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Fig. 1. Heater block assembly.
To accomplish this, it became necessary to discard all types of coil heating elements and to introduce a ribbon type of nichrome wire (Fig. 1 (6)). This ribbon has a cross-section area no greater than 0.006 X 0.047 in. and is capable of dissipating 40.5 watts per linear inch, or an equivalent of 0.1142 Btu. The heater ribbon is sunk into a piece of transite material (Fig. 1 (2)), until it is absolutely flush with the surface of the transite. The accuracy of the positioning of this ribbon wire in the transite is important. The nichrome ribbon carries a current of approximately 16 amp which, in air, would normally cause the ribbon to burn out. This current-carrying capacity of the ribbon changed considerably when the wire was placed under pressure due to the dissipation of heat from the ribbon to other materials in pressure contact with the ribbon. To prevent undue burning out of the heater wire, safety switches had to be provided which would prevent the operator from splicing without first applying pressure to the film and heater element.
Mica (Fig. 1 (3)) is used as one of the heating and cooling gradients. The type of mica used, as well as the thickness of the mica, is extremely important. This mica also serves as
an electrical insulator to prevent contact shorting of the heater ribbon to the stainless-steel platen. The thickness of the mica determines the heating and cooling gradient. The optimum thickness has been found to be between 0.002 and 0.003 in.
In the development stage of the Presto-Splicer, mica was used as a heater platen, but it tended to flake and adhere to the film being spliced, which, of course, was not desirable. To overcome this, a stainless-steel platen (Fig. 1 (7)), 0.005 in. in thickness, having a high electrical resistivity, was placed over the mica. It is interesting to note that resistivity played an important part, and, inasmuch as resistivity is inversely proportional to heat conduction, the use of a specific type of stainless steel became necessary. This steel introduced problems of warping and elongating under heat, so that it therefore became necessary to have the grain of the material perpendicular to the line of heat.
Pressure Requirements
After elaborate testing, it was found that pressure under 200 psi applied to the line of splice allowed gas bubbles to appear in the splice, causing a poor bond and brittleness of the film. When pressure of 200 psi or more was applied to the splicing area, these gas bubbles disappeared and a satisfactory homogeneous bond resulted.
Heat Transfer
With the advent of tri-acetate film, it became necessary to find the material having the poorest heat-conducting characteristics which would allow the concentration of heat to be applied to the film rather than* to be dissipated to the material used for applying the pressure to the film (Fig. 2). Other requirements of this pressure platen were that it should in no way affect or adhere to the emulsion of the film. At present, the material used for this
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February 1953 Journal of the SMPTE Vol. 60