Experimental analysis of selective laser sintering of polyamide powders: an energy perspective

https://doi.org/10.1016/j.jclepro.2010.07.018Get rights and content

Abstract

This paper presents an analysis of Selective Laser Sintering (SLS) from an energy standpoint. Selective Laser Sintering (SLS) has a potential as an environmental benign alternative to traditional processes but only few authors deal with the process optimisation including energy aspects. In the present paper an analysis of the energetic aspect of SLS is proposed. In addition, with respect to the classical technological parameters (resolution, productivity) attention is paid to energetic elements (energetic productivity, laser parameters) showing how the perspective of a sensible development of such a kind of technology could be beneficial not only from a technological point of view, but also for energy saving in a lot of manufacturing fields. A polyamide powder is the material tested to acquire some characteristics data of the process. It is shown that the Volumetric Energy Intensity (VEI) of the process in optimal condition could be of the order of 0.2 J for each mm3 of material agglomerated.

Introduction

The purpose of manufacturing processes is to transform materials into useful products. In the course of these operations, energy resources are consumed. Much of the work characterizing environmental impacts of manufacturing processes and systems has focused on energy consumption patterns. From the analysis of the literature it is apparent that energy and electricity use per unit volume (or mass) of material processed has increased over the past decades (USEPA, 2007, Schifo and Radia, 2004). Energy consumption is a component of environmental impact and it is a critical component in any overall sustainability strategy (Gutowski et al., 2009). A relevant paper about the energy requirements of manufacturing processes could be found in Gutowski et al. (2006).

In order to tackle the issues for achieving sustainable production and improve process performance, in alternative machining technologies, the main objectives under consideration are as follows

  • -

    To reduce machining processes energy consumption.

  • -

    To minimize waste (generate less waste and increase waste reusage or recycling).

  • -

    To use resources efficiently.

Hence reducing energy usage is an essential consideration in sustainable manufacturing. For example, in a recent study, Pusavec et al., 2010a, Pusavec et al., 2010b suggested a number of ways to improve sustainability in manufacturing. Reducing the energy consumed in manufacturing processes was identified as one of the strategies.

Even if the energy consumed for non-cutting operations dominates the total energy consumption in machining (Rajemi et al., 2010).

Optimisation of machining operations has been undertaken for a considerable number of years based on technological and economic considerations. The current and urgent need to reduce energy consumption requires a knowledge base for selection of minimum environmental processing conditions. The optimum condition for minimum costs does not necessarily satisfy the minimum energy criterion.

For this purpose laser assisted manufacturing processes like Rapid Prototyping techniques, when compared with traditional manufacturing processes, have the potential to reduce cost, increase surface resistance to wear and fatigue, extend part/tool life, and expand the range of manufacturable materials. They also offer advantages in terms of energy consumptions reduction since they are based on the concept of fabricating a predefined object by material deposition instead of the conventional chip removal mechanisms.

However, to date, very little research has been conducted to evaluate and compare the environmental performance (e.g. greenhouse gas emissions or carbon footprint) of laser assisted manufacturing processes relative to the traditional manufacturing methods. It is critical for engineers to design and develop novel processing techniques that are more environmentally friendly without sacrificing functionality, performance, and economic viability (Zhao et al., 2010).

Of all RP techniques, Selective Laser Sintering (SLS) seems to be one of the most consolidated processes to create solid objects, layer by layer, from plastic, metal, ceramic powders or pre-coated sands that are sintered using laser energy (Levy et al., 2003). SLS not only reduces the time and cost of prototyping components, obtaining the same dimensional accuracy, surface finish and repeatability as common manufacturing process, but also reduces the energy intensity and the environmental impacts of the process.

