236 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1941 



agents and tackifiers with various organic binding materials in rubber 

 compounding, floor-tile compositions, and other industrial applica- 

 tions. Annual production in 1935 was 8,000,000 pounds and the 

 output is said to have increased appreciably in recent years. 



Molding technique. — The discovery of the fundamental principle 

 involved in the operation of the hydraulic press is generally conceded 

 to have been made by Blaise Pascal in 1653. The adaptation of this 

 principle to a practical machine is credited to Joseph Bramah in 

 1795. Little industrial use was made of it until after the discovery 

 of the vulcanization of rubber by Charles Goodyear in 1839. A 

 simple hydraulic rod-type press, between the platens of which a 

 2-piece mold was inserted, was developed for handling the manu- 

 facture of rubber products and was subseqently employed for 

 molding the thermoplastics. 



The advent of the phenolic thermosetting resin in 1909 provided 

 the stimulus for introducing features in the compression molding 

 press which would increase the output from a given mold. However, 

 realization of the fully automatic compression molding press has 

 come about only in the last 2 to 3 years. These presses perform all 

 the operations of routine molding of thermosetting plastics, consist- 

 ing of measuring the charge of molding powder, preheating it, loading 

 it into cavities, closing the mold, opening it slightly for breathing, 

 that is, expulsion of gases, closing it again for a predetermined 

 curing period, opening the mold, ejecting the finished pieces, blowing 

 flash from the cavities and plungers, and then repeating this cycle 

 hundreds and thousands of times with the only manual labor required 

 being to keep the hopper supplied with the molding powder. 



The original conception of the injection molding principle is com- 

 monly attributed to Edmond Pelouze, who in 1856 developed a die- 

 casting machine for forcing molten metal into a die by mechanical 

 or hydraulic means. The industrial history of the injection molding 

 machine for plastics in the United States is only about 5 years old, 

 a fact which seems almost incredible when one looks at the huge 1940 

 model capable of taking a mold 3 by 4 feet in cross section and 

 turning out four 36-ounce moldings every minute. 



The need for the injection molding machine came with the com- 

 mercial development in 1929 of the heat-stable thermoplastic mold- 

 ing material, cellulose acetate, which required an uneconomical 

 chilling period when molded by conventional compression methods. 

 The cellulose acetate plastic, however, unlike the older cellulose 

 nitrate type, could be kept hot for a relatively long period in a heating 

 chamber and injected hot into a cold mold, wherein it cooled in a 

 few seconds to a temperature at which it would maintain its shape 

 and hence could be ejected from the mold. 



