A MOLD CLAMPING DEVICE

Abstract: Clamping cylinder 22 slidably receives a clamping ram 26 whose front end is fixed on a movable board 20 of (e.g.) an injection molding machine (21,23-25) and which has a chamber opening in its rear face. The interior of the clamping cylinder 22 is divided into a rear chamber W and a front chamber X by the large-diameter piston section of the clamping ram 26. From the inner rear wall of the clamping cylinder 22 there projects, a mold opening piston having a front end which slidably fits in the chamber in the clamping ram 26. The opening piston has a large-diameter piston section which divides the chamber into a rear chamber Y and a front chamber Z which communicates with the atmosphere. A hydraulic controlling device 31-37 controls the connection of the chambers X,Y and Z to each other and to an oil pressure source 40. Thus it connects the chambers X and Y to the chamber W for high-speed mold closure. It connects the chamber W to the oil pressure source 40 and relieves the pressure in the chambers X and Y for tight clamping; and it connects the chamber Y to the oil pressure source and connects the chamber W to both the chamber X and the relief means 32 for opening the mold. Preferably, the effective pressure receiving area of the chamber Y is to be equal to the cross-sectional area of the chamber section of the clamping ram 26.

Description: This invention relates to a mold clamping device, e.g. for injection molding machines and die casting machines. Direct pressure type mold clamping devices for injection molding machines and die casting machines are required to open or close molds at a high speed for reducing the time of the molding cycle. In addition, it is also necessary to clamp molds with a large force to oppose the molding pressure. To satisfy these opposing demands, this particular type of mold clamping device generally has a complex structure. Further, many kinds of mold clamping devices are available. For example, a booster-ram type mold clamping device is shown in Fig. 5. In this device, a smaller-diameter booster ram 11 is a sliding fit into a large-diameter clamping ram 10. Oil is introduced into the smaller-diameter cylinder chamber 13 of the clamping ram 10 via an oil path 12 provided in the booster ram 11 for high-speed mold closure. Oil is also introduced into a rear chamber 14 behind the clamping ram at a negative pressure from an oil tank 16 via a prefill valve 15. After closure of the mold, a high-pressure mold clamping is conducted by closing the prefill valve 15 and introducing oil into the rear chamber 14 of the clamping ram. However, in the above stated booster ram type mold clamping device, the rear chamber 14 of the clamping ram is at a negative pressure with respect to the high-speed advancing of the clamping ram. Therefore, attracting force introduces oil to the rear chamber 14 from the oil tank 16 which results in the following problems: (1) The oil tank 16 requires a larger capacity than the mold clamping cylinder in order to avoid sucking air into the system so that the mold clamping device must be larger than would otherwise be necessary. (2) A high-speed advancing movement will be unstable due to introducing oil into the rear chamber 14 of the clamping cylinder from the oil tank 16 by sucking. The oil path provided between the oil tank 16 and the rear chamber 14 of the clamping cylinder and the prefill valve 15 should be of a large-diameter to reduce fluid resistance. (3) On switching to tight clamping, a shock often occurs because the oil pressure in the rear chamber 14 is changed from a negative pressure to a high pressure very quickly. Thus, the time of the molding cycle is longer because rising pressure requires much time. There is a mold clamping device disclosed in the Japanese Provisional Publication 53-42248 which comprises a clamping piston which slidably fits in a clamping cylinder. A clamping ram of the same diameter and high-speed advancing cylinder are provided on the front and rear faces of the clamping piston, an oil path is provided which can be opened or closed and connects the front and rear chambers of the clamping cylinder, and a small-diameter booster ram slidably fits into the high-speed advancing cylinder. In this clamping device, moving the clamping ram at a high speed by introducing oil into the high-speed advancing cylinder from the booster ram, does not create the problems stated above because the oil travels from the front chamber to the rear chamber without negative pressure in the rear chamber by connecting chambers provided on the front and rear sides of the clamping piston. However, with the structure of this device, the total length of the device is more than twice as long as the clamping stroke so that the device is large, heavy and expensive. A preferred embodiment of the present invention solves the above stated problems by providing a simple, compact, light and low cost device, whose length is half of the conventional length, and which can be quickly switched to a tight clamping condition. The present invention also makes it possible to provide a mold clamping device, which can use a standardized oil circuit or standard parts, and which can be easily designed, manufactured and controlled. The device of the present invention has a clamping cylinder which slidably fits a clamping ram whose front end is fixed on a movable board of injection molding machines, etc. The inner space of the clamping cylinder is divided into a rear chamber W and a front chamber X by the large-diameter piston section of the clamping ram. On the inner rear wall of the clamping cylinder, there is fixed a mold opening piston whose front end slidably fits in the clamping ram from the rear. The inner space of the clamping ram is divided into a chamber Y and a chamber Z opening to the atmosphere by the large-diameter piston section of the opening piston. A hydraulic controlling device which