What is the principle of sandblasting? In short, sand is blown out of a special apparatus with compressed air and, when it hits the surface, it cleans it of paint, rust, putty and everything else that may obscure pure metal from our view. Therefore, a sandblasting machine is simply necessary for the job.
In general, the name “sandblasting” is not entirely correct. Sand is not always used as an abrasive material. But we’ll talk about this later, and now – a few words about the devices.
They are classified in different ways. Most often, sandblasters are divided into ejector and pneumatic. The latter, as a rule, are used to perform a large amount of work - for example, cleaning the hull of a ship. Therefore, we will study more closely only the first type, with which we have to work.
Compressed air performs two tasks here at once. The first is quite obvious: pour sand onto the surface being treated at high speed. The second task is to lift sand from the container. Spray guns work on a similar principle (called the Venturi effect). The disadvantages of the ejector apparatus include not too much power: no more than three square meters can be processed per hour. But for cleaning body parts it is quite enough, especially since these devices have two undoubted advantages. Firstly, they do not require a lot of compressed air; it is consumed very economically. And secondly, ejector-type devices allow you to work very precisely, processing only those areas that require it. Of course, they also need little abrasive material - although it all depends on the area of the surface being treated.
Well, one last thing: a relatively lightweight unit is much easier to carry with you in case the owner of a wonderful device suddenly has a “hack job on the side.” Therefore, most often in garages you can find just such ejector devices - not too powerful, but light and compact. And at the same time, they are still capable of accelerating particles of abrasive material to a minimum speed of 300 km/h.
By the way, it’s time to understand abrasives: why almost no one uses real sand, but they still call it sand?
I went and dug up
Just 60 years ago, the main abrasive was ordinary quartz sand. Most often they used river or quarry, which must at least be sifted, and better yet, washed. However, some fans of sandblasting in their garage do not bother much with finding and preparing sand. Some lucky people who live near sand quarries or sea coasts simply drag buckets of this natural material to themselves, and then use it for business. True, they usually sift it anyway. Some questions arise specifically about sea sand from beaches: it contains salt, which is not very good for the car body. But, as adherents of free beach sand say, such an abrasive doesn’t do anything bad, and it doesn’t cause more rust on cars.
For a long time, quartz sand remained the only abrasive. Despite one feature - very low cost - sand also has serious disadvantages. Firstly, it is still not very hard. When it encounters metal at high speed, it simply crumbles into dust, and is simply not able to remove solid formations. And secondly, sand can cause silicosis, a deadly lung disease. It is for this reason that in some countries it is prohibited for use as an abrasive material. Unfortunately, our people are not afraid of anything, which is why sand is used quite often to this day.
The materials are much more expensive than sand, but much safer and more efficient - these are nickel slag and cooper slag: waste generated during the production of nickel and copper, respectively. At the moment this is the most common material - today we will work with cooper slag. Less popular materials are electrocorundum, steel and cast iron shot.
Experienced craftsmen know that the wrong choice of abrasive can either ruin the surface being treated or fail to remove everything that was intended from it.
Therefore, inexperienced sandblasters may well, for example, ruin a cast disc with too hard or coarse abrasive - this, unfortunately, happens. Some craftsmen use ordinary soda for fine cleaning: the consumption is high, but it is difficult to damage anything.
In any case, one of the main requirements for the abrasive material remains one: it must be dry. But let’s assume that the master was able to correctly select both the abrasive and the speed at which it flies out of the device’s nozzle. What else could go wrong?
It happens that a client, after sandblasting, makes a complaint: I gave you the car, but I took the sieve. Here the situation is usually classic, described by the phraseology “too late to drink Borjomi”: most likely, the part was sandblasted too late. Either it had already been completely devoured by rust, or rust had devoured it even earlier, but the part was covered with a ton of putty and paint.
We just don’t want to wait for this “terminal stage”, so we are going to process the Saab arches on time. In any case, we think it’s on time. We hope so.
In all directions
So, we arrive at the work site. We look at the sky: the sun is our ally, because bare metal should not be left like that for a long time, you need to immediately cover it with at least a primer, and this is inconvenient to do in the rain.
Since we need to work in the area of the wheels, they will have to be removed: we are not going to sandblast the tires. We lift the entire side at once and secure it: leaving the car on jacks for a long time will be too careless, for reliable fixation you need to use stands. That’s why we arrange our Saab to rest on them. But now we have to digress a little from the main theme of the story towards the car.
