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TECHNICAL PAPERS

CO/ DRY ICE PARTICLE  BLAST CLEANING  FOR THE MOLDED RUBBER PRODUCTS INDUSTRY

  1. Introduction

  2. Cleaning Performance Aspects of Dry Ice Blast Rubber Mold Cleaning Systems
  3. Tire and Rubber Mold Cleaning Technologies: Mold Erosion and Damage Studies
  4. Dry Ice Blast Technology - Selecting the Correct System for Rubber Mold Cleaning and Maintenance
  5. Rubber Mold Cleaning Methods for Dry Ice Blasting Technology
  6. Emerging Rubber Mold Maintenance Technologies
  7. Additional Uses for Dry Ice Blasting Technology in Rubber Products Manufacturing Facilities - Further Productivity Gains
  8. Summary

1. Introduction

The use of solid CO2 / dry ice blasting for the non-abrasive cleaning of rubber molds without the removal of the molds from the press or the creation of a secondary waste stream has intrigued molded rubber products manufacturers since the mid-1980's. Today, many of these molded rubber product manufacturers are aware of CO2 / dry ice blasting technology and its potential to reduce production downtime and maintenance labor costs, while enhancing product quality and appearance. In fact, molded rubber products manufacturers make up a major segment of the CO2 / dry ice blast cleaning market worldwide.

While working to develop applications solutions for the molded rubber products 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 molded rubber products industry is overwhelmingly significant and the cost to benefit analysis usually indicates a payback in terms of months, not years.

The following discussion will provide an understanding of the state-of-the-art solid CO2 / dry ice blast rubber mold cleaning technology. From a cost-to-benefit standpoint, solid CO2 /dry ice blast mold cleaning is, more often than not, shown to be the best choice among the many methods and technologies currently available for rubber mold cleaning. There are, however, a variety of technologies that offer different levels of mold cleaning cost and performance within the dry ice blasting industry. Indeed, 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, and the other is the inductive feed, or "two-hose" system. There may not be significant cleaning performance differences between the two types in abrasive blast systems, but there is a dramatic performance differential between the types for CO2 / dry ice blasting systems, and the differences must be fully understood and considered before selecting a mold cleaning system (see point 4 below). There are also two basic forms of the solid CO2 / dry ice blasting media that must be understood. There are discrete "pelletized" dry ice particles, and shaved dry ice "flakes" produced from a block of dry ice (click here for further info).

The mold cleaning performance levels of the different types of CO2 / dry ice blasting systems and CO2 / dry ice media 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.

Why Rubber Curing Molds need to be cleaned
A major problem faced by all molded rubber products manufacturers is that of mold fouling: a residue build-up on the curing surfaces of rubber molds resulting primarily from chemical reactions between the release agent, or the adhesive used for rubber-to-metal bonded parts, and the base polymer under heat and pressure. With injection molding systems, a major concern is keeping mold surfaces that mate, or come into intimate contact during cure, free of residue build-up to minimize flash. For multi-section rubber molds, these are the surfaces between the mold sections that come into contact with one another when the mold is closed and that produce a parting line on the part. If too much residue is allowed to build up in this area of the mold, the mold sections will not mate together completely, even under the extreme squeeze pressure of the press. The result is a noticeable "flash" at the parting line on the surface of the product. An excessive amount of flash results in additional labor costs to remove it. For molds that are designed to be flashless/trimless, keeping the precision mating surfaces free of fouling is even more critically important. Excessive fouling residue build-up in intricate details and in sharp edges or corners of molds can cause the parts to loose details (lettering, logos, etc.) and critical cross sectional shapes (seal lips, o-ring cross section, etc.), all resulting in scrapped parts.

In addition, excessive build up of release agent in the cavities can cause high releasant transfer to the parts, resulting in surface gloss level deficiencies like glazing, etc. As much as the release agent can benefit the rubber molding process, an excess of it in the cavities can be detrimental to the process.

