Shutheight adjuster for a sprue bar

The present invention relates to an improved sprue bar assembly having a mechanism for automatically adjusting the length of the sprue bar in a stack mold, or stack mold carrier assembly, to accommodate molds of different shutheight. The improved sprue bar assembly of the present invention can also be used to facilitate injection compression and/or coining processes in stack molds.

Stack molds typically rely upon hot runner systems for conveying melted plastic to their mold cavities. Generally, a hot runner manifold is mounted in the moving center section of the stack mold or is fastened to a movable stack mold carrier that may be part of the machine. A sprue bar extends from the hot runner manifold and seals to the machine injection nozzle. This sealed connection is typically broken and remade for each molding cycle. When the mold must be opened to eject the molded parts, the pressurized hot runner manifold and sprue bar melt channels must be relieved of pressure in order to prevent drool or leakage of the plastic when the melt channel sections must be separated. However, this pressure relief must not allow air to be introduced into the melt stream when the mold is closed for the next injection cycle.

A stack mold carrier is sometimes installed in a machine to allow a variety of different stack molds to be installed or removed quickly. The carrier typically has a linkage system for opening and closing the various mold sections and sometimes includes the hot runner and sprue bar so that only the mold cavities and cores need be changed. If a mold core and cavity set is installed that has a different shutheight from the set it is replacing, a different sprue bar of suitable length must also be installed to match the new shutheight. This is both expensive and time consuming.

Several techniques have been used to solve the problem of stack mold hot runner depressurization. Swiss patent 625,461 to Hotz illustrates a stack mold hot runner manifold having a retractable pin in the melt channel which when retracted increases the volume of the melt channel, thereby depressurizing it prior to the mold opening. After closing for the next cycle, the pin must be advanced in order to be ready for the next depressurizing action. Japanese Patent Document No. 62-35817 illustrates a similar depressurizing pin.

French Patent No. 2,152,932 illustrates a spring loaded bushing on an offset sprue bar that can slide away from the hot runner nozzle, thereby increasing the volume of the melt channel to allow depressurization as the mold opens. The mold closing action recompresses the springs for the next cycle. Further examples of stack mold applications are shown in U.S. Pat. Nos. 5,458,843 and 5,464,579, both to Brown. Also, FIG. 29 in an article “Progress of Automatic Molds” from Japan Plastics Age shows a spring loaded bushing used in a stack mold application.

Other manifold depressurization mechanisms are illustrated in U.S. Pat. No. 4,299,791 to Aoki, U.S. Pat. No. 3,934,626 to Hall and Japanese Patent Document No. 51-102047. Each of these documents shows spring loaded bushings. U.S. Pat. No. 4,473,347 to Terashima illustrates a pair of opposed sliding blocks that create additional melt channel volume in the manifold to effectively depressurize the manifold.

Allowed co-pending U.S. patent application Ser. No. 09/054,692 to Teng et al., now U.S. Pat. No. 6,027,681 which is hereby incorporated by reference herein, illustrates a stack mold carrier linkage system having a linkage that can accommodate molds of different shutheights. There is no teaching in this application however of adjusting the sprue bar length to suit the differing mold shutheights.

U.S. Pat. No. 5,910,327 to Schad illustrates a telescoping sprue bar that allows a shorter sprue bar to be used in a stack mold. However, the Schad design does not facilitate adjustment to accommodate differing mold shutheights. Japanese Patent Document No. 6-000848 also illustrates a telescoping sprue bar.

U.S. Pat. No. 3,843,295 to Greenberg illustrates an adjustable injection nozzle in a stack mold application whereby the nozzle can be lengthened by using springs or fluid pressure. In this patent, multiple nozzles protrude through the center section of the stack mold to supply resin to the mold cavity sets distant from the machine's injection unit. This impractical mold layout does not work in situations where the molds are to be changed in a manner that minimizes time and cost. Also, the nozzle extensions that pass through the mold set adjacent the machine's injection unit in order to reach the distant mold set are impediments when the mold sets open and must eject their molded parts.

