24 



THE INDIA RUBBER WORLD 



[October 1, 1920. 



ELECTRICAL HEATERS 



Experimenis have been made in using electric current in place 

 of steam for closed heaters, with good results. Its operating 

 cost, however, with cost of current per K. W. and steam per 

 boiler horsepower being equal is more than 100 per cent greater 

 than the steam. Its advantages arc as follows: (1) flexibility, 

 allowing heaters to be placed anywhere in the plant requiring 

 wiring only and no expensive steam mains and returns; (2) 

 lower maintenance expense; (3) better control either hand or 

 automatic. When current is cheaper it would not surprise the 

 writer to sec more electrical heaters used for the closed type. 



For hydraulic presses steam is the better agent because of the 

 ease in maintaining an even temperature in the platens. This is 

 because of the fact that the cast iron platens are small and of 

 uniform thickness and only require a steady circulation of 

 steam at a given pressure. 



VAPOR CURE HEATERS 



Where goods are cured with acid fumes generated from air 

 heated by steam coils, the temperatures are under ISO degrees 

 F. and '.he same general design of closed-type heater can be 

 followed but no mechanical means of air circulation is required 

 because the heaters are generally built considerably smaller. 

 Acid-resisting materials such as asbestos and magnesia should 

 be used on the interior, and if any wood or metal is used it 

 should be thoroughly painted with asphaltum. 



In conclusion it should be borne in mind that since all rubber 

 goods must be cured the importance of vulcanization can hardly 

 be overestimated, and time and money spent for its improvement 

 to insure uniform product will be well repaid by increased orders. 



THE MANUFACTURE OF DENTAL RUBBER 



By Arthur C. Squires 



THE MANUFACTURE of dental rubber is a highly specialized 

 branch of the rubber manufacturing industry. This material 

 is supplied to the dental trade in unvulcanized sheets for the 

 purpose of making individual dental plates. Dental rubber must 

 be absolutely pure and free from all foreign matter. It should 

 possess strength, light gravity, permanent color, the quality of 

 packing easily in the vulcanizing molds, and cure in 55 minutes 

 at a temperature of 320 degrees F. It must finally take a high 

 polish. 



A variety of colors are made for the dental trade including 

 pink, maroon, orange, and jet black in plain and mottled finish. 



Glass Top Cutting Table 



Jet black dental rubber is the strongest and lightest in gravity. 

 Dark orange is the strongest of the colored rubbers, and maroon, 

 comes next. 



Pink dental rubber is used only as a facing to match thei 

 natural gums of the individual wearer. Owing to the amount 



of white pigment used in combination with other colors in pro- 

 ducing a natural pink gum color, the strength of pink dental 

 rubber is not equal to that of the lighter gravity rubbers. A 

 plate would not be strong enough if made entirely of pink rubber 

 and therefore it is utilized only in gum facings. 



The following list shows the great variety of colors used in 

 the dental trade : 



No. 

 1. 



2. 

 3. 

 4. 

 5. 

 6. 

 7. 



Colors 

 Light orange 

 Medium orange 

 Dark oranue 

 Mottled lisht red 

 Mottled dark red 

 Plain maroon red 

 Jet black 

 Coal black 

 Light pink 



No. 

 10. 



11. 

 12. 

 13. 

 14. 

 15. 

 16. 

 17. 

 18. 



Colors 

 Medium pink 

 Kosc pink 

 Snow white 

 Red palate 

 Black palate 

 Sunset orange 

 Mottled maroon 



(1) Weighted 



(2) Weighted 



Numbers 3. 6, 7, 9 and 17 dental rubbers are most used; 

 however, the greater number of colors are made under special 

 brands for the dental depots and sold exclusively by them. 



As a matter of history it is interesting to note that prior to 

 1897. the Imperial Rubber Works of New York City imported 

 a white vulcanite base to which color was added and milled at 

 the works of this company. After many unsuccessful experi- 

 ments the writer succeeded in producing a pink dental rubber, 

 mention of which was made in The India Rubber World, 

 January 1, 1897. Practically the same recipe is now used in this 

 country in the manufacture of standard pink dental vulcanite. 



The following is a brief description of the processes used in 

 the manufacture of dental rubber : 



Bolivian rubber is considered to be the strongest and best 

 rubber for this purpose, and after a thorough washing on a 



Sheet Stock Smoothing Table 



cracker-washer, the thin sheets of rubber are then rewashed 

 on a smooth-roll mixer equipped with water connections for 

 constantly spraying the stock. During this operation the mill 

 rolls are closed as tightly as possible, for the thinner the sheets 

 the better the result. After washing, the thin sheets of raw 

 gum are thoroughly dried in a vacuum drier. 



The washed and dried rubber is then broken down on a 

 regular mixing mill, this operation requiring at least three hour^ 

 time, as the stock must be extremely soft before adding the 

 compounding ingredients in order to insure their perfect distri- 

 bution throughout the batch. 



Two grades of sulphur are generally used in compounding 

 dental rubber, ordinary flour of sulphur for all inottled vul- 

 canite, and lac sulphur for plain colors, such as pink, maroon, 

 light, medium and dark orange rubbers. There are three colors 

 of specially manufactured vermilion, Nos. 2, 3, and G. O., which 

 may be combined to produce certain required colors. In combin- 

 ing sulphur with vermilion it is necessary to sift the mixture in 

 a rotary sifting machine. 



Particular attention sliould be given to the mixing and milling 

 operation which is performed on an ordinary two-roll mixer. 

 The front roll should be coated with the soft, broken down 

 stock, one to two inches thick. In adding the ingredients care 

 must be taken not to allow the rubber to break or the surface 

 of the rolls to be exposed. Nor should the compound be added 

 to the batch faster than the soft milled rubber can absorb it. 



The following typical dental rubber compounds, taken from 

 "Crude Rubber and ConipoHnding Ingredients." are interesting in 



