Anti-drool mechanism for a sprue bushing

A shut-off valve for preventing drool from an injection molding apparatus is provided in a melt channel of a sprue bushing. The shut-off valve includes a fixed member located in the melt channel and a reciprocating member coupled to the fixed member. The reciprocating member is biased toward an extended position in which an inlet of the sprue bushing is blocked thereby. The reciprocating member is movable from the extended position towards a retracted position in which the inlet of the sprue bushing is clear by the force of a melt stream entering the melt channel of the sprue bushing.

FIELD OF THE INVENTION

The present invention relates generally to an injection molding apparatus and, in particular to an anti-drool mechanism for a sprue bushing.

BACKGROUND OF THE INVENTION

Many injection molding systems use a sprue bushing to provide a transfer melt channel between the machine nozzle and a distribution manifold in a mold. In the event that the machine nozzle has to be disengaged from making contact with the sprue bushing, molten plastic will drool backwards from the sprue bushing, and this will adversely affect the next shot of melt. This situation can happen in many applications, such as for example with shuttle molds, rotary molds, and stack molds.

Due to residual pressures in the system, molten material tends to leak, or “drool”, from the sprue bushing at the end of the injection cycle, i.e., when the machine nozzle is retracted from the sprue bushing inlet. This drool represents waste material and increased production costs. Further, the drooled material may collect on the mold and prevent complete closing thereof or cause permanent damage thereto.

Many techniques and devices exist in the art that are designed to prevent or reduce drool in an injection molding system. Examples include various shut-off mechanisms located at the machine nozzle tip or spindle elements for use in a “suck-back” procedure. In many cases, the machine nozzle incorporates an anti-drool mechanism into its structure, as is described in U.S. Pat. No. 3,934,626 to Hall, incorporated herein in its entirety by reference thereto.

However, there exists a need to have an anti-drool mechanism that can be added as a modular component to existing injection molding systems, including stack molding systems.

SUMMARY OF THE INVENTION

According to an embodiment of the present invention there is provided an injection molding apparatus which has a sprue bushing with a melt channel for receiving a melt stream of moldable material under pressure. The sprue bushing melt channel has a shut-off valve to prevent melt from drooling between injection cycles.

The shut-off valve includes a fixed member located in the melt channel and a reciprocating member coupled to the fixed member, the reciprocating member being biased toward an extended position in which an inlet of the sprue bushing is blocked by the reciprocating member. The reciprocating member is movable towards a retracted position in which the inlet of the sprue bushing is clear by the melt stream entering the melt channel of the sprue bushing.

DETAILED DESCRIPTION OF THE INVENTION

Referring now toFIG. 1, an injection molding apparatus10is generally shown. Injection molding apparatus10includes a manifold12having a manifold melt channel14. Manifold melt channel14extends from an inlet16to manifold outlets18. Inlet16of manifold melt channel14receives a melt stream of moldable material from a machine nozzle60through melt channel62of a sprue bushing20and delivers the melt to hot runner nozzles22, which are in fluid communication with respective manifold outlets18. Sprue bushing20is heated by heater21and manifold12is heated by heater13. The sprue bushing20includes a shut-off valve64, which will be described below. The shut-off valve is generally an anti-drool mechanism for reducing the occurrence of drool when the machine nozzle60is de-coupled from the sprue bushing20.

Although a pair of hot runner nozzles22is shown inFIG. 1, it will be appreciated that a typical injection molding apparatus may include only one or a plurality of hot runner nozzles for receiving melt from respective manifold outlets.

Each hot runner nozzle22is received in an opening32in a mold plate34. A collar28surrounds the nozzle22. The collar28abuts a step36, which is provided in opening32to maintain a nozzle head26of the hot runner nozzle22in abutment with a lower surface40of manifold12. A nozzle tip30is received in a downstream end of hot runner nozzle22and is threaded thereto. A nozzle melt channel24extends through hot runner nozzle22and nozzle tip30. Nozzle melt channel24is in communication with manifold outlet18to receive melt from manifold channel14. Hot runner nozzle22is heated by a heater54and further includes a thermocouple56.

A mold cavity50is provided between mold plate34and a mold core52. Mold cavity50receives melt from nozzle melt channel24through a mold gate48. Cooling channels58extend through mold plate34to cool mold cavity50.

Manifold12is maintained in position relative to mold plate34by a locating ring46. Spacers44are provided between an upper surface38of manifold12and a back plate42.

Referring toFIGS. 2 and 3, shut-off valve64includes a pin retaining assembly100having a reciprocating member, which is generally a pin102. The pin retaining assembly100is coupled to an inner wall68of a fixed sleeve66by a collar104. The fixed sleeve66is received in an upstream portion72of bore70, which extends through sprue bushing20. The bore70includes a shoulder76that separates the upstream portion72from a downstream portion74. A shut-off valve melt channel61extends around the pin retaining assembly100and is delimited by the outer surface of pin retaining assembly100and inner wall68of fixed sleeve66. Shut-off valve melt channel61is in fluid communication with sprue bushing melt channel62, which extends within downstream portion74of bore70.

Referring also toFIG. 4, fixed sleeve66includes a threaded portion78that is provided between a rear end80and a forward end82thereof. Threaded portion78mates with a threaded portion86of bore70. A forward surface88of fixed sleeve66abuts shoulder76of bore70in order to locate the fixed sleeve66within bore70. Rear end80is shaped to allow for engagement with a tool, for example a wrench, to facilitate installation and removal of the fixed sleeve66from the sprue bushing20. A projection90extends outwardly from rear end80to act as a stop for the tool.

