Shot sleeve for a die casting machine and a process for removing impurities

Abstract: A shot sleeve for a die casting machine comprises an elongated hollow body for receiving and guiding a shot plunger. This body defines an elongated chamber of predetermined cross-section having a front opening to face a sprue runner of a die. According to the invention, the cross-section enlarges over part of its length in forward direction, i.e. towards the front opening. When combined with the shot plunger, it forms a shot unit, and it is preferred, if the plunger's front surface comprises a conical surface which tapers away from a perimetrical scrapping edge of the plunger, the conical surface having a base of smaller cross-section than the perimetrical edge to form a marginal surface under an angle to said conical surface.

Description: The invention relates to a shot sleeve and a shot unit for a die casting machine, more particularly to a cold chamber die casting machine, especially to those having a horizontal cold chamber or shot sleeve. The invention is particularly intended, although not limited, to the use in casting thixotropic metal. When referring to "die casting machines", this term as used herein should encompass also those machines which often are referred as "forging machines", although forging, in its proper sense, is an operation to process solid metal, while processing semi-solid or thixotropic (sometimes referred as "superplastic") material by forging machines which employ a shot unit is more similar to die casting than to forging. When processing metal either in liquid or in semi-solid form, one problem that may arise is the formation of peripheral shells. Such shells form, for example, when liquid metal of high temperature is poured into a relative cold shot chamber where the periphery tends to solidify more or less, thus forming a peripheral shell which is not desirable and should not enter the cavity of a die. US-Patent No. 4,687,042 shows that a dendritic shell may exist on the peripheral surface of a semi-solid slug. According to the known suggestion the slug is put into a vertical prechamber of the die which has a restricted gate, thereby stripping the dendritic shell and, thus, avoiding heating the slug up to a transformation of the dendrites into globules. However, an investigation of Fascetta et al., published in Die Casting Engineer, September-October 1973, pp. 44-53, entitled "Die Casting of Partially Solidified High-Copper Content Alloys", showed that a dendritic structure tends to be retained in a shot sleeve so that only the interdendritic liquid can-be pressed out of it which, in consequence, leads to bad quality of the parts to be cast. Another problem in connection with a peripheral shell is its contents of oxides and/or other impurities. While dendrites form an interlocking network and, therefore, can be considered to form a kind of felt which, according to the above suggestion can be stripped off like a stocking, oxides and other particulate impurities do not form a network so that they are easier susceptible to be swept into the die cavity causing locations of lower strength and elongation in the shaped part, Although the above-mentioned US-Patent mentions also impurities in general to be retained and discloses also an entrapment ring to receive the stripped impurities, this can work, if it does at all, only in connection with the interlocking network of the dendrites which acts as a kind of filter preventing the oxides on the periphery from entering into the runner system. Without such network, particulate impurities disperse in the metal when becoming liquid due to pressure and shearing forces imposed. Investigations of the inventor have shown that the effect of an entrapment ring is rather random. The JP-59141358 A teaches a casting method to remove oxides and to improve the heat retaining property of a molten metal by dropping a heat insulated material vessel in which the molten metal is charged onto a plunger tip and moving upward the plunger tie after bringing to a top mold into press contact with a bottom mold there by casting the molten metal. The shot sleeve shows a cross-section which enlarges and vessel which is crushed. The JP 0116255 A discloses a horizontal closing, vertical injection die casting machine, teaching an improved productivity by projecting the injection stroke limit position of the plunger chip from the upper side end face of an injection sleeve and rushing it into a biscuit. Due to the space of the biscuit part having the gap caused by the thickness of the sleeve between the outer periphery of a plunger chip and the inner wall periphery of the holes no solidified shell is cut off by the plunger chip. It is therefore an object of the invention to remove a perimetric or peripheral shell from any metal present in a shot sleeve before entering a die cavity and, more particularly, to prevent impurities in form of particles to reach the die cavity. This is achieved according to the invention in a surprising manner by the characteristics of claim 1. The surprising action of such an Arrangement is based on the finding that most of the pistons scrap any shell that might be present in a shot sleeve off the inner walls of the latter, wherein the perimetrical front edge (the "peripheral" if the plunger is more or less cylindrical, as usual, and does not have a polygonal cross-section) of the plunger acts as a scrapper edge. Surprising is that, with a conically enlarging cross-section of the shot