Plastic article or earthenware shaping or treating: apparatus – With mechanical means forming or expanding pores
Reexamination Certificate
2002-08-30
2003-11-25
Davis, Robert (Department: 1722)
Plastic article or earthenware shaping or treating: apparatus
With mechanical means forming or expanding pores
C425S208000, C425S209000, C425S557000, C425SDIG002, C425S81700C, C366S088000, C366S089000, C366S091000
Reexamination Certificate
active
06652254
ABSTRACT:
TECHNICAL FIELD
The present invention relates to molding apparatus for use in a process for preparing thermoplastic resin molded articles, especially thermoplastic expansion-molded articles, or molded articles of thermoplastic resin having a high melt viscosity and difficult to mold in a molten state.
BACKGROUND ART
Expansion-molded articles of thermoplastic resin can be prepared by a process wherein a chemical blowing agent is used (for chemical expansion) or by a process wherein a gas, such as a chlorofluorocarbon, butane, pentane, carbon dioxide or nitrogen, is directly supplied to and dissolved in the resin for use as a blowing agent (for physical expansion). In recent years, there is a growing demand for producing expansion-molded articles of thermoplastic resin with use of carbon dioxide, nitrogen or like inert gas for physical expansion in consideration of sanitation and environment.
In view of the above situation, many studies have been made on processes for preparing expansion-molded articles of thermoplastic resin with use of inert gases (see, for example, JP-A No. 10-230528/1998).
However, since many of the conventional processes intend to solve the problem of how to produce expansion-molded articles of fine cells, they require very complex equipment comprising a plurality of devices. To realize physical expansion, there is a need to use a cylinder, screw and controller which are designed specially, in the gas impregnation step of melting a thermoplastic resin, supplying an inert gas to the molten resin and impregnating the resin with the gas by mixing. The equipment then requires devices which are made anew or substantial modification of existing devices to result in the problem of an increased cost.
In view of the foregoing problem of the prior art, an object of the present invention is to provide a molding apparatus which is adapted to realize a gas impregnation step by continuously supplying carbon dioxide, nitrogen or like inert gas to a thermoplastic resin at a relatively low pressure with good stability and using a screw having such an overall length as to permit the use of, an existing molding machine cylinder as it is.
DISCLOSURE OF THE INVENTION
We have conducted research not from the viewpoint of how to produce expansion-molded articles of very fine cells but from the viewpoint of how to realize physical expansion with use of an inert gas, such as carbon dioxide or nitrogen, with the greatest possible ease and at as low a cost as possible. Consequently, we have realized physical expansion easily at a low cost by causing a screw to perform all the functions of melting a thermoplastic resin, supplying an inert gas to the molten resin and mixing the gas with the molten resin for impregnation, utilizing existing injection molding cylinder, controller, etc. as they are.
The molding apparatus of the invention for preparing molded articles of thermoplastic resin is an apparatus for use in practicing a molding process for producing molded articles of thermoplastic resin which comprises the gas impregnation step of supplying an inert gas from a gas supply opening in a screw to a thermoplastic resin melted by the rotation of the screw within a molding machine cylinder to impregnate the molten resin with the inert gas, and the molding step of obtaining an expansion-molded article from the gas-impregnated molten resin, the apparatus being characterized in that the screw for use in the gas impregnation step comprises a resin melting portion positioned in an upstream region of the cylinder for rendering the resin molten, a molten resin nonfilled-up portion integral with a downstream side of the resin melting portion for making the resin lower than the inert gas at the gas supply opening in pressure, and a gas impregnation portion integral with a downstream side of the molten resin nonfilled-up portion for supplying the inert gas and impregnating the molten resin with the inert gas.
The molten resin nonfilled-up portion of the screw is a portion formed by reducing the screw shaft diameter or increasing the flight pitch thereof. The provision of the molten resin nonfilled-up portion produces an enlarged space defined by the screw fight, the cylinder and the screw shaft, whereby the resin within this space can be made lower than the gas at the gas supply opening in pressure. The molten resin nonfilled-up portion extends from the downstream end of the resin melting portion to a position downstream therefrom where the space defined by the screw fight, the cylinder and the screw shaft is greatest. The space defined by the screw fight, the cylinder and the screw shaft is smaller in the gas impregnation portion extending from the downstream end of the nonfilled-up portion than in the molten resin nonfilled-up portion, so that the impregnation portion becomes gradually filled up with the molten resin which used to be in the nonfilled-up state. Thus, the gas impregnation portion has two states as shown in
FIG. 4
, i.e., a molten resin nonfilled-up state in the upstream region and a molten resin filled-up state in the downstream region. Since the screw has the gas supply opening in the region of such a molten resin nonfilled state, a required amount of the inert gas is supplied to the molten resin with good stability.
If an attempt is made to additionally provide the gas impregnation portion at the forward end of the screw of the conventional apparatus, the screw becomes greater in overall length by an amount corresponding to the added portion, and the cylinder of an existing molding machine can not be utilized, whereas the construction of the invention described above makes it possible to provide the gas impregnation portion while enabling the screw to retain such an overall length as to permit the use of the cylinder of an existing molding machine.
In the molding apparatus described, the screw preferably has a gas inlet channel formed in the upstream end thereof, and a gas supply channel communicating with the gas inlet channel and extending through the screw longitudinally thereof, the gas supply opening being formed in the gas impregnation portion and communicating with the gas inlet channel via the gas supply channel. It is then possible to use an existing cylinder without making almost any modification therein, that is, without modifying the gas inlet, gas supply channel and gas supply opening thereof.
The resin melting portion of the screw comprises a solid (hereinafter “solid” means powder, pellets or the like) transport part disposed at an upstream position and having a small screw shaft diameter, a molten resin transport part disposed a downstream position and having a large screw shaft diameter, and a compression melting part positioned between the two parts and having a screw shaft diameter gradually increasing downstream, the lengths of the screw parts and portions preferably having the following relationships with the cylinder diameter D:
length L
1
of the solid transport part=5D-10D,
length L
2
of the compression melting part=3D-6D,
length L
3
of the molten resin transport part=1D-4D,
length L
4
of the molten resin nonfilled-up portion=0.1D-2D, and
length L
5
of the gas impregnation portion=4D-10D. The resin can then be plasticized with heat and also with the inert gas which affords an additional plasticizing effect, with the result that the molten resin is impregnated with the inert gas positively and plasticized more effectively with the inert gas. The plasticizing effect produced by the inert gas refers to the phenomenon shown in
FIG. 3
that molecules of the inert gas (carbon dioxide in the illustration) dissolving in the resin between molecular chains thereof expand the spaces between the molecular chains, consequently increasing the free volume of the molecular chains to plasticized the resin substantially in the same manner as by the plasticization with heat.
The solid transport part is given a length L
1
of 5D to 10D (optimally about 8D) because a diminution in the solid transport part due to the metering stroke i
Hirano Hiroyuki
Kawauchi Hitoshi
Shimura Satoshi
Davis Robert
Del Sole Joseph S
Rader Fishman & Grauer
Sekisui Chemical Co. Ltd.
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