The present invention relates to a process for manufacturing bodies of polymeric material by means of the blow-molding technique, in particular to provide bodies comprising a core of foamed material enclosed by an external shell.
Such bodies normally display a rigid structure and good properties of heat insulation, so to make the use thereof for such household electrical appliances, as refrigerators, and the like, particularly advantageous. At present, the technique of blow-molding is used in order to manufacture a hollow shell of thermoplastic material, inside which a thermosetting foam is subsequently injected, for example a polyurethane foam obtained by means of the well-known RIM technology. Such an operating way shows problems both in terms of possibility of finished article recycling, and in terms of environmental pollution by the foaming agent.
In fact, the chemical nature of the foamed polyurethanic core makes it impossible for the manufactured article to be recycled, and the chloro-fluoro-alkane foaming agent normally used in RIM technology causes problems of interaction with ozone in the stratosphere. Furthermore, the execution of the process according to the two-step technology known from the prior art (blow-molding and subsequent foam injection) causes the production cycle to be rather slow and expensive.
The purpose of the present invention is of supplying a process of the type as specified in the introduction to the specification, which does not display these drawbacks and which is simple and inexpensive to carry out.
According to the invention, such a purpose is achieved because the blowing is performed by introducing into the interior of a hollow thermoplastic body a foamable thermoplastic polymeric material suitable for bringing the walls of the hollow body into contact with the walls of a mold.
By "foamable thermoplastic polymeric material" any thermoplastic material mixed with a foaming agent, e.g., CO.sub.2, N.sub.2, hydrocarbons, are meant. It is evident that, as in the traditional blow-molding technology, the hollow extruded body (parison) is at a temperature close to the polymer softening temperature, and anyway such as to secure the "blowing" of the hollow body. This feature is particularly advantageous in the case of the present invention, because the foamed thermoplastic material (which also is at a temperature which is close to the polymer softening temperature) can be thermowelded to the walls of the hollow body, thus making it possible to obtain a structurally resistant manufactured article. A further advantage of the present invention relates to the possibility of using the same polymeric material, or materials belonging to the same polymeric family, for both the foamed core and the external shell, making it possible for the manufactured article to be recycled at the end of its useful life time. The process according to the present invention also makes it possible for the production cycle to be speeded up, as compared to the traditional technology of blow-molding and subsequent foaming of the cooled hollow shell.
A further purpose of the present invention is providing a device for carrying out the above said process.
Such a device, of the type which comprises an extruder with an accumulation chamber suitable for cyclically extruding a tubular parison between two mobile half-molds, is characterized in that said device also comprises a second extruder for a polymeric material admixed with a foaming agent, and an auxiliary accumulation chamber in communication with said second extruder, and equipped with an injection nozzle centrally installed relative to a ring-shaped nozzle for the tubular parison extrusion.
Further advantages and characteristics of the process and device according to the invention will be clear from the following disclosure in detail, supplied for merely exemplifying, non-limitative, purposes, by referring to the accompanying drawings, in which: