DE10218634B4 - Water-permeable artificial stone - Google Patents

Abstract

A method of producing an artificial stone (1) having a water permeability of at least thirty liters per minute per square meter, comprising the following steps a) to e) or f) to j):
a) a mold (18) is prepared,
b) in the mold (18) is first introduced a humid fresh first layer (6) layer of a curable mixture and distributed by spreading within the mold (18) and compensated, wherein the first layer (6) consists of
ba) grains (2), which are obtained from broken natural stones and have a particle size of 1 to 10 mm, preferably 1.5 to 6 mm, in particular 2 to 5 mm, and
bb) 80 to 400 kg, preferably 150 to 300 kg, cement (4) per cubic meter of grain,
c) the first layer (6) is briefly compacted with a press (20) by applying a defined force within the mold (18),
d) on the still fresh first ...

Description

The The invention relates to a method for producing an artificial stone, preferably for a floor covering, here for example for outdoor areas, stables and wet areas, and one after the procedure manufactured artificial stone.

Known are relatively large pores, water-permeable Artificial stones from broken natural stones in the form of slabs, paving stones, Composite stones and the like. In these fragments are broken Natural stones connected with cement. On the relatively rough, So not smooth surface accumulating water can pass through interconnected pores flow through the artificial stone. The disadvantage is that the permeability of such artificial stones by in the pores penetrating small parts or dust and dirt Duration strongly decreases. After all can the water permeability even completely prevented by constipation. In addition, that can Pore volume does not always set so that a more even drain of water is achieved by the cross section of the artificial stone. Furthermore is walking with bare feet unpleasant on such a rough surface.

• a) Es wird eine Form vorbereitet,
• b) in die Form wird zunächst eine frische erste Lage einer aushärtbaren Mischung eingebracht und innerhalb der Form verteilt und ausgeglichen,
• c) es wird die erste Lage mit einer Presse durch Aufbringen einer definierten Kraft innerhalb der Form kurzzeitig verdichtet,
• d) auf die noch frische erste Lage wird eine frische zweite Lage einer aushärtbaren Mischung eingebracht und innerhalb der Form verteilt und ausgeglichen, und
• e) es werden die zweite Lage und indirekt auch erneut die erste Lage mit der Presse durch Aufbringen einer definierten Kraft innerhalb der Form verdichtet.

Out EP 1 125 703 A2 For example, a method for producing an artificial stone is known, which comprises the following steps.

• a) a mold is prepared
• b) a fresh first layer of a curable mixture is first introduced into the mold and distributed and balanced within the mold,
• c) the first layer is briefly compacted with a press by applying a defined force within the mold,
• d) on the still fresh first layer, a fresh second layer of a curable mixture is introduced and distributed within the mold and balanced, and
• e) the second layer and, indirectly, the first layer with the press are compacted by applying a defined force within the mold.

Out DE 91 00 380 U1 For example, a porous concrete paving stone is known from a cementitious mixture of 60 wt.% crushed natural stone or gravel with a grain size between 2-8 mm and 40 wt.% washed sand with a grain size up to 2 mm, in which the top of the paving stone is formed by a filter layer which consists of a cement-bound mixture of 92% by weight of double-sieved high-grade chippings with a particle size between 1 and 3 mm and 8% by weight of washed sand with a particle size of up to 2 mm.

Out DE 91 116 773 U1 is a water-permeable surface fastener known whose body of concrete mineral grains, which are bound to release continuous pores and with a body covering this, thinner facing layer, which also has mineral grains, which release between them through pores.

Out WO 94/25676 A1 is a Bodenflächenbefestigung known, which has a filtering effect for fluids flowing through. In the direction of flow from an inlet surface to an outlet bottom, the free flow cross sections become increasingly larger.

Out DE 40 36 445 C2 is a siltstone made of concrete material known in which the aggregates have a size distribution up to 8 mm and is permeable to water.

From the DE 39 09 169 A1 is a water-permeable natural-colored plaster element in the form of plates, paving stones, composite stones and the like, which can be used for paving sidewalks, walking paths, etc. It has a layer comprising a mixture of crushed natural stone particles and a hard urethane resin binder.

From the DE 196 05 990 C2 is a method for producing a floor covering, in particular Wegebelags, as well as a floor covering known, which is constructed in layers. The upper layer is made of a granular material, which is made of a broken natural stone and which is bound by polyurethane. This layer has through pores after curing.

