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bioplasticsMAGAZINE_1406

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bioplasticsMAGAZINE_1406

3D printing What is 3D

3D printing What is 3D printing? Challenges for making bioplastics 3D printable Christian Bonten is the Chairholder and the director of the Institut für Kunststofftechnik (Institute for Plastics Engineering) in Stuttgart, Germany, partner in the BioFabNet project (cf. p. 18) , and here he explains the technology: As soon as you need just one of a kind, or a prototype, it is worth using an additive manufacturing process, which does not need a costly mould like for instance injection moulding. There are different kinds of processes (see Fig. 1), that can all be covered by the umbrella term 3D printing. Commonly, all of these additive manufacturing processes use flowable materials or materials in powder form and build up the final products in the form of layers. Here, layer by layer is deposited on, and connected to, the former layers in different ways. The 3D CAD model is converted into a layer model (STL format) and then forwarded to the controller of the additive process machine. The final part is always stepped and its surface is not smooth (Fig. 2). The original 3D printing is just one kind of these additive manufacturing processes. In this original process, a layer of powder is brought onto a platform where a printing head runs over the layer and glues the powder selectively. It works rather similar to ink jet technology. Today, another process, the Fused Deposition Modelling (FDM), is used widely – even in private households – and hence stands synonymously for the additive manufacturing processes in general. In the FDM process, a heated nozzle delivers a melt strand linearly on a platform (Fig. 3). This thermoplastic strand solidifies after cooling and the next melt strand can be laid down on top of it. Solid Liquid Gaseous Filament Fusing / solidifying Solidify by binder Powder Fusing / solidifying Blanking / glue Film Blanking / polymerisation Polymerisation Chemical reaction Process: lay down of a melt strand filament Fused deposition modeling (FDM) 3D- Printing (3DP) Selektive laser sintering (SLS) Laminated object manufacturing (LOM) Solid polymerisation (SFP) Stereolithography (SLA) Laser chemical vapor deposition (LCVD) contact heating Fig. 1: Different 3D printing processes at a glance (source: 3D Printing, Carl Hanser Publishers) 1 2 prototype nozzle linewise application supporting structure base plate 3 4 Fig. 3: Principle of the FDM process (Source: Fig. 5.66 in Kunststofftechnik, Carl Hanser Publishers) Layered construction Fig. 2: Principal cycle of additive manufacturing processes (Source: Fig. 5.61 in Kunststofftechnik, Carl Hanser Publishers) This QR-Code (or the short-link bit.ly/1uiDvXh) connects to a short video-clip on the IKT-Youtube-channel that demonstrates the FDM process 16 bioplastics MAGAZINE [06/14] Vol. 9

3D printing Fig. 4: Filament from a PLA blend (Source: IKT) The melt strand is not produced by extrusion, as is usual in plastics series processes, but out of a mono-filament (Fig. 4), which is melted completely in the FDM nozzle by contact heat. The nozzle-infeed (depending on the different machine producers) usually has a diameter of exactly 3.0 or exactly 1.75 mm, whereas the nozzle outlet is 0.2 to 1,0 mm, depending on the machine. The production pressure is raised by pushing the filament into the heated nozzle. For this purpose, the machine has pressure rolls or wheels (see Fig. 5). Fig. 5: Detail of the printer head of the FDM process (Source: IKT) There are three process steps to produce 3D printed, biobased, plastics parts (Fig. 6). The first step is the compounding step that upgrades biopolymers to processable bioplastics. The second step is the production of printable monofilaments and the third step is the 3D printing process itself. Compounding: To achieve 3D printable bioplastic filaments IKT Engineer Linda Goebel (Fig. 7) has to develop Bio-Blends on one of the twin screw extruders in the compounding technical centre of IKT. Requirements of the material: The chosen material has to be thermoplastic and needs to consolidate quickly. In the solid state, the filament has to be strong enough, to avoid breakage during its transport and the filament´s surface needs a certain roughness, to prevent slipping effects. In the molten state, the viscosity must be high enough to avoid filament rupture, dripping off of melt from the nozzle as well as keeping the upper new layer on top of the layer laid down shortly beforehand. But, viscosity should not be too high, to allow entanglements across the layers´ surfaces and thus a fusion. The re-solidified state of the material must meet the requirements of the later part. Requirements of the filaments: The filament diameter must be perfectly round to allow pushing by means of the rolls and wheels as well as to make sure that the there is enough contact to the inner nozzle wall. If a filament were slightly oval it would probably neither be pushed into the nozzle, nor would it have enough contact for an efficient and fast heat transfer. In addition the filament’s diameter should not pulsate along its length, i.e. the diameter must be precisely the same over the whole length. This is not easy, since the thermoplastic melt produced through a die contains molecular orientations, which will relax after leaving the nozzle. A so-called die swell occurs and will influence the filament´s diameter even after production. Compounding Production of the filaments 3D-printing Fig. 6: Three process steps from the biopolymer to the 3D part (Source: IKT) Fig. 7: Linda Goebel during 3D printing experiments (Source: IKT) www.ikt.uni-stuttgart.de bioplastics MAGAZINE [06/14] Vol. 9 17

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