62 MISC. PUBLICATION 679, U. S. DEPT. OF AGRICULTURE 



this way during hardening of masses of fluid resin that are more than 

 one-half inch thick, it has been found advisable to restrict the size. 

 of single-layer cast blocks to a point where this %-inch limit is not 

 exceeded. When larger blocks are desired, it is generally necessary to em- 

 ploy the layer-by-layer method described in a patent by Kuettel [29 ». 5 

 This calls for alternately pouring and hardening successive layers of 

 fluid resin that do not exceed the %-ineh limit of thickness required for 

 satisfactory polymerization. If this procedure is carefully carried out 

 with due regard for the elimination of dust and other foreign matter 

 between layers, blocks of practically any desired size that are clear and 

 homogeneous and free from lines of demarcation between the layers 

 can be produced. 



In the embedment of objects that tend to float in the fluid resin, the 

 first layer poured upon the base should be just thick enough to anchor 

 the specimen in place after solidification. It is also advisable to restrict 

 the next layer to a depth that will not tend to cause undue softening of 

 the anchoring layer before polymerization can take place. The remaining 

 layers may be of any convenient thickness up to the %-inch limit. In 

 the case of heavier specimens that sink in the fluid plastic, any desired 

 depth up to the %-inch limit may be employed for the first layer as well 

 as for the subsequent ones. Successive layers should be added in this 

 manner, making sure that each is adequately polymerized before the 

 next is poured on, until the specimen is completely embedded and is 

 covered to a sufficient depth to allow for machining and polishing the 

 finished block. 



To embed the specimen pour the first layer of casting fluid of requi- 

 site viscosity on the base to the proper depth indicated by the nature of 

 the specimen material. Then introduce and orient the specimen, prefer- 

 ably with smooth-tipped forceps or tweezers or with dissecting needles. 

 Extreme care is necessary in arranging fragile objects because they tend 

 to adhere quickly to the base and are liable to be torn or broken if an at- 

 tempt is made to move them afterwards. In the case of specimens that 

 have hollows or cavities on the underside, it is usually advisable to mount 

 them upside-down to facilitate the escape of trapped air, and then to 

 reverse the block when finishing it after the casting has been completed. 



As soon as the specimen has been placed in position, the mold should be 

 effectively covered with tinfoil or cellophane to prevent undue vapor 

 loss and then allowed to stand for an hour or two at room temperature to 

 permit the trapped air to escape. If the bubbles do not rise properly or 

 the fluid plastic does not enter the hollow spaces, it is advisable, except 

 where very viscous casting sirup has been used, to place the mold in a 

 vacuum chamber for a short time (fig. 15). In doing so, however, the 

 process should be watched closely, and the degree of vacuum controlled 

 so that excessive bubbling or frothing does not occur. 



When the fluid plastic surrounding the specimen appears to be freed 

 of air and the submerged part of the specimen is suitably impregnated, 

 the mold, with its covering in place, should be set in a water-bath or in a 

 constant temperature oven of about 45° C. ( 113° F. ) in the case of methyl 

 methacrylate, or 50° C, (122° F. ) for ethyl methacrylate, until 



5 Patent under assignment to E. I. du Pont de Nemours & Co., Inc. The process 

 covered by this patent cannot be used except by consent of the assignee, 



