protoTYPING: The many forms of rubber

During my career in NASCAR, rubber was worshipped.

Rubber in the form of the beautiful black donuts with Goodyear emblazoned on the side.

Of course I am talking about the tires. They were $400 each, lasted about 20 minutes, and were the only connection between the grainy asphalt of Darlington and Daytona and the 3,400 pounds of carefully crafted steel, aluminum and carbon that sat on top of them.

Taking a rest on kinematics rig, Andromeda style, at the Robby Gordon NASCAR shop in2007.

Jeremy taking a rest on a kinematics rig, Andromeda style, at the Robby Gordon NASCAR shop in 2007.

Every week at the track, the tires were measured for their circumference and ranked by their spring rate in order to create harmonious 4-tire sets. Their data was inputted into a carefully designed database, like food and religious preferences in an eHarmony profile, 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 the proper care for the team’s rubber. On my team, the tire specialist’s name was Glen, but everyone called him “Doogie” like the savant doctor from the low-definition television show of the early 90s, and his job was just as important. He set the pressure inside the tires to a tenth of a psi and monitored them throughout the day. He would set them at high pressure and placed 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, their temperature and pressure was recorded in hopes to glean how to tune the suspension for even greater performance.

The importance of rubber is not limited to racing, of course. There are many consumer products that are made in whole or in part with rubber or rubber components. 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 made from different types of rubber.

The following is an overview of the different types of rubber and their properties, and how to prototype with them.

Types of rubber

There are as many different formulations of rubber as there are snowflakes on the peak of Cotopaxi. However, we will focus on a few major groups that are used for the bulk of commercially available products.

Natural rubber

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 used in tires, rubber gloves, and compression hosiery.

Passenger car tires are made with natural rubber that is vulcanized.

Passenger car tires are made with natural rubber that is vulcanized.

Silicone rubber

Silicone rubber is a type of synthetic rubber that requires a chemical reaction to solidify. Single part silicones are formulated to cure under specific environmental conditions like moisture, heat or UV light. Two-part silicones have the reactive ingredients segregated into a part A and a part B and cure when they are mixed together. Silicone formulations are high strength and many have extreme heat resistance. They are also inert in many different chemical environments. They are used for products such as pot holders and oven mitts as well as dog toys and the “soft touch” overmold on some products.

TPE

TPE, short for thermoplastic elastomer, is a broad group of rubber compounds. These compounds get soft and flow under heat like cheese, unlike natural rubber or silicone which hardens under heat like an egg. They are stretchy and very resilient. They have a marked manufacturing advantage as they setup quickly and be can be injection molded with short cycle times. They are used in many consumer products, such as athletic shoes, handles for bikes and knives, and baby products.

A sample kit of different TPE formulations from a TPE manufacturer.

A sample kit of different TPE formulations from a TPE manufacturer.

Properties

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 ways rubber is characterized. 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 hardness. The hardness of the rubber is given as a number on the durometer scale. The lower the number, the softer a rubber is. A pencil eraser is a 40 durometer on the shore A scale, often shortened to 40A. The tricky part about durometer is that there are many different scales, and they are represented by different letters. The 40A pencil eraser is an 80 durometer on the shore OO scale.

Low durometer rubber is soft and stretchy but is 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.

Testing the durometer of silicone rubber with a thumb nail.

Testing the durometer of silicone rubber with a thumbnail.

Another important parameter to know when exploring different rubber options is the tensile strength. The tensile strength is a measure of how strong the rubber is if you tried to pull it apart. It is usually quoted in psi or pounds per square inch. So a chunk of rubber that has a cross section of 1 square inch with a 500 psi tensile modulus will require 500lbs 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.

Prototyping

Many types of rubber require special molding equipment to make production parts. However, there are some convenient ways to prototype them. The easiest way is to find flat sheets of rubber and cut them into the desired shape. Supply houses like McMaster-Carr have a wide variety of sheet rubber in different styles, durometers and thicknesses. Thin sheet can be cut with scissors or a hobby knife, and 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 the layering of the 3D process, these tend to be less strong, less durable, and have less elongation than a molded rubber. However, it is a great way to get a dimensionally accurate part without molding. Both Shapeways and Stratasys Direct offer rubber 3D prints.

Another way to get 3-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. Since 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 is a urethane and silicone supplier that has many different types of rubbers and a library of educational materials to help prototypers.

Rubber molding station

The Enventys molding station with a silicone mold, digital scale,vacuum chamber and mold kits.


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Originally published at Edison Nation Blog.