Rubber Compression Molding Process

What is the Rubber Compression Molding Process?

Vulcanizing rubber turns this… Non-Newtonian Fluids & Oobleck
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Into this…

Any questions? “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.
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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.

Development Process

What is the Concept of Rubber Compression Molding?

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.

What is the Engineering Process for Rubber Compression Molding?

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.

What is the Prototyping Process for Rubber Compression Molding?

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.

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What is the Production process for Rubber Compression Molding?

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:

What is 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.

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