Neil Hopkinson

Top surface and side roughness of Inconel 625 parts processed using selective laser melting

Purpose – Obtaining the required part top surface roughness and side roughness is critical in some applications. Each of these part properties can often be improved to the detriment of the other during selective laser melting (SLM). The purpose of this paper is to investigate the selective laser melting of Inconel 625 using an Nd:YAG pulsed laser to produce thin wall parts with an emphasis on attaining parts with minimum top surface and side surface roughness.Design/methodology/approach – A full factorial approach was used to vary process parameters and identify a usable Inconel 625 processing region. The effects laser process parameters had on the formation of part surface roughness for multi‐layer parts were examined. Processing parameters that specifically affected top surface and side roughness were identified.Findings – Higher peak powers tended to reduce top surface roughness and reduce side roughness as recoil pressures flatten out the melt pool and reduce balling formation by increasing wettabilit...

Investigating mechanical anisotropy and end-of-vector effect in laser-sintered nylon parts

Abstract A study investigating the effects of part-build orientation in the laser sintering process is presented. The investigation uses tensile, flexural, and compression testing methods to assess the changes in the mechanical properties of laser-sintered nylon-12 parts. The test parts were built in the x, y, and z orientations with the x axis parallel to the direction of the laser scanning, the y axis perpendicular to the direction laser of scanning, and the z axis in the direction of powder layers. The results from the tests show that the build orientation of the parts has an effect on the mechanical properties produced. The tensile tests show a maximum difference of 16 per cent and 11.2 per cent in strength and modulus respectively for parts built in the x, y, and z axes. The flexural tests show a 9.4 per cent and 7 per cent maximum difference in strength and modulus respectively for the parts produced in the x, y, and z axes. For the compressive tests, there is a 3.4 per cent and 13.4 per cent maximum difference in strength and modulus respectively for the parts produced in the x, y, and z axes. A statistical analysis of the results obtained highlights the presence of anisotropy in tensile and compression parts owing to their build orientation in the laser sintering machine. The test parts built in the x axis orientation showed the highest strength and modulus values while the parts built in the z axis orientation showed poor strength and modulus values. However, this is not the case for the flexural test parts, which show the highest strength and modulus values are from those built in the y axis orientation. Analysis has shown that this is due to the end-of-vector effect, which is most prominent in the y axis orientation. This effect should always be considered during laser sintering, when mechanical integrity is vital.

Degree of particle melt in Nylon‐12 selective laser‐sintered parts

Purpose – Selective laser‐sintered (SLS) parts are known to include un‐melted regions, where insufficient energy has been input into the powder to fully melt all particles. Previous research has shown the presence of two distinct peaks on a differential scanning calorimetry (DSC), and the purpose of this paper is to demonstrate that these peaks relate to the melted and un‐melted regions of the part.Design/methodology/approach – 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.Findings – DSC analysis of SLS Nylon‐12 parts has shown the presence of two distinct melt peaks. It has been shown that these correspond to the melted and un‐melted regions of the part, and that the amount of energy input in the SLS process affects the degree of melting. It has also been identified, via correlation between DSC charts and optical microscopy im...

Effects of electroplating on the mechanical properties of stereolithography and laser sintered parts

This paper provides a quantitative and qualitative assessment of the effects of electroplating on polymer parts made by stereolithography (SL) and laser sintering. A series of test samples were coated with copper and nickel with varying thickness. Thicker coatings (120 μm) were reproduced with a repeatability that should not adversely affect the tolerances with which such parts may be produced given the tolerances of the initial rapid prototyping processes themselves. Thinner coatings (20 μm) resulted in a smother surface finish than thicker coatings for SL parts, however the converse was true for laser‐sintered parts. Composite theory was used to predict that thicker coating would lead to higher Youngs modulus in parts and this was shown to be true in physical tests although the practical values were lower than the predicted values especially for thicker coatings. Physical tests also confirmed that thicker coatings increased UTS and impact energy but had a minimal effect on the ductility of parts.

Selective laser melting of Inconel 625 using pulse shaping

Purpose – The purpose of this paper is to investigate the selective laser melting (SLM) of Inconel 625 using pulse shape control to vary the energy distribution within a single laser pulse. It aims to discuss the effectiveness of pulse shaping, including potential benefits for use within SLM.Design/methodology/approach – Laser parameters were varied in order to identify optimal parameters that produced thin wall parts with a low surface roughness without the use of pulse shape control. Pulse shape control was then employed to provide gradual heating or a prolonged cooling effect with a variety of peak power/pulse energy combinations. Properties of pulse shaped and nonpulse shaped parts were compared, with particular attention focused on part surface roughness and width.Findings – High peak powers tended to reduce top surface roughness and reduce side roughness as recoil pressures flatten out the melt pool and inhibit melt pool instabilities from developing. Ramp up energy distribution can reduce the maxim...

