Design and Fabrication of Injection Mold Using Gypsum

Rapid tooling (RT) technology has been recognized as a good approach for the initial trial during the product development process. Lots of RT methods are revealed for making tools with different pilot run requests. Gypsum (calcium sulphate dehydrate) is a well-known mineral obtained from natural resources and burned to calcium sulfate hemihydrate or anhydrite to be used as plaster of Paris in different applications such as buildings, art or ceramics. The inherent property of gypsum is weak and porous so that the mold made from gypsum can only be used for low pressure molding process, i.e., casting. In an attempt to provide a rapid and economical alternative for making mold with a low quantity production, the feasibility of gypsum as a mold insert used in injection mold is developed. The design and fabrication process of the mold insert was implemented, starting with the gypsum powder, mixing the powder with water and binder to form plaster. Before the plaster solidified, an additional compression was introduced to enhance its strength. Then the plaster was machined into a mold insert and validated by injection test. The optimal combination of ingredient, 58 wt%/37 wt%/5 wt% of hemihydrate/water/binder, was found to work best. A coin pattern was selected to build the gypsum insert for checking the replication accuracy of the molded part. In addition, the life cycle of the gypsum insert for injection molding was also evaluated in this study.