Silicone mold calculator
Silicone to pour your own mold around any master, the box, minus what the object displaces, weighed out into part A and part B, and costed against what buying the mold instead would come to.
INPUTS
Fills the master with a cavity size from the mold chart, so the make-or-buy figures below start from the same
shape rather than two different guesses.
Laid in a row, one wall apart. Several at once is where the saving is.
Counters, bar tops, boards, trays, panels. Measure the inside of the form, not the outside.
mm
mm
mm
The box is this plus one wall over the top.
mm
About 12 mm of silicone on every side and over the top is the working minimum. Thinner and the mold tears where you flex it hardest, usually on the third or fourth casting. Thicker costs more silicone and stiffens the mold, which makes release harder rather than easier.
Mixed one to one, so it is the forgiving one to measure and the one to start with. It shrinks almost not at all and stays clear. It is also inhibited: latex, sulphur clay, tin-cure silicone and some tapes will stop it curing where they touch it, and the failure is a permanently sticky patch on the master.
Platinum-cure silicone is inhibited by latex, sulphur-bearing modelling clay, tin-cure silicone and some tapes,
where it touches one of those it never cures, and the result is a permanently sticky patch that has to be cut
away. Test a coin-sized blob against an unfamiliar master before you commit a whole box of it.
RESULTS
Silicone to mix
238 ml
262 g 0.24 L $7.34 of silicone
Weigh out
| Part | By weight | Share |
|---|---|---|
| Part A / base | 131.0 g | 50.0% |
| Part B / catalyst | 131.0 g | 50.0% |
| Mixed | 262.1 g | 1:1 by weight |
Build the box
124× 124 × 22 mm
The master displaces
100ml
The box is always rectangular, whatever the master is, so a round or awkward object still pays for four corners
of silicone it never uses. That is why several masters in one box is so much cheaper per mold than one at a time
, the walls are shared and only the row gets longer.
Free printable
Print the pour-depth card
Maximum pour depth and cure windows by resin type, the number that cracks a casting. Free.
How this is worked out
A block mold is a box with something standing in it, so the silicone is the box minus the master, the same arithmetic as a river table, read inside out, and it goes through the same volume function. The box is the master's footprint plus 12 mm of wall on each side and one wall over the top, and it is always rectangular: a round master still needs a square box, and those corners are silicone you buy and never use. Several masters share their walls, so they go in one row a wall apart rather than in separate boxes. Millilitres become grams at about 1.1 g/ml, and the split into part A and part B is done on that weight, because mold silicone is catalysed by weight in every case, 1:1 for platinum-cure (addition) and 10:1 for tin-cure (condensation). Nothing about epoxy's volume-versus-weight problem applies here; there is no volume figure to misread.
box = (master + 12 mm wall each side) × (height + one wall)
silicone = box − what the masters displace
grams = ml × 1.1 silicone is roughly water-dense
part B = grams ÷ (ratio + 1) THE RATIO IS BY WEIGHT
Platinum-cure (addition) 1:1 · Tin-cure (condensation) 10:1
one coaster → 238 ml · four in one box → 214 ml each
Questions makers ask
How much silicone do I need to make a mold?
The volume of the box, minus whatever the master displaces. For a single 100 mm coaster standing 10 mm tall in a box with 12 mm of wall on every side, that is about 238 ml, or 262 g, silicone runs close to water-dense at about 1.1 g/ml, so the two figures are near enough the same number. Set your own master's dimensions above and the tool works it out for any shape.
How thick should the silicone walls be?
About 12 mm of silicone on every side and over the top is the working minimum. Thinner and the mold tears where you flex it hardest, usually on the third or fourth casting. Thicker costs more silicone and stiffens the mold, which makes release harder rather than easier.
Is it cheaper to make your own molds or buy them?
Buy, almost always, for a shape somebody already sells. That single coaster mold costs about $7.34 in silicone at $28 a kilo, before the box, the master and an evening of your time, and a four-cavity coaster mold costs less than that on any marketplace. Pour your own when the shape does not exist: a found object, a carved master, something the size you actually want. That is the case no listing can serve, and it is the whole reason the craft has a mold-making side.
