Dry Ice Blasting Technology for the Tire Manufacturing Industry

CO2 / Dry Ice Blast Cleaning in the Tire Industry
The use of CO2 / dry ice blasting for the non-abrasive cleaning of tire mold sidewalls without the removal of the mold from the press or the creation of a secondary waste stream has intrigued tire manufacturers since the mid-1980’s. The collective experiences of those tire manufacturers who were "early adopters" of this technology have led to a more pragmatic approach regarding the use and benefits of CO2 / dry ice blasting. The result has been the continued success and growth of the CO2 / dry ice blasting industry with significant support from the tire manufacturers. Mainstream technology acceptance has arrived in particular for those CO2 / dry ice blasting technology suppliers who listened to these pioneer customers in the tire industry, and worked diligently with them to provide fully developed tire mold maintenance solutions.

Today, most tire manufacturers are aware of CO2 / dry ice blasting technology and its potential to reduce production downtime and labor costs, and enhance product appearance. In fact, tire manufacturers make up a major segment of the CO2 / dry ice blast cleaning equipment sales and contract cleaning services market. And this will probably be true, at an increasing rate, for many years to come. While working to develop applications solutions for the tire industry, CO2 / dry ice blast cleaning technology suppliers began to find solutions for the general problems of noise, ergonomics, operator safety, work area accessibility, system reliability and operating costs. The economic impact of implementing CO2 / dry ice blast mold cleaning within the tire industry is overwhelmingly significant. The cost to benefit analysis usually shows the payback period in terms of months, not years. In the course of developing supplier-customer relationships in the tire industry, it was discovered that a tire manufacturer could realize over a million dollars per year in savings from labor and mold rework cost reductions, increased mold and press up-time, increased tire production and a reduced scrap tire rate. There is even a significant cost reduction from not having to purchase thousands of drill bits per year to replace the ones broken off while attempting to clear clogged vents and microvents.

The following discussion will provide an in-depth understanding of state-of-the-art CO2 / dry ice blast tire mold cleaning technology. From a cost to benefit standpoint, CO2 / dry ice blast mold cleaning is the best choice among many methods and technologies currently available. Within the CO2 / dry ice blasting industry, however, there are a variety of technologies that offer different levels of tire mold cleaning cost and performance. The mold cleaning performance levels of the different types of CO2 / dry ice blasting systems are very significant. Failing to understand these fundamental differences may lead to the selection of an inappropriate system and a significant reduction in potential productivity increases and cost savings.

Similarly to abrasive blasting equipment, there are two types of CO2 / dry ice blasting systems. One type is the direct feed or "single-hose" system. The other is the inductive feed or "two-hose" system. In abrasive blast systems there may not be significant cleaning performance differences between the two types of systems. However, for CO2 / dry ice blasting systems there is a dramatic performance differential between the types, which must be fully understood and considered before selecting a tire mold cleaning system (for more information you may continue reading or click here to jump further down). There are also two basic forms of the solid CO2 / dry ice blasting media that must be understood. These are discrete "pelletized" dry ice particles, and shaved dry ice "flakes" produced from a block of dry ice (for further information you may continue reading or click here to jump further down). The purpose of this paper to provide the reader with a detailed understanding of these and other aspects of state-of-the-art CO2 / dry ice blast tire mold cleaning technology.

Why Tire Curing Molds Need to be Cleaned
A major problem faced by all tire manufacturers is that of mold fouling, a residue build-up on the curing surfaces of tire molds caused primarily by the chemical reactions between sulfur and zinc oxide under heat and pressure. Excessive fouling in the bead area of a tire mold can cause enough irregularity on the finished tire bead surface that the tire will not seal properly on a wheel. Over time, the tire will slowly leak air, resulting in a highly dissatisfied or irate customer. Furthermore, the finely sculpted alpha-numeric characters of the D.O.T.* information must, by government regulation, remain clear, crisp, and completely legible from cure to cure; the surface of the sidewalls must maintain a uniform texture and gloss level to satisfy the market demands for aesthetics; the tread area must be free of fouling to prevent light spots on the tread lugs; and the brand logo and lettering must remain very crisp and precise because that is usually the focus area from which customers develop an initial perception of a tire company’s product quality.

