First, dry ice cleaning technology
1.Definition of dry ice
The shape of dry ice is shown in Figure 1.
Figure 1 Appearance of dry ice
Dry ice is solid carbon dioxide. At normal temperature and pressure of 6079.8 kPa, carbon dioxide is condensed into a colorless liquid, and then quickly evaporated under low pressure, it will condense into a compacted snow-like solid substance. Its temperature is- 78.5 ° C, this is dry ice. Dry ice storage is more than 1.5 times that of water ice. After absorbing heat, it sublimates into carbon dioxide gas, without any residue, non-toxicity, no odor, and sterilizing effect. It does not liquefy when heated, but directly vaporizes. Dry ice is the solid state of carbon dioxide. Because dry ice has a very low temperature of ﹣78.5 ° C, it is often used to keep objects frozen or cold.
When the carbon dioxide gas is pressurized to about 101325Pa at room temperature, when a part of the carbon dioxide gas is cooled to about 56 ° C, it will freeze into a snowflake-like solid carbon dioxide. The heat of vaporization of solid carbon dioxide is large, at 364.5J / g at ﹣60 ° C. When vaporized at normal pressure, the ambient temperature can be reduced to about -78 ° C, and no liquid is generated, so it is called "dry ice".
Physical properties of dry ice: molecular weight 44.01; density (solid) 1560kg / m3 (-78 ° C); melting point ﹣57 ° C; boiling point ﹣78.5 ° C; liquid to gas ratio 8.726SCF (gas) / LB (liquid ﹣17.8 ° C, pressure 21 kg / cm2); the liquid is converted to a solid ratio of 0.46 (﹣17.8 ° C) and 0.57 (﹣48 ° C).
2.Principle and characteristics of dry ice cleaning
Through the flow of compressed air, the dry ice particles are accelerated to a speed close to the speed of sound and hit the surface attachments being cleaned. The surface attachments suddenly drop in temperature and cause surface embrittlement, and at the same time, these extremely low temperature dry ice gases enter the cracked attachments; When the attachments on the surface are impacted, the dry ice particles quickly rise to the gas, and the energy generated by the gas is about 700 times the original energy. These broken attachments are peeled off and removed from the surface being cleaned.
Figure 2 Principle of dry ice cleaning
The principle of dry ice cleaning is shown in Figure 2. It is divided into three steps:
Step 1: Energy transfer
Dry ice particles accelerate in a high-pressure air stream, impact the surface to be cleaned at a speed close to the speed of sound, and vaporize at the instant of impacting the surface. The momentum of dry ice disappears at the moment of impact, and rapid heat transfer occurs between the dry ice particles and the cleaning surface, causing the solid CO2 to quickly sublimate into a gas.
Step 2: Low temperature cracking
The temperature difference can destroy the combination of materials with different thermal expansion coefficients. Dry ice particles (﹣78.5 ° C) impact the surface being cleaned, causing the surface attachments to freeze and embrittle and burst, resulting in "cracking", which affects the mechanical properties of the surface attachments to be cleaned.
Step 3: Micro explosion
The dry ice particles impacted the cracks between the attachments on the surface to be cleaned, and the volume expanded nearly 800 times within a few thousandths of a second, forming countless "mini explosions" to peel off the attachments from the surface. At the same time, the grinding and impact of dry ice particles and the shearing of compressed air are used to peel off the adhered matter in a solid form from the surface to be cleaned, thereby achieving the purpose of removal. Because of the CO2 volatilization, the dry ice cleaning process does not generate any secondary waste. The only thing that needs to be cleaned next is the removed attachments.
3. Comparison of dry ice cleaning and traditional cleaning
4. Composition of dry ice cleaning system
The complete dry ice cleaning system includes a liquid carbon dioxide storage system, a dry ice manufacturing system, a dry ice storage and transportation system, a compressed air supply system, and a jet cleaning system. The connections between the systems are shown in Figure 3:
Figure 3.Composition of dry ice cleaning system
Figure 4 Dry ice preparation process
The dry ice preparation process is shown in Figure 4. The dry ice produced from the dry ice granulator is shown in FIG. 5.
