Ultra-high precision machining of contact lens polymers
- Authors: Olufayo, Oluwole Ayodeji
- Date: 2015
- Subjects: Contact lenses , Polymers
- Language: English
- Type: Thesis , Doctoral , DPhil
- Identifier: http://hdl.handle.net/10948/3001 , vital:20385
- Description: Contact lens manufacture requires a high level of accuracy and surface integrity in the range of a few nanometres. Amidst numerous optical manufacturing techniques, single-point diamond turning is widely employed in the making of contact lenses due to its capability of producing optical surfaces of complex shapes and nanometric accuracy. For process optimisation, it is ideal to assess the effects of various conditions and also establish their relationships with the surface finish. Presently, there is little information available on the performance of single point diamond turning when machining contact lens polymers. Therefore, the research work undertaken herewith is aimed at testing known facts in contact lens diamond turning and investigating the performance of ultra-high precision manufacturing of contact lens polymers. Experimental tests were conducted on Roflufocon E, which is a commercially available contact lens polymer and on Precitech Nanoform Ultra-grind 250 precision machining. Tests were performed at varying cutting feeds, speed and depth of cut. Initial experimental tests investigated the influence of process factors affecting surface finish in the UHPM of lenses. The acquired data were statistically analysed using Response Surface Method (RSM) to create a model of the process. Subsequently, a model which uses Runge-Kutta’s fourth order non-linear finite series scheme was developed and adapted to deduce the force occurring at the tool tip. These forces were also statistically analysed and modelled to also predict the effects process factors have on cutting force. Further experimental tests were aimed at establishing the presence of the triboelectric wear phenomena occurring during polymer machining and identifying the most influential process factors. Results indicate that feed rate is a significant factor in the generation of high optical surface quality. In addition, the depth of cut was identified as a significant factor in the generation of low surface roughness in lenses. The influence some of these process factors had was notably linked to triboelectric effects. This tribological effect was generated from the continuous rubbing action of magnetised chips on the cutting tool. This further stresses the presence of high static charging during cutting. Moderately humid cutting conditions presented an adequate means for static charge control and displayed improved surface finishes. In all experimental tests, the feed rate was identified as the most significant factor within the range of cutting parameters employed. Hence, the results validated the fact that feed rate had a high influence in polymer machining. The work also established the relationship on how surface roughness of an optical lens responded to monitoring signals and parameters such as force, feed, speed and depth of cut during machining and it generated models for prediction of surface finishes and appropriate selection of parameters. Furthermore, the study provides a molecular simulation analysis for validating observed conditions occurring at the nanometric scale in polymer machining. This is novel in molecular polymer modelling. The outcome of this research has contributed significantly to the body of knowledge and has provided basic information in the area of precision manufacturing of optical components of high surface integrity such as contact lenses. The application of the research findings presented here cuts across various fields such as medicine, semi-conductors, aerospace, defence, telecom, lasers, instrumentation and life sciences.
