Phosphate-bonded Investment Material Research Articles

An In-Vitro Study Effect of Adding Silica Gel on Surface Hardness of Phosphate-Bonded Investment Material

Background: Only a few research have been undertaken to assess the influence of nanoparticles on the surface hardness of thermal investment materials. As a result, the current investigation was carried out to determine the influence of silica gel agents on the surface hardness of dental phosphate investment material at various percentages. Materials and methods: in this investigation, commercially available phosphate-bonded refractory investment material (Zetavest fine) and Silica gel powder (Himedia-India) were used. The specimens were manufactured and split into four groups (n=16): specimens without silica gel substance; specimens with 1% silica gel material; specimens with 2% silica gel material; and specimens with 3% silica gel material. A hardness test was performed with the use of an indenter durometer (Shore D). Surface hardness characteristics were analysed using A way ANOVA (post-hoc Tukey) test with a p-value of ≤0.05 indicating significance. Results: The surface hardness of investment specimens with 1%, 2%, and 3% silica gel powder before heating were comparable and have shown substantial differences from the specimens with non-additive. Conclusion: Silica gel of 1, 2, and 3% can improve the phosphate-bonded surface hardness.

A Lab-Based Study to Assess the Surface Roughness of Various Investments: An Original Research.

Surface roughness of the crowns is dependent on the pattern material used for the making as well as the procedure of investing. The study was aimed to examine the impact of various pattern materials and investment procedures on the surface roughness of nickel-chromium alloy raw castings. An in-vitro study was piloted at a tertiary care hospital. Eighty samples of inlay wax and pattern resin were divided equally. They were invested in phosphate-bonded investment material and kept under normal atmospheric pressure as well as invested under a pressure of 3 bars. The surface roughness was calculated with a Profilometer. The values obtained were compared using statistical tools keeping P < 0.05 as significant. Lowest surface roughness was seen for the wax pattern invested under positive pressure. While the highest was seen for the resin patterns invested at room pressure. A significant variance between the wax 1 and wax 2 (P < 0.01); as well as between the wax 2 vs. Resin 2 specimens (P < 0.01) was noted. Wax patterns can be suggested as the material and method of choice because they showed the least amount of surface roughness when placed under pressure. Resin patterns when invested under pressure can also be suggested as an alternate as they also exhibited similar surface roughness as that of the wax.

Influence of Different Wax Hardening Agents on Surface Roughness of Refractory Cast Investment Materials

Background: Phosphate-bonded investments produce a refractory cast model with rough surface and undesirable properties. Dental Surface hardening agents are then applied to refractory investment materials to enhance its properties.&#x0D; Purpose: compare and evaluate the effects of different dental cast hardening agents (Bees wax, Carnauba wax, and Ceresin wax) on surface roughness of refractory model investment material. &#x0D; Approach: 30 specimens were constructed from commercially available phosphate bonded investment material. Specimens were subjected to a roughness test before dipping in handing agents, then divided into three groups according to the type of wax in which they were dipped. (10 specimens for each type). Surface roughness test was done by a profilometer where three readings were taken for all the specimens' surfaces randomly. The mean was then extracted for the roughness values before and after dipping in waxes. &#x0D; Results: the mean and standard deviation were tabulated and statistically analyzed using students T- test. The roughness values were decreased after the specimens were dipped in waxes (P value &lt; 0.001 HS highly significant).&#x0D; Conclusion: The dipping process with hardening agents has significantly improved the surface roughness of the specimens' investment materials.

Analyse the effect of different investment techniques and pattern materials on surface roughness of raw Ni-Cr castings - An In Vitro study.

