Refining Post-Processing Surfaces for 3D Printed TPU

January 2, 2020 by Bridget O'Neal 3D Printing 3D Printing Materials

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In the recently published ‘Investigations on post processing of 3D printed thermoplastic polyurethane (TPU) surface,’ Abdelmadjid Boualleg has written a Master’s thesis for the Mechanical Engineering Department at Halmstad University, exploring improved use of materials in additive manufacturing.

With a focus on FDM 3D printing, Boualleg is mindful of the challenges in creating strong parts with good surface quality, investigating different methods and materials and experimenting with parameter settings and post-processing methods.

Surface quality is a topic of concern for 3D printing users today, no matter what their area of interest is—whether for industrial production, in the lab, or on the DIY maker/user level. Boualleg specifically considers laser ablation in post processing as a viable solution for refining surfaces of parts made with Thermoplastic Polyurethane (TPU). A range of settings are studied with laser ablation, including power, speed, and pulse per inch—as well as considering problems in FDM 3D printing like layering issues such as the stair-casing effect.

A. Stair case effect, B. Raster pattern

“Thermoplastic Polyurethane (TPU) is a very common form of elastic polymer, or elastomer, capable of being used on any properly equipped FDM 3D printer. TPU is made of a linear segmented block copolymer with mixture of soft and hard segments, 7 making it easier to extrude and use. It is a medium-strength material with very high flexibility and durability, able to withstand much higher compressive and tensile forces than its more common counter parts PLA and ABS,” states Boualleg.

The FlashForge Dreamer was used for this study, allowing for the original printhead to be replaced by a flexion extruder2 for use with test samples for TPU-70A. The Trotec Speedy 400 3 laser machine was used also.

“With the interaction of the laser beam with the material, an amount of laser energy is absorbed by the material. This heat energy is enough to cause melting of the surface peaks,” stated the researchers. “The molten mass then flows into the surface valleys and pores by surface tension, gravity and laser pressure, thus diminishing the roughness. Therefore, the speed and the effective power are the most important parameters on both surface roughness and material removal rate.”

With the addition of 3D printing, less material is used—meaning less waste, along with reduced cycle time in production and greater affordability too.

Principle of FDM process

Like both plastic and rubber, TPU offers both physical and chemical property combinations for applications like auto, electrical, textile coatings, and more. TPU film is usually divided into chemical, polyester, polyether, and polycaprolactone.

Present classification of TPE

Laser technology has also been used in post processing for over 30 years, specifically in modifying material surfaces like ceramic, plastic, adhesives, and a variety of metals. For this study, Boualleg experimented with a range of surfaces printed at angles of 0°, 45°, 90° and 135°. The outcome demonstrated that these build angles were suitable for melting material at peaks.

The effects of the laser energy interacting with material

“Implementation of laser ablation technique has provided a significant improvement in surface roughness of parts manufactured by FDM,” concluded Boualleg.
“The post processing by laser ablation is an effective technique for reducing surface roughness. The fast-moving laser beam provides just enough heat energy according to the given parameters (power and speed) to cause melting of the surface peaks. The molten mass then flows into the surface valleys by surface tension, gravity and laser pressure, thus minimizing the roughness.”