Another problem area, especially in injection and injection transfer molds, is the clogging of sprues, runners, gates, and vents with fouling residue, overbuild-up mold release residue, and semi-cured rubber. Clogged vents can cause non-fills and other serious part defects.

Finally, the overbuild-up of mold release agent residue or adhesive residue used in rubber-to-metal bonded parts can cause defects like knit lines and surface gloss loss, or even cause the parts to stick in the mold and tear upon removal.

Comparing Costs to Benefits
As an example, a current user of CO2 / dry ice blast mold cleaning technology in the rubber industry conducted an in-depth cost-to-benefit analysis to compare the value of CO2 / dry ice blasting with the abrasive blasting method already in use at his facility. This study was conducted after completion of a trial usage period with a CO2 / dry ice pellet blasting system, and before the final purchase of the system. This user took into consideration all aspects of capital and operating costs, including the purchase price of the CO2 / dry ice blasting equipment, CO2 / dry ice pellet usage and cost vs. that of the abrasive media, compressed air and electricity costs for both types of system over time, mold and press downtime for cleaning with both types of system, and other mold maintenance costs relative to his operation.

CO2 / dry ice pellet blast cleaning can be accomplished in-the-press in 1/10 (one-tenth) to 1/4 (one quarter) of the time required for traditional off-line methods. Interestingly, the cost-to-benefit analysis was presented to the company management in terms of "increased product sales dollars per day" by using CO2 / dry ice blasting instead of their existing off-line abrasive mold cleaning method. The study indicated that the "lost sales dollars due to mold cleaning" would be decreased by 75% every day. In other words, whatever total parts sales dollars were being "lost" each day due to press downtime were cut to only 25% of that amount by adopting the CO2 / dry ice blast cleaning technology for all of their molds. Based on this increased productivity ratio, the user determined that complete pay-back on a portable CO2 / dry ice pellet blast mold cleaning system would be realized in under 60 days. Other cost to benefit studies have been conducted by rubber product manufacturers with comparable results, and given the similarities in the manufacturing processes of most rubber molders, this level of increased productivity and payback is typically experienced when abrasive mold cleaning is replaced with CO2 / dry ice blasting technology.

2. Cleaning Performance Aspects of CO2 / Dry Ice Blast Rubber Mold Cleaning Systems - Mold Condition Factors That Affect Cleaning Performance

Temperature
Data gathered since the emergence of CO2 / dry ice blast cleaning in the molded rubber products industry supports the fact that rubber 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 (cold molds). Although the reasons and mechanisms that give rise to this phenomenon are not completely understood, mold cleaning experience in the curing departments of many different rubber products manufacturers have proven this to be true. In case studies involving Rubber Mold Fouling, particularly with EPDM, FKM, NR, NBR, HNBR, Butyl compounds, and flouri-elastomers, it has been determined that the reactive chemicals present in the base polymer, the chemicals in cure accelerators and inhibitors, and the chemicals in many mold release agents 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 fractured into small particles by inducing high levels of thermal stress, or "thermal shock", with CO2 / dry ice pellets and thus easily removed from the mold surface. Since the temperature of solid CO2 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 can become much more difficult to remove from the mold surface because it resembles a very dense visco-elastic material, which absorbs the impact energy of the CO2 / dry ice pellets. The thermal shock mechanism becomes less effective because there is very little temperature differential between the material and the mold surface. The overall result is that it may become very difficult to remove the fouling residue from room temperature - or "cold" - rubber molds, and sometimes the residue will not respond to the CO2 / dry ice blasting at all.

Adhesive Fouling of Molds Producing Rubber-to-Metal Bonded Parts
Many rubber products, especially in the automotive and vibration mount markets, consist of a rubber isolation shape molded around and bonded to a steel mounting plate or a cylindrical center steel tube. These rubber-to-metal bonded parts require a heat activated adhesive to be applied to the metal portion, which creates an incredibly strong bond between the metal and rubber during the curing cycle. The concern with this process is that the heated adhesives become almost liquefied at curing temperatures and the excess adhesive flows out of the part during the high temperature curing cycle, creating a large amount of rapidly built-up fouling on the mold surfaces. Photo 1, below, shows typical rubber-to-metal bonding adhesive fouling on a mold surface.