Aims:This study aimed to analyze the effect of different investment techniques and pattern materials on the surface roughness of raw castings from nickel-chromium alloy.Settings and Design:This is an experimental in vitro study carried out in Bharati Vidyapeeth Dental College and Hospital, Sangli, Maharastra.Materials and Methods:Sixty square-shaped wax patterns, measuring 10 mm × 10 mm × 2 mm, were divided into four groups. A phosphate-bonded investment material (Bellasun, Bego, Germany) was used to invest 15 samples of inlay wax and kept under normal atmospheric pressure and the remaining 15 wax patterns were invested under a pressure of 3 bars for 30 min, and then allowed to bench set for another 30 min. The same investing techniques were carried out for the remaining thirty samples made from pattern resin.Statistical Analysis Used:The surface roughness (μm) of the castings was measured by a profilometer. Student's “unpaired t-test” was used for the statistical analysis.Results:Specimens that were invested at atmospheric pressure had significantly more surface roughness (μm) values than those invested under increased pressure (P < 0.01).Conclusions:Wax patterns exhibited the least surface roughness when invested under pressure and can be recommended as the material and technique of choice. In addition, resin patterns invested under increased pressure produced smoother casting surface than those invested at atmospheric pressure, and the difference is highly significant.

Comparative evaluation of diametral tensile strength of phosphate-bonded investment (ringless) material by using air-drying method, conventional hot air oven, microwave oven, and combination of microwave and conventional oven: An in vitro study.

Aim:This study evaluated and compared the diametral tensile strength (DTS) of phosphate-bonded investment (PBI) material (ringless) used in removable cast partial denture fabrication.Settings and Design:Comparative - Invitro study.Materials and Methods:The PBI material, Wirovest used in this study was subjected to four different drying methods. A total of 80 specimens were prepared in a cylindrical form; 20 specimens were air dried for 2 h, 20 specimens were dried in a conventional oven at 230°C for 1 h, 20 specimens were dried in microwave oven at 600 W for 10 min, and remaining 20 specimens were dried first in microwave oven and then in conventional oven. The dried specimens were tested at 2-h interval for diametral compression at a crosshead speed of 0.5 cm/min.Statistical Analysis Used:Pairwise analysis.Results:The microwave drying technique and drying by combination method resulted in greater DTS, respectively.Conclusion:Within the limitation of this study, PBI specimens dried in microwave oven at 600W for 10 min increased the diametral strength and are also a time-saving procedure.

Effect of phosphate compound on physical and mechanical properties of SiO2 ceramic

This paper reports on the addition of Al(H2PO4)3 binder solution with different vol% ratio and its effects on the mechanical behavior of the formed silicon dioxide ceramic. The starting material, commercial phosphate-bonded investment material, was analyzed by X-ray fluorescence (XRF) spectrometry. After silicon dioxide powder was mixed with a binder, the thick liquid was poured into a rubber mold and allowed to dry at room temperature. Then, the green bodies formed were sintered at 1200 °C for 5 h in a furnace in air atmosphere. Phase and impurities of all the ceramic samples were characterized by X-ray diffraction (XRD) and Raman spectroscopy. The morphology was revealed by scanning electron microscopy (SEM). The local atomic structures of some ceramic samples were investigated by extended X-ray absorption fine structure (EXAFS) to better understand the formation of phosphate compound. Compression test was carried out on the ceramic samples at room temperature. Results showed that quartz-cristobalite transformation in the silicon dioxide structure did not significantly affect the mechanical property of the ceramic samples prepared in this work. Porosity was decreased with Al(H2PO4)3 diluted binder ratio. The presence of pores act as stress concentration points in SiO2-based ceramics in which crack could initiate under loading. We strongly believe that a sharp increase in the compressive strength is due to phosphate compound and its unique morphology.

