This invention relates to an injection foaming process in which a physical foaming agent is injected into an injection molding machine and mixed with a melted thermoplastic resin before injection foaming, and to an injection molding machine and resin composition suitable therefore. More specifically, this invention relates to a process for obtaining a foam by injecting a physical foaming agent into the cylinder of an injection molding machine at a pressure lower than the storage pressure of the physical foaming agent, mixing it with a melted resin and expanding the volume of the cavity of the mold at the time of injection, and to an injection molding machine and resin composition suitable therefore.
In the past, study has been made of the use of an injection molding method using thermoplastic resin in which foaming is conducted with an aim of reducing materials usage and product weight. As the method for carrying out injection foaming, a method in which a chemical foaming agent of the thermal decomposition type such as azodicarboxylic acid amide is added directly to the raw material thermoplastic resin or used as a masterbatch is known.
Since such chemical forming agent is available easily and usable on a common in-line-type injection molding machine, it has been used widely in injection foaming. However, when a chemical foaming agent is added to pellets, a relatively large amount such as 1 to 5 wt % of the chemical foaming agent is needed, and in addition, the foaming agent in power state and the pellets got separated, causing uneven foaming. Besides, in the case of a chemical foaming agent being as a masterbatch, part of the foaming agent is decomposed due to the heat in the extruder when it is produced. This causes a problem of lower efficiency in the generation of gas at the time of injection foaming.
On top of that, chemical foaming agents are expensive, and this is making the materials cost higher. Some of them entails the generation of harmful gases such as carbon monoxide. Besides, in some cases, decomposition residues of a chemical foaming agent remained in the cylinder of the injection molding machine for a long time and appeared on the surfaces of molded articles from time to time, spoiling their appearance.
A proposal has also been made for physical foaming in which propane, nitrogen gas, carbon dioxide, or the like is used as the foaming agent in place of a chemical foam agent. Especially, the extrusion foaming method using these physical foaming agents is used widely for polystyrene paper, etc. In this extrusion foaming method, a physical foaming agent is injected under pressure into the cylinder of an extruder through a hole made in the middle of the cylinder (Japanese Patent Publication HEI7-16450 and Japanese Patent Publication HEI8-81590, for example). In the case of extrusion foaming, products whose dimensions and expansion ratio are stable can be obtained by keeping the feed rates of the raw material resin and the physical foaming agent constant.
On the other hand, in the case of the in-line-type injection molding method using a physical foaming agent in injection molding, it happens in some cases that even if a hole is made in the cylinder for the injection of the physical foaming agent, it cannot be injected into the cylinder, depending on the position of the screw, because the relationship between the position of the injection hole and that of the screw is not always constant unlike in the case of an extruder since the resin is plasticized, metered and injected as the screw moves forward and backward in the cylinder.
In this way, it has been much more difficult to feed a physical foaming agent to an injection molding machine than to an extruder. Because of this, proposals have been made for improving the injection foaming method.
For example, as the injection foaming method using an organic solvent as a physical foaming agent so that the physical foaming agent may not evaporate easily when the resin is transported to the injection molding machine, a method (Japanese Patent Publication SHO 46-2184) in which a mixture of an organic agent and a resin is fed intermittently to an injection molding machine and a method (Japanese Patent Publication HEI 6-41344) in which an organic solvent and a thermoplastic resin are mixed by means of an extruder and then injected directly into a mold have been proposed. These methods require that because of the use of an organic solvent, modifications be made to the equipment so that it will be changed to an explosion-proof type. Furthermore, in the case in which such mixture is injected into the mold from an extruder, melted resin is continuously fed to the mold from the extruder, and consequently the resin is discharged out of the system at the time of the removal of the product while the resin is in the process of being foamed. This presents the problems of a possibility of the molded article being smeared with such discharged resin and a high rate of production of defective articles.
On the other hand, another method in which resin is foamed after it is impregnated with a physical foaming agent in supercritical condition has been proposed. This technology is known as microcellular foam technology (U.S. Pat. No. 5,158,986 (Japanese Patent 2625576) and U.S. Pat. No. 4,473,665, among others). This microcellular foam technology provides products capable of retaining impact resistance, despite being a foam, by making the cell diameter of the foam smaller than that at which cell failure takes place.