These are all excellent and obvious reasons for keeping tire molds clean. However, they are not the only ones. A vast majority of the tire molds in use today have vents and/or microvents to expel air trapped between the green tire surface and the mold surface as the bladder expands the green tire into the mold cavity during cure. Typically, the microvents are between 0.02 and 0.04 inches (0.5 mm to 1.0 mm) in diameter, and the vents are between 0.04 to 0.06 inches (1 mm to 2 mm) in diameter. Both types can extend an inch (2.5 cm) or more in depth into the mold. A typical passenger car tire mold contains thousands of these vents. The problem lies in that the bladder expansion pressure, combined with the elevated curing temperature, causes some of the tire's surface rubber to "extrude" into these vents. When the cure cycle is over and the tire is released from the mold, most of the extruded rubber in these vents remains attached to the cured tire and pulls back out to form the familiar rubber "whiskers" on new tires. However, not all of the microvents release their extruded "whiskers, and over time, more and more of the vents become plugged with rubber and cease to function. When this occurs, air trapped in the molds begins to cause surface irregularities and other faults on the finished tires, which ultimately increases the production scrap rate.

Another area of concern is keeping mold surfaces, which mate or come into intimate contact during cure, free of residue build-up. For two-piece tire molds, these are the surfaces between the two mold halves that come into contact when the mold is closed and that produce a parting line in the mid-cross section of the tread pattern. If too much residue is allowed to build up in this area of the mold, the halves will not mate together completely, even under the extreme squeeze pressure of the press. The result is a noticeable flash around the circumference in the middle of the tread pattern on the finished tire. An excessive amount of flash results in additional labor costs to remove it.

For larger and wider tires, curing is typically done with segmented molds. The fit between adjacent mold segments and the fit between the closed segment "ring" and the sidewall plates have very tight tolerances. If too much residue builds up on these components, small gaps will develop resulting in flash on the tires. For tires cured in segmented molds, flash can be very noticeable and objectionable because it occurs partly on the sidewalls and across the tread pattern. Also, a build up of residue on the mating mold surfaces can cause high mechanical stresses in the fasteners, which attach the segments and sidewall rings to the press. The high mechanical loads can cause fastener failures, which typically result in very costly damage to both the mold and press.

Mold Condition Factors That Affect Cleaning Performance...Temperature
Data gathered since the emergence of CO2 / dry ice blast cleaning in the tire industry supports the fact that tire molds between 300° F and 350° F (149°C and 177°C), can be cleaned 3 to 4 times faster than the same molds at ambient temperature. Although the reasons and mechanisms that give rise to this phenomenon are not completely understood, mold cleaning experience in curing departments at many different tire manufacturers have proven this to be the case. In studies of tire mold fouling, it has been determined that the more reactive chemicals are present in the base polymer, the more the chemicals in the cure accelerators and inhibitors, and the chemicals in many mold release agents, will combine at curing temperatures to form an almost glass-like material at the product-mold interface. This glass-like material is different from the polymer material of the cured product. The glass-like property of this fouling residue at elevated temperatures allows it to be easily removed from the mold surface by fracturing it into small particles by inducing high levels of thermal stress, or "thermal shock " with CO2 / dry ice pellets. Since the temperature of solid CO2 / dry ice is -109°F (-78.3°C), the CO2 / dry ice pellet blast stream is an ideal source for inducing thermal shock in the residue layer. At lower temperatures (below 150°F / 65.6°C), the fouling residue becomes much more difficult to remove from the mold surface because it resembles a very hard visco-elastic material, which absorbs the impact energy of the CO2 / dry ice pellets. The thermal shock mechanism ceases to function because there is very little temperature differential between the material and the mold surface. The overall result is very difficult residue removal from room temperature, or "cold", tire molds, and sometimes the residue will not respond to CO2 / dry ice blasting at all.

Mold Condition Factors That Affect Cleaning Performance... Mold Surface Condition
Abrasive blast media, like plastic or glass beads, typically leave a "bare metal" appearance after residue removal, even on steel tire molds. This "like new" appearance is deceiving because it is achieved at the expense of removing a small amount of metal from the mold surface, and by imparting a much "rougher" surface finish (more micro "peaks and valleys") into the mold from the chiseling effect of thousands of abrasive impacts. The rough surface creates an "anchor pattern" that was not present in the original mold surface. This causes fouling residue to adhere and accumulate at an even faster rate than it did on the original mold surface. This mold surface "erosion" will be in discussed in the next section in more detail, but it is evident that what appears to be a "clean mold" surface is a step toward decreasing the useful life of a very expensive production tool.

