"SEALING DEVICE FOR INJECTION SYSTEM OF DIE-CASTING

MACHINES"

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This invention concerns an injection system for die-casting machines for molten or semi-molten metal alloys, in particular but not exclusively indicated for hot chamber systems, and in particular an injection system provided with non metallic dynamic sealing elements that yield under compression.

In injection systems for die-casting machines of molten non ferrous alloys, and in particular aluminium alloys, for the construction of the piston injector, the liner and some dynamic seal elements placed between them are at present using materials resistant to high temperatures, able to support the corrosive action of the alloys in the molten or semi-liquid state, to resist wear generated by the sliding of the components in relative motion and to prevent or impede possible gripping, plus restrain the alloy leaks so as to achieve an acceptable, economic duration of such components.

In the so-called "hot chamber" systems, in which the injection pump is immersed in the molten alloy, dynamic sealing elements in the form of metal split rings applied coaxially to the injection piston are generally used. The split rings are interrupted in the circumferential direction which enables them to compress and expand elastically in a radial direction and are mounted coaxially to the injector piston in special grooves or circumferential housings provided in its lateral surface.

During mounting of the injector piston in the liner, the split rings are compressed in their housings tightening onto the injector piston and successively they expand due to their flexibility coming into contact with the internal surface of the cylinder. The pressure generated by the sealing elements on the walls of the liner sets the sealing level at the injection pressure of the alloy being worked. Depending on the conformation of the cut in the circumferential direction and the function conditions of the injection piston, as regards to pressure and speed, one or more sealing split rings can be provided. The sliding between the internal surfaces of the liner and the dynamic sealing elements integral with the injector piston, in rapid movement during the injection of the molten alloy, causes considerable wear problems in these components that, over a period of time, bring about defects in sealing against pressure. Therefore the present injection systems are subject to frequent and costly maintenance.

It is well known that rubbing and wear between relative sliding surfaces depend on the chemical, physical characteristics of the materials and coupled surfaces, including the dynamic characteristics of the movement, above all in terms of speed and relative pressure. The best fittings, as regards to shapes and contact materials, are the object of continuous studies and experiments, the objectives of which are to maximise the useful life of the components in contact. At present many studies are being carried out on the coupling between non metallic materials and the mixed couplings between metallic materials and non metallic materials and generally wear is addressed to the surfaces of the least important

components, e.g. in the case of the injection systems for die-casting machines, on the sealing elements between liner and injection piston.

The patent JP 55088966 describes a method for avoiding erosion and damaging of the cylinder of a die-casting machine on the part of the molten metal by making the internal surface of the liner and the sealing ring on the injector piston with two different types of ceramic material.

Patent US 4899804 describes a head of an injector piston for die casting comprising a wear and seal ring the external diameter of which corresponds to the diameter of the liner. The injector piston comprises a cap that is used to hold the annular sealing element on the injector piston. The material of the annular sealing must be harder and thermally less conductive than the material of the head of the injector piston. Notwithstanding the numerous studies on the matter, the state of the technique still does not offer a satisfactory solution to the problem of obtaining wear components with a useful, acceptable and economic period of life together with a good pressure sealing capacity.

There are also other well known methods that permit reduction in wear between the sealing elements and the liner that envisage the adaptation of the radial pressure exercised by the sealing elements on the internal surface of the cylinder based on the different injection pressures during the die-casting cycle.

In fact, the pressure of the alloy is very low in the initial phase of the injector piston stroke that is when the alloy reaches at a low speed the gate in the metal die. In the second phase of the injection he pressure quickly increases to allow filling of the die at a high speed and, in the third phase of

the die casting process, the pressure becomes very high to enable the casting to be compact. Consequently, by varying the radial pressure that the sealing elements exert on the internal surface of the liner to avoid leaking of the alloy due to the pressure of the die casting cycle, it is possible to reduce the radial stresses, and consequently the wear, on the sealing elements and on the liner, with greater benefits for the duration of such components.

In this regard, the patent US 5350007 describes an injection system for die-casting machines that comprises an injector piston in which some circumferential grooves connected to the pressurization surfaces by means of a conduit passage system are provided. The grooves house some semirings with play which are solicited radially towards the exterior by the pressure of the molten metal which, during the injection phase, returns through the passage conduits provided in the injector piston. The rings must meet certain geometrical requirements that depend on the materials used (Young's modulus), on the internal diameter of the cylinder and on the injection pressure of the metal.

Although the idea of adjusting the radial pressure exerted by the sealing elements on the liner is a good indication for achieving an extension of the life of the wear elements, the abovementioned patent only supplies general indications as to the materials to be used, limiting the choice criteria only to Young's modulus of the material and to some cylinder and injector piston geometric parameters.