Figure 5 Dry ice made from dry ice granulator
Dry ice cleaning systems are generally divided into two parts:
The first part is a dry ice preparation system, which is used to make liquid CO2 into dry ice particles or dry ice of a certain size;
The second part is the dry ice blast cleaning system. This system uses compressed air generated by a specially prepared air compressor or a compressed air system prepared by the factory itself. The prepared dry ice particles or dry ice cubes are loaded into the cleaning equipment. Driven by compressed air, high-density dry ice particles or fine powder impact the surface of the object to be cleaned with the compressed air, thereby playing a cleaning role.
According to the needs of users, the dry ice cleaning system can be assembled with the dry ice preparation system and the dry ice blasting system as a set of systems. The dry ice preparation system can also be separated from the dry ice blasting system. (You can even buy dry ice outsourcing), and use a dry ice blasting system, that is, a dry ice washer, for cleaning.
5.Two basic systems for dry ice blast cleaning
Figure 6 Schematic of direct acceleration (single pipe) injection system
The first is a direct acceleration (single pipe) system (Figure 6): This system introduces dry ice particles into the air stream at the funnel instead of the nozzle opening, which allows the dry ice to flow at 900 ft / s (1 ft = 30.48 cm) Faster than the nozzle opening (if using a dual pipe system, its speed is 300ft / s), so as to obtain better cleaning results. If you do not need such a large cleaning energy, you only need to lower the pressure of compressed air to reduce the spray degree of dry ice particles.
The second is the Venturi system (dual-pipe system) (Figure 7): This system uses a vacuum to draw fine dry ice particles into the air stream with the nozzle opening facing the mold surface. This system is less expensive than a single-pipe system, but because it cannot achieve the high speeds that a single-pipe system can produce, it is difficult to remove heavy residues and dirt from the mold.
Figure 7 Schematic diagram of dual-pipe injection system
The appearance of the dry ice washer is shown in Figure 8.
Figure 8 Appearances of two types of dry ice washer
6. Scope of dry ice cleaning technology
Advantages of dry ice cleaning technology:
(1) Dry ice cleaning technology has been approved as a usable cleaning method by the US Environmental Protection Agency, the US Food and Drug Administration, and the US Department of Agriculture.
(2) Dry ice cleaning technology will not release harmful gases into the atmosphere and will not generate secondary waste.
(3) Dry ice cleaning technology is safe and non-toxic, because dry ice particles will sublime and disappear in the air after hitting the surface being cleaned.
(4) Dry ice cleaning technology reduces or completely eliminates the possibility of workers being harmed by chemical cleaning agents.
In recent years, the dry ice blast cleaning technology has changed from large to small, which has enabled each independent company to apply the technology to countless industrial and civilian cleaning services. Cleaning with dry ice is no longer a privilege for large companies.
Dry ice cleaning technology is a "physical cleaning method". So in theory, dry ice cleaning technology can replace traditional physical cleaning processes, such as sand blasting, water spraying, wiping and other technologies.
In terms of industrial applicability, the application of dry ice cleaning technology is related to national environmental awareness and legal requirements, that is, in the case that traditional processes can not meet environmental protection conditions, dry ice cleaning technology can be used instead of traditional cleaning processes. From a process perspective, the principle of any cleaning process is to ensure that the workpiece being cleaned is not damaged, and the integrity of the workpiece must be ensured. Dry ice cleaning processes must also adhere to this principle. Dry ice cleaning sprays dry ice particles onto the surface of the workpiece. The surface of the workpiece must withstand a certain wind pressure. If the surface of the workpiece cannot withstand a certain wind pressure, the dry ice cleaning process cannot be used.
Dry ice cleaning can be widely used in military, nuclear energy, metallurgy, foundry, petroleum, chemical industry, rubber, plastics, ships, automobiles, food, medicine, machinery, power, electronics and other fields. Dry ice cleaning is applied to various molds, such as tire molds, polyurethane molds, rubber molds, alloy die-casting molds, etc., to decontaminate the production line and save time.
