Wilma Polini

Cold plasma treatment of polypropylene surface: a study on wettability and adhesion

Abstract The increasing use of polymeric materials in high technological fields, such as automotive, has forced the need to overcome some of their limitations by means of innovative processing. In the automobile industry a complex and critical process is used in order to enhance both wettability and adhesive properties of polypropylene bumper surfaces. Cold plasma treatment represents an efficient, clean and economic alternative to activate polymeric surfaces. The present work deals with air cold plasma treatment of polypropylene surfaces. Particularly, the influence of AC electrical discharge cold plasma parameters on wettability and adhesion of polymeric surfaces was studied. Also, the nature of the relationship between wettability and adhesion was investigated. Owing to the complexity of plasma–workpiece interaction, an experimental approach was followed. A set of process variables (voltage, time and air flow rate) was identified and used to conduct some experimental tests on the basis of design of experiment techniques. The experimental results show that the proposed plasma process may considerably increase polypropylene wettability and adhesion properties. These outcomes represent the first step in trying to optimise the polymeric adhesion by means of this non-conventional manufacturing process.

A review of two models for tolerance analysis of an assembly: Jacobian and torsor

The dimensional and geometrical variations of each part of an assembly have to be limited by tolerances able to ensure both a standardised production and a certain level of quality, which is defined by satisfying functional requirements. The appropriate allocation of tolerances among the different parts of an assembly is the fundamental tool to ensure assemblies that work correctly at lower costs. Therefore, there is a strong need to develop a tolerance analysis to satisfy the requirements of the assembly by the tolerances imposed on the single parts. This tool has to be based on a mathematical model able to evaluate the cumulative effect of the single tolerances. Actually there are some different models used or proposed by the literature to make the tolerance analysis of an assembly constituted by rigid parts, but none of them is completely and univocally accepted. Some authors focus their attention on the solution of single problems found in these models or in their practical application in computer-aided tolerancing systems. But none of them has done an objective and complete comparison among them, analysing the advantages and the weakness and furnishing a criterion for their choice and application. This paper briefly introduces two of the main models for tolerance analysis, the Jacobian and the torsor. These models are briefly described and then compared showing their analogies and differences. The evolution of these two models, known as unified Jacobian-torsor model, is presented too.

Dimensional errors in longitudinal turning based on the unified generalized mechanics of cutting approach.: Part I: Three-dimensional theory

Abstract During the machining of a part, a new surface is generated together with its dimensional deviations. These deviations are due to the presence of several phenomena (workpiece deflection under strong cutting forces, vibration of the machine tool, material spring-back, and so on) that occur during machining. Each elementary phenomenon results in an elementary machining error. Consequently, the accuracy of the manufactured workpiece depends on the precision of the manufacturing process, which it may be controlled or predicted. The first part of this work presents a new model to evaluate machining accuracy and part dimensional errors in bar turning. A model to simulate workpiece dimensional errors in longitudinal turning due to deflection of the tool, workpiece holder and workpiece is shown. The proposed model calculates the real cutting force according to the Unified Generalized Mechanics of Cutting approach proposed by Armarego, which allows one to take into account the three-dimensional nature (3D) of the cutting mechanism. Therefore, the model developed takes advantage of the real workpiece deflection, which does not lie in a plane parallel to the tool reference plane, and of the real 3D cutting force, which varies along the tool path due to change in the real depth of cut. In the first part of the work the general theory of the proposed approach is presented and discussed for 3D features. In the second part the proposed approach is applied to real cases that are mostly used in practice. Moreover, some experimental tests are carried out in order to validate the developed model: good agreement between numerical and experimental results is found.

To design the cure process of thick composite parts: experimental and numerical results

The cure degree must be as uniform as possible during the manufacture of polymer matrix composite components in order to have a product without defects. For thick composite components, this condition is not often respected; in fact, the cure degree trend between the core and the external surface is different causing structural and geometrical/dimensional unconformities. In most cases, these problems can be solved through a redesign of cure process in terms of thermal cycle, in fact that one recommended by furnisher is usually suitable for thin components. The optimization of cure thermal cycle should include several performance criteria for the production system such as the targeted cure degree, the targeted maximum temperature of the part and the duration of the cure cycle, as well as the production system limitations such as the maximum allowable heating rate, the maximum allowable cooling rate and so on. A previous work shows a method to optimize the cure degree of a thick composite component. The present work presents an indirect way to validate the proposed method: some experimental tests have been carried out by implementing the cure cycle identified by this method, the values of temperatures have been recorded by thermocouples and the obtained temperature trend has been compared with that due to numerical approach. Further considerations on the cure degree and the cure rate have been deduced. The experimental results show a good agreement with the numerical ones.