Mold Before Dry Ice Blasting

Photo 1 - Typical Mold Fouling Caused By Rubber-to-Metal Bonding Adhesive

Fortunately, this type of "baked-on" adhesive fouling also resembles a glass-like residue that responds exceptionally well to the temperature gradient induced "thermal shock" removal mechanism of CO2 / dry ice blasting. Rubber-to-metal bonded part molds can be cleaned in the press as often as once a shift if necessary, so that CO2 / dry ice blasting offers the rubber-to-metal bonded parts manufacturer a safe, easy to use, and extremely cost effective way to keep their molds in continuous production.

Mold Metallurgy and Temperature Effects
Early on, valid concerns were expressed by rubber products molders about the possibility of mold sub-surface damage resulting from thermally induced changes in the metallurgy of the mold material. This concern was strong where martensitic and other heat-treated and hardened alloy tool steels are used, as well as for certain precipitation hardened aluminum alloys. University and independent laboratory metallurgical studies were funded and performed by several "CO2 / dry ice blast cleaning early adopter" rubber molding companies in the mid-1990's. The specific findings in these reports are proprietary to those companies that paid for the studies. In general terms, though, the conclusions drawn were (1) no metallurgical changes to the martensitic or alloy hardened structure of the tool steel molds were found after many cycles of CO2 / dry ice pellet blasting, (2) the sub-surface cool-down (thermal gradient) in tool steel was negligible (low thermal conductivity), so the mechanism to promote micro-cracks was considered non-existent. In aluminum molds, which have higher thermal conductivity and generally smaller thermal mass, the thermal gradient was greater and more pronounced, but no changes in the precipitation hardened structure were discovered after many blasting cycles. (Aluminum mold surface erosion was also considered negligible, and this subject is discussed in greater detail further down on this page.)

Mold Surface Condition
Abrasive blast media, like plastic or glass beads, typically leaves a "bare metal" appearance after residue removal, even on steel molded rubber products 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 discussed later 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.

Flashless/Trimless Tooling
Many rubber product molding companies are currently using, or are considering using, precision designed tool steel flashless/trimless molds. Rubber flash at the tool parting lines requires additional manufacturing processes to remove it, additional quality assurance (inspection) steps to monitor the level and effectiveness of flash removal, creates scrap rubber which must be handled and accounted for, all of which adds to the overall production cost. Also, some types of rubber parts have such critical surface finish and dimensional tolerances that only flashless/trimless mold technology can be used to produce them. Flashless/trimless tooling virtually eliminates part flash and its associated margin-reducing production costs, but the capital cost of the tooling is much higher, and the precision mating surfaces of the molds must be maintained without misalignment, knick damage, or erosion for the life of the tooling.

Abrasive blast cleaning of flashless/trimless tooling typically cannot be tolerated because of the risk of tooling erosion and damage. Non-abrasive CO2 / dry ice blasting can be used for hundreds and even thousands of cleaning cycles with NO tool wear experienced.

Non-Abrasive Aspects of CO2 / Dry Ice Blasting
CO2 / dry ice blasting does not abrade or erode the surface of common mold materials like tool steel and hardened aluminum. Since CO2 / dry blasting only removes the residue on the mold's surface and not any surface metal, any dark stains from cured molded rubber product compounds will remain on the mold's surface, especially on steel tooling. Following CO2 / dry ice blasting, a functionally clean, residue free molded rubber products 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 molded rubber products are cured and inspected for the sharpness of the molded-in details (especially under-cuts and acute angle seal surfaces), and rubber surface gloss level.