U.S. Pat. No. 4,212,626 to Gellert illustrates two opposed hot runner valve gated nozzles that are used to control the melt flow across the parting line of a stack mold, thereby preventing leakage and drool when the mold opens. Decompressing the manifold is not discussed and such a design would not be easily adjustable for molds of differing shutheights.

Co-pending U.S. patent application Ser. No. 09/173,783 now U.S. Pat. No. 6,135,757 illustrates the concept of providing grooves on the outer surface of valve gate stem to facilitate closing the stem against compressed melt and reducing the heat loss from the stem. Providing a channel within the stem to provide a venting route for excess melt is not contemplated.

Injection compression molding and coining are processes whereby melted resin is fed into a mold that is not completely closed and clamped such that after feeding a metered amount of resin into the mold, the supply is shut off and the mold closed and clamped compressing the melt to fill the cavity entirely. This process provides the molded part with enhanced properties, typically less residual stress and sometimes allows parts of thinner cross section to be molded. U.S. Pat. No. 5,130,075 to Hara illustrates such a process in which resin is fed into a mold that is allowed to open during filling and then is closed and clamped to complete the process. Processes like injection compression and coining have not previously been possible to perform in stack mold applications because the movement of the mold during the filling and/or compressing stage could not be facilitated with a fixed length sprue bar.

Accordingly, it is an object of the present invention to provide a sprue bar assembly that accommodates molds of different shutheights.

It is a further object of the present invention to provide a sprue bar assembly as above that decompresses the melt channel.

It is yet a further object of the present invention to provide improved compression and/or coining processes for making molded articles.

The foregoing objects are attained by the sprue bar assembly of the present invention.

In accordance with the present invention, a sprue bar assembly is provided for use with an injection molding machine having at least one mold and for accommodating different mold shutheights. The sprue bar assembly comprises a fixed sprue bar section having a first channel through which molten material, such as a plastic resin, flows. The first channel communicates with the outlet of a source of the molten material. The sprue bar assembly further comprises means for accommodating different mold shutheights. The accommodating means comprises an adjustable sprue bar section having a second channel through which molten material flows and powered means for holding the adjustable sprue bar section in contact with the fixed sprue bar section so that molten material can flow from the first channel to the second channel without leaking. In a preferred embodiment of the present invention, the fixed sprue bar section and the adjustable sprue bar section are each provided with a valve gate for preventing drooling or leakage of the molten material when the two sections are separated.

The adjustable sprue bar assembly of the present invention can be used in a number of different molding machines for performing a plurality of different molding processes. For example, the adjustable sprue bar assembly of the present invention can be used to facilitate injection compression and/or coining processes in stack molds.

Other details of the sprue bar assembly of the present invention and its applications, as well as other objects and advantages attendant thereto, are set forth in the following description and the accompanying drawings in which like reference numerals depict like elements. dr

FIG. 1 is a side view of a stack mold using a stack mold carrier and having a sprue bar assembly in accordance with the present invention;

FIG. 2 is a side view in partial cross section of the sprue bar assembly in accordance with the present invention while a mold is in a closed position;

FIG. 3 is a side view in partial cross section of the sprue bar assembly in accordance with the present invention while a mold having an increased shutheight is in a closed position;

FIG. 4 is a side view in partial cross section of the sprue bar assembly in accordance with the present invention while a mold is in a partially mold open position;

FIG. 5 is a side view in partial cross section of the sprue bar assembly in accordance with the present invention while a mold is in a final mold open position;

FIG. 6 is a top view of a hot runner assembly including a sprue bar assembly in accordance with the present invention;

FIG. 7A is a side view of an alternate sprue bar nozzle assembly with a valve gate in an open position;

FIG. 7B is a side view of the alternate sprue bar nozzle assembly of FIG. 7A with the valve gate in a closed position;

FIG. 7C is a sectional view of a tip of a valve stem used in the valve gate shown in FIG. 7A;

FIG. 8 is a side view in partial cross section of a sprue bar assembly in accordance with the present invention used in an injection-compression molding application in a mold filling stage;

FIG. 9 is a side view in partial cross section of the sprue bar assembly of FIG. 8 in a compressing/coining stage;

FIG. 10 is a side view in partial cross section of the sprue bar assembly of FIG. 8 in a partially mold open position;

FIG. 11 is a side view in partial cross section of the sprue bar assembly of FIG. 8 in a final mold open position; and

FIG. 12 is a side view in partial cross section of an alternative sprue bar assembly in accordance with the present invention.

