International projectionist (Jan-Dec 1937)

©C1B 339653 INTERNATIONAL PROJECTIONIST VOLUME XII NUMBER 5 / MAY 1937 FILM SCRATCHES: SOME CAUSES AND MEANS FOR THEIR ELIMINATION By A. C. SCHROEDER MEMBER, PROJECTIONIST UNION 150, LOS ANGELES, CALIFORNIA SCRATCHED film! Is there a projectionist who has not had to contend with it? We know that there is a reason for everything, but scratches sometimes defy all efforts to explain why or how. In the final analysis, a scratch is caused by a hard or gritty substance which contacts the film, and has moved over the surface, usually lengthwise. This is easily understood; but it is rather puzzling when a film is scratched by sliding over a perfectly smooth and polished piece of metal. Possibly it is caused by dust or dirt caught between the film and the metal, and it is the dirt that does the damage. This condition is disclosed by close examination of the suspected metal object, as a shiny spot will be present which has been caused by the film rubbing against it. Such a spot is sometimes present on the Simplex aperture plate, or the film trap, especially in the older heads in which the aperture hole came closer to the film than it does in the present mechanisms. It has been my experience that nothing can be allowed to touch the emulsion side of the film if scratching is to be avoided, which brings up the problem of the rollers in the magazines. These rollers are supposed to very nearly touch the film, so that no fire will pass them. But if they are too close, the film will occasionally touch the metal, and if it does, it will surely result in scratches, since the part of the roller that causes the trouble is of slightly smaller diameter than the ends of the rollers that contact the edges of the film. Sliding Film Motion Harmful This difference in diameter means that the peripheral speed of the two parts is different, so that the speed of the ends of the rollers is nearly that of the film traveling over it; but the speed of the smaller diameter is quite a bit less, and there will be a sliding motion of the film over the small diameter of the roller for the instant that the two are in contact at this point. The condition is aggravated by buckled film, which is longer in the center than it is along the sprocket holes, or at the edges. For this condition to exist, the added length of film [7] has to go somewhere, so it forms a hump, while the edges may be lying perfectly flat. This can be seen if a buckled film is placed flat on the bench: the center portion humps up every so often. When such a film is bent in a curve, as it is when entering or leaving the magazine fire valves, the center portion travels in a circle a little larger in diameter than do the edges of the film, which are in contact with the roller. If another roller happens to be opposite the first roller, and in a position where the film is bulging out, the bulge will touch the small diameter of the second roller and scratches will eventually result. The remedy is to have the small diameter turned down even more, after which it should be highly polished. One might ask, why not make the roller of the same diameter clear across? One objection to this is that the roller never travels at the same speed as is the film rolling over it. There is always some slippage, and the trouble would persist. Possibly two rollers could be forced together by spring pressure, both rollers being the same diameter clear across, and the film then run between