- Full Text:
- Date Issued: 2015
- Authors: Olufayo, Oluwole Ayodeji
- Date: 2015
- Subjects: Contact lenses , Polymers
- Language: English
- Type: Thesis , Doctoral , DPhil
- Identifier: http://hdl.handle.net/10948/3001 , vital:20385
- Description: Contact lens manufacture requires a high level of accuracy and surface integrity in the range of a few nanometres. Amidst numerous optical manufacturing techniques, single-point diamond turning is widely employed in the making of contact lenses due to its capability of producing optical surfaces of complex shapes and nanometric accuracy. For process optimisation, it is ideal to assess the effects of various conditions and also establish their relationships with the surface finish. Presently, there is little information available on the performance of single point diamond turning when machining contact lens polymers. Therefore, the research work undertaken herewith is aimed at testing known facts in contact lens diamond turning and investigating the performance of ultra-high precision manufacturing of contact lens polymers. Experimental tests were conducted on Roflufocon E, which is a commercially available contact lens polymer and on Precitech Nanoform Ultra-grind 250 precision machining. Tests were performed at varying cutting feeds, speed and depth of cut. Initial experimental tests investigated the influence of process factors affecting surface finish in the UHPM of lenses. The acquired data were statistically analysed using Response Surface Method (RSM) to create a model of the process. Subsequently, a model which uses Runge-Kutta’s fourth order non-linear finite series scheme was developed and adapted to deduce the force occurring at the tool tip. These forces were also statistically analysed and modelled to also predict the effects process factors have on cutting force. Further experimental tests were aimed at establishing the presence of the triboelectric wear phenomena occurring during polymer machining and identifying the most influential process factors. Results indicate that feed rate is a significant factor in the generation of high optical surface quality. In addition, the depth of cut was identified as a significant factor in the generation of low surface roughness in lenses. The influence some of these process factors had was notably linked to triboelectric effects. This tribological effect was generated from the continuous rubbing action of magnetised chips on the cutting tool. This further stresses the presence of high static charging during cutting. Moderately humid cutting conditions presented an adequate means for static charge control and displayed improved surface finishes. In all experimental tests, the feed rate was identified as the most significant factor within the range of cutting parameters employed. Hence, the results validated the fact that feed rate had a high influence in polymer machining. The work also established the relationship on how surface roughness of an optical lens responded to monitoring signals and parameters such as force, feed, speed and depth of cut during machining and it generated models for prediction of surface finishes and appropriate selection of parameters. Furthermore, the study provides a molecular simulation analysis for validating observed conditions occurring at the nanometric scale in polymer machining. This is novel in molecular polymer modelling. The outcome of this research has contributed significantly to the body of knowledge and has provided basic information in the area of precision manufacturing of optical components of high surface integrity such as contact lenses. The application of the research findings presented here cuts across various fields such as medicine, semi-conductors, aerospace, defence, telecom, lasers, instrumentation and life sciences.
- Full Text:
- Date Issued: 2015
Drivers and consequences of residents' satisfaction with off-campus student housing in South-South, Nigeria
- Authors: Bella-Omunagbe, Ojo Cyprain
- Date: 2015
- Subjects: Student housing -- Nigeria , Housing -- Resident satisfaction , Residence and education , Universities and colleges -- Nigeria -- Administration
- Language: English
- Type: Thesis , Doctoral , DPhil
- Identifier: vital:9720 , http://hdl.handle.net/10948/d1021198
- Description: The student housing system worldwide and South-South Nigeria in particular has witnessed an unprecedented transformation, such that private off-campus student housing facilities (SHFs) are now the primary source of accommodation for students in tertiary institutions. A considerable gap exists between the supply and demand for on-campus student housing and the quest to fill this gap has stimulated the creation of a significant student housing market in the areas where these tertiary institutions are located. The prospect for economic investments in the student housing sector is high and private investors are involved in the provision and management of offcampus student housing. The main consequence of this practice in South-South, Nigeria is the delivery of low-quality buildings that are not able to meet the needs and expectations of residents. SHFs that are constructed without due regard to residents needs are characterised by dissatisfaction with attributes of housing and low investment performance. The implication is that residence users are often not satisfied with the attributes of the residential environment that are provided; thus their behaviours often impose some consequences on investors gains and objectives.Therefore, understanding the dynamics among attributes that are important to students, that give the required satisfaction, and the impact of the availability or lack-of on behaviour such as loyalty, willingness to pay for attributes and word of mouth behaviour are critical to profitability. Most often, the relationship among these attributes are treated as linear and symmetrical with the assumed implication that better attributes produce improved behaviours. However, this may not always be the case. This approach is rarely addressed and is little understood in student housing studies. Therefore, the aim of this study was to identify student housing attributes that act as drivers of resident atisfaction and the consequences/effects of these drivers on student behaviour in order to determine appropriate measures that could be used to develop, maintain and upgrade student accommodation. The methodology of the study included an extensive literature review and a field study conducted to obtain the perceptions of students in seven tertiary institutions located in South-South, Nigeria. The main task was to define attributes of student housing facilities based on the symmetric and asymmetric impact of the performance of attributes on satisfaction with residence. The Kano model and importance-performance analysis (IPA) were used to establish sets of criteria that could be used to prioritise attributes that are required in student housing for optimal investor gains. Analysis of the findings lead to the conclusions that different degrees of behaviour were associated to the perception of importance that is attached to attributes by residents and the satisfaction that is derived from the use of such attributes. The implication of the conclusions is that to meet users satisfaction needs, varied improvement strategies are required for different attributes in order to maximise the use of resources for maximum gains. The recommendations for investors in SHFs include among others to segment the SHFs market based on demographic characteristics, prioritise and provide only attributes that add-value to identified groups. Emphasis should also be placed on providing attributes that are not only satisfactory, but with capacity to improve loyalty/retention, willingness to pay and positive word of mouth behaviour. It is also recommended that the local authority should improve critical attributes that are deemed to be outside the scope of the investors.
