Isabel Bagudanch

Polymer incremental sheet forming process: Temperature analysis using response surface methodology

ABSTRACT To reduce costs associated with the manufacturing of customized products, several innovative forming processes have been developed. Incremental sheet forming (ISF) is one of these new technologies, becoming, in the past decade, more interesting for the academic and industrial community. The influence of main process parameters, namely, tool diameter, spindle speed, feed rate, and step down, is studied in depth in this paper. The maximum temperature achieved during the forming process of a truncated pyramid frustum with a circular generatrix using three nonbiocompatible and two biocompatible polymer materials is measured. Box–Behnken design of experiments and the response surface methodology have been utilized to statistically analyze the results and to provide models able to predict the maximum temperatures.

On the Rule of Mixtures for Predicting Stress-Softening and Residual Strain Effects in Biological Tissues and Biocompatible Materials

In this work, we use the rule of mixtures to develop an equivalent material model in which the total strain energy density is split into the isotropic part related to the matrix component and the anisotropic energy contribution related to the fiber effects. For the isotropic energy part, we select the amended non-Gaussian strain energy density model, while the energy fiber effects are added by considering the equivalent anisotropic volumetric fraction contribution, as well as the isotropized representation form of the eight-chain energy model that accounts for the material anisotropic effects. Furthermore, our proposed material model uses a phenomenological non-monotonous softening function that predicts stress softening effects and has an energy term, derived from the pseudo-elasticity theory, that accounts for residual strain deformations. The model’s theoretical predictions are compared with experimental data collected from human vaginal tissues, mice skin, poly(glycolide-co-caprolactone) (PGC25 3-0) and polypropylene suture materials and tracheal and brain human tissues. In all cases examined here, our equivalent material model closely follows stress-softening and residual strain effects exhibited by experimental data.

A functional methodology on the manufacturing of customized polymeric cranial prostheses from CAT using SPIF

Purpose This paper aims to propose a functional methodology to produce cranial prostheses in polymeric sheet. Within the scope of rapid prototyping technologies, the single-point incremental forming (SPIF) process is used to demonstrate its capabilities to perform customized medical parts. Design/methodology/approach The methodology starts processing a patient’s computerized axial tomography (CAT) and follows with a computer-aided design and manufacture (CAD/CAM) procedure, which finally permits the successful manufacturing of a customized prosthesis for a specific cranial area. Findings The formability of a series of polymeric sheets is determined and the most restrictive material among them is selected for the fabrication of a specific partial cranial prosthesis following the required geometry. The final strain state at the outer surface of the prosthesis is analysed, showing the high potential of SPIF in manufacturing individualized cranial prostheses from polymeric sheet. Originality/value This paper proposes a complete methodology to design and manufacture polymer customized cranial prostheses from patients’ CATs using the novel SPIF technology. This is an application of a new class of materials to the manufacturing of medical prostheses by SPIF, which to this purpose has been mainly making use of metallic materials so far. Despite the use of polymers to this application is still to be validated from a medical point of view, transparent prostheses can already be of great interest in medical or engineering schools for teaching and research purposes.

Force Modeling in Single Point Incremental Forming of Variable Wall Angle Components

Prediction of forming forces in Incremental Sheet Forming (ISF) is specially important in the case of using adapted machinery not designed for the process. Moreover, forming force is an important indicator that can be monitored on-line and utilized for real time process control. Besides experimentation, simulations based on the Finite Element Method (FEM) have been utilized as a reliable source of process force data. Nevertheless, the long solution times required to simulate ISF renders difficult its inclusion into a process optimization chain. In consequence, models that predict the forces required to manufacture simple parts have appeared. This work begins with a review of forming force models available for Single Point Incremental Forming (SPIF). Then, an equation recently proposed in the literature is compared with published experimental results of SPIF under different working conditions. The same data is employed to verify our own FEM simulations. Finally, the above-mentioned formula and FEM simulation were applied to predict the forming force of Variable Wall Angle (VWA) geometries where available force information is limited. Besides the applicability assessment of the equation, results will supplement a future experimental campaign focused in modeling geometries of intermediate complexity level by means of Computational Intelligence methods.

Single-Point Incremental Forming of Two Biocompatible Polymers: An Insight into Their Thermal and Structural Properties

Sheets of polycaprolactone (PCL) and ultra-high molecular weight polyethylene (UHMWPE) were fabricated and shaped by the Single-Point Incremental Forming process (SPIF). The performance of these biocompatible polymers in SPIF was assessed through the variation of four main parameters: the diameter of the forming tool, the spindle speed, the feed rate, and the step size based on a Box–Behnken design of experiments of four variables and three levels. The design of experiments allowed us to identify the parameters that most affect the forming of PCL and UHMWPE. The study was completed by means of a deep characterization of the thermal and structural properties of both polymers. These properties were correlated to the performance of the polymers observed in SPIF, and it was found that the polymer chains are oriented as a consequence of the SPIF processing. Moreover, by X-ray diffraction it was proved that polymer chains behave differently on each surface of the fabricated parts, since the chains on the surface in contact with the forming tool are oriented horizontally, while on the opposite surface they are oriented in the vertical direction. The unit cell of UHMWPE is distorted, passing from an orthorhombic cell to a monoclinic due to the slippage between crystallites. This slippage between crystallites was observed in both PCL and UHMWPE, and was identified as an alpha star thermal transition located in the rubbery region between the glass transition and the melting point of each polymer.

