1,721,093 research outputs found
Cooling rate influence in bonded magnet injection moulding
No full textRecently the use of injection moulded bonded magnets has become very widespread. This is mainly due to the interesting price of injection moulded magnets. However the process is actually limited by some critical aspects that are related to rheological and thermal properties of the melt. Compared to a non-filled polymer melt, the high filler content (about 90% in weight), leads to a much higher viscosity, heavily affected by temperature. Furthermore, injection moulding of bonded magnets is characterized by high cooling rate, due to the high diffusivity of the metal filler. This can often lead to a premature gate freezing, negatively affecting the quality of the moulded part. Therefore, a reliable no-flow temperature is a prerequisite for a robust numerical simulation of the process. This paper presents a full characterization of the no-flow temperature of a commercial polyamide-based compound. To this aim, a method based on differential scanning calorimetry (DSC) has been compared with another approach that relies on rheological data. Great attention was paid to evaluate the influence of cooling rate on no-flow temperature and a regression model was proposed for describing the relation between these two variables. Furthermore, a comparison between numerical and experimental results was carried out in order to validate the numerical simulation of the process
Optimization of bonded magnet injection moulding
No full textRecently the use of injection moulded bonded magnets has become very widespread. This is mainly due to their good value and low cost. There are several bonded-magnet compounds commercially available, even with rare hearth alloys that yield magnetic performances comparable to those ones offered by sintered magnets. However the magnetic properties of injection moulded bonded magnets are negatively affected by the thick solidification layer which develops during the filling of a mould cavity. This is due to the lack of orientation that filler particles have during magnetization in a solid matrix and it is stressed by high cooling rates, due to the high diffusivity which is proper of the metallic filler. This paper presents an approach to improve the magnetic performance by applying a fast mould heating and cooling system and by varying some influent process parameters. A fast mould heating and cooling system based on cartridge heaters has been implemented in a CFD code, in order to simulate the mould thermal behavior. A process simulation based on FEM has been used for cost and final part quality prediction. The numerical predictions about skin thickness distribution were compared to experimental SEM micrographs results in order to validate them. Furthermore, a multi-objective optimization algorithm, based on numerical simulations, has been implemented with the aim of minimizing the overall moulding cost, by determining the optimal value of skin thickness that balances material cost against process cost
Magneto-Rheological Characterization of Polymer-Bonded Magnets
No full textIn recent years, polymer-bonded magnets have been widely used for several engineering applications because of their ease of processing, low weight, and low cost compared to sintered permanent metallic magnets. The polymer-bonded magnets are prepared by blending a magnetic powder with a polymeric binder in a mixer or extruder. The resulting composite material can be shaped by the powder injection moulding process into magnets with complex shapes. Generally, the used fillers can have a magnetic isotropic or anisotropic structure. The latter presents a high degree of orientation, which is often desired because it results in high magnetic properties. In order to obtain this structure, the fillers need to be oriented by a magnetic field during the filling of the cavity and before the melt freezes. However, the application of the magnetic field tends to influence the rheological behaviour of the material. Therefore, in order to accurately simulate the injection moulding of polymer-bonded magnets it is important to characterize the magneto-rheological behaviour of the material. To this aim, a new magneto-rheology apparatus was especially developed. The instrument is made of a special capillary, mounted on the die of a laboratory extruder. The melt flowing throw the capillary is subjected to perpendicular magnetic fields. The magnetic fields are generated by an enamelled copper wire solenoid fed by a direct current. The desired geometry of the magnetic flux was obtained by using a mild steel circuit. The distribution of the magnetic flux density in the capillary was simulated and a geometrical optimization of the mild steel circuit was performed. An extensive experimental campaign was carried out to determine the interactions between the presence of the magnetic field and the main process variables, namely temperature, pressure and shear rate
Appropriateness of choice and interpretation of tests for thrombophilic defects: a practical experience
No full textBACKGROUND:
The use of laboratory coagulation tests in some fields of primary medicine is not well defined in available guidelines. In Italy, general practitioners tend to lack a thorough understanding of the utility of knowledge of coagulation inhibitors.
METHODS:
We reviewed the clinical conditions for which outpatients have been referred to our laboratory for antithrombin testing or for inherited thrombophilia tests panels over a 3-week period. Evidence-based guidelines were used to identify the appropriate requests.
