PLASTIC MATERIALS IN TELEPHONE USE 487 



As to the molding compounds, the Bell System obtains from the 

 suppliers whatever materials are needed and this is generally true for 

 the industry. These range from the plain wood flour-filled phenolics 

 to the various thermoplastics depending on the application. Exact 

 compositions are seldom specified in order that the manufacturer be 

 given all possible opportunity to exercise his ingenuity to produce 

 satisfactory quality material. 



The die or mold is such an important item in the molding of a 

 material that several points should be emphasized about it. The 

 dies are always expensive. Every part is an individual design problem, 

 involving flow of material in the cavity, use of inserts, opening and 

 closing, the clamping of die parts under high pressures, and alignment. 

 Everything possible must be done to reduce the complication of the die; 

 eliminate inserts if possible, provide generous fillets, ample taper for 

 removal of parts, and facilitate flow of the compound. The Bell 

 System has found it advantageous to make most of its own dies. The 

 conventional boring, milling and hobbing processes are used. Very 

 little success has been had with other methods, such as casting with 

 hard alloys. 



In spite of the expensive and time-consuming effort that must be 

 encountered in designing and building a molding die, the finished die 

 when properly used represents one of the most indestructible tools of 

 modern manufacture. The parts are finished, require little or no 

 surface treatment, the dimensions are accurate and sub-assembly 

 operations may already be completed as the part emerges from the 

 die. 



The dies used are of the three general types: the open or flash die, 

 the closed or positive die, and the injection die. The open type con- 

 sists of two parts which come together at a cutting edge or ridge that 

 surrounds both halves of the die. Since this ridge or cutting edge 

 must withstand the full pressure of the press it is usually about one- 

 eighth of an inch wide. Flash dies are all relatively simple, readily 

 loaded, and the charge need not be measured accurately, any excess 

 being forced out as the two halves close until the cutting edges come 

 into contact. Such molds may be used for any molding compound not 

 requiring high pressure and which does not have high die shrinkage. 

 Shellac-mica commutators (Fig. 1) are manufactured by means of a 

 flash die. 



The positive type die consists of a plunger and a cavity shaped to 

 produce the finished part. The plunger may aid in shaping the part. 

 Only enough material is placed in the die to make the part. The ma- 

 terial may be weighed in separate charges or preformed by a separate 



