Chemical milling. Chemical milling (contour etching). Advantages of chemical milling

Chemical methods of processing materials are called in which the removal of a layer of material occurs due to chemical reactions in the processing zone. Advantages of chemical processing methods: a) high productivity, ensured by relatively high reaction rates, primarily the absence of dependence of productivity on the size of the surface area being treated and its shape; b) the ability to process particularly hard or viscous materials; c) extremely low mechanical and thermal effects during processing, which makes it possible to process parts of low rigidity with sufficiently high accuracy and surface quality.

Dimensional deep etching (chemical milling) is the most common chemical processing method. It is advisable to use this method for processing surfaces of complex shapes on thin-walled parts, obtaining tubular parts or sheets with a smooth change in thickness along the length, as well as when processing a significant number of small parts or round workpieces with large; number of treated areas (perforation of cylindrical pipe surfaces). By local removal of excess material in unloaded or lightly loaded areas by this method, the overall weight of aircraft and missiles can be reduced without compromising their strength and rigidity. In the USA, the use of chemical milling made it possible to reduce the weight of a supersonic bomber wing by 270 kg. This method allows you to create new structural elements, for example sheets 1 of variable thickness. Chemical milling is also used in the manufacture of printed circuits of electronic equipment. In this case, from a panel made of insulating material, covered on one or both sides with copper foil, the areas specified by the circuit are removed by etching.

The technological process of chemical milling consists of the following operations.

1. Preparing parts for chemical milling to ensure subsequent tight and reliable adhesion of the protective coating to the surface of the part. For aluminum alloys, this preparation is carried out: by degreasing in B70 gasoline; light etching in a bath with caustic soda 45-55 g/l and sodium fluoride 45-55 g/l at a temperature of 60-70 ° C for 10-15 minutes to remove the clad layer; washing in warm and cold water and clarification in nitric acid, followed by washing and drying. For stainless and titanium alloys, parts are prepared by etching to remove scale in a bath with hydrofluoric (50-60 g/l) and nitric (150-160 g/l) acids or in a bath electrically heated to 450-460 ° C in caustic soda and sodium nitrate (20%), followed by washing and drying, degreasing and light etching with repeated washing and drying.

2. Application of protective coatings to areas of the workpiece that are not subject to etching. It is produced by installing special overlays, chemically resistant adhesive-type templates or, most often, by applying paint and varnish coatings, which are usually used perchlorovinyl varnishes and enamels, polyamide varnishes and materials based on non-oprene rubbers. Thus, for aluminum alloys, we recommend enamel PKhV510V, solvent RS1 TU MKhP184852 and enamel KhV16 TU MKhPK-51257, solvent R5 TU MKhP219150, for titanium alloys - glue AK20, thinner RVD. For better adhesion of these coatings to the metal, the surface is sometimes pre-anodized. The application of paint and varnish coatings is carried out with brushes or spray guns with preliminary protection of the etching areas with templates or by immersion in a bath; in the latter case, the contour is marked on the dried protective film, then cut through and removed.

3. Chemical dissolution is carried out in baths in compliance with the temperature regime. Chemical milling of aluminum and magnesium alloys is carried out in solutions of caustic alkalis; steels, titanium, special heat-resistant and stainless alloys - in solutions of strong mineral acids.

4. Cleaning after etching of parts made of aluminum alloys with an enamel protective coating is carried out by washing in running water at a temperature of 50+70 ° C, soaking the protective coating in hotter running water at a temperature

70-90° C and subsequent removal of the protective coating with knives manually or soft brushes in a solution of ethyl acetate and gasoline (2:1). Then they are clarified or lightly etched and dried.

The quality of the surface after chemical milling is determined by the initial roughness of the workpiece surface and etching modes; usually it is 1-2 grades lower than the cleanliness of the original surface. After etching, all previously existing defects on the workpiece are removed. (risks, scratches, irregularities) retain their depth, but widen, acquiring greater smoothness; The greater the etching depth, the more pronounced these changes are. The quality of the surface is also influenced by the method of obtaining workpieces and their heat treatment; rolled material gives a better surface compared to stamped or pressed material. High surface roughness with pronounced irregularities is obtained on cast workpieces.

Surface roughness is influenced by the structure of the material, grain size and grain orientation. Aged hardened aluminum sheets have a higher grade of surface finish. If the structure is coarse-grained (for example, the metal is annealed), then the final processed surface will have large roughness, uneven, and bumpy. The fine-grained structure should be considered most suitable for chemical processing. It is better to process carbon steel workpieces by chemical milling before hardening, since in the case of hydrogenation during etching, subsequent heating helps remove hydrogen. However, it is advisable to harden thin-walled steel parts before chemical treatment, since subsequent heat treatment can cause their deformation. The surface processed by chemical milling is always somewhat loosened due to etching, and therefore this method significantly reduces the fatigue characteristics of the part. Taking this into account, for parts operating under cyclic loads, it is necessary to carry out polishing after chemical milling.

Chemical milling accuracy ±0.05 mm. depth and not less than +0.08 mm along the contour; The radius of curvature of the cutout wall is equal to the depth. Chemical milling is usually carried out to a depth of 4-6 mm and less often up to 12 mm; With a greater milling depth, the surface quality and processing accuracy deteriorate sharply. The minimum final thickness of the sheet after etching can be 0.05 mm, so chemical milling can be used to process parts with very thin bridges without warping; processing can be carried out on a cone by gradually immersing the part in the solution. If it is necessary to etch on both sides, you must either position the workpiece vertically so as to allow the released gas to freely rise from the surface, or etch in two stages - first on one side and then on the other. The second method is preferable, since when the workpiece is positioned vertically, the upper edges of the cutouts are processed worse due to gas bubbles entering there. When making deep cuts, special measures (for example, vibration) should be used to remove gas from the surface being processed, which interferes with the normal process. Control of depth and etching during processing is carried out by immersion Simultaneously with the preparation of control samples, direct control of dimensions using thickness gauges such as an indicator bracket or electronic, as well as through automatic weight control.