connects said chambers X, Y and Z to an oil pressure source and connects said chambers X and Y to said chamber W for a high speed mold closure, which connects said chamber W to said oil pressure source and reliefs the pressure in said chambers X and Y for a tight clamping, and which connects said chamber Y to the oil pressure source and connects said chamber W to both said chamber X and the oil tank for opening the mold. Note that, preferably, the effective pressure receiving area of the chamber Y is designed to be equal to the cross sectional area of the cylindrical section of the clamping ram. The high-speed mold closure occurs by pressure working on the cross section of the cylindrical clamping ram when oil is introduced into the chambers W, X and Y. Following the advancing of the clamping ram, the oil in the chambers X and Y is introduced into the chamber W so that the high-speed mold closure is available; at that time the chamber W is not at negative pressure. The tight clamping occurs by pressure working on the effective pressure receiving area of the chamber W when pressure in the chamber W is increased and pressure in the chambers X and Y are reduced. Upon reducing pressure in the chambers X and Y, this tight clamping is quickly executed. The mold opening occurs by pressure working on the effective pressure receiving area of the chamber Y when the oil is introduced into the chamber Y. Following the retracting of the clamping ram, the oil in the chamber W is introduced back into the chamber X and the rest is discharged to the oil tank. When the effective pressure receiving area of the chamber Y and the cross-sectional area of the cylindrical section of the clamping ram is the same area, the quantity of the oil introduced from an oil pressure source during a high-speed mold closure is the same as the quantity of the oil introduced therefrom during a high speed mold opening so that speed of mold opening is approximately equal to the speed of closure dependent on constant oil outflow from the oil pressure source. Therefore, according to this invention, the quantity of the oil provided from the oil pressure source may be a portion of the capacity of the clamping cylinder in the clamping and opening mold, the high speed operation is available in spite of a small outlfow from the oil pressure source. Especially during a tight clamping, the operation can be quickly changed to tight clamping by only reducing the pressure in the chambers X and Y. Additionally, there is no need to provide a sucking tank or a prefill valve as required in conventional devices. The total length can be reduced to be half of the conventional mold clamping devices which have a clamping ram and a high-speed advancing cylinder with the same diameters provided on the front and rear faces of the clamping piston. The mold clamping device of this invention has a high speed efficiency and a high reliability, and can be a simplified structure, which is compact in size and manufactured at a reduced marketing cost, and so on. Moreover, if the effective pressure receiving area of the chamber Y and the cross-sectional area of the cylindrical section of the clamping ram are substantially equal, the quantity of the oil provided at the mold opening is approximately equal to the quantity at mold clamping so that standardization can be achieved in designing oil circuits and selecting structural parts. Thus design, manufacturing and control can be easily conducted. An embodiment of the invention will now be described with reference to the accompanying drawings, in which: Fig. 1 shows a cross-sectional view during changing to the high-speed mold closure state from mold opening state; Fig. 2 shows a cross-sectional view during the tight clamping state; Fig. 3 shows a cross-sectional view during the state of changing to a mold opening; Fig. 4 shows a cross-sectional view taken on line IV - IV of Fig. 1; and Fig. 5 shows a cross-sectional view of a conventional booster ram type mold clamping device. In Figs. 1 to 3, a movable board 20 is guided to and away from a fixed board 21 by a tie bar, not shown, which is bridged between the fixed board 21 and a clamping cylinder 22. On the opposite faces of the movable board 20 and the fixed board 21, there are provided a movable mold 23 and a fixed mold 24. On a base (not shown) located to the rear of the fixed board 21, a conventional injection machine 25 is provided and is movable to and away from the fixed mold 24. A clamping ram 26 having a front end connected to the rear face of the movable board 20 is a sliding fit in a clamping cylinder 22. The inner space of the clamping cylinder 22 is divided into a rear chamber W and a front chamber X by a large-diameter piston section of the clamping ram 26. The large-diameter piston section is provided at the rear end of the clamping ram 26. A mold opening piston 27 having a rear end is fixed on the inner rear wall of the clamping cylinder 22 and is a sliding fit in the clamping ram 26. The inner space of the clamping ram 26 is divided into a chamber Y and a chamber Z by a large-diameter piston section provided at the front end of the mold opening piston 27. The chamber Z is connected to the atmosphere by an air path 28. Oil is introduced into or discharged from the chambers W and X via a ports opening in the cylindrical face of the clamping cylinder 22. The oil is also introduced into or discharged from the chamber Y via an oil path through the opening piston 27 whose one end opens at the rear end of the opening piston and the other end opens in the cylindrical face of the opening piston in the chamber Y. Because the large-diameter piston section of the clamping ram 26 is provided at a rear end thereof, and a large-diameter piston section is provided on the opening piston 27, the total length of the device including clamping cylinder is shortened as shown in Fig. 1. The effective pressure receiving area S1 (the area of the