In these known floor coverings, a high permeability to water in the upper layer is achieved. This is because the top layer has a large amount of pores that are continuous. As a result, even a strong supply of water is easily coped with, the surface of the upper layer remains moist after water supply, but it never collects water on the surface. This will prevent people from slipping. The surface also has a granular structure, so it is not smooth. A disadvantage is also in these prior art artificial stones, that the discharge of water over a long time can not be guaranteed. In particular, when laying the artificial stones outdoors it is unavoidable that dirt of all kinds, such as dust, sand, etc. acts on the artificial stone and clogs the pores of the artificial stones. In addition, by the use of binders based on hard or polyurethane, the pore volume can not be adjusted so that a uniform Ab flow of water through the cross section of the artificial stone. Another disadvantage of the use of hard or polyurethane is given by the relatively high cost of the binder material. Finally, it has also been found that use of such artificial stones in stables, for example, as a floor in horse stables is not advantageous because the urine of the animals strongly attacks the binder and can even lead to the destruction of artificial stones.

Of these, starting the invention has the task of the well-known Artificial stones and the process for their preparation to that effect to develop that improves a water flow through the artificial stones is, with a deposit of dirt in the artificial stone the flow as possible should affect little and also deposition of dirt controlled takes place and therefore deposited dirt can be eliminated easily can. Here, the artificial stone is resistant to environmental influences and economical be manufacturable and the surface of artificial stone is not coarse-grained.

These Task is by a method of producing an artificial stone solved with the features of claim 1.

Especially have the grains the first layer has a particle size of 1.5 to 10 mm, preferably 2 to 5 mm, and will be 80 to 300 kg of cement per cubic meter of grains used. Preferably, in compliance with all other instructions of claim 1, the grain size the grains the second layer in the range 4 to 9 mm.

Farther the task is solved by an artificial stone, following one of the above Method is made and the features of claim 10 having.

The first position is the upper layer of the installed stone in cross-section and the second layer is the lower layer in cross-section.

The superficial Layer, ie the first layer, has due to their production finer grains smaller pores than the underlying portion of the artificial stone and is possible thin, preferably about 1 cm thick. This ensures that dirt that the Pores of the superficial layer can not happen, collecting at the surface. This dirt can be easily removed, for example by cumshot. Finer dirt that passes through the pores of the superficial layer will not be held in the underlying layer, because there the pores are much bigger are, so a freer pass phrase is possible. The danger of that the artificial stone, especially that of the superficial Layer of artificial stone, added by dirt is low overall. The thickness of the first layer must be such chosen be that when compaction of the second layer no coarser grains through can press the first layer through to the later upper surface.

In addition will improves the flow behavior. Once in the superficial Layer of penetrated water is in any case drained rapidly, because the pores and also the pore volume of the underlying Shares of artificial stone allow rapid derivation. The lower, second Location is for example about 3 cm thick. This leaves the artificial stone largely dry in its lower part. A water jam will avoided. It also becomes the derivative into an underlying layer, for example, a specially prepared pad or even normal soil, simplified. In that regard, that has under the surface layer part of the artificial stone a high storage volume, the cheap at a traffic jam noticeable and a drainage of water in under the artificial stone Improved layers improved.

in the Contrary to the prior art must be artificial stone according to the invention clearly differentiated above and below. The artificial stone after the invention must always be installed so that the superficial layer is actually up is. Here it helps, if the artificial stone a corresponding marking Has. Cheap is also when the superficial Layer a different color has as the underlying share. That way you can the surface clearly distinguish it from the lower part of the artificial stone.

For the bond the grainy Good from broken natural stones has become more commercial Cement, for example, Portland cement proved favorable. Cement is extremely inexpensive and across from environmental influences like UV light resistant. Besides that is it is possible the artificial stone according to the invention to be used in stables or urinals, as cement to urine is resistant.

Essential is that washed the broken grains or cleaned by small parts and / or sand introduced into the mixtures because less binder is needed and also the pores not unnecessary be clogged. Fine components with a diameter of less than 1 mm should not be included in the mixture.

In the inventive method freshly worked in fresh. After the first layer has been compacted in the mold, then the following fresh second layer is placed on the still fresh first layer and also immediately verdich tet. This is favorable, since thereby the production time is kept short and on the other hand, the bonding of the two layers is more intimate.