The productivity of chemical milling is determined by the rate of material removal in depth. The etching rate increases with increasing solution temperature by approximately 50-60% for every 10 ° C, and also depends on the type of solution, its concentration and purity. The solution can be stirred during the etching process using compressed air. The etching process is determined by an exothermic reaction, so the supply of compressed air cools it somewhat, but basically the constant temperature is ensured by placing water coils in the bath.

Etching by immersion has a number of disadvantages - the use of manual labor, partial breakdown of protective films on untreated surfaces. When processing a number of parts, the jet etching method is more promising, in which alkali is supplied by nozzles.

A means of increasing the productivity of chemical milling is the use of ultrasonic vibrations with a frequency of 15-40 kHz; in this case, processing productivity increases by 1.5-2.5 times - up to 10 mm/h. The chemical processing process is also significantly accelerated by targeted infrared radiation. Under these conditions, there is no need to apply protective coatings, since the metal is subjected to strong heating along a given heating circuit, and the remaining areas, being cold, practically do not dissolve.

The etching time is determined experimentally on control samples. The pickled workpieces are removed from the etching machine, washed in cold water and treated at a temperature of 60-80 ° C in a solution containing 200 g/l caustic soda to remove the emulsion, paint and BF4 glue. The finished parts are thoroughly washed and dried in a stream of air.

Improving the conditions for rough cutting of workpieces by preliminary removal of the skin by etching is another example of the dissolving effect of the reagent. Before etching, the workpieces are blasted with sand to remove scale. Titanium alloys are etched in a reagent consisting of 16% nitric and 5% hydrofluoric acids and 79% water. According to foreign literature, for this purpose, etching in salt baths is used, followed by washing in water and then re-etching in acid etchants to finally clean the surface.

The chemical effect of the technological environment is also used to improve conventional cutting processes; Material processing methods based on a combination of chemical and mechanical influences are becoming increasingly used. Examples of already mastered methods are the chemical-mechanical method of grinding hard alloys, chemical polishing, etc.

The essence of the invention: the method includes: applying a protective coating to the surface, marking and cutting out the contour of the chemical milling zone, mesh disruption of the protective coating inside the contour of the milling zone and etching the metal to the required depth while simultaneously removing the protective coating from the milling zone by peeling. 2 ill., 1 tab.

UNION OF SOVIET

SOCIALIST

REPUBLIC (51) S 23 F 1/02

STATE PATENT

No. 990871, class C 23 F 1/02, 1979.

The invention relates to the chemical processing of materials and can be used in mechanical engineering for chemical freezing. There is a known method and device for automatic chemical milling of surfaces of complex curvature, according to which the protective material applied to the surface of the part is cut off by a laser from the area to be chemical milled.

However, laser cutting of protective material from the entire surface to be chemically treated, as provided for in this method, is unproductive, because The energy of the laser beam at the moment of removing the coating is concentrated on a small area.

The closest to the proposed method is the method of producing parts with areas of variable thickness by chemical etching, which includes preliminary application of a protective coating to areas of the surface that are not subject to etching and its subsequent removal by peeling.

",5U 1791467 A1 (54) METHOD FOR CHEMICAL MILLING OF PARTS (57) The essence of the invention: the method includes applying a protective coating to the surface, marking and cutting the contour of the chemical freezing zone, mesh disruption of the protective coating inside the contour of the milling zone and metal etching to the required depth while removing the protective coating from the milling zone by peeling. 2 ill., 1 tab.

In this case, the removal of the protective coating is carried out in two ways: from equal-thickness areas, the coating is removed before the etching process, and during the etching process, the protective coating is peeled off only from a section of variable thickness (wedge), and this operation is performed using a special device that includes an electric motor. , moving platform, frame, shaft for winding the protective coating. ABOUT

The disadvantages of the known method include high labor and energy costs associated with the need to use combined methods for removing the coating, the use of a special device and associated installation work, as well as the limited use only for wedge-shaped parts.

The purpose of the invention is to increase productivity and simplify the process.

This goal is achieved by the fact that in the method of chemical milling of parts, including applying a protective coating to the surface, marking and

1791467 cutting out the contour of the chemical milling zone, etching the metal to the required depth while simultaneously removing the protective coating from the milling zone by peeling; before etching the metal, a mesh disruption of the protective coating is carried out inside the contour of the milling zone.

Distinctive features of the invention are that before etching the metal, a mesh disruption of the protective coating is carried out inside the contour of the milling zone. The positive effect from the use of the proposed invention arises from increasing the productivity of the chemical milling process by eliminating the labor costs associated with removing the protective material from the areas subject to chemical milling, which according to the proposed method, it is carried out simultaneously with the dissolution of the metal without additional influence.

Chemical milling of parts is carried out in a specialized chemical milling line.

Figure 1 shows an example of the technological layout of the line on which the method is carried out; Fig. 2 is an example of fastening a part for transportation through the operations of the chemical milling process. The chemical milling line contains baths 1-4 for process solutions (the number of baths is determined by the technological process and the required productivity and may be more than indicated in Fig. 2), auto operator 5, moving along the line along guides 6, 7, a control stand having two stands 8 and 9, a manipulator 10 with a replaceable working element 11. For fastening the part

12, a frame 13 is used, which has a reversal mechanism 14, with clamps 15, 16, 17.

To transport the part 12, fixed in the frame 13, along the line by the auto operator 5 there is a traverse 18 connected to the frame 13 by flexible suspensions

19. To mount part 12 into frame 13, use the mounting table 20. To apply coating to part 12, there is a special chamber 21. To control the line, a computer (control computer complex) 22 is used, which receives information from devices for monitoring the thickness of parts, monitoring the state of the protective coating, control of parameters of technological solutions, etc.

