1. Field of the Invention
The present invention relates to molding processes for molding compounds which contain as a binder or other ingredient a material which is cured or solidified upon heating with evolving gas by heating the molding compound under pressure in a mold. The present invention also relates to molding devices for use in such molding processes.
2. Description of the Prior Art
In recent years, resinous moldings and fiber-reinforced resinous composite materials have come to be widely used in everyday commodities and industrial components. More recently, carbon fiber reinforced carbon composite materials, or carbon-fiber/carbon composite materials (C/C materials), have attracted attention as materials especially suited for space-craft or air-craft components, automobile components, and medical equipment.
The above-mentioned resinous moldings and fiber-reinforced resinous composite materials are usually produced by a process in which a molding compound or composition containing a thermosetting resin as a binder is molded by the application of heat and pressure. In the production of carbon-fiber/carbon composite materials, a precursor material, obtained by a molding process, is carbonized and sometimes graphitized. The precursor of the carbon-fiber/carbon composite material is produced by molding a molding compound containing a thermosetting resin or a pitch (e.g. coal tar pitch) as a binder. In the case in which pitch is used, the pitch-containing molding compound is molded into precursor moldings by heating the molding compound, under a compressive force, to the solidifying or setting temperature (about 500.degree. C.) of the pitch. The solidifying or the setting of the binder due to heat is referred to hereinafter as the setting of the binder.
However, the production of moldings of larger dimensions by the above-mentioned processes was difficult for the following reasons. When the molding compound is heated to a high temperature in the mold, the thermosetting resin or the pitch present in the compound as the binder generates a gas and water vapor before it is set to form moldings. Thus, if the generated gas accumulates in the middle portion of the molding and remains therein during the setting stage of the binder, laminar fractures are formed therein. If, on the other hand, the generated gas remains scattered over the whole volume of the molding during the setting stage of the binder, gas bubbles are left in the molding. When the thickness of the moldings is large, it is difficult for the generated gas to escape from the interior of the molding. Thus, moldings of larger thicknesses tend to include fractures or bubbles therein.
A more specific mechanism of the laminar fracture formation in the middle portion of the molding along the thickness thereof is as follows. FIG. 1 schematically shows the setting stage of the molding compound 4a, 4b, and 4c in a metallic mold, which comprises a frame 1 having a vertical bore la extending therethrough, and upper and lower dies or punches 2 and 3 slidably inserted in the bore la. The molding compound 4a, 4b, and 4c including pitch or a thermosetting resin as a binder is heated and pressed by the upper and lower punches 2 and 3. For the purpose of effecting uniform setting of the compound, the upper and lower punches 2 and 3 are heated by sheathed heaters to equal temperatures at the surfaces thereof contacting the molding compound. It is inevitable, however, that the temperature varies along the thickness of the molding compound, i.e., along the vertical direction in the figure. That is, the temperatures of the regions 4a near the surfaces which are in contact with the punches 2 and 3 become higher than that of the middle portion 4c along the thickness thereof. Thus, at the early setting stage of the molding shown in FIG. 1, the regions 4a of the molding compound near the contact surfaces thereof are already set, the next regions 4b which are closer to the middle of the thickness of the molding are highly viscous, being just before the setting stage, and the middle region or portion 4c of the molding is still molten and of relatively low viscosity. The entirety of the middle molten portion 4c actively generates reaction gas. As the molten portion 4c has a relatively low viscosity, the generated gas can move within portion 4c relatively easily. The high viscosity regions 4b and the set regions 4a, however, are hardly capable of allowing the gas to pass therethrough, and the gas generated in the middle portion 4c can hardly escape therefrom. Thus, the gas tends to accumulate in the middle portion 4c of the molding. The thickness of the molten middle portion 4c grows thinner as the setting of the compound advances. Therefore, a large quantity of gas is finally accumulated in the middle portion of the molding at the final setting stage thereof. As a result, the middle portion of the molding, which becomes of high porosity and of low strength, is split by the springing-back action of the molding or by the gas pressure to form laminar fractures therein when the compressive force P is removed.
The conventional method for preventing the troubles resulting from the generation of gas during the setting of the molding compound is the degassing procedure. In this procedure, the compressive force against the molding is occasionally removed during the molding process to air the gas accumulated in the molding. However, this degassing is not very effective at removing the gas from the molding and it is difficult to prevent the occurrence of the troubles resulting from the generation of the gas in the molding compound during the setting thereof, especially in the case of moldings having large dimensions.