Sleeve molding

Details Sleeve molding Sleeve molding

1998-03-17

2002-08-06

Ortiz, Angela (Department: 1732)

Plastic and nonmetallic article shaping or treating: processes

Direct application of fluid pressure differential to...

Producing multilayer work or article

C264S537000, C264S255000, C264S328800, C264S904000, C425S130000, C425S534000

Reexamination Certificate

active

06428737

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to a method and apparatus for making multilayer injection-molded plastic articles such as preforms, wherein the successive molding of an inner sleeve and outer layer enables cost-effective production of multilayer preforms for pasteurizable, hot-fillable, and returnable and refillable beverage containers.
BACKGROUND OF THE INVENTION
There is described in U.S. Pat. No. 4,609,516 to Krishnakumar et al. a method for forming multilayer preforms in a single injection mold cavity. In that method, successive injections of different thermoplastic materials are made into the bottom of the mold cavity. The materials flow upwardly to fill the cavity and form for example a five-layer structure across the sidewall. This five-layer structure can be made with either two materials (i.e., the first and third injected materials are the same) or three materials (i.e., the first and third injected materials are different). Both structures are in widespread commercial use for beverage and other food containers.
An example of a two-material, five-layer (2M, 5L) structure has inner, outer and core layers of virgin polyethylene terephthalate (PET), and intermediate barrier layers of ethylene vinyl alcohol (EVOH). An example of a three-material, five-layer (3M, 5L) structure has inner and outer layers of virgin PET, intermediate barrier layers of EVOH, and a core layer of recycled or post-consumer polyethylene terephthalate (PC-PET). Two reasons for the commercial success of these containers are that: (1) the amount of relatively expensive barrier material (e.g., EVOH) can be minimized by providing very thin intermediate layers; and (2) the container resists delamination of the layers without the use of adhesives to bond the dissimilar materials. Also, by utilizing PC-PET in the core layer, the cost of each container can be reduced without a significant change in performance.
Although the above five-layer, and other three-layer (see for example U.S. Pat. No. 4,923,723) structures work well for a variety of containers, as additional high-performance and expensive materials become available there is an on-going need for processes which enable close control over the amount of materials used in a given container structure. For example, polyethylene naphthalate (PEN) is a desirable polyester for use in blow-molded containers. PEN has an oxygen barrier capability about five times that of PET, and a higher heat stability temperature—about 250° F. (120° C.) for PEN, compared to about 175° F. (80° C.) for PET. These properties make PEN useful for the storage of oxygen-sensitive products (e.g., food, cosmetics, and pharmaceuticals), and/or for use in containers subject to high temperatures (e.g., refill or hot-fill containers). However, PEN is substantially more expensive than PET and has different processing requirements: Thus, at present the commercial use of PEN is limited.
Another high-temperature application is pasteurization—a pasteurizable container is filled and sealed at room temperature, and then exposed to an elevated temperature bath for about ten minutes or longer. The pasteurization process initially imposes high temperatures and positive internal pressures, followed by a cooling process which creates a vacuum in the container. Throughout these procedures, the sealed container must resist deformation so as to remain acceptable in appearance, within a designated volume tolerance, and without leakage. In particular, the threaded neck finish must resist deformation which would prevent a complete seal.
A number of methods have been proposed for strengthening the neck finish. One approach is to add an additional manufacturing step whereby the neck finish, of the preform or container, is exposed to a heating element and thermally crystallized. However, this creates several problems. During crystallization, the polymer density increases, which produces a volume decrease; therefore, in order to obtain a desired neck finish dimension, the as-molded dimension must be larger than the final (crystallized) dimension. It is thus difficult to achieve close dimensional tolerances and, in general, the variability of the critical neck finish dimensions after crystallization are approximately twice that prior to crystallization. Another detriment is the increased cost of the additional processing step, as it requires both time and the application of energy (heat). The cost of producing a container is very important because of competitive pressures and is tightly controlled.
An alternative method of strengthening the neck finish is to crystallize select portions thereof, such as the top sealing surface and flange. Again, this requires an additional heating step. Another alternative is to use a high T
g
material in one or more layers of the neck finish. This also involves more complex injection molding procedures and apparatus.
Thus, it would be desirable to provide an injection-molded article such as a preform which incorporates certain high-performance materials, and a commercially acceptable method of manufacturing the same.
FR-A-2538297 to Aoki describes a two-step molding process, wherein a full-length inner sleeve of PET is formed in a first molding step on a first core, and the core and sleeve are transferred to a second cavity where a full-length outer layer of polycarbonate is molded over the sleeve. Aoki is directed to providing a specific molding apparatus in which a plurality of neck molds are mounted for rotational movement on a rotating platen, and a pair of injection cores are disposed for rotational movement above inner and outer molding stations, so that one of the injection cores is disposed above that portion of the rotating platen which is in line with the stopping position at the neck mold so that one of the injection cores can pass through one of the neck molds before it is inserted into the inner molding station.
GB-A-142956 to Bonis describes a two-step molding process, wherein a full-length inner sleeve is formed in a first molding step on a first core, and the core and sleeve are transferred to a second cavity where a full-length outer layer is molded over the sleeve.
JP-5-73 568 to Mitsubishi describes a two-step molding process, wherein a full-length inner sleeve of PET is formed at a first molding step on a first core, and the core and sleeve are transferred to a second cavity where a full-length outer layer of a mixture of PET and gas barrier resin is molded over the sleeve. Mitsubishi describes a specific process having a very long processing time, i.e., the inner layer is formed of a high copolymer which is processed for 30 minutes in order to increase the density. The outer layer includes a gas barrier polymer mixed with PET in order to improve the adhesion with the inner PET layer.
SUMMARY OF THE INVENTION
The present invention is directed to a method and apparatus for making a multilayer injection-molded plastic article, such as a preform, which is both cost-effective and enables control over the amounts of materials used in the various layers and/or portions of the article.
According to a method/embodiment of the invention, an inner sleeve is molded on a first core positioned in a first mold cavity. The inner sleeve is only partially cooled before being transferred while still at an elevated temperature to a second mold cavity where an outer layer is molded over the inner sleeve. By providing the inner sleeve in the second mold cavity at the elevated temperature, bonding between the inner sleeve and outer layer is enabled during the second molding step, such that layer separation is avoided in the final molded article. The inner sleeve may comprise a full-length inner sleeve, extending substantially the full length of the article, or alternatively may comprise only an upper portion of the article, in which case the outer layer comprises a lower portion of the article and there is some intermediate portion in which the outer layer is bonded to the inner sleeve.
In one embodiment, a first thermoplastic material is used to make an

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