Dissertations / Theses on the topic 'Shrinkage of wax pattern'

The introduction part of the thesis focuses on the overview of rapid prototyping in foundry industry. Principles of the most important RP methods are described and the FDM method is analyzed in more detail. This method was used in the practical part for the production of wax patterns with silicone moulds. The wax patterns were used for the production of castings using the lost wax method. The result of this work is determination of dimensional changes during the whole process of casting manufacture from the drawing to the final casting.

This diploma thesis deals with the problem of wax patterns manufacturing in investment casting technology (lost wax process). This technology is very sensitive to the exact execution in each phase of the whole process, from concept design up to the final casting expedition. During most operations, dimensional changes occur, which is affecting the accuracy of the final casting. No matter how big these changes are, but especially if they occur with the least variance. One of the first phases in this technology is making wax patterns. There are several ways of making the wax patterns and this thesis is describing two different methods used in the Alucast foundry. One is based on using “soft waxes” enabling using low pressure machines, the other one on the “hard waxes” where the injection machine with higher presses is being used (SOM). These two above mentioned technologies are compared in this work.

Thesis: S.B., Massachusetts Institute of Technology, Department of Mechanical Engineering, 2018.
Cataloged from PDF version of thesis.
Includes bibliographical references (pages 43-45).
Metal objects were of great cultural significance in pre-Columbian Mesoamerica. Historical and archaeological evidence prove that these items were made by the process of investment casting, or "lost wax" casting, by which a wax model of the object is created, and a ceramic mold is built around it. The wax is melted out to allow for the pouring of the molten metal. Considerable research has focused on the alloy composition of these objects, and some research has been done on the ceramic molds, but little is known about the composition, source, and manufacture of the wax itself. This paper builds upon work by Michael Tarkanian and Dr. Elizabeth Paris, attempting to fill this gap in the literature. Spanish written accounts of ancient Mesoamerican casting processes mention that wax from stingless bees was mixed with copal, or tree resin, and used to form the models for lost-wax casting. Waxes from Mexican stingless bees Melipona beecheii, Scaptotrigona pectoralis, and Melipona yucatanica were considered in this study, in addition to three copals: Bursera copallifera, Protium copal, and Pinus contorta. Thermal data from Differential Scanning Calorimetry revealed no changes in thermal behavior between waxes and their blends with copals, showing that none of the blends considered in this study are miscible. However, hardness testing revealed that the blends of Pinus contorta with Melipona beecheii and with Melipona yucatanica were harder than their respective waxes. This is in line with a Spanish account of the casting process, which mentions that copal was added so the wax may "[become firm and] harden well". Including data collected by Tarkanian and Paris, the most favorable material for investment casting would be Friesomelitta nigra wax mixed with Bursera copallifera, according to the criteria considered in this study.
by Eleni Chrisoula Pitses.
S.B.

Regression tasks belong to the set of core problems faced in statistics and machine learning and promising approaches can often be generalized to also deal with classification, interpolation or denoising problems. Whereas the most widely used classical statistical techniques place severe a priori constraints on the type of function that can be approximated (e.g. only lines, in the case of linear regression), the successes of sparse kernel learners, such as the SVM (support vector machine) demonstrate that good results may be obtained in a quite general framework by enforcing sparsity. Similarly, even very simple sparsity-based denoising techniques, such as classical wavelet shrinkage, can produce surprisingly good results on a wide variety of different signals, because, unlike noise, most signals of practical interest share vital characteristics (such as smoothness, or the ability to be well approximated by piece-wise linear polynomials of a low order) that allow a sparse representation in wavelet space. On the other hand results obtained from SVMs (and classical wavelet-shrinkage) suffer from a certain lack of interpretability, since one cannot straightforwardly attach probabilities to them. By contrast regression, and even more importantly classification, in a Bayesian context always entails a probabilistic measure of confidence in the results, which, provided the model assumptions are reasonably accurate, forms a basis for principled decision-making. The relevance vector machine (RVM) combines these strengths by explicitly encoding the criterion of model sparsity as a (Bayesian) prior over the model weights and offers a single, unified paradigm to efficiently deal with regression as well as classification tasks. However the lack of an explicit prior structure over the weight variances means that the degree of sparsity is to a large extent controlled by the choice of kernel (and kernel parameters). This can lead to severe overfitting or oversmoothing -- possibly even both at the same time (e.g. for the multiscale Doppler data). This thesis details an efficient scheme to control sparsity in Bayesian regression by incorporating a flexible noise-dependent smoothness prior into the RVM. The resultant smooth RVM (sRVM) encompasses the original RVM as a special case, but empirical results with a variety of popular data sets show that it can surpass RVM performance in terms of goodness of fit and achieved sparsity as well as computational performance in many cases. As the smoothness prior effectively makes it possible to use (highly efficient) wavelet kernels in an RVM setting this work also unveils a strong connection between Bayesian wavelet shrinkage and RVM regression and effectively further extends the applicability of the RVM to denoising tasks for up to millions of datapoints. We further discuss its applicability to classification tasks.

