1. Field of the Invention
This invention relates to an apparatus and method for converting the circular flow inside a melt channel to a uniform annular flow. More specifically, this invention relates to an apparatus and method for improving uniform melt flow and elimination of stagnation points as it passes through an injection molding machine and/or hot runner system.
2. Summary of the Prior Art
The large number of variables in the injection molding process creates serious challenges to creating a uniform and high quality part. These variables are significantly compounded within multi-cavity molds. Here we have the problem of not only shot to shot variations but also variations existing between individual cavities within a given shot. Shear induced flow imbalances occur in all multi-cavity molds that use the industry standard multiple cavity xe2x80x9cnaturally balancedxe2x80x9d runner system whereby the shear and thermal history within each mold is thought to be kept equal regardless of which hot-runner path is taken by the molten material as it flows to the mold cavities. These flow imbalances have been found to be significant and may be the largest contributor to product variation in multi-cavity molds.
Despite the geometrical balance, in what has traditionally been referred to as xe2x80x9cnaturally balancedxe2x80x9d runner systems, it has been found that these runner systems can induce a significant variation in the melt conditions delivered to the various cavities within a multi-cavity mold. These variations can include melt temperature, pressure, and material properties. Within a multi-cavity mold, this will result in variations in the size, shape and mechanical properties of the product.
It is well known that providing for smooth flow of pressurized melt is critical to successful molding of certain materials. Sharp bends, corners or dead spots in the melt passage results in unacceptable residence time for some portion of the melt being processed which can cause too much delay on color changes and/or result in decomposition of some materials or pigments of some materials such as polyvinyl chloride and some polyesters or other high temperature crystalline materials. In most multi-cavity valve gated injection molding systems it is necessary for the melt flow passage to change direction by 90xc2x0 and to join the bore around the reciprocating valve stem as it extends from the manifold to each nozzle.
These problems necessarily require fine tolerance machining to overcome and it is well known to facilitate this by providing a separate bushing seated in the nozzle as disclosed in U.S. Pat. No. 4,026,518 to Gellert. A similar arrangement for multi-cavity molding is shown in U.S. Pat. No. 4,521,179 to Gellert. U.S. Pat. No. 4,433,969 to Gellert also shows a multi-cavity arrangement in which the bushing is located between the manifold and the nozzle. Also shown in U.S. Pat. No. 4,705,473 to Schmidt, provides a bushing in which the melt duct in the bushing splits into two smoothly curved arms which connect to opposite sides of the valve member bore. U.S. Pat. No. 4,740,151 to Schmidt, et al. shows a multi-cavity system with a different sealing and retaining bushing having a flanged portion mounted between the manifold and the back plate.
U.S. Pat. No. 4,443,178 to Fujita discloses a simple chamfered surface located behind the valve stem for promoting the elimination of the stagnation point which would otherwise form.
U.S. Pat. No. 4,932,858 to Gellert shows a separate bushing seated between the manifold and the injection nozzle in the melt stream which comprises a melt duct with two smoothly curved arms which connect between the melt passage in the manifold and the melt passage around the valve stem in an effort to eliminate the stagnation points.
Another valve nozzle device has also been known, the device having a number of valve nozzles as shown in FIGS. 7 and 8. The plastic resin is passed through a first passageway 30 and then passed through a second passage 30a extending substantially at right angles with respect to the first passage 30 into valve chambers and then injected into metal molds through nozzles (not shown). Needle valve 32 is provided adjacent to the nozzle.
With the above described construction of the conventional multi-valve nozzle device, since the second passage 30a extending substantially at right angles with respect to the first passage 30 is in a plane including the needle valve 32, resin is caused to stagnate at positions P1 and P2 as shown in FIGS. 7 and 8. The stagnation of the plastic resin causes a pressure loss in each valve chamber as well as inhibits color change and uniform melt velocity. Although the stagnation of the plastic resin may be more or less reduced by the application of the prior art, heretofore the complete elimination of the stagnation or and resulting non-uniform annular flow has been impossible.
Reference should also be made to the following references: xe2x80x9cAnalysis for Extrusion Die Designxe2x80x9d by B. Proctor, SPE ANTEC, Washington, D.C. pages 211-218 (1971); and xe2x80x9cExtrusion Dies for Plastics and Rubberxe2x80x9d by W. Michaeli, Carl Hanser Verlag, Munich, ISBN 3-446-16190-2 (1992).
There exists a need for a method and apparatus that substantially reduces the flow imbalances and stagnation points in an injection molding system and/or hot runner system that occurs as a result of the flow being diverted around a melt flow obstruction such as a valve stem, a nozzle, a nozzle tip, a valve stem guide, a torpedo, etc.
A flow deflector in a melt channel, preferably around a valve stem or other flow obstruction, where the melt flow is converted from circular flow to annular flow. The deflector comprises a cylindrical body with a gradually expanding channel disposed on its outer surface. The channel is such that a first and second wall of the channel form two symmetrical inverted funnel-shaped cavities as the melt travels down the cylindrical body. The walls of the channel are designed to have substantially the same length in the direction the melt travels. In this arrangement, the melt flow is constricted on the near side of the flow as it travels around the cylindrical body which in turn promotes the flow around the back of the cylindrical body. Promoting the flow around the back of the cylindrical body helps to xe2x80x9cwash-outxe2x80x9d any stagnation points whilst also promoting a uniform annular flow rate as the melt exits the large end of the channel.