CARATTERIZZAZIONE MECCANICA, GEOMETRICA E FUNZIONALE DI DISPOSITIVI BIOMEDICALI

Aim of this work is to develop and to validate a structured methodology to investigate mechanical, geometric and functional aspects having a direct impact on the quality and safety of the dental prosthesis. Dental technicians are liable for damages when prosthesis are used. It’s clear from a review of the normative technical documentation as well as of the technical-scientific literature, that safety requirements are not universally accepted and, thus, not yet adopted in the design phase of the prosthesis production. Therefore operators rely on good manufacturing practices more than in inspection and testing investigations. Moreover new methods and technologies are introduced in the production process of dental prosthesis, which have made it increasingly difficult to assess the impact of methodological issues on functional, quality and safety requirements. For these aspects, with reference to technical standards and scientific background, eight type of test were developed that were submitted to a sample of dental laboratories. Tests concern structural properties, geometric precision and functional aspects of dental devices. The proposed tests and the results of the experimental campaign are presented. The results allow assessing the suitability of the proposed method for the characterization of medical devices under study and on that basis, for the definition of quality and safety requirements to be adopted in the design phase and process control.Obiettivo del lavoro è sviluppare e validare metodologie che permettano di investigare in modo sistematico gli aspetti meccanici, geometrici e funzionali che hanno un impatto diretto sulle caratteristiche di qualità e sicurezza dei dispositivi biomedicali. I fabbricanti del dispositivo medico sono i primi responsabili nel caso in cui si manifestassero dei danni arrecati all’utente durante il corretto utilizzo. Da un’analisi della normativa vigente e dalla letteratura tecnica-scientifica del settore è emerso che i requisiti di sicurezza non risultano accettati universalmente e quindi non sono adottati nella fase di progettazione delle protesi. Ciò fa si che gli operatori del settore si affidino alle buone pratiche di fabbricazione in sostituzione dei controlli e prove finalizzate a limitare i rischi di insuccesso. Inoltre l’introduzione di nuovi metodi e tecnologie nel processo produttivo dei dispositivi dentali aumenta le difficoltà nel valutare la ricaduta delle scelte metodologiche sul rispetto dei requisiti funzionali, di qualità e di sicurezza. Per tale ragione, in riferimento alle norme vigenti e al background scientifico, otto tipologie di prove sono state sviluppate e sottoposte a laboratori odontotecnici. Le prove hanno riguardato la verifica strutturale dei dispositivi, il controllo della precisione geometrica e gli aspetti funzionali delle protesi in esercizio. Nel seguito sono descritte le modalità di prova e presentati i risultati della campagna sperimentale effettuata. I risultati consentono di osservare come il metodo proposto sia adatto alla caratterizzazione dei dispositivi medici oggetto dello studio e che su tale base si può pervenire alla definizione di requisiti di qualità e sicurezza condivisi e pertanto utilizzabili in sede progettuale e di controllo di processo

Bite force and contact area evaluation in a physical-virtual environment

An integrated physical-virtual methodology for the in-vitro study of the occlusion is proposed. Both the contact area and the bite force are investigated in order to evaluate the functional characteristics of removable complete dentures and its relation to teeth morphology, position and orientation in the dental prosthesis and to relative position between antagonist teeth. The force is experimentally estimated by a 6-axis strain gauges sensor, while the contact areas are virtually identified by reverse engineering techniques. The proposed method is aimed to assist the functional design of artificial teeth for removable complete as well as the prototyping phases of the prosthesis

Mechanical characterization of polyamide porous specimens for the evaluation of emptying strategies in Rapid Prototyping

Rapid-prototyping is usually considered as a powerful tool in geometric and functional optimization of a product. In such an approach, focus is exclusively on the real component, and not on the rapid prototype which represents it. This work takes part on a wider study which focuses on a rational, systematic approach in obtaining an ???optimized rapid prototype???, with particular regard to emptying strategies without loss of structural and geometric properties. In detail, a set of tensile tests have been performed on different types of specimen, which reproduce a set of corresponding emptying strategies: each type of specimen is characterized by a different percentage of porosity (40, 60 and 80%), obtained by the combination of particular values of two parameters: reticular structure and its density. As a result, the correlation between mechanical strength and geometric structure has been evaluated, allowing the identification of a profitable emptying strategy, in terms of cost and weight