Example, Chemical milling of a part made from aluminum alloy AMg-6 was carried out, with an initial sheet thickness of 8 mm, Alloy composition, wt., 4: copper 0.1, marg. nets 0.5-0.8; iron 0.4; silicon 0.4; titanium

5 0.02-0.1; magnesium 5.8-6.8; beryllium 0.02-0.05; the rest is aluminum.

The part has three chemical milling zones with a depth of 2.1 mm, 3.4 mm, 5.9 mm.

On part 12 in chamber 21 it was applied

10 spray protective coating - enamel KCh 7101 with a viscosity of 45 s to B3-4.

The coating was applied in three layers, drying each layer for 40 minutes at 50 C and final drying for 6 hours at 80 C. Film thickness

15 180-200 microns.

The installation of the coated part 12 into the frame 13 was carried out on an assembly stand 20 in a horizontal position. Next, the chemical milling zone was marked

20 with the maximum depth of metal removal and cutting the outline of the design on the protective coating with an electric burning pencil "Silhouette". A pattern in the form of a grid was applied to the surface of the zone with the coating being cut through to the metal. Size calculation

The mesh cells were carried out taking into account the etching factor (the ratio of the amount of lateral etching a to the etching depth h), determined on the samples, and the depth

30 etching h was taken to be equal to the depth of dissolution of the metal with the smallest depth. bine zone (2.1 mm). – – – – – – – – = 3.11 and 6.53

The cell size was determined;

B=2xf=3.1 1 x2=6.22 mm

The cell size was assumed to be 6

At the end of the process of applying a pattern to the area intended for processing, frame 13 with part 12 was moved by auto operator 5 through the baths in accordance

45 with the technological process, Chemical milling and peeling of the coating was carried out in a bath of the composition, g/l:

Caustic sodium 150-200

Triethanolamine 20-30

50 Thiourea 6-10

Chemical milling temperature - 80 C

Marking, breaking the continuity of the coating and chemical milling of the two subsequent zones were carried out in the same way.

55 The finished part had a clear chemical milling contour with surface finish R

To confirm the reduction in labor costs, experimental work was carried out in a laboratory environment

1791467 of the Hermes organization. The essence of the work was to compare the labor costs by time measurement during chemical reserving using the traditional method (in accordance with

OST 92-4555-75) and the method described in the invention. The work was carried out on samples made of AMg6 alloy, dimensions 100x50x8 mm. KCh-7101 enamel (3 layers) was used as a protective coating. The size of the “window” of the milled area is 50x30 mm, the cutting depth is 3 mm.

The results of the experiment are shown in the table. From the given data it follows that, given the quality of the surface, labor costs in the proposed method are on average less than $10, i.e. and labor productivity increases. With large dimensions of parts and complex contours, the positive effect increases noticeably.

The proposed method allows noeb to increase productivity, reduce labor costs, and simplify the chemical milling process. Formula of invention

A method of chemical milling of parts, including applying a protective coating to the surface, marking and

10 cutting out the contour of the chemical freezing zone, etching the metal to the required depth while simultaneously removing the protective coating from the milling zone by peeling, characterized in that 15 in order to increase productivity and simplify the freezing process, before etching the metal, a mesh disruption of the protective coating is carried out inside the contour of the milling zone, 1791467

Order 135 Circulation Subscription

VNIIPI of the State Committee for Inventions and Discoveries under the State Committee for Science and Technology of the USSR

113035, Moscow, Zh-35, Raushskaya embankment 4/5

Manufacturing and production plant "Patant", r. Uzhgorod, Gagarin st., 101!

Editor A. Egorova

Compiled by I. Skorobogatov

Techred M. Morgenthal Proofreader S. Lisina

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Chemical milling of concrete is the treatment of a concrete surface with special chemicals in order to improve its adhesion. On concrete surfaces, after hardening, almost all the pores are clogged, so paint and sealant adhere to them rather poorly. Chemical milling helps open up the pores in the concrete and prepare it for any type of coating.

The connection of the surfaces of dissimilar bodies depends on their adhesion. From Latin adhesion is translated as sticking. Thanks to this phenomenon, it becomes possible to apply paint and varnish and galvanic coatings, welding, gluing, etc. Therefore, increasing adhesion is a very pressing problem for modern construction.

When performing concrete work, situations sometimes arise that make it impossible to pour the entire object at one time. During subsequent pouring, a so-called cold seam occurs at the point of contact between the new and old layers of concreting.

A cold seam causes a loss of joint strength and a violation of its water permeability

Another problem is created by the difficulties that arise when carrying out finishing work on concrete surfaces (plastering, making self-leveling floors). Indeed, on the surface of the concrete, eight hours after it sets, a cement film (layer of cement laitance) is formed, which prevents the adhesion of the finishing material and concrete. If the cement film is not removed, the connection will become weak. and the likelihood of peeling and failure of the floor or plaster will significantly increase.

Various methods are used to remove laitance, but in recent years chemical milling has become widespread. This method is equally effective for removing laitance from both old and fresh concrete or brick surfaces. The main purpose of chemical milling is to prepare the surface for applying various coatings on a polymer, cement or gypsum base.

Preparing the concrete base for coating

The technology for removing laitance with penetrating compounds is used when performing waterproofing work, eliminating “cold joints,” installing self-leveling floors, as well as other processes that require high-quality adhesion of the concrete base and the applied compound.

It allows you to open pores, microcracks and capillaries of concrete, which ensures the penetration of a chemically active composition into its pore space, the formation and growth of crystals of materials used for waterproofing concrete surfaces and other purposes.

Reasons for the formation of laitance

Cement laitance is a weak and loose crystalline structure on the surface created with concrete. The thickness of its layer can be 20-300 microns, but this layer “lives” separately from the concrete. It does not have a strong physical connection with the concrete base and at the same time prevents the penetration of any liquids into the concrete. Because of this, a dense and durable crystalline structure does not form in the surface layer of concrete.

