486 BELL SYSTEM TECHNICAL JOURNAL 



2. Specific mechanical properties. 



a. Transverse strength. 



b. Impact strength. 



c. Cold flow. 



d. Shrinkage. 



e. Wear resistance. 

 /. Machinability. 



3. Specific electrical properties. 



a. Insulation resistance 



1. as afl^ected by humidity. 



2. as afifected by light. 



b. Dielectric constant. 



c. Power factor. 



d. Dielectric strength. 



4. Moldability. 



a. Free flow at moderate temperature and pressure. 



b. Favorable setting characteristics. 



c. Short molding cycle. 



d. No tendency to stick to die. 



e. No abrasion of die surface. 



/. Minimum shrinkage in mold. 

 g. Low bulk factor. 



5. Economic considerations. 



a. Low density materials preferred. 



b. Cost. 



c. Die life. 



d. Utilization of scrap. ^ 



e. Molding cycle time. 



/. Trimming and finishing characteristics. 

 g. Refinishing or maintenance. 



The objectives of an ideal plastic in the telephone industry depend 

 upon the use to which the material will be put in the telephone plant. 

 The material may be a structural member, an insulator, or both, and 

 may be in the hands of the public or in a telephone exchange. All of 

 the above requirements need not be met but excellence in a majority 

 of these properties is generally desirable. 



The Molding of Telephone Parts 



Molding involves consideration of (1) the molding compound (2) 

 the die (3) the press (4) the heating and cooling system (5) method of 

 ejecting part from mold (6) finning and trimming methods. Regard- 

 less of the type of plastic, these operations are necessary. 



