GB2056338A - Die-casting method and apparatus - Google Patents

Abstract

A die-casting method and apparatus adapted to be used for production of, for example, housing members of compressors, pumps or the like, wherein a molten metal injected into a forging die cavity (30) may be applied with pressure through a location other than an injection port. This invention has for its object to improve the sliding property of a plunger (36) for applying the forging pressure. The pressure plunger (36) is slidably fitted in a pressure sleeve (35) which has an enlarged portion (35a), said pressure plunger (36) being also provided with a reduced portion (36d), so that the pressure plunger (36) may be smoothly moved with the solidified metal layer being kept in the minute gap formed between the pressure sleeve (35) and the pressure plunger (36). Thus, the flash of the solidified metal layer is prevented from clogging in a portion between the pressure sleeve (35) and the pressure plunger (36), so that the sliding movement of the pressure plunger (36) is kept normal. <IMAGE>

Description

SPECIFICATION Die-casting method and apparatus TECHNICAL FIELD The present invention relates to die-casting method and apparatus and, more particularly, to diecasting method and apparatus in which molten metal is injected into a die cavity and is squeezed also at a portion other than the portion through which the molten metal is injected into the die cavity.

BACKGROUND ART A die-casting method in which molten metal filling up a die cavity is squeezed also at a portion other than the portion through which the molten metal is injected into the die cavity is disclosed in Japanese Laid-Open Publication No. 51-12981 7, for example. In the apparatus disclosed in the publication referred to, the arrangement for the slidable engagement between a squeeze plunger for squeezing molten metal injected into the die cavity and a squeeze passage in which the squeeze plunger is disposed snugly and slidably is merely of a cylindrical form. No special consideration has been given to the arrangement concerned.

It has been found through researches and experiments conducted by the inventors that even though a squeeze plunger and a squeeze passage were in close contacting relationship with each other prior to a squeezing operation, the squeeze passage is slightly elastically deformed and expanded during squeezing operation because of a high pressure which exceeds 100 atm., so that a small gap is formed between the inner peripheral surface of the squeeze passage and the outer peripheral surface of the squeeze plunger. Molten metal penetrates into the small gap to form a fin which sticks to the plunger and which rises a problem of adverse affect on a smooth sliding movement of the squeeze plunger. If a bad sliding movement of the squeeze plunger is caused. the frictional resistance produced when the squeeze plunger is slid is drastically increased to decrease the squeezing pressure to be imparted to the unsolidified metal in the die cavity, resulting in an insufficient squeezing effect. Thus, the squeezing movement of the squeeze plunger cannot result in any successful elimination of the formation of cavities or voids in die-cast products. In addition, if such a sticking of the metal fin to the plunger would be developed, there would be caused a serious problem that the squeeze plunger faiis to move.

DISCLOSURE OF THE INVENTION In view of the above-stated result of experimental researches that an inferior sliding movement of the squeeze plunger causes an insufficient squeezing pressure on the unsolidified metal which in turn results in the formation of voids in die-casting articles, the present invention aims to provide improved die-casting method and apparatus in which the squeezing plunger is advanced for the squeezing operation with a solidified layer of molten metal being held in a small gap between the forward end of the squeeze plunger and the squeeze passage to surely avoid the sticking of metal fin to the outer peripheral surface of the plunger for thereby advantageously avoiding any inferior sliding movement of the plunger whereby squeezing effect on unsolidified metal in the die cavity can surely be maintained for a long period of time even during massproduction to assure elimination of the formation of voids.

In order to achieve the object discussed above, the present invention provides "a die-casting method including the steps of bringing a plurality of dies into close contact with each other to form therebetween a die cavity for casting an article, a runner open at its one end to said die cavity and adapted for introducing a molten metal into said die cavity and a squeeze passage communicating with said die cavity at a portion other than the portion at which said runner is open to said die cavity, injecting a molten metal at a predetermined pressure into said die cavity and said squeeze passage through said runner to fill up said die cavity and said squeeze passage, and moving forwardly a squeeze plunger through said squeeze passage to forcibly displace said molten metal from said squeeze passage into said die cavity to effect a squeeze on said molten metal in said die cavity, wherein, when the molten metal is squeezed by the forward movement of said squeeze plunger, said squeeze plunger is advanced with a solidified layer of said molten metal being held in a small gap formed between the outer peripheral surface of the forward end portion of said squeeze plunger and the inner peripheral surface of said squeeze passage." The present invention also provides a die-casting apparatus for carrying out the die-casting method.

According to the present invention, since the squeezing plunger is advanced with a solidified layer of metal being held in the small gap between the outer peripheral surface of the squeeze plunger and the inner peripheral surface of the squeeze passage, the undesirable sticking of the metal fin to the outer peripheral surface of the squeeze plunger can advantageously be avoided to ensure a smooth sliding operation of the squeeze plunger for a long period of time. Consequently, a sufficient squeezing pressure is always exerted to the molten metal in the die cavity to surely prevent the formation of voids.

BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a sectional view of an example of apparatus used for carrying out the method of the invention; Figs. 2 and 3 are sectional views of a part of the apparatus shown in Fig. 1, showing pressurizing plunger 36 and a metal accumulation space 32, Fig. 2 showing the pressurizing plunger 36 in its fully retracted position, and Fig. 3 showing the pressurizing plunger in its fully advanced position; Fig. 4 is an illustration of the relationship between time lag and amount of the squeezing displacement of metal; Figs. 5(a) and 5(b) are photographs of die-cast structures having surface defects and segregations, respectively; Fig. 6 is an illustration of the relationship between the amount of squeezing displacement of metal and the density of the die-cast product; Fig. 7 is a sectional view of a part of the apparatus, showing a solidified layer ss formed in a squeeze passage 17: Fig. 8 is an illustration of the relationship between the thickness of the solidified layer and the time elapsed after the charging; Fig. 9(a) is a sectional view of a die-cast product produced by the apparatus shown in Fig. 1; Fig. 9lib) is a plan view of the die-cast product shown in Fig. 9(a); Fig. 10 is a photograph of the structure of the product produced in accordance with the method of the invention; Fig. 11 is an illustration showing the difference in density between products produced by the method in accordance with the invention and products produced by a die-casting method which does not include the squeezing of cast metal; and Figs. 12 and 1 3 are sectional views of the forward end portion of a squeeze plunger of another embodiment of the invention, wherein Fig. 12 shows the plunger 36 in its fully retracted position and Fig. 1 3 shows the plunger 36 in its fully advanced position.

BEST MODE FOR CARRYING OUT THE INVENTION The most preferred embodiment of the invention will be described hereinafter. An explanation will be made first with respect to the apparatus for use in carrying out the method of the invention with reference to an illustrated example of the apparatus.

Referring first to Fig. 1, a base 1 of the apparatus is fixedly installed on a foundation such as the floor of a factory by means of studs, not shown. Support members 2, 3 are fixed to the base 1 and stationarily support an injection cylinder 4. The injection cylinder 4 has a cylindrical inner surface 4a which slidably holds an injection piston 5 which is adapted to be moved right and left, as viewed in the drawing, by hydraulic signal pressure applied through first and second hydraulic signal pressure pipes 6, 7 open in the opposite ends of the injection cylinder 4.

The hydraulic signal pressures are supplied by an oil pump, not shown, through an input pipe 8 and is selectively distributed to the first and second signal pressure pipes 6, 7 by means of a solenoid controlled hydraulic pressure switching valve 9. The oil forced out from the injection cylinder 4 by the injection piston 5 is discharged through that signal pressure pipe 6 or 7 through which the signal pressure is not applied, and is returned to the pump (not shown) via the pressure switching valve 9 and an output pipe 10. A pressure switch 11 is disposed at an intermediate portion of the first signal .pressure pipe 6 and is adapted to deliver an electrical signal to a hydraulic pressure switching valve 42.

to be discussed later, when a predetermined pressure level (e.g. a pressure which is 50 to 80% of the maximum injection pressure to be discussed later) is exceeded by the hydraulic pressure in the first signal pressure pipe 6.

The movement of the injection cylinder rod 5 is transmitted to a plunger tip 1 3 through a plunger rod 12, so that the plunger tip 1 3 is slidably moved right and left in a shot sleeve 14, as viewed on the drawing. A pouring port 1 5 opens in the upper wall of the shot sleeve 1 4 at the point which is cleared by the plunger tip 13 in its fully retracted position (shown in Fig. 1). A molten metal, such as an aluminum alloy, magnesium alloy, zinc alloy or the iike, is poured by a pouring apparatus, not shown, into the shot sleeve 1 4 through the pouring port 1 5. Thus, the shot sleeve 14 constitutes a part of the injection passage through which the molten metal is injected. A fixed platen 1 6 is fixed to the base 1 and rigidly holds a fixed die 18. Another fixed platen is provided also at the right-hand end of a tie bar 22, although Fig. 1 shows only one fixed platen 16 located at the left-hand end of the tie bar 22. In order to obtained a minute die shape as well as to ensure easy maintenance, the fixed die 1 8 is constituted by two separate parts; a holding block 1 9 made of ductile cast iron (FCD 55) and an impression block 20 made of a hot tool steel (SKD 61). The holding block 19 and the impression block 20 are rigidly connected to each other by means of hexagon socket-headed bolts 21. The aforementioned shot sleeve 1 4 extends through the fixed platen 1 6 and the holding block 1 9 and opens in one end face of the latter.

Two tie bars 22 are fixed to each of the upper and lower portions of the fixed platens 16. These tie bars 22 extend through a movable platen 23. The movable platen 23 are snugly and slidably received on tie bars 22 and are adapted to be moved along the base 1 to the right and left as viewed in the drawing by a driving power of a piston not shown.

A movable die 26 is fixed to the movable platen 23 through a side fixing plate 24 and upper and lower fixing plates 25, 25. As is the case of the fixed die 18, the movable die 26 is composed of two parts: a movable holding block 27 made of ductile cast iron (FCD 55) and a movable core 28 made of a hot tool steel (SKD 61), which are connected to each other by means of bolts 29.

As the movable platen 23 is moved by the piston not shown, the movable die 26 is brought into close contact with the fixed die 1 8. The two dies are shaped such that they define therebetween a die cavity 30 for die-casting the product. a runner 31 through which the molten metal is injected into the die cavity 30 and a squeezing passage 17 which opens to the cavity 30 at a portion of the latter remote from the runner 31. Gaps of from 0.1 mm to 0.5 mm are formed in the abutment surfaces of the fixed and movable dies 1 8 and 26 to define air vents 33 through which the air forced by the injected molten metal is relieved from the cavity 30. The ends portion of the runner 31 adjacent to the cavity 30 is restricted to form a gate 34 so that the molten metal supplied from the runner 31 is injected into the cavity 30 at a high velocity.