CO2 / dry ice blasting does not abrade or erode the surface of most common mold materials. Since CO2 / dry ice blasting only removes the residue on the mold’s surface and not any surface metal, any dark stains from cured tire compounds will remain on the mold’s surface. Following CO2 / dry ice blasting, a functionally clean, residue free tire mold may not at first appear clean by the old standard of a bright, bare metal surface. The proof of the mold’s cleanliness will be seen when the first tires are cured and inspected for the sharpness of the tread, lettering, and logo details, and sidewall surface gloss level.

Tire Mold Cleaning Technologies and Mold Erosion / Damage
The most well known and widely used method of tire mold cleaning is abrasive particle blasting. This method is very cost-effective, easy to install and maintain, and relatively easy to use. All forms of abrasive blasting MUST be done in an enclosed structure to prevent the distribution of fine airborne abrasive dust particles within the factory environment, and to capture and recycle the spent media. The most popular abrasives used to clean tire molds are glass, plastic, metallic and ceramic beads. These media have gained acceptance in the tire industry because they are regarded as only ‘mildly abrasive". Other abrasive particle blast media used in the tire industry include silica sand, steel shot, walnut shells, bicarbonate of soda, and abrasive impregnated sponge. All of these abrasive blast media can typically be captured and recycled for use in more than one cleaning session. However, all of them eventually breakdown (pulverize) into a fine dust, which must be disposed of in compliance with federal regulations. The fact that all of these media types are considered "abrasive" means that the tire molds will ultimately be eroded to a point where they must undergo extensive rework or be scrapped. Abrasive blast tire mold cleaning is, at best, a compromise between a cost-effective cleaning method and reduced tire mold life.

Abrasive or even "mildly abrasive" blasting causes other problems as well. The imbedded fine silica "dust" residual from sand or glass bead blasting, or the imbedded plastic "dust" from PMB blasting, can alter the surface of the tire mold enough to prevent the proper chemical bonding of certain release agents. These mold release agents that depend on a completely metallic surface to bond to for providing many cure cycles worth of release, are actually pulled off of the mold surface and rendered ineffective in fewer cure cycles because of the "bond blocking" effect caused by grit residue imbedded in the mold surface. The same type of chemical "bond blocking" can occur when attempting to apply various mold coatings for long term product release capability. In general, all chemicals applied to a metal mold surface react much faster and more efficiently when all of the metal surface is available and not masked by grit residue.

Other non-abrasive tire mold cleaning technologies that exist, or are emerging, include laser ablation, chemical flushing, and the mechanical adhesive bonding of residual rubber. These methods are still in their developmental stages and do not currently offer a near-term solution to the immediate and near term needs of the tire industry. Of all the generally accepted "off the shelf" tire mold cleaning technologies currently available, only solid CO2 / dry ice blasting has been acknowledged to be non-abrasive, cost-effective, and to not produce a secondary waste stream or bond blocking residue dust on the molds. Tables 1 and 2 below present data from two mold erosion studies that were conducted by a major tire manufacturer in 1991, and again in 1996. Table 1 shows the results of the 1991 tests, where CO2 / dry ice pellet blasting was examined to see what erosion effects were apparent in four types of tire mold materials (steel, forged aluminum, and two types of cast aluminum) using three different CO2 / dry ice pellet blasting nozzle configurations.

All samples were blasted with CO2 / dry ice pellets at a 300 psi (20.7 bar) blast pressure. Pellet mass flow rate was 250 pounds (113.6 kg) per hour. Three different single-hose system blast nozzles were used. Negligible effects of the blasting were seen on the steel and forged aluminum samples, while both cast aluminum samples showed minimal to severe erosion. Nozzle design had the most significant effect on erosion rate and cleaning cycle time.

