Abstract: A polyimide film essentially consisting of polyimide having recurring units of the formula (I): ##STR1## which has a density of 1.335 to 1.390 g/cm.sup.3 at 23.degree. C. and/or a refractive index of 1.605 to 1.680 at 23.degree. C. in the direction of thickness and is transparent; and a preparation process of the polyimide film by extruding the polyimide having recurring units of the above formula (I) through a common melt-extrusion process, casting in a chill-roller to obtain an unstretched film, uniaxially or biaxially stretching the unstretched film to cause molecular orientation, and successively setting the stretched film through heat-treatment.

Abstract: A method for preparing a biaxially stretched polyether ether ketone (PEEK) film having excellent precision in thickness, insulating properties and heat shrinkage from an amorphous PEEK film which includes a first stretching step of roll-stretching the amorphous PEEK film in a temperature range of from 50.degree. C. to (Tg-10).degree. C. in a stretching ratio of from 150% to 350% in the progress direction of the film while a necking phenomenon is caused in the film; a second stretching step of stretching the film in the temperature range of from Tg to 170.degree. C. in a direction at right angles to the stretching direction in the first stretching step in a stretching ratio of from 150% to 350%; and a heat set step of thermally setting the stretched film at two stages in the temperature range of 210.degree.-330.degree. C. and the temperature range of 180.degree.-210.degree. C.

Abstract: A process for the preparation of a polyimide molded form comprising the steps of swelling a primary-molded form consisting essentially of polyimide having recurring units of the formula (I) ##STR1## by using an aprotic polar solvent and subsequently stretching the swelled form.

Abstract: In the preparation of a film by melt-extrusion process of thermoplastic polyimide, static charge is applied to a molten film, primary cooling is carried out by adhering the film to a cooling roller having a surface temperature in the range of from the glass transition temperature of said polyimide -50.degree. C. to the glass transition temperature -15.degree. C., the film is successively cooled to 60.degree. C. and less without bringing the film into contact with rollers, and is taken up under tension to give a polyimide film.

Abstract: A method of electromagnetically stirring molten metal in an unsolidified portion of a continuously cast strand by a magnetic field formed by applying alternating current to at least two exciting coils, the method comprising: supplying to one of the exciting coils a first alternating current of a frequency in the range of 1-60 Hz and to the other one of the exciting coils a second alternating current with a frequency difference in the range of 0.03-0.25 Hz from the first alternating current to form a varying composite magnetic field thereby to induce stirred movement of varying direction and intensity in the molten metal.