Pattern recognition is a term that can be used to cover various stages of the investigation of characterising data sets including contributing to problem formulation and data collection through to discrimination, assessment and interpretation of results. Chemometrics techniques and Artificial Neural Networks (ANNs) are pattern recognition techniques commonly used to visualise and gather useful information from multidimensional datasets i.e. datasets with n-samples with m- variables. Of the many chemometric techniques available, Principal Component Analysis (PCA) and Hierarchical Cluster Analysis (HCA) are the most commonly used in the evaluation of dataset(s) generated from the analysis of samples which have relevance to forensic science. By contrast, Artificial Neural Networks (ANNs) and in particular Self Organising Feature Maps (SOFM) and Multi Layer Perceptron (MLP) have had limited application in forensic science eventhough these pattern recognition techniques have been known for almost 30 years. This study focuses on the applicability of the Artificial Neural Networks (ANNs) to specific datasets of forensic science interest and compares these with 'conventional' PCA and HCA techniques. Datasets generated from the analysis of wax based products and lighter fuels were used. The wax based product data set contained information obtained from Thin Layer Chromatography (TLC), Microspectrophotometry (MSP), Ultra-Violet and Visible Spectroscopy (UV/Vis) and Gas Chromatography with Flame Ionisation Detector (GC-FID) analysis of a variety of products from multiple sources where discrimination by brand was the objective. The data provided for the lighter fuel samples was obtained from analysis of a number of brands, both unevaporated and evaporated by Gas Chromatography-Mass Spectroscopy (GC-MS) and the objective was to discriminate the samples by brand as well as link degraded samples from the same brand together. The wax based product analysis provided simple, straight forward data whilst the lighter fuel analysis provided a more complicated and challenging dataset to investigate in terms of facilitating sample discrimination and/or linkage. In all cases, the 'conventional' Principal Component Analysis (PCA) and Hierarchical Cluster Analysis (HCA) failed to provide any meaningful discrimination of the samples by product type regardless of the nature of the datasets. In contrast, the Artificial Neural Networks (ANNs) techniques provided full discrimination of the samples by product type even when the samples had undergone considerable ageing and weathering. This work has demonstrated the potential use of Self Organising Feature Maps (SOFM) and Multi Layer Perceptron (MLP) to datasets of forensic science relevance. The findings of this work provide avenues for further exploration of Artificial Neural Networks (ANNs) in forensic science.

The aim of this diploma thesis is to ascertain the process of injecting the wax through an injection moulding machine under certain input conditions and with the help of the simulation program Cadmould 3D-F®, to perfect the injection and the following solidification of the wax. Only thanks to a perfect injection and the following solidification of the wax, a precise wax pattern can be made. In order to compare the process of injecting the wax mixture, we are helped by the Cadmould 3D-F® simulation program and the records of the injection taken by a video camera. In this thesis a simulation program used when simulating the injection of plastics was applied, as a means of comparison. In this case the similarity between the rheological characteristics of both wax and plastic was exploited.

