September 1, 1921 



THE INDIA RUBBER WORLD 



899 



The Manufacture of Rubber Stamps 



IT is conservatively estimated that the rubber stamp manu- 

 facturers of the United States employ $15,000,000 capital in 

 producing an output of rubber stamps variously estimated and 

 ranging up to $5,000,000 annually. 



Rubber stamps are almost as indispensable in business activi- 

 ties as the telephone. They are believed to be an American in- 

 vention and are produced and used in the United States more 

 extensively than in any other country. The average usefulness 

 of a rubber stamp is placed at from two to three years. The 

 amount of stamp rubber required to make the annual trade re- 

 quirements would run into many tons. 



The rubber stamp trade includes many devices other than rub- 

 ber stamps and type, such as stencils, dies, checks, tags, presses 

 and other time-savers useful for business purposes. Several in- 

 teresting journals are especially devoted to the development of 

 the stamp trade interests which are quite distinct from the field 

 of rubber manufacturing in general. 



MAKING RUBBER STAMPS 



Rubber stamps may be made from ordinary printers' tv'pe- 

 forms, wood-cuts, electrotypes, line cuts, etc. From any of 

 these printing means a rubber printing stamp is made by first 

 taking an impression of the form in a plastic medium that may 

 subsequently be hardened and used as a mold in which to vul- 

 canize the rubber. To form such a mold one may use a matrix 

 composition or plastic mold-board. 



MATRIX COMPOUND 



The matrix composition is a quick, hard-setting mixture con- 

 sisting of 8 parts of plaster of Paris, 8 parts of China clay, 3 

 parts of talc or French chalk, and enough Indian-red iron oxide 

 to give the matrix a pink tint. Any mineral color may be sub- 

 stituted for Indian-red if desired. The mixed dry ingredients 

 are brought to the consistency of soft putty by stirring uo in 

 a bowl with a hot water solution of dextrine in the proportion 

 of 3 to 4 ounces of dextrine to one gallon of water. The func- 

 tion of the dextrine is important because it regulates the rapidity 

 with which the plastic composition sets into a hard matrix, in 

 other words its plasticity and toughness. The larger the pro- 

 portion of dextrine the slower the setting effect. 



MOUNTING COMPOUND FOR IMPRESSION 



The soft composition is spread on a metal mold-plate, Fig. 1, 

 with raised edges to regulate its depth when 

 struck off with a metal straight edge with 

 which the surface is smoothed and made 

 level with the tops of the bearing edges. 

 If much of the composition adheres to the 

 scraper it is an indication that the composi- 

 tion is too soft and should be allowed to 

 harden more before proceeding with the 

 leveling. If much too soft, dry composi- 

 tion may be well worked into the mass 



The Barton Mfg. Co., and rescraped onto the mold-plate. 



N. V. 



Fig. 1. Mni.i) Plate 



MAKING THE MATRIX 



The type form is set in place on the bed 

 of a hand-press. Its face is covered with a piece of fine muslin 

 and overlaid with tough tissue paper of the same size. Some 

 operators substitute a single thickness of dental rubber dam in 

 place of muslin and tissue. Three impressions are necessary to 

 properly form the matrix, according to the following directions : 

 First Impression. The mold-plate, on which the compound is 

 smoothly spread, is laid face downward on the cloth and paper- 

 covered type. The whole is pushed into a hand-press and light 



pressure applied for a moment. Releasing the pressure and re- 

 moving the cloth and paper from the mold-plate reveals a rough 

 outline of type form. 



Second Impression. After allowing the compound to harden 

 somewhat more, the face of the type is brushed with: naphtha, 

 making certain that each letter is well-oiled. This keeps the com- 

 pound from adhering to the type. The mold-plate is placed again 

 on the type in its previous position but without the muslin and 

 paper covering. The second impression is taken with a little more 

 pressure than the first. When the plate is removed a shallow but 

 exact impression will be found molded by every letter. 



Third Impression. The face of the type having been again 

 brushed with naphtha, the mold-plate is again set in position and 

 a third impression promptly taken, applying sufficient pressure to 

 give the desired depth to the lettering. On removal of the mold 

 the face of the impression should have a nicely polished ap- 

 pearance. If it does not present this condition and is still soft 

 enough to be easily impressed by the finger, another impression 

 may be taken. If, on removing the mold plate from the type 

 form at any time, it is found that any of the compound attaches 

 to the type, it indicates either that the compound was too hard 

 or that too great pressure was used, thus making the mold too 

 deep. 



DRYING THE MOU> 



After the last impression is made and the mold has set until 

 it feels like wax, the surplus compound which has been forced 

 up between the lines of the impressions is trimmed down with a 

 long-bladed sharp knife. The rough particles that result from the 

 trimming and overhang in the type impressions can be brushed 

 out after the mold is thoroughly dried and hardened. A dry 

 scrubbing brush is suitable for this purpose. 



Another method for smoothing down the mold after it has 

 thoroughly dried and hardened is to smooth down the ridges 

 with a piece of medium sandpaper. The type impressions will 

 fill up with the resultant dust, which is removed by turning 

 the mold upside down and sharply tapping the comers of each 

 end on a bench or other solid surface. The remaining dust is 

 blown out with a bellows or by other means. Every particle of 

 dust must be removed, or when vulcanizing, the rubber cannot 

 completely fill the letters, periods, etc. 



Dry the Mold Slowly. The mold can be dried by applying to 

 it evenly a moderate heat, in any convenient way. It should be 

 placed face up over a kerosene or alcohol burner, on a gas 

 heater or an electric vulcanizing plate. The heat must be very 

 moderate, especially at the beginning. If the mold is dried too 

 rapidly it will crack around the edges of the letters or different 

 parts of it may "pop" or burst out and spoil the entire mold. 



After drying the mold at a low heat for about half an hour, 

 much of the moisture will have escaped from the compound. More 

 heat can then be gradually applied until the mold is absolutely dry. 



Should any moisture be left in the compound, the mold would 

 be destroyed during vulcanization. 



To Determine When the Mold is Dry, place a small, dry 

 mirror, glass down, on the face of the mold for a few minutes. 

 .If there is any moisture present the face of the mirror will be 

 steamed. Do not attempt to vulcanize when the mold is in this 

 condition. Have it absolutely dry. In the end much time and 

 expense will be saved. After the mold is absolutely dry, smoothed 

 down and cleaned out, it is ready for vulcanizing. 

 MOLD-BOARD MOLDS 



A very expeditious method of making vulcanizing molds for 

 rubber stamps is by means of a plastic preparation mounted on 

 a card board base. This combination mold-stock comes in sheets 



