BACKGROUND

The present invention relates generally to polymer injection molding systems, and more particularly to an improved system and apparatuses thereof configured to modify the viscosity of the polymer melt to prevent or minimize cold slug within the molding system.

The polymer product industry worldwide is very large and relies upon efficient and successful conversion of raw polymer base materials (i.e. resins usually in the form of liquid or pellets) into fully formed finished plastic products in order to minimize production costs. In hot runner type injection molding systems, this requires precise control of viscosity throughout the manufacturing process to maintain the polymer melt in a molten and flowable fluidic state. This is most often attempted via temperature control. Hot runner systems generally employ heated manifold plates which contain internal flow conduits (“hot runners”) to deliver the hot polymer melt to the mold cavities. But in many cases, polymer temperature cannot be controlled accurately at all locations in the material flow path and also cannot typically be changed rapidly.

In a large majority of cases where products are made via hot runner injection molding, the goal is to produce multiple products of either a single type or multiple types in cavities of a single mold at the same time for efficiency and cost-effectiveness. Examples of products made in this way include electrical connectors, medical syringes, communication device (i.e. cell phone) components, automotive components, and others. In these multi-cavity injection molding processes, precise relative control of the polymer material state is critical to ensure continuous polymer flow progression through all portions of the hot runner manifold and importantly the “hot drop” injection nozzles which form the movable interface with the mold cavities. The need to provide these conditions throughout the entire hot runner molding system is critical to fully fill all of the mold cavities necessary to produce fully formed plastic parts.

In addition, with hot runner based injection molding systems, the processing of some materials such as liquid crystal polymers (LCP) or materials containing colorants can be extremely difficult. LCP materials can change viscosity dramatically in regions such as the tips of hot drop injection nozzles where precise temperature control can be challenging. This can result in “cold slug” polymer formation comprised of partially or fully solidified polymer which can in turn partially or fully occlude the hot runner flow conduits in the manifold, flow channels in the product cavity, and most particularly the small diameter flow passages of the hot drop injection nozzles which are particularly susceptible to this problem. This results in incomplete filling of the mold cavities and wasted defectively molded articles. Moreover, present day colorant additives mixed with the raw polymer can also yield inconsistent product coloration results if the material is kept too hot for too long during the manufacturing process by heating the nozzles and hot runner manifold adding another concern to the problem.

Accordingly, there remains a need for an improved hot runner injection molding systems which minimizes or prevents the foregoing problems.

BRIEF SUMMARY

The present application discloses an injection molding system and related process configured for the imposition of controlled localized shear on the polymer melt inside the injection nozzles which interface with the mold cavities. In one non-limiting embodiments, the molding system may be a hot runner type injection molding system with “hot drop” injection nozzles. The shear rate is selected to maintain the polymer melt in a molten and fluidic state. Inducing shear on the polymer advantageously reduces its viscosity to avoid the foregoing cold slug formation problem (i.e. partially or completely solidified polymer) within the narrow polymer flow passages particularly of the hot drop injection nozzles. This ensures complete filling of the mold cavities with each injection molding cycle thereby producing fully formed end products or articles.

In lieu of past approaches relying upon heating the polymer melt alone to keep the polymer melt in a flowable fluidic state or condition, inducing shear on the polymer melt according to the present disclosure without reliance upon temperature control advantageously avoids cold slug formation while overcoming the foregoing problems associated with solely temperature controlled injection molding systems. However, it bears noting that the present shear-inducing injection molding system is versatile and compatible with temperature control injection molding. Accordingly, shear-inducing components of the present system may be used alone or in conjunction with temperature control in various embodiments.

In one non-limiting implementation, the localized shear strain may be imposed on the polymer melt by a rotatable or otherwise movable shear-inducing element disposed inside the hot drop injection nozzle. The shear-inducing element can be a component that traditionally exists in the hot drop nozzles or a new component that is added to enable the imposition of motion or shear on the polymer melt. In one embodiment, such a shear-inducing element may be the axially movable valve pin that resides inside hot drop nozzle which shuts off polymer flow through the nozzle to the mold cavities between molding cycles. In addition to linearly translating between open and closed positions of the nozzle, the valve pin can additionally be rotated at a desired rotational speed (RPM/revolutions per minute) about its centerline axis in a controlled fashion to induce shear on the polymer melt and maintain a minimum viscosity value selected to prevent polymer solidification and cold slug formation. In one embodiment, a mechanical drive mechanism may be provided to rotate the valve pin, as further described herein. Other methods however are possible to rotate the valve pin shear-inducing element.

In one aspect, a polymer injection molding system comprises: a manifold comprising a plurality of internal flow conduits configured for conveying polymer in a fluidic state; an injection nozzle fluidly coupled to the flow conduits, the injection nozzle configured to interface with a mold cavity and changeable between open and closed positions; the injection nozzle comprising a tubular valve body defining a pin axis, a central axial passage extending along the pin axis and configured to dispense polymer into the mold cavity, and an elongated valve pin movably disposed in the central axial passage; wherein the valve pin is rotatably movable and configured to induce shear on the polymer. A rotary drive mechanism coupled to the valve pin is operable to rotate the valve pin. The valve pin may be rotated when the injection nozzle is in the closed and/or open positions.

In another aspect, an injection nozzle assembly for a polymer molding system comprises: an injection nozzle configured to receive polymer in a fluidic state, the injection nozzle configured to interface with a mold cavity and changeable between open and closed positions; the injection nozzle comprising a tubular valve body defining a pin axis, a central axial passage extending along the pin axis and configured to dispense polymer into the mold cavity, and an elongated valve pin movably disposed in the central axial passage; the valve pin being axially translatable between a projected position when the injection nozzle is in the closed position, and a retracted position when the injection nozzle is in the open position; and the valve pin further being rotatably movable and operable to induce shear on the polymer.

In yet another aspect, a method for operating a polymer injection molding system comprises: providing an injection nozzle fluidly coupled to a source of fluidic polymer, the injection nozzle being in a closed position; coupling the injection nozzle to a mold cavity; changing the injection nozzle to an open position by moving a valve pin inside the injection nozzle to a retracted position; injecting the fluidic polymer into the mold cavity; changing the injection nozzle to the closed position by moving the valve pin to a projected position; and rotating the valve pin about its axis to impart a shear force on the fluidic polymer.

