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TECHNICAL PAPERS

STATE OF THE ART IN CO2 / DRY ICE BLAST TIRE MOLD CLEANING

At ITEC'98, Cold Jet®, Inc. presented a paper about the basic technology behind CO2 / dry ice pellet blast mold cleaning titled "Tire Mold Maintenance with Engineered CO2 Blasting Systems" (click here for link to that paper). As a follow-on to that, this paper discusses important developments in this mold cleaning method that have occurred since 1998.

For manual, hand-held applicator tire mold cleaning, the most notable development has been blast nozzles that are shorter in length, lighter in weight, operate at much lower air flow rates (and are therefore less noisy) without sacrificing performance, and are configured for rapid interchange (from one nozzle to another). All of these new developments make the task of manual CO2 / dry ice pellet blast tire mold cleaning much easier for the operator, and more acceptable and safer for the general plant environment.

For automated and semi-automated (assisted) tire mold cleaning, two significant methods have been developed and implemented using off-the-shelf automation or telemanipulator equipment. These systems were not developed by Cold Jet®, but by a tire manufacturer using Cold Jet® CO2 / dry ice pellet blasting equipment together with engineered machines and components offered by other companies in the automation and manipulator fields. This manufacturer then installed the systems at several of his facilities in the United States. The systems/methods discussed in this paper are (1) the off-line cleaning of two piece and segmented molds in a sound-proof booth with an industrial robot, and (2) the in-the-press cleaning of very large agricultural tire molds with a reach-extending, dexterous telemanipulator arm (manipulator-assisted manual cleaning). Both of these methods are currently used by major tire producers. Both the robotic off-line cleaning cell and the reach-extending telemanipulator arm use commercially available equipment with slight alterations, so these systems are not proprietary and can be discussed in detail.

Manual CO2 / Dry Ice Pellet Blast Tire Mold Cleaning With New Nozzles and Hand Held Applicators

Dry Ice Blasting of a Tire Mold

Several years ago, at ITEC'98, Cold Jet® presented a technical paper that described in great detail the technology of CO2 / dry ice blast tire mold cleaning. In the years leading to 1998, the technology had been transformed from a very expensive, very large, very cumbersome, and very noisy method to a much more acceptable method for cleaning molds in the press and off-line. The 1998 paper described the then state-of-the-art blasting units that were small, inexpensive, reliable, and very portable. The hand-held portion of the system showcased a single lightweight urethane blast hose, an ergonomically-designed applicator "gun" and powerful blast nozzles that required only normal plant air pressure to operate and were now at or below a 100 dBA instead of a 130 dBA sound pressure level (SPL). The two years following ITEC'98 brought further improvements to the hand held portion of the single-hose CO2 / dry ice blasting systems. These improvements will be discussed here.

CO2 / dry ice blast tire mold cleaning is a "line-of-sight" method. The dry ice pellet blast stream exists the nozzle in a focused, high-speed straight-line stream. The nozzle tip must be held close to the surface so the pellets can impact and clean before they dissipate in the air. For these reasons, and because tire molds present a challenging geometry of multi-faceted tread sipes, curved sidewall surfaces, and intricately engraved details, manual mold cleaning requires the smallest, easiest to maneuver nozzles and applicators possible. Single-hose system CO2 / dry ice blasting nozzles have always been either one- or two- dimensional expansion hybrids of the traditional convergent-divergent "DeLaval" type nozzle. As more and more research and development effort has been applied to these nozzles, especially in the last half of the 1990's, these nozzle designs have become even more hybridized and specialized. State-of-the-art computational fluid dynamics (CFD) methods, as well as research into entrained particle flow dynamics, have allowed engineers to optimize the kinetic energy imparted to the CO2 / dry ice pellets as they enter and exit the nozzles, driven by compressed air. These advances have allowed the design of shorter blasting nozzles that impart the same or even more cleaning energy than the older generation of longer nozzles. The newer, shorter nozzles also require less airflow than their predecessors, and are therefore less noisy, and less expensive to operate. A typical evolution for tire mold cleaning nozzles is shown in Table 1 below.

