ASSESSMENT OF MANUFACTURABILITY AND PERFORMANCE OF POLYURETHANE HEART VALVES PRODUCED THROUGH A LOCALLY DEVELOPED DIP MOULDING PROCESS

Conference ProceedingsPolyurethane heart valves have been widely studied as possible replacement for mechanical and bioprosthetic heart valves. The development of an inexpensive routine production technique for manufacturing of polyurethane valves will greatly benefit a very large number of patients in developing and emerging countries. A polymer heart valve shows favourable physical properties and flow dynamics compared to human heart valves, however, the outcome of producing a polymer heart valve with the required flexibility, durability and hemodynamic function is often difficult to predict. The design of the mould, the selection of the material and the fabrication method used are the key factors that influence the achievement of an acceptable heart valve for use in the human body. From their previous work on developing a repeatable, semi-automated dip moulding process for producing tri-leaflet polyurethane heart valves, the authors have shown that the selection of an appropriate set of dip moulding process parameters and mould material properties could result in achieving polyurethane valve leaflets with the required physical and mechanical properties. This paper reports on the progress made with application of the developed dip moulding process to produce polyurethane heart valves suitable for use in human body. The mould, frame and sewing ring were manufactured in Ti6Al4V(ELI) by using a Direct Metal Laser Sintering (DMLS) process and the valve leaflets were moulded directly onto the sewing ring. The heart valve properties obtained are presented and assessed. Conclusions are drawn regarding the prospects of these valves surviving the extensive in vitro simulation trials required to qualify them for subsequent clinical trials

Analysis of the performance of hydrogen maser clocks at the Hartebeesthoek Radio Astronomy Observatory

Hydrogen maser frequency standards are commonly utilised in various space geodetic techniques such as Very Long Baseline Interferometry (VLBI) as local reference clocks. The Hartebeesthoek Radio Astronomy Observatory in South Africa is currently operating two maser frequency standards i.e., an EFOS28 and an iMaser72 for the 15 m and 26 m VLBI radio telescopes respectively, an older EFOS6 is a standby spare. This study utilised the least-squares method to derive clock parameters, which indicates the performance levels of the masers by making use of the offset measurements obtained between hydrogen maser clock 1 PPS and GNSS 1 PPS for a period of 35 days. The masers were also compared using a frequency comparator (VCH-314) for a time period of 100 s. The results indicate that the performances of both Masers are relatively similar to each other, with short-term and long-term results indicating good agreement. The iMaser72 has a better standard error of 0.0039 μs compared to the standard error of 0.0059 μs for the EFOS28 maser clock. In general, both masers performed at an expected level required for radio astronomy and geodetic VLBI applications. The method used in this study proved to be useful in managing local hydrogen maser clocks to ensure accurate VLBI observations are obtained

DIRECT METAL LASER SINTERING, USING CONFORMAL COOLING, FOR HIGH VOLUME PRODUCTION TOOLING#

Published ArticleExisting techniques to manufacture conventional tool steel inserts for the plastic injection moulding process are expensive and time-consuming. Complex mould inserts, difficult to manufacture with conventional processes, can be produced using Direct Metal Laser Sintering (DMLS) with Maraging tool steel (MS1). MS1 is an additive manufacturing (AM) material made available by Electro Optical Systems (EOS) GmbH. Contrary to material removal processes, DMLS can produce MS1 tool steel inserts directly from Computer-Aided Design (CAD) files suitable for high volume plastic injection moulding. Through DMLS it is possible to create conformal cooling channels inside the MS1 inserts that have advantages in reducing heat rapidly and evenly. This can result in a reduction of cycle times, cost per product as well as improving part quality by eliminating defects such as warpage and heat sinks. This paper will present a comparison between Finite Element Analysis (FEA) simulations of the injection mould inserts with actual mould trails of AM and conventional manufactured inserts. It also includes the design and manufacturing of conventional and DMLS inserts and compares the manufacturing costs and lead times. Using FEA simulations, the design of conformal cooling channels is optimised by comparing the mould temperature of different cooling channel layouts. Bestaande tegnieke vir die vervaardiging van matryse vir die plastiek-inspuit giet tegniek is duur en tyd rowend. Verder is dit nie altyd moontlik om konvensionele metodes vir die vervaardiging van matryse vir geomteries komplekse gietstukke te gebruik nie. Vir sodanige gietsukke kan invoegsels relatief vinnig vervaardig word, deur van direkte laser metal sinterings metodes (DLMS) met Maraging-staal (MS1) gebruik te maak. MS1 is ’n laag vervaardings materiaal wat onlangs deur Electro Optical Systems (EOS) GmbH beskikbaar gestel is. Dit is ’n pre-allooi, ultra hoë sterkte metaal met goeie meganiese eienskappe. In teenstelling met materiaal verwyderings prosesse (masjienerings prosesse), kan DMLS MS1 staal matryse of insetsels wat vir hoë volume produksie van plastiek gietsukke bruikbaar is, direk vanaf rekenaar-gesteunde ontwerp prosesse vervaardig word. Die gebruik van DMLS kan ook vir die ontwerp en vervaardiging van vorm getroue verkoelings kanale in matryse voorsiening maak, wat tot laer hitte asook die vinnige en eweredige verspreiding daarvan sal lei. Voorgenoemde behoort tot ’n aansienlike verlaging in produksie siklus tye te lei met ’n dien ooreenkomstige verlaging in die produksie koste asook ’n verbetering in die kwaliteit van die vervaardigde produkte a.g.v. die voorkoming van defekte soos kromtrekking en hitte-putte wat normaalweg deur oneweredige hitte verspreiding veroorsaak word

