Cast resin and 3D novelty magnets

By Fridge Magnets Editorial

Cast resin magnets are poured into silicone molds and finished with a ferrite or neodymium disc glued to the back. Weight is what decides whether one stays up, because a thick piece hangs its mass away from the door and levers the disc off. Epoxy domes yellow in direct sunlight.

Weight is the whole problem

A resin croissant the size of a matchbox weighs about 30 g. The disc glued to its back is usually 20 by 3 mm ferrite, which holds 500 g on a bare steel plate and about 350 g on a painted door. On paper that is more than ten times what it needs to carry, and yet these are the magnets that turn up on the kitchen floor.

The gap between the paper figure and the floor is geometry. Pull force is measured straight out from the face of the magnet, with the load acting through the contact patch. A 3D piece does not load its magnet that way. Its mass sits 10 or 20 mm out from the door, so gravity acting on that offset produces a torque that tries to rotate the whole thing about its bottom edge. The top of the disc gets peeled away first, and once a millimeter of air is in there the magnet has already lost most of what it had, since each tenth of a millimeter of gap costs a small magnet roughly 10 percent of its pull.

That is why our rule for a resin piece is to pick a disc whose painted-door figure is at least ten times the weight of the casting. It is a rule of thumb rather than a measurement, and it exists because the published number is measured in the one direction your magnet is not being loaded. Magnet strength explained covers why the two differ.

Disc size against piece weight

Two discs set wide apart beat one big disc for a heavy piece, and not because of the extra pull. Spacing them puts the upper disc further above the center of mass, which shortens the lever the piece has to work with. A single 20 by 3 mm N42 disc at 3,500 g would give you a bigger number and a worse magnet for the job, because all of that strength is concentrated at one point that the piece can pivot around.

Where the disc sits matters as much as its size. A casting with a domed or uneven back holds its disc away from the door, and the recess that fixes this has to be molded in or milled out; glueing a disc onto a curved back and hoping is how a mass-produced magnet ends up holding 100 g instead of 350 g. Run a fingernail across the back of one in a shop. If the disc rocks or the resin around it stands proud, put it down. Larger art pieces hit the same arithmetic from the other end, and decorative and art magnets works through it at those sizes.

How they get cast

Almost every resin magnet you will handle comes out of a silicone mold. The maker sculpts or 3D prints a master, casts a silicone block around it, and then pours a two-part resin into the cavity: polyurethane for opaque pieces, epoxy for anything clear or dome-topped. The two parts go together by weight, get stirred for a minute or two, and then need degassing in a pressure pot or a vacuum chamber, because a bubble that reaches the surface during cure becomes a permanent pinhole.

Color goes in one of two ways. Pigment mixed into the resin before pouring gives a piece that is colored all the way through, which is what you want on anything that will be handled daily. Paint applied after demoulding gives finer detail, particularly on food replicas where the browning on a crust has to be graded, and it wears off at the high points within a couple of years on a fridge door.

Dome-topped magnets are a different build. Underneath the dome there is a printed insert, usually paper or vinyl, mounted on a flat blank. The maker floods epoxy over it and lets surface tension pull the pour into a lens, which is why the trade calls them doming resins: they are formulated to self-level and to hold that curve while curing. The curve magnifies the print, which is the point, and it also collects every scratch that a wiped-down door will give it.

Yellowing, chipping, and how to clean one

Epoxy yellows under ultraviolet light. Some formulations carry UV inhibitors that slow it down, and none of them stop it. A dome-topped magnet on a door that gets afternoon sun through a kitchen window will discolour, and the print underneath will look brown through it before the print itself has faded at all. Nothing reverses this. The only real answer is to keep clear-topped magnets on a door away from direct sunlight and put the pigmented castings in the sunny spot, since color running through the body has far less to lose.

Chipping is the other failure, and it depends on which resin the maker used. Polyurethane has some give and tends to dent rather than shatter. Epoxy is harder and more brittle, so a dome dropped onto a tile floor takes a bite out of its rim, and there is no filling it invisibly. A ferrite disc on the back is brittle too, and cracks across its diameter without necessarily coming apart, which shows up as a magnet that has suddenly and mysteriously halved in strength.

For cleaning, warm water with a drop of dish soap on a soft cloth is the whole method. Do not soak a painted casting and do not use a scouring pad on a dome, since the polish on a dome is a few thousandths of a millimeter of cured surface and an abrasive takes it straight off, leaving a haze that never comes back. Alcohol and solvent cleaners attack the paint on hand-finished pieces. Neither heat nor a warm kitchen troubles the magnet itself: ceramic ferrite works to about 250 C, and even a neodymium disc, which is the fussier of the two, holds up to about 80 C. Magnets in heat covers where that stops being true.

Putting a magnet back on

Resin magnets shed their discs more often than any other type, because the glue joint has to hold against a torque rather than a straight pull, and because the back of a casting is exactly the surface glue likes least. Cured resin is smooth, and it usually carries a film of mold release that came off the silicone.

So the repair starts with abrasion, not adhesive. Scuff the back with 120 grit until the shine has gone across an area wider than the disc, wipe it with isopropyl alcohol, and let it flash off. Then use two-part epoxy, five-minute or slower, buttered thinly onto both faces. Clamp it with a rubber band or a clothes peg and leave it alone for an hour longer than the packet says.

The glues to skip are the two most people reach for. Hot glue holds beautifully for a week and then creeps, and a fridge in a warm kitchen sits at the top of the range where creep gets quick. Cyanoacrylate grabs instantly and cures hard and brittle, which is fine on a 5 g magnet and a poor bet on a heavy one, since the shock of a door slamming shears a brittle joint far more readily than a slightly flexible one.

If the disc itself came away cracked, replace it rather than gluing the halves back. A ferrite disc broken across its diameter loses a large share of the pull it started with, and a replacement 20 by 3 mm disc at 500 g costs less than the postage on it.

Handmade against mass produced

The two differ in three places, and price is the least interesting of them.

The mold is the first. A small maker pours into silicone that has run maybe two or three hundred pieces, and silicone degrades with every hot cure, so the last casting out of a tired mold has soft edges and a rough parting line. A factory running injection or compression tooling in steel gets the same part every time. If you are buying a set to sell on, the consistency is worth something. If you are buying one magnet, the handmade one is usually the sharper casting, because it came out of a mold with few enough uses on it to still be crisp.

The magnet on the back is the second, and it is where mass production quietly cheats. A factory buys one disc size and glues it to everything, which is fine for a 10 g fridge magnet and wrong for the 80 g novelty on the same order. A maker who is casting by hand can size the disc to the piece, and the good ones will tell you what they used when asked. If a listing does not say what magnet is on the back, assume it is the smallest one that passed the drop test on a workbench.

The third is the finish on the back itself. Look for a molded recess with the disc set flush and a felt or vinyl pad over it, which keeps the disc from marking the door and stops it from being levered off with a fingernail. That last detail costs almost nothing and it is missing from most cheap pieces. If you are buying a souvenir cast in resin rather than a decorative one, souvenir magnets covers what else changes at the low end of that market.