Receptacles – Closures – Pivotable
Reexamination Certificate
1999-12-21
2003-04-15
Tentoni, Leo B. (Department: 1732)
Receptacles
Closures
Pivotable,
C264S328100, C264S328180
Reexamination Certificate
active
06547094
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to injection moulding processes, in particular to a process for injection moulding articles having thin sections such as thin-walled tubular containers as used in the cosmetics industry for lotions, moisturisers and the like.
FIELD OF INVENTION
Thin-walled tubular containers, such as those used in the cosmetics industry, are currently produced by a combination of extrusion, injection moulding and welding processes (generally referred to herein as the extrusion process). The body of the tube is extruded in the form of a continuous cylinder which is then cut into the desired length to form the body of the container. In a separate injection moulding process the “head and shoulders” of the tube are produced. The injection moulded “head and shoulders” are then welded to the extruded tube to form the container. Once the container is filled with product the tail end of the container is sealed by a further welding process. This process for producing tubes has a number of limitations, the main being the high equipment cost, the lack of variety of tube shapes that can be produced using it, no ability to provide various textured surface finishes or embossing as an integral part of the manufacturing process, and no ability to incorporate attachments/components such as closures and hooks during the manufacturing process. Low MFI polyethylene (MFI generally less than 2) is the preferred polymer for tube manufacture as it in general imparts the properties of good feel and flexibility required by customers and is suitable for extrusion processing. In addition, low MFI polyethylene offers sufficient product resistance and barrier properties to make it suitable for most products currently packed into tubes. In cases where the barrier properties of polyethylene are inadequate for particular applications, medium density polyethylene (MDPE), high density polyethylene (HDPE), polypropylene (PP) and multilayer polymerfilms are commonly used.
While the injection moulding of articles such as thin walled containers has been proposed, it has hitherto not been possible to injection mould such articles having relatively long, thin sections without the articles being too susceptible to failure to be of commercial or practical use. The main problems have been associated with the polymers used to injection mould tubes, in that the process of moulding a cylindrical or other shaped tube requires the polymer to simultaneously have a high MFI to enable said polymer to flow down the long, narrow and curved path dictated by the tube shape without the use of excessive injection pressures, yet to have sufficiently good mechanical properties to be able to withstand handling and resist the stress cracking effects of many of the products that will be packed in it. In order to injection mould a tube, conventional techniques would require the polymer to have flow properties capable of forming moulded parts with radii and a length/thickness ratio of 100 and often higher. Forcing a ‘standard’ polymer to flow in a mould with such dimensions introduces severe stresses into the polymer, these stresses being “frozen” into the article thus produced when the polymer rapidly cools below its crystallising temperature before these stresses can be relieved. These stresses result in the tube having surprising different and deteriorated properties relative to the other products moulded from the same polymers under less severe moulding conditions.
Further stresses are introduced into the tubes when they are filled with product and then crimped d sealed—most often by heat sealing or ultrasonic welding. This process involves bending the ‘open’ end of the tube back on itself through an angle of up to 180° to form the fold at the edge of the seal. This fold is in the direction of the flow of the polymer, which direction having been demonstrated to be the direction of maximum weakness of the moulded product. This ‘folded and sealed’ area, where the tube is required to be deformed in order to effect a seal, is an area of the injection-moulded tube particularly susceptible to stress and flex cracking.
The following examples illustrate the special problems of injection moulding such tubes. Tubes were injection moulded using DuPont 2020T polymer, a polymer DuPont describe as “especially suited for injection moulded closure and extruded tubing where flexibility and maximum resistance to environmental stress cracking is required”. These tubes were moulded with extreme difficulty, requiring very high injection pressures and temperatures simply to get the 2020T to fill the mould. In each moulding significant degrees of core shifting/flexing were noted, due no doubt to the extremely high injection pressures that were required. In addition, it was noted that the tubes had virtually no resistance to flexing in the direction of the material flow, with significant cracking being induced with less than 5 manual squeezes of the tube. The environmental stress cracking of the same tubes was tested, and in spite of claims of “maximum resistance” to environmental stress cracking, was found to be totally inadequate for moulding thin-walled tubes by injection moulding.
In another illustration of the difficulty of injection moulding tubes, a Dow ‘Dowlex’ LLDPE pamphlet advises that LLDPE has substantially better ESCR properties than an equivalent high pressure LDPE. To illustrate the difference, the pamphlet states that in one comparative test a high Dow Dowlex LLDPE has an ESCR in oil some 80 times better than that achieved by a high pressure LDPE with the similar density and MFI (5700 hrs compared to 70 hrs). It further states that the LLDPE has an ESCR approximately 10 times better than the LDPE when immersed in a 10% Teric solution at 50° C. (225 hrs vs 26 hrs). However, contrary ro these observations, we have found that when these polymers are moulded in the form of thin-walled tubes and ESCR subsequently tested using a specially designed test method for assessing tube ESCR, both Dow's ‘Dowlex’ LLDPE 2517 and Kemcor's LD 8153 (a high pressure LDPE with similar MFI and density) performed poorly in 10% Teric N9 at 50° C., and both failed within 20 minutes—clearly indicating their unsuitability for tube manufacture by injection moulding. This poor result is illustrative of the highly unusual and difficult nature of manufacturing injection moulded thin-walled tubes acceptable to the market.
SUMMARY OF THE INVENTION
We have now found that it is possible to injection mould flexible thin-walled articles having relatively long thin-walled sections by selection of the polymers used in the injection moulding process having a time to failure of greater than 10 hours when tested according to the following procedure:
i) a plurality (preferably 6 or more) of strips of the polymer blend incorporating any post moulding treatment intended for the final article having cross-sectional dimensions of 0.65 mm in thickness and 10 mm in width are injection moulded under high shear, long flow length conditions, similar to those intended for use in the manufacture of the flexible thin-walled article.;
ii) the strips are bent back upon themselves and stapled 3 mm from the bend;
iii) the bent strips are immersed in a solution of a stress crack agent such as an ethoxylated nonylphenol, eg. a 10% solution of Teric N9 (nonylphenol ethoxylated with 9 moles of ethylene oxide—Orica Australia Pty Ltd) and held at a temperature of 50° C.;
iv) the strips are observed for signs of cracking; and
v) the time to failure is when 50% of the strips show signs of cracking.
Any reference to “an ESCR” throughout the specification and claims which follow, unless specfically stated otherwise, refers to an ESCR determined using the above test procedure. Accordingly, the invention provides a process for the manufacture of thin-walled articles comprising the steps of:
1) selecting a polymer blend having an ESCR of greater than 10 hours;
2) melting the polymer blend;
3) ramming the molten polymer blend into a mould having a cavity which produces
Dorsey & Whitney LLP
Tentoni Leo B.
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