Production of wax pattern model by injection machine is a one of the key phases of Investment casting process. The thesis deals with the development of numerical simulation wax blend injection in to metal die. The possibilities of selection simulation software – ProCast and Cadmould are described in the relation to numerical predictions of wax injection. Creating of material database of selected wax blends is described. The prototype simulations of die filling are made. The special testing die are designed a used for created simulations. The measurements of initial conditions are mentioned.

The thesis continue thematically in the previous development of the wax mixtures for the company Fimes. The general description of the investment casting process from the production of the die to casting of the wax pattern is handled in the first part. It is followed by the summary of the previously carried out experiments on cracking the shells. The experimental part describes the individual operations from the production of tested wax patterns to casting in the Boilercláv. The newly developed wax mixture is compared to the wax mixture used by the company Fimes. The experiments focus on the possibility of cracking the shells during the casting process and the dimensional stability of wax mixtures.

Determining accuracy of wax patterns is an important in wide implementing rapid prototyping methods in investment casting. This experiment is comparing wax pattern accuracy produced in classical metal die and silicone mould. It has been found out, that wax patterns produced by the silicone mould are ounly about one or two accuracy steps worse then wax patterns produced by classical metal die.

The aim of my diploma thesis was to find the possibilities of reaching the dimensional tolerance in the individual phases of casting by the investment casting process. The accuracy of the selected wax patterns and castings were measured by a caliper. At first the selected dimensions of the wax models were measured right after taking them out of the mold and then they were measured during seven days. The castings were measured after pouring. The obtained figures were statistically evaluated. The results show that the large-scale and thick-wall wax model should be coated after seven days of stabilization because the dimensions of the wax model are not stable.

This thesis is to evaluate currently used wax and compare its properties with of newly purchased one. Further, it is to evaluate whether the quality is reflected by using "boiled wax'', which is being used as a runner wax. All tests were carried out at the Fimes a. s. In the first part of this thesis, there is a brief description of the lost wax method. This is followed by a decription of wax blends from technology point of view. The experimental part describes the operations from the production of wax patterns, through their dimensional inspection, up until dewaxing process. All the results are noted and compared.

Denna uppsats syftar till att undersöka användbarheten av metoden Independent Component Analysis (ICA) för brusreducering av bilder tagna av infraröda kameror. Speciellt fokus ligger på att reducera additivt brus. Bruset delas upp i två delar, det Gaussiska bruset samt det sensorspecifika mönsterbruset. För att reducera det Gaussiska bruset används en populär metod kallad sparse code shrinkage som bygger på ICA. En ny metod, även den byggandes på ICA, utvecklas för att reducera mönsterbrus. För varje sensor utförs, i den nya metoden, en analys av bilddata för att manuellt identifiera typiska mönsterbruskomponenter. Dessa komponenter används därefter för att reducera mönsterbruset i bilder tagna av den aktuella sensorn. Det visas att metoderna ger goda resultat på infraröda bilder. Algoritmerna testas både på syntetiska såväl som på verkliga bilder och resultat presenteras och jämförs med andra algoritmer.

The purpose of this thesis is to evaluate the applicability of the method Independent Component Analysis (ICA) for noise reduction of infrared images. The focus lies on reducing the additive uncorrelated noise and the sensor specific additive Fixed Pattern Noise (FPN). The well known method sparse code shrinkage, in combination with ICA, is applied to reduce the uncorrelated noise degrading infrared images. The result is compared to an adaptive Wiener filter. A novel method, also based on ICA, for reducing FPN is developed. An independent component analysis is made on images from an infrared sensor and typical fixed pattern noise components are manually identified. The identified components are used to fast and effectively reduce the FPN in images taken by the specific sensor. It is shown that both the FPN reduction algorithm and the sparse code shrinkage method work well for infrared images. The algorithms are tested on synthetic as well as on real images and the performance is measured.

An overview of the development of the Investment Casting market and Rapid Prototyping equipment is given in the introductory part of this diploma thesis. Basic methods of rapid prototyping and the FDM method are futher described in detail. The FDM method is consequently used in the practical part of this work for the preparation of the silicon mould. The largest part deals with the issue of the simulation of pouring process of wax into the silicon mould. A characteristics of the casting wax is made; the parametres, which are inserted into the simulation software ProCAST and the possibilities of their measurement are described. The process of the simulation of the mould filling and its validation with the real filling process are described in detail.

