Setting expansion of gypsum-bonded investment in dental casting

The values of setting expansion of investment under uniaxial stress have been determined at conditions designed to obtain the difference of setting expansion between that parallel to the loading direction and that perpendicular to the loading axis. The setting expansion curves were represented by $$a(t) = a_0 (1 - P/E')[1 - exp( - kt)]$$ along the loading direction and $$a(t) = a_0 (1 + \nu 'P/E')[1 - exp( - kt)]$$ perpendicular to the loading direction, wherea(t) is a setting expansion,a ₀ = 0.009,ν′ = 0.2,E′ = 5 kg cm^?2,k = 0.032 min^?1,P applied stress, andt the time (min). On the basis of these results, a method to estimate the value of setting expansion under restrictive force was developed. According to this method, the setting expansion of the investment could be calculated by substituting?ε/?t forε, ka ₀ exp (?kt)/E′ for 1/E, ν′ forν, andka ₀ exp (?kt) forαT in the theory of elasticity.     

Finite-element analysis of setting expansion in dental gypsum-bonded investment

A parameter study with use of the finite-element method (FEM) was conducted for the determination of the setting expansion of gypsum-bonded investment. In this study the FEM was applied to the setting expansion within an elastic ring, and the results of FEM analysis were compared with the values measured and the values analytically obtained. The agreement of the results was clarified when the step number for calculation was increased. The present formulae appear to be reasonable for comparison of the value for setting expansion.     

ProCAST软件在熔模铸造工艺优化中的应用

为解决企业生产中某精铸件存在的缺陷问题,利用数值模拟软件ProCAST对其充型过程和凝固过程进行模拟,并预测缩孔和缩松的存在情况.根据数值模拟的结果对该铸件工艺方案进行相应的改进,显著降低了缩孔和缩松缺陷,并提高了产品的合格率.应用表明:铸造模拟软件能够准确地预测充型凝固过程中可能产生的缺陷,从而辅助工艺人员进行工艺优化.     

Mechanical properties of dental investment materials

Measurement of the elastic modulus (E) of investment materials has been difficult because of their low strength. However, these values are essential for engineering simulation and there are many methods available to assess the elasticity of materials. The present study compared two different methods with one of the methods being non-destructive in nature and can be used for specimens prepared for other tests. Two different types of investment materials were selected, gypsum-and phosphate-bonded. Method 1 is a traditional three-point bending test. Twelve rectangular bars with dimension of (70 x 9 x 3 mm) were prepared and placed on supports 56.8 mm apart. The test was conducted at a cross-head speed of 1 mm/min by use of a universal testing machine. The load applied to the test specimen and the corresponding deflection were measured until the specimen fractured. The E value was calculated from a linear part of the stress-strain plot. Method 2 is an ultra micro-indentation system to determine near surface properties of materials with nanometer resolution. The measurement procedure was programd such that the specimens were indented with an initial contact force of 5 mN then followed by a maximum force of 500 mN. Measurement consisted of 10 indentations conducted with a spherical stainless steel indenter (R = 250 m) that were equally spaced (500 m). The E value rose asymptotically with depth of penetration and would approach the three-point bending test value at approximately four times maximum contact depth for both materials. Both methods are practical ways of measuring the E of investment materials.     

Thermoanalytical characteristics of powders in dental cast investment

The present study examined the thermal properties of phosphate-bonded investments, a gypsum-bonded investment and an experimental investment powder when the basic powders were heated to high temperatures by simultaneous differential thermal analysis (DTA) and thermogravimetry (TG). The phosphate-bonded investments showed values of about 59 kcal mol^–1 (247 kJ mol^–1) (thermal decomposition of NH₄H₂PO₄) and about 11 kcal mol^–1 (46 kJ mol^–1) (formation of NH₄MgPO₄). Thermal reactions occurred clearly on the DTA-TG curves for the investment powders, using powders of NH₄H₂PO₄, and MgO with NH₄H₂PO₄/MgO = 1 as main components in the investment.     