Top sidewall: 5 minutes
Bottom sidewall: 6 minutes
Tread: 16.5 minutes

The 1991 and 1996 mold erosion studies were conducted at significantly higher blast pressures (250 to 300 psi / 17.2 to 20.7 bar) and at higher particle velocities than are actually required to clean tire molds and vents. This is because CO2 / dry ice pellet blasting nozzle design technology has advanced significantly since the 1991-1996 era. Nozzle and delivery system improvements now allow tire manufacturers to completely clean tire molds and clear vents and microvents down to 0.03 inches (0.8 mm) in diameter, at blasting pressures no higher than 55 to 60 psi (3.79 to 4.14 bar) . Pellet flux density (i.e. the distribution of CO2 / dry ice particle impacts per unit area per unit time) at the mold surface is now better than it has ever been because of the aerodynamic advances in single-hose blast delivery systems. The tire mold fouling residue removal capability of CO2 / dry ice blasting is better than it was in 1996 (or 1991) and with much lower air pressure and overall kinetic energy delivered to the mold surface. Presently, CO2 / dry ice blasting induced mold erosion, even for cast aluminum molds, is considered negligible, and a significant number of tire manufacturers are now specifying and purchasing CO2 / dry ice blast mold cleaning systems for new production facilities, as well as to replace their existing abrasive blasting systems.

Furthermore, CO2 / dry ice blasting can effectively remove most imbedded grit residue left on the mold surface from abrasive cleaning methods, as well as remove grit residue from vents and microvents. This allows mold release agents to work better, and can even lead to the reduction of the amount of mold release required, which in turn will substantially decrease the rate of deposit of mold fouling substances.

Another benefit related to the low blast air pressure and volume requirements of today’s single-hose CO2 / dry ice blasting systems is that it reduces the costs associated with the required compressed air system. Earlier, high-pressure based single-hose CO2 / dry ice blasting systems required expensive, dedicated air compressors. Most past and current two-hose CO2 / dry ice blasting systems require two or more times the air flow volume, even at "low pressure", of a state-of-the-art low-pressure single-hose system. In a compressed air system, high pressure and high flow volume dramatically increase operating costs in terms of equipment costs and energy (electricity) consumption. Any cost to benefit analysis for a CO2 / dry ice blast tire mold cleaning system should include the compressed air system's fixed and operating costs.

Mold Vents, and Microvents
Vents and microvents present a unique cleaning and maintenance problem for the tire industry. During the cure cycle, as the green tire expands and air is evacuated through the vent system, each vent acts like a small extrusion die hole that allows some uncured rubber at the green tire surface to flow into the hole. The rubber then cures to form a "whisker" on the tire surface. 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. This process repeats to clog a few more vents each cure cycle. Eventually, so many vents become clogged that they have to be cleared to allow enough air to escape when the green tire expands. 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. A subset of the clogged vent problem occurs when these very fragile drill bits break off to permanently plug the vents. Then the vent must be removed and replaced with a vent insert. Typical passenger car and light truck tire molds contain thousands of vents or microvents. The labor required to drill out clogged vents and to repair vents when drill bits break off in them, plus the cost of hundreds of drill bits consumed per month, adds up to a very significant yearly cost for most tire manufacturers.

In recent years it has been discovered that CO2 / dry ice pellet blasting is effective in removing almost 100% of the rubber "whiskers" and other residue from even the smallest diameter microvents in tire molds. Furthermore, the vent clearing can take place in the press at the same time as the surface fouling residue is being removed, so actual "one step cleaning" can be accomplished. This is the result of the state-of-the-art single-hose CO2 / dry ice blasting nozzle design, which allows for the delivery of very high surface impact energy and thermal energy at very low air pressure and volume.

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. CO2 / dry ice pellet blasting possesses enough energy to quickly remove the small amount of tightly adhered rubber at the base of the "whisker." The high-velocity stream of particles and air following immediately after the initial pellets' impact, simply blow the unanchored "whisker" out of the hole, through the vent passages, and completely out of the tire mold. The ability of CO2 / dry ice pellet blasting to accomplish this type of vent-clearing in one step represents a revolutionary productivity gain for the tire industry.

Direct Acceleration Systems vs. Inductive Systems (or "Single-Hose" vs. Venturi / "Two-Hose") —Kinetic and Thermal Energy Effects.
Solid CO2 / dry ice blasting systems are available in two basic configurations. The least complex and least costly to produce is the inductive, "two-hose" system, sometimes called an "inductive, or venturi system". These systems are typical of sand, PMB, and glass bead blasting, and most CO2 / dry ice blast systems available today employ this method. With these systems the blast media is sucked into a chamber in the hand held applicator, or "gun", by the venturi effect and then propelled out of a short nozzle by a high-volume flow of compressed air. Because these systems rely on the creation of a strong suction to bring the blast media from the storage hopper to the nozzle, the length of the interconnecting dual blasting hose is typically limited to fifteen feet (4.6 meters) or less. In the two-hose system, one hose is the media suction hose that can be constructed from lightweight material, and the other hose is the compressed air delivery hose, which is typically heavier in order to withstand pressures as high as 200 psi (13.8 bar) or greater.

