FI79694C - Polycrystalline transparent sinter tubes - Google Patents

Description

1 79694

Polycrystalline, translucent sintering tubes Polycrystalline genomic sintered sinterrör

The invention relates to polycrystalline, translucent sintering tubes with very good surface properties.

Master application 843336 of this application discloses a process for the production of polycrystalline, translucent sintering tubes from alumina powder of a purity of less than 99.5% by weight, having a grain size of less than 1 .mu.m, optionally adding small amounts of magnesium oxide and zirconia, and mixing binders and lubricants and this starting material is drawn into moldings which are sintered by annealing at a temperature between 900 ° C and 1200 ° C and which are fired at a temperature between 1750 ° C and 1900 ° C in a hydrogen atmosphere or under vacuum. Together with a special traction device and certain softening methods, a considerable improvement in the surface properties of the pipes is achieved. This also results in excellent light transmission as well as very good mechanical strength.

Examination of previously known tubes has shown that there is a relationship between grain size, mechanical strength, and light transmission. A small grain size results in high strength and poor light transmission, and vice versa. This fact is also apparent from DE patent publication 3 108 677 or DE application publication 3 201 750, respectively.

In general, the applicant's experiments belonged to two different groups: a) crystal size <30 μm; mechanical strength> 300 N / mm2; total permeability <92%, or b) crystal size> 30 μm, mechanical strength <300 N / mm 2; total permeability> 90%.

Prior art methods have not been performed due to their high workload. Also, most of these methods previously described in 2 79694 cannot be repeated so accurately that reliable, absolute values of the light transmittance and mechanical strength of these products can be obtained.

Patent application DE-OS 29 18 729 discloses a process for the production of such polycrystalline light-transmitting alumina and its use in high-pressure discharge lamps, wherein the average grain size of the sintered body as alumina crystals is not less than 20 μm. In this case, starting from alumina powder with a grain size of less than 1 μm and a purity of more than 99.5% by weight, to which MgO, La 2 O 3 and Y 2 O 3 are optionally added as additives, the mass is mixed thoroughly and the starting material is pressed into a shaped body. , and in which the combustion is carried out in two steps, namely first at a temperature of 1300 to 1500 ° C and sintering at a temperature between 1650 ° C and 1900 ° C for up to 15 hours in a hydrogen atmosphere or under vacuum. In this case, between 1400 ° C and 1700 ° C, the heating rate must be at least 200 ° C / h surface roughness! to achieve an improvement, which roughness in the sintered product is 3 to 7 μm. The two combustion steps are, of course, costly, and given the long delay times, this method is also time consuming. In addition, during combustion, the growth of crystals is only difficult to control, so reproducibility is difficult. According to the table, the average grain size of a pipe tool is 45 ... 80 yum. Thus, the mechanical strength of the product is obviously lower than the strength of the present invention. The surface roughness of the body is about 3-7 μm and is thus relatively high. The form of extrusion of the tubes is not indicated in the description.

Another possibility for improving the in-line transmission of light is disclosed in U.S. Pat. No. 3,935,495, in which the sintered surface of alumina is chemically treated with a molten flux. At the same time, surface irregularities are reduced, but these operations in this method are complex and result in high production costs. In addition, this chemical etching lowers the strength of the sintering tube. The publication does not specify the grain size, composition or organic additives of the batch used in the manufacture of the pipes. If smelting is used in the manufacture of pipes, the surface roughness increases sharply.

In addition, other polycrystalline alumina raw materials are known which have improved the in-line transmission of light by using a special range of additives. In this case, mention should be made in particular of DE-PS 31 08 677, in which process the improvement in the quality of the sintering tube was achieved by adding 0.03 to 0.15% by weight of MgO and 0.002 to 0.07% by weight of ZrO 2, and HfC> 2 together. Overall, it was found that MgO is an essential component in alumina in ceramic products if the sintering process is to achieve a high degree of transparency. In particular, l

MgO controls the increase in granule size in the subsequent sintering steps. The addition of ZrO 2 or HfO 2 is intended to prevent excessive evaporation of the magnesium during the sintering step and at the same time to prevent the formation of Spinel in the final product. Only the experimental experiments G, H and possibly F produced permeability rates (in-line transmission) of more than 64%. Magnesium was used as the stearate additive.

The products thus obtained had an average grain size in the region of 20. . .50 yum.