sleeve, the plunger can no longer act as a scrapper. However, the present inventor has found that with scrapping the shell off the inner wall, the problem arises where to dispose the scrapped shell. If the known entrapment ring of relative short axial length is used, one can never be sure that the shell enters just that narrow gap in radial outwards direction and does not move the other way radially inwards. Moreover, one has to keep in mind that scrapping is a "dynamic" operation in that, when stripping the shell, it becomes thicker and thicker the more the plunger moves to the front opening. A further advantage is that with a conical biscuit remaining after the shot, it is easier to push it out, thus saving some energy. In contrast, by providing a cross-section which enlarges more and more, this "dynamic" effect is taken into account. Of course, this enlargement can be a stepwise enlargement, although it is preferred if the cross-section enlarges steadily in a tapering shape. The present invention, according to a second aspect concerns also a whole shot unit, i.e. a unit which comprises not only the shot sleeve, but also the shot plunger and its drive, and, optionally, the adjacent parts towards and before the cavity, such as those surfaces which define the sprue runner. In such a shot unit, it has been found that a synergetic effect can be achieved, if the plunger's front surface comprises a conical surface tapering away from the plunger's perimetrical scrapping edge, the conical surface having a base of smaller cross-section than the perimetrical edge to form a marginal surface under an angle to said conical surface. In this way, the tapering front cone acts like the tip of an arrow or like a plough, urging the stripped shell into the more and more enlarging lateral space of the shot chamber. Although plunger of this general type are known, e.g. from US-Patent No. 4,144,734, it will be clear that such a synergetic effect cannot be reached with a mere cylindrical shot chamber. Moreover, it is preferred, if the sprue runner extends substantially in alignment with the direction of displacement of the shot plunger. This has a double effect: On the one hand, the stripping action is rendered more uniform, and, on the other hand, improves the flow under shearing action when semi-solid, i.e. thixotropic, metal is used that becomes liquid only under shearing stress. This uniform application of stress is particularly enhanced if the sprue runner extends substantially in alignment with the longitudinal axis of the shot chamber, i.e. is centrally arranged with respect to the longitudinal axis rather than excentrically. In a special embodiment, the walls or surfaces which define the sprue runner form a hollow conical surface facing the interior of the shot chamber to enhance flow of liquefying semi-solid metal. Such an embodiment can be varied in that this hollow cone matches the shape of the conical front surface of the plunger; this results in a double effect: The interengaging cones form a valve-like closure member which establishes a further means to prevent oxides and other impurities from flowing into the runner system. As another effect, the biscuit can be smaller than usually, thus diminishing the losses in the form of scrap metal. Since the effect of the configuration according to the invention is the accommodation of a doubled shell of impurities, such as oxides, or of pre-solidified metal, there is, according to a third aspect of the present invention, a process where the axial length of the enlargement of the chamber of the shot sleeve is chosen as a function of the axial length of a slug to be shaped in a die casting machine, as will become apparent from the following description. The present invention relates also to a process for removing impurities contained in a circumferential region of a heated slug according to claim 9. Further details will become apparent from the following description of embodiments schematically illustrated in the drawings, in which Figs. 1 - 3 represent three different embodiments according to the invention in three different positions of the plungers in a longitudinal cross-section through the respective shot sleeves. A conventional die casting machine, only part of which is shown in Fig. 1, comprises a stationary die mounting platen 1 onto which a stationary die 2 is mounted in a manner known per se and. therefore, not shown in detail. The stationary die 2 has an insert 3 that, together with an insert 4 of a movable die 5 defines a cavity 6 only part of which is shown in Fig. 1. This cavity 6 is to receive metal in liquid state that enters through a gate 7 of restricted cross-section which is in communication with a chamber 8 of a shot sleeve 9 through a sprue runner 10 and a front opening 15 of the chamber 8. The shot sleeve 9 has an elongated opening 11 through which metal (e.g. a semi-solid slug 12) may be inserted when a shot plunger 13 is in a retracted position at the right side of Fig. 1 beyond opening 11. The shot sleeve has, moreover, a projection or flange 14 for fastening it to the platen 1 before the stationary die 2 is mounted on the platen. In this way, the shot sleeve 9 is clamped in a conventional manner between the parts 1 and 2. The cross-section of the chamber 8 will normally be substantially circular, but other cross-sections, such as polygonal ones, are also known in the art. The plunger 13 has a front surface 16 surrounded by a perimetrical or peripheral edge 17 that forms an angle of 90° with