The aim of this work is to compare technology of silicon and aluminium moulds used to make castings using technology of cire perdue with single – piece work. The work itself deals with physical production of silicon mould in vacuum casting system, shaping aluminium mould using CNC machine and subsequent casting of wax models into these moulds. It also handles with physical production of one piece by shaping. It contains background research of the method lost wax casting manufacturing aluminium and silicon moulds and last but not least of Rapid Prototyping technology for producing master model.

Os primeiros estudos sobre pavimentos de concreto continuamente armados (PCCA) foram desenvolvidos em zonas cujo clima característico é o temperado (EUA, Holanda e Bélgica), não se tendo estudos técnicos acerca do comportamento do pavimento em ambiente tropical. A premissa de funcionamento do PCCA está baseada na não execução de juntas de retração, obrigatórias em pavimentos de concreto simples (PCS), ou seja, as fissuras devido à retração do concreto ocorrem aleatoriamente, sendo controladas pela elevada taxa de armadura longitudinal disposta longitudinalmente ao longo de toda a extensão do pavimento. Como essa armadura não possui função estrutural, pois é colocada acima da linha neutra da placa, no banzo comprimido, sua função é manter as fissuras fortemente apertadas, garantindo suavidade ao rolamento e elevada transferência de carga (LTE), a qual ocorre pelo intertravamento dos agregados nas faces fissuradas, proporcionando assim um pavimento de alta durabilidade e que dispende baixíssimos custos com manutenção. No Brasil, a primeira experiência com PCCA teve início no ano de 2010, quando foram construídas quatro seções de 50 m de extensão cada, consideradas curtas em comparação aos 400 m de extensão de um PCCA rodoviário que podem ser alcançados ao final de um dia de trabalho. O monitoramento contínuo de tais seções mostrou haver uma diferença muito significativa de comportamento, comparado aos PCCA tradicionais, no que tange o tempo decorrido para estabelecimento do padrão de fissuração (a primeira fissura surgiu um ano após a construção), bem como espaçamento médio entre as fissuras, devido à curta extensão das placas e a inexistência de um sistema de ancoragem nas extremidades. Assim, para melhor compreender o comportamento de pavimentos de concreto com armadura contínua com padrões condizentes à realidade rodoviária em ambiente tropical, em janeiro de 2016, foi construído no campus da Universidade de São Paulo o primeiro PCCA de longa extensão do Brasil, com 200 m de comprimento. Foram empregados quatro diferentes tipos de concreto, variando o tipo de cimento e o tipo de agregado, bem como foram empregados aço galvanizado e aço comum. Para entender o comportamento do pavimento sob as condições climáticas brasileiras foram realizados três estudos: levantamento de fissuras (espaçamento e abertura), modelagem analítica do espaçamento entre fissuras através de modelos de previsão de retração em concreto e testes com FWD para avaliação da eficiência de transferência de carga (LTE) entre as fissuras. A análise do padrão de fissuração mostrou que o desenvolvimento das fissuras aconteceu conforme a literatura técnica, tendo início na primeira semana após a concretagem e atingindo aproximadamente 60% do número total de fissuras antes do primeiro mês. As seções centrais possuem o maior número de fissuras, pois estão ancoradas pelas seções de extremidade, onde o surgimento das fissuras é mais lento por conta da falta de ancoragem. As aberturas das fissuras mostraram-se maiores nas posições com aço galvanizado devido à menor aderência aço-concreto nesse caso, em comparação às posições com aço comum. O modelo de previsão de retração no concreto que melhor se aproximou do espaçamento médio desenvolvido em campo foi o modelo do Eurocode 2, com diferenças em torno de 30%, aproximadamente. As análises de LTE mostraram um desempenho muito satisfatório, apesar de a LTE nas regiões com aço galvanizado ter resultado menor do que a LTE nas regiões com aço comum.
The first studies on continuously reinforced concrete pavements (CRCP) were developed in areas in which the typical weather is temperate (United States, Netherlands and Belgium), with no technical studies carried out in tropical weather. CRCP structural premise is based on a slab without construction joints which are required in jointed plain concrete pavements (JPCP); i.e., CRCP cracks due to concrete shrinkage occur randomly being controlled by the high longitudinal reinforcement percentage, positioned longitudinally along the pavement length. As the longitudinal reinforcement has no structural role, because it is placed above the slab neutral axis, in the compression zone, its main function is to keep cracks strongly tight, ensuring a smooth ride and high load efficiency transfer (LTE) at cracks, which occurs through aggregates interlocking, thereby providing a highly durable pavement with low maintenance costs. The first experience with CRCP in Brazil started in 2010, when four experimental sections were constructed. Each section is 50 meters long, short when compared to the traditional CRCP that can extend for over 400 meters providing that concrete pouring does not stop. Continuous monitoring of these sections has shown that the short CRCP behaves differently from traditional CRCP, regarding the time taken for the cracking pattern full development (the first crack was visible on the surface one year after the construction), as well as average cracking space due to the slab\'s short extension and lack of anchorage. Therefore, in order to fully analyze the behavior of a traditional CRCP under tropical weather, in January 2016, the first long extension CRCP in Brazil was built, at the University of São Paulo campus, with 200 meters length. Four types of concrete were applied in the construction, varying cement and aggregate type. Galvanized and ordinary steel were used as well. To analyze the pavement behavior under Brazilian climatic conditions, three studies were carried out: cracks surveys (spacing and width), analytical modeling crack spacing through shrinkage prediction models and Falling Weight Deflectometer testing to evaluate the crack\'s load transfer efficiency (LTE). Cracking pattern analysis has shown cracks development consistent with technical literature. The first cracks appeared during the first week after concrete placement and, before the first month, about 60% of the total cracks number had developed. Central sections presented greatest number of cracks as they are anchored by the outer sections, where cracking occurs slower due to the lack of anchorage. Cracks width was higher in areas with galvanized steel due to the weaker steel-concrete bond, when compared to areas with common steel. The shrinkage prediction model for concrete that better approached the average field crack spacing was the Eurocode 2, with mean difference of 30%. LTE analysis has shown a quite satisfactory performance, even though LTE in areas with galvanized steel is lower than LTE in areas with common steel.