piston section of the opening piston 27 minus the cross sectional area of the rod section of the opening piston) is designed to be substantially equal to the cross sectional area of the cylindrical section of the clamping ram. Note that, the effective pressure receiving area of the chamber X (the area of the piston section of the clamping ram 26 minus the cross-sectional area of the clamping ram) is defined as S3. The chambers W and Y are connected to an oil pump 40 and an oil tank 32 via a three-position switching valve 31 which is bridged between an oil path 30 connected to the chamber W and an oil path 29 connected to the chamber Y. The three-position switching valve 31 switches the oil paths 29 and 30 to one of the following three positions: a position for connecting both the chambers W and Y to the oil pump 40 (shown in Fig. 1); a position for connecting only the chamber W to the oil pump 40, and for connecting the chamber Y to the oil tank 32 (shown in Fig. 2); and a position for connecting only the chamber Y to the oil pump 40, and for connecting the chamber W to the oil tank 32 (shown in Fig. 3). The chambers W and X are connected with each other by an oil path 33 branched from the oil path 30 and a switching valve 34 provided in the oil path 33. The oil paths 29 and 33 are connected by an oil path 36 having a switching valve 35 in substantially the middle thereof. A switching valve 37 is provided midway of the oil path connecting the oil pump 40 and the three-position switching valve 31. The switching valve 37 shuts the oil flow to prevent opening or clamping of the mold unnecessarily. Upon receiving a clamping or opening signal, then the switching valve 37 releases the oil flow. This system, including a three-position switching valve 31 and switching valves 34,35 and 37, is controlled to switch to prescribed positions automatically by a control means (not shown) of the injection molding machine, etc. and storing control programs. Next, the action of the device will be described. (i) High-speed mold closure: The three-position switching valve 31, and switching valves 34, 35 and 37 are set at the position shown in Fig. 1. The chambers W, X and Y are connected to the oil pump 40 and then the chambers are pressurized. The pressure working in the chamber W is the value of the product of the area S1+S2+S3 shown in Fig. 4 times the oil pressure P (=S1·+S2·P+S3·P) . But the component pressure S1 P is countervailed by the pressure in the chamber Y and the component pressure S3·P is countervailed by the pressure in the chamber X so that the clamping ram 26 is advanced by the component pressure S2·P. Following an advancement of the clamping ram 26, the oil in the chamber X is introduced into the chamber W via the oil path 33, switching valve 34 and the oil path 30, and the oil in the chamber Y is also introduced into the chamber W via the oil path 29, the three-position switching valve 31 and the oil path 30. Therefore, the oil quantity supplied to the chamber W from the oil pump 40 may be the quantity corresponding to the result of multiplying the cross-sectional area S2 by the travelling length of the clamping ram so that the high speed mold closure can be supplied with oil independently of the capacity of the chamber W, and the chamber W will not be at a negative pressure. (ii) Tight clamping: In the high-speed mold closure, oil outflow from the oil pump 40 is reduced slightly before completing mold closure, then the molds are closed with low speed and low pressure (S2 P). The three-position switching valve 31, switching valves 34 and 35 are changed to the position shown in Fig. 2 by the signal indicating completion of the high-speed mold closure. The chamber W is connected to the oil pump 40 and is pressurized as described before but the chambers X and Y are connected to the oil tank 32 to relieve pressure so that component pressure S1·P and S3·P forcing against pressure in the chamber W disappears, then the pressure working on the clamping ram 26 quickly attains the value of S1·P+S2·P+S3·P . Therefore, tight clamping can be executed, and the switching valve 37 is switched to the position of oil shut to maintain a tight clamping state. (iii) Mold opening: In the tight clamping state, the following steps are executed. They are, the injection machine 25 comes into contact with the fixed mold 24, resin melt is injected into the molding mold, and the resin melt is cooled, and solidified. After the above stated molding steps, a mold opening signal is generated, then the three-position switching valve 31 and the switching valves 34, 35 and 37 are switched to the positions shown in Fig. 3, and the oil outflow from the oil pump 40 is reduced. Therefore, the oil is introduced into the chamber Y via the three-position switching valve 31 then initial mold opening is executed by the component pressure S1·P. After the molded product is ejected from the molding mold, the oil outflow is increased, and high-speed mold opening is executed. Slightly before the end of mold opening, the oil outflow is reduced again to slow down the speed of mold opening. Upon reaching the prescribed position, the oil outflow reduces to zero and the mold opening stops. After completing the mold opening, the molded product can be taken out. Discharged oil from the chamber W following retracting the clamping ram 26 is introduced into the chamber X via the oil path 30, the switching valve 34 and the oil path 33, and the rest of the oil is returned to the oil tank 32. In the above embodiment, we explained a mold clamping device for an injection molding machine, but the mold clamping device, of course, can be used for die casting machines and similar machines. Additionally, the hydraulic controlling device including the three-position switching valve 31, the switching valves 34, 35, 37, etc. is not essential. Of course, this invention is not restricted to the details of the above stated embodiment.

