How much silicone for a 12 × 8 in tray mold?
You need about 6.5 pounds (105 ounces) of silicone rubber to mold a 12 × 8 in tray master that stands 1 inch tall and displaces 20 fl oz of water, with 1/2 inch walls — 52.3 ounces each of Part A and Part B at a specific gravity of 1.18.
How to work it out
- Box 13 × 9 × 1.5 in = 176 cu in − model 36.1 cu in = 139 cu in (77.2 fl oz)
- × SG 1.18 + 10% = 105 oz → A 52.3 oz + B 52.3 oz, 4 × 2 lb kits (23.4 oz left over)
A tray is the case where guessing the model volume goes wrong. The master is mostly air: a 12 × 8 in dish, 1 in tall, whose walls and floor displace only about 20 fl oz of water. The silicone fills the dish as well as surrounding it, so the rubber you need is much closer to the whole box than you might expect.
Adjust for your project
More options: waste
- Part A by weight
- 52.3 oz (1,483 g)
- Part B by weight
- 52.3 oz (1,483 g)
- Silicone volume (no waste)
- 77.2 fl oz (2,284 ml)
- Mold box inside (L × W × H)
- 13 × 9 × 1.5 in
- Kits to buy
- 4 kits
- Left over from the last kit
- 23.4 oz (1.46 lb)
- Mix by the ratio on your data sheet. Many silicones must be weighed; some, like Smooth-On’s Mold Star line, are 1A:1B by volume as well as by weight. Parts A and B here are by weight.
Silicone for a 12 × 8 in tray mold, by how much water the tray displaces
| Tray displacement | Silicone to mix | Kits to buy | Left over from the last kit |
|---|---|---|---|
| 12 fl oz | 115 oz | 4 kits | 12.6 oz |
| 16 fl oz | 110 oz | 4 kits | 18 oz |
| 20 fl oz | 105 oz | 4 kits | 23.4 oz |
| 24 fl oz | 99.2 oz | 4 kits | 28.8 oz |
| 28 fl oz | 93.8 oz | 3 kits | 2.24 oz |
What to know about a 12 × 8 in tray mold
Measure the master by displacement, not by its outline. Treated as a solid 12 × 8 × 1 in block, the calculator would subtract 96 cu in and ask for only about 60 oz of rubber — and you would run out partway up the walls. Sink the tray in a tub filled to the brim and catch the overflow, or work out the walls and floor from their thickness; either gets you close to the real displacement.
Set the tray right side up on the base, glued down around its foot. The silicone fills the dish and rises 1/2 in above the rim. Flipped over, that filling becomes a raised island in the middle of the mold, and the resin you pour later floods the channel around it and levels off as the tray’s flat underside. That is why bought tray molds have a thick block of rubber in the centre.
At 6.5 lb this is four 2 lb trial kits with about 23 oz left over, so it is worth checking the larger unit sizes. Mix it all at once rather than in four separate batches, so the rubber rises evenly — that means a slow product: Mold Star 15 SLOW lists about 50 minutes of pot life, while fast versions such as Mold Star 16 FAST give about 6 minutes, far too little for a pour this size. Degassing is optional for most Mold Star work, according to Smooth-On; a slow, thin pour into one corner does most of the job.
Once it has cured, the resin to fill the new mold is about the tray’s own displacement plus a little waste — see how much resin for a 12×8 tray mold. Smaller first project? Try how much silicone for a 4 inch coaster mold.
Different size? Use the full Silicone Mold Calculator.
What you’ll need
- Silicone mold-making rubberMid-range price
Two-part liquid silicone for pouring block molds. Check the mix ratio (often 1:1 or 10:1 by weight) and specific gravity on the TDS.
See options on Amazon (opens in a new tab) - Graduated mixing bucketsBudget price
Marked buckets make accurate Part A / Part B ratios easy — mismeasured ratios are a leading cause of soft, sticky cures.
See options on Amazon (opens in a new tab) - Digital scale (0.1 g)Budget price
For mixing by weight. Far more accurate than cup lines for small batches — and it doubles for the water method on molds.
See options on Amazon (opens in a new tab)
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Frequently asked questions
How much silicone for a 12x8 tray mold if I treat the tray as a solid block?
The calculator would say about 59.6 ounces — but that is wrong for a hollow tray, because the rubber fills the dish too. Measure the tray’s displacement instead.
How much silicone for a 12x8 tray mold with 3/4 inch walls?
About 141 ounces in a 13.5 × 9.5 × 1.75 in box — 5 kits of 2 lb.
How much silicone for a 12x8 tray mold with 3/8 inch walls?
About 88 ounces. A tray mold this thin flexes a lot, so support it on a board when you fill it with resin.
Other common sizes
Sources
- Smooth-On — How much mold rubber do I need? (opens in a new tab) — Container volume minus model volume; the water method; convert volume to weight with the specific volume (cu in per lb) on the technical bulletin
- Smooth-On — Mold Star 15 SLOW product page (opens in a new tab) — 1A:1B by volume or weight; specific gravity 1.18 (23.5 cu in/lb); 50 min pot life, 4 h cure; sulfur clays, latex and fresh epoxy can inhibit the cure
- Smooth-On — How to make a silicone mold using Mold Star 15 (opens in a new tab) — Glue the model and mold box to a base; pour about 1/2 in (1.25 cm) over the model’s high point