The present invention relates to a method for casting zinc alloys in order to provide structurally complex articles.

Nowadays, in order to provide particular, structurally complex articles, such as for example faucets and valves, it is usual to use the low- pressure casting of copper alloys, such as bronze and brass.

In this type of casting, the molten metal is injected into the cavity of a die, into which a sand-based core has first been inserted.

The molten metal is introduced into the die with a low relative pressure, in the order of 0.5 bar, upward from below, as an effect of the pressure itself.

This molten metal occupies the entire volume of the cavity of the die, except for the volume occupied by the core and, solidifying, it produces a casting, assuming the shape of a semi-finished product that has the almost- final shape of the article.

Once solidified, the casting is extracted from the die and subjected to a mechanical decoring operation, which consists in removing the sand of the core from the solidified casting.

Mechanical decoring takes place using a shot-blasting machine.

A shot-blasting machine is an apparatus comprising a box into which the castings are placed and are then subjected to the action of grit, constituted by abrasive particles, which are thrown onto the surfaces of the castings using compressed air or centrifugal force.

The semi-finished product is then subjected to post-casting finishing processes such as mechanical stock-removal machining and/or galvanic baths, etc., in order to give it determined surface characteristics.

The advantage of this method consists in that the molten metal is introduced into the die directly from the tank of a smelting furnace without being exposed to the air. In this manner porosity and oxidation defects are minimized and, as a consequence, the mechanical properties and the aesthetic finishes of the article are also superior.

Furthermore, the fact that in low-pressure casting the melted alloy rises upward from below:

- also means that air bubbles will not become trapped, as occurs in other types of casting such as, for example, gravity casting,

- makes it possible to vary the speed of filling the die as a function of the size of the cavity that the flow fills as the die is filled.

The choice of copper alloys and of the low-pressure casting method, particularly for faucets, is historically due to:

- the low cost of the raw materials, especially copper, and of the casting method,

- the pleasing visual impact of the article cast from copper alloy using the low-pressure casting method,

- the high resistance to water corrosion of products made of copper alloys,

- the good mechanical properties of the castings, which ensure excellent results in seal tests.

Recently, with the increase in the cost of raw materials, and in particular with the exponential increase in the cost of copper, the need has become felt to find an alternative solution to the use of this alloy in the method explained above, for producing such articles.

A family of zinc alloys is known, for example, under the name “zamak” and this family includes an alloy known as “zamak 5”.

“Zamak 5” is a zinc alloy, and its name derives from the initial letters of its component metals in German: zinc (z) aluminum (a), magnesium (ma) and copper (k).

The number “5” indicates the sum of the percentages of aluminum (approximately 4%) and copper (approximately 1%) in the alloy. Other metals present in the alloy are, for example, lead, iron, tin, cadmium, silicon and nickel, all of which have a percentage of less than 0.1%.

With zamak 5, the costs of raw materials is substantially halved with respect to the costs of raw materials for the usual copper alloys used in low- pressure casting for the production of faucets and valves.

However zinc alloys, and in particular zamak 5, present some drawbacks for the low-pressure casting method.

First of all, it is not possible to subject the solidified castings to the decoring operating using a shot-blaster, because the hardness of zinc alloy castings, and in particular of zamak 5 castings, is appreciably lower than the hardness of copper alloy castings, and the surface would be considerably damaged by the action of the grit.

Furthermore, the viscosity of zinc alloys in the molten state, and in particular of zamak 5, is low and, while on the one hand this would enable an easy filling of the die even in the points furthest from the point of entry, on the other hand such a fluidity of the molten alloy could lead to a sudden change of regime in the flow entering the die, from laminar to turbulent, with the consequent generation of defects in the solidified casting, such as, for example, microporosity.

Another drawback is linked to the fact that many of the components of the apparatus for low-pressure casting, starting from the smelting furnace and the spout for injecting the molten metal into the die, are normally made of G22 cast iron, which is subject to corrosion by the zinc and to the diffusion of carbon.

In particular, the corrosiveness of zinc increases as its temperature increases.

Another drawback is linked to the fact that generally, cores are used with a considerable particle size and a surface layer of graphite, which leads to the formation of an excessive and unwelcome superficial roughness on the solidified zinc alloy casting, owing to the greater fluidity of zinc alloys and in particular of zamak 5, over copper alloys, which causes a greater penetration of the alloy between the grains of sand of the cores and therefore an unacceptable roughness once the casting has solidified.

The aim of the present invention is to provide a method for casting zinc alloys that is capable of improving the known art in one or more of the above mentioned aspects.

Within this aim, an object of the invention is to provide a method for casting zinc alloys, at low pressure, that enables the decoring of the castings without damaging their surfaces.

Another object of the invention is to provide a method for casting zinc alloys, at low pressure, that makes it possible to fill the die without triggering turbulent motions in the flow of molten metal entering the die, thus preventing the formation of microporosity in the casting.