Furthermore it is known that in this type of application the seal between the rings and the piston is never efficient, which penalises the performance of the injection system and the quality of the casting.

The objective of this invention therefore is to supply an injection system for die-casting systems that enables minimizing of wear between the sealing elements and the liner. Said objective is achieved with an injection system whose main characteristics are specified in the first claim, whereas other characteristics are specified in the other claims.

The injection system according to this invention is provided with dynamic sealing elements made of a non metal graphite base material yielding under compression, such dynamic sealing elements being compressed in an axial direction by means of axial compression positioned on the head of the injector piston. The compression in an axial direction causes, due to the compliance of the material, an expansion of the dynamic sealing elements in a radial direction against the internal surface of the liner, necessary to restrain the alloy being worked from leaking. The degree of compression exerted by the axial compression means is proportional to the injection pressure during the die-casting cycle, starting from a static pre-load condition necessary to guarantee sealing under minimum pressure conditions.

A first important advantage of the injection system according to this invention is that the choice of graphite as a base material for the construction of the dynamic sealing elements enables good sliding characteristics to be achieved between the components in relative motion minimizing the wear problems due to sliding.

In addition, the proportionality between the degree of compression exerted by the means of compression on the sealing elements and the die- casting cycle pressure generates variable radial pressures on the internal

surface of the cylinder, resulting in the fundamental advantage of a further reduction of the wear between the elements in relative motion.

Yet another advantage of the injection system according to this invention is that the compression means that act on the dynamic sealing elements can easily be manually and/or automatically adjusted, allowing the regular control of the static preloading necessary to guarantee sealing under minimum pressure conditions.

In addition, the dynamic compression action exerted by the means of compression during the several phases of the die-casting cycle, which can in certain cases be achieved, can be fully automated allowing the further advantage of an authentic optimization of the sealing level of the most important parameters of the die-casting cycle.

Further advantages and features of the injection system for die- casting machines according to this invention will become evident to the experts in the field from the following detailed and not limiting description of some of its preferred possible forms with reference to the attached drawings where: figure 1 shows a longitudinal section view of a first possible form of an injection system for die-casting machines according to this invention; figure 2 shows a longitudinal section view of a second possible form of an injection system for die-casting machines according to this invention; figure 3 shows a longitudinal section view of the injection system for die-casting machines in figure 2, to which has been added

some manual adjustment elements of the static preloading required by the sealing elements; figure 4 shows a longitudinal section view of the injection system for die-casting machines in figure 3, in which the adjustment elements of the static preloading required by the sealing elements are operated by a rotary servomotor; figure 5 shows a longitudinal section view of a third possible form of an injection system for die-casting machines according to this invention, in which the dynamic compression of the sealing elements during the several die-casting phases of the cycle is automatically adjusted in relation to the pressure present in the pressurization chamber of the injector piston actuator; figure 6 shows a longitudinal section view of the injection system for die-casting machines in figure 5, in which the dynamic compression of the sealing elements during the several die-casting phases is actively adjusted by means of an added hydraulic servomechanism; figure 7 shows a longitudinal section view of a fourth possible form of an injection system for die-casting machines according to this invention; and figure 8 shows a longitudinal section view of a fifth possible form of an injection system for die-casting machines according to this invention, in which the compression of the sealing elements is adjusted automatically by means of a servomotor.

In reference to figure 1 , the injection system for die-casting machines according to this invention comprises as is well known, a liner 1 for a molten or semi-liquid metallic alloy and an injector piston 2 inserted coaxially in the liner 1 and connected to an actuator 2a so as to impart the injector piston 2 with an alternative motion. The liner 1 is provided with an input port 3 and a delivery port 4 that is connected to a mould M by means of a pouring channel C. The injector piston 2 is provided with one or more dynamic sealing elements 5 basically annular shaped to come into contact with the internal surface of the liner 1 to prevent the alloy from leaking and consequent drop in pressure during all the die-casting process phases.

According to this invention, the sealing elements 5 are positioned stacked on the injector piston head 2 and are made of a non metallic material with a graphite base and which yields under pressure. This choice is based on the fact that graphite based materials have already been used with success for the construction of static sealing elements such as, for example, those used in valves for the petrochemical industry.

Preferably, the sealing elements 5 are made of foamed graphite, a material which is extremely heat resistant and well suited to the realisation of sealing elements for high temperature applications. The sealing elements 5 can in addition be provided internally and/or externally with elements resistant to the molten alloy and to temperatures from 600 to 800 ⁰ C, both the non metallic type, such as, for example, carbon fibre, and the metallic type as long as they can resist attacks from the molten alloy being worked, and they can also comprise spring equipped elements so as to allow the gathering and release of elastic force, for example of the conical disk springs.