The The order of introduction of the layers is basically arbitrary. The artificial stone is different from at least two layers granulation composed, it can also consist of three or more layers. It is always the grain chosen so that it rises from top to bottom. It will be the pores Increasingly bigger from top to bottom, never smaller.

starts now with the bottom or bottom layer, so it can when applying the next Location happen that their stones in the pores of the bottom layer penetration. If you work the other way around, you start at the bottom of the form with the superficial Location, so is this danger that the next position introduced the Fills pores or clogged, not given.

The Compaction of the two layers is as fast as possible, the press acts on the layers for a maximum of a period of three Seconds, for example about 2 seconds. For one, this is for a fast production conducive to others, however, too strong a telescoping of the grainy goods and thus prevents an excessive reduction of the pore volume.

each Type of press is conceivable, in particular have hydraulic and pneumatic presses proved to be advantageous. For artificial stones with square base has the pressing surface advantageously side edges of 30 cm in length.

The pressure acting on the layers and the amount of cement to be used are interdependent. For example, it has been found that at a pressure of 4,4 kg / m ² (4 tons press) about 250 kg / m ^{3 of} cement should be used to achieve an optimal result. At a pressure of 22 kg / m ² (20 t press) about 100 kg / m ^{3 of} cement should be used. Good results have been achieved when the press pressure for densifying the first, finer-grained layer is about 50% of the press pressure for densifying the second, coarse-grained layer.

Especially It is crucial that the grainy Good from broken natural stones has a minimum grain size. Grains under 1.5 mm should not be used. This ensures that the pores between the grains not by dusty Particles of the grainy Good impaired or can even be blocked.

It is therefore preferable that the minimum grain size is sufficiently high. Preferably, the granular Good the first layer a grain size of 1.5 to 10 mm and the grainy Good the second layer has a grain size of greater than 10 mm on. With these grain sizes is Ensure that sufficient pore volume is sufficient large pores. The measure of the minimum grain size is Kornband the superficial Layer lower than in the grain band of the underlying layers.

For the superficial Layer are particularly suitable platelet-shaped grains. When placing in the Form they prefer to lay flat, so they are parallel to surface of artificial stone. The pore geometry that sets in is for that Deriving dirt particles cheap.

By the use of broken natural stones, the artificial stone has a color, which lies in the range of the colors of the natural stones. Preferably, the Natural stone particles but also be stained, so that any Color designs are achievable.

The Artificial stones are suitable for any use. They are suitable for walkways, less polluted Trajectories but also for walking surfaces in swimming pools, on terraces and balconies. They can be shaped as flowerpots be, you can used in urinals to cigarette butts and the same keep. You can continue as soils in animal stables be used, for. B. in horse boxes. It is advantageous to arrange the plates on a supporting lattice, the high discharge capacity and permeability for water Has. Such animal stables can be cleaned easily, the animals are in the dry and urine expires.

she are suitable everywhere where, on the one hand, you want a quick drainage of water, on the other hand but want to avoid that dirt and the like with is derived. they seem as far as a filter that separates dirt and water from each other.

In an advantageous embodiment variant can the artificial stone have an internal reinforcement.

For the production of artificial stones, a device with press, as z. B. offered by the company Zenith under the name HB 860, proven. In this device, a pressure of, for example, 100 bar is established between the mold and a punch introduced into the mold, with which at least one layer or both layers are pressurized. There is also a compensation. During a very short time, for example one second, it can also be several seconds, now with a vibrating force short-stroke vibration, for example, 1 mm or less stroke applied. From below, the mold is vibrated with a force of a few tons, for example 10 to 25 tons, typically 14, 16 or 18 tons. From the top of the plunger is a corresponding vibration or shaking, but with a force of, for example, 2 to 10 tons, the force can be in the range 1 to 15 tons, it is for example 5, 6 or 8 tons. All this refers to an artificial stone with the dimension 50 by 50 cm and a thickness of a few cm.

critical is the high water permeability of artificial stone. The water permeability is typically 80 liters per minute and square meter, it can also be higher, z. B. 105. Even with pollution over time, for example Years, are registered in the artificial stone, although the water permeability is reduced, but still remains above, even after years especially 30 liters per minute and square meters, mostly above of 50 liters per minute and square meters. In addition, can be clean the stone, z. B. with water under high pressure. Due to the very high water permeability can the cleaning intervals are chosen to be large.

