| The plaster mold casting process is still widely used in the production of non-ferrous metal castings due to its good dimensional accuracy and surface quality of castings.However,the plaster mold tends to damage structures during handling for its low green strength,and produce gas holes defects in castings for its high density and low permeability.Although a higher green strength of mold can be obtained by increasing thickness of the wall of plaster mold,it will lead to lower permeability of mold which is difficult to be dried completely and other problems.So,this traditional plaster mold is not suitable for high precision and high grade casting production.A new technology of high-strength and thin-walled plaster mold is prospective.At present,the reinforced plaster molds are prepared by adding various kinds of refractory powders in slurry.Unfortunately,there is not a notable effect for the refractory materials which do not provide interracial bonding in mold.The water glass with high adhesive strength is widely used as a shell binder in investment casting.Then it is expected that water glass,water-soluble silicate sol,and high compatibility with gypsum slurry,can be employed to reinforce plaster mold and improve its performance.In this case,a high-strength and thin-walled plaster mold with high permeability can provide the same strength as a thick-walled shell so that the quality of the casting can be improved.For this reason,water glass as a reinforcing agent was introduced into the gypsum slurry to prepare a reinforced plaster mold and the effects of content of water glass on the strength,mass ratio of moisture,and fracture feature of the green,and the fired specimens were investigated in this work and then the reinforcement mechanism of water glass in mold was also found out,on which controllable preparation of mold in engineering application depends deeply.Based on this idea,0~2.25%(mass fraction of the mixture of gypsum and mullite powders)water glass with a density of 1.37 g/cm~3 and a modulus of 3.15 was selected as reinforcing agent was introduced,0~2.25%by total weight of the mixture of gypsum and mullite powders,into the slurry to prepare mold.The water glass was prediluted with required water for the preparation of slurry before addition to the mixture,and then mixed with gypsum and mullite powders and pour into the dies to prepared specimens.The specimens was dried for 24h at room temperature for green strength test,and further fired for 30~150 min at 1100?for fired strength test.The phase composition,thermal stability,and fracture surface were observed and analyzed by XRD,TG-DSC,and SEM,so that the effects of water glass on the properties of plaster mold and its loss mass ratio of water were revealed.The results show that the green strength of reinforced mold specimen first increases then decreases with the amount of sodium silicate.And at the mount of1.35%sodium silicate,it reached the maximum,2.16MPa,increased by 58%comparison with the unreinforced plaster mold,only green strength of 1.36MPa.This indicated that sodium silicate has been very effective for plaster molds.In contrast,it is not found that there is a significant different in the fired strength of specimens between the reinforced and the unreinforced.In addition,there is remarkable decrease of the water loss ratio of plaster mold specimens dried at room temperature,resulted in addition of sodium silicate.On the contrary,it is not found that there is a noticeable effect on the water loss ratio of plaster mold specimens fired at 1100?,which achieves almost a constant value in excess of 150 min during firing.The XRD patterns demonstrate that there is the same phase composition in the reinforced specimens as in the unreinforced.However,the peaks in the X-ray diffraction patterns of the reinforced specimens decreased in different level in intensity for adding the water glass.This indicates that the there is an effect of water glass in the reinforced mold on the crystallization process of calcium sulfate.Thermogravimetry-differential scanning calorimetry(TG-DSC)analysis shows that there is an indistinctive effect of water glass on thermal stability of the molds,the endothermic peak temperatures in the DSC scan of the plaster mold reinforced with water glass decrease slightly about 5~9?.Moreover,there is a weak mass loss ratio of the reinforced specimens fired at temperature in excess of 350?observed in TG curves.It is also found by SEM that the grain size of calcium sulfate in the mold specimens,reinforced with a low content of water glass,and dried at ambient temperature for 24 h,reduces,but increases with the increase of water glass content.In conclusion,the performance of plaster mold can be improved significantly by reinforcement of water glass and the reinforced mold can provide higher green strength,beneficial to lower percentage of damaged mold during handling;furthermore,a thin-walled reinforced mold with a same green strength as the thick-walled,can provide a higher permeability of mold for the thinner wall for which the moisture migration in the molds can be accelerated during firing process,and lower percent of casting defects. |
PDF Full Text Request