respect to a longitudinal axis A of the chamber 8 and the inner wall 18 thereof. In this way, the edge 17 will act as a scrapper whenever a shell of metal forms along this inner wall 18. This applies also if liquid metal is filled into the chamber 8 through the opening 11 and freezes at the bottom of the chamber 8. Drive for displacing the plunger 13 is imparted via a plunger rod 19 that is connected to a conventional hydraulic drive in a manner not shown. The present inventor has considered that when the marginal or perimetrical edge 17 scraps any shell off which may be formed on the outside of the metal 12, e.g. a shell of peripheral oxides, at least the major part or substantially the entire shell should be prevented from entering the cavity 6 in order not to deteriorate the mechanical properties of the part to be formed. The more the plunger 13 moves towards the front opening 15, the larger the shell quantity scrapped off and, thus, the greater the probability that the impurities of the shell eventually are shot into the cavity. On the other hand, it will be clear that, when the slug 12 is displaced towards the front opening 15 and finally engages the opposite wall of the insert 4 and sprue runner 10, it will finally be subjected to shearing forces which convert the initially almost solid state of the slug 12 into more of a liquid-like state to create enhances flow into cavity 6 while the edge 17, at the same time, begins or continues to scrap off any impurity shell that might be present on the peripheral surface of the slug 12. According to the present invention, this shell would then ply about itself, becoming gradually thicker and thicker so that some space is needed. The suggestion of the prior art to have a relative small (when measured in the direction of the axis A) entrapment ring just near the front opening 15 did not take this fact into account and, therefore, failed to retain the unwanted impurities. According to the present invention, such a surface impurity accommodating space is therefore provided in that the cross-section of the chamber 8 enlarges more and more over a portion 1 of its length towards the front opening 15. The more such shell accumulates the more space is now provided to accommodate it. The enlargement is shown, in principle, in a linear, tapering way, but could comprise at least one step, particularly an initial step 20 starting from the minimum cross-section before the enlargement of the chamber 8 begins. Just an initial step 20 enhances doubling of any shell that might be present on the slug 12, while avoiding any squeezing effect due to a wedge-like configuration between the peripheral surface of the plunger 13 and the enlarging portion of the chamber 8. Furthermore, the tapering angle ( will be chosen according to the axial length of the slug, the thickness of a possibly existing shell, the type of metal used etc. It has been found, however, that this angle (, in practice, should be in a range of 3° to 20° at least over part of its axial length. Most preferred is it when this tapering angle is about 10° ± 5°. Fig. 2 shows a more advanced condition of a modified plunger 13' in a modified front opening and sprue runner system which is more preferred. In this condition, the plunger 13' presses against the end surface of the slug 12 to press it through a restricted front opening 15' which, preferably, aligned with the longitudinal axis A, but could, in principle, be also excentrical relative to this axis A. The plunger 13' has a front surface which forms a conical surface and tapers away from the perimetrical scrapping edge under a second tapering angle β which has, preferably, the same magnitude as the tapering angle α or is, at least, in the same range. As may be seen from Fig. 2, the conical front surface 16' has a smaller cross-section or diameter than the perimetrical edge 17. Therefore, a marginal surface 16" is formed that is under an angle to the conical surface. 16'. Although this configuration is known from the above-mentioned US-A-4,144,734, it has a certain synergetic effect together with the enlarging cross-section of the shot sleeve 9 in that the marginal surface 16" enhances doubling of any shell (12' in Fig. 2) while at the same time the enlarging inner wall provides a space so that doubling is effected to the radial outside rather than to the inside, as might be the case with a conical surface 16? only. To the contrary, the conical surface 16' acts like a plough urging the shell 12' to the side towards the inner wall 18. It should be noted that the plunger 13 may have a marginal edge which joins the conical surface 16' by a rounding 18', as indicated by interrupted lines, and which forms preferably a peripheral groove (also indicated by interrupted lines). In this way, the outer edge 17 (or its tangent to the rounding) will form a tip, when seen in cross-section, which better scraps any shell off, on the one hand, while the rounding 18' folds or doubles it radially outside. This is particularly advantageous, because it promotes the tendency of such shell to be displaced into the radial outer space provided by the enlargement of the chamber within the shot sleeve 9. Nevertheless, the more the plunger 13 approaches the front opening 15', the less is the shape of the shell predictable. Therefore, it is advantageous, if the cross-section of the chamber enlarges more in a section "s" adjacent the front opening 15'. In Fig. 2 this is accomplished by having the angle α enlarged to form an angle