Further Advantages and features of the invention will become apparent from the following Description and dependent claims, not limiting to be understood. In this drawing show:

1 : a partially sectional view of an artificial stone according to the invention,

2 : a schematic diagram of the method according to the invention for producing the artificial stone,

3 : A schematic diagram of a lower main surface of the second layer with gutters.

Inventive artificial stones 1 consist of grains 2 made of natural stones. These grains derived from mineral material 2 are through cement 4 connected with each other.

A first location 6 in the installed state of the artificial stone 1 serves as an accessible surface has a grain size, which is in the range of 1.5 to 10 mm grain diameter. Typical grains are ground granite, basalt, porphyry, marble, etc. The proportion of cement 4 can be between 80 and 300 kg / m ^{3 of} granular material. cement 4 is suitable because it is relatively insensitive to weathering, in particular temperature-resistant and water-resistant and at the same time extremely cost-effective and easy to process.

The artificial stone 1 is at least two-layered. To the first location 6 closes a second location 8th at. The grain size of the grains of the second layer 8th always lies in a range above the grain size of the first layer 6 , Ideally, the grain size of the second layer 8th at least twice the grain size of the first layer 8th , The grain size of the first layer 6 For example, in the range of 1.5 to 5 mm and the grain size of the second layer can be correspondingly in the range of 4 to 11 mm.

Between the grains 2 and the cement 4 there are pores 10 that are so interconnected that the artificial stone 1 is liquid permeable over its cross section. Overall, the pore volume of the artificial stone should be 1 above 10% by volume, advantageously at 15% by volume.

The grains 2 usually show their natural color. However, it may also be appropriate to the artificial stones 1 or the individual layers 6 . 8th to color. This offers itself, for example, because it is intended to use the first layer as the top layer and a coloring thus the laying of artificial stones 1 facilitated. For staining, UV-resistant inks are preferably used. Natural colors such as Fe ₂ O _{3 have} proved to be favorable.

The artificial stones 1 have the shape of a plate. In the illustrated embodiment of 1 They have a square, flat surface, which also serves as the upper main surface 12 referred to as. They have circumferential narrow surfaces and a lower main surface 16 which forms the lower surface.

By the two-layeredness is achieved that between the grains 2 existing pores 10 not be clogged. It should be achieved that dirt on either the upper main surface 12 remains lying or the entire artificial stone 1 passes.

According to 2 the production of artificial stones takes place 1 in a form 18 , Sidewalls of the form 18 are slightly inclined inwards. The outer angle between a lower major surface and a surface of the side wall is slightly more than 90 degrees, for example 92 degrees. As a result, on the one hand, the demoulding of the mold 18 even from a mold cavity in which the shape 18 is made, simplified. On the other hand, it is ensured that the artificial stones 1 along with their shape 18 can be put together seamlessly, even if the substrate is slightly convex curved for laying. Two artificial stones next to each other can definitely enclose an angle of up to four degrees.

The shape has edge lengths of, for example, 30 cm, the depth is z. B. about 3 cm. It is either a lost form 18 and then stays with the artificial stone 1 connected, or is, as shown here, a form from which the artificial stone 1 taken as soon as it has set sufficiently.

In the form 18 For example, a mixer is first mixed, that is, made of cement 4 and grains 2 existing fresh first location 6 introduced and by brushing out within the mold 18 distributed. It is essential that the distribution or balancing is not done by shaking, because the free pores 10 between the grains 2 should be preserved.

Subsequently, the first layer 6 within the mold 18 with the help of a press 20 compacted. The compaction process should not exceed three seconds, because a longer period of time the permeability of the artificial stone 1 could negatively influence.

In a next process step is on the still fresh first layer 6 a likewise fresh, ready mixed second layer 8th introduced and spread by spreading within the mold. This is followed by a new compaction process, which should also not exceed three seconds. The directly on the second location 8th and indirectly to the first location 6 acting force of the press 20 exceeds that in the previous compression process of the first layer 6 acting force by about twice. This is due to the larger grains 2 the second location 8th and due to the desired co-compression of the first layer 6 advantageous or necessary.

Then the artificial stone cures 1 from and is optionally of the form 18 separated.

