What does Ketchup have to do with Rheocasting?

Instead of the classical HPDC, Rheocasting uses a semi-solid slurry. It is hard to imagine that adding solid particles to a liquid melt doesn’t negatively impact the flow and castability. Let me explain that effect with an everyday item.

Ketchup is an excellent illustration of thixotropic behaviour. When you first squeeze a ketchup bottle, it may seem quite thick and resistant to flow out. Give it a vigorous shake or tap on the bottom to get it to flow.

When you apply that force, the ketchup experiences shear stress. Shear stress is the stress that occurs when adjacent layers of a material slide past each other. In this case, the stress is created when you shake or squeeze the ketchup bottle.

As the Ketchup experiences this shear stress, it starts to change its viscosity. Viscosity is a measure of a fluid’s resistance to flow. Initially, the Ketchup has a higher viscosity, making it more like a solid. However, as the shear stress is applied, the Ketchup exhibits a lower viscosity, becoming more fluid-like.

This decrease in viscosity allows the Ketchup to flow out of the bottle more easily. You can now pour it onto your burger or fries.

But here’s where the thixotropic property comes into play. Once you stop squeezing the bottle or applying force, the Ketchup gradually returns to its original, thicker state. It thickens up again, regaining its higher viscosity.

This recovery of viscosity is due to ketchup’s thixotropic nature. Over time, the ketchup molecules realign or reassociate, forming a more structured network. This reformation of the network increases the viscosity of the Ketchup, causing it to become thicker.

So, in summary, Ketchup behaves thixotropically. Applying force or shear stress by squeezing or shaking the bottle temporarily makes it less viscous and flows more easily. However, once the stress is removed, it gradually regains its original thickness and becomes more flow-resistant.

Thixotropic behaviour is not limited to Ketchup alone. The aluminium semi-solids follow the same principle. So, now you know why the flow behaviour of Rheocasting slurries is so desirable.

Did you enjoy this piece of content? Then look no further than the Rheocasting Workshop where you learn more about Rheocasting and its implementation to acquire new profitable castings.