Evaluating the Suitability of Alumide Tooling for Injection Moulding of Different Polymers

Published ArticleThis paper describes the possibility of using laser-sintered Alumide® as an alternative material for producing rapid tooling (RT) inserts. To determine the durability of Alumide® inserts for the injection moulding (IM) process, a product with geometrical features was developed, and Alumide® inserts were manufactured. Polypropylene (PP), acrylonitrile-butadiene-styrene (ABS), polycarbonate (PC), and polyamide 6 (PA 6) were used for IM trials with the Alumide® inserts. From these trials, it was concluded that polymer materials with a processing temperature of about 230 ºC, similar to PP and ABS, can be used with Alumide® inserts as RT inserts for the IM process

Extracting Independent Local Oscillatory Geophysical Signals by Geodetic Tropospheric Delay

Zenith Tropospheric Delay (ZTD) due to water vapor derived from space geodetic techniques and numerical weather prediction simulated-reanalysis data exhibits non-linear and non-stationary properties akin to those in the crucial geophysical signals of interest to the research community. These time series, once decomposed into additive (and stochastic) components, have information about the long term global change (the trend) and other interpretable (quasi-) periodic components such as seasonal cycles and noise. Such stochastic component(s) could be a function that exhibits at most one extremum within a data span or a monotonic function within a certain temporal span. In this contribution, we examine the use of the combined Ensemble Empirical Mode Decomposition (EEMD) and Independent Component Analysis (ICA): the EEMD-ICA algorithm to extract the independent local oscillatory stochastic components in the tropospheric delay derived from the European Centre for Medium-Range Weather Forecasts (ECMWF) over six geodetic sites (HartRAO, Hobart26, Wettzell, Gilcreek, Westford, and Tsukub32). The proposed methodology allows independent geophysical processes to be extracted and assessed. Analysis of the quality index of the Independent Components (ICs) derived for each cluster of local oscillatory components (also called the Intrinsic Mode Functions (IMFs)) for all the geodetic stations considered in the study demonstrate that they are strongly site dependent. Such strong dependency seems to suggest that the localized geophysical signals embedded in the ZTD over the geodetic sites are not correlated. Further, from the viewpoint of non-linear dynamical systems, four geophysical signals the Quasi-Biennial Oscillation (QBO) index derived from the NCEP/NCAR reanalysis, the Southern Oscillation Index (SOI) anomaly from NCEP, the SIDC monthly Sun Spot Number (SSN), and the Length of Day (LoD) are linked to the extracted signal components from ZTD. Results from the synchronization analysis show that ZTD and the geophysical signals exhibit (albeit subtle) site dependent phase synchronization index

Analysis of the performance of hydrogen maser clocks at the Hartebeesthoek Radio Astronomy Observatory

Hydrogen maser frequency standards are commonly utilised in various space geodetic techniques such as Very Long Baseline Interferometry (VLBI) as local reference clocks. The Hartebeesthoek Radio Astronomy Observatory in South Africa is currently operating two maser frequency standards i.e., an EFOS28 and an iMaser72 for the 15 m and 26 m VLBI radio telescopes respectively, an older EFOS6 is a standby spare. This study utilised the least-squares method to derive clock parameters, which indicates the performance levels of the masers by making use of the offset measurements obtained between hydrogen maser clock 1 PPS and GNSS 1 PPS for a period of 35 days. The masers were also compared using a frequency comparator (VCH-314) for a time period of 100 s. The results indicate that the performances of both Masers are relatively similar to each other, with short-term and long-term results indicating good agreement. The iMaser72 has a better standard error of 0.0039 μs compared to the standard error of 0.0059 μs for the EFOS28 maser clock. In general, both masers performed at an expected level required for radio astronomy and geodetic VLBI applications. The method used in this study proved to be useful in managing local hydrogen maser clocks to ensure accurate VLBI observations are obtained.The National Research Foundation (NRF), the University of Pretoria and the Department of Science and Technology (DST).http://www.sajg.org.za/index.php/sajgam2017Geography, Geoinformatics and Meteorolog

The Hopkins Verbal Learning Test and screening for dementia

The present study investigated the sensitivity and specificity of the Hopkins Verbal Learning Test (HVLT) for demented patients (n=82, using NINCDS criteria) and 114 healthy controls - equivalent in age, years of education and gender–ratio - from the Oxford Project To Investigate Memory and Ageing. The HVLT ‘Total recall’ score had 87% sensitivity and 98% specificity for dementia using a cut-off score of 14.5. Using a 'Memory' score (the sum of the 'Total Recall' and the 'Discrimination Index') with a cutoff score of 24.5 gave a 91% sensitivity and 98% specificity for Alzheimer’s disease cases when compared to controls. Unlike the MMSE, the HVLT has no ceiling effects and does not have to be adjusted for education. We conclude that the HVLT is an easy to administer, quick and well tolerated tool for the screening of dementia