The weight of a plate of an artificial stone 1 with dimensions 50 × 50 cm and a thickness of 28 mm is about 10 kg. Already due to the relatively large weight of the artificial stones are safe on a surface.

The finished shaped stones can be cut, for example with a diamond saw. They leave treat and lay like normal flagstones.

The surface of the artificial stones 1 always stays grippy and does not get slippery. Water never stands on the upper main surface 12 , The up to the upper main surface 12 porous structure ensures a non-slip grip for shoes and feet.

The artificial stones 1 are suitable for terraces, balconies, walk-in roofs, garden paths. It is possible to put a ring around a tree with the help of artificial stones. It can be placed directly on the leveled ground, ie earth. Through the artificial stones through a sufficient water supply of the tree is ensured.

Form 18 , which essentially represents a frame, can be intimately connected with the artificial stone 1 stay connected. It gives the artificial stone 1 then a number of advantages: the corners become more robust, a crumbling or kinking at the corners during transport, handling, etc. is avoided. The artificial stones 1 have constant dimensions in terms of their length, width and height. Due to the shape, it is possible to connect the artificial stones form-fitting with each other. This form-fitting ensures the cohesion of a larger area of installed artificial stones. An exchange of a faulty or defective stone is easily possible. Due to the tailor-made production, there are no passport problems during the subsequent insertion of an artificial stone 1 , The frames stiffen the artificial stones 1 mechanically off. The production of artificial stone 1 is greatly simplified. There are no reusable forms needed that would need to be cleaned. Only a pad is necessary, but can be easily cleaned.

It but it is also possible the frame later to remove. All you have to do is make sure that he does not connects with the artificial stone.

3 shows a further embodiment of the artificial stone according to the invention 1 , In this case, the lower main surface 16 the second location 8th , so in the installed state of the artificial stone 1 on a support surface, gutters 22 for the derivation of the through the artificial stone 1 seeping water. These can, for example, during the pressing process in the second position 8th be pressed. The gutters 22 can, as in 3 shown, be arranged in a grid, but there are also other orientations conceivable. Good results have been achieved with gutters that are about 15-30 mm wide and 10 mm deep.

As an alternative to the method sequence described above, the second layer may also initially be used 8th into the mold 18 be introduced and with the help of the press 20 be compacted. Then it is on the still fresh second layer 8th the likewise fresh ready-mixed first layer 6 applied, there is a renewed, second compression process.

It is not necessary that during the second compression process, a force is applied which is greater than the force in the first compression process. Rather, it has been shown that in both Compaction processes the same force can be applied.

Decisive is a vibration, so shaking, compacting. For this purpose, a vibration force engages the mold and / or the punch of the press 20 at.

The specific weight of the finished artificial stone is in the range of 2000 to 2100 kg per square meter ² , typically 2050 kg per square meter ² , the pore volume is then about 13%.

critical is that no grain is present below the specified grain bands, because by Fine grain reducing the pores and thus reducing the water permeability entry.

The Invention is not limited to the illustrated and described embodiments limited, but includes all equivalent designs.

Claims (14)