- Full Text:
- Date Issued: 2015
- Authors: Bella-Omunagbe, Ojo Cyprain
- Date: 2015
- Subjects: Student housing -- Nigeria , Housing -- Resident satisfaction , Residence and education , Universities and colleges -- Nigeria -- Administration
- Language: English
- Type: Thesis , Doctoral , DPhil
- Identifier: vital:9720 , http://hdl.handle.net/10948/d1021198
- Description: The student housing system worldwide and South-South Nigeria in particular has witnessed an unprecedented transformation, such that private off-campus student housing facilities (SHFs) are now the primary source of accommodation for students in tertiary institutions. A considerable gap exists between the supply and demand for on-campus student housing and the quest to fill this gap has stimulated the creation of a significant student housing market in the areas where these tertiary institutions are located. The prospect for economic investments in the student housing sector is high and private investors are involved in the provision and management of offcampus student housing. The main consequence of this practice in South-South, Nigeria is the delivery of low-quality buildings that are not able to meet the needs and expectations of residents. SHFs that are constructed without due regard to residents needs are characterised by dissatisfaction with attributes of housing and low investment performance. The implication is that residence users are often not satisfied with the attributes of the residential environment that are provided; thus their behaviours often impose some consequences on investors gains and objectives.Therefore, understanding the dynamics among attributes that are important to students, that give the required satisfaction, and the impact of the availability or lack-of on behaviour such as loyalty, willingness to pay for attributes and word of mouth behaviour are critical to profitability. Most often, the relationship among these attributes are treated as linear and symmetrical with the assumed implication that better attributes produce improved behaviours. However, this may not always be the case. This approach is rarely addressed and is little understood in student housing studies. Therefore, the aim of this study was to identify student housing attributes that act as drivers of resident atisfaction and the consequences/effects of these drivers on student behaviour in order to determine appropriate measures that could be used to develop, maintain and upgrade student accommodation. The methodology of the study included an extensive literature review and a field study conducted to obtain the perceptions of students in seven tertiary institutions located in South-South, Nigeria. The main task was to define attributes of student housing facilities based on the symmetric and asymmetric impact of the performance of attributes on satisfaction with residence. The Kano model and importance-performance analysis (IPA) were used to establish sets of criteria that could be used to prioritise attributes that are required in student housing for optimal investor gains. Analysis of the findings lead to the conclusions that different degrees of behaviour were associated to the perception of importance that is attached to attributes by residents and the satisfaction that is derived from the use of such attributes. The implication of the conclusions is that to meet users satisfaction needs, varied improvement strategies are required for different attributes in order to maximise the use of resources for maximum gains. The recommendations for investors in SHFs include among others to segment the SHFs market based on demographic characteristics, prioritise and provide only attributes that add-value to identified groups. Emphasis should also be placed on providing attributes that are not only satisfactory, but with capacity to improve loyalty/retention, willingness to pay and positive word of mouth behaviour. It is also recommended that the local authority should improve critical attributes that are deemed to be outside the scope of the investors.
- Full Text:
- Date Issued: 2015
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