Process for hot drawing hollow extruded or drawn aluminium bodies and equipment for application of said process

Abstract: Drawing of hollow extruded or drawn aluminium bodies is accomplished by forcing the hollow body through a drawing ring pushed by a plunger or mandrel placed inside said body and heated to a temperature not less than 90°C and generally between 105°C and 140°C. The equipment for application of the process comprises one or more toolholder feed bars in which the plungers or mandrels and one or more drawing rings are alternately inserted or coupled and in which each plunger has an inner chamber (22), heating means (14) arranged in said chamber and longitudinal grooves (16) along its outer surface.

Description: The present invention relates to a process for hot drawing hollow extruded or drawn aluminium bodies and equipment for application of said process. More in particular, the present invention relates to hot drawing of hollow aluminium bodies accomplished on drawing machines of the horizontal or vertical single-acting press type by forcing through an extruder of the extruded body fitted over a plunger. As is known, the drawing process is a cold operation used for finishing and reducing the dimensions of the cross section of metal bars, wires, and tubes by plastic deformation of the material through stretching, utilizing the ductility of the metal. The material is passed through the sized opening or extruder created in the tool and/or tools of the machine and having the profile and dimensions which it is desired to give to the cross section of the finished product. The process of drawing a hollow body consists of passing said body through one or more extruders with decreasing diameter, pushed by a mandrel or plunger placed inside said body. As is known, during the drawing operation lubrication is very important and has the primary purpose of reducing friction in the forcing and possible seizure. In drawing of extruded aluminium bodies several basic factors should be considered. The extruded body has a constant inside diameter. The outside diameter is variable, precisely larger at the mouth. This involves tapering of the outer wall toward the end of the body, i.e. a wall thickness which increases progressively on the outside as the mouth is approached. During the drawing operation made on a body of this type there is observed at the portion with constant and smaller thickness considerable sliding and slipping of the extrados of the body due to reduction of the thickness and the difference between the relative speeds between the body fitted over the plunger and the drawing ring. The same thing happens in a less severe form during drawing involving the portion of the body in which the wall is thicker because said thickness is not substantially reduced. As a whole, the drawing process does not show particular disadvantages due to slipping along the inner surface of the extruded body. The diameter of the latter is indeed slightly greater than the plunger and therefore the body lays downs thereon progressively starting from the bottom area up to the head. In view of the foregoing there is a requirement first of all to provide a drawing ring or extruder of material very resistant to wear and capable of maintaining considerable hardness even at the high temperatures it reaches and which are generally around 150°C to 200°C. This provision however leaves unsolved the basic problem concerning sliding of the bodies being drawn, conventionally done cold. To solve this problem, it is known to treat the bodies, especially inside, with a substantial quantity of lubricating substances. As a result the bodies, after drawing, must be subjected to careful washing to eliminate the lubricating substances deposited thereon. The use of such lubricating substances, even if used in small quantities in each operation, leads to significant problems of total elimination and the need to proceed systematically with cleaning of the machines and results in an overall increase in production costs. The object of the present invention is to obviate the above shortcomings. In its most general aspect, the present invention permits achieving said object by heating the hollow aluminium body when it passes through the drawing ring or extruder to not less than 90°C. Accordingly a first subject matter of the present invention is a process for drawing a hollow extruded or drawn aluminium body by forcing through a drawing ring pushed by a plunger or mandrel placed inside said hollow body, in which said hollow body is heated to not less than 90°C when it passes through the drawing ring. Generally the hollow body is heated to between 105°C and 140°C and preferably between 110°C and 125°C. Another subject matter of the present invention is the drawing device for providing the process described above. In accordance with the present invention the drawing device comprises one or more toolholder bars in which there are alternately inserted or coupled the plungers or mandrels of different length and/or diameter and one or more drawing rings or extruders and in which each toolholder bar has an internal chamber extending longitudinally along said bar, with a heating means arranged in said chamber, and with longitudinal grooves made along the outer surface thereof, said grooves having a length at least equal to that of the chamber or of the heating means. The heating means permits heating of the outer surface of the plunger, inserted in or coupled with the toolholder bar, to a temperature not less than 100°C and preferably between 115°C and 150°C. The process of hot drawing hollow bodies, especially of aluminium, of the present invention, achieves the basic result of excluding recourse to any kind of lubrication on the bodies subjected to the above operation on condition that said bodies upon passage in the drawing ring are heated to a temperature above 80°C and generally between 105°C and 140°C. Said heating of the hollow body takes place by the heat exchange effect with the