How to make silicone molds
The half of the mold question a marketplace cannot answer. If you can cut your own, you are no longer limited to shapes somebody else decided to sell.
Build a box around the master with about 12 mm of clearance on every side, mix two-part pourable silicone by weight, pour it in one thin stream from a height so it stretches and sheds bubbles, and demold after the manufacturer's stated cure. A block mold takes roughly the volume of the box minus the master — for a single coaster that is about 238 ml. For small shapes, putty silicone is faster, cheaper and needs no box at all.
There are two mold questions and they have almost nothing to do with each other. One is which mold to buy, which is a shopping problem. The other is how to make one, which is the point at which the craft stops being limited by what somebody else decided to stock — and it is why this cluster is nearly as large as the buying one.
The arithmetic is the same as a river table's, in a different hat: a box with something in it, and the silicone is the box minus the master. Around a single coaster with 12 mm of wall it comes to about 238 ml — $7.34 of silicone against a bought four-cavity mold that costs less. That comparison is the honest core of this page: making molds is worth it for shapes nobody sells, and rarely worth it for shapes everybody does.
What it buys you is originality and repeatability together. Cast a master once — in clay, in wood, in 3D print, or take a found object — and you can make it fifty times. The buying guide covers the other half, and the calculator sizes what goes in whichever mold you end up with.
What a mold costs to make
A poured block mold with 12 mm of silicone on every side and over the top — thinner walls tear on release. Silicone at $28 a kilo, and it is roughly water-dense, so millilitres and grams are close. Compare each figure against what the equivalent bought mold costs before you start.
The two kinds of mold-making silicone
Two liquids, mixed and poured around a master in a box. It captures detail properly, it self-levels into every undercut, and it makes a mold you can use fifty times. It also needs a box, a release agent on porous masters, and enough silicone to fill the whole thing — which is the cost line in the table above.
Two coloured putties kneaded together and pressed around the master by hand. No box, no measuring by weight, no waiting overnight — a pendant mold is done in ten minutes. It picks up less fine detail and it will not do undercuts, and for 3 mm charms that is usually a fair trade.
Soft — easy release, flexes under weight at the soft end and firm — holds edges, fights on release at the firm. Soft silicone releases undercuts easily and bulges under the weight of a deep cast; firm silicone holds a crisp edge and fights you. Small detailed work wants soft, deep or flat-sided work wants firm, and most general-purpose mold silicone sits around 30A because it is the compromise.
What decides whether the mold is any good
Silicone captures everything, including the sanding marks and the layer lines on a 3D print. Whatever the master looks like is what every casting will look like for the life of the mold, so the polishing happens on the master and not on fifty castings afterwards. This is the same rule as buying: a matt mold gives matt castings.
Tin-cure silicone is cheaper and shrinks slightly with age. Platinum-cure holds its dimensions and captures finer detail, and it is inhibited — sulphur-containing clay, latex, some tapes and uncured polyester will stop it setting at the contact surface, leaving a permanently sticky face. Test on a scrap if you are unsure what a master is made of.
About 12 mm of silicone on every side and over the top is the working minimum. Thinner and the mold tears at the point 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.
Silicone is thick and holds air, and a vacuum chamber is the professional answer. Without one, the technique that does most of the work is pouring in one thin stream from as high as you can manage into the lowest corner of the box — the stream stretches and sheds bubbles on the way down, and the silicone rises around the master rather than trapping air under it.
Epoxy does not stick to cured silicone, which is why casting needs no release. Silicone does stick to porous masters — wood, plaster, unsealed clay, 3D prints — so a mold release or even a coat of petroleum jelly on the master is worth it. It is the exact opposite of the rule for pouring resin.
A well-treated silicone mold improves for the first few pours and then declines slowly as the surface accumulates micro-scratches and the castings come out less clear. That is the number to divide the cost by: $7.34 over a hundred coasters is nothing, and over three is not.
Making one
Every millimetre of clearance is silicone you buy and never use. 12 mm on the sides and over the top is enough. Lego, foam board taped together, or a plastic food container cut down all work — it only has to survive one pour and be peelable afterwards.
A master that floats up during the pour gives you a mold with a skin of silicone across the cavity floor, which is not recoverable. A blob of hot glue or a piece of double-sided tape under it settles it, and comes off the master afterwards.
Most pourable mold silicone is 1:1 by weight and some is 10:1 — read the tin, and use a scale rather than eyeballing. An out-of-ratio silicone stays tacky forever, which is the same failure mode and the same cause as it is for epoxy.
A thin stream from as high as you can hold the cup, into the lowest corner, letting the level rise around the master. Never pour directly onto the master — it traps air against exactly the surface that matters, and that air becomes a lump on every casting.
Silicone that is nearly cured tears at the thin sections when you demold it and stays slightly tacky against the master. An overnight cure costs nothing and a torn first mold costs the whole pour.
A mold you made yourself has no listing to tell you its capacity, so measure it once: fill the cavity with water and pour it into a jug. A 100 mm coaster cavity holds 79 ml and takes 86 ml of mixed resin with wastage. The calculator takes either number.
Work out what your own mold holds
A mold you made has no listing to tell you its capacity. Measure a cavity with water, put the number in, and the calculator gives what to mix, the resin and hardener split, and whether the depth needs more than one pour.
Open the epoxy calculator →Print the pour-depth card
Maximum pour depth and cure windows by resin type — the number that cracks a casting. Free.
Estimates, not advice. Every figure on this page is computed from published reference values and standard measures, so your own inputs, equipment and conditions will move it. Nothing here is professional advice — talk to a qualified professional before acting on it.