In the two-hose systems, the media particles are moved from the hopper to the "gun" chamber by suction, where they drop to a very low velocity before being induced into the outflow of the nozzle by the large flow volume of compressed air. Since the blast media particles have only a short distance in which to gain momentum and accelerate to the nozzle exit (usually only 8 to 12 inches / 20.3 cm to 30.5 cm), the final average particle velocity is limited to between 200 and 400 feet (61 and 122 meters) per second. So, in general, two-hose systems, although not as costly, are limited in their ability to deliver contaminant removal energy to the surface of a mold. When the need for more blasting energy is required, these systems must be "boosted" at the expense of much more air volume, usually higher blast pressure as well, leading to much more nozzle back thrust, and very much more blast noise generated at the nozzle exit plane.

The other type of solid CO2 / dry ice media blasting system is like the "pressurized pot" abrasive blasting system common in the sand blasting and PMB blasting industries. These systems use a single delivery hose from the hopper to the "nozzle" applicator in which both the media particles and the compressed air travel. These systems are more complex and a little more costly than the inductive two-hose systems, but the advantages gained greatly outweigh the extra initial expense. In a single-hose solid CO2 / dry ice blasting system, sometimes referred to as a "direct acceleration" system, the media is introduced from the hopper into a single, pre-pressurized blast hose through a sealed airlock-feeder. The particles begin their acceleration and velocity increase immediately, and continue to gain momentum as they travel the length of the hose. At the end of the hose, the spray nozzle "gun" actually consists of a convergent-divergent (isentropic flow) nozzle, which exchanges pressure differential across the nozzle for a huge increase in air and particle velocity. CO2 / dry ice particle velocities have been measured and substantiated in excess of 700 feet (213.4 meters) per second, and up to as high as 950 feet (289.6 meters) per second at the nozzle exit plane, and this is accomplished at less than one third (1/3) of the flow volume required by the most aggressive two-hose systems.

In addition to the lighter weight and less cumbersome hand held applicator and hose of a single-hose system, the contaminant removal energy delivered to the surface is considerably higher than that provided by a two-hose inductive system. Even with solid CO2 / dry ice blasting, a significant component of the of the contaminant removal energy is the kinetic energy delivered to the surface per unit of area. Since kinetic energy is a function of the mass and velocity of the particles in the relation K_e=1/2" mv², it can be seen that a two-fold increase in particle velocity, given equal particle mass and equal nozzle spray area, effectively increases the impact energy delivered to the surface by a factor of four. A three-fold particle velocity increase, from 300 to 900 feet (91.4 to 274.3 meters) per second, increases the blast impact energy nine times!

Table 3 below shows the relative cleaning performance between a single-hose system and a two-hose system at typical "factory air" blasting pressure. The term C_vbe is called the Blast Energy Coefficient, and represents the comparative capability of the CO2 / dry ice blasting systems to remove a volume of soft pine wood from a test specimen within a controlled interval of time.

Table 3. Comparing the Blast Energy Coefficient of Single Hose vs. Two Hose CO2 / Dry Ice Blasting Systems

As the data in Table 3 illustrates, the typical blasting energy of a single-hose system compared to that of a two-hose system, at equal blast pressure, is about three times greater.

CO₂ Blasting Media Types
Solid carbon dioxide (dry ice) blasting media is currently available in two forms, discrete rice grain sized pellets, which are produced by pressure extruding and cutting "strings" of dry ice, and sugar granule-sized flakes of dry ice produced by mechanically shaving the face of a large block of dry ice. To understand the differences in mold cleaning performance between the two CO2 media forms, some technical background discussion is appropriate.

Traditional abrasive particle blasting, and even the "mildly abrasive" blasting technologies, rely on the intrinsic surface hardness and geometry of the media, and the work available at the surface resulting from the kinetic energy of the media acting through the surface hardness and geometry.