plunger, whose outer surface is in contact with the intrados of each body during drawing and is continuously heated to a temperature not less than 100°C and preferably between 115°C and 150°C. Heating the bodies to this temperature regenerates the residual lubricant present on the surface thereof so that no further addition of lubricant is required. The advantages achieved by the present invention are that the semifinished extrusions are drawn in the state in which they are found; there is no requirement for additional lubricating substances, which would have to be removed from the bodies and from the machine and disposed of. The drawn bodies are also easy to separate from the plunger due to the effect of the expansion thereof which takes place due to the induced heating and also the effect of regeneration of the lubricating substances. Said plunger can also be advantageously made of steel instead of special materials such as widia and the like, thus permitting considerable economies. The process for drawing hollow bodies of the present invention is carried out in the presence of temperatures induced in the bodies by the plunger, on which said bodies are fitted automatically from time to time. Heating of the plunger, whose outer surface is heated to a temperature preferably between 115°C and 150°C, is transmitted to the semifinished body during drawing. Basically, there is heat exchange between the plunger and the extruded body upon passage of the latter through the drawing ring or extruder. Heat exchange is helped by the high thermal conductivity of the aluminium and the excellent ratio of the exchange surface to the mass of the extruded body. The latter therefore reaches a temperature not less than 90°C and in particular between 105°C and 140°C. During drawing performed at said temperature regeneration of the residual lubricating substance of the extrusion process takes place. The lubricating substance, in general zinc stearate or sodium stearate, originally spread on the aluminium pellet, has an optimal use temperature around 110°C and 125°C and is not exhausted completely in the extrusion operation. The lubricating substance hence is already present on the surface of the body to be drawn and its effect is therefore utilized again and more completely due to the effect of the above mentioned heat exchange between the plunger and the body. Thus it is not necessary to add further lubricating substance during drawing, especially inside the extruded body, with the result of avoiding dirtying of the drawing machine, simplifying the subsequent step of washing of the body after finishing the operation and, in short, achieving substantial production savings, the direct costs of the added lubricating substance and the costs of subsequent removal thereof being eliminated. The drawing operation performed on a hot hollow body as explained above also results in considerable easing of the final step of the process of removal of the drawn body from the plunger after forming of the end as compared with the conventional cold drawing operation. Said removal is conventionally obtained by means of a removal ring in segments which acts on the drawn body, which appears with a first section of the skirt or wall starting from the end with constant thickness, a second section of skirt characterized by a cylindrical zone with constant thickness and another conical section merging with the constant thickness section undercut in relation to the plunger. The average temperature of the extruded body before insertion of the plunger is around 35°C - 40°C while the temperature of the body after drawing reaches 95°C - 115°C as a consequence of the above mentioned heating. Consequently this temperature difference causes thermal expansion of the drawn body such as to cause a first detachment of the body from the plunger. At the moment the head of the drawn body comes into contact with the removal ring, the force of removal and consequently the force of friction is limited since the body does not completely cling to the plunger thanks to the thermal expansion which took place. Therefore, the interference to be overcome with the plunger is considerably reduced. In addition, the regeneration of the residual lubricant of the extrusion process further lowers the friction coefficient between the drawn body and the plunger. All these concomitant and helpful factors permit removal of the plunger body without input of a volume of air under pressure in the body. This operation is however conventionally used during the cold drawing process to compensate for the vacuum effect which causes the drawn body to remain completely clinging to the plunger. Elimination of the compressed air in the removal step prevents the danger of formation of irregularities which could result from the fall of the drawn body on the unloading conveyor belt. The present invention will be further clarified by the detailed description given below and the annexed drawings which represent an embodiment of the device used in the process as well as the arrangement which the body adopts on the plunger following thermal expansion. In the figures: FIG. 1 shows a diagrammatic view of the cross section of a plunger provided with heating elements, FIGS. 2 and 3 show respectively a diagrammatic view of the hollow body on the plunger after drawing and during removal, and FIG. 4 shows an enlarged diagrammatic view of a part A of FIG. 2 in which can be seen the detachment of the drawn hollow body from the plunger. FIG. 1 shows diagrammatically a unit of equipment specifically suited for application of the process described for hot drawing of hollow extruded aluminium bodies. In its general aspect this unit calls for the use of electrical resistances or similar heating means inserted in a longitudinally extended chamber created inside the toolholder on the drawing machine. In particular it is necessary that the heating of the plungers be achieved homogeneously along the portion on which are fitted the extruded aluminium bodies