Thomas Childs

Selective laser sintering of an amorphous polymer—simulations and experiments

Abstract Thermal and powder densification modelling of the selective laser sintering of amorphous polycarbonate is reported. Three strategies have been investigated: analytical, adaptive mesh finite difference and fixed mesh finite element. A comparison between the three and experimental results is used to evaluate their ability reliably to predict the behaviour of the physical process. The finite difference and finite element approaches are the only ones that automatically deal with the non-linearities of the physical process that arise from the variation in the thermal properties of the polymer with density during sintering, but the analytical model has some value, provided appropriate mean values are used for thermal properties. Analysis shows that the densification and linear accuracies due to sintering are most sensitive to changes in the activation energy and heat capacity of the polymer, with a second level of sensitivities that includes powder bed density and powder layer thickness. Simulations of the manufacture of hollow cylinders and T-pieces show feature distortions due to excessive depth of sintering at downward facing surfaces in the powder bed. In addition to supporting the modelling, the experiments draw attention to the importance of sintering machine hardware and software controls.

Density prediction of crystalline polymer sintered parts at various powder bed temperatures

The effect of powder bed temperature setting on the prediction of density of sintered parts produced by the selective laser sintering (SLS) process is reported. A crystalline polymer, nylon‐12 – commercially named Duraform polyamide – has been used in this work. To study the effect of the powder bed temperature, a two‐dimensional model of the sintering process for crystalline polymers has been developed. Latent heat has been considered in the model. Three powder bed temperature settings, 174, 178 and 182○C, have been applied to study their effect on the sintered parts’ density and size accuracy. This paper only reports on density. Results show that at a powder bed temperature of 182○C, a fully solid density, 970kg/m3, can be obtained at a default energy density of 0.0284J/mm2. By reducing powder bed temperature to 178○C, at the same energy density, density of a sintered part decreases by about 4 per cent.

The selective laser sintering of polycarbonate

Abstract This paper investigates thermal modelling of the selective laser sintering process for amorphous polycarbonate powders. The aim is to develop a simulation for process accuracy and control which are key areas of developement for the new layer manufacturing rapid prototyping technologies. A state-of-the-art adaptive mesh 2D finite difference code is used simultaneously to consider heating and sintering and its results compared with a classical moving heat source model and with experiments. The analysis shows that the change of material thermal properties with temperature and particularly with position as densification takes place must be included for accurate prediction of both densification and of the phenomenon known as ‘bonus z’. The work forms a basis for moving to a 3D simulation.

A Note on Interpreting Tool Temperature Measurements from Thermography

Thermography (thermal imaging) is a well-established experimental method for studying cutting tool temperature distributions. In one form, cutting edge temperatures within the chip / tool contact area are deduced from thermal images of tool faces normal to the cutting edge but offset from the contact region. In general practice, the offset is made as small as possible (<< 1 mm) and it is assumed that the observed temperature is the same as that within the contact. In this short communication an approximate analytical model is developed for the influence of the offset on the observed temperature. The predictions from the model are compared with previously unpublished existing results on the machining of Ti alloys (Ti6Al4V and Ti5Al4V) and on steel (AISI 4140). It is shown that ignoring the offset may introduce underestimates of cutting edge temperature of ≈ 30% or more. This is large compared to the usually considered uncertainties of ± 5% from camera and tool emissivity calibration. There is a need for a dedicated study of this effect.

SIMULATIONS AND EXPERIMENTS ON MACHINING CARBON AND LOW ALLOY STEELS AT RAKE FACE TEMPERATURES UP TO 1200°C

Finite element simulations are reported of the machining of carbon and low alloy steels at speeds from 50 to 300 m/min and feeds from 0.125 to 0.3 mm/rev. Their mechanical models include yield delay (resulting in an upper yield point followed by a yield drop). Predictions are compared with experimentally measured cutting and thrust forces, shear plane angles and rake face temperatures (from tool-work thermocouple measurements) in these conditions. Agreements are generally within 10% for all quantities, except that some uncertainty arises from comparing the semi-orthogonal experimental with the plane strain simulation data.

Shelling strategies to save time in a rapid tooling process

Selective laser sintering can be used to manufacture injection mould inserts using an indirect metal laser sintering process, such as the RapidTool™ process commercialised by 3D Systems. The volume of material to be laser processed for insert manufacturing is very high when compared to that for plastic prototype manufacturing. Consequently, the time involved in the laser processing is also very long. This paper describes the development and assessment of shelling strategies to be applied in an indirect rapid tooling process aimed at reducing time in the process. The feasibility of the shelling idea has been confirmed and although the scanning system offers some limitations to the idea two strategies are presented as successful, open shell and closed shell, with a great potential to save time.