Do I mix mold silicone by weight or by volume?
By weight, always, and that is a real difference from epoxy rather than a preference. Platinum-cure (addition) is 1:1, which is forgiving. Tin-cure (condensation) is 10:1, on the coaster mold above that is 23.8 g of catalyst in 262 g of mix, which is not a quantity anyone can judge by eye. A scale reading to 0.1 g is not optional at that ratio.
What is the difference between tin-cure and platinum-cure silicone?
Mixed one to one, so it is the forgiving one to measure and the one to start with. It shrinks almost not at all and stays clear. It is also inhibited: latex, sulphur clay, tin-cure silicone and some tapes will stop it curing where they touch it, and the failure is a permanently sticky patch on the master. Cheaper, tolerant of almost any master, and mixed ten parts base to one of catalyst, which is a ratio nobody can judge by eye, so a scale reading to 0.1 g is not optional here. It shrinks slightly as it cures and the mold has a shorter life on the shelf than a platinum one.
Why does a round master cost more silicone than its own volume?
Because the box is rectangular whatever the master is. Nobody builds a round pot around a coaster, so the four corners are silicone that gets bought and never used. It is also why several masters in one box is so much cheaper per mold, the walls are shared and only the row gets longer. Four coasters in one box works out about 10% less silicone each than four separate boxes.
How many castings does a silicone mold give?
Around 100 before it goes cloudy and starts losing detail, which is the number to divide the cost by. A mold is a tool rather than an ingredient, <a href="/resin/pricing-calculator">the pricing calculator</a> puts that per-casting figure into what a piece actually costs you.
What can stop silicone curing?
Platinum-cure silicone is inhibited by latex, sulphur-bearing modelling clay, tin-cure silicone and some tapes and adhesives. Where it touches one of those it simply never sets, and the result is a permanently sticky patch that has to be cut out of an otherwise finished mold. Test a coin-sized blob against an unfamiliar master before committing a whole box. Tin-cure is far less fussy, which is most of its appeal.
How much resin will the finished mold hold?
The master's own volume, 100 ml for that coaster, which is what <a href="/resin/epoxy-calculator">the epoxy calculator</a> sizes once the mold exists. <a href="/resin/mold-chart">The mold chart</a> lists the same figure for the molds you can buy instead.
What a mold costs to pour
Four masters people actually make molds of, each in a box with 12 mm of wall, at $28 a kilo. Read the last column against what the equivalent bought mold costs before you start, the answer is usually to buy, and the exception is the shape nobody sells.
| What you are molding | Box | Silicone | By weight | Cost |
|---|---|---|---|---|
| One 100 mm coaster | 124 × 124 × 22 mm | 238 ml | 262 g | $7.34 |
| Four coasters in one box | 460 × 124 × 22 mm | 855 ml | 940 g | $26.33 |
| A pendant or charm | 64 × 54 × 15 mm | 48 ml | 53 g | $1.49 |
| A deep casting block | 174 × 124 × 72 mm | 653 ml | 719 g | $20.13 |
The coaster rows are the argument in one line: four in a shared box come to $6.58 of silicone each against $7.34 on its own. Walls are most of what you buy, and walls are the part that can be shared. A bought four-cavity mold at Shore 30A, medium, the usual compromise, still costs less than either.
Read before your next batch
- How to make resin art Buy the right resin for the depth you are pouring, weigh both parts rather than eyeballing them, and leave it alone once it is poured. Almost every first-batch failure is one of those three.
- UV resin One part, no hardener, no measuring, and set in about a minute. It is a genuinely different product from two-part epoxy, and its limits are not the ones you would guess.
- Resin vs epoxy: which type to buy Four products share the word, and the one thing that separates them is how deep you can pour them. Pick by that, and everything else follows.
- Deep pour vs table top epoxy Two clear liquids that look identical in the bottle, with maximum pour depths about ten times apart. Going past the wrong one is a heat problem, not a patience problem.