Proposal has been made for improvements in the method for injecting such physical foaming agent in supercritical condition and in the molding method so that the supercritical physical foaming agent may be applied to the injection foaming method.
For example, proposal has been made for a method in which a microcellular foam is obtained by saturating melted resin with a physical foaming agent, then injecting it into the cavity so pressurized (counterpressure) that the foaming agent will not be vaporized while operating the process so that the resin will become unstable when it is oversaturated, and subsequently expanding (core-backing) the volume of the cavity (U.S. Pat. Nos. 4,473,665 and 5,334,356).
In the injection molding method for obtaining a microcellular foam, it is necessary to dissolve a large amount of a physical foaming agent in melted resin so that it will come into oversaturated condition, to feed a physical foaming agent at high pressure and at raised temperature in some cases so that the physical foaming agent may come into supercritical condition in the cylinder of the injection molding machine, and in addition to use a pressure pump or the like to inject the physical foaming agent at a pressure level that will enable the foaming agent to overcome the high resin pressure. Furthermore, such injection molding method requires a system that will open and close the physical foaming agent injection hole made in the cylinder in response to the movement of the screw and will control the motion of the pressure pump, many sensors, and an information processing system; and as a result, such injection molding method becomes complex and expressive.
Furthermore, in the mold, it is necessary to seal the metal surfaces of the mold at the time of pressurizing the cavity (counter-pressurization) and backing the core by bringing the metal surfaces into close contact so that the high-pressure gas will not leak. However, it is difficult to accomplish this with the mold clamping force of a common injection molding machine, and because of this, gas tightness so required is normally attained by use of a packing made of rubber or the like. In spite of that, it has been difficult to produce a large volume of products because such packing slowly becomes worn out as the cavity is heated.
Moreover, since the degree of the gas tightness of the cavity is raised, there has been a possibility that the counter-pressurizing gas is not released from the cavity at the time of injection with the result that there will be a gas holdup in the cavity, causing a dent in molded articles and consequently a decline in their appearance.
Further, in Japanese Patent Publication HEI 11-34129, Japanese Patent Publication HEI 11-34130 and WO 98/31521, proposal has been made for a physical foaming agent to be fed under high pressure into the cylinder from the middle of the cylinder of an injection molding machine having an in-line screw so that the foaming agent will come into supercritical condition. Injection of a physical foaming agent under high pressure will require a pressure pump and complex and expressive equipment.
Proposal is also made for a method in which the injection of a physical foaming agent can be carried out independently of the movement of the screw. For example, proposal is made in Japanese Patent Publication HEI 8-258096 for a method in which a physical foaming agent is injected from the end position of the screw through a hole made in the axial direction of the screw and mixed with melted resin. In this method, since a gas-permeable sintered metal or the like is used to prevent the melted resin from flowing back into the aforesaid injection hole, it is necessary to inject the foaming agent at higher pressure as a pressure loss occurs when the physical foaming agent passes through the sintered metal. Moreover in Japanese Patent Publication HEI 8-85128, proposal is made for a method in which injection foaming is carried out after the resin is impregnated adequately with a physical foaming agent such as carbon dioxide under pressure in the chamber set up between the hopper and cylinder of the injection molding machine and then sent to the cylinder. It is difficult to impregnate the resin with a physical foaming agent at a temperature in the vicinity of room temperature in a short time, and such method is not suitable for industrial continuous production.
Proposal is made for a method in which intermittent injection molding operation is made possible by continuously injecting a physical foaming agent into melted resin, feeding the melted resin to the cylinder of an injection molding machine, and storing the resin in the accumulator or discharging the resin out of the system, except at the time of metering (Japanese Patent Publication HEI 10-230528 and Japanese Patent Publication HEI 10-24436). According to this method, there is no need to conduct the injection of a physical foaming agent in synchronization with the motion of the mold and the screw, but since a pump is used for the purpose of pressurization, the manufacturing equipment becomes expensive. In addition, since the yield of the materials declines as the melted resin is discharged out of the system, the application of the method to the manufacture of inexpensive foam products has been difficult.
To meet the above challenges, the inventors conducted a study strenuously and successfully made the present invention.