Plastic and nonmetallic article shaping or treating: processes – Pore forming in situ – Composite article making
Patent
1997-03-27
1998-08-25
Kuhns, Allan R.
Plastic and nonmetallic article shaping or treating: processes
Pore forming in situ
Composite article making
264 50, 264 54, 264572, B29C 4404, B29C 4406
Patent
active
057980632
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
The present invention relates to a process for making an injection molded article having a substantially smooth outer surface and a foamed core, as well as to a molded article so produced.
BRIEF DESCRIPTION OF THE PRIOR ART
In the case of conventional low pressure injection molding systems, foaming occurs virtually instantaneously as the melt enters the cavity and melt pressure decays to atmospheric pressure. As a result, however, articles so molded tend not to have smooth outer surfaces.
Various processes for making an injection molded article with a substantially smooth outer surface and a foamed core are known in the art. One such process is referred to as a "high pressure molding process". This process operates by maintaining a homogeous melt phase of a thermoplastic polymer containing dissolved gas within a molding cavity after injection of the polymer. Foaming of the polymer melt is induced by the controlled decay of melt pressure to levels below the solution pressure of the dissolved gases. The control over melt pressure decay rates is achieved by mechanical movement of a mold face, a retracting pin (in an integral skin molding system) or a control over the decay rate of air pressure within the cavity (in a counter pressure foam molding system); c.f "The Injection Moulding of Cellular Thermoplastics", 1985, a brochure available from Schering Industrial Chemicals, Cambridge, England.
These known processes have certain disadvantages. A molding process which makes use of integral skin molding can only be carried out with equipment of relatively simple design. The mechanical movement of the mold face necessitates that the mold have a simple shape, and thus, such processes are only suitable for preparing articles of simple shape.
A process that makes use of counter pressure foam molding has the disadvantage that permits a maximum reduction in density of only about 8% over a non-foamed article. For certain end uses, it is desirable to have very low density, i.e. highly foamed articles.
Accordingly, it is an object of the present invention to provide a process for making an injection molded article having a substantially smooth outer surface a foamed core which is capable of forming articles having shapes more complex than those possible using integral skin molding technology and having densities lower than those possible using counter pressure foam molding technology (e.g. a reduction in density of 10% and greater).
The object of the present invention is obtained through a process comprising the steps of through an injection unit into a mold and maintaining said melt under a holding pressure to form an article having a substantially smooth outer surface; pressure into the mold to create a hollow space in the melt by returning a portion of the melt from the mold back to the injection unit; mold; and melt. detail below.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 shows a molding apparatus suitable for use in the present invention, consisting of a mold 1, an injection molding machine 3 having a injection unit 2, a gas injection needle 4 and a pressurized gas source 5 for the gas injection needle 4. Thermoplastic melt is indicated as 6.
FIG. 2 depicts the apparatus of FIG. 1 after pressurized gas is introduced into mold 1 to force a portion of the melt back into injection unit 2 to create hollow space 7.
FIG. 3 indicates a pressure release of the gas which allows foaming of the melt.
FIG. 4 shows removal of the molded article after sufficient cooling.
DETAILED DESCRIPTION OF THE INVENTION
As used herein, a "substantially smooth" outer surface of the foamed article of the present invention means an outer surface (skin) that is substantially free from pits, surface irregularities, roughness and other such surface defects, and which approximates the smoothness of a non-foamed article.
The present invention can be practiced with molding devices and equipment that are currently commercially available. Such equipment includes a standard injection molding machine with a shut-off no
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patent: 5474723 (1995-12-01), Horikoshi
Bender Klaus
Leboeuf Philippe
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E. I. Du Pont de Nemours and Company
Kuhns Allan R.
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