What is die casting?

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What is die casting?

Die casting is a metal casting process characterized by the application of high pressure to the molten metal through the die cavity. Molds are usually made of stronger alloys, a process somewhat similar to injection molding. Most die-casting castings are iron free, such as zinc, copper, aluminum, magnesium, lead, tin and lead tin alloys and their alloys. According to the type of die casting, cold chamber die casting machine or hot chamber die casting machine is needed.
The cost of foundry equipment and molds is high, so die-casting techniques are generally used only for mass production. It is relatively easy to make die-casting parts, which typically require only four major steps, with a low cost increment. Die casting is particularly suitable for the manufacture of a large number of small and medium-sized casting, so die casting is the most widely used of various casting processes. Compared with other casting techniques, die-casting has a flatter surface and a higher dimensional consistency.
Based on the traditional die casting process, several improved die casting processes were developed, including the non - hole die casting process to reduce casting defects and eliminate pores. It is mainly used to process zinc, which can reduce waste and increase yield. Also invented by general dynamics precision pressure casting technology and semi - solid die - casting and other new die - casting process.

Die casting is a kind of precision casting method to force molten metal into complicated metal mold by high pressure. In 1964, the Japan die casting association defined die casting as "pressing molten alloy into precision casting molds at high temperature to produce high-precision casting with excellent surface quality in a short period of time". Die Casting is called ”Die Casting” in the United States, “Pressure Die Casting” in the United Kingdom, and most of the domestic industry is familiar with the Japanese version, called Die Casting. 
The tensile strength of these materials is nearly twice as high as that of ordinary casting alloy, which is of more positive significance for the parts produced by aluminum alloy automobile wheels, frames and other parts that hope to use higher strength and impact resistant materials.

In 1838, die - casting equipment was invented to make moulds for movable type printing. The first patent related to die casting was issued in 1849. It was a small manual machine used to produce the type of printing press. When Otto Mergenthaler invented the Linotype typesetter in 1885, which could press a line of text into a single type, it revolutionized the printing world like never before. After the printing industry entered the large-scale industrialization, the traditional manual pressure has been replaced by die-casting. Around 1900, the introduction of typesetting into the market further improved the printing industry's automation technology, so sometimes more than a dozen die-casting machines could be seen in the newspaper office. As consumer products grew, Otto's invention gained more and more applications. Die-casting can be used to mass produce parts. In 1966, general dynamics developed the precision pressure die-casting process, sometimes called double punch die-casting

Die-casting process 
Traditional die casting process mainly consists of four steps, or high pressure die casting. These four steps include mold preparation, filling, injection and sand removal, which form the basis of various improved die casting processes. A lubricant is sprayed into the mold cavity during preparation. The lubricant helps control the mold temperature as well as the casting unmold. The mold can then be closed and the molten metal injected at high pressure, with a pressure range of about 10 to 175 mpa. When the molten metal is filled, the pressure remains until the casting solidifies. The push rod then rolls out all the castings, and since there may be multiple cavities within a mold, multiple castings may be produced during each casting process. The process of sand removal requires the separation of debris, including mold opening, runner, gate and flying edge. This process is usually done by extrusion of the casting by a special finishing die. Other sanding methods include sawing and grinding. If the gate is relatively fragile, you can directly break the casting, which can save manpower. The excess mold opening can be reused after melting. The usual production is about 67 percent.
High-pressure injection causes the filling die to be filled very quickly, so that the molten metal can fill the entire die before any part solidifies. In this way, even thin walls that are hard to fill can avoid surface discontinuity. But it can also trap air, which is hard to escape when the mold is quickly filled. This problem can be reduced by placing the vent on the parting line, but even very sophisticated processes can leave a hole in the center of the casting. Most die casting can be done through secondary processing to complete some structures that cannot be done through casting, such as drilling, polishing.
Defects can be checked after the sand has been removed. The most common defects include stagnant flow (underwatering) and cold scars. These defects may be caused by insufficient temperature of the mold or molten metal, impurity in the metal, too few air vents, and too much lubricant. Other defects include pores, shrinkage holes, thermal cracks and flow marks. Flow marks are marks left on the surface of a casting by a gate defect, sharp corners, or excessive lubricant.
Water-based lubricants, called emulsions, are the most commonly used type of lubricant for health, environmental and safety reasons. Unlike solvent-based lubricants, if the minerals in the water are removed by proper processes, it will not leave a by-product in the casting. If the process of treating water is not proper, the minerals in the water may cause surface defects and discontinuity of the casting. There are mainly four water-based lubricants: water blending, oil blending, semi-synthesis and synthesis. Water-oil lubricants are best because when using lubricant, water deposits oil and cools the surface of the mold through evaporation, which helps to unmold. Typically, this type of lubricant consists of 30 parts of water and 1 part of oil. In extreme cases, the ratio can be as high as 100:1.
Oils that can be used as lubricants include heavy oils, animal fats, vegetable fats, and synthetic oils. The heavy residual oil is viscous at room temperature and becomes a thin film at high temperatures during die casting. The viscosity and thermal properties of the emulsion can be controlled by adding other substances into the lubricant. These materials include graphite, aluminum and mica. Other chemical additives can prevent dust and oxidation. Emulsifiers can be added to water-based lubricants so that oil based lubricants can be added to water, including soap, alcohol and ethylene oxide.
Solvent-based lubricants have long been used, including diesel and gasoline. They facilitate the casting to come out, but small explosions occur during each die-casting process, leading to the accumulation of carbon in the cavity wall. Solvent based lubricants are more uniform than water-based lubricants.