7.Fate of the removed matter by dry ice cleaning
As for the final destination of the object to be cleaned, because dry ice cleaning is a physical cleaning, in principle, the object to be cleaned only peels off the object and does not disappear. Therefore, it is "existent". So where did the object to be cleaned go? In fact, dry ice quickly freezes the object to be cleaned, forming a crack. When dry ice expands in the crack, the instantaneous explosion causes the object to be cleaned into particulate matter, which blows away with compressed air. Peel attached objects and diffuse into the air. Of course, after cleaning, if there are no other measures, these particles dispersed in the air will settle on the cleaned ground like dust, which can be removed by vacuuming. Under certain specific conditions, some oil or water traces may be scattered on the site. This is because there are some floating residual oil or water in the workpiece being cleaned. These residual oil or water are not attached to the surface of the workpiece. It is free, so when compressed air is blown into the workpiece, these residual oil or residual water is immediately sprayed away from the workpiece in the form of a mist and scattered to the surroundings. In actual operation, the residual oil in the workpiece should be sucked off with a cloth before cleaning the workpiece with dry ice.
8.The cost of dry ice cleaning
The key issue of whether dry ice cleaning technology can replace the traditional cleaning process is the process cost, which must be related to the consumption of dry ice, the pressure of air pressure, the consumption of the displacement, and the resulting cost issues. However, the process cost of any cleaning process is directly related to the original state and cleaning accuracy of the object being cleaned. Different cleaning accuracy requirements and different cleaning conditions will cause significant cost differences.
The dry ice cleaning process raw materials are mainly dry ice and compressed air. The sources of compressed air are inexhaustible and do not require transportation, so there is no procurement and transportation cost of raw materials. The cost of dry ice is basically that 3 tons of liquid carbon dioxide can produce 1 ton of qualified dry ice, so the cost of dry ice can be calculated.
The consumption of dry ice is a matter of great concern to users. In fact, the consumption of dry ice is closely related to the cleaning conditions and accuracy. For thick oily dirt and strong adhesion, such as paints, scales, carbon deposits, welding slag and other removed objects must be deep-frozen and effective impact to crack the removed objects layer by layer and separate layer by layer, so the ice consumption Inevitably, the correspondingly higher air pressure requirements and correspondingly higher costs. However, for oily soils with low adhesion, strong compressed air is not required, and low-pressure cleaning can be used. The corresponding dry ice consumption is much less, and the cost is much reduced.
9. Safety of dry ice cleaning
Dry ice is solid carbon dioxide, non-toxic and non-flammable. Therefore, on the whole, it is a safe cleaning method, which is harmless to human bodies and workpieces. However, the carbon dioxide atmosphere is formed after the dry ice is evaporated during the dry ice cleaning process. Under ventilation, the diffusion of carbon dioxide into the atmosphere does not create an adverse environment. However, in an unfavorable environment with poor ventilation conditions, carbon dioxide sinks due to air circulation. In this environment, people feel uncomfortable due to lack of oxygen, so air circulation should be ensured without carbon dioxide accumulation.
In a dry environment, high-speed air jets can also produce electrostatic effects. Therefore, dry ice cleaners (dry ice sprayers, dry ice sprayers, dry ice spray cleaners, mold washers, mold cleaners) should be equipped with protection devices to avoid static electricity. Adverse reactions.
10.Three related matters needing attention in dry ice cleaning
Although the dry ice cleaning method has many advantages in all aspects, there are still some issues that need attention in actual operation:
(1) Removal of rust and residue on dry ice cleaning: Although dry ice cleaning does not cause wear on the surface of the object, it also limits its ability to remove stubborn dirt and thick rust. For example, for those soft and elastic residues remaining in natural rubber molded operating systems, dry ice cleaning is less effective, as these dry ice particles will bounce back when sprayed onto such residues. (Of course, the use of dry ice blasting to remove liquid silicone rubber (LSR) works well, especially for those mold cavity surfaces that have patterns). In addition, dry ice blast cleaning can only remove those residues that can be seen by the eyes.