A study on the material removal mechanisms in ball polishing

The manufacturing process of freeform glass components for precision optics is usually based on contour CNC grinding and polishing operations. To predict the geometrical precision of the production process, a correlation between the geometrical error and the process parameters is required. This is even more important in the polishing operation which is the final stage of the process. In this work a model for material removal estimation in deterministic polishing of glass moulds is proposed and validated. The model is developed for CNC ball polishing of free-form surfaces, where the pad, made of a polyurethane layer superimposed to a rubber bulk, moves along a scanning path, in a suspension of cerium oxide. As many models in literature the removed material can be estimated by pressure and sliding velocity between polishing pad and workpiece. Adopting the Hertz theory these physical characteristics can be related to the CAD-CAM-CNC parameters, e.g. tool and workpiece shape, dimension and modulus of elasticity, feed rate, feed step, tool rotational speed and radial tool deformation. The model validation was performed on ground glass flat samples polished with different process parameters, measuring the removed material by a contact probe profilometer. The developed model shows a satisfactory estimation of removal material as a function of the process parameters

An Innovative Approach to Geometric Characterization in Profile Tolerancing

Profile tolerances are used to control form or combinations of size, form, orientation and position of free form features (lines or surfaces) usually described by a CAD 2D or 3D model. The procedures reported in the standard ISO/TR 5460 do not address the problem of identifying and separating the size error contribution in profi le error assessment. This issue has become an industrial need in years because complexity and precision requirements of parts functional surfaces are increasing. In order to improve geometric tolerance verification, ISO 14660 and ISO 17450 Standards have been developed, where the \u201cassociation\u201d operation is introduced. For standard geometries the association operation \u201cis used to fit ideal feature(s) to non-ideal feature(s) according to specific rules which are called criteria\u201d [1]. The associated feature has the same shape of the nominal model and differs from it in size, orientation and position. Following the diffusion of measuring instruments, as mechanical or optical coordinate measuring machines, and their integration with CAD systems, the geometric characterization of a free form profi le is currently obtained by fi tting algorithms that minimize the distance among the measured points and the virtual model. This method doesn\u2019t allow the separation of size errors from geometric errors in profi le error assessment. However this information could be very useful to adjust the manufacturing process, thus increasing the possibility of manufacturing good parts. As an example, it could be possible to correct size errors resulting from wrong tool\u2019s set-up in cutting operation, or from an overestimation or underestimation of material\u2019s shrinkage in extrusion operations, or to regulate the gap in a mechanical system, by mean of correcting errors due to wear phenomena. In this paper a new method for geometric characterization of free-form profi le that allows the separation of size contributions from form, orientation and position errors is presented. The algorithm consist in an iterative process where at every cycle a resizing (offset) of the nominal model and a repositioning (fitting) of the points measures are calculated. The method has been validated with several simulated test-case and the results confirm both the correctness of the proposed approach and its utility in conformity assessment operations

La Precisione di Accoppiamento in Sottostrutture Fresate a Supporto Implantare

Le terapie dentali implanto-protesiche che prevedono l\u2019utilizzo di impianti osteo-integrati impongono specifici requisiti sulla precisione di accoppiamento tra le strutture di supporto. Nel paziente edentulo totale, tali requisiti risultano ancor pi\uf9 difficilmente conseguibili per la presenza contemporanea di pi\uf9 impianti. I processi produttivi tradizionali, fino ad oggi ancora i pi\uf9 diffusi, non riescono a soddisfare pienamente la domanda clinica di sovrastrutture ad accoppiamento passivo, in grado di preservare e favorire i meccanismi di osteointegrazione mentre i nuovi processi CAD/CAM sembrano venire incontro a questa esigenza primaria. Da qui l\u2019esigenza di valutare e confrontare, mediante l\u2019individuazione di opportuni parametri geometrici, la precisione di fabbricazione di strutture protesiche ottenute con differenti processi produttivi e materiali alternativi: leghe nobili, Co-Cr e Ti. I risultati ottenuti per via sperimentale designano senza possibile dubbio il processo CAD/CAM come l\u2019unico in grado di consentire il raggiungimento di accoppiamenti passivi entro i limiti tecnologici legati alle fasi ed alle attrezzature proprie del processo medesimo

Form errors estimation in free-form 2D and 3D geometries.

The aim of this work is to present a method for the verification of form tolerances in free-form geometries. Method: New free-form error parameters, in accordance with ISO GPS standards are defined. Geometrical errors in 2D e 3D geometries are calculated using a new fitting method consisting in an association operation where nominal profile/surface is resized (with an offset), located and oriented on the non-ideal geometry. Form tolerance is estimated using peak-to-valley deviation after the best fit. Result: Automatic algorithms are implemented to analyse 2D free-form profiles of moulds for ophthalmic lenses obtained with grinding and polishing operations and to analyse 3D free-form surfaces of an injection moulded lamp lens for automotive industry. Discussion & Conclusion: The comparison between results obtained using this approach and using traditional fitting methods shows differences on the fitting and errors parameters estimation. In particular in the traditional fitting there is a repartition of the size error contribution in the translation and form errors. The differences increase proportionally to the offset value estimated. The calculation of offset parameter allows the separation of size from geometric errors in free-form profiles/surfaces and the “true” form error estimation