Die-casting machines can be divided into two different types: hot chamber die-casting machines and cold chamber die-casting machines. The difference lies in their strength. The typical pressure range is between 400 and 4000 tons.
Hot chamber die-casting
Hot chamber die - casting, sometimes known as goose neck die - casting, is a pool of molten liquid and semi-liquid metals that fill the mold under pressure. At the start of the cycle, the machine's pistons are contracting, and molten metal fills the neck of the goose. The pressure or hydraulic piston squeezes the metal into the mold. The system's advantages include quick circulation (about 15 cycles per minute), easy automation, and the ability to melt metal. Disadvantages include the inability to die-cast metal with a high melting point, as well as the inability to die-cast aluminum, which takes iron out of the molten pool. Thus, generally speaking, hot chamber die-casting machines are used for zinc, tin, and lead alloys. Furthermore, hot chamber die casting is difficult to use for large casting, which is usually a small casting.
Cold chamber die casting
Cold chamber die-casting, including aluminum, magnesium, copper, and zinc alloys with higher aluminum content, can be used when die-casting is not available for hot chamber die-casting. In this process, the metal is first melted out of a separate crucible. A certain amount of molten metal is then transferred to an unheated injection chamber or injection nozzle. Through hydraulic or mechanical pressure, the metal is injected into the mold. The biggest drawback of this process is the long cycle time due to the need to transfer molten metal into the cold chamber. Cold room die-casting machine is divided into vertical and horizontal, vertical die-casting machine is usually a small machine, while horizontal die-casting machine has a variety of models.

