Rheological Behaviour Of Silica Sol Slurry For Short Polypropylene Fibers-reinforced Shell For Investment Casting

It is expected that fiber-reinforced shell, as a replacement of a single-layer shell with low mechanical properties, can be used to puring melt into mould for investment casting. For preparation of fiber-reinforced shell, a very convenient approach to the introduction of fibers being mixed with sllury into shells is suggested. However, it is obvious that fibers intrduced into slurry will have an effect on the rheological behaviour of slurry, further on quality of shell. So, it is important for rheological behavior of slurry containing fibers to be investigated. And the successful application of fiber-reinforced shell in production of investment casting strongly depends on controlling of rheological behavior of slurry.Selectived polypropylene-, or the modified fibers, with a diameter of around 50.0 μm and and length of 2.0 mm to 6.0 mm, as reinforcers, were introduced respectively at 0 wt%~1.0 wt%(based on mass of Mullite powders) into the silica sol slurry for preparation of fiber-reinforced shells for investment casting, and the influence of the length and content of fibers on rheological behavior of slurries containing short polypropylene fibers was investigated with a rheometer, type R/S(Brookfield Co.), in this work.It is found that the selectived polypropylene fibers in the mixture of dry powders milled with planetary ball mill bended and deformed, moreover, well-dispersed in the slurry.The results indicate that the apparent viscosity, thixotropy, and suspensibility of the coatings increase with the amount and length of selectived polypropylene-, or the modified fibers in the coatings. On the contrary, they decreased with the amount and length of fibers under condition of fiber glomerating in the slurry.The results indicate that there is a obvious increse in apparent viscosity of the coatings containing polypropylene fibers treated with KH550 and free-organobentonite thixotropy, a little in thixotropy. Besides, it is not found that there is an evident variation in suspensibility of these coatings. The selectived polypropylene fibers well-dispersed in the sllury for the addition of 0.8 wt% organobentonite as suspending agent so that rheological behavior of slurries varied noticeably. The apparent viscosity, thixotropy, and suspensibility of the coatings increase with the amount and length of fibers. The slurry containing 1.0 wt% selected fibers with length of 5.0 mm has the highest apparent viscosity of 642.22 Pa·s, increased by 544%, and a maximum area of thixotropy loop of 525.43 Pa·s-1, increased by 220%, as well as a suspension ratio of 95%, increased by 17%, separately, compared with these without fibers. In additions, there is a sharp fall in the amount of fiber floating in the upper surface of sllury. It is not found fiber floating in the coatings containing fibers with length in excess of 5.0 mm.These indicate that the selectived polypropylene-, or the modified fibers have a more significant influence on rheological behavior of the coatings with 0.8 wt% organobentonite as suspending agent than without organobentonite under the same length and amount of fiber as reinforcers.