Influence of Nd: YAG Laser Parameters on Tensile Behaviour, Microhardness and Surface Roughness of Ni-Cr Alloy for Dental Prostheses

The paper presents the results of tensile behaviour, microhardness, surface roughness and microstructure of Ni-Cr alloy surface treated using high power Q-switched Nd: YAG laser has been studied. The major clinical disadvantage of the Ni – Cr is their lack of adequate ductility and yield strength. These properties combined made finishing, polishing and burnishing of conventional base metal alloys rather difficult. The dumbbell shaped tensile and cylindrical specimens of Ni-Cr were cast with a phosphate bonded investment material, using an induction melting centrifugal casting machine. The microhardness values of the surface melted layers increased as compared with Ni-Cr as - cast condition. The results of experiment showed that the surface treatment process has improved the % elongation, and surface roughness and microhardness as compared with as-cast. The improvement of the mechanical properties may be attributed due to grain refinement imparted by laser shock peening. The microstructure and changes in crystal orientation presented in the surface layer of the laser treated material were analyzed by optical, SEM as well as XRD. The chemical composition of laser treated surface was determined by EDAX attached along with SEM. Vickers microhardness was measured as per ASTM E384 11el standard test method. The data were compared using ANOVA and post hoc –Tukey test. In our present study, the laser shock peening process shows, a substantial increase in surface roughness from Ra = 0.440 μm before LSP treatment to 1.781 μm and surface hardness of Ni-Cr was achieved by 53.5% the base material hardness (i.e. from284 HV to 436 HV) and the mean values of % elongation of Ni-Cr alloy was higher (double) than that of after laser shock peening. The mean values of UTS, YS, modulus of elasticity of Ni-Cr were significantly lower after LSP. The experimental results showed that the mean values of percentage elongation of Ni-Cr increased by 200 % after LSP. It is evident from the above experimentation, increase of ductility of Ni-Cr alloy facilities workability which could produce a reliable removable partial denture (RPD) metal framework for dental prostheses.

Effects of setting under air pressure on the number of surface pores and irregularities of dental investment materials

Surface roughness and irregularities are important properties of dental investment materials that can affect the fit of a restoration. Whether setting under air pressure affects the surface irregularities of gypsum-bonded and phosphate-bonded investment materials is unknown. The purpose of this study was to investigate the effect of air pressure on the pore size and surface irregularities of investment materials immediately after pouring. Three dental investments, 1 gypsum-bonded investment and 2 phosphate-bonded investments, were investigated. They were vacuum mixed according to the manufacturers' recommendations, then poured into a ringless casting system. The prepared specimens were divided into 2 groups: 1 bench setting and the other placed in a pressure pot at 172 kPa. After 45 minutes of setting, the rings were removed and the investments were cut at a right angle to the long axis with a diamond disk. The surfaces of the investments were steam cleaned, dried with an air spray, and observed with a stereomicroscope. A profilometer was used to evaluate the surface roughness (μm) of the castings. The number of surface pores was counted for 8 specimens from each group and the means and standard deviations were reported. Two-way ANOVA was used to compare the data. Specimens that set under atmospheric air pressure had a significantly higher number of pores than specimens that set under increased pressure (P<.05). No statistically significant differences for surface roughness were found (P=.078). Also, no significant difference was observed among the 3 different types of materials tested (P>.05). Specimens set under positive pressure in a pressure chamber presented fewer surface bubbles than specimens set under atmospheric pressure. Positive pressure is effective and, therefore, is recommended for both gypsum-bonded and phosphate-bonded investment materials.

The effect of four sprue shapes on the quality of cobalt-chromium cast removable partial denture frame-works.

