Venting is crucial for Rheocasting

“When we’re going to use Rheocasting, we don’t need a vacuum system.” This is one of the most common sentences I hear from people interested in Rheocasting.

When looking at the information material, their claim is backed up. You always see these uniform melt fronts in plates to describe the differences between HPDC and Rheocasting. As long as the slurry flow is laminar, the air doesn’t get mixed with the slurry. Therefore, the air is transported into the overflows.

So, here comes the hard truth:

If you cast a simple plate-like structure, the air is pushed in front of the slurry out of the casting. Yet, rarely does someone do that. Castings are usually more complex. They have wall thickness jumps, rips, openings, etc. All these lead to metal meetings somewhere in the casting.

At these metal meetings, the air in front of the slurry prevents the recombination of the two melt fronts, resulting in an oxide bifilm within your casting. These oxide layers lead to poor mechanical and thermal properties.

In the picture, you can see the extreme form of this principle. This tool is designed to generate venting issues. Both cavities are connected to the vacuum system. When you closely look at the one in the front, you can see the base of the additional vents on top of the fins. These help to evacuate air from the casting further. The other cavity doesn’t have them, and the fins are barely filled as the entrapped air builds up a counterpressure.

The additional vents are a fraction of a millimetre thick. Liquid metal in HPDC would fill them outright and permanently lock the casting in the die. However, the thixotropic properties in Rheocasting and the stable slurry front hinder it from entering the opening. Yet, air can escape through these gaps.

So, outstanding part qualities can be achieved by understanding the flow of the slurry and the venting mechanism of Rheocasting parts. Or if you turn it around: Due to poor cavity venting, all efforts can be useless as the air works against the casting. So, don’t make your life harder by not implementing venting in Rheocasting tools!

Adjusting the venting concept is often an issue when using old HPDC tools for trial castings. Implementing venting in the tool at a later point is nearly impossible and breaks the project’s budget. So, managing expectations is crucial for trial castings.

Adding venting to a later production tool is easy, especially when considering the tool’s increased lifetime. Spending more on a well-designed tool is worth it. Then you will have the perfect parts, which you can see in all the information.

If you want help reaching the perfect parts in Rheocasting or HPDC, reach out. Schedule a Free Consultation Call today. Casting-Campus GmbH is happy to help you improve the overall quality of your castings!