Does Mr. Leidenfrost destroy your Tools?

Before we lean into our suspect that causes tools to fail early, let’s give you some background information. 

When we look at a typical HPDC cycle, the spraying and solidification times account for approximately two-thirds of the time. The tool’s cooling is straightforward. The channels behind the cavity with water or oil take the heat out to a heat exchanger. In addition, spot cooling could have been implemented, too.

There are two general directions for spraying the release agent. Microspray, where you apply the pure release agent in a nebula, and conventional spraying, where you mix the release agent with water and spray it on the surface of the tool. 

Now, it gets complicated, as conventional spraying is often used to cool the tool’s surface. Depending on the spray head, it moves along the surface and sprays the mix, or the spray head masks the tool. The exact amounts can be set for each location. 

Each tool has a hotspot somewhere, and that is precisely the point where the most amount is sprayed to cool it down. But what if you tell me you don’t really cool down the hotspot; you instead cool down the surrounding area? 

Then, the next casting cycle comes, and the liquid metal gives the cooled steel surface a thermoshock. This causes your tool not to reach the expected 100.000 cycles. After a few thousand shots, it will start to crack.

Why is it so problematic to cool down the hot spots by spraying?

Let me introduce Johann Gottlob Leidenfrost. He discovered this effect in 1756, so long before HPDC was a thing.

Imagine you’re cooking in your kitchen and accidentally splashing a drop of water onto a hot frying pan. Instead of immediately boiling away, the drop of water skitters around the pan, almost like it’s dancing. When a drop of water hits the hot pan, the bottom layer of water that touches the pan immediately turns into steam. This steam forms a little cushion of vapour that keeps the rest of the water drop from touching the pan directly. This is kind of like how a hovercraft floats on air, except it’s happening with the water drop on the hot pan.

This layer of steam insulates the water drop, preventing it from immediately evaporating. The water drop doesn’t stick to the pan; it kind of hovers over it, protected by this vapour layer. This effect is called the Leidenfrost effect.

So why does this happen?

When a liquid, like water, comes into contact with a surface much hotter than its boiling point, it creates an insulating vapour layer. This vapour layer drastically reduces the heat transfer rate between the liquid and the hot surface.

The same thing happens with your tool. The water never touches your tool’s hotspot, and the release agent is never applied to that location. This can also lead to your part getting stuck in the tool, in addition to the constant thermal shocks.

So, don’t blame Mr. Leidenfrost for your destroyed tool! Instead, tackle these hotspots already in the tool design and keep your surface temperature as high and evenly as possible when casting! Did you enjoy information like this? Then schedule a Free Consultation Call and inquire about the Casting Expert on Demand from Casting-Campus GmbH