The attentive reader has already noticed that there is no rust on the arches - instead we see a primer. Yes, that’s all true: a not-too-handed specialist has already worked on the car. Now we need to open the seams (there are welded fragments in the wings) and those places that were not touched at all by the previous masters, but in which there are defects. To do this, you will need to remove a layer of paint, putty, primer - in general, everything that you managed to apply to the metal. The fate of this Saab is too complicated to talk here about the continuous process of its restoration, so I will limit myself to only the information that I gave above: we clean what we have already cooked and tried to clean.
First, take a marker and apply markings. Our device allows us to work carefully, without removing the paintwork where it will be useful to us.
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Sandblasting is used for surface cleaning of metal using a natural abrasive - sand, sifted and cleaned to obtain a homogeneous fraction. The principle and operating technology of the metal sandblasting process is based on the supply of a sand mixture under pressure, which is directed to the processing object.
During the cleaning procedure with an abrasive, not only the rust, scale, traces of molding material and paint coating residues are “knocked off” from the metal, but also fatty deposits. That is why the technology of sandblasting metal is rightfully considered an extremely effective and relatively inexpensive way to prepare metals for subsequent paint coating or other work. The most important aspect in preparing a metal surface is the surface profile. As a result of research, it has been established that for high-quality adhesion and reliable protection of the metal, before applying a covering agent, it is necessary to comply with the requirements for sandblasting. Below is a photo of metal sandblasting.
Uniform and high-quality notching guarantees a more reliable and durable connection between the prepared surface and the coating. Sand particles form a peculiar relief on the metal surface, the depth of which depends on the parameters of the sand, pressure, distance and angle of attack of the nozzle.
Metal cleaning is divided into 3 successive stages:
- Preparation,
- Cleaning and finishing,
- Hardening with a shot blasting unit.
At stage 1, the surface is freed from old paintwork, oxidation products and other surface contaminants. In addition to the above, carbon deposits formed on rolled steel are removed. Sharp grains of sand form relief and create roughness to ensure greater protective effectiveness of the applied products.
Sandblasting and finishing are different from preparation, but in the first case the essence is to improve the overall appearance of the part and increase its level of suitability, and not to prepare the part for coating. Finishing sandblasting involves removing burrs from cast elements, as well as improving its appearance.
Shot peening is a process that is designed to significantly improve the performance properties of parts that experience significant loads during operation.
Equipment for sandblasting metal
An apparatus for sandblasting metal must have such properties as decent performance and efficiency, achieved by the fact that once used sand is prepared in a special way and subsequently reused. The general scheme of these systems includes a compressor, a sand container, a nozzle, hoses for compressed air and a sand mixture.
Equipment for sandblasting metal is divided into 3 types:
The suction (ejection) effect, in which air moves into the spraying element, where, due to the formation of vacuum, the treatment substance from the reservoir is sucked into the mixing tank of the sandblaster. The material combines with the air mass and the resulting working mixture is directed through a nozzle onto the material being processed. This type of unit is trouble-free in production, easy to operate, with minimal wear of pipelines and nozzles, but not very productive.
Pressure, in these systems the sand is placed in a bunker, from where, through a special lock, it moves into a reservoir under pressure. Next, by means of injected pressure, the sand mass is directed into the mixing tank, where the air flow directs it to the outlet device, from where it is ejected onto the surface being processed. Injection units have decent productivity, but their weak point is the complexity and excessive wear of mixture pipes and nozzles.
Gravitational, in them sand is poured from a special cavity into a container, where it is combined with compressed air and redirected to the output head. The composition of individual copies of this device provides for mixing the sand component with air immediately before exiting the feeding device. The described unit has very high operational reliability, they are simple in layout, and more economical than the two types described above.
There are 4 basic types of units:
- low-power,
- medium-power,
- high-performance
- and high-volume.
Low-power - has a capacity of 10 - 30 liters. As a rule, they are used in cases where there is a need to clean elements that are unrealistic or impractical to transport to production workshops. You need to understand that the air pressure in all devices is approximately identical, and the low power in this particular case indicates limited capabilities in terms of the volume of work performed. The main advantage of this type is its lightness, which facilitates its use in almost all conditions and on objects of varying complexity.
Medium-power sandblasters are delivered to the work site by transport. However, despite this, it is structurally endowed with decent operating efficiency. Mainly due to the capacity of the reservoir for sandy material (100-140 l.). However, the sand processing system with a capacity of more than 150 liters is in greatest demand. Its mobile modifications are equipped with a wheel system for mobility. To install a local location, you need a specially allocated and prepared room.
The maximum capacity of the sand processing complex is from 2000 to 22000 liters. sand. Its maximum productivity is obtained, among other things, due to the presence of not one, but several working outputs.
