Working with a Castable Resin: The Optimal Burnout Cycle
The Optimal Burnout Cycle for Castable Resin in Precision Jewelry Casting
Burnout is a critical phase in the investment casting (lost-wax casting) process used to produce metal jewelry from a wax or castable resin model. Essentially, a burnout cycle suitable for castable resins can also be applied to wax, although there are important differences between processing castable resin and wax resin.
Achieving an effective burnout requires precise control of both temperature and timing to ensure the complete elimination of the resin without damaging the mold. Of course, a good result starts from the resin post-printing process (you can read more about castable resin post-processing here [relevant internal link]) and from perfect investment preparation (you can read more about investment preparation for resin for jewelry casting here [relevant internal link]).
This post explores the key phases involved in castable resin burnout, shedding light on the essential changes that take place throughout the process when using resin for jewelry casting. The information provided here refers specifically to BlueCast resins, which offer high performance and cannot be compared to other products in the realm of castable resins.
What an Effective Burnout Should Achieve for Resin in Jewelry Casting:
- Optimize and reinforce the investment to resist the thermal and mechanical stress of molten metal flow during jewelry casting.
- Completely remove wax/resin for jewelry casting without leaving ash or residue.
- Prevent casting defects such as porosity, cracks, or inclusions when using models made from castable resin or wax resin.
Throughout all burnout stages for castable resins, proper ventilation is essential to evacuate volatile organic compounds (VOCs) and prevent mold contamination.
It's important to note that the set temperatures do not always correspond to the actual temperatures inside the oven. Therefore, the use of probes/thermocouples is recommended to accurately determine these differences and set the burnout cycle temperatures based on real, not assumed, data for your chosen castable resin. Also, different zones within the oven have varying temperatures. The front and lower parts tend to be colder.
The Casting Burnout steps
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Preheat – Wax Removal (Less Relevant for Castable Resin):
Temperature Range: ~110–140°C
This phase, typical of the casting process for wax trees, is also common when using resins. The purpose of this step is to remove residual moisture from the investment, making it harder, and to prepare the flask/mold for the higher temperatures that follow in the castable resin burnout cycle. During this stage, any water present in the investment gradually evaporates.
It is mistakenly believed that castable resin behaves like wax. Normally, wax becomes liquid at these temperatures, begins to flow out of the mold, and is partially absorbed by the investment, whereas resin for jewelry casting remains mostly inert and only partially begins to transform into liquid and gaseous byproducts.
This phase is particularly critical for ensuring the integrity of the mold when processing castable resin. The investment must reach full mechanical strength and thermal maturity, while temperature ramping must be carefully controlled to prevent the resin from undergoing premature phase transitions or initiating degradation reactions before the investment structure is fully stabilized and resistant.
Heating must proceed slowly to avoid thermal shock, which could cause the mold to crack during the burnout of the castable resin pattern.
Core Component: Rapid thermal ramping may induce the violent expansion of residual moisture or initiate premature thermochemical degradation of the castable resin, occurring before the investment matrix has achieved sufficient sintering or structural integrity. Such conditions significantly increase the risk of microfractures, mold compromise, and contamination of the casting due to the chemical reaction between the mold and the resin for jewelry casting patterns.
If you want to perform this step with a dewaxing furnace separate from the burnout furnace, it is possible to do so, but it is advisable to set the temperature to a value below 120° Celsius, especially when dealing with certain types of wax resin.
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Pyrolysis Phase: Castable Resin Decomposition for Jewelry Models
Temperature Range: ~150–450°C
What happens in this phase is the castable resin undergoes pyrolysis, breaking down into smaller molecules. It transitions from solid to liquid, and only at higher temperatures to vapor. Carbon-rich byproducts form, especially under low-oxygen conditions during the decomposition of the resin for jewelry casting model.