Also the shape of the transversal section is of great importance for the good operation of the sealing elements 5. In preference, the sealing elements 5 have a quadrangular shaped ruling transversal section, with ratios between the sides of between one and two and internal angles of between 60° and 120°.

The injection system according to this invention comprises axial means of compression so as to compress the sealing elements 5 in an axial direction to increase the radial pressure that the latter exert on the internal surface of the liner 1. In this way the cohesion and hardness of the material increase, indispensable characteristics to avoid the molten alloy attacking and eroding the graphite material and rapidly deteriorating the pressure sealing capacity.

The axial means of compression are positioned on the head 6 of the injector piston 2 and generally comprise a plate 7 positioned on the free end of the battery of sealing elements 5 and a screw register 8 axially attached to the injector piston 2.

In the form illustrated in figure 1 , the plate 7 has an annular shape and is inserted to slide on a cylindrical pin 6a positioned on an axis with the injector piston 2 on its head 6. The cylindrical pin 6a centres the battery of sealing elements 5 with respect to the injector piston 2 and is provided with a threaded end 6b. The screw register 8 is a locknut that engages with the threaded end 6b of the cylindrical pin 6a causing pressure of the plate 7 on the sealing elements 5. The compression in an axial direction, as stated above, determines a radial pressure of the sealing elements 5 on the internal surface of the liner 1.

When mounting the compression means 7, 8 establish a static preload pO on the sealing elements 5, necessary to guarantee sealing in minimum pressure conditions.

According to this invention, the axial compression means are also suitable to dynamically compress the sealing elements 5 in a mode directly proportional to the injection pressure. In fact, during injection the plate 7 is solicited in the axial direction directly by the molten alloy which is in the injection pressure p, schematized in the figure by means of the dotted arrows. When the pressure p exceeds the static preload p0 setting for the sealing elements 5, the plate 7, that is free to move axially, increases compression on the sealing elements 5, causing greater radial pressure of the same on the internal surface of the liner 1. Thanks to this, the sealing of the system is dynamically adapted to the instantaneous pressure of the alloy being worked, enabling a reduction in contact pressure each time the instantaneous pressure drops so as to achieve less wear of the components and in particular of the internal surfaces of the liner 1.

In the possible form illustrated in figure 2, the plate 7 is provided with a hollow stem 7a with internal threading which is inserted coaxially in the injector piston 2, in an axial cavity 2b provided in its head 6, and axially centres the battery of sealing elements 5 with respect to it. The screw register 8 is a tie rod provided with a head 8a and a threaded end 8b and lodged in the injector piston 2 in an axial seat 2c provided at the opposite end to its head 6. The threaded end 8b of the tie rod engages with the hollow threaded stem 7a of the plate 7, causing in this way the compression of the sealing elements 5 between the plate 7 and the head 6.

The static preload pO necessary to guarantee the seal in minimum pressure conditions is achieved by adequately tightening the screw register

8 in the hollow stem 7a of the plate 7. Preferably, the screw register 8 comprises in addition a number of springs, for example some Belleville washers, positioned between its head 8a and the axial seat 2c. The springs

9 stress the screw register 8 in an axial direction maintaining the static preload p0 set during assembly on the battery of sealing elements 5, constant. To guarantee holding of the static preload pO, the springs 9 are made of a material able to support temperatures of about 600 ⁰ C. During the adjustment of the static preload pO, to avoid the relative rotation between the plate 7 and the head 6 of the injector piston 2 with torsion of the sealing elements 5, a torsion constraint can be provided, such as for example a key 10 or a grooved joint, positioned between the hollow stem 7a and the axial cavity 2b provided in the head 6 of the injector piston 2. Figure 3 shows the possible form exemplified in figure 2, in which the axial seat 2c of the screw register 8 has a diametral through opening 2d that houses a transverse element 11. The transverse element 11 supports the head 8a of the screw register 8 and, if present, the springs 9. The ends of the transverse element 11 project out of the injector piston 2 and are in contact with a ring nut 12 positioned coaxially to the injector piston 2, between the transverse element 11 and the head 6, and turning on a threaded portion 2e provided on the injector piston 2 near the through diametral opening 2d. The rotation of the threaded ring nut 12 stresses the transverse element 11 in an axial direction allowing the manual adjustment

of the static preload pO of the battery of sealing elements 5 without the need to dismantle the injector piston 2 from the actuator 2a and from the liner 1.

