Most casts are made of either plaster of Paris or fiberglass, wrapped over several inner layers of padding and fabric. The specific material your doctor chooses affects how heavy the cast feels, how long it takes to harden, whether you can get it wet, and how clearly X-rays can see through it. Here’s what goes into each layer and how the two main types compare.
The Layered Structure of a Cast
A cast isn’t a single material. It’s built up in layers, each with a different job. The innermost layer is a stockinette, a soft knitted tube typically made from unbleached, absorbent cotton. This sits directly against your skin and gives the cast a smooth, finished edge at the top and bottom.
Over the stockinette comes the padding layer, usually several wraps of soft roll made from cotton, viscose rayon, or polyester. Viscose rayon is actually more absorbent than cotton and does a better job wicking moisture away from skin. Synthetic polyester padding is sometimes used instead for people with sensitivities to natural fibers. This padding protects bony areas like your wrist, elbow, or ankle from pressure against the hard outer shell.
The outermost layer is the rigid shell, either plaster or fiberglass, that actually immobilizes the bone. This is the part most people think of as “the cast.”
Plaster of Paris
Plaster of Paris is the traditional casting material, and it’s chemically simple: calcium sulfate. It starts as a dry powder made by heating gypsum rock to about 120°C, which drives off most of the water in the mineral’s crystal structure. The resulting powder is embedded into rolls of gauze bandage. When your doctor dips those rolls in water and wraps them around the padded limb, the calcium sulfate reabsorbs water and recrystallizes into a hard, porous solid.
This chemical reaction gives off heat, which is why a fresh plaster cast feels warm. With room-temperature water (around 24°C), the cast surface generally stays below 48°C. But warmer dip water or very thick casts can push temperatures close to the burn threshold of 49.5°C, which is one reason technicians pay attention to water temperature during application. The initial setting takes 5 to 15 minutes, though the cast continues to harden and strengthen over the next day or so.
Plaster’s main advantages are its low cost and the ease with which it can be molded precisely around a limb. Its drawbacks are real, though: it’s heavy, it falls apart when wet, and it blocks X-rays. That dense mineral structure scatters radiation and creates a thick opaque layer on imaging, which can make it harder to check how a fracture is healing without removing the cast.
Fiberglass
Fiberglass casting tape replaced plaster in many settings after its introduction in the 1970s. The material is a woven fiberglass fabric impregnated with a polyurethane resin. When exposed to water, the resin cures and hardens, similar in concept to plaster but with very different results.
A fiberglass cast on a forearm weighs roughly 325 grams compared to 457 grams for an equivalent plaster cast, about 30% lighter. The difference is even more noticeable on a full leg cast. Fiberglass is also dramatically stronger. In mechanical testing, fiberglass casts showed roughly five to six times the tensile strength of plaster and nearly five times the impact resistance. Plaster casts also lose their shape under repeated stress: after just a few cycles of bending, the force a plaster cast could resist dropped by nearly 60%, and by 80% after a few more. Fiberglass showed no similar fatigue pattern.
Fiberglass is more water-repellent than plaster (the shell itself won’t dissolve), though the cotton padding underneath can still trap moisture. It’s also more radiolucent, meaning X-rays pass through it more easily. Doctors can often check fracture alignment through a fiberglass cast without cutting it off, though the woven mesh pattern of the tape can still create some interference on images.
Waterproof Cast Liners
Standard cotton padding under any cast needs to stay dry, which is why you’re told to keep a cast out of water. But waterproof liners exist as an alternative. One common type uses expanded polytetrafluoroethylene (the same family of material as nonstick cookware coatings) laminated with polyurethane padding. Another uses a blend of polypropylene, polyamide, and polyester fibers with an acrylic adhesive.
These liners let water drain through and air circulate, so you can shower or even swim without damaging the cast. They’re typically paired with a fiberglass shell rather than plaster. Not every fracture is a good candidate for a waterproof setup, and they cost more, but for active kids or anyone who needs to be around water, they can make the weeks in a cast significantly more manageable.
3D-Printed Casts
A newer option uses 3D printing to create a custom-fitted lattice shell, usually from PLA (polylactic acid), a rigid biodegradable plastic. These open-weave designs are lightweight, fully ventilated, and don’t trap moisture at all. Some designs use flexible thermoplastic polyurethane (TPU) for sections that need to bend slightly, or carbon-fiber-reinforced nylon for areas that need extra strength.
3D-printed casts require a scan of the limb and time to print, so they aren’t practical in an emergency room for a fresh fracture. They’re more commonly used for follow-up immobilization once swelling has gone down, or for stress fractures and other injuries where the initial days of plaster or fiberglass have passed. The lattice structure means you can scratch an itch, wash the skin underneath, and see the limb directly, which solves several of the most common complaints about traditional casts.
How Material Choice Affects Your Experience
Plaster is still used when precise molding matters most, such as immediately after a fracture when swelling is expected to change. It conforms to the limb’s shape better during application, and it’s inexpensive. Once swelling stabilizes, many doctors switch to a fiberglass cast for the remaining weeks because of its lighter weight and greater durability.
Fiberglass is the default for most routine fractures. It comes in colors, sets faster, holds up better over weeks of daily use, and allows for X-ray monitoring without removal. The trade-off is slightly higher cost and a rougher surface texture that can snag clothing or scratch furniture.
Whichever material is used, the padding layer underneath does most of the comfort work. A well-padded cast with proper technique matters more to your day-to-day experience than whether the shell is plaster or fiberglass. The cast’s job is simply to hold the bone still while it heals, and both materials accomplish that reliably.