Patent application DE-OS 28 10 128 discloses that an ordinary binder or lubricant is further mixed into the starting mixture to enable extrusion in the form of a sintered tube of a compressed, compact body. In addition, a pre-sintering step is then necessary to remove the organic constituents before sintering. Accordingly, the annealing combustion is carried out in an oxygen-containing atmosphere at temperatures of about 900 to 1200 ° C. According to the invention, the overall permeability of the pipes produced is at least 93%. The light transmittance <79694 can be further improved with the molten flux according to U.S. Pat. No. 3,935,495. The tubes can be made by a method known per se from U.S. Pat. No. 3,036,210. In this case, however, cleaner Al2O3 (purity of at least 99.9%) and less MgO (maximum 0.1%) should be used. The publication lacks a precise indication of which liquid binder suspension is used in the examples and an explanation of the die of the extrusion means.

Furthermore, on the basis of patent application DE-OS 32 01 750, a method for producing alumina-transmitting sintered bodies is known, in which special attention is paid to the traction device. In this process, zirconia, hafnium oxide, cerium oxide or magnesium oxide are preferably added to the powder as additives. Plasticizers corresponding to 12% by weight of paraffin and 1% by weight of stearin are then added to the powder. The density of the shaped pipe is 40 to 70% by weight, preferably 60% by weight of the theoretical density of corundum. When pulling the pipes, special attention is paid to the fact that the material flow in the pulling device is advantageous. The implementation of the shaping is described in great detail and is carried out in two steps, first the preparation of the preform by injection molding and then the final shaping by thermal extrusion. The individual tubes are then immersed in alumina powder to remove organic matter in the next step. The pre-firing is performed for one hour at 1100 ° C; the subsequent sintering is then carried out in a vacuum oven at 1800 ° C. This method has two disadvantages, firstly because the extra, expensive Al2O3 powder is used for immersion and because the adhering powder must then be removed again in the next step. In addition, during sintering, contact reaction of the tubes may occur due to horizontal layering. The tubes thus prepared have a light transmittance of 93% and a bending strength of 600 N / mm 2, but no measurement method for determining the bending strength was shown, so these values are not comparable with others.

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The density is 3.98 g / cm 3 and the average crystallite size is about 10, which in itself indicates poor in-line light transmission. The tubes obtained according to the examples have an average particle size in the range of about 10 to 15 μm. The mechanical strength is thus high. Thus, the formulation avoids the use of commonly used isostatic presses, but does not obtain an optimal sintering tube in terms of light transmission and strength; these are the main difficulties for the subsequent use of the tube as a high-pressure gas-discharge lamp.

In principle, the difficulty is the crystallite size of the sintered Al2O3 body. As the crystallite size increases, the light transmittance improves, but then the mechanical strength decreases. As the crystal size becomes smaller, both of these parameters have the opposite effect. The proportion of plasticizers, 13% paraffin and stearin, is also high, which also results in a considerable shrinkage, which in turn carries the risk that the sintered tube contains too many micropores. In addition, the straightness of the sintering tube may suffer if large amounts of plasticizer are not homogeneously mixed with the alumina powder. In principle, it has also been shown that, in addition to the use of a suitable alumina powder and similar additives, the sintering process is also important, but above all the design has a considerable effect on the sintered end product.

It is therefore an object of the invention to achieve further improved properties in terms of transparency, strength and surface roughness.

A polycrystalline, translucent sintering tube with low surface roughness and thus also better translucency, which also has improved mechanical strength, can be produced by adjusting the addition of additives and the sintering process, as well as by selecting lubricants and binders for the raw material to achieve the best possible homogeneity.

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Further optimization can be performed by the manufacturing method and the structure and properties of the traction device.

The invention relates to polycrystalline, translucent alumina sintering tubes as defined in the claims, which are characterized in that the sintered moldings of alumina have an average crystallite size of 20 to 40 μm; their surface roughness is in the range of 0.1 to 1 μm, preferably about 0.2 μm; They have a flexural tensile strength of 320 N / mm or more, measured by the three-point method at a distance of 40 mm from the measuring points; their in-line light transmission is greater than 64%, measured with a light beam of 0.94 μm directed at the concave surface of a 0.6 mm thick sample, with an angle of impact of 6 ° and an outgoing light measured at an exit angle of 60 °; and their total permeability is 93 ... 96%.