Statement of Problem:Sprue design is a factor that controls the velocity and adequate supply of metal to the mould. Currently various manufacturers recommend different shapes of sprue, which have not been advocated in textbooks and literature is lacking for their routine applications.Purpose:This in vitro study was carried out to determine the efficacy of four sprue shapes in producing complete, void free cobalt-chrome removable partial denture frameworks.Materials and Methods:A brass metal die with a Kennedy class III, modification 1, partially edentulous arch was used and four sprue shapes (Group A-Ribbon, Group B-Square, Group C-Round and Group D- Round with reservoir) were evaluated. 40 refractory casts were made, 10 wax patterns for each sprue design were waxed up, invested with phosphate bonded investment material and castings done with induction casting machine by the same operator under standardized protocols. The cast frameworks were evaluated for 1. The defects observed visually before finishing and polishing procedures, 2. Fit on the master die as seen with naked eye and 3. Defects on radiographic evaluation. Data were tabulated and statistically analyzed with 1-way ANOVA followed by Student ‘t’ test.Results:The results differed significantly (P < 0.0001) between the Groups with maximum defects in the castings of Group A followed in decreasing order by Group B, Group C and Group D. When comparing between the Groups (P < 0.05), the defects in Groups C and D was significantly lower than Group A and Group B.Conclusions:Round sprues with reservoir produced most satisfactory fit of castings with minimum number of internal and external defects.

Dimensional accuracy of castings fabricated with ringless and metal ring investment systems for implant supported fixed dental prosthesis : An in-vitro comparative study

Background: The ring-less investment system is in use for dental casting, although there was no adequate scientific data to support its use either for conventional fixed dental prosthesis or implant retained fixed dental prosthesis. Methods: An in-vitro study was undertaken to compare the vertical marginal accuracy of single full coverage metal restorations, between ring-less and metal ring investment techniques, using two different investment materials, for implant supported fixed dental prosthesis. Three groups were made of ten samples each. Group I consisted of metal ring with PCT® FlexVest (phosphate bonded investment material). Group II consisted of metal ring with Bellasun® investment material. Group III and the final group consisted of ring-less investment system and Bellasun® investment material. The wax patterns were prepared on a metal die, cast and finished. The cast restorations (samples) were again seated on the metal die and the accuracy of fit was evaluated by measuring the gap between the finish line on the die and the margins of the sample at four specific sites using a profile projector (Helios-350H, Microtecnica, LTF, Italy) having accuracy of 1µm. Result: Mean marginal accuracy for Group-III was found to be the least (58.87 + 17.87µm) followed by Group-II (97.23 + 16.37[m), and Group-I (109 + 7.55 µm). However, Group I showed the least variability among the readings (SD=7.55). Conclusion: Ring-less system of casting can be recommended for use in fabricating implant supported fixed dental restorations.

Microstructure, mechanical performance and corrosion properties of base metal solder joints

Alloys have been considered to be of paramount importance in the field of prosthodontics. Long span prosthesis may often require joining of one or more individual castings to obtain better fit, occlusal harmony and esthetics in comparison to one-piece casting. This study was undertaken to evaluate the mechanical properties of base metal alloys joined by two different techniques, namely, gas oxygen torch soldering and laser fusion, compared to a one-piece casting. Mechanical properties evaluated were tensile strength, percentage of elongation and hardness of the solder joint. In addition, corrosion properties and scanning electron microscopic appearance of the joints were also evaluated. The samples were prepared according to American Society for Testing Materials specifications (ASTM, E8). Specimens were made with self-cure acrylic and then invested in phosphate-bonded investment material. Casting was done in induction casting machine. Thirty specimens were thus prepared for each group and compared with 30 specimens of the one-piece casting group. SPSS software (version 10.0, Chicago, IL, USA) was used for statistical analysis. ANOVA and Benferroni post hoc tests were done for multiple comparisons between the groups and within the groups for mean difference and standard error. Results showed that tensile strength of the one-piece casting was higher than laser fused and gas oxygen torch soldered joints. Laser fused joints exhibited higher hardness values compared to that of gas oxygen torch soldered joints. Scanning electron microscopic examination revealed greater porosity in the gas oxygen torch soldered joints. This contributed to the reduction in the strength of the joint. Gas oxygen torch soldered joints showed less corrosion resistance when compared to laser fused joints and one-piece casting. Laser fusion, which is a recent introduction to the field of prosthodontics, produces joints which have properties between those of one-piece casting and the gas oxygen torch soldering.