α'. However, the invention is not restricted to a mere enlargement of the widening angle, but can also be in the form of a step. Moreover, the angles α and α', rather than forming an edge, can join in a curved manner. It may be seen from Fig. 2 that the angles α and β are measured with respect to a line L that runs parallel to the axis A. Although Fig. 2 shows different magnitudes of those angles α and β, it should be understood that it is preferred if these tapering angles are mirror symmetrical with respect to the line L, at least over part of their axial length, i.e. with exception of the section "s" with the angle α' in the embodiment shown. While the foregoing relates mainly to the peripheral shell 12' of the slug 12, it will be understood that impurities may also cover the front surface thereof. Although this front surface will ordinarily have a much smaller area than the peripheral surface, it is preferable, if the sprue runner 10' comprises an impurity trap formed as a blind hole 21 which is substantially in alignment with the direction of displacement of the shot plunger 13 along its axis A. The sprue runner of Fig. 2 has then a branch conduit 10" leading upwards and sidewards into the cavity (not shown). By the blind hole 21, any impurity that may be on the front surface of the slug, while being pushed directly out of the front opening 15', will engage the inner wall of this blind hole and will, thus, be trapped therein. It should be noted that a squeeze piston 22 may form the back wall of this blind hole 21. Alternatively or cumulatively, cyclone-like traps may be provided along the sprue runner, e.g. along the branch conduit 10". In order to remove any biscuit out of the shot sleeve at the end of the shot, either the front surface of the plunger 13 may have an undercut to grip the biscuit and to tear it off when the plunger 13 is retracting, or the parts delimiting the front opening 15' can be displaced apart to release the biscuit such as by a sliding plate arrangement disclosed in DE-A-19 50 795. Both approaches are well known to those skilled in the art. In the embodiment of Fig. 3, the sprue runner system and trap are much the same as shown in Fig. 2. The conical front surface 16' is, in this case larger as compared with that of Fig. 2, i.e. it has an axial length at which is substantially equal to that of the hollow cone formed by the end section of the chamber that is defined by the shot sleeve 9. Since the axial length at shall act in the manner of a plough, it is preferable if the axial length at amounts to at least 50% of its width b measured normally to the longitudinal axis A. This is much more than the prior art used with the same objective to retain impurities or a pre-solidified shell. It has been found that it is not critical, if the axial length is about as long as the width b or even longer. It is, however, more preferable, if the axial length al amounts to at least 66% of width b, and in the most preferred case, the axial length al amounts to about 70% to 80% of said width. Furthermore, it can be seen that the angles α and β are mirror symmetrical with respect to line L. To facilitate the manufacture, the marginal surface is flat forming a 90° angle with the axis A.. An important modification of the function of the conical front surface 16' resides in that it acts as a kind of valve body that co-operates with a hollow conical surface 24 formed in the piece 23 which defines the front surface 15'. This hollow conical surface 24 faces the chamber of the shot sleeve 9 and forms a predetermined angle with the longitudinal axis A. Preferably this angle corresponds to the angle β so that the cone 16' can (almost) engage the hollow conical surface 24 when the plunger 13 is in the end position shown in Fig. 3. When the term "almost" is used in this connection, it should be noted that, unless there is a very precise control system which controls the displacement of the plunger 13, an adjustment of the plunger has to be made in such a way that its front surface, in the end position, is in some distance to the surface 24. This distance may serve to exert a certain after-pressure to the metal in the sprue runner, but in the present embodiment this can be done by squeezing piston 22. When using a shot sleeve 9 according to the present invention, one should select the length 1 (Fig. 1) in relationship to the axial length of the slug 12 so as to take into account that a possible shell of impurities has to be doubled within the space provided by the enlargement of the chamber 8, on the one hand, and that, in most cases, a biscuit of a certain axial length shall remain (to provide a stock of material for an after-pressure after filling the cavity and before the metal is solidified). Therefore it is preferred, if the length 1 of the shot sleeve 9 amounts to at least the length of the slug 12 used, but, ordinarily, it will be still more favorable if length of said shot sleeve amounting to approximately the two-fold of said predetermined length of said slug, or to accommodate . at least twice the volume of the portion of the metal to be retained. The use of interengaging conical surfaces 16' and 24 reduces the loss of metal due to a biscuit of significant length, on the one hand, and closes the chamber of the shot sleeve 9 so as to prevent safely entering of impurities from this chamber into the runner system, on the other hand. It will be appreciated by those skilled in the art that the invention can be subject of numerous modifications.