Such sintering tubes can be made from more than 99.5% by weight of alumina powder with a grain size of less than 1 .mu.m, to which small amounts of magnesium oxide and zirconia can be added, whereby binders and lubricants are mixed into the oxides, this starting material is drawn into molds, moldings into molds. By annealing at a temperature between 1750 ° C and combustion at a temperature between 1750 ° C and 1900 ° C in a hydrogen atmosphere or under vacuum. The tubes are prepared by mixing a water-soluble PVA adhesive or a water-soluble, low-viscosity, low-swelling methylcellulose adhesive, grease or oil, glycerin, a wetting agent, and forming a plasticizer mixture. This mixture is combined with Al 2 O 3 powder and magnesium oxide and possibly zirconium dioxide, the mixture thus obtained is homogenized and cold-formed by means of a piston press using an extrusion device having a surface roughness Ra of less than 0.15 μm and consisting of a housing and a drawing core. is 50 to 70% by weight of the theoretical density of the alumina and its surface is glossy.

li 7 79694 In the present invention, the improved properties of the polycrystalline, translucent sintering tube depend in particular on the use of binders and lubricants. To obtain the preferred result, alumina is started from a powder having a specific surface area of about 14 m 2 / g. Further optimization is achieved by dispersing in water by adding 0.75 to 2.0% by weight of a liquefying agent, such as, for example, sodium-free ammonia, in a plate grinder whose grinder plates consist of 99.7% alumina ceramics.

Thus · no impurities are introduced into the mixture due to wear, which is possible when grinding in a conventional drum grinder. This is followed by screening the suspension to remove lumps that later cause white spots in the sintered ceramic product.

It has proved particularly advantageous to add a mixture of 0.1 to 0.5% by weight of glycerin and 0.3 to 2.5% by weight of oil in a ratio of 1: 3 to 1: 5 to the dispersed vitrification mass. after screening, followed by foaming. The drying of the glazing mass is carried out at temperatures between 80 and 100 ° C, after which the residual moisture is about 3%. After these operations, a very soft and loose, non-sedimented powder is obtained, which is very well distributed in the subsequent mixing steps. Thus, this powder already contains the ingredients of a water-soluble, pasty plasticizer mixture to be used later, so that the optimum affinity of the fat mixture is later obtained in the powder and thus a very high homogeneity of the plasticizable mass is achieved. Particular care must be taken to ensure that the temperature does not rise above 100 ° C during drying to prevent charring or hardening of the organic constituents. Disintegration of the agglomerate in a disc mill may be omitted if the alumina powder is loose and has a small lump size. This technological step is followed by softening or possible blending with additives.

Essentially, the individual plasticizer ingredients are mixed into a paste before being added to the powder mixture. Al 2 O 3 powder 8 79694 is preferably mixed with 11.2 to 19.5% by weight of a plasticizer mixture consisting of 3.0 to 4.5% by weight of PVA or a low-viscosity, poorly swellable methylcellulose adhesive. 5 to 8.5% by weight of oil, 2.0 to 4.5% by weight of fats, 0.5 to 1.5% by weight of glycerin and 0.2 to 1.0% by weight of wetting agent. In this case, PVA and poorly swellable methylcellulose adhesives are water-soluble. They thus allow good wetting and the breaking up of lumps. The chemical basis of PVA adhesives is polymeric hydroxyl compounds, and the viscosity of a 5% solution is 100 cp. Fats contain short-chain fatty acids and the oil is a mixture of naphthenic, paraffinic and aromatic hydrocarbons with natural fat derivatives and polar additives. This pasty plasticizer mixture is well miscible with dry alumina powder. Good miscibility is the basis for the very smooth surface of the sintering tube to be obtained later.

If these measures are not carried out, and if the binder is added without first reacting it with the other components, the formation of lumps can hardly be avoided, since it is not possible to cover all the alumina particles with an adhesive film with the finest possible additions or intensive mixing of adhesives. Rather, whole pieces of powder glue into large lumps. Attention must also be paid to the order in which the individual plasticizers are added. Namely, if the oil were first added, then the Al 2 O 3 powder would immediately absorb the oil due to its high specific surface area, and when the glue was added, sufficient wetting of the powder would not occur.

