**6.2 Semi-solid casting**

When the film constituent is an acid-insoluble polymer, this approach is favoured. The water-soluble polymers are initially dissolved in water in this step. The resulting solution is mixed with an acid-insoluble polymer solution that has been prepared

*Thin Films: A Promising Approach for Drug Delivery System DOI: http://dx.doi.org/10.5772/intechopen.103793*

**Figure 1.** *Methods of preparation for thin films.*

separately. Both solutions have been thoroughly combined. Following the mixing of the two solutions, a sufficient quantity of plasticiser is added to the final solution to get the gel's mass. Finally, using heat-controlled drums, the gel mass is cast onto the films or ribbons. The film thickness should be between 0.015 and 0.05 inches. The acid-insoluble polymer should be used at a 1:4 ratio with the film-forming polymer.

**Figure 3.** *Industrial machine for film formation by solvent evaporation [15].*

Cellulose acetate phthalate and cellulose acetate butyrate are examples of acidinsoluble polymers [65].
