Rubber Compression Molding Process
Rubber Compression Molding Process
Vulcanizing rubber turns this…
“Yeah – a ton of questions” you say. Of course, that’s why you’re reading this in the first place.
Ok, let’s start with some basics about the materials and science involved to give a foundation of understanding.
Raw rubber in its many forms consists of long chainlike strands of carbon and hydrogen atoms. These strands could be likened to a large bowl of cooked spaghetti noodles.
Twirl a fork in it and you can grab some of the noodles and twist them around. Some of those noodles will intertwine and pull in other noodles not directly touching the fork. They (kind of) stick together. You end up with a ball of noodles on your fork that usually ends up slowly unraveling off with a malicious splat as the tsunami of tomato sauce hits your shirt.
Raw rubber feels like something between a liquid and a gelatin. It’s gooey and not very useful. Back in the 1840s though, Charles Goodyear discovered that treating this goo with a bit of sulfur and heat actually changed it into something useful. This new process called vulcanization made the rubber solid, yet quite stretchy and durable and was originally used to make shoes and waterproof materials. Chemists have since described why this happens.
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When activated by a moderate amount of heat, the sulfur bonds the long carbon-hydrogen chains (noodles) to each other at random intervals along their length to form a large entangled three-dimensional web. Instead of being able to slide past each other and change the overall shape of the mass of material, the whole arrangement becomes locked together in the shape it was in when the sulfur took hold.
There has certainly been quite a bit more development in chemistry and materials science since Goodyear’s day, but you can enroll for some classes at your local university for that. This is enough of an understanding for the scope here.
Gooey Stuff + Sulfur + Heat = Tires, Shoes, etc.
Now you know where tires and shoes come from, but what else is this useful for?
Since these parts are not produced by melting materials and cooling them back to a solid like many other molding operations, the resultant parts are typically able to handle very high temperatures. You’ll likely have some spatulas in your kitchen that were made with this process. Vulcanized rubber compounds are used heavily in the automotive industry and related sorts of industrial products. You’ll find a lot of things like gaskets, dust boots, plenums, belts, and the like that benefit from the scale and attributes of compression molding. In such parts like the ones shown above, the application requires some amount of flexibility and lacks very intricate detailed geometry.
Developing a product that would use the rubber compression molding process will need to start with defining the goal to work towards first, then fleshing out the details. In this, designers will usually start with some sort of sketch or constraining geometry for a mating component. In a way similar to an artist’s sketch that start with some basic reference lines and shapes before filling in the details, compression molded rubber parts can be progressively coaxed into shape without too much risk of running into manufacturing problems. Compression molding is forgiving of a lot of things that would be strict taboos in other molding processes.
Design engineers will convert the concept sketches into CAD geometry. With this model, things can be optimized for the characteristics needed in the part. This may be for weight, flexibility, pressure, vibration dampening, thermal decoupling, or any sort of mechanical property. Engineers will adjust the geometry and the material specifications in order to achieve the balance needed. The weight of a compression molded rubber part significantly contributes to its cost, so this will be minimized in most cases. Experienced designers will know where to trim material without negatively impacting the part’s performance and CAD software can instantly calculate the part weight for cost assessments throughout the process.
Production tooling for compression molded rubber parts is relatively economical; however, other good options exist for prototyping. In some cases now, flexible parts can be directly 3D printed. These are available in a range of durometers as well and can give an indication of the fit and feel of a flexible rubber part design. With the range of other properties desired in a part, flexible 3D printed materials can often fall short.
Once the design is validated to a comfort level supporting the time and financial investment, production compression mold tooling can be designed and built. At this stage, the designers and toolmakers will work together to create the right type of tooling for the part. Two categories exist:
Rubber Compression Molding
Traditional compression molding involves a two-part tool containing one or more cavities for the finished part. A precise amount of unvulcanized rubber compound is somewhat pre-formed and placed onto the lower half of the tool in the cavity. Then, the upper half is clamped down over it with heat and pressure to squeeze the rubber to fully fill the cavity and cure the part. After a specified curing pressure and time have been satisfied, the mold is opened and the part is removed.
Ruber Transfer Molding
Transfer molding differs from the standard rubber compression molding process in that the two-part mold form is closed around the cavities before the raw rubber material is added. The unvulcanized rubber is placed on top of the upper mold half-covering channels that run down to each of the cavities. A third plate is then pressed down over the top to force the raw rubber down into the individual cavities. This method can allow overmold insert components to be firmly clamped in place. It also economically produces multiple parts in each press cycle; however, the material in the sprue channels needs to be trimmed from each part and a small blemish is typically left behind.
The compression molding process, in either case, involves a manual process cycle where a production technician loads uncured raw rubber material into a mold tool assembly, then operates the machine to press and cure the parts, then removes the parts from the tooling. Since the parts are cured by chemical bonds and flexible, they can be safely stretched and pulled out of the tool with some amount of force. Because of this, compression molding can accommodate undercuts in tooling design. If the durometer of the material is low, it can accommodate quite sever undercuts and complex geometry where removing the part from the tool is less like popping an ice cube out of a tray and more like peeling off a tight latex glove. In fact, tool inserts can be used that are positioned between the top and bottom halves of the cavity which become contained within the compression molded part once cured. The finished part is then stretched and peeled off of the insert and perhaps assisted with a jet of compressed air.
Compression molding is a unique type of molding process that produces flexible parts in shapes that would be very costly to accomplish using other processes. The process is only made possible by the raw materials though, and developers are continually innovating in this area. This is true both in areas of prototyping and production material properties. Consider where compression molded rubber products might bring the right mix of benefits for solving problems on your products. For some benchmark comparisons, take a look at a few real examples of costs and timing for bringing compression molded rubber parts to life from concept through production.