Additive manufacturing using materials that form a weak gel

A method of additive manufacturing of a three-dimensional object having a cavity, the method comprising sequentially forming a plurality of layers in a configured pattern corresponding to a combined shape of the object and a sacrificial object, in a manner that said sacrificial object is enclosed by a sacrificial shell, and said sacrificial shell is enclosed in said cavity;

• and
• removing said sacrificial object and said sacrificial shell from said cavity;
• wherein said sacrificial object comprises a hardened modeling material, and said sacrificial shell is made of a hardened weak gel material formed of a flowing gel formulation,
• said flowing gel formulation comprising:
• at least one monofunctional curable material;
• at least one hydrophilic multifunctional curable material;
• and
• at least one water-miscible non-curable material,
• wherein:
• a total amount of said at least one monofunctional curable material ranges from 1 to 10, % by weight, of the total weight of the formulation;
• a weight ratio of a total weight of said at least one monofunctional curable material and a total weight of said at least one hydrophilic multifunctional curable material ranges from 1:1 to 10:1;
• and
• a total amount of said non-curable materials is at least 85% by weight of the total weight of the flowing gel formulation.