The main source of the formation of laitance is an aqueous solution of calcium hydroxide, which comes to the surface of concrete along with water. Reacting with carbon dioxide present in the air, it forms a film of calcium carbonate, which in chemical composition is limestone and is insoluble in water.

The formation of cement laitance is also promoted by:

alkali metal salts, which are present in free form in the composition of cement;
ash waste thermal power plants, which are added to cement and release alkalis;
gravel, crushed stone, sand containing halogen compounds;
antifreeze and modifying additives, used in the production of concrete mixtures.

Cement laitance in its composition is a mixture of carbonates, nitrates, sulfates and chlorides, soluble and insoluble in water

Soluble alkalis, when cement is combined with water, form solutions that chemically bond with aluminates and silicates of cement. Upon contact with carbon dioxide, these alkalis become carbonized and form a dense cement laitance, insoluble in water. Another reason for the formation of milk may be the water used for mixing cement, if its composition does not meet regulatory requirements.

Cement laitance is a loose, fragile structure that fills the pore space of concrete to a certain depth. When applying any coating to concrete with a cement film on the surface, instead of the expected monolithic connection, a three-layer system “surface coating - cement laitance - concrete” is formed. The strength between the layers of this “pie” is half as much as expected.

In this case, each of the layers works independently of the others and separately perceives mechanical loads. The weakest point in terms of strength is the cement film. Obviously, with increasing stress, destruction will occur here. The cement film is a kind of boundary at which shrinkage compressive stresses turn into tensile stresses. That is why the cold weld zone immediately becomes prestressed.

Concrete, as you know, works well in compression, somewhat worse in bending and very poorly in tension. The joint zone, due to tensile stresses, has much lower strength and density than monolithic concrete. That's why cracks under equal stresses form primarily along cold seams.

To avoid the “cold joint” effect and make the concrete surface able to accept a protective layer of sealant or paint, it is necessary to remove the cement film and open the pores in the concrete. For this purpose, various mechanical and chemical methods are used.

Methods used to remove laitance

Mechanical cleaning

Mechanical cleaning of concrete surfaces is carried out using mechanical wire brushes, milling and grinding machines. To avoid damage to the underlying layers of the screed, dry mechanical cleaning of hardened concrete can be carried out only after it has gained a certain strength. But as strength increases, cleaning becomes significantly more difficult.

The use of milling machines and driven metal brushes is justified only when the concrete gains a strength of no more than 2-3 MPa. In the case when the concrete becomes more durable, the cleaning efficiency will noticeably decrease due to a significant increase in processing time and increased tool wear.

Disadvantages of mechanical methods for cleaning concrete from cement laitance:

the possibility of cleaning only after the concrete has reached the required strength leads to rather long technological interruptions;
internal stresses may arise, manifested by microcracks;
only a layer of cement laitance is removed, and the pores of the concrete do not open;
the formation of large amounts of dust, which requires the use of industrial vacuum cleaners;
high labor intensity;
high cost of equipment.

When mechanically cleaning chemical milk, it is difficult to even control the quality of the work performed

Hydrosandblasting

The use of hydrosandblasting allows you to remove the cement film and open the pores of concrete only in the surface layer.

The process has the following disadvantages:

the impossibility of cleaning before the concrete gains a strength of 5 MPa;
the occurrence of internal stresses due to the impact of the working jet, as well as their subsequent relaxation, leading to the formation of microcracks;
restrictions on use during existing production and internal work;
high cost of equipment (high-pressure compressors, abrasive blasting systems, air filtration units).

Cleaning with air or water jet

This treatment is carried out with water or water-air jets under a pressure of 0.5-0.7 MPa. This method is the simplest and allows for cleaning almost immediately after pouring concrete (with a strength of 0.3 MPa). With such strength, you can walk on the surface of concrete, but marks will remain on it.

In this case, the concrete has a fairly strong structure, so there is no danger of breaking the adhesion of the mortar part and the coarse aggregate. The time to achieve this strength ranges from 4 to 18 hours and depends on the temperature and humidity of the surrounding air, as well as on the properties of the concrete mixture.

The disadvantages of this method include:

impossibility of use on vertical surfaces and at negative air temperatures;
a cement film remains on the surface of the concrete, insoluble in water;
The compressor oil contained in the compressed air forms an anti-adhesive film on the surface.

Chemical etching

Chemical etching is carried out using hydrochloric acid. This cleaning process is technically unjustified and even harmful. The use of hydrochloric acid reduces the durability of concrete.

Disadvantages of chemical etching:

slight increase in adhesion strength compared to an untreated surface;
surface destruction of not only cement laitance, but also cement stone, which causes the destruction of the cold seam between new and old concrete during operation;
the need for additional treatment with alkali to neutralize the acid.

Application of hardening retarders

In order to increase the time interval between pouring the concrete mixture and removing the cement laitance, as well as to facilitate the cleaning procedure, various hardening retarders are used, for example, SDB (sulfite-yeast mash). The SDB solution is applied to the concrete surface with a paint sprayer.

The weakened surface layer is removed with drive brushes or a jet of water under high pressure.

The disadvantages of this method include:

impossibility of carrying out processing immediately after pouring concrete. Depending on the air temperature, the start time of treatment can be from 2 to 4 days;
the need for careful control of the strength of the base concrete;
impossibility of using hardening retarders when carrying out concreting in the autumn-winter period.

The low technical level and uneconomical nature of existing methods for cleaning concrete surfaces from cement film have led to the search for new ways to solve this problem. As a result of research, a completely new method of removing laitance was developed - chemical milling.

Advantages of chemical milling

The method of chemical milling consists in sequential processing of the concrete surface with compositions made on the basis of complex polyfunctional acids. This method completely eliminates the use of mechanical cleaning, shot-, hydro-, sand- and hydro-sandblasting, and in some cases the need to install a plaster mesh.

