Plastic and nonmetallic article shaping or treating: processes – Mechanical shaping or molding to form or reform shaped article – Shaping against forming surface
Reexamination Certificate
1998-06-15
2001-09-18
Heitbrink, Jill L. (Department: 1732)
Plastic and nonmetallic article shaping or treating: processes
Mechanical shaping or molding to form or reform shaped article
Shaping against forming surface
C264S328120, C425S555000
Reexamination Certificate
active
06290893
ABSTRACT:
BACKGROUND OF THE INVENTION
The field of the invention pertains to the injection molding of plastic materials and, in particular, to means and methods of improving the strength of plastic to plastic weld lines or knit lines in molded products and improving the ability to pack out various areas of a molded cavity. The invention also addresses methodology for improved venting of molds and the improvement of gateblush in molded parts.
Often in the injection molding manufacture of plastic products multiple gates or multiple flow paths or both are required in the mold. Welds or knit lines occur in the plastic where the flow paths join. Because of certain mold or product designs and injection molding techniques a “cold” weld or knit line can occur because the plastic does not fuse together completely. The long chain polymeric molecules from each flow front at the knit line fail to fully re-entangle and regain the integrity of the solidified melt. The result is a very weak area in the plastic product. Many polymeric materials, especially filled materials and liquid crystal polymers, exhibit an extreme loss of knit line strength relative to the balance of the product. Thus, knit line strength failure is a major cause of production rejects and the structural failures of plastic products in service.
U.S. Pat. No. 2,191,703 discloses means and method for the elimination of knit lines and welds by accurately controlling the location of the knit lines and by providing cavities in communication with the mold cavity at the knit line locations. The continued application of injection pressure forces the newly formed knit lines out of the mold and into the cavities provided. The result is a product substantially devoid of weak knit lines. Unfortunately, after ejection from the mold the product must be trimmed from the plastic formed in the cavities and then the trim line polished if needed for appearance.
U.S. Pat. No. 2,199,144 discloses the use of tangential injection about a toroidal shape such as a steering wheel thereby creating a continuous flow about the mold and a smooth merger of flows at each tangential gate.
Both of the above disclosures are directed to toroidal shapes with toroidal shaped strengthening inserts. These approaches to solving the weld or knit line problem are only applicable to a limited number of geometries and not generally applicable.
U.S. Pat. No. 4,399,093 discloses the use of differing cooling rates in the two flows of the molten plastic resin that merge at the knit line. Initially the resin flows fill the mold at relatively low pressure. Upon filling the mold cavity a high packing pressure is applied causing the colder less viscous resin to protrude into the warmer more viscous resin at the knit line. The result is a knit line of greater surface area and interlocking shape to improve the strength at the knit line. However, the differing temperatures of the resin flows are dependent on product shape and cooling rates within the mold and, therefor, limited to product shapes and molds that can produce the differential cooling rates in the resin flows without affecting product quality.
With a view toward providing a method of injection molding that has wider applicability in improving the knit line strength of plastic injection molded products, applicant has developed the new method and apparatus disclosed below.
SUMMARY OF THE INVENTION
The invention contemplates the use of packing pins in the mold which in construction are similar to the ejector pins in most conventional plastic injection molds. The new packing pins are sequentially actuated during or after filling of the mold or during both phases of the process. The actuation of the packing pins is normally continued during cooling as necessary to cause flow of plastic parallel or transverse to the original direction of flow. (The primary flow causing the knit line to be strong will be the flow parallel rather than transverse.) The to and from kneading action, especially internal to the frozen plastic skin, results in the equivalent of a scarf joint at the knit line. The layering effect perpendicular to the knit line greatly expands the surface layer contact between the two flows. The irregular shape of the layers enhances the mechanical strength of the joint in addition to increasing the likelihood that long chain molecules will re-entangle between the two melt flows. Further, the anisotropic characteristics in the direction of flow that occur with some plastics can be improved by the scarfing and layering at the knit line.
Other applications of the packing pin concepts are in areas of relatively small cross-section or other hard to pack areas in molded parts. The actuation of packing pins in a localized area or any number of areas can be either in concert using a common mechanism such as an ejector plate or separately using air or hydraulic cylinders, cams or electrical devices. With separate actuation, each area can be packed a different amount.
Packing amounts or degrees can also be adjusted or modified by adjusting the packing pin diameter or stroke or both depending on the application.
The are other areas of packing pin applications in insert molding where locator pins for insert location protrude into the cavity of the mold. These pins are retracted at a proper time, generally after filling. The function of the pins are to support and locate the insert in the cavity while the plastic flows around it. After the part is full, the pins are retracted at a proper time which is long enough to ensure that the insert stays at its desired location, but soon enough to allow plastic to flow into the void created by the retracting pin. Many times the pins must be pulled prematurely to allow flow into the void area. This results in movement of the insert and a resultant substandard or reject part. Using a packing pin in concert with a locator pin can eliminate or reduce the movement by providing additional localized material and displacing it into the locator pin void as or after the locator pin is pulled allowing the void to be filled with the displaced material.
The packing pin concept can also be used in a gate area to reduce gateblush caused by plastic entering the mold. Gateblush is caused by displacement movement of plastic and impinging on the cavity wall thus eroding the partially solidified plastic on the cavity surface. A packing pin can be used to retract the cavity wall from the mold surface in the blush area. After the mold filling is complete, the packing pin can return the cavity surface to the proper position. During fill, the material in the “well” area caused by the retracted packing pin will not be eroded and thus the blush area on its surface will not be present. In this situation, when a packing pin is pushed forward excess plastic may be trapped in the cavity if the gate has been sealed. Excess material can be allowed to discharge out of the gate and back into the sprue or runner of the mold if gate seal has not been effected. The method used (gate seal or non gate seal) depends on the application.
A packing pin can also be utilized in conjunction with a subgate or other type of gate allowing plastic to enter or leave a cavity and thus provide a valve gating action coincident with packing a part.
In the case of a vent exiting a part, the packing can be used as a “powered vent” shut off within the exit gate runner to enhance the ability to remove air and other gases from the mold cavity. The pin can be in a fully retracted position allowing generous venting action of the mold. In an intermediate position the pin will shut off the vent and still be retracted below the mold's surface and available for the final packing. The pin can then be activated to the final flush position within the cavity to complete the packing process as previously described.
Pins which are only used to shut off vents as described above and not used for additional packing are also considered part of this invention.
A tapered egress from the vent pin to the outside of the mold surface is needed to allow the clearing of t
Deimen James M.
Heitbrink Jill L.
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