Effect of the degree of particle melt on mechanical properties in selective laser-sintered Nylon-12 parts

Selective laser sintering (SLS®, a trademark of 3D systems Inc.) is a manufacturing process which has emerged from numerous other technologies as the leading process considered viable for rapid manufacturing (RM). SLS of polymers has found use in a wide range of industries ranging from aerospace to medicine. The ability to manufacture easily parts that previously have been difficult or impossible to produce, without tooling, has proved invaluable for many applications. A major area of focus within RM is the requirement to produce parts with more repeatable mechanical properties than can currently be achieved. This research has investigated the use of a novel method of interpreting a differential scanning calorimetry curve to indicate the level of melting within semicrystalline selective laser-sintered parts, or the degree of particle melt (DPM). The DPM has been shown firstly to be affected by the amount of energy input into the process whereby, as the energy input increases, the DPM also increases. Results have also shown that, as the DPM increases, the tensile strength and elongation at break also increase, whilst there is no significant effect on the Youngs modulus. These findings will enhance the ability to optimize and predict the properties of the SLS process, an area which is critical when producing end-use parts, particularly when considering demanding applications such as in the aerospace and automotive industries.

Effect of section thickness and build orientation on tensile properties and material characteristics of laser sintered nylon‐12 parts

Purpose – The purpose of this paper is to describe work carried out as part of a £350,000 project aimed at improving understanding of polymer sintering processes. This particular package of research was performed in order to identify the effects of different section thicknesses (and therefore different thermal conditions) in parts produced by laser sintering (LS), on the resultant mechanical properties of these parts.Design/methodology/approach – Laser sintered nylon‐12 parts were produced in a range of thicknesses between 2 and 6 mm, and in three different orientations, to identify the effects of each on the tensile properties of these parts.Findings – Results indicated that, at any of the orientations tested, the section thickness had no significant effect on any of the main tensile properties, or on the repeatability of these properties. Crucially, this is in direct contradiction with the trends identified previously in this project, whereby changes in section thickness have been shown to correlate wit...

Process repeatability and sources of error in indirect SLS of aluminium

Purpose – The purpose of this paper is to investigate the accuracy and repeatability of the indirect selective laser sintering of aluminium process.Design/methodology/approach – This work characterised the shrinkage of indirect SLS aluminium parts during the various stages of production. Standard scale parts were measured using a Giddings and Lewis co‐ordinate measuring machine in both the green and infiltrated condition.Findings – The experiments conducted show that most accuracy is lost during the furnace cycle and that the greatest loss of accuracy occurred in the Z dimension. Additionally the position of parts within the part bed in both X, Y and Z is shown to influence accuracy, with smaller parts being built closer to the edge of the bed later in the build. These results have been interpreted as being a result of the phenomenon of “Z‐growth”. Finally, the research shows that the overall accuracy of the indirect selective laser sintering of aluminium process is comparable with many existing processes...

Layer thickness and draft angle selection for stereolithography injection mould tooling

The introduction of rapid prototyping has allowed engineers and designers to generate physical models of required parts very early on in the design and development phase. Further to this, the use of stereolithography (SL) cavities as a rapid tooling method has allowed plastic prototype parts to be produced in their most common production manner -- by injection moulding. The process is best suited to small production runs where the high costs of conventionally machined tooling is prohibitive. One of the major drawbacks of the SL injectionmoulding process is the susceptibility of the tools to premature failure. SL tools may break under the force exerted by part ejection when the friction between a moulding and a core is greater than the tensile strength of the core, resulting in tensile failure. Very few justified recommendations exist about the choice of mould design variables that can lower the part ejection force experienced and reduce the risk of SL tool failure. This research investigates the ejection forces resulting from SL injection moulding tools which are identical in all respects except for their build layer thickness and incorporated draft angles in an attempt to identify appropriate evidence for recommendations with respect to these design variables and SL injection moulding. The results show that adjustment of draft angle results in a change of part ejection force as a reasonably linear relationship. An adjustment of the build layer thickness results in a change in part ejection force as a more non-linear relationship. The adjustment of build layer thickness had a greater effect on ejection force than the adjustment of draft angle. The results also show that the surface roughness of all tools remains unchanged after moulding a number of parts in polypropylene. A mathematical model was used in an attempt to predict ejection forces according to the moulding material used. This model reflected the experimental results in terms of relative values but not in absolute values, which may be due to inappropriate specific values used in their calculation. Finite element analysis (FEA) was used in an attempt to identify the factors involved in the ejection process. Results indicate that the effect of draft angle on ejection force is due to elastic deformation of the surface roughness. A fact borne out by the lack of damage to the surface after ejection.

Predicting stereolithography injection mould tool behaviour using models to predict ejection force and tool strength

The work reported involved Finite Element Analysis (FEA) modelling of heat transfer in a stereolithography (SL) tool and then performing a series of experiments to measure true heat transfer in the tool. The results from the practical measurement of heat transfer were used to validate and modify the FEA model. The results from the modified FEA model were then used to predict the tensile strength of the tool at various stages after injection of the thermoplastic melt. Previously developed equations to predict ejection forces were used to estimate the ejection forces required to push the moulding from the SL core. During the practical experiments the true ejection forces were measured. The combination of predicted tool strength and ejection forces were intended to be used a basis for to determine whether certain SL tool designs will fail under tension during part ejection. This would help designers and manufacturers to decide whether SL tooling is suitable for a specific application. The initial FEA heat transfer model required some modifications and the measured ejection forces were higher than the predicted values, possible reasons for these discrepancies are given. For any given processing conditions there was an inherent variance in the ejection forces required however longer cooling periods prior to ejection resulted in higher ejection forces. The paper concludes that, due to the variations in required ejection forces, a reliable tool to predict tensile failure will be difficult to produce however improved performance may be gained by adopting processing conditions contrary to those recommended in the current process guidelines.