F. M. Duarte

Thermal conditions affecting heat transfer in FDM/FFE: a contribution towards the numerical modelling of the process

The performance of parts produced by Free Form Extrusion (FFE), an increasingly popular additive manufacturing technique, depends mainly on their dimensional accuracy, surface quality and mechanical performance. These attributes are strongly influenced by the evolution of the filament temperature and deformation during deposition and solidification. Consequently, the availability of adequate process modelling software would offer a powerful tool to support efficient process set-up and optimisation. This work examines the contribution to the overall heat transfer of various thermal phenomena developing during the manufacturing sequence, including convection and radiation with the environment, conduction with support and between adjacent filaments, radiation between adjacent filaments and convection with entrapped air. The magnitude of the mechanical deformation is also studied. Once this exercise is completed, it is possible to select the material properties, process variables and thermal phenomena that should be taken in for effective numerical modelling of FFE.

IR sheet heating in roll fed thermoforming: Part 1 - Solving direct and inverse heating problems

Abstract Computational methods for modelling the heating stage in thermoforming of roll fed plastic sheet (direct problem) and for setting the temperatures of the heater bank to produce a prescribed sheet temperature (inverse problem) are presented, discussed, and experimentally validated. The algorithm computes the relevant local shape factors, and takes into account the effects of sag and of the surrounding air. The importance of some boundary conditions, such as air temperature, is discussed and quantitatively estimated.

Evaluation of Properties and Biodeterioration Potential of Polyethylene and Aliphatic Polyester Blends

Abstract Blends of high density polyethylene (HDPE) and biodegradable polymers – polylactic acid, PLA, poly(∊-caprolactone), PCL and Mater-Bi® (thermoplastic starch (TPS) with PLA or PCL) – were prepared in a co-rotating twin-screw extruder, together with polyethylene modified with maleic anhydride (PE-g-MA) used as compatibiliser. The mechanical and rheological properties, morphology and potential for biodeterioration of polymeric materials were evaluated. Blends with PLA showed a reduced elongation at break but an increased Youngs modulus while the addition of PCL led to materials with a greater elongation at break and a lower Young modulus. The rheological results evidenced that HPDE and the blend with the highest TPS level exhibited the highest viscosity. The microbial growth test carried out to evaluate the potential for biodeterioration of the blends, using a pure culture of Pseudomonas fluorescens, indicated that HDPE/PCL had a lower resistance to bacterial attack than the blend of HDPE/PLA. This was verified by a higher cell number on its surface after 10 weeks of incubation. The addition of 30% starch to the HDPE/PLA blend enhanced its biodeterioration potential, the same was not observed in the case of the HDPE/PCL blend containing just 18% starch.

Infrared sheet heating in roll fed thermoforming: Part 2 - Factors influencing inverse heating solution

Abstract Two methodologies for solving the inverse heating problem in thermoforming, i.e. setting the temperature of the heaters that provide a prescribed temperature of the sheet to be formed against the mould, are compared in terms of temperature gradients across the thickness and sensitivity to the most important process parameters. The influences of sheet thickness, sheet emissivity, room temperature, and distance between the heater bank and the forming station on the thermal homogeneity of the sheet are also discussed.

Cure and rheological analysis of reinforced resins for stereolithography

The rising of consumers’ demands and an ever increasing pressure of international markets imposed a deep change in the product development process to respond to an increasing product complexity and higher quality, as well to the need to promptly introduce products into the market. Stereolithography plays an important role on this new product development context. This technology produces models for thermosetting resins through a polymerisation process that transforms liquid resins into solid materials. In this work, a new route to produce metallic parts through stereolithography is explored. The curing analysis of hybrid reinforced polymeric systems, polymerised through radicalar or/and cationic mechanisms, is investigated. The rheological behaviour of these polymeric systems is also evaluated due to its importance for recoating. The influence of other processing and material characteristics like light intensity, initiator concentration, low powder size of metallic powders, degree of dilution, etc. is also investigated.

