Polyolefin synthetic resin in-mold foam molding method

Details Polyolefin synthetic resin in-mold foam molding method Polyolefin synthetic resin in-mold foam molding method

2000-11-15

2002-12-31

Kuhns, Allan R. (Department: 1732)

Plastic and nonmetallic article shaping or treating: processes

With measuring, testing, or inspecting

Controlling heat transfer with molding material

C264S051000, C264SDIG001, C425S143000

Reexamination Certificate

active

06500368

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an in-mold foam molding method employing prefoamed beads comprising polyolefin synthetic resin, and to in-mold foam molded articles.
2. Description of the Related Art
Existing in-mold foam molding apparatuses for producing molded articles using prefoamed beads comprising thermoplastic synthetic resins include apparatuses comprising a core mold and a cavity mold; filling means for filling the mold cavity formed between these two molds with prefoamed beads; steam feed means for passing steam through the prefoamed beads packed into the mold cavity in order to heat, foam, and fuse the preformed beads; and cooling means for cooling the molded article by spraying cooling water onto the back faces of the core mold and cavity mold. Chambers are defined to the rear of the core mold and of the cavity mold, and the core mold and the cavity mold are provided with air orifices, such as core vents or core vent holes, which communicate with the mold cavity. During the prefoamed bead filling operation, the chambers function as outlet spaces for air entering the mold cavity together with the prefoamed beads; during heating, foaming, and fusing thereof, they function as chambers for supplying steam to the mold cavity; and during cooling, they function as cooling compartments wherein cooling water may be sprayed onto the back faces of the core mold and cavity mold.
The method for molding a molded article using this type of molding apparatus is basically composed of the following four steps.
During the initial filling process, a pressure differential is created between the mold cavity and a starting material tank that contains prefoamed beads, and the prefoamed beads are carried from the starting material tank into the mold cavity on a stream of air so as to fill the mold cavity. Typical filling methods include cracked filling, compression filling, pressurized filling, and the like.
In a subsequent heating/fusing step, steam pressure is caused to act on the chambers, whereupon the prefoamed beads are heated and fused by steam entering the mold cavity via the air orifices. Since air remaining in the spaces between prefoamed beads can make it difficult for the prefoamed beads to fuse, steam is passed through the mold cavity to replace any air remaining in the spaces between prefoamed beads with steam before proceeding with heating/fusing.
In a subsequent cooling step, cooling water is directed onto the backs of the molds in order to cool the molded article. During this process the molded article is cooled indirectly via the molds by cooling water directed onto the molds from the back, and is also cooled directly by cooling water that penetrates into the mold cavity via the air orifices.
In a subsequent mold release step, the molds are parted and the molded article is released. The timing of mold parting is typically set with reference to time elapsed since the cooling water spray commences. When just released from the mold, the molded article has adequate shape retention due to the high vapor pressure and air pressure inside the beads, but with time the steam condenses and the molded article tends to contract. Accordingly, it is desirable that the molded article be adequately cooled, and where there are strict requirements as to the shape and dimensions of a molded article, the molded article may be set in a fixture until shape and dimensions stabilize.
The foam molding method described above is the principal molding method used currently. However, this molding method has a number of drawbacks, such as the following.
(1) To compensate for lower mold strength resulting from the molds being perforated by a multiplicity of air orifices, mold wall thickness in molds consisting of aluminum alloys must be of the order of 8-12 mm, for example. However, this has the effect of increasing the heat capacity of the mold, lowering thermal efficiency during heating/cooling so that the rate of temperature rise or temperature drop is slower, resulting in lower precision of control.
(2) Typically, a pair of molds is provided with some 2000 to 4000 air orifices, so the process of making the orifices is complicated and results in higher fabrication costs. Since the core vents are installed by hand in mounting orifices provided in the mold, the operation is quite complicated and damage to mold surfaces is unavoidable, thus requiring a retouching operation.
(3) Since clogging of air orifices such as core vents or core vent holes by scale or the like can result in heating defects, mold release defects, or cooling defects, the core vents must be replaced or periodically subjected to cleaning with high pressure water or to some other maintenance procedure.
(4) Since air orifices leave marks on foam molding surfaces, the visual appeal of molded articles suffers, and when the exterior surface is subjected to printing or the like, air orifice marks are an obstacle to attractive printing.
(5) Since the foam molded article is cooled by spraying cooling water into the chamber after molding, water infiltrates into the molding cavity through the air orifices, resulting in water content of about 6-10% in the molded article, necessitating a drying process. Further, since cooling water comes into direct contact with the molded article, cooling water quality must be controlled in order to produce uncontaminated molded articles.
(6) As all of the prefoamed beads are heated, expanded and fused under the same heating conditions by steam passing from the chamber into the mold cavity, molded articles produced in this way (hereinbelow referred to as isothermal molded articles) develop different surface qualities depending upon the extent of fusion of the beads. Specifically, lower fusion rates are associated with poor surface qualities, whereas higher fusion rates are associated with good surface qualities. For isothermal molded articles, higher bead fusion rates improve physical properties such as the mechanical strength of the molded article, but require longer heating, expansion/fusion times and cooling times, creating the problem of longer molding cycle times overall and reduced throughput.
For such reasons, in the molding technique described earlier, bead fusion rates in molded articles are typically set to 40%-80%, for example, in order to give good surface qualities and assure attractive appearance as well as assuring a fusion rate adequate to assure mechanical strength. However, even where mechanical strength requirements for a molded article are not particularly stringent, the need to assure an attractive appearance requires a moderately high fusion rate, which will result in a correspondingly longer molding cycle time and reduced throughput. Fusion rate as used herein is ascertained by splitting the molded article and observing the condition of the beads on the sectional face, specifically, by measuring the proportion of beads experiencing breakdown of the bead per se, deeming beads having cracking along the bead surface but no bursting of the bead per se to be unfused and deeming beads experiencing bursting of the bead per se into fragments to be fused.
The foam molding process described hereinabove is designed such that air orifices such as core vents and core vent holes are used to deliver steam, air, or other service fluids to the mold cavity or to evacuate same from the mold cavity during production of foam molded articles. However, as noted, the provision of air orifices creates a number of problems.
With the goal of providing a fundamental solution to these problems, the inventors conducted extensive research concerning development of a foam molding process which would employ molds devoid of air orifices, and conducted tests of various kinds. While the goal is a mold “devoid of air orifices,” it is of course necessary to provide, in lieu of core vents and core vent holes, some kind of passages for delivering/evacuating steam, air, or other service fluids to and from the mold cavity, which gives rise to the issue of where and how to form

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