A squeeze sleeve 35 is press-fitted into the central part of the movable core 28 so as to be positioned opposite substantially to the center of the die cavity 30. This squeeze sleeve 35 has a cylindrical shape and is made of a hot tool steel (SKD 61). The squeeze sleeve 35 closely and slidably receives a squeeze plunger 36 which is also made of hot tool steel (SKD 61). The aforementioned squeeze passage 17 is defined by the portion of the inner peripheral surface of the squeeze sleeve 35 extending beyond the inner end surface of the squeeze plunger 36. By the squeeze plunger 36, the molten metal filling the squeeze passage 1 7 is forced out to a portion of the cavity 30 opposed to the squeeze passage 1 7 (this portion will be called hereunder "molten metal accumulation space 32"). For easy maintenance, the squeeze plunger 36 is composed of two members 36a, 36b which are connected to each other by a connecting ring 37, so that only the part slidably movable in the squeeze passage 35 can be replaced.

Figs. 2 and 3 show the squeeze sleeve 35, the forward end of the squeeze plunger 36 and the molten metal accumulation space 32. As will be seen in Figs. 2 and 3, the forward end portion 35a of the squeeze sleeve 35 of the apparatus of the invention has an inner diameter which is somewhat (0.1 mm, for example) increased over that of the other portion of the squeeze sleeve 35. In relation to the diameter d of the squeeze plunger 36, the diameter D of this increased-diameter portion 35a is selected preferably to fall within a range given by: d + 0.05 mm < D < d + (d x 0.05) mm Due to the presence of this increased-diameter portion 35a, the solidified layer formed on the surface of the molten metal takes a form of a ring-shaped film A which, when the squeeze plunger 36 is advanced, is positioned in a small annular gap 35b provided by the increased-diameter portion 35a.

The squeeze plunger 36 is slidably moved in the squeeze sleeve 35 with the thin filmA being held between the outer peripheral surface of the forward end portion of the squeeze plunger 36 and the inner peripheral surface of the squeeze sleeve 35. The length y of the increased-diameter portion 35a is determined dependent on the amount of displacement of the molten metal effected by the squeeze plunger. More specifically, the increased-diameter portion 35a preferably extends to the forward end 35e of the squeeze sleeve 35, i.e., to the end of the squeeze sleeve 35 adjacent to the die cavity 30, from a point 35d which is forwardly displaced a predetermined distance x (this distance x is not greater than 10 mm and, preferably, is from 2 to 3 mm) from a point 35c which is occupied by the squeeze plunger 36 in its fully retracted position (shown in Fig. 2). In practical point of view, however, no problem will be caused by eliminating the distance x by increasing the length y in such a manner that the point 35d is displaced to a point which is slightly rearward of the fully retracted position 35c of the squeeze plunger 36.

Preferably, the stroke of the squeeze plunger 36 is so determined that the end surface of the squeeze plunger 36 does not project beyond the forward end 35d of the squeeze sleeve 35 even when the squeeze plunger 36 is in its fully advanced position 35f (shown in Fig. 3), so that the squeeze plunger 36 does not project directly into the metal accumulation space 32 of the die cavity 30. In practical point of view, however, no problem is caused if the squeeze plunger 36 projects slightly into the space 32.

In the illustrated embodiment, the space 32 is formed at a portion of the cavity 30 around an area opposite to the squeeze passage 1 7. The size of the space 32 is so selected that the area of the crosssection of the space 32 perpendicular to the travel of the squeeze plunger 36 is greater than the crosssectional area of the bore of the squeeze sleeve 35. The metal solidified in this space 32 is usually cut and removed from a resultant die-cast article.

A squeeze piston 38 is connected to the outer end of the squeeze plunger 36 and is adapted to slide within a squeeze cylinder 39 so as to advance and retract the squeeze plunger 36. As is the case of the injection cylinder 4, third and fourth hydraulic signal pressure pipes 40,41 are open in the squeeze cylinder 39. A solenoid-controlled oil pressure switching valve 42 is adapted to control the transmission of the signal pressure from an oil pump (not shown) to the signal pressure pipes 40, 41 thereby to control the movement of the squeeze plunger 38. This squeeze cylinder 39 is fixed to the fixing plate 24 by means of bolts 43 so that the cylinder 39 is movable together with the movable die 26.

Ejector pins 44 extend through the holding block 27 and the movable core 28 and have ends which are exposed to the die cavity 30 from the surface of the movable core 28. These ejector pins are adapted to separate and eject from the movable die 26 a die-cast product solidified in the cavity 30 after the movable die 26 is retracted to open the die. These ejector pins are driven to the right and left as viewed in the drawing by an ejector piston 49 and through an ejector plate 45, ejector rods 46, ejector plate 47 and an ejector actuating rod 48. These ejector rods 46 are slidably received by respective bores (not shown) formed in the holding block 27. The ejector cylinder rod 49 is adapted to slide within an ejector cylinder 50 in which are opened fifth and sixth hydraulic signal pressure pipes 51, 52, as is the cases of the injection cylinder 4 and the squeeze cylinder 39. A solenoid-controlled oil pressure switching valve 53 is adapted to control the hydraulic signal pressure from an oil pump (not shown) thereby to effect the forward and rearward movement of the ejector cylinder rod 50.

Hereinafter, the sequential steps of die-casting method of the invention will be described in detail.

[First step] The movable platen 23 is moved to the left as viewed in Fig. 1 by driving a piston which is not shown, so as to bring the movable die 26 into intimate contact with the fixed die 1 8. thereby to form the die cavity 30 for the die-casting of a product. runner 31, squeeze passage 1 7 and air vents 33.

[Second Step] Molten metal is poured from a pouring device, not shown, through the pouring port 1 5 into the shot sleeve 14 and further into a part of the runner 31. Then, the oil pressure switching valve 9 is operated to direct the signal pressure to the first signal pressure pipe 6,a so that the injection piston 5 (and, accordingly, the plunger tip 13) are advanced at a predetermined pressure which is determined by the level of the signal pressure. By this forward movement of the plunger tip 1 3, the molten metal in the shot sleeve 1 4 is forced into the runner 31 and is injected to fill up the die cavity 30 and the squeeze passage 1 7. The injection is made at a high velocity because the molten metal is accelerated when it passes through the gate 34. The level of pressure applied to the molten metal in this step (i.e., the injection pressure) is 500 to 1 500 atm. The air present in the cavity 30 and the metal accumulation space 32 would cause undesirable cavities or voids in a resultant product if the air is entrapped in the molten metal at the injection stage. Therefore, a part of air stayed in the die cavity 30 and the squeeze passage 1 7 is relieved through the air vents 33 disposed at predetermined points of the abutment surface of the movable and fixed dies 26 and 1 8.

[Third Step] After the filling up of the die cavity with the molten metal, the squeeze plunger 36 is driven to forcibly displace the molten metal from the squeeze passage 1 7 into the space 32 before the molten metal in the gate 34 is solidified.

If the time period from the moment when the filling of the die cavity is completed to the moment when the squeeze is commenced (this time period will be referred to hereinafter as "time lag") is too long. the molten metal in the die cavity would be solidified. The solidified layer formed during this period of time lag is not squeezed and thus cannot be free from the production of cavities or voids, with the result that the resultant die-cast product includes portions which fail to provide sufficient strength and airtightness. Such cavities or voids, if formed once, must be removed or eliminated by squeezing the solidified layer at a very high pressure. In other words, for a given squeeze pressure, the increase in the time lag results in the effectiveness of the squeeze. This fact has been confirmed also through experiments made by the present inventors on the relationship between the time lag and the squeezing displacement of metal, the result of the experiments being shown in Fig. 4. In Fig. 4, a full line curve L shows the result of the experiment conducted at a squeezing pressure of 2750 Kg/cm2, while a dot-anddash curve M and a broken-line curve N respectively show the results of experiments conducted at squeezing pressures of 2125Kg/cm2 and 1500 Kg/cm2.

Further, when the time lag is too long, the solidified layer of the metal is shorn by the squeezing operation, so that the resultant die-cast product is liable to involve surface defects which undesirably lower the mechanical strength of the product. in addition, the metal which has been crystallized before the squeezing is locally concentrated to cause a segregation. The segregation adversely affects the workability (particularly for cutting? of the product and makes it difficult to precisely work the product.

Figs 5(a) and 5(b) are photographs of structures of die-cast products having surface defects and segregations, respectively. These faults were both observed in the products produced with too long time lags.

It is, therefore, desirable to shorten the time lag as much as possible in order to avoid the surface defects and segregations.

In the described embodiment, the time lag is shortened by controlling the timing of commencement of the movement of the squeezing plunger 36 in the following manner.

Namely, when the die cavity 30 and the squeeze passage 1 7 have been completely filled with the molten metal, the forward movement of the injection plunger 13 is stopped with a resultant abrupt pressure rise in the first signal pressure pipe 6. Then, the pressure rise in this pipe 6 is detected by the pressure switch 11. The pressure switch 11 is adapted to deliver an electric signal to the oil pressure switching valve 42 when the pressure in the first signal pressure pipe 6 is increased beyond a predetermined pressure level. The oil pressure switching valve 42 then switches the transmission of the signal pressure to the third signal pressure pipe 40. It will be understood that, with the above-stated arrangement, it is possible to actuate tI,e squeeze plunger 36 promptly (usually about 0.5 second or so) after the completion of the injection.

With the die-casting machine having the above-described construction, it usually takes about 5 to 6 seconds for the molten metal in the gate 34 to be solidified completely. Thus, according to the time lag employed in the described embodiment of invention, the squeeze action of the squeeze plunger 36 is commenced in a period of time which is sufficiently short as compared with the time required for the complete solidification of the molten metal in the gate 34.

Fourth Step] As the squeeze plunger 36 is driven promptly, the molten metal in the squeeze passage 1 7 is forced into the space 32 to displace the molten metal from the space 32. The pressure exerted to the molten metal in the squeeze passage 1 7 causes the molten metal to flow back not only into the die cavity 30 but also toward the runner 31 and the shot sleeve 1 4 to also squeeze the molten metal in the runner and the shot sleeve because the molten metal present in the gate 34 is still unsolidified at this time.

Therefore, a squeezing displacement of molten metal equal only to the amount of metal required for the compensation of the shrinkage of molten metal in the die cavity 30 and the squeeze passage 17 is insufficient.