to be drawn, i.e. the final section of the plunger. The equipment unit is also the subject matter of the present invention and comprises a toolholder bar (10) to which are coupled or in which are inserted the plungers (12) with various dimensions depending on the diameter and/or length of the bodies. Inside the toolholder (10) is a longitudinally extended chamber (22) in which is inserted an electrical resistance (14) which heats the plunger (12) to the previously determined temperature. Although for this purpose a resistance (14) is preferred as a heating means the alternative use of other heating systems cannot be excluded, for example blown air, diathermic oils and the like, made to flow in said longitudinally extended chamber (22). The outer wall of the toolholder (10) is advantageously provided with longitudinal grooves (16) of limited depth. Said grooves (16) have the function of creating a ventilation with redistribution of heat along the plunger (12) during drawing. The hollow body (18) is fitted on the plunger (12) and removes therefrom by heat exchange a limited amount of heat. This does not take place along the exposed portion of the plunger (12). The grooves (16) help keep a constant temperature over the entire surface of the plunger (12) thanks to the air circulation which continually redistributes the heat. The presence of the grooves (16) also gives the further advantage of preventing the vacuum effect on the drawn body (18), i.e. the curling of said drawn body, at the time it is passing the undercut during removal. FIGS. 2 and 3 show diagrammatically the arrangement which the body (18) takes on the plunger (12) after drawing and during removal respectively. FIG. 4 shows the limited play (24) on the order of a few hundredths of a millimetre which is formed immediately after drawing between the plunger (12) and the drawn body (18). This play is the effect of the thermal expansion on the body (18) following the heat exchange between the plunger (12) and the body. The existence of said body (24) is in itself a helpful condition for removal of the plunger (12) from the drawn body (18). To this are added other positive factors, already mentioned above, to wit: a) regeneration on the body (18) of the residual lubricant from the extrusion process, and b) ventilation along the longitudinal grooves (16) of the toolholder which prevents occurrence of the vacuum effect on the body (18). These concomitant factors, added to the detachment from the plunger (12) of the body (18) following thermal expansion, lead to easy final removal of the drawn body (18). FIG. 3 illustrates the easy removal of the drawn body (18) which, already tending to be mobile on the plunger (12), must only overcome the resistance offered by the undercut to be able to be withdrawn by the removal ring (20). The regenerated lubricant and the ventilation mentioned above contribute substantially to easing said operation which requires no further specific provisions, especially not input of compressed air in the body (18). From the above the many advantages of the process which the present invention offers are evident. The operation of drawing semifinished parts does not require the use of additional lubricants, which would subsequently have to be removed from the bodies and the machine and disposed of. The drawn bodies are also removed readily from the plunger thanks mainly to the thermal expansion thereon and the regeneration of the residual lubricant of the extrusion process. The plunger, for these reasons, is not subject to any particular wear and can thus be made of steel instead of special materials. The drawing process and associated device were described above by way of example.

Claims: Process for drawing a hollow extruded or drawn aluminium body by forcing it through a drawing ring pushed by a plunger or mandrel inserted in said hollow body, characterized in that said hollow body is heated to a temperature not less than 90°C at the time it passes through the drawing ring. Drawing process in accordance with claim 1 wherein the hollow body is heated to a temperature between 104°C and 140°C and preferably between 110°C and 125°C. Drawing process in accordance with claim 1 or 2 wherein the hollow body is heated by heat exchange with the plunger whose outer surface is heated to a temperature not less than 100°C. Drawing process in accordance with claim 3 wherein the outer surface of the plunger is heated to a temperature between 115°C and 150°C. Drawing process in accordance with any of the above claims wherein the hollow body has a temperature between 35°C and 40°C before insertion of the plunger and between 95°C and 115°C after drawing. Drawing device for extruded or drawn hollow bodies made of aluminium comprising one or more toolholder bars (10) with which there are alternately coupled or inserted plungers (12) of different length and/or diameter and one or more drawing rings or extruders, characterized in that each toolholder bar (10) has an internal chamber (22) extending longitudinally along said bar (10), with a heating means (14) arranged in said chamber, and with longitudinal grooves (16) made along its outer surface, said grooves (16) having a length at least equal to that of the chamber (22) of the heating means (14). Drawing device in accordance with claim 6 wherein the heating means is one or more electrical resistances (14). Drawing device in accordance with claim 6 wherein the heating means is hot air blown into the longitudinal chamber (22). Drawing device in accordance with claim 6 wherein the heating means is diathermic oil. Drawing device in accordance with any one of claims 6 through 9 wherein the heating means is heated to a temperature such that the outer surface of the plunger (12) has a temperature between 115°C and 150°C.

Inventors: FRATTINI ROBERTO, FRATTINI, ROBERTO
Applicants: FRATTINI COSTR MECC, FRATTINI S.P.A.-COSTRUZIONI MECCANICHE
Application year: 1991
Main classification: B21D51
IPC codes: B21C9, B21C3, B21D51, B21C1, B21C45