A further object of the invention is to provide a method for casting zinc alloys, at low pressure, in which there is no risk of corrosion of the apparatus by the molten metal.

Another object of the invention is to provide a method for casting zinc alloys, at low pressure, that makes it possible to obtain castings with no surface defects deriving from the composition of the cores of the dies.

A further object of the present invention is to overcome the drawbacks of the background art in a manner that is alternative to any existing solutions.

Another object of the invention is to provide a method for casting zinc alloys, at low pressure, that is highly reliable, easy to implement and of low cost.

This aim and these and other objects which will become better apparent hereinafter are achieved by a method for low-pressure casting of zinc alloys that comprises the following steps:

- arranging one or more cores inside a die, - closing said die and positioning it on a casting spout,

- pressurizing a crucible melting furnace which contains a zinc alloy in the molten state, said zinc alloy rising through said spout,

- filling said die with said zinc alloy in the molten state, obtaining a casting as a consequence of the solidification of said zinc alloy,

- opening said die and extracting said casting,

- subjecting said die to operations of cooling and/or cleaning and/or depositing of a layer of release/protective agents,

- subjecting said casting to an operation of removal of the residues of said one or more cores, said method being characterized in that the operation of removal of the residues of said one or more cores occurs by thermal decoring.

Further characteristics and advantages of the invention will become better apparent from the detailed description that follows of a preferred, but not exclusive, embodiment of the method for casting zinc alloys, at low pressure, according to the invention, which is illustrated by way of nonlimiting example in the accompanying drawings wherein:

- Figure 1 is a diagram of the operating steps of a method for casting zinc alloys, at low pressure, according to the invention;

- Figure 2 is a perspective overall view of a portion of an apparatus for carrying out the procedure of a method for casting zinc alloys, at low pressure, according to the invention;

- Figure 3 is a cross-sectional side view of the portion of apparatus of Figure 2.

With reference to the figures, a method for low-pressure casting of zinc alloys, and in particular of zamak 5, according to the invention, is shown schematically in Figure 1, and generally designated with the reference numeral 10.

The method for low-pressure casting of zinc alloys, according to the invention, is particularly advantageous for providing structurally complex technical articles, such as for example faucets and valves, which need to be obtained by casting in a complex die.

The method for low-pressure casting of zinc alloys, and in particular of zamak 5, comprises the following steps:

- arranging one or more cores inside a die, step 11 ,

- closing the die and positioning it on a casting spout, step 12,

- pressurizing the crucible melting furnace which contains a zinc alloy, in particular zamak 5, in the molten state, such alloy rising through the casting spout, step 13,

- filling the die with the zinc alloy in the molten state, obtaining a casting as a consequence of the solidification of such zinc alloy, step 14.

- opening the die and extracting the casting, step 15,

- subjecting the die to operations of cooling and/or cleaning and/or depositing of a layer of release/protective agents, step 18,

- subjecting the casting to an operation of removal of the residues of the one or more cores, step 16.

One of the peculiarities of the invention consists in that the step 16, the operation of removal of the residues of the cores from the die, occurs by thermal decoring.

With particular reference to Figures 2 and 3, the thermal decoring occurs in a decoring furnace 100 inside which the castings, not shown in the figures, are placed for a period of time substantially comprised between 30 minutes and 90 minutes, at a temperature in the order of 300°C.

The decoring furnace 100 comprises a thermally insulated chamber 101 provided with a plurality of electric resistance heaters 102, and a motorized conveyor belt 103, which extends and can slide between an inlet 104 and an outlet 105, of the chamber 101, for the castings of zinc alloy.

The electric resistance heaters 102 are arranged above the belt 103.

In particular, the cores used for the method of casting zinc alloy, and in particular zamak 5, according to the invention are obtained by mixing sand with urea resin which, at approximately 300 °C, evaporates, releasing the sand and so facilitating its removal from the solidified casting.

The casting method according to the invention can include an auxiliary decoring operation, step 17, downstream of the thermal decoring (step 16).

Such auxiliary decoring is carried out using a vibrating belt 200 and is adapted to the removal of any residues and sand of the cores still present in the castings after the thermal decoring.

In particular, the vibrating belt 200 is arranged downstream of the outlet 105 of the furnace 100 and comprises vibrating motor means 201 arranged below and connected to a surface 202 for advancing the castings, which ends with a perforated plate 203, for the sand detached from the castings to fall through.

The conveyor belt 103 is arranged so as to allow the castings to fall onto the vibrating belt 200, after passing through the outlet 105 of the furnace 100.

Another peculiarity of the casting method, according to the invention, consists in that the step 18 of cooling and/or cleaning and/or depositing of a layer of release/protective agents on the die does not include, as is the case with copper alloys, graphitizing.

In fact graphite would have negative effects on the superficial roughness of castings of zinc alloy, and in particular on castings of zamak 5.

For these reasons, the step of depositing release/protective agents on the die entails the use of an agent based on semi-colloidal graphite, such as for example the product known commercially as “DYCOTE 11” of the company named Vesuvius Italia - Foseco Foundry Division, located at Route Ravello 5/7, Vermezzo (MI).

