Precision Casting Process: Ceramic Slurry FAQs
Welcome to part three of our series of frequently asked questions on the precision casting process. This article is on the ceramic slurry stage, where wax patterns from part two are further prepped for metal pouring. Let's get into it!
Wax patterns being dipped into a ceramic slurry during the casting process.
What Happens in this Stage?
The ceramic slurry is the third major step of the precision casting process. During this step, our wax patterns and their trees are dipped in a ceramic slurry and blanketed in a sand/stucco coating. This process happens several times to ensure adequate coverage and a strong outer coating for metal pouring at a later stage.
You can analogize this process to dredging when frying foods. Think of the wax patterns from our previous step as a piece of chicken. To make an on-point piece of fried chicken, you'll repeatedly dip/dredge the chicken in a mixture of a wash (egg wash or buttermilk) and breading (cornmeal, flour, panko, corn flakes, etc.) Depending on how crispy you want your chicken, you may repeat this process several times. Similarly, wax patterns will be dipped in a wet ceramic slurry and coated in a dry sand/stucco material several times to ensure a solid outer layer for the eventual metal pouring.
What is the Purpose of the Ceramic Slurry?
As you can imagine, if we introduced our wax patterns to molten metal, we'd only end up with a melted mess. The ceramic slurry enables foundry workers to create a negative 1:1 replica of your part that is strong and stable enough to house superheated molten metal. The wax patterns function as the blueprint, and the ceramic shell serves as the mold for metal pouring.
How Many Times Are Parts Dipped?
In the casting process, the number of times a pattern is dipped in ceramic slurry depends on the desired thickness of the ceramic shell and the specific requirements of the casting. Generally, larger format castings are dipped more times than small castings. On average, parts are dipped 5-9 times.
During onboarding, foundry workers will test and optimize the process specifically for your part, ensuring that the coating is strong enough for metal pouring while eliminating as much wasted material (and time) as possible.
How Long Does the Slurry Take?
The slurry process in investment casting typically takes 1 to 3 days, depending on the number of layers, introduced automation, and ambient drying conditions. For example, IPC Foundry Group has introduced robotic dipping, temperature control, and an automated drying carousel that keeps production moving 24/7 and significantly reduces our turnaround times.
On average, each layer requires 2 to 4 hours of drying time, and the final drying/curing can take an additional 12 to 24 hours.
Precision Casting Process in Action
We've covered the basics; let's walk through the slurry process with a few pictures from inside our foundry.
Robotics dip a wax pattern into a ceramic slurry bath. This image shows trees that have already been dipped several times.
Step One
Wax patterns are dunked into a tank of ceramic slurry by robotics. Our slurry tanks are constantly agitated, keeping the slurry consistent throughout.
Robotics place freshly dipped wax patterns into our rainfall sander.
Step 2
Immediately after the slurry dip, patterns are lifted into our rainfall sander and coated with a stucco-like material. When combined, the slurry and sand fall create the necessary ceramic layers for future metal pouring.
Robotics add freshly dipped/sanded parts back onto the conveyor and prepare for another dip.
Step 3
Our robotics then attach the recently dipped and sanded pattern onto our conveyor system and collect the next part in line. Each finished dip travels through our climate-controlled conveyor system so it can dry for any subsequent dips.
Numerous parts drying on our conveyor/carousel track between slurry dips
Step 4
Parts will dry for a few hours between dips. At IPC, we have installed a carousel system to facilitate larger production capacities. This system enables production cycles to move around the clock with zero downtime.
After several slurry dips, parts will cure and dry in preparation for the next step in the precision casting process.
Step 5
After several passes and a final drying/curing cycle, the ceramic shells are complete and are ready for the next steps.
Why is it called Investment Casting?
The name "investment casting" alludes to this step of the process. The word "investment" comes from the Latin word "investire," which means "to surround" or "to cover."
In investment casting, the wax pattern is surrounded (or invested) by a ceramic slurry and sand/stucco to create the mold. This process of enveloping the pattern gave rise to the name. Other metalworking methods, like machining or sand casting, don't use a slurry.
More FAQs About the Slurry Process
What is the Ceramic Slurry Made of?
The ceramic slurry in precision casting is made of refractory fillers (such as silica, alumina, and zircon), binders (like colloidal silica or ethyl silicate), and various additives to enhance its properties. The exact composition can vary depending on the requirements of each part. However, the goal is always to create a strong, heat-resistant shell that can accurately replicate the pattern while withstanding the high temperatures of molten metal.
What is the Purpose of Adding the Sand/Stucco After Each Dip?
Adding the sand to the slurry coating creates a texture that allows for better adhesion of subsequent layers. It also adds to the structural integrity of the ceramic shells to prevent cracking or other failures during metal pouring.
What are Common Failures During this Step?
Most failures during the slurry process stem from improper drying or an incorrect ceramic mixture. Either of these inadequacies can lead to cracking, sloughed or bulged surface patterns, or metal penetration during pouring. Quality foundries will have QC checks throughout the casting process to catch failures and ensure top-notch parts.
What Happens Next?
After the ceramic shells cure, they are ready for the next step of the process, Dewaxing/Burnout. During this step, wax patterns are melted away, leaving behind hollow ceramic shells for metal pouring. We'll cover FAQs on this step of the process in an upcoming blog!
Conclusion:
The slurry step of the precision casting process is when wax patterns are coated in several layers of ceramic slurry to create a hard outer shell for metal pouring. Success during this step helps ensure dimensional accuracy and controlled metal pouring & cooling in later steps of the casting process.
If you'd like to learn more about the casting process, tour one of our foundries in person, or see how IPC can help save you time and money on your precision casting project, contact us today!
