Frequently Asked Questions

General FAQs

What is the purpose of the primary slurry?
The primary slurry has two main functions: to capture the detail of the wax pattern and to provide the surface that the alloy will be cast against.

What is the purpose of a backup slurry?
The backup slurry is used to create the bulk of the shell. It will provide the strength for the shell to survive the dewax and casting process. The backup shell coats will also determine the dimensional and heat transfer properties of the shell.

What is the recommended primary slurry formulation?
The primary slurry formulation is chosen based on the parts being cast, the alloy used and the process. A good starting point can be found on R&R primary binder application instructions or by contacting Ceramic Shell Technical Support.

How many primary coats will I need to use?
This depends on the slurry used, the surface requirements and the parts being dipped. In general, primaries containing high percentages of zircon are used for ferrous and other reactive alloys. One or two coats would be required with zircon stucco. For non-ferrous applications, slurries with lesser amounts or even no zircon could be used. The number of primary coats required is typically two; however, parts with a lot of detail and with critical geometry may require more primary coats to fill tight areas.

What refractory should I use?
This depends on the metal being cast. When casting a ferrous metal, a zircon slurry can be used for primary coats. In some cases, fused silica can be blended in as well. For non-ferrous metals, fused silica can be used in primary slurries, although even in these cases some zircon may be blended in as the density of the zircon will help the flow properties of the slurry. For backups, the typical choices are fused silica (Ranco-Sil™ fused silica) and aluminosilicates (Mulgrain 47, 60, 70).

What is the advantage of using fused silica as a backup refractory in my shell?
Fused silica has a low density (about 2.2 g/ml) and pound for pound will go a longer way. Shells made with fused silica are lighter. Fused silica is very rigid at casting temperatures and is not prone to shell bulging during casting. Fused silica is also much easier to knock out as it goes through a volume/phase transformation during the casting process.

What is the advantage to using aluminosilicates as a backup refractory in my shell?
The primary advantage of using aluminosilicates is that it is very abundant and is more economical than fused silica.

What is the benefit of having a two slurry system instead of using one type of slurry through the entire process?
The benefit of a two slurry system is that you can formulate each slurry to optimize the process of shell building. The goal of the primary slurry is to capture the detail of the pattern and provide a protective layer that the metal can be cast against with minimal reaction. Backup slurries are formulated to provide bulk and strength to the shell, typically consist of larger refractory particles and are formulated to have a lower viscosity.

Shell Building Process FAQs

What is the recommended process for shell coats (number of primaries, number of backups)?
This depends on many factors (part geometry, wax, shell system, etc.), so to give one answer would not be completely accurate. A typical process would be two primaries with four or more backup coats and a seal. The goal is to create a shell that will withstand the pressures that are applied during the dewax and casting process.

How much time should I allow for drying between coats?
There are four factors that impact the shell drying time. First is part geometry. A simple part will dry faster than a part with holes and passageways in any similar conditions. Next, the shell system used will have an impact. Some shell systems apply thinner layers that can be dried quicker. The last two factors are related to the drying environment, more specifically relative humidity and airflow. Lower humidity and increased airflow will speed drying. From a drying standpoint, the most bang for the buck is from increasing airflow.

How do I know when a shell is dry enough for another coat?
There are many ways to determine shell dryness. Some methods are more experience-based, like scratching the area and seeing if it is dry (a wet shell will be softer when scratched with a rigid object). There are drying indicators, like R&R's ReDip™ indicator that is one color when it is wet and one color when dry. Another method is to weigh the shell; once the shell stops losing weight, it is dry. Some foundries use a laser pyrometer to tell if the shells are dry. In this case, as the shells reach ambient temperature, then they are dry enough to coat.

What is the shelf life of my slurry?
This depends greatly on slurry maintenance and turnover. There is no set rule for shelf life. For primes, slurry life in “controlled” slurries typically ranges from three months to one year. With backup slurries, if turnover is adequate, the slurries will last indefinitely. The key to slurry life is a good slurry control program. For complete details on required tests and specifications for slurry parameters, refer to our Slurry Control Technical Tip.

What is the recommended process for burnout cycle?
A minimum burnout at 1600-1800°F for 1 hour after all organics have been burned out is suggested. Typically burnout for shells that have been dewaxed in an autoclave is longer than those who use a FlashFire Dewax Oven.

Slurry Testing FAQs

What is slurry control and why do I need to do it?
Slurry control is the process of running a series of tests on the slurry and comparing results to the specifications. Once this is done, action can be taken and additions will likely be required to bring the slurry parameters back into specifications. By keeping a slurry in control (i.e. in specifications) the slurry will perform at its best and will have the maximum slurry life.