Claims: A mold clamping device comprising: a clamping cylinder (22) slidably fitted with a clamping ram (26) whose front end is adapted to be associated, in use, with a movable element (20) of a mold assembly (20,21,23,24); the clamping ram (26) having a rear-opening cavity constituting a ram cylinder chamber; a mold opening piston fixed on an inner rear wall of said clamping cylinder (22) and having a front end portion which is a slidable fit in the ram cylinder chamber of said clamping ram 26; and wherein the clamping ram (26) has an enlarged portion constituting a piston by which said clamping cylinder (22) is divided into a rear chamber (W) and a front chamber (X); characterised in that said mold opening piston has a large diameter piston section at a front region so as to divide the ram cylinder chamber into a first chamber (Y) and a second chamber (Z), said second chamber opening to the atmosphere; and wherein said rear (W), front (X) and first (Y) chambers are connected to a hydraulic controlling device (31,34,35,37) adapted to provide the following modes of connection: (A) for high speed mold closure, said rear (W), front (X) and first (Y) chambers are connected to an oil pressure source (40) and said front (X) and first (Y) chambers are connected to said rear chamber (W); (B) for tight clamping, said rear chamber (W) is connected to said oil pressure source (40) and said front (X) and first (Y) chambers are connected to pressure relief means (32); and (C) for opening the mold, said first chamber (Y) is connected to said oil pressure source (40) and said rear chamber (W) is connected to both said front chamber (X) and said pressure relief means (32). A mold clamping device according to claim 1 wherein, the effective pressure receiving area of said first chamber (Y) is designed to be substantially equal to the cross-sectional area of the ram cylinder chamber. A mold clamping device according to claim 1 or 2 wherein the enlarged portion of the clamping ram (26) for dividing said clamping cylinder into said rear (W) and front (X) chambers is provided at the rear end of said clamping ram. A mold clamping device according to claim 1,2 or 3 wherein, an oil path operatively connected to said first chamber (Y) provided by a bore in said mold opening piston, said bore having one end at the rear end of said mold opening piston, and the other end on the outer face of said mold opening piston locating in said first chamber (Y). A mold clamping device according to claim 1,2,3 or 4 wherein: said hydraulic controlling device includes a three-position switching valve (31) for coupling the oil path connected to said rear chamber (W) and the oil path operatively connected to said first chamber (Y); a switching valve (34) is provided in an oil path connecting said rear chamber (W) to said front chamber (X); and a switching valve (35) is operatively provided in an oil path connecting said first chamber (Y) to said front chamber (X); and said three-position switching valve (31) is switchable between positions for connecting both said rear (W) and first (Y) chambers to an oil pump (40); for connecting only said rear chamber (W) to said oil pump (40) and connecting said first chamber (Y) to said pressure relief means (32); and for connecting only said first chamber (Y) to said oil pressure source (40) and connecting said rear chamber (W) to said pressure relief means (32). A method of operating a mold assembly including a mold clamping device according to any preceding claim, including the steps of: (A) effecting high speed closure of the mold assembly by connecting said rear (W), front (X) and first (Y) chambers to an oil pressure source (40); (B) effecting tight clamping by maintaining said rear chamber connected to the oil pressure source (40) while said front (X) and first (Y) chambers are connected to pressure relief means (32); and (C) effecting mold opening by connecting said first chamber to the oil pressure source (40) and connecting the rear chamber (W) and the front chamber (X) to the pressure relief means (32).

Inventors: TAKADA, MINORU C/O NISSEI KOGYO K.K.
Applicants: NISSEI JUSHI KOGYO KABUSHIKI KAISHA
Application year: 1988
Main classification: B29C45
IPC codes: B29C45, B29C33, B22D17