A New Phosphate-Bonded Investment Material for Rapid Ceramic Molding of Medium-Size Castings

Phosphate-bonded investment（PBI）ceramic mold materials have outstanding properties, such as rapid setting rate , moderate green strength and high temperature strength, and could be widely utilized in dental restoration and micro-casting of artistic products. However, fast hydration and gel rate of the phosphate-bonded ceramic slurry has been an obstacle to the wide applications of ceramic mold material for medium and large-size precision castings. This paper presents a new phosphate-bonded ceramic mold material by modifying the composition of refractory aggregates and additive agents, which is promising to be used for rapid ceramic mold casting for medium-size super alloy components. The results indicate that the properties of ceramic mold slurry including initial setting time and fluidity are suitable for complete embedding medium-scale patterns and get accurate duplicate of external configurations when the amount of boric acid is 1.11wt% and sodium tri-poly phosphate (STP) is 0.92wt%. The strength of ceramic mold material can resist impact force when pouring molten metal and easy take casting out from ceramic mold.

Differing Expansion Contributions of Three Investment Materials Used for Casting Titanium

This laboratory study aimed at investigating the effect of setting expansion (SE), which could enlarge a wax pattern by concurrent exothermic reaction. Two phosphate-bonded investment materials (Rematitan Plus, T-invest C & B) and alumina-magnesia-based investment material (Titavest CB) were subjected to setting temperature and SE measurements (n = 10). Full-crown wax patterns were prepared from metal dies having the same design. Crown castings (n = 6) were made using a one-chamber high-pressure casting machine. Commercially pure titanium ingot (Japanese Industrial Standard Class II) was used for each casting. Dimensional accuracy of the cast crowns was expressed by marginal discrepancy for a pair of wax pattern and casting prepared from the same die. All results were then subjected to regression analysis. Rematitan Plus gave the highest setting temperature (about 70 degrees C) and the highest SE (1.16 +/- 0.01%) almost concurrently. The result was that Rematitan Plus produced oversized crowns from the lowest thermal expansion (TE) value (0.53 +/- 0.05%) reducing the linear correlation to 0.80 (p < 0.05). No significant linear relationship was found between SE of investment material and crown-casting accuracy. The almost concurrent occurrence of high SE and high setting temperature generation in Rematitan Plus enlarged the wax pattern and effectively supplemented its low TE to produce oversized crowns. Despite this adverse effect, the correlation between TE and crown accuracy was still high. It is generally expected that the TE values given by manufacturers predict crown accuracy, when uniform TE, small SE, and minimum heat generation during setting are assured.

Thermal expansion of prepared dental base-metal casting alloys

Background: The aim of the study was to evaluate the linear thermal expansion of prepared Ni-Cr and Co-Cr dental base-metal casting alloys which were prepared locally in Iraq and compare with other conventional alloys. Materials and methods: Cylindrical shape wax specimen were prepared of 15mm length and 5mm diameter which were invested in phosphate-bonded investment material and casted by using induction casting machine. The liner thermal expansion was tested by using Iraqi made dialometer in which the furnace was set to reach from room temperature to 950 °C in a rate of 10 °C per minute. Results: The prepared Ni-Cr and Co-Cr alloys showed a higher linear thermal expansion than the conventional alloys, however, statistical analysis revealed non significant differences regarding the Co-Cr alloys and highly significant difference regarding the Ni-Cr alloys. Conclusion: In comparison of locally made and other conventional alloys, Co-Cr alloys statistically revealed no difference while Ni-Cr alloys showed a difference.

