Resin Exotherm Calculator

How It Works

Epoxy and polyester resins cure through an exothermic polymerization — the reaction releases heat as chemical bonds form. In a thin coat, that heat dissipates fast enough into the mold and the air that the resin cures at close to ambient temperature. In a deep pour, heat generated in the interior has nowhere to go, and temperature climbs until the reaction accelerates itself (each additional degree makes the reaction faster, which produces more heat, which raises the temperature further). This runaway is called thermal runaway and it can crack the piece, yellow the resin, produce smoke, or in extreme cases ignite mold materials.

Each resin product has a maximum safe pour depth published on its data sheet. Standard table-top epoxy is typically limited to 6 mm per layer, art resin to 12 mm, slow-cure deep-pour epoxy to 25 mm, fast-cure 5-minute epoxy to only 3 mm, and polyester casting resin to 10 mm. Values here are drawn from the WestSystem Epoxy Technical Manual, ArtResin Deep Pour data sheet, and the Ecopoxy Liquid Plastic FlowCast product data sheet.

The calculator estimates peak temperature from three inputs: pour thickness, ambient temperature, and mold material's heat-sink coefficient. Silicone molds are near-adiabatic (heat trapped inside), wood pulls heat away slowly, and aluminum acts as a strong heat sink. The rough model is:

Peak Temp ≈ Ambient + Base_Exotherm + (Thickness − Safe_Depth) × Exotherm_Coefficient

where the base exotherm is the temperature rise at the safe depth (~10-25 °C above ambient depending on formulation), and the exotherm coefficient adds heat for every millimeter beyond the safe depth. Values above 60 °C are marked cautionary, and above 90 °C the tool flags critical risk of cracking, smoking, or fire.

The safe strategy for deep pours is layered pouring: multiple pours at or below the safe-depth limit, waiting between layers for each layer to reach a firm gel state before the next pour. The tool recommends the minimum number of layers and typical wait time.

How to Use This Calculator

1. Select your resin type — from the dropdown of common formulations (table-top epoxy, art resin, slow-cure deep-pour, fast-cure 5-minute, polyester casting).
2. Enter total pour volume — in milliliters, and the intended pour thickness in millimeters.
3. Set the ambient workshop temperature — the tool uses this as the exotherm baseline.
4. Choose mold material — silicone, wood, aluminum, or a custom heat-sink coefficient.
5. Review the estimated peak temperature and safety level indicator — green under safe depth, amber for cautionary depths, red for critical.
6. Follow the recommended layer count and wait times — the tool computes the minimum number of pours and estimated time between them.

Real-World Use Cases

Deep pour for a river table

A woodworker plans a 40 mm slow-cure epoxy pour for a 1200 × 400 mm river-table channel (total volume 19,200 ml). The product's safe depth is 25 mm, so the tool warns that 40 mm exceeds the limit. Estimated peak temp at 40 mm is 62 °C at a 22 °C ambient — cautionary but not critical. The safe strategy is two 20 mm layers with an 8-hour gap.

Small jewelry mold

A jeweler has a 15 mm deep pendant mold and wants to use fast-cure 5-minute epoxy (safe depth 3 mm, exotherm coefficient 8 °C/mm). A single 15 mm pour would drive peak temperature above 100 °C, well into critical territory. The calculator recommends five 3 mm layers, waiting for each to fully cure (~30 minutes at 5-minute product timing) before the next.

Multi-layer geode piece

A resin artist wants a 60 mm thick geode piece in art resin (safe depth 12 mm, exotherm coefficient 2.5 °C/mm). A single 60 mm pour would push peak to over 130 °C — the epoxy would smoke and crack. The tool recommends 5 layers of 12 mm each, waiting 8 hours between pours for each layer to reach a firm gel state. Total elapsed time: 40+ hours.