Aim of this diploma thesis is to choose new pattern wax for Fimes a.s. foundry. New wax should substitute old not fully satisfactory pattern wax which caused problems in technological process resulting in increased number of rejects. New pattern wax should eliminate current problems and help foundry with producing large, thin-walled, high-quality castings. Numbers of pattern waxes supplied by world’s leading wax manufacturers were subjected to laboratory tests and pilot study. This diploma thesis is part of project Alfa TA01010766: „Research and development of production technology large, thin and high quality castings of aluminium alloys “

碩士
中原大學
機械工程研究所
99
Product size accuracy in investment casting depends on the techniques used in the wax injection molding process. This paper discusses the effect of process parameters on size shrinkage rate and how to obtain an accurate computer-assisted analysis method. The wax injection process includes injection pressure, packing pressure, packing time, injection time, injection speed, injection wax temperature, etc. These related factors will affect the accuracy during the formation of wax embryos. During stage 1 of the research, we perform simulation analysis and testing of the accuracy of experimental results. We need to identify the parameters of properties of the wax elements, but due to the lack on data in the current internal databases of simulation analysis programs, we cannot directly obtain the parameters needed for this research. Once the materials are obtained, we can then proceed with the simulation and testing of stage 1, and only after the simulation process method is confirmed we can continue with stage 2 of the research. Stage 2 uses the simulation method from stage 1 as a basis to perform Taguchi method, with the objective of identifying the significant factors affecting size shrinkage rate. Stage 3 of the research makes use of the research data from stages 1 and 2 to perform evaluation tests of current molds in mass production. It also utilizes simulation methods and real wax embryo data to perform cross over validation, with the expectation of obtaining a size shrinkage rate simulation error of within 1%. By making use of simulation techniques to provide mold size scaling factor and combining with adjustment of parameters during the wax injection process, we can significantly increase stability of wax embryo size and correctness of mold design size scaling.