Chemical milling makes it possible to effectively dissolve cement laitance, open the pores of concrete, and create a monolith. This method increases the adhesion strength of monolithic concrete layers by 1.5-3 times, gypsum, cement and magnesium screeds, penetrating waterproofing materials, epoxy, polyurethane, acrylate and cement self-leveling floors, tile adhesives, joint sealants, plasters, facade and interior cladding made of artificial and natural stone.

The main advantages of chemical milling:

dissolution and removal of laitance without destroying the cement stone;
elimination of the cold seam, which contributes to the creation of a monolith;
increasing the penetration depth of waterproofing materials and other coatings;
reducing the labor intensity of cleaning concrete from cement film;
reducing the cost of work.

Materials used for chemical milling

During chemical milling, concrete is sequentially treated with various compounds, for example, the Crystallizol Himfrez complex. This complex includes two compositions: the acidic cleaner Kristallizol Cleaner and the alkaline adhesion activator Kristallizol Active. First, Crystallizol Cleaner is applied to the concrete surface, which dissolves cement laitance. and opens the pores of concrete, but does not react with cement stone and does not disrupt its structure.

An hour later, when foaming stops, Crystallizol Active is applied to the concrete, which enhances adhesion. The use of this complex increases the depth of penetration of active chemicals into the concrete surface.

Chemical milling creates conditions for organizing a monolithic connection between concrete - polymer floor or concrete - waterproofing layer

Advantages of Crystallizol Himfrez formulations:

the compositions are harmless to nature and humans. They meet all environmental safety requirements;
they do not have a strong odor, so working with them is convenient and easy;
the chemical composition does not contain acetic, hydrochloric, orthophosphoric, citric acid and other elements that negatively affect the surface of concrete;
the complex can be used in the construction of any objects, including food industry enterprises, swimming pools, and drinking water reservoirs.

Such materials as Lepta Himfrez, Gambit Frez (N-1) Complex, Elakor-MBZ, ArmMix Cleaner, Dezoxyl STOP, Tiprom Plus have similar properties. All these materials are manufactured according to the same principle and have an identical physical principle of operation. The chemicals contained in their composition destroy the cement film and open the pores of concrete. The use of these materials increases the adhesion strength of concrete to self-leveling floors and other coatings by 1.5-3 times.

Technology of applying the composition for chemical milling

As an example, consider the technology for applying the Elakor-MBZ composition. This composition is used to remove cement film, the top contaminated or weakened layer of cement-containing surfaces indoors or outdoors.

General requirements and recommendations:

base: concrete surfaces, sand-cement screeds;
the humidity of the base should be no more than 6%;
air and base temperature not less than +5 degrees;
relative air humidity – not standardized;
The curing time of concrete after pouring before treatment is at least 14 days.

Process stages:

foundation preparation. At this stage, remove dust, dirt, old paint, oil stains, etc.;
preparing material for work. As a rule, Elakor-MBZ is sold ready for application, but is also available in the form of a concentrate, which must be diluted with water in a ratio of 1:3. The consumption of the finished composition is 0.4-0.5 liters per square meter;
application. The composition is evenly applied to the surface to be treated. This can be done with a roller, brush, pneumatic spray gun or shotcrete method. Shotcrete is the application of solutions using a special shotcrete installation. This installation supplies the solution at a speed of 90-100 m/s. The air pressure is 150-350 kPa;
curing of the composition on the concrete surface until the cement laitance and efflorescence are completely dissolved;
removal of reaction residues using water;
waiting time before applying plasters, mineral screeds, sealants, tile adhesives, self-leveling floors should be at least one hour.

Safety at work

All chemical concrete milling compounds should be handled with great care. If they come into contact with the skin, they can leave a rather painful chemical burn.. You should especially beware of getting these compounds on your face or eyes, as this can cause disfiguring scars and even permanent blindness.

If the compositions come into contact with the skin or eyes, rinse them with plenty of water.

When working with compounds, you should always wear protective clothing with long sleeves, closed shoes, safety glasses with a mask and gloves. Avoid inhaling chemical vapors, as they may cause a burn to the throat or mouth.

For the same reason, you should make sure that the workplace is well ventilated. If the vapors are very strong, a respirator with an acid vapor cartridge should be used to avoid injury. Before using any composition, it is recommended to carefully study the instructions, which are usually indicated on the labels.

Cost of materials for chemical milling of concrete

Estimated cost of compositions used for chemical treatment of concrete surfaces:

Name	Purpose	Cost, rub/liter
Himfrez Complex	Two in one. Removes laitance, opens pores and adhesively activates the surface.	180
Himfrez Cleaner	Dissolves cement laitance and opens the pores of concrete.	140
Himfrez Activator	Increases adhesion (adhesion strength) of concrete with cement and polymer coatings.	140
Scraper	Gentle cleaning of concrete surfaces from cement deposits and efflorescence.	120
ArmMix Cleaner	Cleaning concrete from efflorescence, plaque, cement film, etc., as well as improving adhesion.	65
Deoxyl STOP	Cleaning surfaces made of concrete, metal and other materials. The product removes ceramics, enamel, concrete, glass, salt deposits, etc. Safe for tungsten, titanium, and chemically resistant plastics.	95
Tiprom Plus	Cleaning facades made of brick, artificial and natural stone from salt deposits, cement mortar residues, and atmospheric pollution.	90

conclusions

Chemical milling is characterized by high productivity, low labor intensity and cost-effectiveness. With its help, you can very quickly and quite simply remove cement laitance, the top weakened or contaminated layer of a cement-containing coating from a concrete surface. Experts say that chemical milling is the most effective way to remove cement film from concrete.

When using mechanical methods for cleaning concrete, care must be taken not to polish the pores of the material with settling dust. This can cause the surface to become very smooth, which significantly reduces adhesion. Chemical milling compounds are high-performance, low-consumption solutions that are ideal for creating rough edges on smooth concrete. They open the pores of concrete and increase its adhesion by 1.5-3 times. Besides, Chemical milling is a less labor-intensive procedure than mechanical milling.