GOST for sandblasting of metal
GOST 9.402-80 indicates quality indicators and requirements for surfaces prepared for painting or other protective coating. It contains requirements for sandblasting:
- At 5x magnification, using special devices, there are no signs of metal rusting or soot
- Upon visual analysis, without special magnifying devices, there are no traces of oxidation and rusting of the metal, soot, and other layers of non-metallic origin. No more than 4% of the surface areas show signs of oxidation and rusting of metal and soot.
There are no signs of metal oxidation or rusting or soot on the surfaces of the sandblasted material.
The informative component about the quality and standardization of cleaning is indicated:
Controllers of systems and devices must be instructed in accordance with the specified GOST standardization for sandblasting of metal. Not so much in order to pass the work inspector, but in order to confidently guarantee that the anti-corrosion coating will be applied to a well-prepared surface.
The formation of deposits, deforming and simply undesirable coatings on various products and structures, as a rule, is not compatible with the requirements for their operational properties. The way out is through processing means, including grinding and polishing. But such surface cleaning methods may not be effective in removing scale, rust, and in some cases, old paint. The consequences of deep deformation processes can only be dealt with by sandblasting the surface, based on the principles of abrasive action. The tool uses sand, which acts as a cleaning component. However, this principle of operation also imposes strict safety requirements during operation.
Sandblasting guns
There are two groups of such equipment. It's worth starting with open-type models. As a rule, this is a small price which averages from 3 to 7 thousand rubles. By design, it resembles one powered by pneumatics. Such devices can be used in cleaning stone, concrete and metal surfaces from old rust, dirt, etc.
The advantages of the gun include mobility, ease of use and the ability to accurately work with structures of complex shapes. Thanks to the low power, cleaning practically does not damage the surface base. On the other hand, it is impractical to use pneumatic guns for some types of work in workshops, so in such cases closed-type equipment is used.
Sandblasting chambers
This is the second category of abrasive equipment, the function of which also provides sandblasting. Such devices are used for dry processing of metal parts in order to rid their surfaces of old paint, rust, scale and carbon deposits. Electrocorundum or river sand are usually used as working crumbs - the fraction on average varies from 0.1 to 0.9 mm. The specific parameters of the abrasive are determined by the tasks that the sandblasting machine must perform. The price of such models is 15-20 thousand rubles. At the same time, there is also more expensive equipment, which is characterized by large dimensions and, accordingly, high power.
The principle of operation of sandblasting
During operation, the device directs a jet of abrasive particles onto the surface being treated. In most devices, the force that accelerates the movement of sand is generated by the energy of compressed air. A typical model of such equipment includes three basic elements, including the jet device itself, the compressor and abrasive particles. It should also be taken into account that sandblasting can be performed with different degrees of pressure, that is, the force that acts on the abrasive. The operator regulates this indicator, achieving optimal results. The fact is that a high degree of pressure can not only eliminate the coating being cleaned, but also cause damage to the main surface. Therefore, the user is required to have certain skills and abilities in adjusting processing parameters.
Cleaning Levels for Metal Processing
Cleaning metal surfaces using sandblasting equipment is considered the most common. Today, special requirements have been developed for the quality of surfaces ready for further use. They are worth focusing on if you plan to sandblast cleaning of metal structures with abrasives. So, GOST standards distinguish four levels of surface cleaning from corrosion and scale:
- During inspection under sixfold magnification, rust and scale are not detected.
- Upon visual inspection with the naked eye, scale, particles of molding sands, traces of corrosion, burnt marks and other non-metallic deposits are not noticeable.
- Up to 5% of the working area is affected by stains of scale and casting edge, which can be seen with the naked eye.
- There are no obvious signs of corrosion or flaking scale on the product.
Safety precautions
The basic set of personal protective equipment includes a helmet, gloves and, if necessary, overalls. At the same time, at the workplace where sandblasting is carried out, there should also be spare glasses for the helmet in case the main set is damaged. As for the device itself, it should be grounded, otherwise there is a risk of electric shock. Today, more and more often, inexperienced craftsmen are paying attention to models with remote control, believing that they are safer and more convenient to use. But this is not entirely true, since the design of such devices is more susceptible to contamination by abrasives and requires careful maintenance.
It is also necessary to provide a supply of consumables at the work site. Especially if you are sandblasting metal, you should have rubber hose seals and nozzle rings on hand. If necessary, it will be possible to replace these elements without stopping the organized workflow.
