Journal of the Society of Motion Picture and Television Engineers (1950-1954)

v considerably depending upon the e and diameter of a particular mill, its tnperature rise, the type of oxides being lied. etc. Major progress has been ade in improving this process to get tter dispersion. Contrary to common lief, the tremendous sheering force of 5 steel balls in the ball mill has little, if y, effect on oxide particle sizes, tually, the particle size range is alady so small that it would be a very ficult, if not impossible, task to reduce further by any mechanical means. lis operation is purely a means of thorghly dispersing and wetting the oxide rticles. All raw materials are purchased on a 1 tch basis, and in the case of some criti('1 components, samples are tested bel*e shipment is authorized. A pro(dure for laboratory-scale milling has len developed which produces reliable ; vance information on stock lots. The manence and coercive force of each 1 tch of iron oxide pigment are deterined and test coatings of laboratory;ile mixes are run completely through i s process to provide uniformity of con1)1. Each milled batch is test coated i the standard production equipment i determine magnetic and physical (aracteristics. After approval the mix ipiped through commercial-type paper liters to an agitated storage tank. This ia sealed tank to prevent the evapo:tion of solvents and is constantly agi'ted to prevent the agglomeration of any ; rticles, after the tedious milling operaon just completed. Such a storage •nk is shown in Fig. 1 . 'tating At the present time we have in operton several coating machines. The ingest is capable of coating base maials up to 12 in. wide. This coater is n in P'ig. 2. Here again the mix from the storage nks is filtered to insure coating free of *eign matter or coagulated lumps. To e left, immediately after the feed roll, will be noted a coating head which is applying a very thin but precisely controlled layer of precoat. This is thoroughly dried before the base reaches the second coating station where the magnetic mixture is applied. Here the base passes through a different type of coating head where a controlled thickness of the magnetic mixture is applied. The base is made to pass smoothly (without wrinkles) through this head so that a uniform layer is applied. Drying When the coated tape leaves the liead, it is, of course, wet, just like freshly applied paint, and the rest of the machine is actually no more than a means of drying this wet coating by removal of the volatile solvents. Since only solvent removal, but no chemical change, is involved in the drying, there is no danger of over or under-curing or oxidizing. The wet ribbon of magnetic material is now fed past controlled infrared lamps and space heaters and then into a more or less conventional-type drying cabinet. Final take-up of the complete 5000-ft roll is seen at the left in Fig. 3. During this operation every effort is made to eliminate all traces of dust and dirt. Air taken into the cabinet for drying purposes is thoroughly filtered beforehand. Inclusion of even minute particles of dust in the coating or on the surface will produce typical dropouts such as are seen on Brush high-speed charts. On these coaters, an adjustable record, reproduce and erase head assembly is mounted so as to sweep the full width of the base being coated. This monitors a test frequency through the base side of the tape and gives a rapid indication of the electrical output. Polishing Following the coating operation these wide rolls are buffed by high-speed nylon brushes to remove any traces of surface dirt and to burnish the actual coating to as smooth a surface as possible. The Schmidt and Franck: Magnetic Recording Materials 457