What is the difference between top down and bottom up projection?

In the Top down projection technology the amount of resin used is equal to the height of the object, the object is formed without particular types of stress because it simply grows on a moving tray. No change of resin  vat is required. After the use the vat is simply closed and can be safely stored. In the Bottom up projection technology the amount of resin used is less, however, the object undergoes more stress due to the suction effect happening at each layer. You must also make a periodic change of the resin vat or the fep film with additional costs.