear can become too much, and the die must be replaced. There are many ways a die can fail to produce quality parts; a full run down would fill a hefty textbook, and is beyond the scope of this article. Instead, let's look at the causes of the cracks, lines, and bumps we found on our Volkswagen 01M casting.

Cracking of the die surface and the resulting defect on the part surfaces is typically referred to as heat check. When the hot liquid metal is injected into the die, it hits certain surfaces in the die before others. These areas of the die heat up more than their surroundings, and undergo greater thermal expansion and contraction with each shot of molten aluminium. These cycles cause cracking and damage over time. The cause is thermal, hence the name. Repair is possible by polishing back the cracked surfaces of the die, or by welding fresh metal onto the die and grinding back to the original profile. However, such a repair does not treat the root cause, and heat check will reoccur as it racks up further shots.

Pitting of the die tends to be caused by cavitation. As hot metal flows into the die at high speed, tiny bubbles or pockets of vacuum can form and then collapse. This happens in specific areas due to the nature of the fluid flow through the die cavity. Typically, it's in a region where the flow direction changes or encounters an obstruction, and a low-pressure area is surrounded by high pressure liquid metal. As the pressure increases around the low-pressure bubble, it eventually collapses. The collapsing bubble can generate an intense shockwave and localized high temperatures, damaging the surface of the die. This starts on the microscopic level, but over time, the damage increases and the pits grow deeper as the number of shots made on the die increases. Similar to heat check, the damage can be repaired by welding fresh metal to fill the pitted area. But without redesign to the die geometry to reduce the cavitation problem, it's not a permanent solution and the damage will reoccur.

The damage to the dies evident on our finished transmission housing is understandable in context. The 01M transmission was used in a multitude of Volkswagen vehicles in the 1990s and early 2000s, ending up in hundreds of thousands, if not millions of cars. Volkswagen likely had multiple casting machines running round the clock with many dies on hand to pump out the necessary parts over the years. Some wear is to be expected at these high production levels. Given that the anomalies spotted were in a non-functional location, the engineers would have cleared the parts to flow down the line while monitoring the die's condition for further problems.

Hopefully this article has taught you a little something about how high-pressure die cast parts are made, and will enable you to smartly show off to your friends next time you’re at the junkyard. For those interested in process control, fluid mechanics, and manufacturing efficiency, die casting can be an interesting field to work in. The world loves aluminium parts, so the skills you learn should serve you well into the future!