The inventors have made a series of experiments to examine the densities of the die-cast products obtained under various squeezing displacements of molten metal. A tendency was observed in the results of the experiments, as shown in Fig. 6 wherein points shown by A represent the densities of products produced by a die-casting method without squeezing step, while points shown by 0 represent the densities of the products obtained by the die-casting method of the invention, i.e., the densities of the bodies of the die-cast products from which the parts solidified in the squeeze passage 1 7 and the runner 31 have been cut away. p0 represents the true density of the metal used for the die-casting (in the illustrated example, die-casting aluminum alloy was used), while V0 represents the maximum squeezing displacement of molten metal which is determined by the area of the pressurizing surface of the squeeze plunger 36 and the maximum stroke of the squeeze plunger 36.

From Fig. 6, it will be seen that the density of the product is increased up to a predetermined squeezing displacement of metal V, (this region will be referred to as "first region 0", hereinafter).

Within a region between the above-mentioned predetermined displacement V, and the maximum displacement VO, densities of the products are substantially close to the true density pO. This region will 'be referred to as "second region P", hereinafter. At the maximum squeezing displacement V0 of the molten metal, there appears a variety of product densities ranging from a value substantially equal to the density of the non-squeezed die-casting to a value substantially equal to the true density p0 (this region will be referred to as "third region Q", hereinafter).

The variety of the produdt densities observed at the third region Q is believed to be due to the fact that the actual squeezing pressure in the die cavity 30 varies with different pressures applied by the squeeze plunger 36, even with the same squeezing displacement of molten metal. Namely, when the squeezing pressure exerted by the squeeze plunger 36 is unnecesarily high, the injection plunger tip 1 3 is forcibly moved back. Since the plunger tip 1 3 usually has a much larger diameter than that of the squeeze plunger 36 and thus, if the plunger tip 13 is forced back, the squeeze plunger 36 is instantaneously moved to its inner stroke end without effecting a substantial squeeze on the molten metal in the die cavity 30. As a result, even with the same squeezing maximum displacement VO, the densities of the products largely fluctuate depending on whether the backward movement of the injection plunger tip 13 takes place or not, and on the degree of progress of the solidification attained at the moment when the backward movement of the injection plunger tip takes place.

It will be seen that the squeezing displacements of molten metal should preferably fall within the second region P.

The inventors have investigated the minimum value of squeezing displacement of molten metal V, which falls within the second region P. As a result, it has been found that there is a relationship expressed by the following equation; Po - P Va . a (1 PO where Va represents the amount of molten metal in the die cavity 30 and the squeeze passage 17; and p represents the mean value of the densities of products obtained by die-casting without squeeze, as indicated by A in Fig. 5.

Namely, this predetermined displacement V, is of the value at which the squeezing pressure imparted by the squeeze plunger 36 balances the force which is the sum of the injection pressure imparted by the injection plunger tip 13, flow resistance imparted by the gate 34 and other counteracting forces. In other words, the above-mentioned predetermined displacement is of the amount which is required to assure that the molten metal filling up the die cavity 30 and the squeeze passage 17 is solidified within the die cavity 30 without being caused to flow back into the runner31 through the gate 34. For making thepractical value V of squeezing displacement of molten metal coincident with the predetermined amount V1 obtained by the equation (1), how where K represents a maximum squeezing molten metal factor approximately equal to 1 (one). The factor K has been determined to be approximately equal to 1 for the following reasons. Namely, a too large maximum amount of squeezing displacement of molten metal would require an excessively high load on the squeeze piston 38 as well as impractically large sizes of the squeeze plunger 36 and the metal accumulation space 32. Thus, taking into consideration the difficulty in designing the die-casting machine and also the yield of the material (ratio of the amount of molten metal solidified in the die cavity 30 to the total amount of molten metal injected by the injection plunger tip 13), it is not preferred to employ a too large maximum amount V0 of the squeezing displacement of molten metal.

Thus, the practical amount V of squeezing displacement of molten metal should be greater than the amount determined by the equation (3) but smaller than the amount v0 determined by the equation (4). The practical amount V is, therefore, given by the following equation: #o - # #o - # V= Va + Vb # K (5) #o #o where K represents a practical squeezing molten metal factor which ranges from 0.3 to 1.

As will be understood also from the foregoing explanation, it is necessary to set the squeezing pressure by the squeeze plunger 36 at a predetermined level in order to obtain a squeezing displacement of metal which falls in the second region P shown in Fig. 6. Namely, a too small squeezing pressure will result in such an insufficient squeezing displacement of metal as is the case of the first region 0. On the other hand, a too large squeezing pressure will undesirably force back the injection plunger tip 13, resulting in a squeezing in the third region P.

Therefore, a minimum pressure Pain. is required which is at least high enough to force the part a of molten metal from the squeeze passage 17 into the space 32. This minimum pressure Pain. must be higher than the injection pressure P0 exerted by the injection plunger tip 13, by a value which corresponds to the sum of the frictional resistance produced by the friction caused between the inner wall of the squeeze sleeve 35 and the solidified layer ss (See Fig. 7) in the squeeze passage 17 during the forward movement of the squeeze plunger 36, and of the resistance produced as a result of the shearing deformation of the solidified layer P formed at the inner end 35e of the inner peripheral surface of the squeeze sleeve 35.

Namely, the minimum pressure Pain. is given by the following equations: Pmin##r = Po#r2 + Po#2#r#L# + 2#r##2##(t1)## (6) PO(r + 2L,u) + 2#2#(t1)## Pmin = (7) r where r represents the radius of the squeeze plunger 36, while L represents the length, in the direction of movement of the plunger 36, of the area of contact between the solidified layer ss in the squeeze passage 17 and the inner peripheral surface of the squeeze sleeve 17; The symbol represents the coefficient of sliding friction between the squeeze plunger 36 and the squeeze sleeve 35; The coefficient U in the described apparatus was found to be 0.3 and usually is between 0.2 and 0.4. :(t1) represents the thickness of the solidified layer ss measured t, seconds after the filling of the squeeze passage 17; The symbol T represents the magnitude of stress which is required for shearing the solidified layer p and which ranges from 2 to 3 Kg/cm2 in the case of an aluminum alloy.

The inventors have made experiments under various squeeze pressures to seek for the relationship between the time t elapsed from the moment of completion of die filling and the thickness E of the solidified layer ss. As a result, a tendency as shown in Fig. 8 was observed. Through the experiments for obtaining the tendency as shown in Fig. 8. it has been found that the thickness r (t = 0.5) is about 1 mm in the case where the time lag t is 0.5 second.

The thickness of the shearing surface y was determined to be - E(t,) because the shearing surface y is produced in a direction which is at an angle of 450 to the thicknesswise direction of the solidified layer,3 in the case where the molten metal is aluminum.

The squeeze plunger 36 is allowed to move forward if the pressure is determined to exceed the minimum pressure Pm,n obtained by the above equation. Once the forward movement of the squeeze plunger 36 is started, the length L of the surface of the contact is decreased, so that the pressure required for the squeezing is maintained higher than the minimum pressure Pmin - On the other hand, the upper limit or maximum allowable pressure P is the pressure which is the highest within such a range of pressure as would not cause a backward movement of the injection plunger tip 13. The pressure actually transmitted to the injection plunger tip 13 is lower than the pressure Pa imparted by the squeeze plunger 36. by a pressure corresponding to the pressure drop AP caused when the molten metal passes through the gate 34 and other part. Therefore, this pressure may be of such a level as not to shear the solidified layer ss formed around the inner end of the injection plunger tip 13. More specifically, it is necessary that a balance of pressures at the end of the injection plunger tip 13 is obtained as follows: Po#R2=(Pa-#P)#(R-#(t2))2-2#(R-#(t2))##2##(t2)## (8) where R is the radius of the plunger tip 13.

Then, the following equation is derived from the equation (8):

In addition, the relationship between the pressure Pa of the molten metal squeezed by the squeeze plunger 36 and the maximum pressure of the same plunger 36 is represented by: Pmax###r2=Pa#r2+Pa#2#r#L'# +2#r#2 #(t2)##(t2) (10) 1200

where L' represents the length of the surface of contact at the time of t2.

Therefore, the maximum pressure of the squeeze plunger 36 is given by the following equation:

However, in actual use of the method of the invention in an industrial scale, it is thought that, if the maximum pressure P max determined by the equation (12) is used, the squeezing pressure would in many cases be unduly high to produce fluctuations of the pressure drop AP, thickness E of the solidified layer ss and so on in die-casting certaih kinds of products. It is therefore necessary that the practically used maximum pressure Pm,X' is made smaller than the maximum pressure P max obtained from the above equation. The pressure drop AP is difficult to quantitatively determine as compared with other factors.

Therefore, the pressure obtained by subtracting the term of pressure drop AP, i.e., (r + 2L)AP from the maximum pressure P max obtained by the above equation is used as the practically usable maximum pressure Pmax'.

In the determination of the thickness E of the solidified layer ss and the shearing stress T, the results of experiments made by the present inventors show that a practical value of the time t2 after the filling of dies can be made approximately equal to the time required by the squeeze plunger 36 to travel to the midway of the squeeze passage 17.

The molten metal in the squeeze passage 17 is squeezed at a predetermined pressure between the aforementioned minimum pressure Pm,n and the maximum pressure Pm,X. The squeezing pressure is maintained until the solidification in the die cavity 30 and the metal accumulation space 32 is completed, i.e., until the molten metal on the side of the gate 34 adjacent to the die cavity 30 is completely solidified.

The most characterized feature of the invention resides in that the smoorn sliding movement of the squeeze plunger 36 in this fourth step is assured for a long period of time to ensure that the level of the squeezing pressure imparted by the squeeze plunger 36 is maintained within the afore-mentioned predetermined range. For this purpose, the increased-diameter portion 35a is provided at the end of the squeeze passage 17 adjacent to the die cavity 30. Due to the presence of this increased-diameter portion 35a, an annular small gap 35b is formed between the outer peripheral surface of the forward end of the squeeze plunger 36 and the inner peripheral surface of the squeeze passage 17. Therefore, the squeeze plunger 36 is advanced with a ring-shaped thin film A of solidified layer being held in the small annular gap. Consequently, the solidified layer is prevented from penetrating between the intimately contacting sections of the squeeze plunger 36 and the squeeze sleeve 35. More specifically, the portion of the squeeze sleeve 35 at the squeeze passage 1 7 is elastically deformed and expanded to increase its inner diameter due to the pressure existing in the squeeze passage 1 7, so that the solidified layer of the metal in the squeeze passage 1 7 tends to be forced into a small gap between the squeeze plunger 36 and the squeeze sleeve 35. Hpwever, the velocity of the forward movement of the squeeze plunger 36 is maximum in the beginning of the forward movement. Therefore. the squeeze plunger 36 travels the afore-mentioned distance x shown in Fig. 2 at a high speed without providing any time period for the solidified layer to get into the small gap between the squeeze plunger and the squeeze sleeve. Therefore, the solidified layer of metal is forced with the squeeze plunger 36 into the increaseddiameter portion 35a to form the ring-shaped thin filmA as shown in Fig. 3 around the forward end of the squeeze plunger 36. This ring-shaped thin film A is caused to have a structure similar to a forged structure due to the pressurizing action of the squeeze plunger 36 and, thus, acts as a good seal preventing leak of the molten metal. This thin metal film A is moved with the squeeze plunger 36 forwardly to the position shown in Fig. 3 while the metal film is held between the plunger 36 and the inner peripheral surface of the increased-diameter portion 35a, to reliably eliminate an inferior sliding movement of the squeeze plunger 36 which would otherwise be caused by the penetration of a solidified metal layer between closely contacting sections of the squeeze plunger 36 and the squeeze sleeve 35.

The predetermined distance x shown in Fig. 2 should preferably be not greater than 10 mm because, if the distance x exceeds 10 mm, the squeeze plunger 36 will take a considerably long time to move over this distance to disadvantageously allow a penetration of a solidified metal layer. In the case where this distance x is eliminated and the fully retracted position 35c of the squeeze plunger 36 is located within the region of the increased-diameter portion 35c, the forward end of the squeeze plunger 36 projects into the increased-diameter portion 35a even at the beginning of a squeeze stroke of the plunger. If the squeeze plunger projects into the increased-diameter portion a too large distance, it is likely that the solidified thin metal layer is broken. It is, therefore, necessary that the end 35dof the increased-diameter portion 35a is located in the vicinity of the fully retracted position 35c of the squeeze plunger 36.

[Fifth Step] After the injected metal on the side of the gate 34 adjacent to the die cavity 30 is solidified, any further application of pressure by the squeeze plunger 36 will not be effective to squeeze the metal.

Thus, the oil pressure switching valve 42 is operated to feed the signal pressure now to the fourth signal oil pressure pipe 41 thereby to retract the squeeze plunger 36.

The time required for the solidification of the metal in the die cavity 30 varies with the volume and spatial height of the die cavity. It is therefore preferred to experimentally retract the squeeze plunger 3'6 at various timings to preliminarily measure the time required for the solidification and to operate the oil pressure switching valve 42 by means of a timer after the elapse of a time period which is the sum of the above measured time and a predetermined additional time (which may be 1 or 2 second). The thin film A has a strength similar to that of a forged structure and also has a reasonable thickness. This film therefore, is never broken when the squeeze plunger 36 is retracted.

[Sixth Step] After the retraction of the squeeze plunger 36, the piston not shown is actuated to move the movable platen 23 to the right as viewed in Fig. 1 to separate the movable die 26 from the fixed die 1 8.

The separation of the movable die 26 may be made at such a timing when the outer surface of the molten metal on the side of the injection passage has been solidified to such an extent as to maintain the shape of the die-cast product. In the described embodiment, the movable die 26 is separated at a timing of 0.5 to 1 second after the retraction of the squeeze plunger 36.

The pressure signal applied to the first signal pressure pipe 6 is still maintained when the movable die 26 is separated. so that a die-cast product solidified in the shot sleeve 1 4 may be forced out therefrom.

Then, the signal pressure is switched to the second signal pressure pipe 7 by the oil pressure switching valve 9 thereby to retract the injection plunger tip 1 3. Subsequently, the pressure switching valve 53 is operated to switch the signal oil pressure to the fifth signal pressure pipe 51 so as to move the ejector piston 49 to the left as viewed in Fig. 1. This leftward movement of the ejector piston 49 is transmitted to the ejector pins 44 through the ejector actuating rod 48, ejector plate 47, ejector rods 46 and the ejector plate 45. As a result, the die-cast product which has been solidified in the die cavity 30, runner 31 and the squeeze passage 1 7 is ejected by the ejector pins 44.

The process of the die-casting method of the invention is now completed. The product obtained by this die-casting method has a shape as shown in Figs. 9(a) and 9(b). After the die-casting. the portions which have been solidified in the shot sleeve 14, the runner 31 and the air vents 33 (portions R shown by grid-pattern hatching in Figs. 9(a), 9(b)) are cut away by a press and the portion which has been solidified in the metal accumulation space 32 (portion S shown by grid-pattern hatching in Figs. 9(a), 9(b)) is removed by maching to complete a product.

It is possible to use a part or the whole of the portion S solidified in the space 32 as a part of the final product. This portion, however, is preferably removed by cutting for the following reason.

The molten metal in the space 32 is directly squeezed by the squeeze plunger 36 and the solidification proceeds under this condition. The solidified layer p generated in this portion is thus subjected to shearing before it grows sufficiently, with resultant occurrence of undesirable surface defects. In addition, since the time required for the crystallization of molten metals varies with kinds of the metals cast, the metal which is still in fluid state is forced out of the space 32 by the squeezing plunger 36 while the metal which has been crystallized in the space would remain therein, with resultant generation of segregation.