This is followed by the addition of additives as magnesium oxide and possibly zirconia. It has proved particularly expedient to add magnesium oxide in an amount in the range from 0.03 to 0.08% by weight, preferably 0.05% by weight, and zirconia in an amount in the range from 0.002 to 0.05% by weight, preferably 0.01% by weight, these oxide g 79694 being in the pre-stage as chloride, carbonate and acetate. After mixing, the gases are sucked out of the pulp in the press cylinder and pre-lined. The piston press is then shaped, with a compression pressure of 200 to 500 kg / cm 2 and using a nozzle according to the invention.

An essential condition for the design of the traction device is that the friction between the mass and the traction device be kept to a minimum; thus, the raw material and the surface properties of the traction device are crucial. Equipment made of hard metals has the advantage that no large abrasion occurs when in contact with the abrasive Al 2 O 3 powder. If abrasion occurs, as with standard steel tools, there will be various sliding properties on the inner surface of the traction device, resulting in uneven compaction of the rod. In addition, a rough surface is created on the rod at the rubbing point, whereby optimal surface properties are not achieved for the drawn rod.

A particularly preferred device for drawing a sintering tube is characterized in that the extrusion means is a polished carbide extrusion device with a surface roughness Ra of less than 0.15 μm, consisting of a housing and a pull core connected to the housing by six successively arranged three-star prongs and twisted relative to each other 60 ° from the overlapping position so that a hexagon forms in the axial projection of the draw core.

This provides additional smoothness to the surface of the protruding rod if the softened alumina powder slides along the wall of the traction device. This results in an optimal surface gloss. On the other hand, the structure of the traction device is crucial. Even then, attention must be paid to the effect of friction.

• · The traction device must be designed to be particularly advantageous for the flow, ie the pressure generated must be applied to the smallest possible surface and distributed to the forward-tapering traction device. For this reason, the conical surfaces are kept as small as possible, since the friction on the cylindrical surfaces is substantially lower. In addition, the structure must be symmetrical so that the towed mass is not subjected to an uneven pressure distribution. For this reason, the prongs of the three-star intended to hold the traction core are positioned so as to form a hexagon star. Although the forced rod is split once more, it is then re-welded in the last part of the draw nozzle. Likewise, the mouthpiece has a long outlet period, so that any differences in sealing disappear from the mouthpiece during outlet. In addition, due to the good compaction and sliding properties of the mouldable mass, the leaf-shaped structures of the alumina crystals orient in the same way in extrusion. This is facilitated in particular by a specially designed traction device made of polished carbide.

The tubes are placed on a perforated aluminum sheet to achieve even drying. The drying itself is performed in dust-free chambers at low overpressure, at ambient temperature. The annealing takes place in an oxidizing atmosphere in an electric oven at a temperature between 900 and 1200 ° C with a delay time of 12 hours. This measure is justified because all organic constituents can be removed from the sintered material before sintering in a vacuum or humid atmosphere. Under no circumstances should the temperature rise above 1250 ° C to prevent pre-sintering of the material. From this, it can also be concluded that the heating rate is essential when sintering the shaped bodies, since the grain size can thus be adjusted once again. It has proved expedient that the optimal sintering product is obtained at a heating rate of at least 800 ° C / h, preferably between 800 and 100 ° C, in the range 1200 i ... 1900 ° C. Sintering under high vacuum also takes place at temperatures up to 1900 ° C.

Sintered alumina sintering tubes have the following properties: in-line light transmission of better than 64% and total light transmittance of between 93 and 96, crystal 79694 has a connector size of 20 to 40 μm, a sintering density of 3.98 / cm 3 and a surface roughness of about 0.1 ... 1 μm, preferably 0.2 μm. The strength exceeds at least 320 N / mm2.

Pipes in the raw space do not have to be treated in any other way, as is the case, for example, in isostatic pressing, where rolling is carried out to a thinner stage, whereby considerable mass waste is generated. Post-treatment of sintered pipes is also unnecessary due to the excellent properties of the plasticizer paste as well as the good compaction and sliding properties of the pulp during extrusion. When polishing with flux, there is a risk that the structure will be damaged by long etching times. In addition, sintering requires very little energy and time, as the firing cycle only lasts 8 to 10 hours. Surprisingly, it has also been found that these measures have a very advantageous effect on the surface roughness, mechanical strength and optical properties of the sintered Al 2 O 3 shaped body. The smaller the average crystallite size, the better values can be observed for mechanical strength, inverting for optical properties.