The hard, sharp abrasive particles actually break into smaller pieces that "ricochet" into mold surface features for additional residue removal action. The total kinetic energy of abrasive media particles is therefore spread out over more than a single impact per particle. With solid CO2 / dry ice media however, the particles completely disintegrate and sublimate to CO2 vapor upon initial impact, so all of the solid CO2 / dry ice particles' kinetic energy is spent in one impact per particle. There is no ricochet or secondary impact effect in solid CO2 / dry ice blasting. Therefore, CO2 / dry ice blast mold cleaning performance is determined by a parameter called flux density. Flux density is defined as the number of particle impacts at the mold surface per unit of area per unit of time. In other words, assuming that the particles from each of two dry ice blasting systems possess sufficient and equivalent kinetic energy, and that these systems have nozzles of equal exit plane area, the system that can deliver more particles to the surface in the same amount of time, or the same amount of particles in less time, will generally remove fouling residue faster and more completely than the other system.

CO2 Particle Dynamics
As mentioned earlier, CO2 / dry ice blasting harnesses two types of energy to accomplish mold fouling residue removal. CO2 / dry ice particle size directly influences the levels of kinetic (velocity or impact) energy and thermal (temperature gradient or thermal stress) energy available at the surface. The sugar grain sized particles resulting from shaved dry ice block are roughly spherical and 0.5 mm to 1 mm in diameter, whereas the large pelletized CO2 / dry ice media is typically 3 mm in diameter and between 5 mm and 8 mm in length. However, by the time the pellets are accelerated through the blast hose and nozzle, they are fractured into roughly uniform sized irregular spheres of dry ice of about 2 mm in diameter. Given that the solid CO2 / dry ice is of the same density in both particles, the fractured pellet "spheres" possess about 4 times the mass of the individual shaved flakes or granules. Referring back to the kinetic energy equation, each pellet, if traveling the same velocity as each granule, will deliver 4 times the impact energy of the granule at the surface. Since the fractioned pellet spheres in the single-hose blasting system typically travel 3 times faster than the shaved block granules in the two-hose system, the kinetic energy increases by a factor of 4 X 3² = 36. This is the significant underlying factor in the ability of the high-velocity, single-hose CO2 / dry ice pellet blasting systems to dislodge and remove the rubber "whiskers" from tire mold vents and microvents.

Thermal energy is dependent upon the mass (number and size of particles) of solid CO2 / dry ice delivered to a given area of the surface per unit of time. There is tremendous latent heat transfer as the solid CO2 (dry ice) changes phase to vapor CO2 at the mold surface (246 BTU per pound (0.45 kg) of solid CO2 / dry ice). This heat exchange that occurs with each impacting CO2 / dry ice particle happens within a few milliseconds and the heat is given up mostly from the thin layer of residue, though some comes from the surface of the mold. It is this instantaneous "surface only" heat transfer effect that imparts the thermal stress into the residue to fracture it from the mold surface. Having already described the particle velocity and mass delivery characteristics of the single-hose, isentropic nozzle systems relative to the two-hose inductive nozzle systems, it is evident that the single-hose systems deliver more thermal mass per unit of area per unit of time. Therefore, it produces the best thermal "shock" or residue fracturing effect. If the CO2 / dry ice blasting system cannot deliver this effect efficiently and instantaneously, and if the nozzle traverse rate over the surface is reduced to "make up" for a lower thermal mass delivery rate, the effect will be lost as the mold cross section begins to lose heat from "too much CO2 striking it too slowly". This is typically why two-hose inductive systems fail to give rise to the thermal fracturing effect in tire mold cleaning applications. It is a matter of too little kinetic and thermal energy available in a given instant on the mold surface to be fully effective.

Table 4 below presents the results of a comparison study in which mold sidewalls on both sides of a two-tire press were cleaned, one side with a single-hose, CO2 / dry ice pellet blasting system, and one side with a two-hose, shaved CO2 / dry ice block system, using the same operator.

Object to be cleaned: 2 - Piece 36x12.5/16.5 LT Steel Mold

Table 4. Comparative Results of Tire Mold Cleaning with Single-Hose CO2 / Dry Ice Pellet Blasting System vs. Two-Hose, Shaved CO2 / Dry Ice Block System

CO2 / dry ice blasting is a "line-of-sight" cleaning technology. Like with any blasting method, tread area "shadowing" and missing micro vents in the tread and bead areas are problems with manual mold cleaning in a tire curing press. Furthermore, in presses that open vertically rather than "clam shell" or tilt back type tire curing presses, it is impossible to position the head and torso to see and aim the stream of CO2 / dry ice into all the complex cavities that make up the tread pattern portion of a tire mold, not to mention attempting this while crouching inside a 300°F (148.9°C) lower tire mold cavity.

