L. Carrino

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.

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.

A posteriori optimisation of the forming pressure in superplastic forming processes by the finite element method

In order to optimise the superplastic forming processes, it is necessary to control the strain-rate induced in the material by the pressure gas. This control ensures high material deformability. In this paper, to determine the optimum pressure-time profile in superplastic forming processes, the authors will show an original technique based on finite element method. The proposed technique was tested with reference to the superplastic forming of a thin circular plate clamped in a rigid die and formed by a pressure differential. The experimental activity was carried out using a Pb/Sn-based metallic alloy. Final results of the finite element modelling agree with the experimental ones.

On the optimisation of superplastic forming processes by the finite-element method

Abstract This paper presents an original algorithm, with a finite-element interface capability, that can calculate, for superplastic forming processes, the load curve to be applied during forming. The algorithm makes it possible to maintain the maximum strain rate as near as possible to the optimum characteristic value of the material thereby reducing forming time. The proposed algorithm was validated by calculating the optimum pressure curve to be assigned in the case of both a conical-bulging process and a cap-forming one. Parallel experimental activity was carried out using a Pb/Sn-based metallic alloy that is superplastic at room temperature. For both geometries examined, comparison between numerical and experimental results proved to be acceptable. In particular, the relative error was less than 6% for measurements of both the strain–time relationship and the thickness distribution. Different measurements were carried out during each process: the shift in the sheet centre-point for the conical-bulging process and the thickness distribution corresponding to a particular sheet configuration for the cap-forming one.

Modelling of superplastic blow forming

Abstract Some metals and metallic alloys, when deformed in particular conditions, manifest exceptional ductility giving tensile elongations of up to 1000%. This behaviour, known as “superplasticity”, could undergo considerable development in sheet metal production. The aerospace industry has already taken the opportunity of producing complex-shaped objects in a limited number of mechanical operations. It makes use of superplastic characteristics to noticeably reduce the weight and cost entailed in manufacturing some components, including structural ones. To better take advantage of the superplastic characteristics of the material, it is necessary to control the temperature and the strain rate during the manufacturing process. Since the material undergoes significant elongation, it also, necessarily, undergoes extreme thinning. The latter can prove not to be uniformly distributed because of (i) the particular geometry of the manufactured product, (ii) the characteristics of the material used, (iii) the lubrification and (iv) the process parameters adopted. The design stage should take account of the real thickness distribution in order to avoid critical areas. At this stage, thus, it is necessary, not only to design the product, but also to design the process in order to establish the optimum production parameters and to foresee the real geometry of the product. Numerical modelling is used since “in the field” analysis could prove to be expensive, and, analytical modelling would be limited only to some forms and to the use of largely approximated assumptions. The finite element method can be considered to be the most dependable both for analysing complex geometries, and for taking into consideration all the phenomena involved in the manufacturing process. One of the most delicate operations in this method is sub-dividing the continuum into elements since this discretization can have an influence both on the reliability of the results, and on computational requirements. The objective of this paper is to verify the approximation of the results that can be obtained compared to the different options possible both in terms of element type and number. The production of an axisymmetric cup in commercial aluminium based alloy Al 7475 using blow forming technology was taken as a reference case. Comparison between the results of the different simulations showed a substantial equivalence and a good correspondence to the measured thickness values. Since the computational resources required are very different for the cases examined, it can be stated that the best solution is discretization of the start-off sheet with a row of 55 axisymmetrical four node elements.

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.

Finite Element Modelling and the Experimental Verification of Superplastic Forming

FEM analysis has proved to be a powerful investigative tool capable of encompassing all the aspects that characterise an SPF process. However, despite the high potential of FEM programs they do not allow one to directly and suitably obtain the thickness of a sheet product for high deformation values, as commonly occurs in SPF processes. Many papers have been published on finite element analysis of S.P.F. process but the question of calculus accuracy in thicknesses of a sheet product has not been directly investigated. This problem has been already considered by the authors in a previous study which proposed an algorithm to determine thicknesses for a specific application. The software set up starts out with the results of the FEM modelling, keeps track of the “deformation” undergone by each element of the mesh and calculates to a good approximation the thicknesses at the end of the forming. Although the original version of the algorithm could only be used for the application studied an updated version is introduced in this study that can be used for any case. In other words, the software generates the thickness profile at the end of the analysis independently of technological set up, item shape and type of simulation (3D and 2D). The proposed algorithm was tested with reference to the superplastic forming of an item of simple geometry beginning with a thin circular plate blocked at the edges and put under constant hydrostatic pressure on one side. The test material, made superplastic by means of a series of repeated laminations, was characterised using an alternative method to the traditional tension test. The results of the experiments are in good accordance with the numerical predictions both in terms of thickness distribution and forming times.

Oxygen cold plasma treatment on polypropylene: influence of process parameters on surface wettability

Abstract Polypropylene is one of the most versatile polymers. The wide range of physical properties and relative ease of processing make it an extremely attractive material capable of competing with more expensive resins in a number of demanding applications. Its key limitation is its poor adhesion towards paints. The low polarity of the molecules in polypropylene is the cause of the low surface energy of these plastics. Increasing the surface energy is one of the major purposes of pretreatment for such plastics. The cold plasma process represents a more efficient, cleaner and cheaper treatment than the flaming traditional treatment to activate polypropylene surfaces. The aim of this work has been to evaluate the influence of oxygen cold plasma parameters on wettability of polypropylene surfaces treated. The wettability aging time has been also assessed because it represents a fundamental step to plan the insertion of the plasma process inside an industrial system. A set of process variables (voltage, time and pressure) has been identified and used to carry out some experimental tests on the basis of Design of Experiment techniques. The polypropylene wettability has been quantitatively characterised by the standard procedure detailed in ASTM D724 along the aging time.