Die casting mold
Die - casting molds consist of two parts, the covering part and the moving part respectively. In hot chamber die - casting, the cover part has gate, while in cold chamber die - casting is injection port. Molten metal can enter the mold from here, and the shape of the site matches the shape of the injection nozzle in hot chamber die casting or the injection chamber in cold chamber die casting. The moving part usually includes a push rod and a runner. The runner is the channel between the gate and the cavity through which molten metal enters the cavity. The covering part is usually connected to a fixed platen or front platen, while the movable part is connected to a movable platen. The cavity is divided into two cavity insert blocks, which are independent parts that can be removed or installed relatively easily through bolts.
The mold is specially designed and the casting will remain in the active part when the mold is opened. In this way, the movable part of the putter will push out the casting. The putter is usually driven by the platen. It will accurately drive all the putter at the same time with the same force, so as to guarantee that the casting will not be damaged. When the casting is pushed out, the pressure plate shrinks to pull back all the push rods for the next die-casting .Since the casting is still at high temperature when it is unmoulded, only the number of push rods is enough to ensure the average pressure on each push rod is small enough to avoid damage to the casting. However, the putter will still leave a mark, so careful design must be made so that the position of the putter does not affect the operation of the casting too much.
Other parts in the mold include core slide and so on. A core is a part that opens or opens on a casting and can be used to add detail to the casting. There are three types of core: fixed, moving and loose. The direction of the stationary cores is parallel to the direction of the casting leaving the mold, and they are either fixed or permanently attached to the mold. Movable cores can be arranged in any direction other than the direction of release. The removable cores must be removed from the mold cavity by means of a separation device before the casting can be opened after solidification. The slider is very close to the movable core. The biggest difference is that the slider can be used to make the concave surface. The use of cores and sliders in die - casting increases costs significantly. Loose cores, also known as extractors, can be used to make complex surfaces, such as threaded holes. Before each cycle begins, the slider needs to be manually installed before it is finally pushed out with the casting. Then remove the loose core. Loose cores are the most expensive cores because it takes a lot of work to make and it increases cycle time.
The outlets are usually thin and long (about 0.13mm), so molten metal can quickly cool to reduce waste. There is no need to use risers in die casting because the molten metal pressure is so high that it is guaranteed to flow from the sprue into the mold.
Due to the temperature, the most important material characteristics of the mold are thermal shock resistance and softness. Other features include hardenability, cutting resistance, thermal cracking resistance, weldability, availability (especially for large moulds) and cost. Die life depends directly on the temperature of the molten metal and the time of each cycle. Die molds used for die casting are usually made from hard tool steel, and because cast iron cannot withstand high internal pressure, the mold is expensive, which also results in high costs to open. Metal die-cast at higher temperatures requires the use of harder alloy steel.
Major defects in die casting include wear and erosion. Other defects include thermal cracking and thermal fatigue. Hot cracking occurs when defects occur on the surface of the mold due to large temperature changes. After using too many times, the defects on the surface of the mold will cause thermal fatigue

Die-casting metal 
The minimum cross-sectional area and the minimum die-pulling Angle corresponding to various materials shall be less than 13mm as listed in the table below .
Metal               Minimum cross-sectional area         Minimum drawing Angle
Aluminum alloy       0.89 mm (0.035 in)                  1:100 (0.6 °)
Brass and bronze     1.27 mm (0.050 in)                  8 0 (0.7 °)
Magnesium alloy      1.27 mm (0.050 in)                  1:100 (0.6 °)
Zinc alloy             0.63 mm (0.025 in)                 1:0 (0.3 °)
The main metals used in die casting include zinc, copper, aluminum, magnesium, lead, tin and lead tin alloys. Some of the more unusual die-cast metals include ZAMAK, aluminum and zinc alloys, and Alcoa standards: AA380, AA384, AA386, AA390, and AZ91D magnesium. Die casting of various metals is characterized as follows:
Zinc: the easiest metal to die casting, very economical when making small parts, easy to coating, high compressive strength, high plasticity, long casting life.
Aluminum: lightweight, complex and thin-walled castings with high dimensional stability, corrosion resistance, good mechanical properties, high thermal and electrical conductivity, and high strength at high temperatures.
Magnesium: easy to machine, high strength and weight ratio, usually die - casting metal is the lightest.
Copper: high hardness, strong corrosion resistance, mechanical properties of commonly used die - casting metals, wear resistance, strength close to steel.
Lead and tin: high density, high dimensional accuracy, can be used as special anti-corrosion parts. For public health reasons, this alloy cannot be used as food processing and storage equipment. An alloy (and sometimes a bit of copper) of lead, tin, and antimony can be used to make hand-printed type and gilding in letterpress printing.
The maximum mass limit for die casting using aluminum, copper, magnesium and zinc is 70 pounds (32 kilograms), 10 pounds (4.5 kilograms), 44 pounds (20 kilograms) and 75 pounds (34 kilograms), respectively.

