Continuous 3D Printing of Hierarchically Structured Microfoamed Objects

Abstract

We report the design and results of a novel process combining 3D printing and foaming to produce microfoamed polymeric structures, from simple strands to more complex architectures, using physical blowing agents. Foaming processes are extensively operated in polymeric cellular materials industry to produce pores, yet without spatial control of their positioning. This intrinsic stochasticity may introduce imperfections, which reduce the mechanical properties of the material, thus regular (e.g., periodic) porous structures would be more desirable. 3D printing allows to fabricate polymeric cellular materials with empty spaces in a well‐defined periodic structure. To this end, very expensive 3D printers are required to achieve micron‐resolution pores. Correspondingly, the production time is dramatically large and becomes a bottleneck to the industrial scale‐up. Herein, an innovative technique combining the simplicity of polymer foaming with the precision of 3D printing is presented. The resulting materials have the advantages of both the techniques: they have a micron‐controlled cell structure and can be printed at reasonable costs and time. The proposed approach is validated using a bio‐based and compostable polymer, namely, polylactic acid (PLA). The resulting foamed strands and hierarchical structures are novel in terms of morphology and show a controlled local porosity and superior mechanical properties.

Expanded Beads of High Melt Strength Polypropylene Moldable at Low Steam Pressure by Foam Extrusion

“…The observation of the exiting strands supplies important insights into the non-linear rheological behavior (e.g., die swell) that are directly linked to the extrusion of the material through the die plate for bead production. In particular, the observed die swell is used to calculate the first normal stresses difference, N 1 , (shown in Figure 2 b), using the empirical equations [ 25 , 26 , 27 ]: where are the shear stresses at the wall and the die swell is defined as the ratio between the diameter of the polymer strand and the capillary diameter. It is worth noting that the swell measurements, B , can be considered independent of the geometry because the capillary design is characterized by Dr/D < 20 and L/D = 21 (where Dr is the diameter of the cylindrical reservoir) [ 27 ].…”

“…In particular, the observed die swell is used to calculate the first normal stresses difference, N 1 , (shown in Figure 2 b), using the empirical equations [ 25 , 26 , 27 ]: where are the shear stresses at the wall and the die swell is defined as the ratio between the diameter of the polymer strand and the capillary diameter. It is worth noting that the swell measurements, B , can be considered independent of the geometry because the capillary design is characterized by Dr/D < 20 and L/D = 21 (where Dr is the diameter of the cylindrical reservoir) [ 27 ]. Moreover, the direct observation of the polymer flow provides qualitative information on the final foamed beads’ shape and size.…”

An Experimental and Numerical Investigation on Bubble Growth in Polymeric Foams

“…Based on an existing foaming visualization device [ 13 , 14 ], we have designed and realized an experimental apparatus, shown in Figure 2 a, composed of a pressurized vessel for the execution of the gas foaming process and of a system to allow its visualization. Experiments are conducted by using the following procedure: a spherical-shape PCL pellet with a mass of around 10 is gently laid on a substrate in the middle of the vessel, taking care to ensure that the substrate is level; polymeric pellets are saturated with for 4 h at 80 °C and saturation pressure , as shown in Figure 2 b; after saturation, the vessel is cooled to the foaming temperature °C with a controlled, repeatable cooling history; at the foaming temperature, the sample is pressure-quenched to ambient pressure with a pressure drop rate (PDR) in the order of 10 MPa s −1 [ 15 , 16 ].…”

Microfoamed Strands by 3D Foam Printing

“…The characteristic size of the pores, their shape, and their hierarchical organization are important factors in determining the structure–property relationship in these materials. For instance, hierarchical porous structures outperform their non-hierarchical counterparts with respect to mechanical behavior and accessible active surface [ 2 , 3 ]. As a matter of fact, nature has often chosen optimized hierarchical foams to shape life on our planet.…”

“…The current solutions are based on a two-stage approach: in the first step, the structures are printed with inter-strand porosity, then the intra-strand porosity is produced by freeze-drying or batch-foaming [ 12 ]. Recently, a solution has been proposed where inter- and intra-strand porosities are produced in one step by means of an inline or discontinuous solubilization of a physical blowing agent [ 2 ]. However, those approaches do not enable the fine control of the foamed-strand morphologies and limitations exist in designing innovative morphologies [ 13 ].…”