Laser Beam Polishing of Polymers

Laser beam polished surfaces of injection-molded (left) and FLM-printed (right) PLA specimen (Source: EAH Jena)

Fused layer manufacturing (FLM) is an additive manufacturing process in which parts are built in layers by localized extrusion of molten polymer through a nozzle. It offers a high degree of geometrical flexibility as well as a wide range of suitable materials. Unfortunately, a characteristically high surface roughness is limiting the number of possible applications.

Nowadays, FLM systems are able to process almost every type of thermoplastic polymer. While ABS and PLA remain the most popular material in FLM printing, research has been conducted on PA6, PET, PETG, PMMA, PVA, PC, PE and PP as well as support materials, thermoplastic elastomers, polymer blends and composites. Therefore, one limiting factor regarding industrial applications of FLM printed parts cannot be found in the availability of materials than rather in achievable surface quality. A typical layer thickness of 0.3 mm leads to a roughness of approximately 15 µm Rq (rms roughness). Conventional post-processing methods for surface smoothening include bead blasting, coating and etching. Besides those, optimization of print parameters such as temperature, build-direction or layer thickness may lead to an improved surface quality as well. Nevertheless, such methods are often very time-consuming or result in a significantly reduced dimensional accuracy of the part.

Laser beam polishing principle (top), material accumulation as result of laser-polymer interaction at low intensities (bottom; click to enlarge; source: EAH Jena / Wiley)

An alternative approach of post-processing deals with laser beam polishing of printed parts. The process is based on partially melting the surface with a laser beam. Due to this kind of energy input, material flow is enabled and the subsequent solidification leads to a reduced roughness. Unlike many conventional methods, this approach allows geometry-independent treatment and low processing times without additional resources. The final surface is free of any residual polishing agents and typically defined by a very low micro roughness. Results are largely dependent on achieved temperatures, interaction times, material melt properties and the initial surface structure.

Currently, a huge variety of laser systems is applied for the laser beam processing of different kinds of materials. Laser sources provide certain wavelengths and are therefore specifically chosen for certain applications. The wavelength-dependent range of absorption of polymers is very different. The aim of the depicted process deals with laser beam polishing techniques applied to standard polymers without any tailored additives.

Since there is a huge variety of filaments for FLM available already, not only common polymers (ABS and PLA) but also materials for special applications (e.g. PA6, PC, PMMA) were investigated. A preliminary experiment was designed to study the interactions between laser irradiation and injection molded polymer samples. Experiments were conducted with focused and defocused laser beam and showed that maximum values of feed rate and energy density are limited by material decomposition and the incipient formation of blisters. Best surfaces were achieved at the lowest melt viscosity before the material begins to evaporate. Roughness measurements confirm the greatly improved visual appearance and reveal a decrease in roughness of up to 99.96 %. Although the obtained Rq does not consider uniformity and dimensional stability, it clearly indicates that laser beam polishing of polymers is feasible. Michel Layher

Reference: M. Layher, A. Hopf, L. Eckhardt & J. Bliedtner (EAH Jena): Laser Beam Polishing of Polymers: Selection of polymers suitable for laser‐based post processing of FLM‐printed parts, PhotonicsViews 16(3), 83–87 (June/July 2019); DOI: 10.1002/phvs.201900025

Link: Laser – optics – 3D printing (Bliedtner lab), Ernst-Abbe-Hochschule Jena, Jena, Germany

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