Chemical treatment of concrete is used to eliminate the “cold joint” effect, to activate the action of dust-removing compounds and penetrating waterproofing materials, to create a monolithic connection between a concrete base and a self-leveling floor. This processing has virtually no restrictions. It can be used to remove cement film from both old and fresh pouring. from porous and dense, from wet and dry concrete surfaces, both indoors and outdoors.

More details about chemical milling of concrete are shown in the video:

The essence of the chemical milling process is the controlled removal of material from the surface of the workpiece by dissolving it in an etchant due to a chemical reaction. Areas of the workpiece that are not subject to dissolution are covered with a protective layer of chemically resistant material.

The removal rate of many materials is up to 0.1 mm/min.

Advantages of the process:

· high productivity and quality of processing,

· the ability to obtain parts of complex configurations of both small and significant thickness (0.1-50) mm;

· low energy costs (chemical energy is mainly used);

· short production preparation cycle and ease of automation;

· waste-free due to regeneration of process products.

During processing, material removal can be carried out from the entire surface of the workpiece, to various depths or to the entire thickness of the part (through milling). Chemical milling includes the following main stages: preparation of the workpiece surface; applying a protective layer of the pattern; chemical etching; removal of the protective layer and quality control of products (see Fig. 3.1).

Surface preparation means cleaning it from organic and inorganic substances, for example, using electrochemical degreasing. The degree of purification is determined by the requirements for subsequent operations.

The application of the protective layer of the design is carried out using the following methods: manual and mechanized engraving on the mistaken (varnish, wax) layer, xerography, screen printing, offset printing, as well as photochemical printing.

In instrument making, the most widely used method is photochemical printing, which ensures small product sizes and high accuracy. In this case, to obtain a protective layer of a given configuration, a photomask is used (a photocopy of the part on an enlarged scale on a transparent material). Liquid and film photoresists with photosensitivity are used as a protective layer. Liquid ones, which are the most widely used in industry, require high-quality cleaning of the workpiece surface. To apply them to the surface, one of the following methods is used: immersion, watering, spraying, centrifugation, roller rolling, spraying in an electrostatic field. The choice of method depends on the type of production (continuous application or on individual workpieces); requirements for the thickness and uniformity of the film formed, which determine the accuracy of the pattern dimensions and the protective properties of the resist.

Rice. 3.1. General diagram of the technological process of chemical milling.

Photochemical printing of a protective pattern, in addition to the operation of applying photoresist and drying it, includes the operations of exposing the photoresist layer through a photomask, developing the pattern and tanning the protective layer. During development, certain areas of the photoresist layer dissolve and are removed from the surface of the workpiece. The remaining layer of photoresist in the form of a pattern defined by a photomask, after additional heat treatment - tanning - serves as a protective layer during the subsequent chemical etching operation.

The chemical etching operation determines the final quality and yield of the product. The etching process occurs not only perpendicular to the surface of the workpiece, but also sideways (under the protective layer), which reduces the processing accuracy. The amount of etching is assessed through the etching factor, which is equal to , where H tr is the etching depth, e is the amount of etching. The rate of dissolution is determined by the properties of the metal being processed, the composition of the etching solution, its temperature, the method of supplying the solution to the surface, the conditions for removing reaction products and maintaining the etching properties of the solution. Timely termination of the dissolution reaction ensures the specified processing accuracy, which is approximately 10% of the processing (etching) depth.

Currently, etchants based on salts with an amine oxidizer are widely used, among which the most commonly used are chlorine, oxygen compounds of chlorine, dichromate, sulfate, nitrate, hydrogen peroxide, and fluorine. For copper and its alloys, kovar, steel and other alloys, solutions of ferric chloride (FeCl 3) with a concentration of 28 to 40% (by weight) and a temperature within (20 - 50) C, which provide a dissolution rate of (20 - 50) µm/min.

Among the known methods of etching there is immersion of the workpiece in a calm solution; into a stirred solution; spraying the solution; spraying the solution; jet etching (horizontal or vertical). The best processing accuracy is ensured by jet etching, which consists in the fact that an etching solution under pressure is supplied through nozzles to the surface of the workpiece in the form of jets.

Quality control of parts includes visual inspection of their surface and measurement of individual elements.

The chemical milling process is most beneficial in the manufacture of flat parts of complex configurations, which in some cases can also be produced by mechanical stamping. Practice has established that when processing batches of parts in quantities up to 100 thousand, chemical milling is more profitable, and over 100 thousand, stamping is more profitable. For very complex configurations of parts, when it is impossible to make a stamp, only chemical milling is used. It should be taken into account that the chemical milling process does not allow the production of parts with sharp or right angles. The radius of curvature of the internal corner must be at least half the thickness of the workpiece S, and the external corner - more than 1/3 S, the diameter of the holes and the width of the grooves of the parts must be more than 2 S.

The method has found wide application in electronics, radio engineering, electrical engineering and other industries in the production of printed circuit boards, integrated circuits, in the manufacture of various flat parts with complex configurations (flat springs, raster masks for picture tubes of color TVs, masks with circuit patterns used in thermal spraying processes , grids for razors, centrifuges and other parts).

TO category:

Chemical treatment

Chemical milling, stamping, polishing

In chemical milling, metal removal is carried out by immersing the workpiece in an etching solution. Areas not subject to etching are isolated with appropriate protective coatings. The contours through which the metal dissolves are left uninsulated. This process makes it possible to produce parts of increased hardness of miniature and very large sizes and thin-walled, the mechanical processing of which is very labor-intensive.