Claims: A shot sleeve for a die casting machine comprising: an elongated hollow body (9) for receiving and guiding a shot plunger (13), said body (9) extending along a longitudinal axis (A) and defining an elongated chamber (8) of predetermined cross-section and width, when measured transversely to said longitudinal axis (A), and having a front opening (15) to face a sprue runner (10) of a die (2, 5), said predetermined cross-section enlarging over part (I) of its length towards said front opening (15); and fastening means (14) on said body (9) for holding the body (9) in fixed relationship with said die (2, 5) and said predetermined cross-section enlarges more and more over said part (I) of its length towards said front opening (15), and the fastening means comprise a flange (14) on an outer periphery of said shot sleeve (, 9), the enlargement of said predetermined cross-section being between said front opening (15) and said flange (14); characterized in that : a) the enlargement of said predetermined cross-section starts with a step (20) from a minimum cross-section, wherein said step (20) is in the range of 2 to 8% of its minimum width; b) said chamber (8) includes a first portion of a circular cross-section engaging a scrap edge of the shot plunger (13) and a second portion of circular cross-section, the second portion having a diameter larger than the first portion and the second portion extending a distance along said longitudinal axis (A) greater than the diameter of the first portion; c) the body (9) includes an elongated opening (11) in a sidewall thereof, the elongated opening (11) having a length parallel to said longitudinal axis (A) and width perpendicular to said longitudinal axis (A) sufficient to receive a slug (12) of semi-solid material sized to die cast one or more parts, the enlargement of said predetermined cross-section being sufficient to accommodate oxides of the slug (12) and prevent oxides from passing through the front opening (15) during die casting of the one or more parts. A shot unit for a die casting machine comprising: a shot plunger (13) displaceable along a predetermined path from a shot starting position to an end position, said plunger (13) having a front surface defined by a perimetrical edge (17), and a rear surface; drive means (19) for driving said shot plunger (13) along said predetermined path, said drive means including a plunger rod (19) connected to said rear surface; an elongated hollow shot sleeve (8, 9) for receiving and guiding said shot plunger (13), said shot sleeve (8, 9) defining an elongated chamber (8) of predetermined cross-section along a longitudinal axis (A) and having a front opening (15) averted from said plunger rod (19) to face a sprue runner (10) of a die (2, 5); said predetermined cross-section enlarging over part (I) of its length towards said front opening (15); and fastening means on said sleeve for holding the'sleeve in fixed relationship with said die and said part (I) of the length of the shot sleeve' body (9) enlarges more and more towards said front opening (15), characterized in that said predetermined cross-section forms a tapering angle (α; α') with respect to a line parallel to said longitudinal axis (A), and enlarges monotonously in a tapering shape and said predetermined cross-section enlarges to a greater degree (α') in a section (s) adjacent said front opening (15) and/or the enlargement of said predetermined cross-section starts with a step (20) from a minimum cross-section, said step (20) being in the range of 2 to 8% of its minimum width. Shot unit as claimed in claim 2, wherein at least one of the following characteristics is provided: a) said chamber (8) includes a first portion of circular cross-section engaging a scraping edge (17) of the shot plunger (13) and a second portion (I) of circular cross-section, the second portion (I) having a diameter larger than the first portion and the second portion (I) extending a distance along said longitudinal axis greater than the diameter of the first portion; b) the fastening means comprises a flange (14) on an outer periphery of said shot sleeve (8, 9), the enlargement of said predetermined cross-section being between said front opening (15) and said flange (14); c) the shot sleeve (8, 9) includes an elongated opening (11) in a sidewall thereof, the elongated opening (11) having a length parallel to said longitudinal axis (A) and width perpendicular to said longitudinal axis (A) sufficient to receive a slug (12) of semi-solid material sized to die cast one or more parts, the enlargement of said predetermined cross-section being sufficient to accommodate oxides of the slug (12) and prevent oxides from passing through the front