The basic concept, common to all rapid prototyping techniques, is that any complex shape can be produced by the superposition of small thickness layers. In case of SLS, layers of 0.05–0.3 mm thickness are obtained by thermal binding of small particles that are agglomerated together by the action of a laser source whose wavelength depends of the powder adopted. A CAD software is commonly used to decompose a 3D drawing into a sequence of thin layer whose shape is used to set the working area of the laser beam. After a layer is sintered at a predefined focal length, new uncured powder is levelled on a platform whose vertical adjustment (governed by a step-motor) allows re-focusing of the laser beam.

The inherent versatility of SLS technology combined with a large variety of powder materials allows a broad range of advanced rapid prototyping and manufacturing applications. As a matter of fact, the powder consolidation mechanisms depend on the nature of the material used and on the mechanical properties to be obtained (e.g. porosity) as reported by Kruth et al. (2007).

  • -

    Solid State Sintering is a consolidation process occurring below the material’s melting temperature and governs the agglomeration of most ceramic and metallic powders.

  • -

    Liquid Phase Sintering is common for mixture of two-components powder, composite powder particles and coated particles.

  • -

    Partial Melting can be adopted for metals and for polymers under the threshold value represented by the glass transition temperature.

  • -

    Full Melting is a third major consolidation mechanism often applied to metals to achieve fully dense parts.

  • -

    Chemical Induced Binding is also possible for polymers, metals and ceramics even though not commonly used in commercial applications.

As reviewed by Kandis and Bergman (2000), polymers are the most processed type of powders in SLS but the consolidation phenomena related to these materials seem to be the most complex among all those described in literature. They concern a deep interaction between heat, mass and momentum transfer as well as chemical modifications of the materials and variation of mechanical and thermophysical properties. This hinders a comprehensive treatment of the physical phenomena of grain agglomeration and makes the analysis of the process very complex. As a direct consequence, even if a wide range of amorphous and semi-crystalline thermoplastics have been experimentally tested for the SLS process, commercial applications today are limited to a small number of thermoplastic polymers: mainly polyamide (PA 12 and PA 11), polycarbonate (PC), polystyrene (PS), Polyether-etherketone (PEEK) and variants of those. Polymers are characterised by quite low sintering temperature (<200 °C) and thermal conductivity (below 1 W/m K) so that they can be considered as insulating materials. Polymer powders have a peculiar behaviour with respect to a heat transfer dominated process like SLS and differs from the case of metal based powders which are characterised by thermal conductivity and thermal diffusivity of one or two orders of magnitude higher.

Polymers have been widely used in the selective laser sintering (SLS) process. It is well-known that one of the advantages of the SLS process is to make very complex parts without need of molds as compared with traditional manufacturing methods. Consequently, accuracy is very important for SLS parts, and part accuracy must be preferentially considered in addition to the energy related data when processing parameters are selected. Very few studies are present in the literature considering this connection.

The effects of laser energy density on the relative density, mechanical properties like flexural strength and microscopic morphology, and dimensional accuracy of the SLS parts were studied in Yan et al. (2010). The authors found that SLS specimens made with a quite reduced value of energy density (of the order of 0.08 J/mm2) have relatively higher dimensional accuracy. Therefore, the authors suggested energy density of 0.08 J/mm2 to make green SLS parts.

In Zarringhalam et al. (2009) SLS specimens were produced under different build parameters, in order to vary the amount of energy input, and DSC traces produced for each. DSC results were also compared with optical microscopy images to confirm the findings.

The main purpose of this work is to develop and enhance the knowledge regarding the energetic aspect of SLS of common polymer powders. The first step will represent the analysis of the SLS process that includes productivity and energy aspects and simple process modelling for preliminary parameter estimation. For this purpose a standard version of the process that cannot be referred to any specific industrial version, and without preheating has been considered.

In addition it will point out the thermo-physical properties, such as the thermal conductivity and diffusivity, which represent key parameters in the SLS of polymer powders.