Evaluating Material Constitutive Equations for the Simulation of Incremental Sheet Forming Applied to Form Thermoplastic Materials

Incremental Sheet Forming (ISF) is able to produce highly customized products at a reasonable manufacturing cost and it has gain importance in the last years, becoming the focus of interest for many researchers and institutions. Some recent publications have revealed an increasing interest in forming thermoplastic materials. There is a tremendous amount of effort put in developing a model that may describe the equilibrium hysteresis and rate-dependence of thermoplastic materials in ISF. This paper will present a brief review of the most common constitutive equations that are able to model the behaviour of glassy polymers. It will be shown that by using a small number of material parameters defined in the Marlow model, it is possible to accurately predict experimental data collected on samples of PVC subjected to simple uniaxial test performed at room temperature. Additionally, some parts have been formed with ISF in order to verify whether the material is incompressible or not. It can be concluded that Marlow model might be used in future work to model the ISF manufacturing process.

Fabrication of a biopsy meso-forceps prototype with incremental sheet forming variants

This study aimed at developing an application and comparison of performance between the two main incremental sheet forming (ISF) variants, single-point incremental forming (SPIF) and two-point incremental forming (TPIF), on a scaled prototype of a biopsy micro forceps (BMF). Experimental tests were carried out initially with the SPIF set-up. Rupture of specimens and low accuracy of them due to bending of the material led to the more complicated TPIF variant. Prior to the physical support tool construction, FEM simulations allowed validating the precision gains. After several adjustments to the tool-path in the new configuration, the BMF was successfully manufactured by TPIF on AA1050-H24 and AISI 304 05 mm thick. Therefore, the presented results demonstrate the feasibility to use ISF for the rapid prototyping of sheet metal medical devices.

Process Parameter Effects on Biocompatible Thermoplastic Sheets Produced by Incremental Forming

There has been increasing interest in the processes that enable part customization and small-batch production in recent years. The prosthetic sector, in which biocompatible materials are used, is one of the areas that requires these types of processes; Incremental Sheet Forming (ISF) technology can meet these requirements. However, the biocompatible thermoplastic polymers formed by this technology have not yet been tested. Hence, the aim of this paper is to cover this gap in our knowledge by analyzing the effects of process parameters on the ISF process with the aim of optimizing these parameters before the actual production of, in this case, customized prostheses. Tests with polycaprolactone (PCL) and ultra-high molecular weight polyethylene (UHMWPE) were performed. Maximum force, surface roughness and maximum depth were statistically analyzed by means of response surface methodology and survival analysis. Spindle speed and tool diameter were shown to be the most influential process parameters in terms of maximum forming force and surface roughness for both materials. In contrast, survival analysis applied to maximum depth showed a greater influence of tool diameter in PCL sheets and a greater influence of spindle speed in the case of UHMWPE.

Customized cranial implant manufactured by incremental sheet forming using a biocompatible polymer

Purpose The purpose of this paper is to demonstrate the feasibility of incremental sheet forming (ISF), using the most common variants, single-point incremental forming (SPIF) and two-point incremental forming (TPIF), to produce prototypes of customized cranial implants using a biocompatible polymer (ultrahigh molecular weight polyethylene, UHMWPE), ensuring an appropriate geometric accuracy and cost. Design/methodology/approach The cranial implant is designed based on computerized tomographies (CT) of the patient, converting them into a 3D model using the software InVesalius. To generate the toolpath for the forming operation computer-aided manufacturing (CAM) software is used. Once the cranial implant is manufactured, a 3D scanning system is used to determine the geometric deviation between the real part and the initial design. Findings The results corroborate that it is possible to successfully manufacture a customized cranial implant using ISF, being able to improve the geometric accuracy using the TPIF variant with a negative die. Originality/value This paper is one of the first research works in which a customized cranial implant is successfully manufactured using a flexible technology, ISF and a biocompatible polymer. The use of polymeric implants in cranioplasty is advantageous because of their lightweight, low heat conductivity and mechanical properties similar to bone. Furthermore, the cost of the implant has been calculated considering not only the raw materials and manufacturing time but also the environmental impact, revealing that it is a cheap process with a low lead-time.

Tool Path Strategies for Single Point Incremental Forming

In the last few years the interest in Incremental Sheet Forming (ISF) has considerably grown due to the possibility of obtain small production batches and high customized products. Despite the increasingly knowledge of this processing technique, there are still some important process parameters that require further development. One of the most important parameters when experimental or numerical studies of ISF are carried out is the tool path programming. The decision of the strategy that will be followed can affect the accuracy, the surface finishing, the forming forces, etc. The analysis of these relationships has been studied for several authors using different tool path. In the present paper different tool path strategies for the same geometry are compared. The geometry has two inclination wall angles. Also, different commercial softwares are employed to develop the tool path of the desired geometry. The main objective of the paper is to identify some indicators that can assist in the choice of which strategy is the best, not only depending on the complexity of the geometry or the software used but also on qualitative criteria. With all this work it is possible to provide some useful guidelines that permit to establish a selection criterion of the tool path, which will be very interesting before developing an experimental work.