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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
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
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.
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.
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.
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.
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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