As final step of the research, sintering will be carried out to fabricate single lines on uncured layers of DuraForm™ Polyamide powder in order to assess if the process modelling can be useful to obtain some preliminary information of the energy requirements. This type of powder was chosen firstly because of its common use and secondly for its thermophysical properties which are comparable to many other amorphous (PS, PMMA, PC) or semi-crystalline (PE) thermoplastics.

The energy consumption discussed in this paper is limited to the theoretical optimal process energy which is also just a part of the total consumed energy of a manufacturing process. The rather low efficiency of the laser source as well as the energy consumption of auxiliary energy consumers (e.g. drives, servos, pumps of process gasses, exhaust units, cooling, pc, etc) in productive and non-productive modes, ensure that the necessary practical energy consumption is significantly higher for this type of processes. During an environmental or energy based comparison of different manufacturing processes, it is consequently more relevant to take these practical (complete) consumptions into account.

Section snippets

Selective Laser Sintering of powder material: general elements

The application of SLS is often based on empirical and technological experience especially for polymers whose thermophysical properties are covered by industrial patents. In these cases, one of the main problems concerned with the application of the SLS technology is the selection of optimal parameters. For a given kind of polymer powder, the selection of optimal parameters firstly needs the definition of thermophysical properties like density, specific heat, thermal conductivity, glass

Modelling of the SLS process

SLS is characterised by a deep interaction between heat, mass and momentum transfer along with chemical transitions and variations of the mechanical and thermophysical properties of the materials. According to the optimisation method outlined in Fig. 1, models can be used to estimate preliminarily the process parameters testing range. This indicates the necessity of development of simple phenomenological models.

In this paper the attention is primarily focused on the energetic aspects of the

Experimental methodology

It has been shown how a sintered structure is fabricated by a predefined sequence of layers. These layers are basically constituted of sintered lines flanked one on the other or partially superimposed. Thermal models have been introduced to give an estimate of sintered line cross-sectional profile: the sintered depth determines the allowable layer thickness and is thus correlated with process resolution. The sintered width represents the transversal influence of laser radiation and limits the

Experimental results

In this section the results of the experimental investigation carried out are exposed and briefly discussed. Each testing condition in Fig. 6, Fig. 7, Fig. 8, Fig. 9 is represented as the average value obtained in three different measurements, as explained in previous section. Fig. 6 contains the dimension of the lines obtained as a function of Energy Density. From Fig. 6 it can be noticed that width and depth of line scanned increase with the Energy Density. For low values of ED (0.02–0.03 J/mm2

Conclusions

The SLS process has been investigated from an energy standpoint paying particular attention to the correlation between the technological results and the energy input in the process characterized by the Energy Density parameter. The results obtained show that the energetic intensity of the process is quite low because it ranges between 0.2 and 0.6 J/mm3 operating with energy density in the range between 0.015 and 0.3 J/mm2. In particular a range of parameters for a SLS process of a common

References (27)

  • A. Bejan

    Heat Transfer

    (1993)
  • T.H.C. Childs et al.

    Selective laser sintering of a crystalline and a glass-filled crystalline polymer: experiments and simulations

    Proceedings of IMechE, Part B: Journal of Engineering Manufacture

    (2001)
  • T.H.C. Childs et al.

    Selective laser sintering of an amorphous polymer: simulations and experiments

    Proceedings of IMechE, Part B: Journal of Engineering Manufacture

    (1999)
  • Cited by (86)

    • Effect of Zircoat-M coating on SS304L for designing high temperature process chamber in selective laser sintering

      2022, Materials Today: Proceedings
      Citation Excerpt :

      To establish sustainable production technology, they needed to spread their unique working approach, which included quick prototyping techniques. Although SLS has the potential to be an environment friendly alternative to existing methods, only a few writers have focused on process optimization, including energy considerations [13]. McWilliams et al (1992) worked on improving the quality of parts produced using the SLS method, which is directly dependent on managing heat transfer to the part bed.

    View all citing articles on Scopus
    View full text