1. Technical Field
The present invention relates to a method for manufacturing monolithic hollow bodies by means of a casting or injection moulding process. The term “casting” in intended as indicating high-pressure casting processes (“pressure die casting”), low-pressure casting processes (approximately 1-2 bar) and gravity casting processes (including casting processes with sand moulds and casting processes with metal or “shell” moulds).
The present invention finds advantageous application in the manufacture of articles for use in the automotive sector, to which the treatment that follows shall make explicit reference, but without any loss of generality.
2. Prior Art
The advantages deriving from making manufactured articles in metal alloys by means of pressure die casting or in polymeric materials by means of injection moulding are well known.
These processes enable high industrial productivity deriving from very low moulding cycle times, the production of thin thicknesses (2-3 mm) and achieving finished shapes (“net-shape” or “near-net-shape”) due to the effect of injecting under pressure into metal moulds; in substance, these procedures enable the manufacture of low-cost articles for mass production and types of production commonly used in the automotive sector.
However, significant limits exist regarding the manufacturing processes of articles for which hollow and geometrically complex shapes are required: limits represented by the need of having to use only metal cores that, as they must be constrained to the mould, necessitate being extracted from the manufacture article by withdrawal before ejection of the piece. Thus, due to the requirement of being extractable, these cores do not allow the production of undercuts and so, ultimately, design flexibility is significantly penalized in terms of the internal geometric configuration of the pieces to be made. The use of metal cores is necessary in pressure die casting processes because high mechanical strength is required to support the heavy stresses exerted by liquid metals or technopolymers during the steps of filling the mould and the considerable compression pressures (500-1500 bar) during solidification of the piece.
All the same, obtaining hollow monolithic bodies in metal materials is feasible with casting techniques that do not require high moulding pressures, such as gravity casting for example and which, given the lack of particular stress in the casting step, permit the use of sand cores, which can be removed from the casting after the step of ejecting the piece from the mould with known and conventional methods of thermal, mechanical and/or chemical removal. Obviously, in the case of these casting techniques, the components produced still lose the previously-described advantages deriving from the use of high moulding pressures, especially in terms of weight (the minimum thickness of the walls is 5 mm) and cost (due to the considerable lengthening of production times).
In the case of polymeric materials, there are known techniques that allow the production of hollow monolithic bodies (even in the presence of high moulding pressures) by means of, for example, the use of fusible metal cores: however, in this case, the prohibitive industrial costs of the technology have effectively prevented mass industrial development.
In recent years, some of the limits mentioned above have been overcome in the automotive sector: in fact, pressure-die-cast aluminium solutions have been developed based on the production of castings characterized by undercuts made by means of cores in a refractory material of sufficient mechanical strength (produced with the shell-moulding technology for example) able to adequately resist the stresses exerted by the molten metal during the moulding process of the castings. On the other hand, this has been made possible through the onerous utilization of special semi-solid casting processes (known as “rheocasting”) that enable the injection of molten metal at low velocities, thereby significantly reducing the tensional stresses in play.
Although adequate in relation to certain specific applications, the mechanical strength values of the cores employed are, in any case, generally limited (10-15 MPa at most) and, in consequence, the mould filling conditions are still restrictive (in terms of gate positioning and injection parameters) in order not to compromise the structural stability of the cores themselves.
The methods of consolidation of these cores are based on the utilization of organic or inorganic binders that, under the effect of temperature, enable the cohesion of the refractory powders in which they are mixed. According to the various technologies in use, these binders can be added separately to the refractory material or can constitute an integral part (pre-coated powders). In any case, the bonds are relatively weak and, in consequence, the mechanical characteristics of the cores cannot offer particularly good performance and are therefore not suitable for all applications.
In addition, the organic binders generate gases during casting that must be adequately evacuated to prevent them remaining trapped inside the mould and causing the formation of undesired porosity in the metal. Furthermore, organic binders have quite a significant environmental impact, while on the other hand they are not soluble in water (unlike inorganic binders) and removal of the corresponding cores requires heat treatment on the castings or energetic mechanical action by hammering on the actual castings. Unlike cores using organic binders, cores using inorganic binders have the advantage of not generating gas residues in the casting step; however, such cores using inorganic binders are only made as solid ones, by means of processes (for example, the so-called “hot box”) that do not allow shell cores to be obtained.
U.S. Pat. No. 5,387,280A1 describes the utilization of a lost ceramic core for a casting process of the “investment casting” type; the ceramic core comprises a high percentage (between 20% and 50% by weight) of acid-soluble borate binder and therefore acids are used for removing the ceramic core after forming the piece. However, the use of acids for core removal has a non-trivial environmental impact, especially when a large number of pieces are produced, as occurs in the automotive sector (where the production of more than a million pieces every year is not infrequent).
Patent applications JP06023505A and EP1293276A2 describe the utilization of lost sintered ceramic cores in casting processes. However, the removal of ceramic cores produced according to these patent applications is normally complex, and therefore expensive.
U.S. Pat. No. 3,688,832A1 describe the utilization of lost ceramic cores in casting processes. To strengthen and harden the ceramic cores (to be able to use these ceramic cores in pressure die casting processes) and at the same time to enable simple removal of the ceramic cores from the finished piece after the casting process, the ceramic cores are impregnated beforehand with a hot mixture of at least one organic compound that has a melting point not below 77° C., can be melted to a liquid state and then resolidified following cooling, has a density of at least 1 gram per milliliter and volatilizes (vaporizes) when heated beyond its melting point. Before the ceramic cores are used in the casting process, they are heated to volatilize the organic impregnant through the pores of the ceramic cores. However, the use of organic compounds to impregnate the ceramic cores beforehand considerably increases the environmental impact of the process, as these organic compounds are highly polluting. In addition, the ceramic cores must be heated to volatilize the organic impregnant in a sealed environment that allows all fumes to be recovered, after which they must be adequately treated and not discharged into the atmosphere, with a significant impact on the overall cost of the process. Organic impregnant may remain in the ceramic cores and then volatilize inside the mould, generating gas that can cause the formation of undesired porosity in the metal. In addition, the ceramic cores produced in this way have a high surface porosity and therefore the molten metal that is fed under pressure into the mould tends to penetrate quite deeply inside the ceramic core (even up to 1-1.5 mm); this is big drawback because it makes removal of the ceramic core from inside the metal piece more complex and makes the surface of the metal piece that has been in contact with the ceramic core much rougher.