Table 1 - Dry Ice Blast Nozzle Evolution Comparison Data

Nozzle Type & Length Year Exit Swath Air Flow Noise @ 1 Meter
Linear, High Power
23 inches (58.4 cm) long
1997 1 inch / 2.50 cm 175 scfm @ 80 psi
(4.96 m3/min @ 5.5 bar)
112 dBA
@ 80 psi (5.5 bar)
Linear, Med-Hi Power
5 inches (12.7 cm) long
1998 0.8 inch / 2.03 cm 150 scfm @ 80 psi
(4.25 m3/min @ 5.5 bar)
109 dBA
@ 80 psi (5.5 bar)
Linear, Med-Hi Power
13 inches (33.02 cm) long
1999 1.8 inch / 4.57 cm 100 scfm @ 80 psi
(2.83 m3/min @ 5.5 bar)
105 dBA
@ 80 psi (5.5 bar)
Linear, High Power
23 inches (58.4 cm) long
2000 1 inch / 2.50 cm 99 scfm @ 80 psi
(2.80 m3/min @ 5.5 bar)
99 dBA
@ 80 psi (5.5 bar)
Linear, High Power
23 inches (58.4 cm) long
2000 2 inch / 5.08 cm 150 scfm @ 80 psi
(4.25 m3/min @ 5.5 bar)
98 dBA
@ 80 psi (5.5 bar)

As the table above shows, advances in dry ice blast nozzle design have provided up to twice the blast swath width at the same cleaning level, same pressure, same nozzle length, but at even lower air flow and noise, than a state-of-the-art nozzle from two or three years earlier. Also, much shorter length nozzles have been developed to provide almost the same cleaning capability as the earlier longer nozzles, and these new short nozzles also use less air and produce less noise.

CO2 / Dry Ice Pellet Blast Tire Mold Cleaning Off-Line With An Industrial Robot

Robotic Dry Ice Blasting of a Tire Mold

Many tire production facilities are dedicated primarily to producing after-market, specialty brand or special promotion tires. The majority of after-market or special brand tires are for passenger cars and light truck/SUV's, so the size of the molds typically being cleaned are 14, 15, and 16 inch (35.6, 38.1, and 40.6 cm) radial, usually two-piece, with a growing number of segmented molds now entering the market for the increasingly popular wider, lower profile "high-performance" tires. In an after-market/special brands tire factory, the molds are removed from the presses for change-out much more often than in an OEM plant. This is an ideal situation for replacing abrasive blast cabinets with an automated robot-based CO2 / dry ice pellet blast tire mold cleaning system in the mold processing area.

An industrial robot is an ideal tool to use for automatically aiming the CO2 / dry ice pellet blast nozzle at the complex surface geometries of a tire mold. Although today's industrial robots are very flexible and easy to teach and program, the need for the robot to move the nozzle around the entire mold circumference many times is eliminated by pre-mounting the mold on an industrial turntable. This exploits the circular symmetry of the tire mold to minimize the nozzle motion required, thereby also minimizing the mold's cleaning time. Most robot controllers can be configured to consider the turntable rotation as an extra robot "axis" of motion. The nozzle motion can be "taught" to clean all the sidewall, bead, and tread sipe surfaces with the indexing of the nozzle automatically "timed" to the rotation speed of the turntable. Furthermore, by using a powered turntable to rotate the mold under the CO2 / dry ice pellet blast nozzle, the robot arm reach does not have to extend across the entire diameter of the mold. Therefore, a smaller, less expensive robot can be used.

For a typical robotic CO2 / dry ice blasting system, an industrial robot with at least a 35-pound (15-16 kg) payload capacity, and a reach of about 60 inches (1.5 meters) is needed. The turntable capacity should be between 1500 and 2000 pounds (700 to 900 kg) in order to be able to hold the molds, steam jackets, etc. The nozzle motion path, especially when the mold is rotating under the nozzle, is typically simple for blast cleaning the details of a tire mold. The robot system does not require expensive options like off-line programming, etc. - the most basic off-the-shelf robot and controller configuration should suffice for this type of system.

Industrial Robot Type for use with Dry Ice Blasting System

The hardware required for adapting an industrial robot to perform CO2 / dry ice blasting includes the blast nozzle, a simple attachment fixture for the nozzle, a commercially available torque sensitive break-away device to protect the mold, a length of blast hose, and one or more festooning attachments for the hose (spring stand-offs or bungee cords). All of this equipment mounted on the robot is the "payload", including the 2 or 3 pounds (1 or 1.5 kg) of thrust developed by the CO2 / dry ice pellet blast nozzle.

Of course, when the molds are removed from the presses, they cool down to ambient temperature in a matter of hours. CO2 / dry ice blast mold cleaning works best when the molds are at or near cure temperature. The reasons for this are explained in detail in the 1998 ITEC technical paper (click here for that detail). For robotic mold cleaning, the molds need to be pre-heated immediately before blasting with the dry ice pellets. Experience dictates that the molds can be pre-heated on steam platens to about 450°F / 232.2°C (about 100°F / 37.8°C higher than normal curing temperature) so that the cooling effect of the dry ice blasting will not significantly reduce the mold's temperature during the 30 to 45 minutes it takes to completely clean the mold. Most tire factories heat their molds with superheated steam or electric heaters, or a combination of both. It is a simple matter to use existing surplus factory equipment to fabricate one or two heating tables for the mold shop.