An inlet92extends through rear end80of fixed sleeve66for receiving melt from machine nozzle60. A seat94, which is shaped to mate with the machine nozzle60, surrounds inlet92. Fixed sleeve66further includes a step96that is provided in inner wall68.

Pin retaining assembly100includes a connector114having a bore or chamber126that extends therethrough. The pin102is slidable within the bore or chamber126between an extended upstream position, which is shown inFIG. 2, and a retracted downstream position, which is shown inFIG. 3. In the extended, or closed, position, a tapered head128of the pin102engages the inlet92of the fixed sleeve66to generally prevent melt flow therethrough. In the retracted, or open, position, the pin102is clear of the inlet92to allow melt flow therethrough.

A flange132extends inwardly from a wall134of bore or chamber126inside connector114. The flange132is sized to allow a neck130of the pin102to slide therethrough. A shoulder136is provided upstream of the flange132. A bolt138is threadably received in a bore140that is provided in a downstream end142of the neck130of pin102. A bolt head144telescopes within the bore or chamber126downstream of the flange132as the pin102moves between the retracted downstream position and the extended upstream position.

A spring146is provided between the tapered head128of the pin102and the flange132to bias the head128of the pin102towards the extended position. The spring146can be any conventional spring known in the art, such as a series of stacked Belleville washers, a helical compression spring, or the like. Spring146must have a spring constant sufficient to consistently return pin102to the extended position. Spring146must also be sufficiently rigorous to withstand a high number of cyclic compressions.

With reference toFIGS. 5 and 6, the collar104, which couples the pin retaining assembly100to fixed sleeve66, includes an internal ring106and an external ring108. The internal ring106is coupled to the external ring108by a plurality of bridging portions110. Four separate channels112extend between the internal ring106and the external ring108. The channels112form part of the melt channel61of shut-off valve64.

The external ring108of the collar104is sandwiched between step96of inner wall68of fixed sleeve68and shoulder76of bore70of the sprue bushing20. The internal ring106of the collar104is sandwiched between a shoulder116that is provided in an outer surface118of connector114and an end surface122of retainer120. The retainer120is threadably coupled to the outer surface118of the connector114and includes a pin portion124that is shaped to facilitate smooth melt flow past the pin retaining assembly100. The retainer120includes a cavity164having an end surface168. Axial movement of the pin102into the retracted position is limited by abutment of the bolt head144with end surface168of cavity164of a retainer120. Pin portion124includes flat portions166to allow for engagement with a tool, such as a wrench for example, in order to facilitate removal of retainer120during disassembly of shut-off valve64.

The retainer120generally prevents melt from entering the cavity164thereof. Similarly, the pin102is sized to generally prevent melt from entering bore or chamber126of connector114. Melt leakage into these areas could compromise the performance of the shut-off valve64. Further, disassembly of the shut-off valve may be more difficult.

It will be appreciated by a person skilled in the art that the collar104is not limited to having four bridging portions110. Any number of bridging portions110including a single bridging portion110or a pair of bridging portions110may be used.

A groove148is provided in outer surface98of the fixed sleeve66at forward end82thereof. A passage150extends through the fixed sleeve66to allow groove148to communicate with a groove152that is provided in an outer surface154of the collar104. Similarly, passages156extend through bridging portions110of collar104to allow groove152to communicate with a groove158that is provided in an inner surface162of the internal ring106. A passage160extends through the connector114to allow groove158to communicate with bore or chamber126, which houses the spring146. The series of grooves148,152,158and passages150,156,160is provided to allow air to escape from the spring area when the pin102is forced into the retracted downstream position and the spring146is compressed.

In operation, machine nozzle60is moved into contact with seat94of shut-off valve64and melt flow from the machine nozzle60is initiated. The pressure of the melt stream exiting the machine nozzle60forces the pin102to telescope within pin retaining assembly100and move from the extended position ofFIG. 2to the retracted position ofFIG. 3. With the shut-off valve64in the open position, melt flows from the machine nozzle60, through melt channels61,62of shut-off valve64and sprue bushing20, respectively, and into manifold channel14of manifold12. The manifold12distributes the melt through manifold outlets18to nozzle melt channels24of nozzles22. The melt flows through the nozzle channels24, past mold gates48and into mold cavities50. Once the mold cavities50have been filled, melt flow from the machine nozzle is halted. The relief of the melt pressure allows the pin102to return to the extended position under the force of the biasing spring146. As the tapered head128extends, some melt is forced out of melt channel61and into the machine nozzle60. The machine nozzle60then performs a “suck-back” operation to decompress the melt. Once the pressure of the melt in the machine nozzle60drops below a predetermined value, the machine nozzle60is disengaged from the shut-off valve64. The mold cavities50are then cooled and the molded parts are ejected from injection molding apparatus10.

It will be appreciated that the shut-off valve64is not limited to being used in injection molding apparatus10, as shown inFIG. 1. The manifold12could distribute the melt stream through manifold channel14to other hot runner systems or an injection nozzle, a mold, a stack mold, or a combination of these elements.

In another embodiment, tapered head128of pin102is biased toward the extended position pneumatically. In this arrangement, the series of grooves148,152,158and passages150,156,160are used to conduct compressed air into and out of the connector114. The tapered head128of pin102may further be biased toward the extended position hydraulically.

It will be appreciated by a person skilled in the art that the shut-off valve64may be retrofitted into a sprue bushing of an existing injection molding apparatus or incorporated into a new sprue bushing of an injection molding apparatus.