Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Cellular products or processes of preparing a cellular...
Reexamination Certificate
1999-08-17
2001-08-21
Kuhns, Allan R. (Department: 1732)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
Cellular products or processes of preparing a cellular...
C264S045300, C264S045500, C264S050000, C264S053000, C264S054000, C264S510000, C521S134000, C521S143000, C522S157000
Reexamination Certificate
active
06277896
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to a method for manufacturing a monolithic plastic object which is constructed as structural foam and define a sandwich-like structure with a foam core and closed skin.
2. Description of the Prior Art
Such a method and board obtained therewith is known for instance from U.S. Pat. No. 4,096,218.
Known from the '219 patent is a method according to which a few seconds after completion of injection of the plastic the mould cavity is enlarged in a predetermined time and in a controlled manner. By making use of a foaming agent and application of gas counter pressure, it can thus be achieved that the foam structure is of good quality, while the skin of the obtained board has a closed and smooth structure.
According to the said '218 patent, the mould is held for a predetermined time in its closed starting position, whereafter the mould undergoes a determined expansion in a subsequent specified period. After reaching maximum expansion cooling takes place for a time, whereafter the formed board can be removed.
Attention is also drawn to U.S. Pat. No. 4,133,858, which deals with a related technique and wherein mention is also made of a number of possible plastics for use in the context of this technique and substances such as foaming agents for adding thereto.
Applicant's own European patent application EP-A-0 692 358 relates to a further development of the technique as described in the above stated two references. According to this patent application, opening of the mould takes place in a very well defined and controlled manner, whereby in principle a plastic board of good quality can be obtained. The drawback of this technique however is that the various parameters involved must be controlled within very narrow limits, which cannot always be realized in practice or does not result in an optimum product or process.
Since it has been found that with the known method boards of a sufficiently high and constant quality cannot be obtained, it is an object of the invention to provide a method with which an object, for instance a board, of improved quality can be manufactured.
It has been found that with the prior art method there is only a very limited freedom to vary the relevant parameters in order to obtain objects with different compositions and dimensions. As a result it cannot always be prevented that, where there is insufficient control or freedom of a relevant parameter, the obtained product does not comply with the required quality standards.
It is therefore an object of the invention to increase the freedom of choice and the tolerance limits of the various relevant parameters.
It is a further object of the invention to make the skin thickness better controllable and even practically fully controllable.
Yet another object of the invention is to shorten a production cycle.
Finally, it is an object of the invention to make the foam density distribution considerably better controllable than is the case with the prior art.
SUMMARY OF THE INVENTION
In respect of the above, the present invention provides a method for manufacturing a monolithic plastic object, for instance a board, which object defines a sandwich-like structure with a foam core and closed skin, and wherein the method may comprise the following steps of:
(1) providing an injection moulding device with a plastic plasticizing and injection device and a mould which includes at least two mould parts displaceable relative to each other by displacing means, which mould parts in a closed first extreme position define a mould cavity connecting onto the plasticizing and injection unit and in an open second extreme position release a formed object for removal, wherein the displacing means are adapted to move the mould parts at a chosen speed;
(2) providing foaming means for causing foaming in the mould cavity of plastic injected into the mould cavity;
(3) providing gas feed means for feeding gas under pressure to the mould cavity to temporarily render the foaming means effectively inactive;
(4) energizing the feed means;
(5) energizing the plasticizing and injection unit in the first extreme position of the mould in order to inject heated plastic into the mould cavity, which plastic is a mixture of two plastics, wherein first plastic is a random plastic or mixture of plastics, and, in particular, a non-type A plastic, and wherein the second plastic is type A that constitutes a maximum of 20 percent by mass of the mixture, wherein the type A plastic is a plastic with high melt strength compatible with the first plastic, for instance HMS PP which is high-melt-strength polypropylene, that is obtained by subjecting polypropylene to one of electron bombardment, electron beams, chemical modification and direct polymerization;
(6) ending the energizing of the gas feed means;
(7) ending the energizing of the plasticizing and injection unit;
(8) energizing the displacing means such that the mould parts move apart in accordance with a chosen velocity progression; and
(9) causing the thus formed object to cool to a chosen temperature, fully opening the mould cavity and removing the cooled object.
REFERENCES:
patent: 4096218 (1978-06-01), Yasuike et al.
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patent: 5116881 (1992-05-01), Park et al.
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patent: 5180751 (1993-01-01), Park et al.
patent: 5567742 (1996-10-01), Park
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patent: 0692358 A1 (1996-01-01), None
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Database WPI, Week 9306, Accession No. 93-049320, Abstract of JP 5000429, Jan. 8, 1993, 1 page.
Boshuis Johan
de Jong René
Eidhof Andreas Bernardus
Jagers Christiaan Antonius
Richters Franciscus Egbertus
Corell Resin Technology B.V.
Kuhns Allan R.
Webb Ziesenheim & Logsdon Orkin & Hanson, P.C.
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