A sound-proof and ventilated blasting enclosure (booth) is required to contain the noise, the dry ice pellet blast stream and small amount of CO2 gas generated, and the airborne tire mold contaminant. The booth should be large enough to contain one or two turntables with the mold(s) mounted, as well as the robot arm with the nozzle and blast hose. Adequate ventilation for this process is one air change per minute in the booth. Disposable filters in the air plenum can collect the mold fouling debris that become airborne during the blasting process. The small amount of CO2 gas in the air stream can simply be vented out of the factory with the filtered exhaust air from the booth. Example: for a booth roughly 15 feet (4.6 m) long by 12 feet (3.7 m) wide by 12 feet (3.7 m) high, a 2000 CFM (56.6 m3/min) capacity air handling system should be adequate.

Although current CO2 / dry ice pellet blast nozzles clean tire molds very well at normal plant compressed air pressure (100 psi / 6.9 bar or less), to be effective on the most stubborn mold fouling and to ensure that all of the microvents are cleared during the cleaning process, experience once again shows that blasting at a higher pressure, between 225 psi (15.5 bar) and 275 psi (19 bar) will offer acceptable results 100% of the time. Since the blasting is performed inside a sound proof booth, the increase in blasting air pressure and noise does not affect the plant environment. A reliable and cost effective way to increase air pressure for the robotic mold cleaning system is to install a "booster compressor". Booster compressors are also available "off-the-shelf" and are the most efficient way to locally increase the existing plant air pressure. A booster compressor sized for CO2 / dry ice blasting will be 25 to 30 HP (example: 100 psi (6.9 bar) inlet, 250 psi (17.2 bar) outlet, 250 SCFM (7.08 m3/min)).

The two illustrations below show a possible layout for a robotic tire mold cleaning station. The industrial robot and turntables with the mold fixturing devices are inside the sound-proof blasting booth. The air handling system extracts effluent at the ceiling of the booth. An overhead electric hoist is shown to move molds in and out of the booth, and onto and from the heating tables. The heating tables are outside the booth in a safe area. The CO2 / dry ice pellet generator/blaster integrated system is adjacent to the booth, with the blast hose running through the booth wall to the robot arm and nozzle. Entry doors on the booth are sealed to reduce noise and effluent escaping into the environment, and the doors can be electrically interlocked to stop robot motion and blasting if they are opened during the cleaning process. The robot controller is placed immediately outside the booth entrance so that the operator can see inside the booth through clear windows in the doors.

Simple tables or non-powered conveyor sections can be placed in the area to stage the dirty molds near the heating tables, and similarly queue the clean molds for inspection.

An additional system component, not shown in the illustration, is a storage tank for the liquid CO2 to be supplied to the pellet generator. The liquid CO2 storage tank is typically located outside the factory wall as close as possible to the cleaning system and pellet generator, and in a location accessible to the supply tanker truck. The tank must be sized to the expected system duty cycle. For example, a typical CO2 / dry ice blasting system will consume 450 pounds (205 kg) of liquid CO2 every hour. For 80 hours of cleaning time per week, this is 36,000 pounds (or approx. 18 tons) of liquid CO2 each week. It is usually economical to be re-supplied each week, and it is also prudent to never allow the tank to fall below 1/3 full in order to maintain the vapor head pressure (275 psi to 300 psi / 19 bar to 20.7 bar). So a 30 ton tank would be ideal for this example. The liquid CO2 storage tank is typically leased or rented on a monthly basis from a local supplier of liquefied industrial gases. The end user usually has responsibility for the installation of a poured concrete mounting structure for the tank, as well as the insulated piping from the tank to the pelletizer inside the building. The piping run should be kept as short as possible in order to reduce installed cost and reduce heat gain that would boil the liquid CO2 into unusable vapor. It is therefore advantageous to locate the robotic cleaning system near an outside plant wall, as close to the storage tank as possible.

