Properties and Fracture Surface Features of Plaster Mold Reinforced with Short Polypropylene Fibers for Investment Casting

In this work, plaster molds for investment casting, reinforced with short polypropylene fibers, were prepared and their properties and fracture surface features were investigated. Fiber contents ranging from 0.10 to 0.50 wt% were introduced into a powder mixture of gypsum and mullite (ratio of 3:7) as reinforcements in order to prepare thin-walled plaster molds with high strength and permeability. The strength of green and fired plaster mold specimens was tested, the permeability was also tested, and their fracture surfaces were observed by scanning electron microscopy (SEM). The results show that the strength of the molds increases initially and then decreases with increasing fiber content. The green flexural strength of the specimens containing 0.20 wt% polypropylene fibers reached the maximum, c.a. 1.94 MPa, increased by 36.6%, and particularly, there was a slight increase in their fired strength, a maximum value of 0.45 MPa, in comparison with the fiber-free specimen. This low increase in the fired strength the reinforced specimen will produce a favorable effect on the collapsibility of shells in the casting cleaning process. In addition, the fired permeability of fiber-reinforced fired specimens exhibited significant enhancement, compared to that of the unreinforced ones. With increasing fiber content, the permeability of specimens increased from 8.6 to 11.5%. The improvement of green permeability was limited substantially. SEM observations indicate that the failure of the green specimens is a result of brittle fracture of the plaster substrate and/or fiber debonding from the matrix during loading. On the contrary, in fired specimens, the failure occurs by holes in the plaster substrate resulted from fiber ablation in the matrix during the firing process. The arrangement of polypropylene fibers can also affect the fracture behavior of molds. This proposed approach can reduce mold thickness without loss of strength and provide defined judgment basis for fracture features.