Rubber, what is it? This marvelous material has found its way into just about everything in our lives. Found originally in Central and South America, this natural, organic plant abstract was first given its name 230 years ago by a British chemist Joseph Priestley. He received a bouncy ball of unfamiliar material from an American friend. Priestley noticed that he could rub away pencil marks with it. Thus came the name rubber. Many years later (1839), a Connecticut hardware merchant, Charles Goodyear, accidentally dropped a mixture of rubber and sulfur into a fire. When he retrieved the material, it was no longer sticky. It didn’t get brittle when cold. If stretched, it snapped back to its original shape. This was the discovery of vulcanization, the process that has made rubber one of the most useful materials today.
Since that time, rubber has been mixed with various chemical agents to give it a wide range of properties, such as greater strength, resistence to aging, and more resiliency. In most of today’s rubber products, very little if any natural rubber is used. Compounds have been developed to address just about any environment and capability. Among the most common are styrene-butadiene rubbers (SBR), ethylene-propylene rubbers (EPDM), butyl rubber (Buna-N), acrylic elastomers, and silicone rubbers. SBR’s were widely used by car manufacturers for weatherstripping for many years. It had good properties for the job, except for longevity. Most manufacturers were not concerned about making rubber last ten to fifteen years, five years was considered good enough. Fortunately for the Mopar owners, especially the ’50 to ’60 , the engineers were more concerned with quality than price. These owners will find much of their original rubber is still soft and pliable today.
Most of the rubber compounds we use today for reproduction parts are the EPDM and Buna-N compounds. EPDM is used on all parts that require resistance to weather and ozone. It has an excellent age-resistor, but has poor resistance to gas and oil. This is why you should never clean your rubber with gasoline or kerosene. It’s best to use lacquer thinner or MEK because it will strip the surface clean and evaporate quickly before doing any damage. Buna-N is used on parts such as shock absorbers, engine mounts, etc. It has good resistance to gas and oil and very good metal to rubber bonding properties. Rubber can be formed into many shapes by rolling, cutting, molding, extruding, and dipping then vulcanizing with heat or chemicals to a finished product. Once vulcanized the process can not be reversed to claim the excess rubber. It has in some cases been ground down to a fine filler but not useable by itself.
But, you ask, how does this help me in my restoration? By knowing the rubber compound being used, one can make the right decisions about the proper types of adhesives to use or what chemical or environment might harm it. For example, EPDM has very good properties for wear on your vehicle but it doesn’t work well with all types of contact cements. Have you ever tried to cement runningboard rubber to the metal panel only to have the contact cement hold only to the metal? There are specially formulated contact cements for EPDM. Also gasoline should never be used to clean EPDM. It will be absorbed, making it swell and breakdown.
There are quite a few steps in the process of making restoration parts. The types of materials and processes developed by the car manufacturers were designed primarily for economical production and assembly in large numbers. Many parts are nearly impossible to reproduce exactly the same way today,because of the much lower quantities the market can absorb. Certain processes were also designed without consideration of the longevity of a part (latex skin over foam, for example). Without the economies of scale generated by large production numbers, the tooling is very expensive relative to the value of the part to most customers. This is the biggest stumbling block in making many needed parts for restoring our automobiles.
To first consider a project, one must find a good original. If there is one piece of advice I can give a fellow restorer; it is “never throw any old part away until you complete your restoration”. Many a restorer has made this fatal error, including myself.
We research the tooling cost, market size and materials required to make each part. Once a decision is made to tool a part, it must then be designed. Drawings are made for the toolmaker to follow. More complex parts are digitized. These digitized surfaces are then transported to the CAD program to design the mold and tooling specs. Finished designs are then run through a tool path program to similate making the mold. This checks for any cutting issues before cutting. Metal is not cut until all mold problems are worked out. Depending on the complexity of the mold or the number of pieces required, the tooling process can take weeks to complete. Parts with cores must also have tooling for the cores.
Once the tooling and cores are completed, then it goes to production. There it is vulcanized and inspected by the operator. Then it is sent to the trimming for clean up and quality control for another inspection. Now it is ready for the packaging department to give the part one final inspection before packaging. Parts are inspected three times before going to the warehouse as ready for shipping. There they wait for you to order.
If it wasn’t for Charles Goodyear’s clumsiness, we would never be able to appreciate all of the wonderful things this South American plant has brought us, from tires to helping to keep the rain out of our cars.