Referring now to the drawings, FIG. 1 illustrates a stack mold carrier and two mold sets installed therein. The stack mold carrier includes a center platen 10 positioned between a stationary platen 12 and a moving platen 14 . The center platen 10 has mold halfs 16 and 18 respectively attached to opposed faces 20 and 22 . The stationary platen 12 has a mold half 24 attached to it and the moving platen 14 has mold half 26 attached to it. The mold halfs 16 and 26 form a first mold set 36 and the mold halfs 18 and 26 form a second mold set 34 . Typically, the mold halfs 16 and 18 comprise mold cavity halfs and the mold halfs 24 and 26 comprise mold core halfs; however, if desired mold halfs 16 and 18 could be mold core halfs and mold halfs 24 and 26 may comprise mold cavity halfs. As shown in FIG. 1, the mold sets 34 and 36 are each in a mold closed position.

A linkage system (not shown) is attached to the platens 10 , 12 and 14 to effect movement of the molds and platens in a known fashion. For example, the linkage system can operate the molds so that they simultaneously open or close. Alternatively, the linkage system could operate the molds in a desynchronized cycle where each mold set opens and closes at a different time from the other. Any suitable linkage system known in the art may be used to effect movement of the molds and platens.

As shown in FIG. 1, the center platen 10 contains a hot runner melt transfer system comprising a manifold 28 and sprue bushings 30 and 32 for feeding molten material, such as molten plastic resin(s), to the mold sets 34 and 36 respectively. Beneath or beside or above the mold set 34 is a sprue bar assembly 38 that connects the manifold 28 to the outlet nozzle 39 of the injection molding machine's injection unit 40 . As will be discussed hereinafter, sprue bar assembly is adjustable in length to accommodate molds of different shutheights and/or to accommodate different operational cycles of the molds.

The adjustable length sprue bar assembly 38 in accordance with the present invention includes a sprue bush 42 , a fixed manifold 44 connected to the sprue bush 42 , a fixed sprue bar section 46 connected to the manifold 44 , a movable or adjustable sprue bar section 48 , and a sprue bar mounting section 50 attached to the center platen manifold 28 . The fixed manifold 44 may be mounted within the stationary platen 12 or onto a surface of the stationary platen 12 . Alternatively, the fixed manifold 44 may be mounted within the mold set 34 .

The adjustable sprue bar assembly 38 , in accordance with the present invention, has the ability to increase or decrease its length while maintaining a sealed molten material passage throughout. In FIG. 1, this ability to extend is indicated by the dotted line portion 52 , which indicates that the sprue bar assembly 38 has been extended to accommodate the increased shutheight thickness, indicated by the dotted line portion 54 in mold set 34 .

FIGS. 2-5 illustrate how the adjustable sprue bar assembly 38 of the present invention operates in a conventional stack mold injection molding application. In FIG. 2, the mold set 34 is shown in the mold closed position. The adjustable sprue bar section 48 is fastened to a movable sprue bar sleeve 56 , which slides over manifold spigot 58 which is fastened into the manifold 28 . Movable sleeve 56 is guided by sprue bar mounting bushing 60 which is fixed to the manifold plate 62 which, in turn, is bolted or otherwise fastened to a mold shoe (not shown) or the center platen 10 . Sleeve 56 is caused to move by operation of a powered unit such as cylinder 64 by a control unit 65 .

