A look at the different types, properties, and how to prototype with rubber.
During my career in NASCAR, rubber was worshipped in the form of the beautiful black doughnuts with “Goodyear” emblazoned on the side. The tires cost $400 each, lasted about 20 minutes, and were the only connection between the grainy asphalt of Darlington and Daytona and the 3,400 lbs. of carefully crafted steel, aluminum and carbon that sat on top of them.
Every week, tires were measured for circumference and ranked by spring rate in order to create harmonious four-tire sets. This information was inputted into a carefully designed
data base, like food and religious preferences in eHarmony, in hopes they would work well together under the stresses of speed and 2.5gs of cornering force.
Each team has a dedicated specialist assigned to care for the rubber. The tire specialist on my team was named Glen, but everyone called him Doogie—like the savant doctor from the low-definition television show of the early 1990s—and his job was just as important. He set the pressure inside the tires to one-tenth of a psi (pounds per square inch) and monitored them throughout the day. He would set them at high pressure and place them into the sun in hopes of getting them to stretch and grow to help the car’s handling. They were purged with nitrogen to limit their pressure buildup before getting bolted to the car. Upon returning to the pit, the tires’ temperature and pressure were recorded to determine how to tune the suspension for greater performance.
Many consumer products have, in whole or in part, rubber or rubber components in them. The elastic and insulating properties of rubber and its family members provide helpful properties to a
product. Dog toys, balls, cooking products, O-rings and high-grip surfaces, are all from different types of rubber.
There are as many different formulations of rubber as there are snowflakes on the peak of Cotopaxi. Here is an overview of the different types of rubber and their properties, and how to prototype
with rubber. We will focus on a few major groups that are used for the bulk of commercially available products.
Natural rubber is tapped like maple syrup from the Para tree. The latex compound comes out of the tree as a gooey white sap. It is of little value in this form and needs to be vulcanized (treated with additional compounds at high temperature and pressure) to get useful properties. Natural rubber has good elasticity but relatively poor chemical resistance. It is often in tires, rubber gloves and compression hosiery.
This is a type of synthetic rubber that requires a chemical reaction to solidify. Single-part silicones are formulated to cure under specific environmental conditions such as moisture, heat or UV light. Two-part silicones have the reactive ingredients in two parts, a Part A and a Part B, and cure when they are mixed together. Silicone formulations are high strength; many have extreme heat resistance. They are also inert in many different chemical environments. They are in products such as pot holders and oven mitts, as well as dog toys and the “soft touch” overmold on some products.
Short for thermoplastic elastomer, TPE is a broad group of rubber compounds. These compounds get soft and flow under heat, unlike natural rubber or silicone that hardens under heat. They are stretchy and very resilient. TPE rubbers have a marked manufacturing advantage, as they set up quickly and be can be injection molded with short cycle times. They are used in consumer products such as athletic shoes, handles for bikes and knives, and baby products.
When looking at a data sheet for a specific type of rubber, there is a laundry list of properties to describe it. Words like modulus, durometer, specific gravity and viscosity are just some of the characteristics of rubber. Despite all of the technical jargon, there are a couple of properties that will help you understand different rubber types without needing an engineering degree.
One of the most important properties is the rubber’s hardness, as a number on the durometer scale. The lower the number, the softer the rubber. A pencil eraser is a 40 durometer on the shore A scale, or as 40A. The tricky part about durometer is that there are many different scales, each with different letters. The 40A pencil eraser is an 80 durometer on the shore OO scale.
Low-durometer rubber is soft and stretchy but more prone to wearing out or tearing. Getting a proper durometer rating requires a special gauge. However, it is possible to get a rough idea of the difference between the durometer of different rubbers by pressing your thumbnail into the surface. The further it penetrates, the lower the durometer.
Another important parameter to know when exploring different rubber options is the tensile strength, a measure of how strong the rubber is if you try to pull it apart. It is usually quoted in psi. So a chunk of rubber that has a cross section of 1 square inch with a 500 psi tensile modulus will require 500 lbs. before it breaks. Note that different durometer rubbers can have similar tensile strength; however, lower-durometer materials will stretch a lot farther before they break.
Prototype with Rubber
Many types of rubber require special molding equipment to make production parts. However, there are some convenient ways to prototype them. The easiest is to find flat sheets of rubber and cut them into the desired shape. Supply houses like McMaster-Carr (mcmaster.com) have a wide variety of sheet rubber in different styles, durometers and thicknesses. Thin sheet can be cut with scissors or a hobby knife. They can also be cut with a laser cutter to make more precise shapes.
For parts that require a 3D shape, there are a couple of options. If you have a CAD file, there are 3D printing bureaus that can print in rubber. Because of layering in the 3D process, these are often less strong, less durable, and less elongation than molded rubber. However, it is a great way to get a dimensionally accurate part without molding. Both Shapeways (shapeways.com) and Stratasys Direct (stratasysdirect.com) offer rubber 3D prints.
Another way to get three-dimensional rubber parts is to mold them. Room-temperature vulcanized or RTV rubbers are easy to work with and do not require expensive equipment. There are
many types of urethanes and silicones with different strength, stretch and durometer. Because urethane and silicone do not stick to each other, it is common to pour a silicone mold first, then inject urethane into the silicone tool to form the parts. It is also possible to 3D print a mold in hard plastic and then pour urethane or silicone into it to mold parts. Smooth-On (smooth-on.com) is a urethane and silicone supplier that has many different types of rubbers and a library of educational materials to help prototypers.