原位TiB/Ti复合材料的熔铸制备及其显微组织

结合Ti-B-Al体系的热力学及Ti-B相图,提出了可制备薄壁、复杂形状原位自生钛基复合材料构件的SMIF工艺.采用XRD、SEM和TEM等手段研究用该工艺制备的复合材料的相组成和显微组织.结果表明,钛基复合材料中生成了TiB增强相,且在基体中分布均匀,呈短纤维状;并且Al的加入使得TiB相具有较高的长径比,最高可达110.TiB增强相/基体界面清洁、无污染.受熔模精铸陶瓷型壳的激冷作用,钛基复合材料铸锭表层中TiB相垂直于铸锭的表面分布.与基体合金相比较,钛基复合材料的力学性能有了很大程度的提高.     

Standardization of investment casting process using modified binder hydrolysis

A full factorial 2³ matrix was designed to study the bending strength, permeability and hoop-stress of investment moulds. A novel method of ethyl-silicate hydrolysis; two-step hydrolysis, was used. The results indicate that the bending strength is directly proportional to the quantity of filler material in the slurry. Grain size of stucco material appears to exert the least effect on bending and hoop strength, whereas permeability increases slightly when coarser grain is used.     

Mechanical properties of as-cast microalloyed steels produced via investment casting

Tensile and room temperature Charpy V-notch impact tests were used to evaluate the variations in the as-cast mechanical properties of a low-carbon steel produced via shell mould investment casting and containing combinations of vanadium, niobium and titanium. Tensile results indicate that the yield strength and ultimate tensile strength (UTS) have increased up to respectively 615 MPa and 770 MPa due to the fine-scale microalloy precipitates in the microalloyed samples. Room temperature impact test results show that while addition of vanadium individually has not changed the impact energy, Nb has decreased it considerably. However, examination of fracture surfaces reveals that all microalloyed samples have failed by transgranular cleavage. Based on the transmission electron microscope (TEM) studies, it seems that carbonitrides being greater than 50 nm in size and formed along prior austenite grain boundaries before γ transformation are responsible for the observed reduction in impact energies and brittle fracture. In comparison to sand mould casting, the yield and UTS obtained from investment casting are superior. Furthermore, although the impact energies of Nb-containing alloys are approximately the same as those obtained from sand moulds, the impact energy of the alloy containing only vanadium has improved considerably.     

Regulation of setting time for improving casting conditions in slip-form concreting

The optimal conditions in slip-form concreting can be achieved when the initial compressive strength of concrete in the remoulding zone under the form lies between 0.2 and 0.3 MPa. High environmental temperature and low rate of slip-form movement frequently require retardation of the setting process and, on the contrary, low environmental temperature and high rate of moving can require acceleration of the setting. In order to give the constructors an effective tool for regulating the hardening process, well planned laboratory work has to be carried out before the worksin situ begin. This paper presents an approach and method of testing based on an example of a specific project, where the construction was poured by means of slip-forms in the high-temperature conditions of an Israeli summer. It is shown that laboratory data obtained can be used for improving casting conditions if the constructors are able to plan the rate of casting to the nearest shift and to comply with their plan.     

Enhancing mechanical properties and permeability of ceramic shell in investment casting process

In the present investigation, the mechanical properties and hot permeability of the ceramic shell used in investment casting process were improved by modifying the ingredients of conventional slurry. The modifications were made by adding a varying content of nano alumina, camphor and a mixture nano alumina and camphor (hybrid mixture) individually into the conventional slurry. The properties, viz. green strength, fired strength, corner strength, load bearing capacity, self-load deformation and permeability of the modified shells were investigated and compared with conventional shell. The experimental results revealed that the green strength of all the shells increased with an increment of the additives into the slurry. The maximum green, fired and corner strengths were exhibited by the nano alumina modified shell with 1 wt% nano alumina addition. The lowest self-load deformation at elevated temperature was also attained by the nano alumina modified shell with 1 wt% nano alumina addition. The addition of camphor significantly improved the permeability of the shell among all the developed shells. But, addition of camphor decreased the strength of the shell. It was also found that addition of hybrid mixture into the slurry simultaneously increased the mechanical properties and permeability of the shell.     

