. The Bell System technical journal . high die commutators (Fig. 1) are manufactured by means of aflash die. The positive type die consists of a plunger and a cavity shaped toproduce the finished part. The plunger may aid in shaping the 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 488 BELL SYSTEM TECHNICAL JOURNAL operation. The cavity is loaded and the plunger forced down, formingthe part. Multiple cavity molds cannot be loaded exactly alike andhence some provision must be ma
. The Bell System technical journal . high die commutators (Fig. 1) are manufactured by means of aflash die. The positive type die consists of a plunger and a cavity shaped toproduce the finished part. The plunger may aid in shaping the 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 488 BELL SYSTEM TECHNICAL JOURNAL operation. The cavity is loaded and the plunger forced down, formingthe part. Multiple cavity molds cannot be loaded exactly alike andhence some provision must be made for the escape of excess material,forming a flash that must be subsequently removed. Such dies mustbe carefully designed so that the fin is located for easy trimming and toprovide the best appearance. These multicavity molds are frequentlycalled semi-positive molds to differentiate them from a truly positivemold where there is little flash. A typical telephone part made in asemi-positive mold is the handset handle shown in Fig. Fig. 2—Mold for handset handle. The recently developed process of injection molding which consistsin forcing plastic material through a nozzle into a completely closed diefrom an external compression chamber is also being used and promisesto alter many of the present operations. Since the opening and theclosing of the die are not related to the application of pressure, greaterfreedom of design is possible. Furthermore, since full pressure is notapplied by this method until the die cavity is completely filled withmaterial, the material already in the die tends to support inserts and PLASTIC MATERIALS IN TELEPHONE USE 489 hold them in their true position. Hence more delicate inserts arepossible by this method of manufacture. Since the material is en-closed in an auxiliary pressure chamber and is not exposed to theatmosphere, greater freedom from room dust is possible, renderingthis method ideal for colored plastics. A terminal block used totermina
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Keywords: ., bookcentury1900, bookdecade1920, booksubjecttechnology, bookyear1