Many rubber molds must be "seasoned" with release agent immediately after abrasive cleaning. This usually involves running three to five cycles of parts with sprays of release agent in the cavities after each cycle until enough of a very thin layer of release coat fills in the "valleys" of the roughened surface. All of the parts run during the seasoning process are scrapped because of surface gloss defects, etc. Using CO2 / dry ice blasting to clean molds will, over time, shorten or even, as reported in some instances, eliminate the mold seasoning time and resulting scrap. This is because the mold surface will not be severely roughened during each cleaning cycle, and because the energy level of CO2 / dry ice blasting can be adjusted to allow for the retention of the seasoning coating.

As stated earlier, chemical stains in the mold metal typically cannot be removed by non-abrasive CO2 / dry ice blasting. Severe chemical stains, which can leach out of the more porous mold metals to cause "blooming" on the parts, may be beyond the ability of CO2 / dry ice blasting to counteract. Severe chemical staining of the molds may, in some cases, be significantly reduced over time by using CO2 / dry ice blast cleaning that does not continually open up the surface pores like abrasive blasting does. As the surface roughness goes down and the pores close up over time, stain leaching and stain related defects should diminish as well.

3. Tire and Rubber Mold Cleaning Technologies: Mold Erosion and Damage Studies
The most well known and widely used method of rubber and 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 rubber molds are glass, plastic, metallic, and ceramic beads. These media have gained acceptance in the rubber industry because they are regarded as only "mildly abrasive".

Other particle blast media used in the rubber industry vary in their degree of abrasiveness and subsequent mold erosion, and 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 break down (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 molds will ultimately be eroded to a point where they must undergo extensive rework or be scrapped. Abrasive particle blast mold cleaning is, at best, a compromise between a cost-effective cleaning method and reduced mold life/continuously declining rubber product quality.

Abrasive or even "mildly abrasive" blasting causes other problems as well. The imbedded fine silica "dust" residue from sand or glass bead blasting, or the imbedded plastic "dust" from plastic media blasting (PMB), can alter the surface of the rubber 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 in order to provide 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 mold cleaning technologies exist, or are emerging, including laser ablation, chemical flushing, and mechanical adhesive bonding of residual rubber. However, these methods are still very much in their developmental stages and do not really offer a viable solution for the immediate and near term needs of the molded rubber products industry. Of all the generally accepted "off the shelf" rubber 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 dust residue on the mold surfaces. 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 blast nozzle configurations.

All samples were blasted with CO2 / dry ice pellets at 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 blasting were seen on the steel and forged aluminum samples, while both cast aluminum showed minimal to severe erosion. Nozzle design had the most significant effect on erosion rate and cleaning cycle time.

The 1991 and 1996 mold erosion studies were conducted at a significantly higher blast pressure (250 to 300 psi / 17.2 to 20.7 bar) and at higher particle velocities than are required to clean typical tire and rubber molds with today's technology. This is because CO2 / dry ice blasting nozzle design technology has advanced significantly since the 1991-1996 era. Nozzle and delivery system improvements now allow molded rubber products manufacturers to completely clean rubber molds at blasting pressures between 70 and 90 psi (4.8 and 6.2 bar). Pellet flux density (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 the single-hose blast delivery system.

Rubber-mold specific blast nozzles now exist to focus the blast energy only where it is needed in the complex geometry of rubber mold cavities. The rubber mold fouling residue removal capability of CO2 / dry ice blasting is significantly better than it was even only 3 or 4 years ago, and with much lower air pressure and overall kinetic energy delivered to the mold surface. Presently, CO2 / dry ice blast induced mold erosion, even for cast aluminum molds, is considered negligible, and a significant number of molded rubber products manufacturers are now specifying and purchasing CO2 / dry ice blast mold cleaning systems to replace their existing abrasive blasting systems and methods.

Also, 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, thereby substantially decreasing the rate of deposit of mold fouling substances.