Robotic Dry Ice Blasting Setup - Elevation View

Robotic Dry Ice Blasting Setup - Plan View

CO2 / Dry Ice Pellet Blast Cleaning of Agricultural and Off-Road Tire Molds with a Reach-Multiplier Manipulator

Very large agricultural and off-road tire curing molds present a unique opportunity for CO2 / dry ice pellet blast cleaning. These molds are typically so large that they are integral with the press and removing them for cleaning is not practical. Dry ice blasting is an ideal solution to the problem of cleaning these molds because it can be done in the press with no secondary waste stream. Since dry ice blasting performs better on a hot surface, this is an additional reason why in-the-press cleaning works so well.

The major drawback to in-the-press dry ice blast cleaning is that these molds require a technician to stand inside the hot mold for almost one hour while cleaning the upper and lower mold surfaces. This is too harsh of an environment to expect people to work in every day. Also, the upper mold half in the open position is usually too high for the technician to safely reach its uppermost surfaces without climbing on the press components and subjecting himself to a high risk of injury. A readily available and practical solution to this problem does exist, however, and some tire manufacturers are currently using it.

The large mold cleaning problem can be addressed by using a device called a telemanipulator. The telemanipulator is a device that mechanically extends the reach of the human arm by factor of six or more. Telemanipulators were developed to remotely handle objects and perform tasks in a hostile environment without the use of auxiliary equipment. These devices have found widespread use in nuclear power facilities and nuclear weapons manufacturing facilities for the remote handling of radioactive components.

Telemanipulator for Dry Ice Blasting of Tire Molds

Nozzle for Dry Ice Blasting Large Tire Molds

Telemanipulated Dry Ice Blasting Applicator and Nozzle

Telemanipulators use mechanical leverage and electric motor assist to extend the operator's reach and dexterity. The operator manipulates a joystick control that directly simulates the weight and feel of the device at the payload end of the telemanipulator "arm". In the case of CO2 / dry ice blasting, the operator control simulates the handle of the hand-held blasting gun and nozzle combination. The operator senses the same weight and feel as if the dry ice blasting applicator was in his hands. The system allows the operator to stand safely back from the tire mold (as far back as 15 feet (4.6 meters) or more) while manipulating the cleaning nozzle in the mold. This keeps the operator away from the heat, dirt, and noise associated with the cleaning process. Operator fatigue is reduced significantly so that the mold upper and lower halves can be cleaned as quickly as possible. It is thus possible for one operator to clean several molds in his shift without becoming tired or exceeding the allowable noise exposure.

Telemanipulator System for Dry Ice Blasting Agricultural Tire Molds

Illustration 2 - Diagram of a telemanipulator system used to clean large agricultural tire molds

Illustration 2 above shows the large frame of the telemanipulator mounted on wheels for portability in the factory environment. Two operators can push the entire assembly into place in front of the tire press. To increase operator safety and decrease noise at the operator station and in the press row, a clean Lexan window can be mounted on the side of the frame facing the mold. The operator's vision must be unobstructed, however, so the window must be kept clean. To assist the operator further, high-intensity lights can be mounted on the frame to illuminate the inside surfaces of the mold. A further refinement of the system is to mount a small industrially hardened camera on the blasting nozzle, with the camera monitor mounted at the operator's station in the telemanipulator frame. This feature allows the operator to see areas of the molds that are not readily visible from his station, such as the near front portion of the lower mold, and the hidden sides of some of the agricultural tire mold tread lugs.

A large tire mold telemanipulator-based cleaning system requires only standard single-hose CO2 / dry ice blasting equipment. No special components are required, and a normal hand-held portable dry ice blasting system can be readily mounted and adapted to function with the telemanipulator unit. Additional engineering requirements are minimal, no robot programming training or knowledge is required by the operator, system maintenance is not complex, and equipment cost is low relative to other types of in-the-press mold cleaning technologies for tire molds.

Summary

CO2 / dry ice blasting technology has matured to the point where it has become accepted by the tire industry as the most cost-effective method to clean tire curing molds. The tire manufacturers themselves have directly, by initiating their own engineering projects, and indirectly, by putting market pressure on the suppliers of dry ice blasting equipment to continuously improve the technology, fostered the ongoing implementation and widespread use of dry ice blast tire mold cleaning. The dry ice blasting equipment suppliers have responded to the tire industry's needs by developing smaller, more powerful, quieter, and more efficient blasting nozzles and applicators. The tire manufacturers have taken commercially available technologies like industrial robots and telemanipulators, and tailored them to the requirements of mold cleaning for very different circumstances, such as frequently changed aftermarket passenger / LT (light truck) molds and very large fixed in place agricultural tire molds. CO2 / dry ice blasting technology has thus far proven to be capable of further refinement, improvement, and application development so that it is and will continue to be the best choice among the many alternative curing mold cleaning technologies available for years to come.

 

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