Rubber molding works by forcing uncured rubber into a shaped cavity, then applying heat and pressure until the material permanently holds its new form. The specific method depends on what you’re making: simple shapes like gaskets need only basic compression, while complex parts with tight tolerances call for injection molding or transfer molding. Whether you’re working at an industrial scale or making silicone parts in a workshop, the core principle is the same.
The Three Main Molding Methods
Industrial rubber molding falls into three categories, each suited to different production needs.
Compression molding is the simplest. A technician places a pre-shaped piece of uncured rubber directly into an open mold, then closes it under heat and pressure. The rubber fills the cavity and cures in place. Because tooling costs are low and the equipment is straightforward, compression molding works well for low-to-medium production volumes and simple geometries like O-rings, seals, and gaskets. It’s also the go-to for large parts that would be difficult to inject.
Injection molding heats raw rubber to a liquid consistency, then forces it into a closed mold under high pressure. This produces parts with excellent precision, making it the best choice for tiny or complex shapes, parts with internal features, and anything with tight tolerances. The process is fast and easy to automate, so it dominates high-volume production runs.
Transfer molding sits between the two. Pre-formed rubber goes into a chamber above the mold, and a plunger pushes it through channels into the heated cavity. This gives manufacturers more control over how much material enters the mold, which reduces waste. Transfer molding also handles high cavity counts well, making it a strong option for complex designs and parts that will undergo secondary processes like assembly or the addition of fasteners.
How Rubber Actually Cures
Raw rubber is soft and elastic but won’t hold a permanent shape on its own. Curing, also called vulcanization, creates chemical bridges between the long rubber molecules so they lock into a stable structure. There are two main ways to trigger this.
Sulfur curing is the classic method, used with natural rubber and other “unsaturated” rubbers that have reactive double bonds in their molecular chains. Sulfur atoms form short chains linking one rubber molecule to another, creating a single cross-link per reaction. The result is a flexible, resilient material. Most conventional rubber products, from tires to industrial seals, use sulfur-based curing.
Peroxide curing works differently. When a peroxide molecule breaks apart under heat, it generates reactive fragments that can trigger a chain reaction, forming multiple cross-links from a single starting molecule. This makes peroxide curing more efficient in some cases, and it’s the only option for saturated rubbers like silicone and EPDM that can’t react with sulfur. The tradeoff is that higher processing temperatures can cause the rubber chains themselves to break, so temperature control matters more with peroxide systems.
Choosing the Right Rubber
Different rubber compounds behave differently in the mold and in the finished product. One of the most important variables is shrinkage: every rubber part shrinks as it cools after curing, and the amount varies by material. If you don’t account for it, your finished parts won’t match your design dimensions.
- Silicone (VMQ): 2.0–5.0% shrinkage. Excellent heat resistance, flexible across a wide temperature range, but the highest shrinkage of common rubbers.
- EPDM: 1.9–3.5% shrinkage. Good for outdoor applications and weather seals because it resists UV and ozone well.
- Nitrile (NBR): 1.5–3.5% shrinkage. Strong resistance to oils and fuels, commonly used for gaskets and hydraulic seals.
- Natural rubber (NR): 2.0–3.5% shrinkage. High tensile strength and elasticity, but less chemical resistance than synthetics.
Hardness also varies widely. Rubber hardness is measured on the Shore A scale, where higher numbers mean stiffer material. A tire tread, for example, sits around 60 Shore A. Soft seals and joint padding might be 30–40 Shore A, while load-bearing rubber bumpers can reach 80 or higher. Matching hardness to the application is just as important as getting the chemistry right.
Designing the Mold
A well-designed mold accounts for how rubber behaves during and after curing. Two details matter most for part quality.
First, draft angles. These are slight tapers on the vertical walls of the mold cavity that help the cured part release cleanly. A draft of 1 to 3 degrees is standard. Draft angles become especially important for deeper cavities and complex geometries, where a straight-walled mold can grip the rubber and make removal difficult or damage the part.
Second, wall thickness. Parts with varying wall thicknesses don’t shrink uniformly, which can introduce warping or distortion. Keeping walls as consistent as possible across the part reduces this risk. Where thickness changes are unavoidable, gradual transitions perform better than abrupt ones.
Mold Release Agents
Without a release agent, cured rubber bonds to the hot metal mold surface and tears during removal. Release agents create a barrier between the rubber and the mold, and the right choice depends on your process.
Water-diluted silicone is the most common option for general rubber molding. It prevents sticking and lubricates the rubber so it slides over the hot mold surface during loading. The downside is that it typically needs to be reapplied every cycle.
Semi-permanent release agents build up a silicone matrix on the mold surface that acts as a longer-lasting barrier. Depending on the rubber compound and mold quality, a single application can last anywhere from one cycle to a full day of production. These are more cost-effective for high-volume runs.
One important exception: if you’re molding silicone rubber, you need a non-silicone release agent. Silicone-based releases will bond with the part material instead of creating separation.
Making Silicone Molds at Small Scale
For prototyping, small-batch production, or hobbyist work, room-temperature vulcanization (RTV) silicone lets you create molds without industrial equipment. The process uses a two-part liquid silicone that cures at room temperature rather than requiring a heated press.
You start by securing your master pattern (the object you want to replicate) inside a mold box, then mix the silicone base and catalyst following the manufacturer’s ratio exactly. Pour the mixed silicone over the pattern slowly to minimize trapped air. Most RTV silicones skin over in 5 to 30 minutes and fully cure in about 24 hours at room temperature, though fast-cure formulas can set in a few hours.
Air bubbles are the biggest enemy of a clean RTV mold. A vacuum chamber pulling 25 to 29 inches of mercury for 2 to 5 minutes after mixing removes most trapped air. If you don’t have a vacuum chamber, pouring in a thin stream from a height and working slowly helps, but you’ll likely get some surface imperfections. For casting into the finished mold, pressure casting can also push bubbles out of the liquid material before it sets.
Removing Flash From Finished Parts
Flash is the thin film of excess rubber that squeezes out along the mold’s parting line during curing. Almost every molded rubber part has some. For simple parts, it can be trimmed by hand with a blade, but this is slow and inconsistent at volume.
Cryogenic deflashing is the industrial solution. Parts are tumbled in a chamber while liquid nitrogen freezes the thin flash until it becomes brittle and snaps off. The thicker body of the part stays flexible enough to resist damage. Combining the freezing with media blasting (small pellets fired at the surface) removes both internal and external flash uniformly across all parts in a batch. This produces cleaner results than hand trimming and scales well for production quantities.
Safety During Rubber Processing
Heating rubber releases volatile organic compounds that pose real health risks with repeated exposure. Air sampling in rubber manufacturing facilities has detected benzene, toluene, xylene, and fine rubber dust at various production stages. In some facilities, benzene exposure has measured nearly six times its occupational safety threshold, and toluene levels have reached four times the recommended limit.
Adequate ventilation is non-negotiable when heating or curing rubber. Industrial operations use local exhaust systems at each workstation. For smaller-scale work, a well-ventilated space or a fume extraction setup keeps exposure manageable. Respiratory protection, chemical-resistant gloves, and eye protection are baseline precautions whenever you’re working with heated rubber compounds or liquid curing agents.