This Ultimate F-3A (originally produced by Fred Schorr, introduced in 1949) will serve as a good example and is actually part of what could have been the last run made of any OO scale model in sand castings in the late 1960s; a brief history of their production is laid out in this previous post, which included in particular the best OO wheelsets ever! A set of their F-3 castings are laid out in this photo (the most commonly seen model of their production, besides the wheelsets), and note especially the extra “sprue” material on the bottom of the locomotive shell.
This gets at how the parts were made, as sand castings. The ever useful Wikipedia has a good, brief overview of the process, which begins as follows:
There are two main types of sand used for molding. Green sand is a mixture of silica sand, clay, moisture and other additives. The air set method uses dry sand bonded to materials other than clay, using a fast curing adhesive. The latter may also be referred to as no bake mold casting. When these are used, they are collectively called "air set" sand castings to distinguish these from "green sand" castings. Two types of molding sand are natural bonded (bank sand) and synthetic (lake sand), which is generally preferred due to its more consistent composition.he describes his trials in this article. And after the war M. P. Davis had a large line of sand-cast OO models as well (described here, with more links).
With both methods, the sand mixture is packed around a master pattern forming a mold cavity. If necessary, a temporary plug is placed to form a channel for pouring the fluid to be cast. Air-set molds often form a two-part mold having a top and bottom, termed cope and drag. The sand mixture is tamped down as it is added, and the final mold assembly is sometimes vibrated to compact the sand and fill any unwanted voids in the mold. Then the pattern is removed with the channel plug, leaving the mold cavity. The casting liquid (typically molten metal) is then poured into the mold cavity. After the metal has solidified and cooled, the casting is separated from the sand mold. There is typically no mold release agent, and the mold is generally destroyed in the removal process.
The accuracy of the casting is limited by the type of sand and the molding process. Sand castings made from coarse green sand impart a rough texture on the surface of the casting, and this makes them easy to identify. Air-set molds can produce castings with much smoother surfaces. Surfaces can also be ground and polished, for example when making a large bell. After molding, the casting is covered in a residue of oxides, silicates and other compounds. This residue can be removed by various means, such as grinding, or shot blasting.
During casting, some of the components of the sand mixture are lost in the thermal casting process. Green sand can be reused after adjusting its composition to replenish the lost moisture and additives. The pattern itself can be reused indefinitely to produce new sand molds. The sand molding process has been used for many centuries to produce castings manually. Since 1950, partially-automated casting processes have been developed for production lines.
For those seriously interested in the methods used, Red Adams also provides a lot of practical information in two later articles. The first is in the January, 1940 issue of The Model Craftsman on the topic of casting a boiler, and the second is a two part series in the November and December, 1941 issues of The Model Craftsman on making from scratch a modern diesel locomotive by sand casting the body. This photo is from the November, 1941 article and shows the patterns and core boxes for an E unit locomotive, using the same methods used by Schorr for their model. The finished model may be seen in this article.
Returning to the F-3 model featured in this article, this last photo shows the top of the model in both the original Schorr version and the later Ultimate version, which are easy to tell apart as on the underside the word Ultimate is cast right in. The Ultimate version is actually just a bit cleaner than the original and also note that the bronze itself is of a slightly different quality.
And they are heavy! This is part of the charm of American OO. Have to love those classic sand cast models.