Conclusion
The formation of corrosion on the surface of the car body drives many car enthusiasts into despair. This means that you will have to contact a special service and shell out a significant amount for repairs. Of course, there are alternative means of cleaning metal surfaces in the form of sandpaper and a grinder, but sandblasting is much more effective and faster. And this applies not only to metal, but also to stone, concrete products and building structures. The main thing is to choose the appropriate model of sandblasting machine. Pneumatic devices have gained the most popularity. At a low cost, they guarantee complete removal of any surface from unwanted coatings without harming the useful structure of the material.
Abrasive blasting is almost universal and effectively helps in cleaning the surface of various materials from contaminants. Which in turn extends the life of the materials.
Scale is particles of metal oxides that form on the surface of a metal when the metal is heated and subsequently reacted with oxygen. Since scale has magnetic properties, when it is present on the metal surface, corrosion processes occur several times faster, which leads to rapid wear of the material.
Sandblasting effectively allows you to clean the metal surface from scale for subsequent anti-corrosion treatment and painting.
Bitumen is a petroleum product that is actively used in industry, in particular in construction (for example, in road construction). Often the problem arises of cleaning tanks in which bitumen is heated from its residues.
In such cases, sandblasting comes to the rescue, effectively removing bitumen particles from the surface.
Removing rust and corrosion
Corrosion is the process of destruction of materials under the influence of the external environment. Rust is the corrosion of metal. Rust forms when metal comes into contact with an oxidizing agent (metal, oxygen or acid).
There is no need to talk about the negative consequences of corrosion. There are effective ways to remove rust from a metal surface, for example, sandblasting metal. After which an anti-corrosion coating is applied to the cleaned surface.
Such methods can significantly increase the lifespan of metal structures.
Removing paint and varnish coatings
Varnishes and paints are susceptible to atmospheric precipitation. Therefore, painted surfaces need to be repainted periodically. But you cannot put a new layer of paint (varnish) on top of an old one - the old one must be removed. Sandblasting does an excellent job of this task.
After which the surface is ready for repainting.
Soot is scale on metal that is formed as a result of burning of any lubricant.
The sandblasting method is used for removal.
Graffiti is a type of street art. Most often, the objects of application are places not intended for this purpose, the appearance of which such drawings significantly spoils.
Using sandblasting, it is not difficult to remove this type of art from building facades.
Preparing the surface for painting
Sandblasting is the initial stage prior to painting. This is necessary for two reasons:
- the surface must be cleaned so that corrosion does not form under the paint layer;
- the surface is sandblasted for better adhesion (adhesion of paint to material).
Paving stones, paving slabs, bricks (and other products that are made with molds) sometimes, due to a lack of molds, have defects in the form of relief formations on the surface.
Sandblasting allows you to clean faster than you can do by hand.
Removing cement and mortar
Cement and mortar in dried form often remain on the surface of building materials (bricks, for example), on tools, on equipment (concrete mixer).
Cement is characterized by hard properties in its hardened state, so manual cleaning is time-consuming and difficult.
Efflorescence is a white residue on stone surfaces. Efflorescence is salt deposits that form as a result of walls getting wet or salts coming out of the solution.
Efflorescence can be effectively removed using sandblasting.
Thanks to sandblasting, you can clean not only concrete, metal and stone, but even such delicate surfaces as glass. A type of sandblasting is also used in dentistry - using this method, tooth enamel is cleaned of yellow plaque and tartar.
What is the essence of sandblasting? With this processing method, abrasive particles are accelerated by a portion of compressed air. This leads to the fact that dirt is literally knocked out from the surface by these particles.
The method itself arose completely by accident. Back in the distant 19th century, namely in 1870, an inventor from the United States became interested in how, when gusts of strong wind in the desert, sand hit the window glass, scratching it, thereby cleaning the glass surface. This scientist's name was Benjamin Chu Tilghman, and it was he who became the inventor of the first sandblasting machine. Improved analogues of which we now see in industry.
Sandblasting: advantages and disadvantages
One of the main advantages of this method are:
1). High surface cleaning speed.
2). Relatively quick achievement of results - due to the high speed of bombardment of the surface with particles, dirt disappears literally before our eyes; such an effect is unattainable when using liquid cleaning solutions.
3). Long lasting cleaning effect. Cleaned surfaces take much longer to become dirty compared to other methods.
4). Sandblasting gently cleans metal surfaces, this is ensured due to the excellent adhesion of the jet composition to the surface, which can be made of various types of metals, which prevents it from coming off and cracking.
However, it would seem that such a good method also has a number of disadvantages:
1). When blasting a huge amount of dust is generated, in order to protect the sandblaster itself, it is necessary to take care of a very high degree of insulation of the equipment, otherwise it may very soon fail.