Table 1. - 1991 Tire Mold Material Erosion Rate Study Results CO2 Mold Cleaning Abrasion Trials 9.19.96

Material Test Conditions Average Change in Surface Roughness (µm) Maximum Change in Surface Roughness (µm) Rate of Change of Surf Rough per Clean Cycles Average Change to Coupon Weight (grams) Maximum Change to Coupon Weight (grams)
Steel
1
-0.82
0.2
-036
0.01
-0.02
2
-0.26
1.8
-038
0.00
0.0
3
-0.94
0.0
-014
0.02*
-0.02
Forged Aluminum
1
0.24
1.0
-005
0.00
0.0
2
0.88
1.6
.022
0.02*
-0.02
3
0.28
1.1
.001
0.02*
-0.02*
Cast Aluminum A
1
2.88
5.2
.132
-1.57
-5.43
2
-0.08
0.6
.014
-0.60
-4.02
3
1.10
3.4
.027
-1.02
-4.55
Cast Aluminum B
1
2.30
4.0
.102
-0.93
-2.01
2
1.90
1.9
.183
0.18*
0.18*
3
6.85
6.85
.078
0.0
0.0

*weight gain attributed to oxide formation and/or foreign material

Table 2. - 1996 Tire Mold material Erosion Rate Study Results

Sample # Pre-Cleaning Weight (grams) Post-Cleaning Weight (grams) Weight Loss (grams) Cleaning Cycle Time (minutes) Number of Cleaning Cycles Equivalent Cleaning Duration
1 301.64 301.54 0.10 2 18 6 months
2 304.43 304.40 0.03 2 18 6 months
3 295.85 295.78 0.07 2 36 1 year
4 302.37 302.28 0.09 2 36 1 year
5 298.71 298.62 0.09 2 72 2 years
6 298.71 298.64 0.07 2 72 2 years

For the purposes of this trial, the following parameters were used:

  • 10-day pull schedule
  • 250 psi (17.2 bar) blasting pressure (compressed air)
  • 3" X 5" (7.6 cm x 12.7 cm)engraved 2618 - T6 Aluminum coupons
  • 2-minute cleaning cycle
  • CO2 pellets
  • 360 production days
  • 36 cleanings per year

NOTE: Current segmented mold cleaning cycle times are as follows:

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

Compressed Air Usage and Cost Considerations
Another benefit related to the low blast air pressure and volume requirements for 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 dry ice blasting systems required expensive, dedicated air compressors. Current single-hose direct acceleration systems use "shop air" at 70 to 80 psi (4.8 to 5.5 bar) and between 150 and 200 scfm (4.2 and 5.7 m3/min). Most past and current two-hose dry ice blasting systems require two or more times the air flow volume, even at "low pressure", than 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 rubber mold cleaning system should include the compressed air system's fixed and operating costs.

4. CO2 / Dry Ice Blasting Technology - Selecting the Correct System for Rubber Mold Cleaning and Maintenance.

Direct Acceleration Systems vs. Inductive Systems (or "Single-Hose" vs. Venturi / "Two-Hose") —Kinetic and Thermal Energy Effects.
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 blasting 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, and can be constructed from lightweight material, and the other hose is the compressed air delivery hose, which is typically heavier to withstand pressures as high as 200 psi (13.8 bar) or greater.

Further, in the two-hose system, 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 particle average 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, with much more nozzle back thrust, and very much more blast noise generated at the nozzle exit plane.

The "single-hose" or direct acceleration system, on the other hand, 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 extra advantages greatly outweigh the extra initial expense. In a single-hose solid CO2 / dry ice blasting system, or "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. This is accomplished at less than 1/3 (one third) 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 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 Ke=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 Cvbc is called the Blast Energy Coefficient, and represents the comparative capability of CO2 / dry ice blast 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

NOZZLE MODEL NOZZLE CAPABILITY DELIVERY SYSTEM

PRESSURE

psi (bar)

PELLETS
lbs/hr (kg/h)
TRAVERSE
in/sec. (cm/sec.)
523 SF High Single Hose 80 (5.5) 160 (72.7) 0.75 (1.9)
508 SL Medium Single Hose 70 (4.8) 200 (90.9) 0.75 (1.9)
EA-145 High Two-Hose 80 (5.5) 200 (90.9) 0.75 (1.9)
WOOD REMOVED
inch2 (cm2)
BLAST ENERGY Cvbc SWATH
inch (cm)
DEPTH
inch (cm)
ENVELOPE
inch (cm)
POWER INDEX
0.37 (2.387) 0.278 1.2 (3.05) 0.11 (0.28) 20 (50.8) 3.05
0.186 (1.2) 0.140 0.8 (2.03) 0.19 (0.48) 11 (27.9) 2.65
0.135 (0.871) 0.101 0.8 (2.03) 0.11 (0.28) 23 (58.4) 1.11

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 / Dry Ice 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 dry ice media forms, some technical background discussion is appropriate.

