Tire mold quality and manufacturing process crucial for the performance characteristics of premium products
Construction of high-quality molds for complex winter tires is a complicated process
In addition to the compound and structure of the tire carcass, tire molds used in the curing process are among the most important aspects in the manufacture of premium summer and winter tires. Due to their complex tread structures, winter tires in particular require a very precise mold ("tread negative") for the curing process. In recent years, designs – particularly for tire tread strips – have become increasingly complex, meaning that producing extremely precise molds poses a huge challenge for today's tire manufacturers. On the other hand, these designs also provide an opportunity to use leading mold technologies to stand out from the competition.
Experts make a distinction between two mold manufacturing processes: A mold can be either cut according to 3D information provided by the developer or manufactured using a sand casting process. While the former method is comparatively less complicated, even making the mold for the light alloy casting is a complex, multi-phase process that requires a lot of experience in mold manufacturing. The "base frame" for the subsequent "tread negative mold" is formed by cutting out the individual segments of the tire mold; holes for the vent valves of the mold are then drilled using a variety of very precise operations, and the individual sipes are welded to the grooves milled beforehand. After an extremely thorough, mandatory final inspection, the mold is ready and can be used in production.
Producing a casting mold is far more complicated. This process involves translating all the 3D data for the tire to be produced, including the attachments for the sipe plates, into CNC data for the mold to be produced and using this data to cut an element out of wood. Technical sipes are inserted into this wooden model so that the position and height of the subsequent production sipes can be defined flexibly and precisely. This model can therefore be used repeatedly to produce a flexible cast as only the predetermined heights and positions of the production sipes are required. This flexible model can be used to make a gypsum cast that includes the future production sipes. This mold is then used as a negative for the casting mold to be used to produce the tire mold. After the casting process, which must be performed to the highest level of accuracy, the casting ring is cut into segments of varying lengths and inspected. The segments are then adjusted to the optimum length to allow for heat-related expansion when the tires heat up and to ensure optimal concentricity. The mold is ready for use once holes for the valves and vent valves have been drilled.
Both processes have their advantages and disadvantages. While using the cutting process to produce the mold is faster, the casting process is able to produce significantly more complex molds. The casting process also has the advantage of the flexible tread profile cast, which can be used in mold production again and again. A tire mold produced by cutting, on the other hand, is a one-off.
Continental has many years of experience in the manufacture of complex tire molds. At the beginning of 2015, in a bid to strengthen its expertise in the manufacture of these cutting-edge tools, Continental in Lower Saxony acquired AZ Formen- und Maschinenbau GmbH, based in Runding, Bavaria. Together with its additional factory in the Czech Republic, the AZ Group employs around 350 employees and is now part of the Tire division at ContiMachinery.