Making a hollow mold, or more precisely, using a mold to produce hollow castings, involves pouring a small amount of material into a mold and coating the interior walls rather than filling the entire cavity. The most accessible method is called slush casting: you pour liquid resin into a flexible mold, rotate it by hand until the walls are evenly coated, and let it cure into a lightweight, hollow shell. This technique is widely used for props, figurines, cosplay armor, and decorative objects where a solid casting would be too heavy or waste too much material.
There are several approaches depending on your budget, the size of your piece, and how many copies you need. Here’s how each method works and what you need to get clean, consistent results.
Slush Casting by Hand
Slush casting is the simplest way to produce a hollow part. You mix a small batch of liquid casting resin, pour it into your mold, then tilt and rotate the mold in every direction so the resin flows across all interior surfaces. As the resin begins to thicken and gel, it clings to the mold walls and builds up a shell.
A single pour rarely gives you enough wall thickness for a sturdy part. Plan on two to three coats, using enough resin each time to build up roughly 1/4 inch (6 mm) of total wall thickness. Between coats, wait about 10 minutes for the previous layer to gel so the fresh resin doesn’t pull the earlier coat away from the mold surface. The entire rotation process for one coat typically takes 5 to 10 minutes of continuous movement.
Resin choice matters here. You want a product that thickens gradually over the last 30 to 45 seconds of its working time rather than one that snaps from liquid to solid with no warning. That gradual thickening helps the resin spread evenly and stick to vertical and overhead surfaces inside the mold. Fast-setting resins tend to pool at the bottom before you can distribute them, leaving thin spots at the top of the piece.
Choosing the Right Mold Material
For most hollow casting projects, a two-part silicone rubber mold gives the best results. Silicone is flexible enough to peel away from complex shapes without tearing, it captures fine detail, and resin doesn’t stick to it. A Shore 25A hardness silicone (soft and stretchy, similar to a rubber band) works well for pieces with undercuts, horns, or other geometry that would lock into a rigid mold.
Brush-on silicone is particularly useful for hollow models. You mix equal parts of the two components by volume, then brush the silicone directly onto your original sculpture in thin layers. It self-thickens so it stays put on vertical and inverted surfaces. Once cured, you back the flexible silicone skin with a rigid shell (called a mother mold) made from plaster bandages or fiberglass so it holds its shape during casting.
If you’re casting into plaster, fiberglass, or rigid plastic molds instead, you’ll need a release agent to prevent the casting from bonding permanently to the mold. For silicone molds, a release agent is often optional since most resins don’t adhere to cured silicone. For polyurethane rubber molds over clay, plaster, or wax originals, a general-purpose mold release spray applied before each pour will keep things separating cleanly.
Controlling Wall Thickness
Uneven walls are the most common problem in hollow casting. Thin spots crack under stress, and thick spots add unnecessary weight and use extra material. A minimum wall thickness of 2 mm will hold together for small decorative pieces, but for anything structural or larger than your fist, aim for that 1/4 inch (6 mm) range built up over multiple coats.
Several things help you achieve uniform thickness. First, keep the mold moving constantly during each coat. Pausing with one side down lets resin pool there. Second, work in a warm room. Cold temperatures slow the curing process and give the resin more time to run and drip before it gels. Third, if you notice a thin area after one coat cures, you can target it with the next pour by holding the mold so that area faces down during the early part of your rotation.
Using a Core Insert for Precise Walls
When you need exact, repeatable wall thickness (for production runs or mechanical parts), slush casting by hand isn’t precise enough. Instead, you use a two-part mold with a core: an outer mold that defines the outside shape and a smaller inner piece (the core) that defines the hollow interior. You pour resin into the gap between them, and the space between the two determines your wall thickness exactly.
The core needs to be removable after the casting cures. For simple shapes like cylinders or boxes, you can pull the core straight out. For complex shapes, you might use a collapsible core made of multiple pieces, or a “lost core” made from a material you dissolve or melt out after casting (wax is the classic choice, which is where the term “lost wax casting” comes from).
Cores that fit into the outer mold need tight tolerances. The fit between core and mold should be precise to within 0.05 mm to prevent resin from leaking past the seal points. If the core has an irregular shape that could shift during pouring, adding a slight 1 to 2 degree taper around its perimeter helps it seat firmly. Round cores that could rotate out of alignment need a flat milled into one side to lock their orientation.
Rotational Casting for Larger Pieces
For bigger or more complex hollow parts, a rotational casting machine automates the process. The mold is mounted inside a frame that spins on two axes simultaneously while being heated. Powdered or liquid plastic melts and coats the mold interior, then the mold is cooled and opened.
Industrial machines spin the main axis at up to about 30 rpm, with the secondary axis running at 60 to 100 percent of that speed. The ratio between these two speeds (called the rotation ratio) affects how evenly material distributes. A 1:1 ratio works for simple, symmetrical shapes. More complex shapes need an unequal ratio, something like 1.25:1 or 1.67:1, to ensure material reaches all areas of the mold.
A typical cycle involves 20 minutes in the heating chamber followed by 25 minutes of cooling. This is obviously a different scale than hand slush casting, but understanding the principle helps even if you’re building a DIY rig: you need simultaneous rotation on two axes, not just one, to get full interior coverage.
Preventing Air Bubbles and Voids
Trapped air is the enemy of a clean hollow casting. When resin flows over a pocket of air, it creates a void in your wall, weakening it and leaving a visible defect on the surface. Venting the mold properly solves most bubble problems.
The key principle is that air vents belong at the highest points and at the ends of material flow, the spots where air naturally gets pushed as resin fills the mold. For slush casting, this is less of an issue since you’re rotating an open mold and air can escape freely. But for core-and-shell molds that are closed during pouring, you need small channels cut into the mold at those high points.
Vent channels should be narrow enough that resin doesn’t flow through them freely. A depth of 0.05 mm to 0.16 mm works for most casting resins, with softer or more fluid materials needing shallower vents. Space vents about 25 mm apart around the mold cavity for thorough air evacuation. If your mold has complex geometry with deep pockets or dead-end chambers, you may need to experiment with vent placement over a few test casts to find the right configuration.
Safety While Casting
Casting resins release vapors that can irritate your lungs, eyes, and skin, and some components are genuinely hazardous with repeated exposure. Working safely requires more than just cracking a window.
You need local exhaust ventilation, meaning a fan or hood that pulls fumes away from your face and pushes them outside. General room ventilation (like an open door) isn’t sufficient to control vapor exposure during mixing and pouring. At minimum, wear a half-face respirator with organic vapor cartridges and a dust pre-filter while mixing, pouring, sanding, or finishing any resin product.
Wear chemical-resistant gloves matched to your specific resin. Butyl rubber gloves work for epoxy resins. Polyvinyl alcohol gloves handle styrene-based products. Safety goggles are necessary whenever there’s a splash risk, and safety glasses should stay on during any cutting or sanding of cured castings. Disposable coveralls keep resin off your skin and clothes. If you’re working with any product containing isocyanates (common in some polyurethane systems), a respirator isn’t enough. Those require a supplied-air breathing system.