Dry Ice Pellets

Traditional abrasive blasting, and even the "mildly abrasive" blasting technologies like PMB and glass bead, rely on the intrinsic surface hardness and geometry of the media, as well as 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 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, in the case of two CO2 / dry ice blasting systems with nozzles of equal exit plane area, assuming that the particles from each system possess sufficient and equivalent kinetic energy, 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.

CO2 / Dry Ice Blasting Media Dynamics
As mentioned above, CO2 / dry ice blasting harnesses two types of energy to accomplish mold fouling residue removal. CO2 / dry ice particle size directly influences the level of kinetic (velocity or impact) energy and thermal (temperature gradient or thermal stress) energy available at the surface. Large pelletized CO2 / dry ice media is typically 3mm in diameter and between 5mm and 8mm in length. The sugar grain sized flake particles resulting from saved dry ice block are roughly spherical and 0.5mm to 1mm in diameter. In the pelletized CO2 / dry ice blasting system, by the time the pellets are accelerated and have traveled through the blast hose and the nozzle, they are fractured into roughly uniform sized irregular spheres of dry ice of about 2mm 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 at the same velocity as each granule, delivers 4 times the impact energy of a granule or flake at the surface. Since the fractured 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 residue anchored into the undercuts and corner profiles, as well as the vents of rubber molds.

Thermal energy is dependent upon the mass (number and size of particles) of the solid CO2 / dry ice delivered to a given area of surface per unit of time. There is a 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). This heat exchange with each impacting CO2 / dry ice particle occurs within a few milliseconds and the heat is given up mostly from the thin layer of residue, and some from the surface of the mold. It is this instantaneous "surface only" heat transfer effect that imparts the thermal stress into the residue and fractures it from the mold surface.

Having already described the dry ice 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 dry ice striking it too slowly". This is typically why two-hose inductive systems fail to give rise to the thermal fracturing effect in molded rubber products mold cleaning applications. It is a matter of too little kinetic and thermal energy available in a given instant on the mold surface.

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

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

Single Hose CO2 / Dry Ice Pellet Blasting System Two-Hose Dry Ice Block Shaving Blast System  
Mold cavity 269 Mold cavity 270
Top half sidewall cycle 4 min. 10 sec Top half sidewall cycle 11 min. 40 sec
Nozzle Isentropic (low press) Nozzle Inductive (round)
Blasting pressure 60 psi / 4.13 bar Blasting pressure 70 psi / 4.83 bar
Pellet feed rate 50% CO2 block feed rate 60%
Mold Cavity 269 Mold Cavity 270
Bottom half sidewall cycle 4 min. 25 sec Bottom half sidewall cycle 11 min. 0 sec
Nozzle Isentropic (low press) Nozzle Inductive (round)
Blasting pressure 60 psi / 4.13 bar Blasting pressure 85 psi / 5.86 bar
Pellet feed rate 50% CO2 block feed rate 60%
Total Cleaning cycle 8 min. 35 sec Total cleaning cycle 22 min. 40 sec
Total pellet usage 27.6 lbs / 12.5 kg Total CO2 block usage 44 lbs / 20 kg

Notes: Cleaning of the tread area was attempted. However, the operator was unable to access all areas of the tread because of the size of the inductive nozzle and applicator and the awkwardness of the two-hose blast line. Also, the time-weighted average of the noise generated by the two-hose system was higher than OSHA* allowable limits for eight hours of continuous blasting with dual hearing protection.