(2) Noise level for dry ice cleaning: Just like other compressed air systems, when using dry ice spraying for mold cleaning, the general noise will reach 102dbA. For this reason, better ear protection facilities are required. However, when using dry ice blast cleaning equipment for a long time, even if earplugs are inserted, the noise is still very loud, and it will bring more and more irritation to the human body over time. At this time, use earmuffs Can play a role.
(3) Dry ice cleaning staff safety: If dry ice particles sprayed at a speed of 900 ft / s per second are sprayed onto the human body, the human body cannot resist at all. Therefore, for safety, all protective equipment must be worn before operation, including: durable gloves, full face masks and long sleeves. Generally, there are many workbenches and equipment in the production workshop, and the distance between them is usually 3 to 5ft. Therefore, in the cleaning operation, pay attention to anyone nearby in order to avoid accidentally falling dirt to hurt others.
Application of dry ice cleaning technology in the field of printing and packaging
Printing equipment has a complicated structure and is expensive. Once the surface is damaged, the printing task will have to stop. Choosing a fast and efficient cleaning method can not only clean every tiny corner of the printing equipment without disassembly, but also reduce the downtime caused by cleaning. Printing machinery is difficult to remove ink, and the accumulation of ink on gears and guides can lead to poor print quality.
The working principle of dry ice cleaning is to allow dry ice particles to accelerate to high speed in high-pressure airflow and spray them on contaminated materials. Dry ice particles will vaporize at the moment of impact, so they will not wet the equipment and cause no wear and corrosion to the equipment. Dry ice cleaning can remove a variety of oil-based, water-based inks and varnishes, clean up oil stains, ink buildup and dyes on gears, guide rails and nozzles, to avoid the discharge of hazardous waste and solutions, and personal injury caused by hazardous solvents. Has the advantages of simple and fast, wash old as new, never wear and so on.
Dry ice cleaning can effectively clean rubber residues on the surface of industrial equipment, such as oil, grease, carbon powder, paint, etc., without disassembling the equipment and without using chemical cleaners. It is suitable for large-scale cleaning tasks.
Printing companies such as Advanced Paper Box Company, Bell Paper Box, Inc., and Cincinnati Enquirer are using dry ice cleaning technology. Cincinnati Enquirer, Eastern Color Printing and other foreign printing companies generally use dry ice cleaning technology to clean the bearings, nozzles, magnetic drums and other core parts of the printing machine, which not only solves the problem of color ink cleaning, but also guarantees the printing quality. Without any wear.
Figure 9 An example of dry ice cleaning for printing equipment
Fig. 10 Another example of printing equipment using dry ice cleaning
Aluminized film is a material often used in composite flexible packaging products. Aluminized film is generally processed by a roll vacuum coating equipment. When the vacuum coating equipment is rolled for vacuum aluminum coating, a thick aluminum layer will inevitably accumulate on the surrounding parts of the coating area, and these aluminum layers must be removed regularly, otherwise the quality of the aluminum coating will be seriously affected. influences. Those who are engaged in vacuum aluminum plating know that it is very labor-consuming and time-consuming to remove the aluminum layer accumulated in these parts. We are currently trying to remove the aluminum layer on the parts around the coated area by dry ice cleaning.
Fourth, concluding remarks
This article gives a comprehensive introduction to dry ice cleaning technology, which aims to arouse everyone's interest in dry ice cleaning technology. Perhaps, it is still new in China to apply dry ice cleaning technology to the field of printing and packaging. However, in fact, everyone must have seen the smoke when performing in various theaters, troupes, and wedding banquets. In fact, this smoke is generated by a dry ice fog machine. Dry ice is now readily available in various cities, so I believe it is not difficult to clean equipment with dry ice cleaning technology in the printing and packaging field.