496 BELL SYSTEM TECHNICAL JOURNAL 



fast colored parts with porcelain-like surface luster are possible with 

 these materials. The molding cycle is slightly shorter than for phenol 

 plastics and the material at first is somewhat more fluid. Tight molds 

 are therefore necessary in order to get sufficient pressure. 



The principal difficulty with urea-formaldehyde plastics has been 

 that on exposure to heating and cooling or humidification and drying 

 cycles, there is a tendency toward cracking, particularly at changes in 

 section and around inserts. Molded into uniform thin sections with- 

 out inserts they are reasonably satisfactory plastics. 



At present there are practically no urea-formaldehyde plastics em- 

 ployed in telephone apparatus because the wide continental climatic 

 conditions and exacting requirements will not permit their use. Re- 

 cently there have been improvements made from a stability stand- 

 point and it is believed future application may be found for these 

 plastics, particularly in view of their color permanence. 



Applications of Thermoplastic Materials 

 Cellulose Acetate 



The principal use of cellulose acetate is for interleaving in coils. 

 Various relay coils are made of layers of cellulose acetate over which a 

 layer of enamel coated copper wire is wound. Layer upon layer of 

 wire and acetate sheet form a coil. These are then assembled on a 

 core and spoolheads attached. One of the phenol fiber spoolheads 

 has a surface coating of cellulose acetate and the winding is pressed 

 against this spoolhead and dipped in acetone. This dissolves or 

 softens the exposed edges of acetate and the whole coil is firmly secured 

 to the spoolhead. 



Cellulose acetate is used for this purpose since it is practically inert 

 as regards corrosion of the fine copper wire in contact with it and in 

 this respect it is superior to any known material. It is permanent, 

 reasonably fireproof and has high insulation resistance. Two grades 

 of cellulose acetate sheet are used in telephone practice. These are 

 the window grade used as a window or covering over designations and 

 an electrical grade for coil use. The principal tests for the electrical 

 grade are insulation resistance, shrinkage, and resistance to burning. 



The principal use of molding grade of cellulose acetate is for the 

 terminal block (Fig. 3) mentioned above. Here the application is 

 mainly structural since it has more than adequate electrical insulation. 

 Another application for cellulose acetate is a test strip where a surface 

 layer of acetate over phenol plastic avoids carbonization of the latter. 