Chemical milling is used for controlled removal of material to obtain parts of given sizes, mainly with shaped surfaces, reducing the thickness of the ribs to values that cannot be obtained by mechanical processing, stamping and casting: for processing corrugated walls, forming ledges for connecting several parts into one, for producing holes various shapes, deep and narrow grooves, processing of thin strips and steep bevels and for producing parts with variable cross-sections. In addition, chemical treatment is used to remove the damaged layer remaining on the surface after mechanical treatment and to obtain a desired relief on the surface (chemical branding).

This method can process all metals and alloys, including chemically resistant, heat-resistant, as well as aluminum and copper based. However, depending on the composition of the material being processed, both the composition of the solution and the processing modes change.

Processing of welded parts is possible if the welding is carried out without any defects, otherwise chemical milling of the weld can lead to the formation of pits or local etching.

Chemical milling allows tolerances from ±0.015 to ±0.5 mm. The roughness of the chemically milled surface is within grades 4-5. The average productivity is 0.025-0.1 mm/min.

It should be borne in mind that fluctuations in the thickness of the workpiece and the waviness of its surface are reproduced after chemical milling. Therefore, if the tolerances of finished parts are tight, it is necessary to first eliminate the difference in thickness of the workpieces by mechanical processing.

Equipment. Etching solutions for processing are very aggressive, so containers for them must be made of rigid polyvinyl chloride (vinyl plastic) or fluoroplastic-4. It is advisable to make large-sized baths made of steel, lined with chemically resistant silicate enamel of grade LK-1 or 105 or fluoroplastic ZM.

The thickness of the enamel lining should be in the range of 0.8-1.2 mm, and the ZM fluoroplastic - 400-500 microns. The technology of coating with enamel and fluoroplastic has been mastered by Leningrad enterprises, in particular Lenkhim Food Processing Plant successfully lining large containers with chemically resistant enamel. Etching solutions are heated to 60-70 ° C using a steam-water jacket or tubular heaters and immersing them in the working solution. Heaters must also be protected with a chemical-resistant coating.

Vapors resulting from metal dissolution must be reliably sucked out through hydrofilters. The exhaust system and filters must also be made of chemically resistant materials. Flange connections must be equipped with gaskets made of chemically resistant rubber or combined gaskets made of fluoroplastic and rubber.

Device for regulating the speed of immersion of parts into the bath

The workpiece subjected to chemical treatment is immersed in a bath of solution at a certain speed. The value of the immersion speed does not always have to be the same; it changes depending on the given mode.

To ensure the required speed of immersion of workpieces into the solution, as well as to implement a smooth and stepless change in speed, innovators V. K. Samotesov and A. P. Popov introduced a device.

Its design is simple and allows you to adjust the speed of immersion of workpieces into the solution in the range from 0 to 10 m/min. The device consists of a cylinder with a piston, a centrifugal pump, a two-way valve and a throttle. The cylinder is fixedly mounted on the bathtub body or mounted on a special rack supplied to the bathtub.

The operation of the device when immersing a workpiece consists of the following operations. The handle of the two-way valve is moved from position I to position II. In this position, the water supply to the cylinder stops. At the same time, water leaves the lower cavity of the cylinder through pipelines and a throttle. Under the influence of the load and the workpieces attached to the device, the piston will begin to slowly lower. The lowering speed is controlled by the throttle. When lifting parts from the bath, the handle of the two-way valve from position II is moved to position, and then water under a pressure of 0.6-0.8 atm is supplied to the lower cavity of the cylinder under the piston from the centrifugal pump through a two-way valve, which ensures rapid lifting of parts from the bath .

Solutions. For chemical milling of steel blanks from carbon steels, satisfactory results were obtained by using solutions of copper sulfate, potassium, sodium and ammonium persulfates, ferric chloride and sodium chloride.

For processing copper and brass, 10% solutions of potassium perchlorate mixed in equal volumes with a 4% solution of hydrochloric acid are used.

For chemical milling of X18N9T steel, a mixture of nitric and hydrochloric acids is required.

Rice. 1. A device for immersing workpieces during chemical processing.

For chemical milling of aluminum and its alloys, a mixture of copper sulfate, hydrochloric acid and sodium chloride or solutions of caustic soda and hydrochloric acid should be used.

Process research shows that for each metal and alloy there is an optimal concentration. Thus, for aluminum the optimal concentration of caustic soda is 300 g/l, for silumin - 400 g/l, for duralumin - 500 g/l. Further increase in the concentration of the solution reduces the productivity of the process.

The optimal concentration of hydrochloric acid is 30% and the temperature is 40 °C. As the temperature rises, the productivity of the process increases. However, it should be borne in mind that with increasing temperature, the activity of releasing harmful gases, especially chlorine, increases. Therefore, it is practically impossible to carry out treatment in hydrochloric acid at temperatures above 40 °C.

The technological process of chemical milling may vary somewhat depending on specific conditions. Meanwhile, the basic operations and their sequence remain unchanged.

The first operation in the technological process of chemical milling should be a control one. After machining, workpieces must be carefully inspected to ensure that there are no material or machining defects. The thickness of the sheet blanks is determined. After measuring the thicknesses, processing modes are assigned. Then the workpieces are thoroughly degreased with an organic solvent. If there are places covered with a layer of scale, it must first be removed. After cleaning, a protective coating is quickly applied. Materials for protective coatings are selected depending on the required depth of treatment and the aggressiveness of the solutions used.

In the case of the use of strong acids and deep processing, fluoroplastic ZM is suitable as a protective coating. The fluoroplastic coating is applied by pouring, dipping or using a spray. To increase the viscosity of the suspension up to 14 sec. using a VZ-4 viscometer, a plasticizer is introduced into it. This allows you to apply a layer 30 microns thick at a time, which is quite enough to produce many parts. In exceptional cases, 6-8 layers (0.18-0.24 mm) are required for very reliable protection. After applying each layer, the coating is dried at 120 °C and heat treated at 260 °C. The duration of heat treatment is 45 minutes.

