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Crahtree and Ives — Static Markings on Film 77
Laboratory Static
In the motion picture laboratory static discharges may occur during the following operations (a) Winding the film on the developing racks, (b) Development on the processing machine (c) Cutting of the negative and (d) Printing.
1. Rack Static.
Film is usually wound on the rack by holding the roll of film in one hand and winding with the other. The slack film is then tightened by pulling on each loop which results in severe friction between the slat and the film base, which may result in static markings.
Static discharges may also occur at the point where the film leaves the roll at a tangent as a result of induction and friction, especially if the film has been humidified excessively causing the convolutions to adhere slightly, while if the roll is gripped at all tightly, friction between the hand and the film or between the convolutions of the film may be sufficient to cause static. The latter difficulty may be overcome by the use of a roll holder illustrated in Fig. 12 during winding. The arm AB is lifted up, the roll placed on core C and the arm AB again lowered. The holder is then grasped by handle H, and by exerting a slight pressure with the thumb at A the film may be fed with a uniform tension and speed.
Static markings produced during winding and tightening may -be minimized by humidification of the film before it enters the dark room, and in severe cases by also humidifying the air in the dark room. A suitable relative humidity is from 70% to 80% at 70° to 75° F.
2. Developing Machine Static.
On a processing machine static markings can only be produced up to the point where the film enters the developer, and may be caused by too much tension on the take-off roll, malalignment of the sprockets, or by running the machine at too high a speed. Humidification of the film previous to or during printing, and correction of mechanical defects will prevent such trouble.
3. Electrification of Negative Film in the Cutting Room.
Since electrified film has a powerful attraction for dust particles, it is important to maintain a fairly high humidity in the cutting room in order to minimize the propensity of the electrified film to attract dust. Such humidification also tends to prevent printer static.
4. Printer Static.
The largest proportion of static markings encountered in the laboratory are produced during printing, and especially with step printers. Static is rarely encountered with all-metal continuous printers.
In a step printers the film is subjected to excessive friction during the pull-down movement, especially with shrunken negatives. Static markings may, however, be prevented :