As stated before, the surface defect and segregation adversely affect the strength and workability of the die-cast product. Thus, the portion solidified in the well 32 should preferably be removed particularly in the cases where the die-cast product is intended for used under a high pressure or is subjected to precision-working.

In contrast to the above, the portion solidified in the die cavity 30 does not include any faults such as surface defect and segregation because the molten metal in the die cavity 30 is not directly squeezed by the squeeze plunger 36. Fig.10 is a photograph showing the structure of the portion of the die-cast product solidified in the die cavity 30. It will be seen also in this drawing that the die-cast product produced by the die-casting method of the invention is free from the faults such as cavities or voids, surface defect, segregation and so forth.

Fig. 11 shows the distribution of densities (marked at O) of products of an aluminum alloy, produced by the die-casting method of the invention and the distribution of densities (marked at O) of products of a similar aluminum alloy produced by the conventional die-casting method without squeezing step. The density distribution was measured by cutting each die-cast product into 136 pieces, measuring the densities of respective pieces, and counting the number of pieces belonging to each of a plurality of density values. The numbers of pieces counted for respective density values are shown in Fig. 11. As will be clearly seen from Fig. 11, the product obtained by the die-casting method of the invention has a density value which is approximately close to the true density. In addition, the generation of voids or cavities which most adversely affect the mechanical strength and gas-tightness is avoided almost completely by the present invention.

Figs. 1 2 and 13 show another embodiment of the invention in which the squeeze plunger is provided, at its forward end, with a reduced-diameter portion 36d of an axial length Z so that a small gap 35b is formed between the reduced-diameter portion 36d and the squeeze sleeve 35 so as to effectively prevent the sticking of the solidified layer to the outer peripheral surface of the squeeze plunger 36. In relation to the diameter d of the squeeze plunger 36, the diameter D' of the reduceddiameter portion 36d is determined preferably to fall within a range given by: d-(d x 0.05) mm < D' < d-0.05 mm The reduced-diameter portion 36d can be tapered or conical other than being cylindrical.

Also, the end face 36c of the squeeze plunger 36 is not limited to flat shape and may be of convex, concave or any other desired shape.

Needless to say, it is not essential for the invention to dispose the squeeze plunger 36 in the movable die 26. The squeeze plunger may alternatively be incorporated in the fixed die 1 8 installed for sliding movement along the abutment surfaces of the fixed and movable dies 18 and 26.

7. INDUSTRIAL APPLICABILITY The die-casting method of the invention, which can remarkably suppress and diminish the generation of cavities or voids which adversely affect the gas-tightness and mechanical strength of the cast products, can suitably and effectively be applied to the production of articles which are intended for use under high pressure and products which must be precisely worked. The method may be applied to the manufacture of, for example, housings of compressors. pumps and so on.

Claims (5)

1. A die-casting method including the steps of bringing a plurality of dies into close contact with each other to form therebetween a die cavity for casting an article, a runner open at its one end to said die cavity and adapted for introducing a molten metal into said die cavity and a squeeze passage communicating with said die cavity at a portion other than the portion at which said runner is open to said die cavity, injecting a molten metal at a predetermined pressure into said die cavity and said squeeze passage through said runner to fill up said die cavity and said squeeze passage, and moving forwardly a squeeze plunger through said squeeze passage to forcibly displace said molten metal from said squeeze passage into said die cavity to effect a squeeze on said molten metal in said die cavity, wherein, when the molten metal is squeezed by the forward movement of said squeeze plunger, said squeeze plunger is advanced with a solidified layer of said molten metal being held in a small gap formed between the outer peripheral surface of the forward end portion of said squeeze plunger and the inner peripheral surface of said squeeze passage.

2. A die-casting apparatus comprising: a die cavity for die-casting an article, said die cavity being formed by a plurality of dies placed in close contacting relationship with each other; a runner open at its one end to said die cavity and adapted to introduce a molten metal into said die cavity; a squeeze passage communicating with said die cavity at a portion other than the portion where said runner is open to said die cavity; a squeeze plunger snugly and slidably disposed in said squeeze passage and adapted to forcibly displace the molten metal from said squeze passage back into said die cavity to effect a squeeze on said molten metal in said die cavity: and an increases-diameter portion formed in the inner peripheral surface of said squeeze passage and extending from the end of said squeeze passage adjacent to said die cavity to a point located in the vicinity of a point occupied by said squeeze plunger in its fully retracted position.

3. A die-casting apparatus as claimed in Claim 2, wherein said squeeze plunger when in its fully advanced position does not project beyond the end of said increased-diameter portion adjacent to said die cavity.

4. A die-casting apparatus as claimed in Claim 2 or 3, wherein, relative to the diameter d of said squeeze plunger, the diameter D of said increased-diameter portion is selected to fall within a range defined by: d + 0.05 mm < D < d + (d x 0.05) mm

5. A die-casting apparatus comprising: a die cavity for die-casting an article, said die cavity being formed by a plurality of dies placed in close contacting relationship with each other; a runner open at its one end to said die cavity and adapted to introduce a molten metal into said die cavity; a squeeze passage communicating with said die cavity at a portion other than the portion where said runner is open to said die cavity; and a squeeze plunger snugly and slidably disposed in said squeeze passage and adapted to forcibly displace the molten metal from said squeeze passage back into said die cavity to effect a squeeze on said molten metal in said die cavity; said squeeze plunger having a forward end portion of a reduced diameter and of a predetermined length.