Multiscale modeling of titanium investment cast dental prostheses

Titanium alloys are attractive materials for biomedical applications due to their superior biocompatibility. However, the use of titanium alloys for dental and maxillofacial prostheses is particularly challenging because each prosthesis has a unique, complex shape. A multiscale model for the processing of Ti prostheses was developed that combines a commercial macrocode for process simulation with an in-house code to explicitly track the development of the microstructure including surface reactions. The model was applied to simulate the dental titanium investment casting process. The macroscopic heat transfer model is coupled with a microscale simulation incorporating the release and diffusion of impurity elements at the mold–metal interface. The penetration depth of impurity species was simulated to determine the amount of alpha-case formed. The microstructural modeling results indicate that the critical factor is the time for which liquid Ti is in contact with the mold and that the incorporation of silicon from the mold increases this time by depressing the solidus temperature.     

A novel alternative to the additives in investment casting pattern wax compositions

In this research, modifying effects of some soybeans on the properties of an investment casting sprue wax (B97) have been studied. For this purpose, soybeans of Wilken-3480, NS-6, RHS-6592 and SGI-3308 varieties have been used as the additives. The effects of these soybeans on the surface roughness, solidification shrinkage, viscosity, hardness, tensile strength and wettability of B97 have been investigated. It has been conclusively shown that these soybeans improve the properties of B97 to such an extent that it converts into a pattern wax having properties superior to those of a commercial investment casting pattern wax (B140).     

Evaluation of different bonded investments for dental titanium casting

The properties of several different investments were investigated including phosphate bonded, magnesia bonded, and alumina cement investments.Measurements included the setting expansion, thermal expansion, and compressive strength of investments, as well as the tensile strength, elongation, Vickers hardness (VHN) and surface roughness of titanium castings. For phosphate bonded investment, the setting expansion after being mixed with its own mixing solution was 2.10%, which was larger than the other investments; the thermal expansion was −0.25% at 200 ^∘C, the compressive strength 14 and 5 MPa after heating. For titanium cast in phosphate bonded investment, the hardness on its top surface was 655 Hv, the tensile strength was 379 MPa, the elongation was 19.4%, and the surface roughness was 2.29 μ m. Athough the thermal expansion of phosphate bonded investment is small, the setting expansion is large enough to compensate for the shrinkage of titanium castings. As its thermal expansion at T ≥ 600^∘C was constant and its heating-cooling cycle was almost reversible, these two properties can reduce the thermal shock and thus avoid cracking of the investment.     

Evaluation of different bonded investments for dental titanium casting

The properties of several different investments were investigated including phosphate bonded, magnesia bonded, and alumina cement investments. Measurements included the setting expansion, thermal expansion, and compressive strength of investments, as well as the tensile strength, elongation, Vickers hardness (VHN) and surface roughness of titanium castings. For phosphate bonded investment, the setting expansion after being mixed with its own mixing solution was 2.10%, which was larger than the other investments; the thermal expansion was −0.25% at 200°C, the compressive strength 14 and 5 MPa after heating. For titanium cast in phosphate bonded investment, the hardness on its top surface was 655 Hv, the tensile strength was 379 MPa, the elongation was 19.4%, and the surface roughness was 2.29 μm. Athough the thermal expansion of phosphate bonded investment is small, the setting expansion is large enough to compensate for the shrinkage of titanium castings. As its thermal expansion at T ≥ 600°C was constant and its heating-cooling cycle was almost reversible, these two properties can reduce the thermal shock and thus avoid cracking of the investment.     

The influence of casting technique on elongation and other mechanical properties of low-ductility dental casting alloys

A study was made of the effect of mould orientation on the mechanical properties of eight dental cobalt-chromium alloys prepared by centrifugal casting. In general, tensile test bars cast with their length horizontal rather than vertical to the induced gravitational field yielded superior elongation and ultimate tensile strength values. Proof stress was less affected by casting conditions.