Using MATLAB to Compute Heat Transfer in Free Form Extrusion

Rapid Prototyping (RP) is a group of techniques used to quickly fabricate a scale model of a part or assembly using three-dimensional computer aided design (CAD) data (Marsan, Dutta, 2000). A large number of RP technologies have been developed to manufacture polymer, metal, or ceramic parts, without any mould, namely Stereolithography (SL), Laminated Object Manufacturing (LOM), Selected Laser Sintering (SLS), Ink-jet Printing (3DP) and Fused Deposition Modeling (FDM). In Fused Deposition Modelling (developed by Stratasys Inc in U.S.A.), a plastic or wax filament is fed through a nozzle and deposited onto the support (Perez, 2002; Ahn, 2002; Ziemian & Crawn, 2001) as a series of 2D slices of a 3D part. The nozzle moves in the X–Y plane to create one slice of the part. Then, the support moves vertically (Z direction) so that the nozzle deposits a new layer on top of the previous one. Since the filament is extruded as a melt, the newly deposited material fuses with the last deposited material. Free Form Extrusion (FFE) is a variant of FDM (Figure 1), where the material is melted and deposited by an extruder & die (Agarwala, Jamalabad, Langrana, Safari, Whalen & Danthord, 1996; Bellini, Shor & Guceri, 2005). FFE enables the use of a wide variety of polymer systems (e.g., filled compounds, polymer blends, composites, nanocomposites, foams), thus yielding parts with superior performance. Moreover, the adoption of coextrusion or sequential extrusion techniques confers the possibility to combine different materials for specific properties, such as soft/hard zones or transparent/opaque effects.

Waste Fibre Reinforced Ecocomposites

With a significant production of waste fibrous material, textile companies are now looking for applications where waste materials could be an added-value material. One viable application of these waste materials is in the combination with polymeric matrices, producing composite materials with interesting properties for specific applications, from furniture to thermal and acoustic insulations. The aim of this work was to study the physical and mechanical properties of waste fibre reinforced composites and the influence of different parameters on their mechanical behaviour. Results show that a wide range of different properties and performances may be designed by altering various production parameters, such as thickness of the nonwovens used, time and temperature of the compression moulding, relationship between fibre/matrix ratio, polymeric film used and number of layers.

Preparation of biodegradable materials by reactive extrusion

This work aimed to prepare biodegradable polymeric materials based on blends of a synthetic high density polyethylene (HDPE) and biodegradable polymers such as polylactic acid (PCL) and poly(caprolactone) (PLA), in a co-rotating twin-screw extruder. A polyethylene modified with maleic anhydride was used as compatibiliser. The mechanical results showed that the addition of PLA improves the blends stiffness while the addition of PCL leads to materials with a greater elongation at break and a lower Young modulus. This feature is related with the mechanical properties of each material as well as the adhesion between them. Concerning the biodegradability tests, it was found that HDPE/PCL blend presents the highest degree of biodegradability.

Simulating Human Physiological Response with a Thermal Manikin Testing Different Non Active Medical Devices

The thermal insulation of a clothing system represents a quantitative assessment of the way cloth provides thermal barrier to the user. One of this clothing systems, the surgical gown used in the operating theatre, is considered as a non-active medical device and obeys the Medical Device Directive 93/43/EEC. New materials and gowns are being developed, fitting the level of the barrier function with the comfort issues and therefore the selection of the most suitable gown is vital. During the last 60 years, thermal manikins have been used to measure clothing insulation and to assess the thermal environment regarding comfort issues. The main goal of the present study is the comparison of the thermal insulation values during the objective evaluation using the dry thermal manikin with the results obtained using an Infra-Red camera ThermaCAM, monitoring the temperature development of different surgical gowns at a constant skin temperature of 33 °C in neutral environmental conditions.

A computational study on the influence of the rheological behavior of polystyrene and its blends on their thermoforming ability

The present work aims at understanding the relationship between heating conditions, rheological behavior and thickness distribution that lead to the optimization of the latter in thermoforming. The materials used in this study were polystyrene, PS, high-impact polystyrene, HIPS, and a 50/50 w/w % blend of the two. The study was done by investigating computationally the influence of the material thermo-rheological properties on sheet temperature and final thickness distribution of a vacuum-produced part and relating the sheet heating conditions with the forming stage. When sheet temperature is uniform, the degree of strain hardening and the failure behavior in extension are the most important parameters in controlling the kinetics of the process and the thickness profile. In the case of nonuniform sheet temperature, the results show that an increased degree of strain-hardening is more relevant to the dynamics of the process than relatively small differences in sheet temperature. However, the solution of the inverse thermoforming problem (determining the heater temperature that induces a certain thickness distribution) showed that under practical processing conditions the effect of differences in thermal properties are predominant over the rheological ones.