Mold Vents
Mold vents that become clogged with rubber or other fouling residue prevent the trapped air or outgassing from the compound being cured from exiting the mold cavities. Trapped air or other gasses in the cavities cause non-fill and flow line defects in the parts, and these defective parts must be scrapped. Keeping the vents continuously clear of rubber and fouling residue is a major concern for most rubber product manufacturers in order to reduce or even nullify their scrap rate. The high kinetic energy of the CO2 / dry ice particles in direct acceleration single-hose CO2 / dry ice blasting systems has proven very effective at clearing vents as small as 0.03 inch (0.08 cm) in cross sectional diameter. The rapid kinetic chiseling action on the residual rubber/release agent chemical residue in the vents tends to dislodge and blow out the plug of material, even while the surface of the mold is being cleaned. Even vents that are normal to the surface, like tire mold pin vents (micro vents) are readily unplugged with the CO2 / dry ice pellet blasting process.

5. Rubber Mold Cleaning Methods for CO2 / Dry Ice Blasting Technology

Special Nozzle Configurations for Common Types of Rubber Molds and Presses
CO2 / dry ice particle blast mold cleaning is a "line of sight" process. The high-speed CO2 / dry ice particles must impinge on the fouled surface at as close to 90° as possible. Additionally, the exit - or "tip" - of the blasting nozzle must be kept at 1 to 1.5 inches (2.5 to 3.8 cm) from the surface being cleaned in order to obtain maximum energy from the CO2 / dry ice particles. These fundamental requirements, plus the fact that most rubber presses have 4 to 8 inch (10.2 to 20.3 cm) throat openings with 20 inches (50.8 cm) or more of "depth" (length of the tooling), dictate that the high-speed nozzles and the hand-held applicators must be compact, able to turn the flow of high speed CO2 / dry ice particles at angles of 90° or greater, produce little or no nozzle backthrust, and provide the ability for the mold cleaner to see the details of the molds as he cleans them. Furthermore, the nozzle design must be able to accomplish all of this using the lowest air CFM (m3/min) and producing the least noise possible, without sacrificing significant blast energy delivered to the surface of the mold.

The single-hose, direct acceleration method of CO2 / dry ice blasting provides the correct physical attributes to allow the design of high energy "flow turning" and "retro jet" rubber mold cleaning nozzles. State-of-the-art computational fluid dynamics computer modeling and design programs are used to optimize blast energy while minimizing air flow and associated noise. A high-energy flow-turning nozzle for a 5 inch or more (12.7 cm or more) press opening is illustrated below in Photo 3.

Dry Ice Blasting of a Rubber Mold Press

Noise Associated with Manual CO2 / Dry Ice Blasting
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. To a lesser extent, another component of the total noise is the aerodynamic interaction of the individual CO2 / dry ice pellets or particles with the air stream. In rubber mold cleaning operations, the noise from the nozzle is effectively reflected back to the operator by the flat surfaces of the mold tooling and/or the inside walls of the press. 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 to allow operators to use CO2 / dry ice blasting to clean rubber molds in the presses, and still meet OSHA regulations that require 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 particles with minimum shock or turbulence at the nozzle exit. Thus, cleaning performance is high, and generated noise is very low, typically below 102 dbA at the nozzle exit. Studies have indicated that the noise (SPL) field to which the operator's ears are exposed, while using the new low-noise single-hose CO2 / dry ice blasting systems and wearing an approved blasting helmet or headphones AND state-of-the-art ear plugs (dual hearing protection), is well below the required 84 dbA for 8 hours of continuous blasting per day.