2). Danger for workers performing cleaning operations. The operator of sandblasting equipment assumes the greater risk, but the operating personnel may also be in the danger zone. The culprit is the already known dust, which penetrates the respiratory tract, mucous membranes, skin and hearing organs. To protect themselves from the negative effects of dust, workers must wear mandatory clothing special for such cases and wear a respirator. To minimize the danger, care should be taken to ensure that the products being processed and sand together with compressed air are free of various impurities in the form of liquids, oils and fats. Dust formation is a great danger to humans, therefore, for greater safety, it was decided to use instead of ordinary quartz sand, which during processing creates silicate dust harmful to humans, metal sand extracted from hardened cast iron.
3). Working noise. Sandblasting is a fairly noisy process; workers who have been involved in this type of production for a long time have experienced hearing impairment. However, working at a noise level of 85 decibels is also dangerous because over time it causes stress to the body and leads to increased blood pressure, which can provoke more serious diseases.
4). Danger of jet contact with persons. During the cleaning process, abrasive particles reach speeds of over 650 km/h, which is a very high speed. It is quite obvious that a jet of such power, if it hits the body, can cause injury or even cause death.
Application area
The following are the main areas of application of sandblasting:
1). Cleaning metal surfaces from paint residues, old varnishes, rust, scale and other types of contaminants typical of metals.
2). Degreasing of metal samples or blanks before applying paint, varnish, spraying performed by gas-thermal methods, various galvanic and similar operations.
3). Cleaning of fittings and metal rods for devices involved in an electrovacuum environment immediately before assembling and pumping out the contents from the cylinder.
4). Cleaning glass for decorative purposes, such as frosting.
5). Creating “roughness” on the surface to achieve a decorative “antique” effect.
It is according to the last point that they are increasingly turning to the sandblasting method. There are more and more people who want to not only clean the brickwork while maintaining its decorative appearance, but also age the surface; for example, a new wooden beam can be treated in such a way that it takes on a worn-out effect, as if it were an old surface.
Sometimes this method is used by dishonest antique lovers, aging finds and a collector ignorant in such matters is given a new, but simply aged item for fabulous sums.
Technologies
As stated above, the method was invented back in 1870, and at that time ordinary quartz sand was used for cleaning, which was washed and sifted through special sieves until the contents were homogeneous.
The dust that was generated during processing by this method adversely affected people, which led to a large number of silicosis diseases.
Silicosis is a severe, incurable disease associated with damage to the respiratory tract by silica sand dust, which subsequently leads to fatal diseases such as tuberculosis, euzema and lung cancer.
Despite all the precautions taken, such as increased ventilation and the use of respirators, the number of victims from the dust did not decrease. As a result, the increase in diseases led to the fact that quartz sand was replaced by less dangerous components: glass beads, steel shot, corundum powder or other synthesized materials.
In the modern world, the method sandblasting surfaces has evolved and currently uses the following technologies:
1). Gas-dynamic cleaning. With this cleaning method, abrasive materials in the jet stream are accelerated to a speed of 300 m/s. To understand: this speed is comparable to the speed of a bullet from a gun, i.e. is very high.
2). Hydro jet cleaning. With this type of cleaning, a stream of water is applied to the dirty surface with a pressure from 100 (10 MPa) to 7500 bar (750 MPa). With this pressure, dirt is simply squeezed out from the surface.
3). Hydro-jet cleaning with various inhibitors or abrasives, carried out under different water pressures. If abrasive particles are relatively known, why is the inhibitor so good? The main property of an inhibitor is that it delays or suppresses the occurrence of physiological, enzymatic and physicochemical processes. They have received practical application in the field of protecting metal from corrosion.
By interacting with the metal surface, the inhibitor gives it a positive charge, which leads to a slowdown in the corrosion process. That. we can conclude that an inhibitor is a substance aimed at reducing the spread of corrosion by forming a protective film on a metal surface.
4). Cleaning with dry ice. Carbon dioxide gets its name “dry ice” due to its appearance. Its peculiarity is that under normal natural conditions, for example, at room temperature of 20 degrees Celsius, carbon dioxide turns into a vapor state, skipping the liquid phase.
Efficiency of the method
Due to natural and operational conditions, almost all current paint and varnish and other materials require constant abrasive sandblasting to remove stains and dirt and roughen the surface.
Sandblasting extends the service life of all kinds of coatings up to 6 times, thanks to this you can significantly save on current or major repairs of structures made of metal.