In the possible form illustrated in figure 4, the ring nut 12 is operated in rotation by means of a servomotor 13 and a clutch 14, for example a friction clutch or claw clutch, positioned coaxially to the actuator 2a.. This disposition is advantageous in that it does not require any manual intervention on the part of the operator and enables adjustment of the static preload p0 to be carried out, for example, at each stamping cycle when the piston is in the top end of stroke position. Figure 5 shows a third possible form of the injection system according to this invention, in which the axial seat 2c of the injector piston 2 is hermetically sealed and the head 8a of the screw register 8 forms a lower chamber 15 and an upper chamber 16 provided respectively with a first hydraulic conduit 17 and a second hydraulic conduit 18 suitable for supplying a fluid under pressure to them such as, for example, hydraulic oil. The screw register 8 acts therefore as a hydraulic plunger allowing continuous adjustment of the static preload p0 and the dynamic compression of the battery of sealing elements 5 during all the die-casting phases.

The continuous adjustment can be carried out in the automatic mode, for example, by connecting the first and second 17, 18 hydraulic conduits respectively to two pressurization chambers 2a', 2a" of the actuator 2a. As shown in figure 5, the first hydraulic conduit 17 is connected to a first pressurization chamber 2a' that sets the injection stroke of the injector piston 2, whereas the second hydraulic conduit 18 is connected to a second pressurization chamber 2a" that sets the return stroke of the injector piston 2

after the injection of the molten alloy. In this way the battery of sealing elements 5 will be compressed and decompressed in ratio with the pressure present in the actuator 2a.

As an alternative, as shown in figure 6, the hydraulic conduits 17, 18 can be fed separately by means of a pump 19 and connected to a hydraulic servomechanism 20, for example a proportional group, and said elements 19, 20 can be advantageously controlled in an active mode by a control unit CU to carry out the adjustment of the static preload p0 and the dynamic compression of the sealing elements 5 depending on the fundamental parameters of the die-casting cycle, such as strokes, pressures, times and speeds. These parameters can be obtained from the ideal operating tables memorised in the control unit CU, or can be detected at any moment by appropriate sensors (not shown) connected to the control unit CU.

Figure 7 shows a fourth possible form of the injection system for die- casting machines according to this invention, in which the plate 7 has an annular shape and a stopper element 21 is inserted coaxially in the battery of sealing elements 5. The head 21a of the stopper element 21 is lodged on plate 7 and its stem 21 b is threaded and screwed in a slot 6c provided in the head 6 of the injector piston 2. The screw register 8 is a threaded ring nut positioned in a turning mode on the threaded end 2f of the injector piston 2 screwed to the actuator 2a. A screw down element 22 is placed coaxially to the injector piston 2 between the threaded ring nut and the battery of sealing elements 5. The rotation of the threaded nut causes a variation of the pressure of the screw down element 22 on the battery of sealing elements 5 to enable the manual adjustment of the static preload pθ.

To avoid torsion of the battery of sealing elements 5 during the adjustment of the static preload pθ, the annular pressure element 23 can be coupled to the stem 21b of the stopper element 21 by means of, for example, a key or a grooved joint. Figure 8 shows a fifth possible form of the injection system according to this invention. Analogous to the possible form illustrated in figure 7, the plate 7 has an annular shape and a stopper element 21 is inserted coaxially into the battery of sealing elements 5. The head 21a of the stopper element 21 is lodged on the plate 7 and its stem 21 b is threaded and screwed into a slot 6c provided in the head 6 of the injector piston 2. The register 8 is a transverse element placed in a through diametral opening 2g provided in the injector piston 2 and connected at the top to the threaded stem 24 of a servomotor 25 coaxial to the actuator 2a. A screw down element 22 is placed between the transverse element and the battery of sealing elements 5. The drive of the servomotor 25 causes a variation of the pressure of the screw down element 22 on the battery of sealing elements 5 to allow the continuous adjustment of the static preload p0 of the dynamic compression of the battery of sealing elements 5.

To achieve a better distribution of the contact pressures, in both of the last two possible forms, between the screw down element 22 and the battery of sealing elements 5, an annular pressure element 23 is preferably positioned which has a transversal section basically coincident with that of the sealing elements 5.

It is clear that the possible forms of the invention described and illustrated above are only examples which can undergo numerous variations.

In particular, for example, the sealing elements 5 can have a generatrix section which is not quadrangular but for example trapezoidal, to vary the rigidity characteristics, resistance to wear and sealing of the battery pressure, and it is also possible to combine together sealing elements with different generatrix section shapes. In addition, in the case of an automatic control of the compression of the sealing elements, it is possible to use control algorithms based, other than on the fundamental parameters of the system, also on the time constants characteristic of the die-casting cycle, being able in this way to adjust the compression of the sealing elements with greater precision and in advance with respect to the individual phases of the production process.