There are essentially three types of casting method: fully weighted, semi weighted, and non-weighted. Each has advantages and disadvantages.
I have experienced/used all three.
Non-weighted footbeds are typically made by vacuum-bagging the footbed blank, and the foot while the foot hangs in space. This type of casting is notorious for providing excessive arch fill. While the ‘pressure’ of vacuum bagging provides a reasonably good contour match to the plantar surface of the foot, the development of a toe ridge, which is a byproduct of the molding process, means the forefoot material needs to be reheated and flattened, which often defeats any medial/lateral posting achieved in the initial molding (failure to flatten any contours ahead of the ball of the foot will result in forefoot numbing in use). What you are left with is a fairly solid arch support which is often more obtrusive than it is supportive.
Another means of non-weighted cast involves what is known as a foam-box impression. The foam box uses a type of easily compressed foam to establish the foot contour. Plaster is then poured into this ‘negative’ mold, and the final footbed blank is molded off of this plaster ‘positive’ mold. The impression foam is similar to that which is common to floral arrangements. Plaster molds are rather convenient if you know you will require a number of identical footbeds, but again, the area at the forefoot is often overlooked and underposted.
The weighted casting method is more user friendly for the inexperienced fabricator, as the chances of having excess posting material poking the foot is significantly reduced. Unfortunately, a partially or fully collapsed foot generates the contour, and so there is not much to be gained in the area of structural integrity or restoration of range of movement. Despite the fact that many weighted casting stands have hand rails to assist the balance of the consumer while the casting is made, any time weight is borne by a foot, the body will try to stabilize the foot via muscle activity and joint articulation, which will ultimately affect the contour of the mold. So, you get a relatively unobtrusive contour, but you don’t do much to account for and stabilize the more mobile bones of the foot. Some weighted casts are made with foam casting pillows, while others are made via digitized imaging techniques. While the computer systems offer a bit of a Gee-Whiz factor for the consumer, it is not that accurate, and the additional layers of technology create opportunities for making mistakes. At the same time, I wonder about the rationale behind the analog to digital to analog conversion process. If you were recording music, that certainly wouldn’t be desirable. Seems like a long way around the barn. Given the choice, I would go with the casting pillows.
The semi-weighted method is the one that I prefer, not simply because that is what I was taught to use, but because it offers at least one significant advantage. If used carefully, the casting media will provide an extremely accurate contour of the plantar surface, revealing just how much posting is required to accurately support the foot, without creating excessive arch fill. Due to this accuracy, it is possible to use fairly rigid posting materials, which means the support will be more uniform, and the footbed will have a significantly longer service life. (It has been commonly accepted wisdom that the human foot changes at about four year intervals. This really isn’t true. It is more accurate to say that the average footbed will change to the point of being useless after about four seasons of use. The shape of the foot doesn’t change much unless subjected to trauma, or, if you are female, hormonal flux in the presence of additional body weight, as in the case of pregnancy.)
The primary drawback to semi-weighted casting is that it requires a fair amount of practice and care, as well as a lot of finish work and proper tuning to the foot. Due to the intensity of labor and skill involved, this is not a good method for the high volume shop, in that too much time and operator skill is not a recipe for profit. Creating duplicates requires a return visit by the client, and the final tune of the posting requires a time commitment by both the fabricator and the client.
It is worth noting that a very ‘solid’ foot can be cast successfully using any of these methods, although I am the least fond of the non-weighted method.