As shown in FIG. 2, the cylinder 64 communicates with a source of fluid 67 and has a piston head 63 which reciprocates between a retracted position shown in FIG. 2 and an extended position shown in FIGS. 3, 4 and 5 . The control unit 65 for operating the cylinder 64 may comprise any suitable closed loop servo control unit or control circuit known in the art for causing the cylinder 64 , and hence the piston head 63 , to move as a function of the position of the mold(s) during the molding cycle. For example, the control unit 65 can be a computer which is programmed to operate the cylinder 64 . Inputs to the control unit 65 may include some or all of the following: (1) positions of the machine's moving platen and stack mold carrier (from these one would know the positions of the mold sections that are bolted to these items); (2) position of the movable sleeve 56 with respect to the manifold 28 ; (3) the injection unit position of the feedscrew and/or injection cylinder and injection pressure; and (4) timing of injection start sequence, switch to hold pressure, end of hold pressure, start of mold opening, and end of mold closing. These inputs may be transmitted from appropriate sensors (not shown) and/or other control units to the control unit 65 using any suitable conventional means (not shown) known in the art. From some or all of these inputs, the control unit 65 determines when and how much the cylinder 64 should move and in which direction and with what degree of force. When the control unit 65 comprises a computer, any suitable program may be used to convert the input(s) into the desired output(s). The particular program that is used does not form part of the present invention.

As shown in FIGS. 2-5, the piston head 63 has a piston rod 69 which is connected or joined to the sleeve 56 via crosshead 66 and one or more connecting rods 68 that pass through clearance holes 70 in the manifold 28 . In a preferred embodiment of the present invention, each of the connecting rods 68 is threaded into the sleeve 56 .

While it is preferred to use a power unit, such as cylinder 64 , to reciprocally move the sleeve 56 , other devices such as a rack and gear drive or a lever mechanism driven by a servo motor could be used in lieu of a cylinder.

If desired, a position sensor 72 can be provided to monitor the position of sleeve 56 with respect to the bushing 60 . Signals from the sensor 72 are transmitted to the control unit 65 and are used in the closed loop servo control system for controlling cylinder 64 to position the adjustable sprue bar section 48 in certain positions as needed.

If desired, the cylinder 64 can remain energized throughout the operation to constantly urge sleeve 56 to maintain a sealed connection between the adjustable sprue bar section 48 and the fixed sprue bar section 46 . In operation, as the mold set 34 closes, the injection machine's clamp drive mechanism (not shown) overcomes the force of cylinder 64 and re-establishes a sealed connection upon closing. Unlike a spring, when used in the fashion of the present invention, a cylinder and a properly designed hydraulic or pneumatic circuit will provide a constant, and if required adjustable, sealing force over the entire stroke of movement, whereas a spring will create an increasing force as it is compressed further. This non-linearity of force severely limits the practical stroke over which a spring can be used. In an application such as providing an effective sealing action over a substantial distance, the spring approach will create wear and damage from excessive spring compressive forces.

In operation, when the mold set 34 is closed and clamped as shown in FIG. 2, control unit 65 energizes cylinder 64 to urge movable sleeve 56 to move in a direction so that adjustable sprue bar section 48 maintains a sealed connection with fixed sprue bar section 46 such that when molten material is injected through the melt channels 74 , 76 , 78 , 80 and 82 in the sprue bar assembly at injection pressures, there is no leakage at the connection between the fixed and adjustable sprue bar sections 46 and 48 . During this injection phase, it is possible for the mold to breath or for its parting line 84 to separate briefly by some small amount such as a distance “A” depicted by the arrows in FIG. 3 . Should this occur, the energized cylinder 64 maintains a force on the sleeve 56 so that a sealed connection continues to be maintained in the sprue bar assembly 38 .

If the mold structure should compress slightly under clamp tonnage, or a thicker or thinner mold shutheight is installed in the machine than was anticipated, the distance between the stationary platen 12 and the center platen 10 may increase or decrease. This distance change is represented in FIG. 3 by the dotted line and the arrows “B”. Should the distance decrease, severe damage, such as bending the sprue bar, may result unless a similar decrease is made in the assembled length of the sprue bar assembly between these two platens. If the distance increases between the platens 10 and 12 , the sprue bar sections in a conventional sprue bar assembly may not meet at all, allowing molten material leakage to occur. To compensate for these potential events and to eliminate the consequences caused by them, the cylinder 64 and sleeve 56 used in the sprue bar assembly of the present invention are provided with sufficient stroke to maintain melt channel sealing throughout a wide range of mold shutheight changes. In fact, this distance compensation feature of the present invention can be provided to accommodate molds of significantly different shutheights, up to 500 mm. for example.

