Patent Application
20060071368
The present invention relates generally to molding plastic articles, and more particularly to a method and apparatus for compression molding plastic articles.
Various plastic articles, such as plastic closures and preforms for containers, have been formed by a compression molding process. Some compression molding machines have a plurality of tools mounted in a circumferential array on a rotatable turret in a plurality of opposed coacting pairs. The tools of each pair carry opposed male and female mold sections that when closed together form a cavity mold for compression molding the desired articles.
During compression molding, a molten plastic mold charge pellet is disposed within a mold cavity of the mold tooling. Initial compression of the mold charge pellet distributes the material throughout the mold cavity to define the part shape. However, the volume of molten plastic is significantly greater than solidified plastic. Accordingly, as the plastic solidifies within the molten cavity it takes up less space or volume. If the mold cavity volume is maintained constant throughout the compression molding cycle, voids, sinks or other variations may appear in the final product due to the material shrinkage.
A method of compression molding preforms for blow molding containers includes the steps of providing a preform mold that includes a mold cavity and a mold core, placing a mold charge of plastic resin into the mold cavity, advancing the mold core into the mold cavity to cause the mold charge resin to flow and fill a preform mold volume between the core and cavity, and then as the plastic resin cools and shrinks, advancing the mold core further into the mold cavity. Accordingly, in this method of compression molding plastic articles, the mold core is advanced into the mold cavity as the mold charge material cools and shrinks, to reduce the volume of the preform mold.
According to another aspect of the present invention, an apparatus for compression molding plastic articles includes at least one female mold section defining at least part of an open cavity, at least one male mold section including a mold core selectively receivable at least partially within the open cavity to define at least part of a preform mold cavity, and an alignment plate engageable with the female mold section to provide a seal between them and having an opening generally aligned with the opening of the cavity for receipt of the mold core through the opening. A mold core is received for reciprocation relative to the alignment plate and into the open cavity of the female mold section to change the volume of the preform mold cavity as the mold core moves relative to the female mold section.
These and other objects, features, advantages and aspects of the present invention will be apparent from the following detailed description of the preferred embodiments and best mode, appended claims and accompanying drawings in which:
Referring in more detail to the drawings,
The female mold section 16 preferably includes a main body 22 having a cavity 24 with an inner surface 26 defining an outer surface of a plastic preform 12. One or more coolant passages 28 may be formed in the main body 22 through which a fluid is directed to cool the main body 22 in use. The main body 22 may be fixed to a housing, or may be yieldably biased in the housing, such as to protect the mold section 16 from a machine failure or other system fault. The female mold section 16 may be constructed substantially as shown in U.S. patent application Ser. No. 10/822,299, the disclosure of which is incorporated herein by reference in its entirety.
In one presently preferred embodiment, at least one neck ring mold element, such as split neck ring sections, or such as a unitary neck ring plate 30, is carried by and may be integral with or fixed to the main body 22. The neck ring plate 30 has a opening 32 or passage therein with an inner surface 34 designed to define part of the preform mold cavity 20, and form at least a portion of a neck 36 of a preform 12. In the embodiment shown in
The male mold section 18 includes a mold core assembly 50, a carrier plate 52, and an alignment plate 54 carried by the carrier plate 52. The mold core assembly 50 includes a mold core body 56 with a central bore 58 and a mold core 60 at one end sized to be received in the cavity 24 of the female mold section 16 and the opening 32 of the neck ring plate 30 to define between them the preform mold cavity 20 in which a preform 12 is molded. A counterbore 62 is formed surrounding the blind bore 58 and an inner sleeve 64 is disposed within the counterbore 62. The outer diameter of the inner sleeve 64 is preferably smaller than the inner diameter of the counterbore 62 providing a gap 66 between them which may provide an air gap for insulation, or may receive a cooling fluid to facilitate cooling the mold core assembly 50. The inner sleeve 64 preferably has a through bore 68 aligned with the blind bore 58 and through which a coolant may be passed to facilitate cooling the mold core assembly 50 in use. The inner sleeve 64 may be retained on the mold core body 56 by a retainer 70 fixed to one end of the mold core body 56. The mold core assembly 50 may be constructed substantially as shown in U.S. patent application Ser. No. 10/822,299, the disclosure of which is incorporated herein by reference in its entirety. In addition, the mold core body 56 may have a radially outwardly extending flange 72 in which a plurality of holes 74 are provided to receive fasteners that connect an annular spacer plate 76 to the mold core body 56.
The carrier plate 52 is preferably annular with an opening 78 having an inner diameter that is preferably slightly larger than the outer diameter of the corresponding portion of the mold core body 56 so that a portion the mold core body 56 is slidably received for reciprocation through the opening 78. To facilitate assembly, the opening 78 may have a tapered entrance portion 80. A plurality of circumferentially spaced openings 82 are provided through the carrier plate 52 to receive fasteners that attach the alignment plate 54 to the carrier plate 52.
The alignment plate 54 is preferably generally annular, and carried by the carrier plate 52. A central opening 86 through the alignment plate 54 preferably has a minimum diameter sized to define a sealing surface that closely slidably receives the corresponding portion of the mold core body 56 permitting slidable reciprocation of the mold core body 56 relative to the alignment plate 54, while also providing a seal between them. As best shown in
The mold core body 56 is preferably connected to a first actuator 104 (
The carrier plate 52 is preferably associated with a second actuator 106 (
In a compression molding cycle, the carrier plate 52 and alignment plate 54 may move at the same rate and at the same time as the mold core body 56 to a first position, as shown in
The actuator or actuators 104, 106 that move the mold core assembly 50 and the alignment and carrier plates 52, 54, may include cams 110,112, respectively, engageable with appropriately contoured cam surfaces 114, 116, respectively, to cause the axial displacement of these components relative to the female mold section. The alignment and carrier plates 52, 54, as well as the mold core assembly 50 may be yieldably biased toward their retracted positions, such that when the cam assembly is not active to advance these components, they may retract to open the mold cavity 20 and permit a formed part to be removed therefrom and a fresh mold charge pellet to be added. Of course, other arrangements and actuators can be used, and another cam surface may be used to move the components toward their retracted positions. In addition to or instead of the cam actuator 104, the mold core assembly 50 may be yieldably biased by a spring or may be acted on by a fluid cylinder, servo actuator, cam or other device that advances the mold core as the plastic material shrinks to maintain a generally constant force, or other desired force in the preform mold cavity 20. The mold core 60 can also be advanced by an actuator at a predetermined rate generally equal to the rate of material shrinkage.
While certain preferred embodiments, constructions, arrangements, and aspects of particular components of the compression molding apparatus have been shown and described herein, one of ordinary skill in this art will readily understand that modifications and substitutions can be made without departing from the spirt and scope of the invention as defined by the appended claims. Further, relative adjectives like “upper”, “lower”, “radial”, “axial” and the like are used to describe features of the apparatus and method with respect to the position and orientation of such features as shown in the accompanying drawings of the presently preferred embodiments, and are not intended to limit the scope of the invention.
