**1. Introduction**

Sheet metal forming can be used to create a variety of complicated three-dimensional parts from flat sheet metal by, e.g., bending and deep drawing. Some of the advantages of sheet metal forming are the beneficial mechanical properties of the formed part, the minimal amount of scrap that is generated and the high production capacity that can be attained. The main disadvantage of the method is the cost and time necessary for setting up a new process line. To ensure economic production, sheet metal forming is therefore often limited to mass production [1].

One of the major costs related to sheet metal forming is the development and production of the forming tools. Conventionally, the tool is designed by a tool designer and machined out of metal, which can take a long time and be quite costly. During tool development, a tool may then go through one or more revisions before reaching its final form. Each revision would necessitate a new round of machining or other processing steps each time. This quickly adds up to massive costs, both money and time-wise.

Additive manufacturing has gained popularity as a method of rapid prototyping (RP) tool concepts. These prototypes can then serve as either a proof-of-concept or as tools for small series production [2]. Adopting this method reduces the time and money spent on the tool, and allows for increased flexibility in sheet metal forming [1]. Zaragosa et al. [3] found that implementing an RP solution is more expensive than fabricating a single metal tool due to the initial costs of purchasing equipment and configuring it. However, the cost of printing further tools becomes much smaller than the cost of machining metal tools. It is unlikely that a first tool design is perfect due to the increasing complexity of parts and new sheet materials, which, as noted by Durgun [4], makes the experience of conventional

**Citation:** Tondini, F.; Basso, A.; Arinbjarnar, U.; Nielsen, C.V. The Performance of 3D Printed Polymer Tools in Sheet Metal Forming. *Metals* **2021**, *11*, 1256. https://doi.org/ 10.3390/met11081256

Academic Editor: Golden Kumar

Received: 30 June 2021 Accepted: 7 August 2021 Published: 9 August 2021

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**Copyright:** © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

tool makers insufficient. Therefore, inevitable revisions and iterations on the tool design become cheaper with 3D printing of tools.

Various researchers have investigated the suitability of using 3D-printed tools for sheet forming. Zaragosa et al. [3] studied the use of 3D-printed tool inserts in a metal frame for use in V-bending and found the spring-back to be similar to using fully metal tools. Nakamura et al. [5] used fully plastic tools for V-bending and found that the accuracy of the final bend angle is reduced compared to metal tools due to the increased elastic deflection, nevertheless, the spring-back was repeatable for each material. They further noted that the spring-back remained mostly constant for the plastic tools over 100 bending operations. Nakamura et al. [5] also found, and Zaragosa et al. [3] later confirmed, that using plastic tools results in a better surface finish on the formed part compared to using metal tools. This is because the workpiece is unlikely to be scratched by the plastic tools. Klimyuk et al. [1] optimized their printing strategy and managed to deep draw a cup, noting that no wear was visible on the surface of the printed tools. Schuh et al. [6], on the other hand, after optimization of the printing strategy using a Design of Experiments approach, managed to deep draw a cup but noted that most of the wear occurs during the first operation. Aksenov and Kononov [7] fabricated heat exchanger plates using printed tools and noted that lubrication was not necessary, as the plastic had anti-friction properties. In all the above references, there is agreement that 3D printing of forming tools is suitable for rapid prototyping and small series production.

V-bending is one of the simplest sheet forming operations. It involves a V-shaped punch that presses a sheet into a die, forcing the sheet to bend to a predefined angle. The V-shape is typically designed to account for spring-back, and so that the radius of the corner of the bend is within the tolerance of part design. The ability to bend the sheet close to the nominal angle and dimension depends on the tool, meaning that the spring-back that occurs and the accuracy of the formed radius can be used as an indicator of tool performance [3].

In this work, the performance of 3D-printed tools is investigated under sheet metal forming conditions. After optimizing the printing strategy through V-bending tests and surface characterization, the optimized printing strategy is used to fabricate tools for a groove pressing process. This process is used to form a geometry that is more complex than the basic V-bending geometry to demonstrate the types of geometries that it is possible to form using polymer tools. Further, it serves as another way of evaluating the accuracy of formed parts when using polymer tools, bringing this method closer to an industrial application.
