Rubber is shaped using heat, pressure, or both, with the specific method depending on the scale of production and the complexity of the part you need. The main approaches fall into three categories: molding (for discrete parts), extrusion (for continuous profiles), and cutting or casting (for prototypes and custom work). Each method works differently, and choosing the right one comes down to what you’re making and how many you need.
Molding: The Most Common Approach
Most rubber parts, from gaskets to grommets to automotive bushings, are made by pressing raw rubber into a mold and curing it with heat. There are three main types of rubber molding, each suited to different situations.
Compression molding is the simplest. You place a pre-measured chunk of raw rubber (called a preform) into an open mold cavity, then close the mold under high pressure and heat. The rubber fills the cavity, cures, and holds its new shape permanently. This method requires less machinery than the alternatives, making it a good fit for lower-volume runs or simpler part geometries.
Transfer molding adds a step. Raw rubber is heated in a separate chamber, then forced through a channel into a closed mold cavity. The mold closes, pressure is applied, and the rubber cures in place. This gives you better control over how the rubber fills complex shapes, since it flows into the cavity as a warm, pliable material rather than being squeezed cold.
Injection molding is the most automated option. Raw rubber is melted in a heated barrel and injected into a closed mold under high pressure. This process is fast, repeatable, and well suited to high-volume production where consistency matters. It also requires the most equipment and tooling investment.
Extrusion: Shaping Continuous Profiles
If you need a long, continuous rubber shape, like a door seal, a window gasket, medical tubing, or a hose, extrusion is the standard process. Compounded rubber is forced through a shaped opening called a die, which gives the rubber its cross-sectional profile. Think of it like squeezing toothpaste through a nozzle, except the nozzle is precisely machined to produce a specific shape.
After the rubber exits the die, it passes through a continuous curing system (typically using microwave energy, infrared heat, or a hot air tunnel) that vulcanizes it on the fly. One challenge with extrusion is that the rubber swells as it leaves the die, so the die opening doesn’t match the final part shape exactly. Engineers account for this by designing the die geometry to compensate for how the material deforms during the process.
Extrusion is one of the most economical ways to produce rubber profiles, which is why it’s used extensively in automotive manufacturing, construction, and medical device production.
How Vulcanization Locks the Shape
Raw rubber is soft and stretchy, with no permanent form. What gives a shaped rubber part its lasting structure is vulcanization: a chemical process where heat and sulfur (or other curing agents) create permanent bonds between the long polymer chains in the rubber. These bonds, called crosslinks, turn a pliable material into one that snaps back to its molded shape after being stretched or compressed.
Standard sulfur-based vulcanization is typically performed at 140 to 150°C, with curing times ranging from 10 to 30 minutes depending on the rubber type and the chemical accelerators in the compound. Thicker parts take longer because heat needs time to penetrate to the center. The degree of crosslinking determines the final hardness: more crosslinks produce a stiffer, harder part, while fewer crosslinks leave the rubber softer and more flexible.
Silicone Casting for Prototypes and Small Runs
If you’re making a small number of rubber parts or prototyping a design, room-temperature vulcanizing (RTV) silicone casting is a practical alternative to industrial molding. Instead of metal molds and high heat, you use a liquid silicone that cures at room temperature around a master model.
The process starts with a master model of the part you want to reproduce. Clean it thoroughly (isopropyl alcohol works well) and apply a thin layer of mold release so the cured silicone separates cleanly. Build a simple box around the model to contain the liquid silicone. Mix the two-part silicone slowly to minimize air bubbles, then brush a thin first coat onto the model to capture fine details before pouring the rest. Use a toothpick or needle to work silicone into tight corners and release trapped air. Pouring from the bottom up helps keep the surface smooth.
Air bubbles are the enemy of a clean mold. If you have access to a vacuum chamber, degassing the mixed silicone before pouring removes most of them. After pouring, leave the mold undisturbed for about 24 hours at room temperature to fully cure. Once set, carefully demold, taking your time to preserve fine details, and trim any excess material with a sharp blade.
The finished silicone mold can then be used to cast rubber parts using pourable urethane or additional silicone. This approach won’t match the speed or precision of industrial molding, but it’s accessible with minimal equipment and works well for runs of a few dozen parts or fewer.
Cutting and Machining Cured Rubber
Sometimes you need to shape rubber that’s already been vulcanized, whether you’re cutting sheet rubber to size, trimming molded parts, or creating custom shapes from stock material. Water jet cutting is one of the most effective methods for this. A high-pressure stream of water slices through rubber cleanly, with no heat generated in the process. That matters because rubber is sensitive to heat: a saw blade or laser can melt or distort the cut edge, while a water jet leaves it clean.
For soft materials like rubber, the water jet doesn’t even need abrasive particles mixed in. Pure water at moderate pressure cuts through cleanly. You can also stack multiple sheets of rubber up to 8 inches thick and cut them simultaneously, which multiplies output for batch work. Water jet cutting delivers very tight tolerances, making it suitable for precision parts.
For simpler cuts, die cutting (using a shaped steel blade, like a cookie cutter) works well for high-volume flat parts. Manual cutting with a sharp utility knife or rotary cutter handles basic shapes in sheet rubber. Freezing rubber before cutting can make it temporarily rigid and easier to machine with conventional tools.
Accounting for Shrinkage
One detail that catches people off guard when molding rubber is shrinkage. Rubber shrinks as it cools and finishes curing after being removed from a mold. The typical shrinkage rate ranges from 1% to 3%, depending on the specific rubber compound and the molding conditions. That might sound small, but on a 100mm part, 3% shrinkage means the finished piece is 3mm smaller than the mold cavity.
Mold designers compensate by making the cavity slightly oversized. If you’re designing your own mold for casting or 3D printing a mold form, scale your cavity dimensions up by the expected shrinkage percentage. The exact rate varies by rubber type and formulation, so if precision matters, make a test part first and measure the actual shrinkage before committing to a final mold design.