For shallow metal removals, you can use a coating based on rubber glue 88 or nitro glue AK-20. After applying the protective coating, the workpieces are prepared for processing. To do this, first, a rigid overlay stencil is fixed above the coated workpiece, along which certain areas to be treated are outlined with a sharp knife. The protective coating is then removed from these areas, and the workpieces prepared for chemical treatment are immersed in the solution.

The process of chemical dissolution can be divided into three periods. First, the oxide film present on the metal surface dissolves, and this dissolution, as a rule, proceeds slowly. Next, gaseous reaction products accumulate on the metal surface (usually hydrogen). The formation of gaseous products is significantly influenced by the condition of the surface. The accumulation of the number of cathodes of local elements on the metal surface due to corrosion of the metal and exposure of more noble impurities or due to electrochemical exchange with the electrolyte also plays a serious role. The second period is characterized by a fluid equilibrium between factors and is a steady process. The third period is characterized by a drop in the reaction rate, a decrease in the electrolyte concentration and the formation of a thick layer of reaction products on the metal.

It should be borne in mind that during chemical treatment the rate of dissolution of the alloy over the entire surface is not the same. One reason for this is that it is difficult to maintain the same temperature at all points on the surface being treated.

During the dissolution process, the metal surface heats up, and the temperatures of the solution and the metal do not coincide. Unevenly released hydrogen worsens heat transfer conditions. In places where the temperature is higher, the rate of dissolution increases. All this leads to inaccurate dissolution, sometimes beyond the required tolerance. Therefore, in some cases it is necessary to take a number of measures (stirring or circulating the solution, reducing the concentration and temperature of the solution, correct placement of heaters, controlling the temperature in certain places, etc.) to reduce the temperature difference.

After unloading the parts from the solution, they must be thoroughly rinsed with cold water, the remaining salts must be neutralized, rinsed again with cold and hot water, dried and the protective coating must be removed with a solvent or by peeling off the coating.

Photochemical stamping. Its essence lies in the application of a protective sublayer, followed by the application of a photosensitive layer, copying and etching.

The technological process of manufacturing parts consists of the following main operations:
1. Preparatory operations (cutting the material into blanks, polishing the plates on both sides, degreasing with an organic solvent, rubbing with chalk, washing with running water, drying).
2. Coating the workpieces on both sides using a sprayer with AK-20 nitro glue.
3. Applying a thin layer of photosensitive emulsion in a centrifuge. The emulsion consists of photogelatin - 140 g/l, ammonium dichromate - 15 g/l and 25% ammonia solution.
4. Copying an image from a negative (film) in a photocopier. If it is necessary to expose on both sides of the workpiece, then to ensure an exact match of the image on both sides, two negative films are first glued to a narrow strip of material of the same thickness as the workpiece. The blank is inserted between the negatives.
5. Development in water at a temperature of 60-70 ° C for 2-3 minutes.
6. Staining with purple dye for 2 minutes. at 20 °C and fixation in a solution of the following composition (in g):
7. Rinse with warm and cold running water and air dry.
8. Removing glue from gaps with a swab moistened with acetone.
9. Etching the workpiece with a solution of ferric chloride. weight 1.33-1.55 followed by washing and drying. The duration of etching is determined experimentally.
10. Removing the protective layer by immersing the part in acetone.

The photochemical stamping process has found application at a number of factories, in particular at the Leningrad Vibrator plant for the manufacture of various instrument parts from copper, brass and bronze with a thickness of up to 0.2 mm.

At the Novosibirsk Electrotechnical Institute, this process is slightly modified for the manufacture of parts from aluminum alloy D16. Protection of areas not subject to etching is carried out by electrolytic copper plating. To do this, after fixing the emulsion, the workpieces are fired at 350-400 °C, the oxide layer is removed with a solution and contact copper is deposited in the composition: and after thorough washing, electrolytic copper plating is performed at a density of 2-3 a/dmg.

Remove the emulsion with a 15% caustic soda solution. Etching in places unprotected by copper is carried out in 30% hydrochloric acid at 25-30 °C. At the Leningrad Optical-Mechanical Association, precision photographic equipment parts are made from steel foil using photochemical stamping. For this purpose, sheets of steel foil 0.05-0.2 mm thick are coated with a layer of photosensitive emulsion. The contours of the parts are printed by contact on the emulsion layer through a negative. Then the emulsion layer is tanned and developed in warm water with the addition of 1% methyl violet until the outline of the part is detected in the untanned areas. Dissolution is carried out in the following solution:

The solution temperature should be 15-20 °C, the anodic current density should be 20 A/dm2, the distance between the electrodes should be 10 mm, the etching duration should be 15-20 minutes.

This method is also used for the manufacture of filter meshes made of X18N9T steel. Potassium dichromate 10 g/l and polyvinyl alcohol 70 g/l are used as a photosensitive emulsion. The mesh pattern printed by contact is processed in a solution of chromic acid. Insulation is carried out with perchlorovinyl varnish. Etching is carried out in orthophosphoric acid 600 g/l at a current density of 100 a/dm2 (based only on the area of the holes). The cathodes are plates made of steel X18N9T. Solution temperature 40 °C. Removal of the protective coating is carried out with a 10% solution of caustic soda at 60-70° C.

Photochemical stamping is increasingly used in the production of electronic parts, in the optical-mechanical and aviation industries. This method is very economical for the production of thin-walled metal foil parts with a thickness of 0.01 to 0.2 mm. Manufacturing accuracy - 0.01 mm with smooth edges, without burrs. The production of parts with complex shapes does not require skilled workers. There is no need for stamps either.

Chemical polishing. One of the most promising methods for finishing machine parts and devices is chemical polishing. The possibility of simultaneous processing of a large number of parts of complex shapes and any size, high productivity of the process, the uselessness of direct current sources and contacting devices and a number of other advantages force the development of research work in order to improve chemical polishing.

The best results have now been achieved in the field of polishing aluminum and its alloys, as well as chromium-nickel stainless steels.

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