Manual CO2 / dry ice blasting in the press is only suitable for relatively quick spot cleanings of mold sidewalls (D.O.T. lettering, logo, and some sidewall vent clearing). In-the-press sidewall spot cleaning can increase the number of cure cycles by a factor of 2 to 3 until total mold cleaning (pulling the mold) is necessary. Routine sidewall spot cleaning with CO2 / dry ice blasting generally allows a tire manufacturer to produce blemish free tires for an entire production run, or until normal press maintenance requires that the mold be removed.

The benefits inherent to CO2 / dry ice blast tire mold cleaning are derived from (1) establishing a manual "sidewall touch-up / cleaning" maintenance routine that keeps the operator’s exposure and risk to a minimum, and (2) installing an automated, robotic CO2 / dry ice blast cleaning system for total mold cleaning when the molds are pulled out of the presses for scheduled press maintenance.

A robotic CO2 / dry ice blast mold cleaning system is pictured in Illustration 1 below. This is a "generic" layout based on an actual operating system. This configuration could vary significantly from one tire manufacturing facility to another. The common requirements are (1) a robot located in a sound-proof and adequately ventilated "booth", (2) a source of 200 + psi (13.8+ bar) clean, dry, compressed air, (3) a means to heat the out-of-press fouled molds to 300°F (148.9°C) or higher, (4) a CO2 / dry ice particle generator (pelletizer) and a CO2 / dry ice blasting system, with a robot adapted nozzle, (5) a means and method to stage the fouled molds for cleaning, move the molds through the dry ice blast cleaning system, inspect the molds for complete removal of all fouling, as well as for the clearing of all vents and/or microvents, and stage the molds for reinstallation into the presses.

Illustration 1. Robotic CO2 / Dry Ice Blasting Total Tire Mold Cleaning System

Noise Associated with Manual Blasting
The noise created by the CO2 / dry ice blasting equipment is another factor to be considered. All compressed gas (air) based particle blasting technologies are inherently noisy. The power level of the noise generated at the blast nozzle exit is largely a function of the compressed air outflow volume and velocity. Another component of total noise, though to a lesser extent, is the aerodynamic interaction of the individual CO2 / dry ice pellets or particles with the air stream. Furthermore, In tire mold cleaning operations, the noise from the nozzle is effectively reflected back to the operator by the "dish" shape of the tire mold itself. Noise, specifically the Sound Pressure Level (SPL) in decibels (dbA), is a very real concern and problem to overcome in manual CO2 / dry ice blasting.

Very significant advances have been made in the last two years to allow operators to use CO2 / dry ice blasting to clean tire molds in the presses, and still meet OSHA regulations requiring less than 84 dbA SPL exposure for an eight hour period in a day. In the single-hose constant acceleration system, the physics of isentropic flow have been enhanced by state-of-the art aerodynamic theory and design practice to produce media delivery systems (hoses, applicators, and nozzles) that provide maximum acceleration and velocity to the dry ice particles with minimum shock or turbulence at the nozzle exit. Thus, cleaning performance is high, and generated noise is very low, typically below 98 dbA at the nozzle exit. Studies have proven that with the new low-noise single-hose CO2 / dry ice blasting systems, if the operator wears an approved blasting helmet AND state-of-the-art ear plugs (dual hearing protection), the noise (SPL) field to which the operator’s ears are exposed is well below the required 84 dbA for 8 hours of continuous blasting per day.

Table 5 shows the results of noise level data taken during in-the-press mold cleaning tests at a major tire manufacturer in mid-1996. The data presents SPL levels resulting from blasting with a single-hose system at various pressures measured at the operator’s ear level, outside the blasting helmet.

Table 5. Noise (SPL) Generated While Cleaning Tire Mold Sidewalls with a
Single-Hose CO2 / Dry Ice Pellet Blasting System.

