Rheological Behaviours Of Silica Sol Slurry Containing Short Glass Fibers For Investment Casting Shell

Silicon sol shell processing technology is still widely used in investment casting. However,it also has lots of problems to be resolved,such as a low green strength, slow rate of drying, and long shell-making period. Fortunately, there is a promising approach, fibers-reinforced technology, successfully used in reinforced concrete, to resolve them in shells. It is a very simple technology for fibers to be introduced into the silica sol slurry for preparation of fiber-reinforced shells. But meanwhile, it must be paid attention to variation of rheological behavior of silica sol slurry with short glass fibers, on which technical properties of slurry for shells strongly depend. So, it is necessary to investigate effects of glass fibers on rheological characteristics of silica sol slurry, so that it can be controlled effectively for preparation of fiber-reinforced shells with excellent properties.In the present work, the selectived glass-, or the modified- fibers, with diameter of 13.0 um and length of 1.0 mm to 5.0 mm, as reinforcers were introduced respectively at 0.1~0.9 wt%(based on mass of Mullite powders) into the silica sol slurry for preparation of fiber-reinforced shells for investment casting. The rheological behavior of coatings containing short glass fibers was characterized with a rheometer, type R/S(Brookfield Co.), and their areas of thixotropy loop at a varying shear rate were calculated. As a result, the effects of amounts and lengths of the fibers on the rheological behavior of the coatings were investigated.The results show that the apparent viscosity, thixotropy, and suspensibility of the coatings increase with the amount and length of fibers in the coatings, and on which the influence of the former is more notable than the latter. The apparent viscosities of coatings containing the selectived glass fibers, with less than a length of 2.0 mm, at the same share rate, and their high shear viscosities at 150s-1 all increase with increase of the amount of fibers. Meanwhile, the thixotropy and suspensibility of coatings are improved. However, the apparent viscosities of coatings increases initially, but later decreases with the increase of the amount of the selectived fibers with length in excess of 2.0 mm, but still higher than that without fibers. Compared with the coatings without fibers, the silica sol slurry containing 0.7 wt% fibers with length of 4.0 mm has the highest apparent viscosity of 0.22 Pa·s, increased by 214%, and a maximum area of thixotropy loop of 246.44 Pa·s-1, increased by 91.95%, as well as a suspension ratio of 86%.It is found that the glass fibers treated with KH550 disperse more uniformly in the coatings than the untreated. And the apparent viscosities of coatings containing the treated fibers with less than a length of 4 mm at the same share rate and their high shear viscosities at a rate of 150s-1 all increase with increase of the amount of fibers. Meanwhile, the thixotropy and suspensibility of coatings are improved, higher than those of coatings with same length and content of the untreated. Besides, the apparent viscosities of coatings increases increase initially, but later decrease, with increasing amount of the modified fibers with a length of 5.0 mm in the silica sol slurry. The high shear viscosity and area of thixotropy loop of the coating containing 0.7 wt% the modified fibers with a length of 5.0 mm increased separately by about 85.7% and 114.7%, in comparison to the coating without fibers, meanwhile, the suspensibility of the coating achieves the maximum of 91%.This indicates that the modified fibers have a more significant influence on rheological behavior of the coatings than those of the unmodified in silica sol slurry under the same length and amount of fiber as reinforcers.