Advantageously, the step of depositing release/protective agents on the die is performed every four/five casting cycles.

The die is constituted by two complementary half-dies, which have a temperature in the order of 190 °C at the start of each casting cycle, which corresponds substantially to the temperature at the end of the cycle, when the die is opened.

This avoids the need to cool the die, as occurs for similar methods for casting copper alloys.

The crucible melting furnace keeps the zinc alloy, and in particular zamak 5, at a temperature:

- in the order of 440°C - 460°C between one work shift and the next,

- in the order of 510°C - 530°C during the casting method.

This difference in temperatures between the two half-dies and the molten zinc alloy ensures the perfect filling of the die and a considerable grading and surface quality of the casting.

Advantageously, the casting spout is made of ferritic steel, preferably AISI 430, and is kept at a temperature in the order of 530°C - 550°C, in order to prevent the molten zinc alloy from solidifying when passing through it, and at the same time it ensures an entrance into the die at a temperature substantially comparable to that of the zinc alloy inside the crucible melting furnace.

In particular the step of pressurization of the smelting furnace must be such as to confer a pressure on the zinc alloy, and in particular on the zamak 5, entering the die a pressure comprised between 0.28 bar and 0.48 bar.

With a pressure comprised in this range, from experimental tests it has been observed that:

- it succeeds in filling the die completely, avoiding uneven solidification,

- it avoids triggering turbulent motions in the molten alloy, and therefore the development of microporosity in the casting.

Furthermore, such pressure ensures an excellent dimensional and geometric tolerance, since in such pressure range the volumetric shrinkage of zinc alloys, and in particular of zamak 5, are substantially absent. It should be noted that the absence of volumetric shrinkage also avoids the formation of cracks in the casting.

As already mentioned previously, the cores used in a method of casting, according to the invention, do not entail the use of graphite in order to guard against superficial defects on the castings, and they comprise:

- sand with an AFA fineness index in the order of 50/55, for example desiccated silica sand AFA 50/55 “LA32” supplied by the Hiittenes- Albertus company,

- urea resin, in the order of 2% by weight of the sand, for example the urea resin with the commercial name “THERMOSET PF 10”, supplied by the Hiittenes- Albertus company,

- a catalyst, in the order of 25% by weight of the resin, for example the catalyst known with the commercial name “HBC 050 LP”, supplied by the Hiittenes- Albertus company.

In some variations of embodiment of the method, if it is necessary to increase the susceptibility to crumbling of the cores during the step of decoring, it is possible to add a decoring agent to the composition of the cores, such as for example the agent known with the commercial name “FERANEX 82D84”, supplied by the Hiittenes- Albertus company.

From experimental tests it has emerged that urea resin is the most indicated for the method according to the invention.

However, as an alternative to urea resin it is possible to use furan resins and/or phenol resins, additivated with one or more decoring agents.

It should be noted that a temperature of 190°C of the die enables:

- an adequate difference in temperature from the temperature of the molten alloy which enables the total filling of the die and a considerable quality of the obtained casting,

- an excellent depositing of semi-colloidal graphite-based lubricant/release agent.

Furthermore, with a die temperature of the order of 190°C a massive cooling operation is not necessary, i.e. by immersing the die in cooling water, as occurs for similar copper alloy castings, because from experimental tests it has been observed that the temperature of the die substantially stabilizes around this value after some casting cycles.

In practice it has been found that the invention fully achieves the intended aim and objects by providing a method for casting zinc alloys, at low pressure, that enables the decoring of the castings without damaging their surfaces.

With the invention a method for casting zinc alloys, at low pressure, has been devised that makes it possible to fill the die without triggering turbulent motions in the flow of molten metal entering the die, thus preventing the formation of microporosity in the casting and avoiding volumetric shrinkage of it, and therefore the formation of cracks.

Furthermore, with the invention a method for casting zinc alloys, at low pressure, has been provided in which there is no risk of corrosion of the apparatus by the molten metal.

In addition, with the invention a method for casting zinc alloys, at low pressure, has been devised that makes it possible to obtain castings with no surface defects deriving from the composition of the cores of the dies and, with the indicated fineness of the sand, an acceptable superficial roughness is ensured.

The invention thus conceived is susceptible of numerous modifications and variations, all of which are within the scope of the appended claims. Moreover, all the details may be substituted by other, technically equivalent elements.

In practice the materials employed, provided they are compatible with the specific use, and the contingent dimensions and shapes, may be any according to requirements and to the state of the art.

The disclosures in Italian Patent Application No. 102022000001367 from which this application claims priority are incorporated herein by reference.

Where technical features mentioned in any claim are followed by reference signs, those reference signs have been included for the sole purpose of increasing the intelligibility of the claims and accordingly, such reference signs do not have any limiting effect on the interpretation of each element identified by way of example by such reference signs.