Redeposition of Mold Fouling Residue
It is certainly true that the use of solid CO2 / dry ice as a surface cleaning media creates no significant secondary waste stream. Over time, however, the fouling residue "dust" that leaves the mold surfaces will redeposit on other parts of the presses and machinery, the floor, and even the walls of the curing room. Although redeposited residue build up may take weeks or months to even become noticeable, it is in the best interest of the tire manufacturer to deal with it upfront. To date, the most effective and proven method to curtail residue redeposit is to provide adequately sized (CFM / m3/min) air extraction hoods or returns close-in to the presses, so that the normal facility air handling system can capture most of the airborne residue particles and bring them to a central filtering station. As discussed earlier, if only mold sidewall maintenance is performed in the press rows, then the amount of airborne residue is dramatically reduced. The bulk of the residue will be captured in the dedicated robotic total mold cleaning system. A less appealing alternative to reduce redeposited residue is a point-of-application effluent capture system. These systems are available from the CO2 / dry ice tire mold cleaning system suppliers, and can capture residue during the blasting process. They do, however, add bulk and weight to the hand-held nozzles, and they require an additional piece of equipment (vacuum system and filter module), which must be moved in and out of the press rows.

Coated Molds
The most promising R & D work being carried out in view of reducing mold fouling is the development of "permanent" coatings for aluminum and steel mold surfaces, which will significantly minimize residue adherence and build-up. Early testing with proprietary coatings applied to production tire molds have shown that the molds can remain unfouled for more cure cycles than identical uncoated molds. When the coated molds eventually do accumulate residue build-up, the fouling can easily be removed by CO2 / dry ice blasting, without damaging the coating layer. Once again, with the advent of coated molds, the single-hose direct acceleration CO2 / dry ice blast systems will offer the most benefit because these systems can utilize blast nozzles up to six (6) inches wide at the low pressure and kinetic energy level required for the coated molds. Cleaning coated passenger, light truck, and even large commercial truck and rear farm vehicle coated tire molds will be fast and easy at very low noise, air pressure and CO2 / dry ice pellet flow rates by employing the very wide nozzles available only with single-hose systems.

Ventless Molds
A major quality driver in the tire industry is aesthetics, specifically product appearance. The trend today is to produce tires devoid of the "whiskers" produced by vents or microvents. While many customers, especially the Japanese, are pushing the tire industry toward "whisker-free" tires, the majority of consumers aren’t convinced that the tires they are buying haven’t been re-capped unless they can see the "whiskers" on them.

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 will still remain the preferred way to clean these types of molds for all the reasons previously mentioned in this paper.

Laser Mold Cleaning
Laser tire mold cleaning technology is now available in its "early adopter" stages. This technology has shown adaptability for cleaning the surfaces of passenger and light truck tire molds, even while they are in the press, but it is questionable whether it is able to consistently unplug vents and/or microvents, particularly in the tread and bead areas of the molds. Based on current information, the investment required for a press-row-capable mobile laser system is two to three times that required for an automated CO2 / dry ice blast robotic system, and 25 to 30 times the cost of a portable, manual, sidewall spot cleaning CO2 / dry ice blasting system.

The very same portable CO2 / dry ice blasting systems that are used primarily for tire mold sidewall spot cleaning have many other proven uses in tire manufacturing facilities also. Currently, tire industry users of this technology are applying it to the cleaning and maintenance of Banbury mixers, extruders, and tire building machines, to cleaning residue build up from load wheels, and to the general cleaning of the presses during downtime maintenance. Another possibility being explored is white sidewall grinding dust removal.

When assessing existing and newly emerging tire mold cleaning technologies, the adaptability of CO2 / dry ice blasting technology to many other aspects of the tire manufacturing process, on top of the overall favorable impact to product quality and manufacturing productivity, should not be overlooked.

From the standpoint of fixed (purchasing the system and equipment) and operating (electricity, compressed air, CO2 / dry ice pellet media) costs, cost-to-benefit ratio studies conducted by major tire manufacturers have proven that CO2 / dry ice blasting technology is currently the best choice for tire mold cleaning.

The single-hose "direct acceleration" solid CO2 / dry ice blasting system is preferred over a two-hose inductive system as the most capable for maintaining tire molds or mold sidewall areas in an unfouled condition throughout a tire production run. It is the high level of dry ice pellet kinetic energy provided that is capable of clearing clogged vents and microvents, and removing relatively thick residual rubber in the corners of tread sipes and lettering. And, it is the thermal effect of the CO2 / dry ice media that allows for the quick removal of the glass-like overall fouling residue, that gives the single-hose system its "double punch" for quick, efficient, and complete tire mold cleaning.

*DOT: Department Of Transportation
**OSHA: Occupational Safety and Health Administration