In addition, the very advantageous design for the extrusion nozzle flow has the advantage that the very smooth surface of the pulp is not damaged during the extrusion process. In addition, the risk of the pipes coming out of the traction device is very small, because the frictional resistance is kept to a minimum. In this way, a very good match of the dimensions of the sintered products is also achieved.

Other details are explained in the exemplary embodiments, in particular in Figures IA and B, the traction device is shown.

As shown in Table 1, the effect of alumina powder dispersion and foaming, as well as pre-reaction between adhesive and oil, as well as doping on surface roughness was investigated in several experiments.

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table 1

Experiment Sintering Middle-Middle Light Refraction No. conditions Vol. def. in-line fracture (temperature / crystal roughness transmission strength delay time) connector assembly (jum) (jum) (%) (N / nun2) VI 1830/2 32 0.20 69 380 V2 1830/2 37 0, 50 62 320 V3 1830/2 23 0.90 51 280 V4 1830/2 40 0.25 65 265 V5 1850/1 26 1.9 59 310

The average crystallite size, average surface roughness, in-line transmission of light, and refractive index were measured from type 70-Watt sintering tubes such as those used in high-pressure, sodium vapor discharge tubes. The average crystallite size was obtained by examining the sintered surface with an electron beam microscope. Surface roughness was measured with a Hommel tester so that the tube was touched axially for a length of 5 mm. In the measurement of the in-line transmission of light, a light beam with a wavelength of 0.94 μm was sent to the concave surface at an impact angle of 6 °. The light intensity was measured with a measuring probe at an exit angle of 60 °.

The wall thickness of the sintered bodies was 0.60 mm + 0.3. The measurement of the breaking strength was performed according to the 3-point method. The distance between the measuring points was 40 mm.

Example 1

Experiments 1 (VI) and 2 (V2) show the effect of alumina powder dispersion and subsequent foaming on the properties of the sintered product.

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In Experiment 1 (Vl), 4000 g of Al2O3 powder with a purity of more than 99.5%, a grain size of less than 1 μπι and a specific surface area of about 14 m ^ / g are placed in a plate mill already prepared with 10 kg of distilled water, containing is 1% liquid, such as sodium-free ammonia. This suspension is then ground for 20 minutes and sieved through a plastic sieve with a sieve mesh size of 10 μm. This glazing mass is then mixed with 0.2% by weight of glycerin and 0.8% by weight of oil with a screw mixer to obtain a creamy rigid glazing mass with a large number of air bubbles. Drying on dampers is carried out at 80 ° C until the residual moisture is 3%. the individual plasticizer components are mixed separately into a paste, the paste itself comprising 3.75% by weight of a water-soluble PVA binder dissolved in water in a ratio of 1: 4, 7% by weight of oil and 3% by weight of fats and 0.3% by weight of wetting agent, and 1% by weight of glycerin. This paste is then added to the dried Al 2 O 3 foam while mixing 0.265% by weight of magnesium acetate and 0.07% by weight of zirconium chloride. The total mixing time is about 1.5 hours, in which case distilled water is continuously added up to a total of 7% by weight to obtain a dough-like, mouldable composition. The pulp was then aspirated from the pulp in a piston press and pre-compacted and compressed into rods with a diameter of 3 mm by means of a multi-hole plate, which rods were in turn placed in a press cylinder, the gases were aspirated and pre-compacted.

Figures IA and B show a traction device according to the invention. It consists of an outer housing 2 in which a double three-star 3 is mounted as shown in Fig. 1B. In this case, the dimensions of the press device in nominal dimensions are $ 6.1 x 4.35 mm. The mandrel 4 is held above the claws 5.

The compression pressure is greater than 200 kg / cm 2, and the pressed tubes are cut to a length of 800 mm and placed on a routed press damper. After drying, these bars are cut into separate tubes, while removing the cutting dust, 14 79694, after which these raw pieces are annealed for two hours at 1100 ° C in an oxidizing atmosphere as a stack. In this combustion, magnesium acetate and zirconium chloride are converted to magnesium oxide and zirconia. Other components / which may affect the optical properties are removed at a sintering temperature of 1830 ° C, which is maintained for 2 hours, and at a heating rate of 800 ° c / h, the raw materials reach a translucent appearance in a humid hydrogen atmosphere during sintering.