碩士
國立高雄應用科技大學
模具工程系碩士班
96
The restrictive shrinkage of the pattern wax by the mold cavity of the wax injection mold has been studied. The basic ideas of designing wax injection mold, the pattern wax, the shrinkage characteristics of ceramic shell mold and casting alloys have been investigated, and then, the shrinkage factors were adjusted to calculate practically the mold cavity dimension. The shrink allowances of pattern waxes and casting alloys were obtained by the test of standard test blocks during the experiments. The experiment was divided in three parts: (1) In order to obtain the shrinkage allowances of the pattern waxes and casting alloys of the investment casting process, the standard test blocks of wax injection mold have been made. (2) The whole shrinkage idea has been applied to the design of wax injection mold; therefore, the factors of dimensional errors were investigated, and the errors were then converted into restrictive shrinkage factors. (3) The restrictive shrinkage idea was employed to the design of wax injection mold, and according to the obtained restrictive shrinkage factors, the dimensions of mold cavity were designed. The results showed, with the proper stripping time, the entire restrictive states of wax pattern can be preserved. Since the restrictive shrinkage type are classified as: (1) complete restrictive shrinkage, (2) partial restrictive shrinkage, and (3) unrestrictive shrinkage; therefore, for the dimensional accuracy, the idea of restrictive shrinkage type shall be considered when designing the wax pattern mold.

碩士
國立臺北科技大學
材料科學與工程研究所
97
The ceramic mold for precision casting in formed by repeatedly dipped in ceramic slurry and coated with fine ceramic sand. The shape of wax pattern may affect the thickness of the ceramic shell at the positions of different of radius and filets. In this research, the wax patterns with different radius and filets(R5、R3、R1) were made into different wax trees(vertical cluster, horizontal cluster and the 45 degree cluster). The slurries of different viscosity (light, medium and thick) were used for dipping and the dipping were use by manual operation and robot. The thickness of ceramic mold at locations of different radius or filets were measured and the factors. Affecting the results were discussed in this study. It was found through this study, the slurry of medium viscosity is batter then other for dipping, and wax tree of 45 degree is the most appropriate way to make wax tree. It was also found, robot operation may be suitable for larger casting but manual operation is batter for complex shape casting due to even thickness throughout the ceramic mold.

碩士
崑山科技大學
機械工程研究所
99
This thesis is intended to present a new technology in the domain of manufacturing. It presents a numerical study of the possibility of replacing, in the domain of plastic injection molding technology, the traditional method being used for the past thousands of years to produce a wax pattern through injection. To accomplish this, a mold made of a RP plastic material combined with a PCM material will be used to produce wax patterns instead of a mold made out of steel or iron. The system being studied can be described as a one dimensional transient state heat transfer involving a PCM (Phase Changing Material), a mold made of ABS material and melted paraffin wax as the injection material. The process proposed and studied here uses melted paraffin wax as the injection material. The PCM is placed between two layers of RP plastic material and will be the heat extractor. It is to be noted that once set, the system will be cooled down to a temperature of about 10˚C before the paraffin wax is injected. The step that follows is injecting melted paraffin wax at a temperature of 70˚C into its cavity through a nozzle and then leaves the system exposed to natural internal conduction. Latent heat analysis and numerical methods in heat conduction are used to solve this system in two parts. The first method used, latent heat analysis, involves the law of enthalpy (internal energy) to determine the total cooling time and temperature gradient in the material. In the process, it was observed that the main heat transfer is done through conduction thus heat transfer through convection and through radiation is neglected in this method. The second method uses the numerical methods in heat conduction for a one-dimensional transient heat conduction system. This involves infinite series which are difficult to deal with. However, the terms in the solution converge rapidly with increasing time, and for τ=0.2, keeping the first term and neglecting all the remaining terms in the series results in an error under 2 percent. This system will be simulated using COMSOL under the transient heat transfer physics module. COMSOL is very efficient in solving such systems with a high relativity error. The appearance and the behavior of the melting front can be simulated by modifying the specific heat of the PCM to account for the increased amount of energy, in the form of latent heat of fusion, needed to melt the PCM over its melting temperature range. Numerical simulation of transient heat transfer was conducted considering both a system exposed to room temperature and to adiabatic condition. The behavior of the melting and heat absorption of PCM was simulated by modifying the specific heat of the material to account for the increased or decrease of the amount of energy in the form of latent heat of fusion over or under its melting temperature range. ABS plastic mold was made through FDM Rapid Prototyping process and a carefully prepared experiment was successfully conducted. To confirm the validity of the work, the data acquired during the experiment was compared with the numerical results through a graph. The outcome was a success. Another experiment with a more complex geometric shape was conducted with satisfying results. An optimization of the current process settings revealed a possibility to have better solidification time result (approx. 10 minutes) than that of the traditional wax pattern process (approx. 15 minutes).