Taxonomy of models for tolerance analysis in assembling

The dimensional and geometrical variations of each part have to be limited by tolerances in order to guarantee quality and, at the same time, a decrease of manufacturing costs. Therefore, how to allocate the tolerances among the different parts of an assembly is the fundamental tool to ensure assemblies that work rightly at lower costs. Tolerance analysis allows evaluation of the cumulative effect that the single tolerances assigned on the components has to satisfy the functional requirements of the whole assembly. The results of the tolerance analysis are meaningfully conditioned by the adopted mathematical model. The purpose of this work is to analyse the most significant models for tolerance analysis. The first part of the paper describes a description of the most common criteria used to categorise existing models for tolerance analysis. Taxonomy is then suggested that may be a useful tool to help evaluate, compare and select such models. Five of the most representative models are explained and discussed in detail in order to identify their strong points and their limitations.

A method to characterise superplastic materials in comparison with alternative methods

Abstract Superplastic materials show a very high ductility, i.e. maximum elongation of about 5000%, even if they are lowly stressed. This is due to both peculiar process conditions and material intrinsic characteristics. The aerospace industry has shown that, in order to produce complex parts requiring large tensile elongations that cannot be formed by conventional processes, superplastic forming can be used. A detailed design of technological process is necessary so as to exploit at best the peculiar potentialities of superplastic forming. The aim of the present work is to show a method to characterise superplastic materials. This method is based on the approximate analysis of a superplastic forming process in a triangular indefinite prismatic-shaped die. It has been experimentally validated through laboratory samples on material formed by room temperature; moreover, it has been compared to several methods proposed by other authors.

Modular structure of a new feed-deposition head for a robotized filament winding cell

This work shows the modular structure of a new feed-deposition head for a robotized cell able to manufacture complex shape parts in composite material by means of the filament winding technology. The new feed-deposition head assembles, in a unique structure, the four critical subgroups or modules involved in roving winding: the main frame, the roving-guide system, the roving tensioner and the deposition system. Each component is deeply discussed in this paper in terms of geometry, i.e. shape, dimensions and tolerances, of interfaces with other modules, of constitutive material selection and of related manufacturing processes. The adopted solutions have allowed to increase both the filament winding efficiency and the composite part quality by controlling the process parameters, such as the roving tension, the deposition speed and the winding trajectory through a very compact and flexible frame, as proved by the results of some experimental tests. Moreover, the resulting head is easily adaptable to every robot or machine used to wind. Finally, the modular structure enables an easy maintenance and updating.

Dimensional errors in longitudinal turning based on the unified generalized mechanics of cutting approach. Part II: Machining process analysis and dimensional error estimate

The model presented in the first part of this work is used here to estimate the diameter error in the most common turning operations. In fact, the diameter error is considered as a variable depending on the deflections of the tool, workpiece holder and workpiece, which are considered the main factors responsible for the machining accuracy. The proposed model has been applied to the three most common turning schemes related to workpiece fixturing, where the workpiece is clamped in a chuck, or supported between two centers, or clamped in a chuck at the spindle and supported on a center at the tailstock. Some numerical examples have been computed using the proposed model to predict the diameter error along the workpiece and the cutting force along the workpiece axis, as well as the influence of the cutting force components on the error prediction. The results provide additional insight into error formation in the turning process. Finally, some experimental tests have been carried out in order to validate the developed model. Good agreement has been obtained between numerical and experimental results. The proposed model represents a first step towards accuracy control in machining operations and, thus, towards optimization of the manufacturing process.

Review of variational models for tolerance analysis of an assembly

The tolerance analysis of assemblies, which is carried out during the design stage, is the basic tool to simulate the effects of the assigned tolerances on the required functionality, and then, to define the appropriate allocation of the tolerances. This tool has to be based on a mathematical model able to evaluate the effect that each single tolerance has on the whole assembly. There are some different models proposed in the literature to carry out the tolerance analysis of an assembly, but none of them is completely and univocally accepted. Some authors focus their attention on the solution of single problems found in these models or in their practical application in computer-aided tolerancing systems. None of them has done an objective and complete comparison among them, analysing the advantages and the weakness and furnishing a criterion for their choice and application. The present paper briefly introduces one of the main models for tolerance analysis, the variational model. The variational model is a family of schemes developed by different authors; the two most developed schemes are that involving rotational and translational parameters and that involving only translational parameters. These two models are briefly described herein and then compared to show their analogies and differences.

Knowledge based method for touch probe configuration in an automated inspection system

Abstract Coordinate measuring machine (CMM) dimensional inspection is an integral part of modern manufacturing processes, used to control the production quality of mechanical workpieces. An automatic planning system, linking CAD with CMM, can increase inspection efficiency and effectiveness, reducing or eliminating human influence. Developing a computer aided inspection planning (CAIP) system involves interpretation of part design (geometry and tolerance specifications), representation of inspection actions and their effects on the parts, reasoning about the effects of sequence of such actions, controlling the search and data manipulation, as well as determining details of inspection and programming code for CMMs. In this way it is possible to automatize the different steps involved in a dimensional inspection: determining the measuring points, selecting inspection devices and sensors, determining a detailed inspection sequence, etc. Touch probe selection is one critical step for inspection to be carried out on a coordinate measuring machine. The present paper reports the preliminary study and the prototypical realisation of an expert system to generate touch probe configurations.