Even the casual driver realizes that tires are complex objects, subjected to severe constraints, yet required to perform consistently. The composition of tires has evolved dramatically since the early days, and modern tire manufacturing technologies are in a state of constant evolution. As with so many other industries, the tire industry must be able to quickly design newer and better products at a reduced cost. To face this challenge, it is no longer possible to rely only on empirical know-how, or build-and-test methods for product development. Advanced tools and technologies are now being introduced at several key stages of the design cycle with the hope of making this process much more efficient. The POLYFLOW CFD code has been used successfully for analyzing several steps involved in the making of a tire.

Twin-screw extruder modeling

Rubber compounds are currently produced almost exclusively by means of internal mixers that combine the raw materials and form homogeneous rubber. The quality of the mixing determines the uniformity of the product, and is key to ensuring homogeneity in the resulting compound. To achieve a quality standard at a reasonable cost, an understanding of the mixing can be obtained through the numerical simulation of batch mixers. To simplify the set up of such a simulation, a technique referred to as the mesh superposition technique (MST), has been implemented in POLYFLOW. This is a very powerful technique to simulate flows with moving and intermeshing parts, such as in mixers or twin-screw extruders. To assess the mixing, a tracer can be introduced into the flow, and the extent of its spreading can be evaluated after a certain period of time.

After the mixing phase, additional processing occurs in single- or twin-screw extruders, devices that perform many functions. The screws provide a pumping function to deliver a consistent through-put and to build the pressure required for downstream extrusion tools that shape the part. A careful thermal balance must be maintained to ensure even heating and homogenization, while controlling the temperature rise to avoid scorching. Additives may also be added at this stage, so the screws must be able to provide a significant amount of mixing as well.

Shear rate distribution in twin-screw extruder simulation

The final shaping is done by continuous extrusion through the use of dies. Numerical modeling of rubber extrusion is used by tire manufacturers to improve the geometrical die data and processing conditions, and to reduce the number of experiments on the production line. The unique die design functionality of POLYFLOW has been used to predict the die lip shape required to obtain a prescribed tire tread. In one example, three iterations in the numerical design phase were required to develop a new die shape, whereas six to ten (or more) trial-and-error iterations are usually required for the same purpose. Once the extrusion is complete, curing, the final step in tire manufacturing, takes place. Numerical modeling is used to optimize the cure cycle, thus minimizing capital and energy expenditures.

Using CFD, the design and manufacturing process of new tires can be modernized, with a substantial shift from prototype building and testing to innovation and product quality by computer aided engineering. Therefore, better tires can be manufactured in a more expedient manner. However, the use of CFD does not stop after the tire production is complete. Once the tires are fitted on a vehicle, CFD can also be used for safety analysis, exploring such issues as aquaplaning.

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