Venting HolesVent cleaning and maintenance are a major concer for the tire industry.

During the cure cycle, the green tire expands until it completely fills the mold. The segments push in from outside while the bladder pushes out from the inside. As rubber is pushed into the mold, air must be evacuated through vent systems, allowing the rubber to completely fill in each mold detail: venting holes and venting valves in molds allow the air to escape.

Venting holes

In case of air holes, each vent acts like a small extrusion die hole that allows some uncured rubber at the green tire surface to flow into the hole. As the rubber fills the mold, some of it is forced into these air holes, creating hair-like projections called vent spews. Most of the rubber whiskers pull out of the vent holes and remain attached to the new tire upon removal from the mold, but several may mechanically adhere to the vent hole interior bore and separate from the tire. Most of the mechanical adhesion of the extruded rubber “whisker” in a clogged vent occurs at the entrance “lip” of the vent hole where the “squeeze” pressure on the rubber is high, and the extrusion flow velocity of the uncured rubber is low. The remaining majority of the whisker extending into the vent hole has very little adhesion to the walls of the hole.

The traditional vent clearing method has been to remove the mold from the press, grit blast the sidewall and tread surfaces, then drill the cured rubber out of each vent hole with very small drill bits and air tools.

This manual cleaning is a labor-intensive operation: clogged vents must be drilled out and vents repaired when drill bits break off in them.

For this reason it is important to use high quality drill bits that combine high mechanical performances in terms of elasticity and resistance to high temperature, with very specific sizes both in length and in diameters required. Typically these drills extend from a range of diameters between 0.4 mm to max 1 or 2 mm, while their cutting helix length ranges between 20 mm up to 100 mm for the longest drills. Materials used are also extremely important, with a prevalence of Cobalt High speed steels or coatings to improve the overall elasticity of the tools. All these features place these drilling tools outside of the standard range of commonly used drilling items, making them industry-specific. When these drill bits break off, they can permanently plug the vents: then the vent must be removed and replaced with a vent insert, creating a subset of the clogged vent problem. Therefore, the choice of a supplier who is conscious of the specificity of the usage requested becomes a critical factor for all tire manufacturers adopting air venting systems.
Alternative technologies are in use in the industry too. Sometimes a high-velocity stream of Co2 particles and the air following immediately after the initial pellets’ impact; generally also a laser flow can blow the unanchored “whisker” out of the hole, through the vent passages, and completely out of the tire mold.

Air Spring Valves (Ventless Molds)

A major trend today is to produce tires devoid of the “whiskers” produced by vents or microvents, through the aid of air spring valves. Another way to achieve this is through the use of puzzle mould technology; but this technoloy poses no specifical cleaning problem compared to other molds. Especially Japanese customers and manufacturers are pushing the tire industry toward “whisker-free” tires – although a great percentage of consumers still isn’t convinced that the tires they are buying haven’t been re-capped unless they can see the “whiskers” on them. However, these “Ventless Molds” bring along some advantages, such as no need to remove rubber pins and flashes, an operation that that may complicate work-flow in production, while adding to the esthetic look of the tire: only slight witness marks are visible on the tire surface.

The cleaning operations of Air Spring valves poses a few problems, for which it is importanto to clearly understand their function and characteristics.

Vent valve for venting bores of vulcanization moulds, especially for the tread profile area, consisting of a valve sleeve, in which a valve insert and a wound spring are located. These vaves are pre-assembled precision vent valves which are press-fitted into straight holes drilled into the mold. While the rubber is flowing in the mold and air needs to be vented, the valve stays open; it closes automatically before rubber is squeezed into the valve bore. These valves allow for increased intervals between mold cleaning cycles may become possible, increasing line availability. Valve-pin and spring can be pre-assembled and permanently fixed together, or separated and mounted manually on the tire mold. In both cases, if a valve is finally worn out it can easily be removed, the installation bore redrilled to the next larger diameter and the next size valve inserted.

The assembly into the mold is done with hand-tools, allowing sensitive adjustment to the curvature of the mold; diameters of each single valve varies between 2 and 4 mm. Additionally, in the sidewall area and in tight radius-areas, the rubber flow inside the mold is not head-on towards the valve but rather in an tangential direction. This flow pattern, combined with sometimes softer rubber compounds, has a tendency to clog vent valves.

These valves feature special hardened surfaces and wear- resistant materials for extended service life despite high pressure and process temperatures and abrasive additives in the rubber compound. When tire mold is made of Al material, Air vent should be also in Al instead of other material such as SUS or Fe, in order to have an equal coefficient of thermal expansion.

While ventless molds may address the requirements of OE customers, they may go against the desires and beliefs of those customers in the replacement market. Regardless of what the tire industry does, though, CO2 / dry ice blasting technology still remains the preferred way to clean these types of molds.

While providing the best solution to the cleaning specificalities of air spring vents, however, Co2 cleaning does not solve the main additional problem with particular silica compounds. In fact, single sand particle is able to block the spring vent mechanism; therefore the use of additional ultrasonic tools is very important for the maintenance of these vents.

Typically, a cleaning ultrasonic tool is vibrating after switching on. After placing the vibrating tip to the vent stems head, it will vibrate the whole vent and allow for all remaining particles to drop off. This operation is not so much time consuming on the single vent; however, being a manual operation that needs to be performed on each valve of the mold at the end of each cleaning cycle, it adds to a significant increase in labour work. It is recommended to use ultrasonic tool as a last operation during mould cleaning, and not to clean mould by air from outside a mould, always using the same direction as the rubber flow.

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