Tips & Safety Notes

• Layer pouring is the safest way to build depth. Wait for the previous layer to reach a firm gel state (typically 6-12 hours for slow-cure epoxy) before the next pour.
• Cool ambient temperature buys margin. Working at 18-20 °C gives more thermal headroom than a warm garage at 30 °C; hot workshops accelerate the reaction and reduce safe depth.
• Aluminum mold plates pull heat away and can allow slightly deeper single pours than silicone. Wood is intermediate.
• Watch the surface during cure. Rapid smoke, bubbles rising quickly, or the surface visibly hardening in seconds are all early signs of thermal runaway — cool the piece by moving to a colder space or aiming a fan at it.
• A resin fire is a hydrocarbon fire. Do not use water; smother with a wet towel or Class B extinguisher.

Limitations & What This Tool Cannot Do

• Ambient-temperature assumption. Assumes a 20-22 °C workshop; garages at 30 °C+ accelerate cure and reduce safe pour depth further, and cold environments below 15 °C extend cure but may cause improper mixing. The tool exposes ambient as an input, but the coefficient values are calibrated to typical workshop conditions.
• Mold material simplification. Uses average heat-sink coefficients for common materials. A metal mold in contact with a large heat sink (like an aluminum table) pulls heat away faster than the coefficient suggests; a silicone mold on a wooden bench with an air gap traps more heat than modeled.
• Manufacturer variation. Different manufacturers of "the same" product (e.g., generic "table-top epoxy") have different safe depths — one brand's 6 mm limit may be another's 10 mm limit. The tool uses conservative published averages. If your specific product's data sheet is more permissive, trust the manufacturer.

FAQ

What is exotherm in resin curing and why is deep pouring dangerous?

Exotherm is the heat released by the polymerization reaction as resin cures. In a shallow pour that heat escapes into the air and the mold, so the resin cures at close to ambient temperature. In a deep pour, heat accumulates in the interior, temperature climbs, and each degree makes the reaction faster — the thermal runaway that follows can crack, yellow, smoke, or in extreme cases ignite the piece.

What is the maximum safe pour depth for standard casting epoxy?

Standard table-top epoxy is typically 6 mm per layer, art resin about 12 mm, slow-cure deep-pour formulas 25 mm, fast-cure 5-minute epoxy only 3 mm, and polyester casting resin around 10 mm. Always check your specific product's data sheet — the tool uses conservative averages and your product may be more or less permissive.

Can I speed up a cure by adding heat externally?

No. Adding heat accelerates the exotherm and reduces your safe depth. If you need faster cure, use a fast-cure formulation from the start; do not warm a slow-cure formulation in the mold. The one exception is a post-cure at 60 °C for 4-8 hours after the piece is already fully hardened — this improves final properties but is done after the exotherm risk is over.

How does mold material (silicone, wood, aluminum) affect exotherm?

Aluminum is the strongest heat sink and lets heat escape fastest. Wood is intermediate. Silicone is essentially insulating — heat generated in a silicone mold has almost no path out except through the top surface. Silicone molds see the highest peak temperatures for the same pour depth.

What is thermal runaway and how do I recognize it before damage occurs?

Thermal runaway is the self-accelerating cascade where heat from the reaction raises the temperature, which speeds up the reaction, which produces more heat. Early signs: visible steam or smoke rising from the piece, the surface hardening in seconds instead of minutes, bubbles rising quickly and popping, and the piece feeling hot to a nearby thermometer. If you see these, immediately move the piece to a colder space and increase airflow.

Should I do multiple thin pours or one big pour for a deep casting?

For anything approaching or exceeding the manufacturer's safe depth, always do multiple thin pours. A single thick pour that exceeds the safe depth almost always cracks, yellows, or smokes. Multiple thin pours take more elapsed time (6-24 hours between layers depending on product) but produce a clear, defect-free piece.

Related Tools

• Epoxy Resin Ratio Calculator — Compute exact Part A / Part B weights for the same pour
• Varnish Dilution Calculator — Adjacent solvent and product chemistry
• Wax-to-Metal Casting Calculator — Adjacent mold-and-pour work in a different material system

References

• WestSystem Epoxy Technical Manual, "Mixing and Curing" section — safe pour depth guidelines
• ArtResin Deep Pour technical data (artresin.com) — layered-pour recommendations
• Ecopoxy Liquid Plastic FlowCast product data sheet — deep-pour safe depth for 25 mm formulations

Reviewed by the Craft Calc Lab team on August 1, 2026.