Spin casting is a manufacturing process that uses a spinning rubber mold to force liquid metal or resin into shaped cavities, producing small, detailed parts quickly and cheaply. A disc-shaped mold rotates at speeds typically between 100 and 1,000 RPM while molten material is poured into a central opening, and centrifugal force pushes the material outward into every detail of the mold. It’s one of the most accessible casting methods available, widely used for everything from fishing lures to trophy figurines.
How the Process Works
The process starts with a flexible rubber mold, usually vulcanized silicone, that’s shaped like a thick disc. The mold is clamped into a machine called a spin caster, which holds it horizontally and spins it around a vertical axis. Once the mold reaches the target speed, the operator pours molten metal through a central opening called a sprue. Centrifugal force drives the liquid outward through channels carved into the mold, filling every cavity and fine detail under pressure.
After a short spin cycle, the machine stops, the mold is opened, and the finished parts are removed. Compressed air is blown over the mold to cool it before the next pour. The entire cycle from pour to part removal takes just minutes, making it practical to produce dozens or hundreds of pieces per hour from a single mold.
The Mold: Vulcanized Rubber
What sets spin casting apart from other casting methods is the mold material. Instead of rigid steel or sand, spin casting uses vulcanized rubber, which is cured at around 350°F for 50 to 60 minutes. To make a mold, a master pattern (the original part you want to copy) is placed between layers of uncured rubber, then pressed and heated in a machine called a vulcanizer. The rubber flows around the pattern during curing, capturing surface details with high fidelity.
Once cured, the mold is sliced open, the master pattern is removed, and channels are cut by hand to allow molten material to flow from the central sprue to each cavity. A single mold can contain multiple cavities arranged in a radial pattern, so one pour produces several parts at once. Rubber molds are far cheaper to produce than steel dies, and they can be made in a day rather than weeks. The tradeoff is durability: a rubber mold wears out after hundreds or thousands of shots, while a steel die can last indefinitely.
Why Rotation Speed Matters
Getting the RPM right is critical. If the mold spins too slowly, gravity overpowers the centrifugal force and the molten metal doesn’t distribute evenly, producing lopsided or incomplete parts. Research on vertical centrifugal casting found that at 650 RPM, the downward pull of gravity dominated and created uneven castings. At 850 RPM, the centrifugal force still wasn’t strong enough to fill the mold completely.
Spinning too fast creates its own problems. At 1,250 RPM in the same study, the molten metal slammed into the mold walls with too much force, producing coarse grain structures and uneven shapes. The sweet spot was around 1,050 RPM, where centrifugal force and gravity balanced correctly, the metal distributed evenly, and the finished parts had fine grain structures with uniform hardness throughout.
In practice, the ideal speed depends on the specific alloy, part geometry, and mold size. Most commercial spin casters offer adjustable speeds, with some machines capable of anywhere from 30 to 3,000 RPM.
Materials Used in Spin Casting
Spin casting works best with low-melting-point metals, since the rubber molds can’t withstand extreme heat. Zinc alloys, tin, lead, and pewter are the most common choices. These metals melt well below the temperature that would damage the rubber mold, and they flow easily into fine details at relatively low centrifugal forces.
Some spin casting operations also use thermoset resins, polyurethanes, and other liquid plastics instead of metal. The principle is the same: centrifugal force pushes the liquid into the mold cavities, where it cures and hardens. Resin spin casting is popular for producing detailed miniatures, prototypes, and decorative items where metal isn’t necessary.
Common Products Made by Spin Casting
Spin casting is especially popular for small, detailed, decorative, or novelty items. Fishing lures and weights, toys, board game pieces, trophies, belt buckles, medals, medallions, and decorative emblems are all commonly spin cast. The fashion accessories industry uses it for costume jewelry components, and some manufacturers produce small industrial parts for machinery this way.
The process excels at parts that need fine surface detail but don’t face heavy mechanical stress. If you’ve ever held a small metal figurine, a zinc-alloy keychain, or a pewter chess piece, there’s a good chance it was spin cast.
Spin Casting vs. Die Casting
Die casting uses steel molds and high-pressure injection systems to force molten metal into rigid dies. It produces stronger, more dimensionally precise parts, but the tooling costs are significantly higher because machining a steel die is expensive and time-consuming. Die casting typically makes economic sense for medium to large orders, roughly 100 to tens of thousands of pieces.
Spin casting fills the gap below that threshold. With lower tooling costs and faster mold turnaround, it’s the more practical choice for runs of about 25 to 1,000 pieces. If you need a few hundred custom medals or a short run of promotional items, spin casting delivers acceptable quality at a fraction of the setup cost. For high-volume production where every part needs tight tolerances and high structural strength, die casting is the better investment.
Equipment for a Spin Casting Setup
A basic spin casting operation requires four categories of equipment. The spin caster itself is the core machine: a motorized turntable with a clamping system that holds and spins the mold. Beyond that, you need a metal melting furnace to bring your alloy to pouring temperature, a vulcanizer to cure rubber molds from raw rubber and master patterns, and finishing equipment for trimming, deburring, and polishing the cast parts after they come out of the mold.
Compared to most other metal casting methods, the startup costs are low. Small tabletop spin casters are available for hobbyists and small shops, while larger commercial units handle higher-volume production. The relatively simple equipment list is one reason spin casting remains popular with small manufacturers, jewelers, and hobbyists who need to produce detailed metal parts without a major capital investment.