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

Trial # Mold Code Cycle Time (min:sec) Threshold Setting (80 dBA) Threshold Setting (90 dBA) Nozzle Pressure
psi / bar
Pellet Flow Rate (%)
Segmented
1 P225/50R16 13:53 98.08 97.61 40 / 2.76 55
2 P255/50R16 8:25 97.73 97.14 50 / 3.45 55
3 P255/60R16 7:00 97.49 97.1 60 / 4.14 55
4 P255/60R16 6:49 97.67 97.08 60 / 4.14 55
Two-Piece
1 LT265/75R16 13:03 86.48 74.37 50 / 3.45 55
2 P275/60R15 8:27 95.93 96.28 55 / 3.79 55
3 P275/60R15 7:50 95.91 96.38 60 / 4.14 55

Table 5, above, shows the results of noise level data taken during in-the-press mold cleaning tests at a major tire manufacturer's facility in mid-1996. The data presents SPL levels measured at the operator's ear level, outside the blasting helmet (see Photo 4, below), while blasting with a single-hose system at various pressures. The noise levels at the operator's ear were within the OSHA standards for several hours of mold cleaning time.

Dry Ice Blasting of Tire Molds

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", which is detached from the mold surfaces, will redeposit on other parts of the presses and factory production area. Although redeposited residue build up may take weeks or months to even become noticeable, it is in the best interest of the rubber parts 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 air returns located close to the presses. The normal factory air handling system can capture most of the airborne residue particles and bring them to a central filtering station.

6. Emerging Rubber Mold Maintenance Technologies

Coated Molds
The most promising R and D work currently being carried out to reduce mold fouling and parts sticking in the molds is the development of "permanent" Teflon based coatings for aluminum and steel mold surfaces that 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 accumulate residue build-up, the fouling can typically be removed with reduced energy 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 benefits, because these systems can utilize blast nozzles with widths of up to six (6) inches (15.2 cm) at the low pressure and kinetic energy level required for the coated molds. Cleaning coated rubber molds can be fast and easy, and 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.

Laser Mold Cleaning
Laser rubber mold cleaning technology is now available also. Laser technology has shown adaptability for cleaning the surfaces of passenger and light truck tire molds and other rubber products molds, even while they are in the press, but it is questionable if it is able to consistently clean deep cavities, undercuts, vents, and hard to access complex mold tooling. Many trial demonstrations and longer term tests of the laser mold cleaning technology have been accomplished in the tire industry over the last few years, and in the tire industry, laser mold cleaning has been determined to be non-viable for several reasons, including personnel safety, high cost, inability to thoroughly clean the molds, inability to clean precision details within the molds, lack of industrial robustness of the equipment, and high electric power consumption. Laser technology may evolve to address these problems, but this may be years away. For now, CO2 / dry ice blasting technology offers the best solution for productivity gains in the rubber molding industry.

7. Additional Uses for CO2 / Dry Ice Blasting Technology in Rubber Products Manufacturing Facilities - Further Productivity Gains

The very same portable CO2 / dry ice blasting systems that are primarily used for rubber mold cleaning, have many other proven uses in molded rubber manufacturing facilities. Currently, users of this technology are applying it to the cleaning and maintenance of Banbury mixers, extruder screws and barrels, sprues, runners, gates, ejection pins, and the general cleaning of the presses during downtime maintenance. When assessing existing and newly emerging rubber mold cleaning technologies, the possibility of applying CO2 / dry ice blasting technology to many other areas of the rubber products manufacturing process, and the overall favorable impact on product quality and manufacturing productivity, should not be overlooked.

8. Summary

From the standpoint of fixed (purchasing the system equipment) and operating (electricity, compressed air, CO2 / dry ice pellet media) costs, cost-to-benefit ratio studies conducted by major molded rubber products manufacturers have proven that CO2 / dry ice blasting technology is currently the best choice for rubber 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 rubber molds in an unfouled condition throughout a rubber product production run. It is the high level of kinetic energy provided that is capable of removing fouling residue from deep cavities, undercuts, sharp part details, and of removing the quickly built-up residue resulting from rubber-to-metal bonded parts and excess release agent. In addition, the thermal effect of the CO2 / dry ice media, which allows the quick removal of the glass-like fouling residue, gives the single-hose system its "double-punch" for quick, efficient, and complete rubber mold cleaning.

*OSHA: Occupational Safety and Health Association

 

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