碩士
高雄醫學大學
牙醫學研究所碩士在職專班
99
Abstract Objective: It has been a customary or routine standard to produce a multiunit long-span fixed partial denture through the processing of one-piece casting over removable individual dies and inlay wax, where it usually yields distortions, twists and significant gaps on the margins of the fabricated protheses. Subsequently, these tedious laboratory multi-steps will require further re-processing such as sectioning or/and re-soldering of the products to be re-oriented into the patient’s mouths before final delivery, which often end up with more clinical problems. The aim of the present study was to introduce and to assess the feasibility of a revised or re-modified wax-pattern combined with self-cured resin technique (called: pattern resin) featuring one-piece casting over the individual dies with solid cast without soldering prior to producing the multi-unit fixed partial dentures, which is expected to manifestly fulfill ― 50 ?慆 ‖ professional standards set for assessing the ―absolute marginal discrepancy‖(AMD). on the fabricated crown margins as recommended by the ADA (American Dental Association). Materials & Methods: For fabricating the multiunit fixed partial denture via one-piece casting on individual dies with solid cast without soldering, each of the five replicates per group was designed as follows: the traditional approach using a wax-pattern method on individual dies and solid casts was ― the control ‖ group (WI/WS, n=5) and our revised/ re-developed wax/pattern resin method was set as ― the experimental ‖ group (MI/MS, n=5), each of which in total of 40 corresponding units were then further processed for subsequent embedment, casting into metal frameworks and the final refinements. The resulting ― maximal ‖ AMD on each of the buccal (B), lingual (L), mesial (M) and distal (D) margins on the abutments, (WI/MI), (WS/MS): 12, 13, 23 & 24 (FDI system) were separately recorded by using the optical stereo-microscope for images, then by using computer through the build-in software for scanning & image analyses. The resulting data sets were analyzed by SPSS13.0 software and the paired-t test was employed for statistical comparisons between both groups where α value was set 0.05. The results: Based on the computer images acquired and subsequent analyses obtained from both groups, the results of assessing the ― maximal ‖ absolute marginal discrepancy (AMD) in the four areas: B, L, M & D, a totaling of 320 measurements, showed that: on individual dies, the mean AMD value for WI in the control group (total=80) was 73.18 um with the averaged success rate of 90% (by reaching 50 ?戥m); whereas the mean AMD for MI in the experimental group (total=80) was 33.82 ?慆 with the averaged success rate of 100% (by reaching 50 ?慆). And, on solid casts, the mean AMD value for WS in the control group (total=80) was 433.09 ?慆 with the averaged success rate of 36.25% (by reaching 50 ?慆), whereas the mean AMD for MS in the experimental group (total=80) was 51.65 ?慆 with the averaged success rate of 98.75% (by reaching 50 ?慆). Meanwhile, the results of paired-t test involving all four B, L, M & D areas showed that there were statistically significant differences between WI-MI, WS-MS, WS_B-WS_L, WS_M-WS_D & MS_B-MS_L for comparisons with P<0.05, respectively; while others showed no significant difference, individually. Conclusion: Under the experimental conditions studied, our preliminary pilot analyses with each of five replicates per group provided a rationalized support briefly summarized as follows: for the multiunit fixed partial denture fabricated by one-piece casting over the individual dies with solid cast without soldering, our analyses on assessing the resulting feasibility and accuracy indicated that: i) it is very difficult to achieve the satisfactory outcomes without significant marginal gaps or/and distortions(i.e., AMD values) via the traditional fashion of wax-pattern method on the individual dies with solid casts, and ii) our revised/modified mixed wax/pattern resin method demonstrates quite significantly less marginal discrepancy with much higher success rates for satisfactory laboratory results. Therefore, this revised new technique may have the potential to be applied in the modern dental laboratory to achieve better clinical outcomes and efficacy in the future.

