After we discussed thermoset injection molding in the last blog, this blog deals with elastomers.
An elastomer is a plastic that can be made elastic by vulcanization. And it was the exploration of elastomers that decisively influenced the entire development of plastics. Because the scientific run of artificially produced substances, i.e. plastics in general, began when people wanted to break the monopoly of rubber. Thousands of researchers wanted to invent the first artificially produced rubber and get rich doing it. However, along the way, they first discovered many other plastics – such as acrylic glass.
Rubber is Vulcanized
Until then, rubber was the only material that had some elasticity. Even Aztecs were playing with rubber balls, and after the material was discovered in Europe and the USA, people were fascinated by its possibilities. in 1839, a certain Mr. Frank discovered an industrially applicable process that could transform rubber from its original state into an elastic form. The so-called vulcanization process made possible the first mass production of tires, which, among other things, one can easily infer from the name of the inventor. Vulcanization remains a decisive factor in the injection molding of elastomers, which must be transformed from free-flowing to elastomeric molded parts in the injection molding machine. Only a precise understanding of this process later allowed the development of synthetic rubber.
Initially, Goodyear used a boiling method in which sulfur was added to the liquid rubber. Today, it is only necessary to heat a suitable elastomer to a certain temperature. Elastomers, such as rubber, are made up of large molecules linked together in long chains. When heated, the macromolecules of the new elastomer are connected to each other by bridges, which in Mr. Goodyear’s rubber are still formed by sulfur molecules. Vulcanization remains a decisive factor in elastomer injection molding, which must be transformed from a trickle to an elastomeric molded part in the injection molding machine. Only a precise understanding of this process later allowed the development of synthetic rubber.
How Elastomer Injection Molding Works
Injection molding of elastomers is similar to injection molding of thermoplastics in that the raw material is strongly liquefied by heating and pressed into the mold. However, the cross-linking of the molecules, which is essential to the elasticity of the plastic part, occurs through vulcanization, which is preceded by the second stage of heating in the mold and then cooling. The density of cross-linking plays a decisive role in the degree of elasticity.
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Characteristics of Elastomer Injection Molding
When injection molding elastomers, this phenomenon must be carefully observed and approached accordingly. Unlike thermoplastics, elastomers do not simply liquefy by heating, but rather solidify as they cool in the mold, which is a hollow mold that gives the part its shape. Instead, the process is carried out in discontinuous stages. Vulcanization is combined with molding. In a screw molding machine, the elastomer is drawn into the machine as a trickle (or ribbon) and heated to approximately 176 °F (80°C) in a preheated screw, which liquefies it. In the screw, the elastomer is kneaded and formed into a uniform mass with a uniform temperature distribution, resulting in the preparation of a higher quality part. When the elastomer is injection molded with a plunger injection unit, this task is performed by a specially preheated barrel. The final very thin mass is then pressed through a channel into the cavity of the mold, where the final vulcanization and curing take place.
During the pressing process, additional frictional heat is generated to prepare the vulcanization, thus shortening the process. During the construction process, the nozzles, inlets, and molds must accommodate the low viscosity of the elastomer. At the same time, the thin liquid makes the work extremely precise. During the vulcanization process, a certain amount of material expansion and the resulting pressure occurs, which the closed mold resists and thus additionally supports the formation of precise shapes. After a calculated service life, the phase ends and the part can be ejected or blown out of the reopened mold. The length of this service life occupies most of the process. For this reason, special injection molding machines that can operate multiple clamping units and therefore molds are often used for elastomer injection molding.
High-tech Rubber for Automotive Tires
Elastomers are versatile because their elasticity varies depending on the cross-linking of the molecules. In addition, the composition of elastomers varies. The most famous manifestation of elastomers in everyday life is of course, on the one hand, the very mundane rubber band in the kitchen. On the other hand, we find today’s automotive tires, which are truly high-tech products with many pre-calculated and specially developed properties. For example, the wear resistance and temperature resistance required for high-speed tires require the use of silicone or so-called fluoroelastomer types.
In the case of silicone rubber, the cross-linking of the macromolecules required for elasticity is supported by additional reinforcing agents such as organic peroxides. In the case of high-temperature cross-linked silicones (HTV), this results in a plastic that retains its elasticity, and thus its shape, even at large temperature differences. As a result, this plastic is suitable for use at temperatures between -104 °F (40 °C) and 482 °F (250 °C) without losing its most important properties. Other elastomers solidify into a glass-like substance at low temperatures. Therefore, the moment in the temperature range at which elastomers lose their elasticity is also known as the glass transition temperature.
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If you would like to learn more about the process of injection molding plastics or especially Elastomer Injection Molding processing, please contact us or visit our blog. As mentioned earlier, many episodes cover these topics. We look forward to getting to know you!