FIG. 4 illustrates how the sprue bar assembly 38 of the present invention can be used to decompress the melt channel prior to separating its sections. In the case of a sprue bar assembly having an open melt channel configuration, as shown in FIG. 4, the mold 34 can begin opening prior to the sprue bar sections 46 and 48 separating. Additional stroke is provided in the assembly so that the sleeve 56 can extend until crosshead 66 contacts stop 86 positioned on the end of manifold spigot 58 . This additional stroke provides an increase in volume in the melt channel as sleeve 56 slides over spigot 58 , thereby reducing the melt pressure in the still sealed melt channel passages 74 , 76 , 78 , 80 and 82 throughout the entire hot runner system from the outlet or injection nozzle 39 in the injection unit to the gates at the mold cavities. Sufficient stroke is provided to decompress the melt such that when the mold set 34 continues to open and the sprue bar sections 46 and 48 separate, as shown in FIG. 5, the molten material drooling from the open channels is minimized. Because the sleeve 56 can be moved by the control unit 65 , the stroke of sleeve 56 can be adjusted to suit this condition. Overstroking sleeve 56 can introduce air into the melt channel which can cause undesirable molding problems. Therefore, the control unit 65 preferably is one where the operator can tune the distance sleeve 56 strokes to optimize the system's performance.

To complete the molding cycle, after the molded part(s) has been ejected from the open mold, the mold is closed again and the sprue bar assembly 38 resealed at the connection between sections 46 and 48 . The position of the sleeve 56 is reset by cylinder 64 and any excess melt in the channel displaced by the reset is moved toward the mold gate. Timing the reset coincides with the initiation of injection of the molten material for the next molding cycle so that there is no interruption in the build up of injection pressure as the molten material is fed to the mold cavity or cavities.

FIG. 6 illustrates a hot runner assembly in the mold open position. The hot runner assembly includes the sprue bar assembly 38 of the present invention. In the configuration shown in this figure, there is a moving side 88 and a stationary side 90 . Additionally, the moving side hot runner manifold 92 is mounted in a mold set mounted to the moving platen of the machine. Using this configuration, in combination with a second injection unit (not shown), two resins could be fed to the same mold cavity mounted between the two mold platens for the purposes of making coinjected multilayer parts, or parts having adjacent areas of differing materials. See U.S. Pat. No. 5,651,998 to Bertschi, which is incorporated by reference herein, as an example of this type of application. In this application, a stack mold is not being used but the sprue bar assembly 38 is being used to supply one of the resins to a moving platen as in a stack mold configuration.

In a preferred embodiment of the sprue bar assembly of the present invention, valve gate assemblies are mounted at the opposed ends of the fixed sprue bar section 46 and the movable sprue bar extension 48 such that the melt channel can be shut off when these sprue bar sections separate. The valve gate assemblies positively close the melt channels 76 and 78 to eliminate any chance of drool when sections 46 and 48 separate and also to prevent the ingress of air. In some molding applications in which cycle time or type of resin being used promotes drooling, the additional expense of this embodiment is justified in order to eliminate drool and save the cost of the wasted resin.

Referring now to the drawings, FIG. 7A illustrates a valve gate assembly 94 to be used with fixed sprue bar section 46 and movable sprue bar extension 48 . The valve gate assembly 94 includes a passageway 99 through which molten material is transported and a reciprocally movable valve stem 96 for closing the passageway 99 . A piston cylinder unit 98 and a pivotable link 100 connected to the piston rod of the unit 98 and the valve stem 96 move the valve stem 96 between the open position shown in FIG. 7 A and the closed position shown in FIG. 7 B. As shown in FIGS. 7A and 7C, the valve stem 96 , installed in the end of each of the sections 46 and 48 , is provided with a channel 102 at its tip through which molten material can travel to vent grooves 104 that surround the periphery of the stem 96 . This is to allow any melt or molten material trapped between the ends of the valve stems after mold filling is complete to escape as the stems approach each other to shut off their respective channels in their respective sprue bar sections. The use of a valve gated approach in lieu of the open channel approach shown in FIGS. 2-5 helps ensure substantially drool free operation.