Core Component: This step is generally discouraged for many castable resins. Certain formulations, particularly when the duration is prolonged, can leave sticky residues fused with the investment or ash, thus necessitating extra cleaning. Nevertheless, burnout-engineered resins—such as those from BlueCast—minimize these problems and can undergo standard wax burnout cycles; consequently, it might be preferable to adopt a two-step burnout process, proceeding directly to phase 3 for optimized resin for jewelry casting.
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Burnout Peak: Carbon Residue Removal from Castable Resin Molds
Temperature Range: ~450–740°C
This is arguably the most critical phase of the entire cycle, in which remaining carbon residues from the castable resin must be fully combusted and removed to ensure a clean cavity for jewelry casting.
It is commonly believed that gypsum-bonded investment cannot withstand temperatures over 720°C. This is a myth. In reality, above 720°C, a chemical reaction occurs:
2CaSO4→2CaO+2SO2+O2
This reaction produces oxygen (which helps convert carbon residues to CO₂) and sulfur dioxide (SO₂), a gas that escapes slowly from the mold during the final burnout phase and is almost entirely removed during vacuuming before casting. This is relevant regardless of whether you are using a castable resin or a wax resin pattern.
Keep in mind that pouring molten metal into the flask always generates SO₂, which is the main reason for oxidized surfaces in cast pieces. Therefore, there is no issue running burnout cycles up to 740°C for molds prepared with castable resin patterns.
If your oven lacks proper airflow, it is recommended to flip the flask cone-side up during the final hour at peak temperature to promote oxygen access and assist gas evacuation during the castable resin burnout.
For some noble alloys like platinum, the burnout peak can be even higher. In such cases, always follow the alloy and investment manufacturers' recommendations for your specific resin for jewelry casting process.
Attention: Inadequate burnout at this stage, or the use of entry-level castable resins, may result in inclusions or imperfections in the final casting.
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Cooling Phase After Castable Resin Burnout
Temperature Range: ~450–680°C (variable depending on casting metal)
What Happens: Once the mold reaches its peak burnout temperature, ensuring the cavity is free of contaminants from the burned-out castable resin, it must be brought down to the ideal casting temperature, which is usually lower.
For some precious metals such as platinum, this target temperature can be around 900°C. As noted earlier, always refer to the manufacturer’s guidelines for both investment and alloy when casting jewelry from castable resin models.
Cooling is gradual to avoid thermal shock and prepare the mold for casting. The final step is to stabilize the flask at the desired casting temperature for at least one hour, so the core reaches thermal equilibrium with the furnace after the castable resin burnout cycle.
If multiple casting temperatures are needed, cast high-temp flasks first. Then lower the oven temperature and let the remaining flasks sit for at least 30 minutes before casting at lower temperatures.
Core Component: Some metals require highly precise hold temperatures and times to achieve optimal casting results after the burnout of a castable resin pattern.
Final Notes on Working with Castable Resin for Jewelry Casting:
The practices described in this article are only applicable to BlueCast resins and other high-quality castable resins. Hobby-grade, low-cost resins typically require much longer burnout cycles and higher temperatures, often with inconsistent results in jewelry casting.
- Use High-Quality Investment Powders: Always use reliable investment materials like Optima Prestige, Plasticast, or X-Vest for best results with castable resins. Check out the blog article: The Ultimate Guide to Casting X-One Castable Resin for tips on how to properly prepare investment.
- Carefully Plan Your Burnout Cycle for Each Resin Type: Each resin for jewelry casting and investment type has its own optimal heating curve. Controlled stepwise heating prevents mold damage and ensures thorough burnout. The cycle described in this article is optimized for BlueCast resins.
- Ensure Adequate Ventilation: Proper airflow in the kiln helps safely evacuate vapors and VOCs produced during the burnout of castable resins, keeping the mold clean and uncontaminated for successful jewelry casting.
- Use Resins Designed for Investment Casting: Choose resins specifically formulated for clean burnout; they burn with less residue and help achieve high-quality casting results when using castable resin in your jewelry casting