How do I conduct the tests required to control a slurry?
There are many tests that can be run to control a slurry. Each of these tests is described in detail in our Slurry Control Technical Tip.

How do I correct my slurries once testing indicates they are out of specification?
The adjustments to bring a slurry back into control are easily calculated. For your convenience, automated Slurry Control Worksheets (Microsoft® Excel®) are available for download to calculate slurry parameters as well as slurry additions. These are used in conjunction with our Slurry Control Technical Tip.

How do I handle pH drift in my slurry?
If your pH is low, the addition of reagent grade ammonium hydroxide can be added at 0.5-1.0% by weight (150 ml/gallon of water). An alternative to that is using Triethanolamine (TEA) at 80 ml/gallon. This dilute solution should be used when the slurry requires normal water additions and the use of this dilute solution should begin when the slurry binder pH reaches 9.50 or below.

What does it mean if your pH is low?
It means that the colloidal is aging and becoming unstable. The colloidal is stabilized by -OH ions that repel the colloidal particles. Think of the pH as a measure of the OH ions in the binder. As the -OH ions decrease, so does the stability of the colloidal. This is a natural occurrence and it cannot be avoided, however, it can be slowed through proper slurry control and by making pH adjustments.

What does it mean if your binder solids test high?
This means that the silica particles within the binder are at too high of a concentration. When the concentration level of the colloidal particles increases beyond the point that the particles can repel each other, then the particles start to condense or agglomerate. This destabilization of the colloidal is known as “gelling”. This will affect the performance of the slurry and the quality of the shells that are produced with the slurry. Once a slurry begins to gel, it cannot be reversed and slurry life and performance will be diminished. For complete details on how to test for binder solids, see our Slurry Control Technical Tip.

What does it mean if your binder solids test low?
When the binder solids are too low, the colloidal within the slurry has been diluted too much and, while this will not affect life, it will impact the strength of the shells that are produced. There will be fewer colloidal bonds in the shell layers and the strength of the shell will be reduced.

What does it mean if your refractory solids are high? Low?
High refractory solids can make the shell brittle, causing hot tear defects and non-fill. Low refractory solids result in weak shells, shell leaks, run outs and bulge.

How do I measure the viscosity of a slurry?
The viscosity is measured using a flow cup typically known as a Zahn cup. There are various manufacturers of Zahn cups and each can provide different results, so be sure you always use the same cup type for consistency. Cups are designated by the size of the hole at the bottom of the flow cup. The smaller the number, the smaller the hole (a #2 cup has a smaller hole than a #4 cup and for the same slurry the #2 will give a higher reading). Viscosity is determined by the amount of time it takes for all of the slurry to exit the cup. A Zahn cup should be inserted into the slurry, filled and emptied to “wet out” the cup. The cup is reinserted into the slurry and filled. As the cup exits the slurry, a stopwatch is started. When the cup is empty, the stop watch is stopped and the viscosity is the time on the stop watch.

How do I know when to stop the stop watch when checking viscosity?
The endpoint of a viscosity test can be subjective. Sometimes there is a distinct endpoint when slurry no longer flows. Other times the endpoint is determined to be when the exit stream of slurry begins to break into droplets at 1 inch below the cup; and yet other times the endpoint is determined to be when the hole in the bottom of the cup is visible as the slurry exits when looking into the cup from the top. Any of these methods is acceptable as long as the method is performed consistently time and time again by all operators on all shifts.

Where can I get a viscosity cup?
R&R recommends the Zahn Signature series of cups. These cups are available by contacting R&R Customer Service.

Shell Testing FAQs

What is MOR? Why is it important?
Modulus of Rupture (MOR) is for determining shell strength at green, hot and post fired states. Wax test bars are dipped using a determined shell sequence, then tested in an MOR unit. Shell strength is normally recorded as MOR because the amount of load strength to break a test specimen is recorded per area of the specimen at the break site. Theoretically, no matter how thick or wide the test bars may be, the MOR strength of identical ceramic material should remain the same. This data is important because it gives a glimpse as to how the shell system is going to perform through the process.

What is shell permeability? What does it tell you about your shell system?
Shell permeability is the measurement of a shell’s ability to allow fluids to pass through the material. The higher the reading, the more permeable your shell. Testing shell permeability is important as air trapped in the shell cavity during casting can lead to gas and non-fill defects.

Does R&R test MOR and permeability?
Yes, we test both MOR and permeability.