Conventional urea-based moulding compounds for investment casting patterns are manufactured using a slow “cooking” process. Nowadays in industrial processes the use of a faster process is highly recommended to increase throughput levels. At the same time, for quality control purposes, the requirements of an investment caster must be met. This study is therefore focused on:

• Finding the appropriate conventional process and conditions to prepare urea-based investment casting moulding compounds.
• Optimising the composition variables to meet the mechanical, thermal, surface, flow and cost properties needed in investment casting.
• Characterising the moulding compounds to meet the requirements of an investment caster by comparing them with an industrial, “cooked” urea-based compound.

Polyvinyl alcohol (PVOH) and ethylene vinyl acetate (EVA) urea-based moulding compounds were prepared using a two-roll mill and a conventional extrusion processes respectively. It was possible to injection mould PVOH urea-based moulding compounds with a urea content of up to 90 wt % which had been compounded using a two-roll mill. Using the conventional extrusion process, it was also possible to compound and injection mould EVA urea-based moulding compounds containing up to 70 wt % urea. The effects on composition variables on the properties of the moulding compound were studied and compared to those of the existing “cooked” urea-based moulding compound (Benchmark). The mechanical properties were characterised using the three-point bending test and Charpy impact test. The thermal properties were determined using simultaneous differential thermal analysis and thermogravimetric analysis (SDTA/TGA) and differential scanning calorimeter (DSC). The thermo-mechanical and visco-elastic properties were determined using a dynamic mechanical analyser. A scanning electron microscope was used to study the surface texture of the mouldings. The EVA urea-based moulding compounds showed two endothermic melting peaks and multiple exothermic crystallisation peaks in the DSC curves. The peak at ca. 55 - 66°C corresponds to the melting of the wax/EVA blend, while the large peak at 130 - 132°C corresponds to the melting of the urea. The DSC heating curve of the PVOH urea-based moulding compounds showed two endothermic peaks. The small peak corresponds to the melting of the wax, while the large peak corresponds to the melting of the urea/PVOH blend. PVOH urea-based moulding compound had better mechanical properties than the industrial benchmark. The mechanical properties of the EVA urea-based compound were generally lower. The effect of the wax and polymer content on the mechanical properties was as follows:

• Increasing polymer content produced weaker but tougher moulding compounds.
• Increasing wax content improved the strength and stiffness but gave compounds that were less tough.
• Two-way Analysis of Variance (ANOVA) indicated significant polymer-wax interactions.

The urea content determined the stiffness (elastic modulus) of the compounds. PVOH mouldings had superior stiffness compared with the EVA and cooked urea-based mouldings. The Dynamic mechanical analysis (DMA) results confirmed the result obtained from the modulus of elasticity determination in the three-point bending test. The impact strength increased with an increase in polymer content and reduced with an increase in wax content. The linear thermal expansion coefficient decreased as the urea content was increased. Measured values (100 to 156x10-6°C) were comparable to those of the benchmark. The cooked urea-based moulding compound had the lowest melt viscosity at 110°C, as indicated by its melt flow index (MFI). Fluidity increased with the polymer content. The thermo gravimetric analysis (TGA) results confirmed that both the PVOH and EVA urea-based moulding compounds decomposed readily and left less than 1 wt % ash after combustion. From the SEM results apparent surface roughness appeared to increase with wax content. The EVA urea-based moulding compound had an irregular surface texture. Based on the criteria of cost-effectiveness and environmental friendliness, the synthesis of PVOH urea-based patterns is preferable. The use of a conventional extrusion process to prepare PVOH urea-based patterns is recommended.
Thesis (PhD)--University of Pretoria, 2011.
Chemical Engineering
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