Deformation of phosphate-bonded investment materials at elevated temperatures

Objectives To determine deformation in phosphate-bonded investment (PBI) materials for “in-service conditions” and to investigate the influence of air bubble pores on deformation at 900 °C. Methods Surviving high and low strain-rate disc-rupture strength test samples were assessed for deformation. A dental centrifugal casting machine and a dental superplastic-forming machine were used to apply the loads. An indirect technique was used to measure the deformation of investment diaphragms with both a wide pore size distribution and with very low porosity for four investment materials. A total of 128 high strain-rate and 29 low strain-rate samples were investigated. A one-way analysis of variance was carried out. Results All but one of the high strain-rate test samples showed measurable deformation. All of the low strain-rate test samples containing pores showed measurable deformation. Six out of 14 samples, in which pores had been eliminated, showed no deformation. There was no statistically significant difference in deformation between samples with no pores and those with a large pore size distribution for either the high or low strain-rate tests ( P > 0.05). Significance PBI must survive all forming procedures without deformation if the resultant prosthesis is to fit. One hundred micrometres is generally recognized as the acceptable tolerance of fit for removable metal-based prostheses. The vast majority of deformations measured during this investigation were considerably greater than 100 μm.

The effect of mold temperature on castability of CP Ti and Ti–6Al–4V castings into phosphate bonded investment materials

Objective The aim of this study was to evaluate the castability of CP titanium and Ti–6Al–4V alloy castings into Rematitan Plus investment at three different mold temperatures. Methods A nylon mesh pattern (20 mm with 64 squares and wire of 0.7 mm in diameter) was used for the castability testing. Initially, an image of the wax pattern was obtained by means of a digital camera and the total extension of filaments (mm) was then measured, using the Leica Qwin image analysis system. The mesh sprued was placed in the Rematitan Plus investment material and the castings were made in a Discovery Plasma machine at three different mold temperatures: 430 °C (control group), 480 °C or 530 °C. Ten castings were made for each temperature. The images of the castings were analyzed (Leica Qwin) and the castability index determined by the number of the completely cast segments as a percentage of the wax pattern. Data were analyzed by two-way ANOVA and Tukey's multiple comparison test ( a = 0.05) using materials and temperatures as discriminating variables. Results The Ti–6Al–4V alloy (60.86%) presented a better castability index than CP Ti (48.44%) ( p < 0.000001). For CP Ti, the temperature of 530 °C (23.96%) presented better castability than at other temperatures, 480 °C (14.66%) and 430 °C (12.54%), with no difference between them ( p < 0.001). For Ti–6Al–4V alloy, there was a statistically significant difference among the three temperatures: 530 °C (28.36%) > 480 °C (19.66%) > 430 °C (15.97%) ( p < 0.002). Significance Within the limitations of this study, the increase in the mold temperature of the Rematitan Plus investment resulted in a better castability index for both materials, and Ti–6Al–4V presented a better castability index than CP Ti.

Measurement of the setting expansion of phosphate-bonded investment materials: Part I - Development of the Casting-Ring Test.

The setting expansion is an important property for a phosphate-bonded investment material. This research was undertaken to investigate a test that might be suitable for its measurement when used in a Standard. In the 'Casting-Ring Test', the investment sample is contained in a steel ring and expands to displace a precisely positioned pin. Variables with the potential to alter routine reproduction of the value were investigated. The vacuum-mixer model is a production laboratory variable that must not be ignored and for this reason, experiments were repeated using a different vacuum-mixer located at a second test site. Restraint by the rigid ring material increased expansion, while force on the pin reduced it. Expansion was specific to the lining selected. Increased environmental temperature decreased the final value. Expansion was still taking place at a time at which its value might be measured. However, when these factors are set, the reproducibility of values for setting expansion was good at both test sites (coefficient of variation 14%, at most). The results revealed that with the control that is available reliable routine measurement is possible in a Standard test. The inter-laboratory variable, vacuum-mixer model, produced significant differences and it should be the subject of further investigation.