Advantage and disadvantage
The advantages of die casting include excellent dimensional accuracy. Typically this depends on the casting material, typically with an error of 0.1 mm for an initial 2.5 cm size, with an increase of 0.002 mm for each additional cm. Compared with other casting processes, its castings have a smooth surface and a circular radius of about 1-2.5 microns. Casting with a wall thickness of about 0.75 mm can be made relative to sandbox or permanent mold casting. It can directly cast internal structures, such as wire sleeves, heating elements, high strength bearing surface. Other advantages include the ability to reduce or avoid secondary machining, the speed of production, the tensile strength of the castings up to 415 mpa, and the ability to cast highly mobile metals.
The biggest drawback of die - casting is its high cost. Casting equipment and molds and mold-related components are more expensive than other casting methods. Therefore, it is relatively economical to produce a large number of products when manufacturing die castings. Other drawbacks include that the process is only suitable for highly mobile metals and that the casting quality must be between 30g and 10kg. In normal die - casting, the last batch of castings always have pores. Therefore, any heat treatment or welding cannot be carried out, because the gas in the gap will expand under the action of heat, resulting in internal micro defects and surface peeling.
Pressure casting, or die casting for short, is a type of alloy solution that is poured into a pressure chamber to fill the steel mold cavity at high speed and causes the alloy solution to solidify under pressure
Casting method of solid casting. The main features of die casting are high pressure and high speed.
The metallic liquid is filled with the cavity under pressure and crystallized under higher pressure. The common pressure is 15-100mpa.
Demerit fluids fill the cavity at high speed, usually between 10 and 50 m/s, some more than 80 m/s, (linear velocity through the gate into the cavity -- gate speed), so the filling time of the metal fluid is very short, about 0.01 -- 0.2 seconds (depending on the size of the casting) to fill the cavity.
Die casting is a precise casting method. Die castings made by die casting have small dimensional tolerances and high surface accuracy. In most cases, die castings can be assembled without turning, and threaded parts can be cast directly. Die-casting is used to make most of the small parts from ordinary cameras, typewriters, electronic calculators and ornaments, as well as the complex parts of vehicles, locomotives and airplanes.
Die casting production, die again and again by the chilling effect of thermal forming surface and its internal deformation, with each other and appear repeatedly cyclic thermal stress, lead to structure damage and loss of resilience, cause the emergence of micro cracks, and continue to expand, once the crack expand, fluid diapir and molten metal, and repeated mechanical stress to accelerate the extension of crack. For this reason, on the one hand the die must be fully preheated at the beginning of die casting. In addition, the die must be kept within a certain operating temperature range during die casting production to avoid early crack failure. At the same time, to ensure that the mold before production and manufacturing internal causes do not occur. In actual production, most of the die failure is thermal fatigue crack failure.
Under the action of injection force, the mold will produce cracks at the weakest part, especially the marking marks or electroforming marks on the mold surface are not polished, or the clear Angle of the mold will first appear fine cracks, when there is brittle phase or large grain in the grain boundary, that is, easy to break. When brittle fracture, the crack expands rapidly, which is a very dangerous factor for the failure of mold cracking. For this reason, on the one hand, the scratches on the die surface, the marks made by electrical machining, etc., must be polished. In addition, high strength, good plasticity, impact toughness and fracture toughness are required.
As mentioned above, common die casting alloys include zinc alloy, aluminum alloy, magnesium alloy and copper alloy, as well as pure aluminum die casting. When mold hardness is higher, corrosion resistance is better, and if there are soft spots on the molding surface, corrosion resistance is adverse.
Many factors which would invalidate the mould, both external (case whether pouring temperature, mould preheating, water-based paint spraying quantity of how many, size matches, high pressure die casting, die casting machine tonnage gate open not too fast, cooling water and die casting production casting material, the type and composition of Fe height, the size of the casting shape, wall thickness, size, coating type, etc.).There are also internal factors (for example, metallurgy quality of mold material, forging process of blank, rationality of mold structure design, rationality of casting system design, internal stress generated during mold machine (electrical machining) processing, heat treatment process of mold, including various requirements of matching precision and finish).In case of early failure, it is necessary to find out the internal or external causes for future improvement. However, in actual production, corrosion is only a local part of the mold. For example, the parts directly washed by the inner gate (core and cavity) are prone to corrosion, as well as the aluminum alloy in the soft part.
Pouring line overflow
Requirements for straight runner of the mold for horizontal cold chamber die casting machine:
The diameter size of the pressure chamber shall be determined according to the required specific pressure and pressure chamber saturation. At the same time, the deviation of the inner diameter of the sprue bush shall be appropriately amplified by a few threads, so as to avoid serious problem of punch sticking or wear caused by the deviation of the sprue bush and the pressure chamber diameter, and the wall thickness of the sprue bush shall not be too thin. Generally, the length of the sprue bush should be less than the delivery lead of the injection punch, so that the paint can get out of the chamber.
The inner hole of the pressure chamber and sprue bush should be polished after heat treatment and then ground along the axis direction.
(3) shunt concave cavity and formation of coating, the concave depth is equal to the depth of the runner, its diameter with a gate set of inner diameter, along the parting direction have 5 ° slope. When using the painted direct sprue, the volume of the effective length of the pressure chamber can be shortened and the filling degree of the pressure chamber can be increased.
Mould requirements
Generally, the entrance of the transverse runner of the cooling horizontal mold should be located at the upper inner diameter of the pressure chamber more than 2/3, so as to avoid the metal liquid in the pressure chamber entering the transverse runner early under the action of gravity and start solidification in advance.
The cross-section area of the chute runner shall gradually decrease from the straight runner to the inner gate. In order to expand the section, negative pressure will appear when the metal liquid flows through, easy to inhale the gas on the parting surface, and increase the eddy current wrapping in the metal liquid flow. The average exit section is 10-30% smaller than the inlet section.
The mesh runner should have a certain length and depth. The purpose of maintaining a certain length is to stabilize the flow and guide it. If the depth is not enough, the metal liquid will cool down quickly and the depth is too deep.
The cross-gate section area of the belted runner should be larger than the cross-section area of the inner gate to ensure the speed of the metal injection mold. The cross-gate area of the main runner should be larger than the cross-gate area of each branch.
(5) on both sides should make rounded corners on the bottom of the runner, lest appear early crack, two sides can make about 5 ° slope. The surface roughness of the transverse runner is less than or equal to ra0.4m.