RESULTS:
Numerous unnecessary tests were requested for both antithrombin and thrombophilia, particularly in obstetric and gynaecological situations in the setting of hormonal contraceptive treatment.
CONCLUSIONS:
Laboratory specialists must improve upon their communication with physicians in order to achieve an appropriate use of coagulation tests
Damage initiation and evolution in composite laminates under cyclic loading: a damage-based modelling framework
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Characterisation and analysis of transverse crack-induced delamination in cross-ply composite laminates under fatigue loadings
No full textThe delamination growth represents one of the main phenomena involved in the fatigue damage evolution in composite laminates. With the aim of characterising qualitatively and quantitatively this mechanism and its interaction with the other damage modes occurring during the fatigue life, an experimental campaign was carried out on glass/epoxy infused laminates. Two lay-ups were adopted, namely [02/904]s and [0/902]s. The damage evolution was characterised at the macro and micro-scales. The adopted macro-scale damage indicators were the laminate stiffness drop, the crack density and the delamination ratio, of which the evolution along with the fatigue cycles is shown. Damage evolution and final failure were seen to be sensitive to scale effect. Micro-scale edge observations were useful to understand the interaction between transverse cracks, delamination and fibre failure, all these mechanisms concurring in the laminate final failure
Alteration of plasma proteinase-antiproteinase system in type 1 diabetic patients. Influence of sex and relationship with metabolic control.
No full textThe aim of the present study was to investigate whether or not alterations of the plasma proteinase-antiproteinase system were present in type 1 (insulin-dependent) diabetic patients and, if so, whether or not they were related to sex, age at onset and duration of the disease as well as to short- and long-term diabetic control. The plasma concentration of trypsin-like activity and two of the most important plasma serine proteinase inhibitors, alpha 1-antitrypsin and alpha 2-macroglobulin, were determined in 95 type 1 diabetic and 67 control subjects. The plasma concentration of alpha 1 antitrypsin was found to be markedly decreased (P < 0.001), whereas plasma alpha 2-macroglobulin and trypsin-like activity were increased in diabetics compared to controls (P = 0.009 and < 0.001, respectively). Sex also influenced the values of both proteinase inhibitors in diabetics, women showing higher values of plasma alpha 1-antitrypsin (P = 0.004) than men. In women, HbA1c was also positively correlated with blood glucose (P < 0.001), daily insulin dosage (P < 0.001), and trypsin-like activity of plasma (P = 0.02). On the contrary, in men, HbA1c appeared to be negatively correlated with plasma alpha 2-macroglobulin (P = 0.02). In addition to sex, age at onset (but not duration) of the disease revealed differences in plasma alpha 1-antitrypsin among diabetics, the lowest mean value of this inhibitor being present in men with age at onset below 15 years, who also showed a significant negative correlation between this inhibitor and HbA1c (P = 0.01)
Influence of manufacturing-induced defects on the fatigue performances of autoclave moulded laminates
Full text linkIn the present work, cross-ply and multidirectional laminates were produced by autoclave moulding. Changes in the process parameters led to different microstructural features in terms of fibre volume fraction, global void content, and void size. Fatigue tests revealed a strong influence of the microstructure on the long-term performances of the laminates, in terms of life to crack initiation, crack propagation, crack density evolution and associated stiffness drop. A criterion recently proposed by the authors to predict the formation of the first fatigue cracks accounting for the actual material microstructure, including voids, was then validated on the new experimental data. The results show the need to properly account for the manufacturing induced defects for a more efficient and safer design of composite parts, and remark the necessity of developing models that link manufacturing process parameters, micro-scale morphology, and mechanical performances to enable a cost-effective production that maximizes the performance/cost ratio
Fatigue damage evolution in glass/epoxy cross-ply laminates under spectrum loadings
No full textFatigue tests were carried out on a glass/epoxy cross-ply laminate under constant amplitude, two-stage repeated blocks and three different types of spectrum loadings. The damage evolution, in terms of transverse crack initiation, propagation and crack density evolution, was analysed. It was found that the linear damage accumulation rule, previously validated for long block loadings, is not suitable for a generic spectrum. Indeed, it leads to an underestimation of the number of cracks and their propagation rate. The results are discussed to identify the main spectrum features influencing the fatigue damage evolution. A theoretical framework is derived to interpret the results and predicting the crack density evolution under spectrum loadings
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