Method for producing an artificial stone ( 1 ) having a water permeability which is at least thirty liters per minute per square meter, comprising the following steps a) to e) or f) to j): a) a mold ( 18 ), b) into the form ( 18 ), a fresh, moist first layer ( 6 ) Layer of a curable mixture and by spreading within the mold ( 18 ) and balanced, with the first situation ( 6 ) consists of ba) grains ( 2 ), which are obtained from broken natural stones and have a particle size of 1 to 10 mm, preferably 1.5 to 6 mm, in particular 2 to 5 mm, and bb) 80 to 400 kg, preferably 150 to 300 kg, cement ( 4 ) per cubic meter of grain, c) it becomes the first layer ( 6 ) with a press ( 20 ) by applying a defined force within the mold ( 18 ) condensed briefly, d) on the still fresh first layer ( 6 ) is a fresh moist second layer ( 8th ) of a curable mixture and by spreading within the mold ( 18 ) and balanced, with the second situation ( 8th ) consists of da) grains ( 2 ), which are obtained from broken natural stones and whose grain size is the grain size of the grains of the first layer ( 6 ), and db) 80 to 400 kg, preferably 150 to 300 kg of cement ( 4 ) per cubic meter of grain ( 2 ), e) the second layer ( 8th ) and, indirectly, the first situation ( 6 ) with the press ( 20 ) by applying a defined force within the mold ( 18 ) or f) a shape ( 18 ), g) into the form ( 18 ), a fresh damp second layer ( 8th ) of a curable mixture and by spreading within the mold ( 18 ) and balanced, with the second situation ( 8th ) consists of ga) grains ( 2 ), which are obtained from broken natural stones and whose grain size is the grain size of the grains of the first layer ( 6 ), and gb) 80 to 400 kg, preferably 150 to 300 kg, of cement ( 4 ) per cubic meter of grain, h) the second layer ( 8th ) with a press ( 20 ) by applying a defined force within the mold ( 18 ) briefly compressed, i) to the still fresh second layer ( 8th ) is a fresh damp first layer ( 6 ) of a curable mixture and by spreading within the mold ( 18 ) and balanced, with the first situation ( 6 ) consists of ia) grains ( 2 ) obtained from crushed natural stones and having a particle size of up to 10 mm, preferably 1.5 to 6 mm, in particular 2 to 5 mm and ib) 80 to 400 kg, preferably 150 to 300 kg, cement ( 4 ) per cubic meter of grain ( 2 ), and j) the first location ( 6 ) and, indirectly, the second situation ( 8th ) with the press ( 20 ) by applying a defined force within the mold ( 18 ) compacted. Method according to claim 1, characterized in that that the duration of the compression processes is a maximum of three seconds. A method according to claim 1 or claim 2, characterized in that the grain size of the grains ( 2 ) of the second layer ( 8th ) the grain size of the grains ( 2 ) of the first layer ( 6 ) exceeds by at least two times. Method according to one of claims 1 to 3, characterized in that by the press ( 20 ) to the location to be compacted ( 6 . 8th ) applied force is about two to 30 kg / m ² . Method according to one of claims 1 to 4, characterized in that the on the layer to be compacted ( 6 . 8th ) is 4.4 kg / m ² , when about 250 kg / m ³ cement ( 4 ) in the layer to be compacted ( 6 . 8th ) is included. Method according to one of claims 1 to 4, characterized in that the on the layer to be compacted ( 6 . 8th ) is 22 kg / m ² , when about 100 kg / m ³ cement ( 4 ) in the layer to be compacted ( 6 . 8th ) is included. Method according to one of claims 1 to 6, characterized in that at least one of the layers ( 6 . 8th ), in particular the first location ( 6 ) is colored. Method according to one of claims 1, characterized in that the directly on the second layer ( 8th ) acting force of the press ( 18 ) in process step e) which in the preceding process step c) to the first layer ( 6 ) exceeds acting force. Method according to one of Claims 1, characterized that the shape, in particular the bottom of the mold and / or the stamp, with which the compression force is applied, for a short time Vibrate time. Artificial stone produced by the process according to one of claims 1 to 9, which is a) plate-shaped, b) granules ( 2 ) obtained from broken natural stones and cemented 4 ), wherein about 80 to 400 kg, in particular 150 to 300 kg of cement ( 4 ) per cubic meter, c) from a first location ( 6 ) and a second layer ( 8th ), the grains ( 2 ) of the first layer have a particle size of 1 to 10 mm, preferably 1.5 to 6 mm, in particular 2 to 5 mm and the grain size of the grains ( 2 ) of the second layer ( 8th ) the grain size of the grains ( 2 ) of the first layer ( 6 ) and the d) interconnected pores ( 10 ) such that an outflow of water through the cross section of the artificial stone ( 1 ) is possible. Artificial stone according to claim 10, characterized that its pore volume is at most 30%, typically 15%. Artificial stone according to claim 10 or claim 11, characterized in that the lower main surface ( 16 ) of the second layer ( 8th ), ie in the installed state of the artificial stone ( 1 ) on a support surface, gutters ( 22 ) for the derivation of the artificial stone ( 1 ) has seeping water. Artificial stone according to claim 10, characterized that it has a water permeability, the at least thirty Liters per minute and square meter and preferably forty liters per minute and square meter, especially fifty liters per minute and Square meters. Artificial stone according to claim 10, characterized in that the thickness of the first layer ( 6 ) is significantly less than the thickness of the second layer ( 8th ), in particular that the thickness of the first layer ( 6 ) is selected as thin as possible.