Do not close the parting surface immediately after the injection of the metal liquid of the left side. The flow direction of the metal liquid after entering the mold is as close as possible to the rib and heat sink, which is filled from the thick wall to the thin wall.
When choosing the location of the inner gate, make the metal liquid process as short as possible. In the case of multi-strand internal sprue, it is necessary to prevent several strands of metal liquid from confluence and impact each other after entering the gate, thus causing defects such as eddy current inclusion and oxide inclusion.
The thick part of the inner gate of the thin wall part should be smaller to ensure the necessary filling speed. The setting of the inner gate should be easy to remove, and the casting body should not be damaged (eat meat).
The overflow tank
The overflow groove should be easily removed from the casting without damaging the casting body.
When opening an exhaust tank on the overflow tank, the position of the overflow port should be noted to avoid premature blocking of the exhaust tank, making the exhaust tank useless.
The overflow shall not open several overflow ports or a very wide and thick overflow port in the same overflow tank, so as to avoid cold liquid, slag, gas, paint, etc. in metal liquid from the overflow tank to return to the cavity, causing casting defects.
The design 
On the premise of satisfying the function of the product, rational design die casting, simplify die casting structure, reduce the cost of die casting, reduce the defect of die casting and improve the quality of die casting parts. The injection molding process comes from the casting process, so the die casting design guide is similar in some respects to the plastic part design guide. Detailed die casting parts design reference mechanical industry press published "towards manufacturing and assembly product design guide".
Rounded corners
(including Angle) the casting drawings often indicate the requirements such as no rounded Angle R2. We should not ignore the function of these unmarked angles when making the mold, and we should never make clear or too small rounded corners. The casting round Angle can make the metal liquid filling smoothly, make the air in the cavity discharge in order, reduce the stress concentration and prolong the service life of the die.(it is not easy for the casting to have cracks or defects due to poor filling).Standard oil disc mould clean Angle on more, relatively speaking, at present brother oil disc mould open best, heavy machine oil disc more.
The side concavity caused by someone is strictly prohibited in the direction of mold release (usually the casting is stuck in the mold during mold testing, and the local indentation is caused by drilling, hard chisel, etc.).
Molding parts, casting system should be seriously as required, should be along the direction of stripping. As the metal liquid enters the pouring system from the pressure chamber and fills the cavity, the whole process takes only 0.01 to 0.2 seconds. In order to reduce the resistance of metal fluid flow and reduce the pressure loss as much as possible, both of them need high surface finish. At the same time, the conditions of heating and erosion in the casting system are relatively harsh, and the worse the finish, the more vulnerable the mold is.
5, hardness of the aluminum alloy molding parts: HRC46 ° around copper: HRC38 ° around processing, mold should leave as far as possible to repair allowance, do the size of the ceiling, avoid welding.
Fluidity refers to the capacity of alloy liquid to fill mold. The fluidity of the alloy determines whether the alloy can cast complex castings. The fluidity of recrystallized alloys in aluminum alloys is best.
There are many factors affecting the fluidity, mainly including composition, temperature and solid particles of metal oxides, metal compounds and other pollutants in the alloy liquid.
In the actual production, in addition to strengthening the smelting process (refining and slag removal), the technology of casting (sand mold permeability, metal mold exhaust and temperature) must be improved, and the casting temperature should be raised without affecting the casting quality to ensure the fluidity of the alloy.
Notes for forming conditions of die casting:
Die - casting machine, die - casting alloy and die - casting die are three main factors of die - casting production. The so-called die-casting process is to use these three elements together organically, so as to steadily and efficiently produce qualified castings with good appearance, internal quality, size and pattern or agreement requirements, and even high quality castings.
Melting temperature of material, mould temperature and molten temperature during injection;
Finally, die - cast products to the status of repair to obtain perfect manufacture. Selection principle of operating temperature of die casting mold:
1) the mold temperature is too low, the internal structure of the casting is loose, and the air discharge is difficult and difficult to shape;
2) the mold temperature is too high and the internal structure of the casting is compact, but the casting is easy to be "welded" into the mold cavity, and the mold is not easy to be unloaded. At the same time, the high temperature will cause expansion of the mold itself and affect the casting size accuracy.
3) the mold temperature should be selected within the appropriate range. Generally, the constant temperature control is good after proper test. Notes for forming conditions of die casting can be simply summarized as the following two aspects:
* melting temperature of materials, mold temperature and molten temperature during injection;
The essence of pressure casting (die casting for short) is the method to make liquid or semi-liquid metal fill die casting cavity at a high speed under high pressure and to form and solidify under pressure.
The characteristics 
High - pressure and high - speed filling die - casting die - casting is two characteristics. Its commonly used injection ratio pressure is from thousands to tens of thousands of kPa, even up to 2 times 105kPa.The filling speed is about 10~50m/s, and in some cases it can even reach more than 100m/s.The filling time is very short, generally within the range of 0.01~0.2s.Compared with other casting methods, die-casting has the following three advantages:

The product is of good quality
Casting size accuracy is high, generally equivalent to 6~7 levels, even up to 4 levels; Good surface finish, generally equivalent to 5~8 levels; The strength and hardness are higher, the strength is generally 25~30% higher than sand mold casting, but the elongation decreases about 70%.Stable size and good interchangeability; A thin - walled complex die - casting. For example, the minimum wall thickness of current zinc alloy die casting can reach 0.3mm.Aluminum alloy castings up to 0.5mm;The minimum casting aperture is 0.7mm.The minimum pitch is 0.75mm.
2. High production efficiency
Machine productivity high, such as domestic brand J â…¢ type 3 horizontal cold chamber die casting machine work 600 ~ 700 times on average eight hours, small hot chamber die casting machine work 3000 ~ 7000 times on average every eight hours. Easy to realize mechanization and automation.
3. Good economic results
Due to the precision die - casting size, surface gloss and cleaning. Generally, the machine is no longer used directly, or the processing volume is very small, so it not only improves the metal utilization rate, but also reduces a lot of processing equipment and time. The casting price is cheap. Composite die casting may be used for other metallic or non-metallic materials. Save both assembly time and metal.

Although die casting has many advantages, it also has some disadvantages which need to be solved.
Such as:
1). During die casting, due to the high filling cavity speed of liquid metal and the unstable flow state, general die casting method is adopted, which is easy to produce pores in the casting and cannot conduct heat treatment;
2). It is difficult to die-cast complex castings with inner pits;
3). High melting point alloy (such as copper and ferrous metal), with low die casting life;
4) it is not suitable for small batch production, which is mainly caused by high cost of die-casting mold manufacturing, high production efficiency of die-casting machine and uneconomical small-batch production.

Application range and development trend of die casting
Die casting is one of the most advanced metal forming methods, which is an effective way to achieve low chip and no chip. The size and weight of the casting depends on the power of the die-casting machine. Due to the increasing power of the die-casting machine, the casting size can range from a few millimeters to 1 ~ 2m.Weight can range from a few grams to tens of kilograms. Overseas die-casting aluminum casting with diameter of 2m and weight of 50kg.