A novel experimental setup for in situ optical and X-ray imaging of laser sintering of polymer particles

We present a unique laser sintering setup that allows real time studies of the structural evolution during laser sintering of polymer particles. The device incorporates the main features of classical selective laser sintering machines for 3D printing of polymers and at the same time allows in situ visualization of the sintering dynamics with optical microscopy as well as X-ray scattering. A main feature of the setup is the fact that it provides local access to one particle-particle bridge during sintering. In addition, due to the small scale of the device and the specific laser arrangement process, parameters such as the temperature, laser energy, laser pulse duration, and spot size can be precisely controlled. The sample chamber provides heating up to 360 °C, which allows for sintering of commodity as well as high performance polymers. The latter parameters are controlled by the use of a visible light laser combined with an acousto-optic modulator for pulsing, which allows small and precise spot sizes and pulse times and pulse energies as low as 500 μs and 17 μJ. The macrostructural evolution of the particle bridge during sintering is followed via optical imaging at high speed and resolution. Placing the setup in high flux synchrotron radiation with a fast detector simultaneously allows in situ time-resolved X-ray characterizations. To demonstrate the capabilities of the device, we studied the laser sintering of two spherical PA12 particles. The setup provides crucial real-time information concerning the sintering dynamics as well as crystallization kinetics, which was not accessible up to now.

Laser sintering of polymer particle pairs studied by in situ visualization

Merging of particle pairs during selective laser sintering (SLS) of polymers is vital in defining the final part properties. Depending on the sintering conditions, polymers can undergo full or partial sintering whereby incomplete sintering results in poor mechanical properties. At present, the underlying mechanisms and related conditions leading to various consolidation phenomena of polymer particles are not well understood. In the present work, a novel in-house developed experimental setup is used to perform laser sintering experiments on polystyrene (PS) particle doublets while performing in situ visualization of the sintering dynamics. From the recorded images, the evolution of the growth of the neck radius formed between both particles is analyzed as a function of time. Sintering conditions such as heating chamber temperature, laser pulse energy and duration, laser spot size and particle size are precisely controlled and systematically varied. A non-isothermal viscous sintering model is developed that allows qualitative prediction of the observed effects of the various parameters. It is shown that the sintering kinetics is determined by a complex interplay between the transient rheology caused by the finite relaxation times of the polymer and the time-dependent temperature profile which also affects the polymer viscosity. The combination of a full material characterization with sintering experiments under well-defined conditions has resulted in a general understanding of the effects of material and process parameters on laser sintering. Thereby a strong foundation is laid for the route towards rational design of laser sintering.

Scaling behavior in the convection-driven Brazil nut effect

The Brazil nut effect is the phenomenon in which a large intruder particle immersed in a vertically shaken bed of smaller particles rises to the top, even when it is much denser. The usual practice while describing these experiments has been to use the dimensionless acceleration Γ = aω(2)/g, where a and ω are, respectively, the amplitude and the angular frequency of vibration and g is the acceleration due to gravity. Considering a vibrated quasi-two-dimensional bed of mustard seeds, we show here that the peak-to-peak velocity of shaking v = aω, rather than Γ, is the relevant parameter in the regime where boundary-driven granular convection is the main driving mechanism. We find that the rise time τ of an intruder is described by the scaling law τ ~ (v-v(c))(-α), where v(c) is identified as the critical vibration velocity for the onset of convective motion of the mustard seeds. This scaling form holds over a wide range of (a,ω), diameter, and density of the intruder.