388 APPLIED SCIENCE 



When a pattern is rapped to loosen it in the sand before 

 drawing, it is impossible to produce castings of uniform size, 

 as the rapping is left to the molder's judgment. Rapping 

 a long and narrow pattern results in making a casting longer 

 than the pattern unless great care is taken. Such a pattern 

 moves endwise easily because of its narrowness, but moves 

 sidewise very little because its greater surface is not so easily 

 moved. A casting of this kind is often found to be wider on 

 one end because of carelessness in rapping. 



433. Molding in Cores. Many large castings, the pat- 

 terns of which would require a great amount of material and 

 labor, may be molded in cores. In this process no pattern 

 is used, the practice being to make a core-box of some part of 

 the casting and, after baking, to assemble the various parts 

 in a flask. In many cases this is not the most economical 

 method, but it is sometimes used of necessity because the 

 peculiar shape of the required casting makes it impossible to 

 draw the pattern if it be made in the same shape as the 

 required casting. 



As few castings are obtained without the use of cores, it is 

 essential that the pattern-maker give this branch of molding 

 close study. The proper location of core prints on the pat- 

 tern may make the difference between success and failure. 



Any pattern could be so designed that it might be molded 

 in cores, but such designing is economical only where a dupli- 

 cation of parts formed from one core-box is possible. 



434. Weights of Materials and Castings. The weight of 

 a casting may be approximately estimated from the weight of 

 the pattern, as shown in the table below. In all cases where 

 there are core prints or batteries to sustain the pattern, or 