In Experiment 2 (V2), the same charge as in Experiment 1 is used, but the alumina powder is added to the mixer without dispersing and foaming.

As can be seen from the results in Table 1, dispersing and foaming the alumina powder provides an improvement in surface properties. When dispersed in a disc mill, the agglomerate is divided into individual crystals without impurities and foaming prevents the individual crystals from clumping together, so that a very easily distributed powder is obtained in the mixer, into which the plasticizer components can be easily mixed. If dispersion and foaming are not carried out, the agglomerate will not be distributed and the surface values will deteriorate due to the unevenness of the grain size. In addition, the molding mass is inhomogeneous, which is seen as white spots in the sintered tube. White spots lead to reduced light transmission as well as deterioration of mechanical properties.

Example 2

Experiment 3 (3V) takes a closer look at the pre-reaction between adhesive and oil in relation to surface roughness. Again, 4000 g of Al 2 O 3 powder having the same known properties as in Experiment 1 are dispersed and dried, then 0.3% by weight of wetting agent and 3.75% by weight of PVA binder are added to the mixer, and at intervals of 5 minutes each. The following components are mixed one by one: 7% by weight of oil, 3% by weight of fat, 1% by weight of glycerin and about 7% by weight of distilled water. Additives of 0.265% by weight of magnesium acetate and 0.07% by weight of zirconium chloride are also added. The other process steps are performed as in Experiment 1.

Table 1 shows a clear deterioration in surface roughness in Experiment 3 (V3). If the plasticizer components are added in this order, there will be inhomogeneity in the moldable mass, since the binder is not evenly distributed in the fine Al 2 O 3 powder. In this case, the moldable mass can no longer be compacted evenly, whereby the extrusion properties are also deteriorated due to the uneven sliding effect. This unevenness is seen as white spots in the sintered tubes.

Example 3

In Experiment 4 (V4), surface values are evaluated for alloying, with sintering tubes being prepared analogously to the method of Experiment 1, except that the alloying is performed with only 0.265% by weight of magnesium acetate and that the sintering is performed in vacuo.

As can be seen from Table 1, no effect on surface roughness could be observed, but a slightly uneven growth was seen in the crystallite structure, which, however, did not affect the in-line transmission of light. Thus, the alloying method does not appear to have an effect on the surface roughness of the sintering tube.

Once again, in order to highlight the advantages of the extrusion method according to the invention compared to the conventional isostatic pressing method, Experiment 5 (V5) was included in the table. In this experiment, as in Experiment 1, 4000 g of Al 2 O 3 powder was similarly ground, and this vitrification mass was spray-dried. The spray-dried granulate was then isostatically compressed at 1400 bar into tubular shaped bodies. The outer jacket surface of the tube compressed into one 16,7964 mandrel is then rolled thinner.

The disadvantages of this method are obvious when looking at the surface roughness. Despite careful finishing with abrasive discs, surface quality is not achieved. Turning marks are created which cause light to scatter on the surface and thus impair the light transmission. In addition, in isostatic compression, due to the pressure acting in each direction, the arrangement of the leaf-shaped crystallites parallel to the surface of the tube does not occur, leaving the surface rougher.

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Claims (1)

17 79694 Claim Polycrystalline, translucent alumina sintering tubes, characterized in that the sintered shaped bodies of alumina have an average crystallite size of 20 to 40 microns; their surface roughness is in the range of 0.1 to 1 μm, preferably about 0.2 μm; they have a flexural tensile strength of 320 N / mm2 or more measured by the three-point method at a distance of 40 mm from the measuring points; their in-line light transmission is greater than 64%, measured with a beam of 0.94 μm directed at the concave surface of a 0.6 mm thick sample, with an angle of impact of 6 ° and an outgoing light measured at an exit angle of 60 °; and their total permeability is 93 .. .96%. In the form of a polycrystalline aluminum oxide flange, in the form of sinks, the sintered form of the alumina is crystalline at a temperature of 20 to 40 [mu] m, a maximum of 0.1 to 1 [mu] m is obtained, with a yield of about 0.2 [mu] m; a total strength of up to 320 N / mm2 of the three-point method with a maximum span of 40 mm; the in-line transmission is not more than 64% of the time required to reach the level of 0.6 mm / s, the shear is not more than 0,94 / im, the colors are not more than 6 ° and the maximum distance is not exceeded. the surface is under 60 °; and total transmission is 93 .. .96%.