Sensitivity of a disc rupture strength test to air bubble pores in phosphate-bonded investment materials at elevated temperatures

Objectives: To investigate the sensitivity of a modified disc rupture test to variables affecting the strength of four phosphate-bonded investment materials under conditions closely resembling the in-service environment. This study examined the influence of air bubble pores on the strength of phosphate bonded investment materials at a temperature of 900°C. Method: A modified disc rupture configuration was used to test a circular investment diaphragm with clamped edges, in tension, initiated by bending. A sequentially-varying molten metal mass was used to apply the load using an electronic centrifugal casting machine. The staircase method was used to determine the increase or decrease of the applied load with the standardized increment of load being no greater than 1 or 2 g. The mean load applied to 30 samples defined the transition of survival to failure of the investment disc and was taken as a measurement of investment strength. Two material handling techniques, one giving virtually no pores and one showing a wide pore size distribution were used. Results: The diaphragm clearly either failed or survived the test with the pattern of fracture indicating brittle failure. A statistically significant difference in strength ( P<0.050) of diaphragms with and without air bubble pores was demonstrated for three materials. Fewer pores resulted in increased strength of those materials and produced a much reduced scatter of strength in the fourth. Significance: The modified disc rupture test is sufficiently sensitive to identify variations in strength of phosphate bonded investment materials caused by differing pore size distributions.

Determination of the strength of dental phosphate-bonded investments by 2 and 3 parameter Weibull analysis

Objective: Phosphate-bonded investments are being investigated for use as die materials for dental superplastic forming. The effects of handling technique on the strengths of these investments needs to be determined. The purpose was to use methods of Weibull analysis to fit measured 4 point bend strength data and determine strength and Weibull modulus, where modulus represents the scatter of the data. Materials and methods: Weibull parameters were determined using methods of regression and maximum likelihood using 2 and 3 parameters. The parameters were x 0 the lower bound of strength, m the Weibull Modulus, and θ the characteristic strength. x 0 is zero for the 2 parameter models. In addition to plots of the measured data and curves generated using the Weibull parameters, the fit of the models to the measured data was determined using correlation coefficient and the χ2 statistic. Results: The results showed that characteristic strength was similar whether determined using regression or maximum likelihood, 2 parameter or 3 parameter models. However, Weibull Modulus was significantly lower for 3 parameter models than for 2 parameter models. Visually the 3 parameter models give the best fit for the majority the data sets. Correlation coefficient, r, showed whether the method of maximum likelihood or the method of regression was more appropriate. χ2 and correlation coefficient did not necessarily give the same indication of goodness-of-fit of these data sets. Significance: Strengths of dental phosphate-bonded investment materials can be described well using weibull analysis which gives characteristic strengths and quantitative measures of scatter of the data in the form of Weibull Modulus. These may then be used to select a handling technique to produce dies for dental superplastic forming.

The influence of handling technique on the strength of phosphate-bonded investments

Objectives: This study examined the influence of six handling techniques on the strength of four phosphate-bonded investment (PBI) materials. The size and distribution of pores produced by the handling techniques were investigated and their effect on room temperature strength determined. Methods: Specimens were tested for strength using four point bend at a cross-head speed of 1 mm min −1. The fracture surface of randomly selected specimens was viewed through a light microscope to determine the pore size. A three-parameter Weibull distribution was used to describe strength giving values of Weibull modulus and characteristic strength. Results: The handling technique that produced the smallest pore size resulted in the highest strength. Significant differences in strength (90% confidence level) were found between most handling techniques. The reduction of pore size by mechanical spatulation and setting under pressure was found to be the most effective method of increasing the strength of PBI. Significance: Previous studies have related handling technique to the inclusion of air bubbles in dental investments but have not reported any consequent effect on strength. PBI materials are used in the production of a range of dental appliances with a variety of different material forming processes and are required to withstand the loads applied without fracture or distortion to ensure accuracy of fit of these prostheses. The use of a handling technique that reduces pore size can significantly increase the strength of PBIs, thereby decreasing the possibility of die fracture or degradation.