opening (15) during die casting of the one or more parts; d) the sprue runner (10) extends perpendicularly to said longitudinal axis (A), at least a portion of the sprue runner (10) being defined by part of an axial end face of the shot sleeve (Fig. 1). Shot unit as claimed in any of claims 2 or 3, wherein said front surface of said shot plunger (13) comprises a conical surface (16') tapering away from said perimetrical edge (17) under a second tapering angle (β) with respect to a line (L) parallel to said longitudinal axis (A), said conical surface (16') having a base of smaller cross-section than said perimetrical edge (17) to form a marginal surface (16") under an angle to said conical surface (16'), wherein at least one of the following characteristics is preferably provided: a) said first and second tapering angles (α, β) preferably being mirror symmetrical with respect to said line (L), at least over part of their axial length; b) at least one of said first and second tapering angles (α, β) is in a range of 3° to 20° at least over part of its axial length, at least one of said first and second tapering angles (α, β) being preferably about 10° ± 5° at least over part of its axial length; c) the axial length of the conical front surface (16') and of said hollow conical surface (18) are substantially equal; d) the axial end surface of the plunger (13) forms substantially a right angle to said longitudinal axis (A). Shot unit as claimed in any of the preceding claims, wherein said front surface of the shot plunger (13) comprises a conical surface (16') tapering away from said perimetrical edge (17) under a second tapering angle (β) with respect to a line (L) parallel to said longitudinal axis (A), said conical surface (16') having a base of predetermined width (b), when measured normal to said longitudinal axis (A), and an axial length (al) amounting to at least 50% of said width (b), preferably at least 66% of said width (b), particularly about 70% to 80% of said width (b). Shot unit as claimed in any of the preceding claims, further comprising means (23) defining said sprue runner (10') to extend substantially in alignment with the direction of said displacement of the shot plunger (13), at least one of the following characteristics being preferably provided: a) said means (23) defining said sprue runner (10') extend substantially in alignment with said longitudinal axis (A); b) said means (23) defining said sprue runner (10') form a hollow conical surface (24) facing said chamber (8) and forming a predetermined angle with said longitudinal axis (A), said front surface of said shot plunger (13) comprising preferably a conical surface (16') tapering away from said perimetrical edge (17), the conical surface (16') of said front surface and said hollow conical surface (24) of said means (23) defining said sprue runner (10') tapering substantially under the same angle so as to interengage each other when said plunger (13) reaches its end position; c) said means (23) defining said sprue runner (10') comprise an impurity trap formed as a blind hole (21) substantially in alignment with the direction of said displacement of the shot plunger (13). A process for removing impurities contained in a circumferential region of a heated slug in a die casting machine, said process comprising the steps of: heating a slug (12) of a predetermined length; placing the heated slug (12) in a shot sleeve for said die casting machine, said shot sleeve including an elongated hollow body (9) for receiving and guiding a shot plunger 13), said body (9) extending along a longitudinal axis (A) and defining an elongated chamber (8) of predetermined cross-section and width, when measured transversely to said longitudinal axis (A), and having a front opening (15) to face a sprue runner (10) of a die (2, 5); said predetermined cross-section enlarging over part (I) of its length towards said front opening (15); advancing the plunger (13) in the shot sleeve (8, 9) such that a portion of the slug (12) enters the die (2, 5) and forms a shaped part, while another portion of the slug (12) forms a biscuit which remains in the shot sleeve (8, 9), the biscuit being located in the enlarged portion (I) of the chamber (8); and removing a shaped part tram the die (2, 5) and removing the biscuit tram the shot sleeve (8), characterized in that said predetermined cross-section enlarges more and more over a part (I) of its length and over a distance at least about equal to said predetermined length of said slug (12) taking in said biscuit, wherein said biscuit remains as doubled shell.

Inventors: JUNG PAUL, JUNG, PAUL
Applicants: BUEHLER AG
Application year: 1998
Main classification: B22D17
IPC codes: B22D17