The Science Behind Ketchup Pouring Could Lead to Ceramic 3D Printing

3D printing has matured from a speculative sci-fi fantasy into a practical tool. Plastic is the most commonly used material for 3D printing, but NIST research is helping expand 3D printing into a new area - ceramics. Small details matter when it comes to printing ceramics, and those details can only be perfected if there's a reliable way to measure them. Good measurements from NIST will help the industry.

Technical ceramics - as opposed to those used in more common applications like pottery - have unique properties. They are lightweight, durable, excellent electric insulators and able to withstand extreme temperatures. Common technical ceramics include alumina and zirconia. Their unique properties make them ideal for electronic components, ballistic armor and rocket engine parts.

3D printing offers a new way to create these parts. Dental implants are already commonly printed in ceramic. 3D printing is ideal for making medical implants that replace bone because they can be made of the same mineral as bone and replicate the complicated internal latticework that bones have, resulting in a replacement part that's unique to each person and as close to real bone as possible.

"Anytime something new comes along, people are skeptical it will work," said Russell Maier, a NIST materials scientist who studies ceramic 3D printing. "Part of my research is to help make ceramic printing more consistent and reliable."

One of Maier's big challenges is accurately measuring how wet ceramic flows as it's pushed and pulled. But to understand why that's important for 3D-printing ceramics, you need to know a little bit about ketchup.

A Tale of Two Condiments

There's something weird about ketchup. If you turn a ketchup bottle upside down, the condiment doesn't pour. But as any french fry enthusiast knows, a sharp tap causes a glob of ketchup to fall out easily.

Compare that to a more normal fluid, such as honey. No amount of tapping will make honey pour faster; it will still pour slowly.

Before a tap, ketchup is around five times thicker than honey. After a tap, it becomes twice as thin. The force of that tap momentarily changes how easily ketchup flows.

Viscosity is another word for how thick or runny a fluid is. Newton's law of viscosity states that the more quickly a fluid flows, the greater the resistance to that flow.

Fluids such as ketchup that change their thickness when a force is applied are called non-Newtonian fluids because the law of viscosity doesn't apply to them.

The basic materials used to 3D-print ceramics are also non-Newtonian. And measuring how their viscosity changes is essential to making the technology work.

3D-Printing Ceramics

There are a few ways to 3D-print ceramics, each with its own advantages and disadvantages. They usually start with a thick liquid made of tiny particles suspended in water or a polymer. The type of ceramic 3D printing Maier uses is for technical purposes, not something the average hobbyist would likely do at home. This is due to the size and expense of the required equipment.

One method uses a nozzle to lay down ceramic paste, like a baker frosting a cake, one layer at a time. This creates a soft shape that can then be dried and fired in a kiln to make the final ceramic part. For this to work, the ceramic must behave like a liquid as it flows through the machine, then retain its shape like a solid after it's laid down. (See the video for this method in action.)

Another method starts with a tank of liquid ceramic slurry. The mixture also contains polymers and a chemical that causes those polymers to solidify when exposed to UV light. To create a 3D part, UV light is projected through the tank's transparent bottom in a specific pattern to solidify each layer. Then the part is lifted slightly to allow a new layer of liquid slurry to flow underneath and solidify. This method allows for finer details, but it uses more expensive raw material. The polymers inside the ceramic must also be removed before the part is finished, which can take several days.

"Understanding how the slurry flows is key to getting a good final result for both of these methods," said Maier. "Sometimes you want the slurry to flow easily, and other times you want it to keep its shape."

Scientists measure these non-Newtonian properties using a machine called a rheometer. In a typical test, a small amount of ceramic paste is placed between two plates. The rheometer gently squeezes the sample, then rotates one plate while measuring how much force is needed to make the paste flow. Anyone using a ceramic printer will need to use a rheometer regularly to check the quality of their raw materials. But it's a difficult measurement to get right.

Many of these ceramic slurries are water-based. They tend to dry out over time, which changes their properties. Another challenge is that these fluids have a kind of memory.

Just as a memory foam mattress takes a few minutes to return to its original shape, these materials take time to go back to their original viscosity. Simply touching the wet ceramic to put it into the machine can dramatically change its viscosity, ruining the measurement. The ceramic can also act as its own lubricant, causing the plates to slide across the sample rather than smooshing it. This makes the measurement less reliable.

With all these tricky details, it can be difficult for ceramic printer users to know whether their rheometer is accurate.

A Ceramic Slurry

Getting rheometer measurements right is the perfect job for a reference material. A reference material is a special sample that comes with a list of known properties. By making measurements on the material and comparing them with NIST's, scientists can make sure their results are accurate.

Maier and his fellow NIST researchers Ran Tao and Samuel Hales are creating a slurry of tiny zirconia particles floating in a liquid polymer - similar to wet clay. The slurry reliably yields the same rheometer measurements over and over again. To prevent it from drying out, the sample uses polyethylene glycol - the same stuff lip balm is made of - which won't evaporate.

The material comes with instructions for resetting its memory: First, twist it rapidly until it becomes as runny as possible, then let it relax completely. This helps ensure that every measurement is the same.

Maier's work could one day help you recover from a serious injury. Moving beyond dental applications, new medical implants are being developed that have the same shape, microstructure, material and performance as bone. This type of implant knits together with living tissue more easily than titanium implants do, potentially leading to an easier recovery for the patient.

Now that Maier and team have created this material, the next step is to test it by sending it to other research labs around the country to make their own rheometer measurements and see if everyone gets the same answer.

After the results are in, the ceramic slurry has taken its first steps toward becoming a fully-fledged standard reference material.

Maybe Maier will celebrate with some fries and ketchup.