Properties Of Cattail Fiber-Reinforced Plaster Molds For Precision Casting

The plaster mold casting process is widely used in production of nonferrous alloy with a complex structure and thin walled. The castings formed by plaster mold casting are characterized by fine surface, good size and shape precision. But the aluminum alloy castings produced by plaster mold casting easily generated degree of pinhole which led to the casting quality was underproof, which was related to the properties of plaster molds. In the process of drying, the moisture content in mold samples migrated difficultly because of the good compactness of the plaster mold. Then, the high temperature and long time resulted in weak properties. Usually, increasing the thickness of plaster mold was applied to ensure the sufficient properties. However, it is difficult to dry the plaster mold because of the increased thickness. So, high-strength and thin-walled plaster mold fundamentally solved unthoroughness in dehydration of plaster mold at the drying process and reduced or eliminated pinhole flaws of aluminum alloys produced by plaster mold casting.In order to improve the plaster molds with high strength and thin walled, the cattail fibers were introduced into slurry for plaster molds with the content of 0.1% to 0.6%. In the present work, the thermostabilities and the phase composition of gypsum powder(α-hemihydrated plaster) and plaster mold specimens were analyzed by TG-DSC and XRD. The effects of various amounts of cattail fibers on the bending strength about green strength and dry strength and residual humidity of specimens of plaster molds were investigated by testing the green strength and drying strength of cattail fiber-reinforced plaster molds and observed the fracture surface of plaster mold.The results show that the cattail fibers disperse into the plaster slurry difficultly with the content of 0.4%. The ball-milled fibers could disperse into the plaster slurry uniformly with the content of 0.6%. Compared with the unreinforced specimens of plaster molds, the bending strength reinforces significantly. When the fibers were ball milled, addition amount was 0.4%, the specimens reached the highest green strength of 1.842 MPa, an obvious increase about 17.8%, the dry strength reached 3.321 MPa, increase by 32% than the unreinforced specimens. The thermal analysis indicated the free water content in plaster mold dehydration process generates at 123.1℃~175.5℃, the crystal water dehydration process generates at 324.0℃~365.8℃. The drying quality of the fiber-reinforced plaster mold in the process of drying was studied by the method of weight-loss. The results showed that the plaster mold with cattail fibers can bring a fast dry ratio. The loss mass of plaster mold specimens increases with fiber addition.As the results, the properties of specimens of fiber-reinforced plaster molds have definitely improved. The green strength and dry bending strength of specimens of cattail fiber-reinforced plaster molds with the content 0.4% enhanced 17.8% and 32%. And,the moisture content in specimens migrated easily because the addition of fibers could increase moisture diffusion channels in the specimens of plaster molds.