FIGS. 8 through 11 illustrate how the sprue bar assembly 38 of the present invention operates in a stack mold injection compression and/or coining molding application. FIG. 8 shows the initial mold position for a coining operation in which a mold 110 , formed by mold halfs 112 and 114 , is partially closed. After a metered amount of a molten material, such as a plastic resin, is injected into the mold 110 , the closing is completed to compress the melt and force it to complete the filling of the cavity in mold half 112 . An alternate process is to start with the mold 110 closed but not clamped and to allow the mold 110 to open as molten material is fed into it under pressure. Then after a metered amount of molten material has been supplied to close the mold, once again compressing the molten material to force it to fill the mold cavity. This latter process-injection compression molding and to some extent coining can allow a thinner part to be molded than otherwise could be obtained. Because the distance between the machine platens changes in these processes, it has been difficult to perform them in a stack mold configuration when a fixed length sprue bar has been used to conduct the molten material to the center platen manifold 28 . By using the adjustable length sprue assembly 38 of the present invention, it now becomes possible to provide a trouble free, reliable means for performing injection compression and coining processes in a stack mold.

In operation, FIG. 8 shows the mold condition in which the metered amount of molten material is fed into the mold cavity via the sprue bar assembly 38 . FIG. 9 shows the closing and compression stage of the mold where the distance the mold has moved is indicated by the dotted lines. Accordingly, the sprue bar assembly 38 has shortened in length by having the cylinder 64 allowing sleeve 56 to retract into the housing or mounting bushing 60 . The sensor 72 keeps track of the position of the sleeve 56 . FIG. 10 shows the melt decompression stage where the mold 110 is slightly open. As can be seen from this figure, the cylinder 64 is operated to move the sleeve 56 so that the extension 48 stays in contact with the sprue bar section 46 . FIG. 11 shows the mold 110 open and the sprue bar sections 46 and 48 separated as previously described. If desired, the sprue bar sections 46 and 48 may be provided with the valve gate shown in FIGS. 7A-7C when the process or the plastic material being used justifies its benefits.

While the figures in the instant application show an offset sprue bar assembly, wherein the center line of the sprue bar is not the same as that of the mold, the present invention is equally applicable to sprue bars mounted with their channel centerlines coincident with machine clamp or mold centerlines. Similarly, the sprue bar assembly of the present invention can be applied to a machine injection unit mounted off-center, such as a Tandem machine wherein the injection unit may be positioned on top or to the side of the platen.

FIG. 12 illustrates an alternative sprue bar assembly in which the fixed sprue bar section 46 has been replaced by a second movable or adjustable sprue bar section 48 ′. By providing two adjustable sections 48 and 48 ′, the range of available motion is effectively doubled. As a result, the assembly has more flexibility in adjusting height for various molds.

As can be seen from FIG. 12, the adjustable sprue bar sections 48 and 48 ′ are aligned with each other. Adjustable sprue bar section 48 operates in the manner discussed hereinabove and is connected to power unit 64 in the manner discussed hereinabove. Adjustable sprue bar section 48 ′ is connected or joined to a movable sleeve 56 ′ positioned within a fixed sprue bar section or manifold 44 ′. Power unit 64 ′ is provided to extend or retract the sleeve 56 ′ relative to the manifold 44 ′.

Power unit 64 ′, such as a cylinder unit, has a piston head 63 ′ which is reciprocally movable between the retracted position shown in FIG. 12 and an extended position (not shown). The sleeve 56 ′ is connected to the piston head 631 via the piston rod 69 ′, the crosshead 66 ′ and one or more connecting rods 68 ′. The power unit 64 ′ communicates with a fluid source 67 ′. Operation of the power unit 64 ′ is controlled by a control unit 65 ′ such as the closed loop servo control unit previously discussed. If desired, the control unit 65 ′ may be omitted and both power units 64 and 64 ′ may be controlled by a single control unit.

It is apparent that there has been provided in accordance with the present invention a shutheight adjuster for a sprue bar assembly which fully satisfies the means, objects, and advantages set forth hereinbefore. While the present invention has been described in the context of specific embodiments thereof, other variations, alternatives, and modifications will become apparent to those skilled in the art after reading the foregoing description. Therefore, it is intended to embrace such variations, alternatives, and modifications as fall within the broad scope of the appended claims.