Why Resin Molds Often Short-Pour:
Specific Gravity & Density Explained
Published on March 28, 2026 • By MakerRatio Studio Team • Tested in Workshop
Studio Comparison: A short-poured sunken coaster (left) vs a chemically balanced 1.12 density casting (right).
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My workshop table was covered in sticky, ruined resin coasters.
It was 2:00 AM on a Tuesday, and I had just short-poured my third coaster mold set in a single week. The edges were bowed downward, the surfaces had a 2mm concave lip, and I had effectively thrown $240 worth of premium casting epoxy straight into the scrap bin.
If you’ve poured resin, Jesmonite, or craft concrete for more than a month, you know this exact heartbreak. You carefully measure your silicone mold cavity, calculate 500ml of volume, weigh out 500 grams of liquid on your digital scale... and yet, when the casting cures, the top edge is noticeably sunken.
I spent three frustrating weeks trying to figure out where the missing volume went. Did the resin evaporate? Did the silicone expand?
Then I talked to an industrial materials chemist, and she told me something that changed my entire workshop process: "Water math is lying to you."
The Hidden "Density Deficit" (Why 1ml Does NOT Equal 1g)
When crafters estimate resin volume, almost everyone relies on the standard universal rule: 1 milliliter = 1 gram.
That rule works perfectly if you are pouring pure tap water at room temperature. Water has a specific gravity (density) of exactly 1.00 g/ml.
However, liquid epoxy resins, hardeners, silicone rubber, and Jesmonite composites are significantly heavier and denser than water:
| Material Type | Specific Gravity (Density) | 500ml Mold = Real Weight Needed | Hidden Deficit |
|---|---|---|---|
| Pure Tap Water | 1.00 g/ml | 500.0 grams | 0g (Baseline) |
| Tabletop Epoxy Resin | 1.12 g/ml | 560.0 grams | -60.0g (Short Pour!) |
| Deep Pour River Table Epoxy | 1.08 g/ml | 540.0 grams | -40.0g |
| Jesmonite AC100 Composite | 1.75 g/ml | 875.0 grams | -375.0g |
| Craft Mortar / Concrete | 2.10 g/ml | 1,050.0 grams | -550.0g |
Look at that table again.
If you pour a 500ml mold using standard epoxy resin (1.12 g/ml) and mix 500 grams by weight on your scale, you are short by 60 full grams of material. That 60-gram deficit is the exact reason your edges collapse and your surfaces curve inward as the epoxy cures.
The 3-Second Density Formula
To get a crystal-clear, perfectly flat casting every single time, you must correct for your material's specific density and add a 5% safety buffer for container wall cling.
Real-World Studio Test Example:
- Mold Volume: 250 ml (Circular Coaster Mold)
- Resin Brand Density: 1.10 g/ml (ArtResin Preset)
- Math: 250 × 1.10 = 275g × 1.05 (wall buffer) = 288.7 grams total mixed weight.
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3 Workshop Rules to End Short Pours Forever
1. Never Trust Printed Graduation Marks on Plastic Cups
Printed volume lines on cheap plastic mixing cups can be off by up to 10%. A 5ml error in Part B hardener can leave your casting rubbery or tacky forever. Always weigh your Part A and Part B on a 0.01g digital scale.
2. Overfill by 1mm (The Meniscus Rule)
Liquid surface tension pulls epoxy slightly up silicone walls. Overfill by 1mm past the rim to compensate for thermal shrinkage as the exothermic reaction cools.
3. Use Brand-Specific Density Profiles
Deep-pour resins are thinner (lower density ~1.08 g/ml), while quick-cure dome resins are thick (higher density ~1.14 g/ml). Always check density specs before pouring large batches.
Stop guessing your resin weights. Calculate your exact recipe before you crack open your next bottle of hardener!