Citation: Waalkes, L.; Längerich, J.;
Imgrund, P.; Emmelmann, C.
Piston-Based Material Extrusion of
Ti-6Al-4V Feedstock for
Complementary Use in Metal
Injection Molding. Materials 2022, 15,
351. https://doi.org/10.3390/
ma15010351
Academic Editor: Jun Liu
Received: 29 November 2021
Accepted: 31 December 2021
Published: 4 January 2022
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Article
Piston-Based Material Extrusion of Ti-6Al-4V Feedstock
for Complementary Use in Metal Injection Molding
Lennart Waalkes
1,
*, Jan Längerich
1
, Philipp Imgrund
1
and Claus Emmelmann
2
1
Fraunhofer Research Institution for Additive Manufacturing Technologies IAPT, Am Schleusengraben 14,
21029 Hamburg, Germany; jan.laengerich@iapt.fraunhofer.de (J.L.); philipp.imgrund@iapt.fraunhofer.de (P.I.)
2
Institute of Laser and System Technologies iLAS, Hamburg University of Technology TUHH, Denickestr. 17,
21073 Hamburg, Germany; c.emmelmann@tuhh.de
* Correspondence: lennart.waalkes@iapt.fraunhofer.de
Abstract:
Piston-based material extrusion enables cost savings for metal injection molding users
when it is utilized as a complementary shaping process for green parts in small batch sizes. This,
however, requires the use of series feedstock and the production of sufficiently dense green parts
in order to ensure metal injection molding-like material properties. In this paper, a methodological
approach is presented to identify material-specific process parameters for an industrially used Ti-
6Al-4V metal injection molding feedstock based on the extrusion force. It was found that for an
optimum extrusion temperature of 95
◦
C and printing speed of 8 mm/s an extrusion force of 1300
N ensures high-density green parts without under-extrusion. The resulting sintered part properties
exhibit values comparable to metal injection molding in terms of part density (max. 99.1%) and
tensile properties (max. yield strength: 933 MPa, max. ultimate tensile strength: 1000 MPa, max.
elongation at break: 18.5%) depending on the selected build orientation. Thus, a complementary use
could be demonstrated in principle for the Ti-6Al-4V feedstock.
Keywords:
additive manufacturing; material extrusion; Ti-6Al-4V; feedstock; metal injection molding;
green parts
1. Introduction
Metal injection molding (MIM) is a production technology that is primarily suitable
for high production volumes since molds are required that only amortize with increasing
quantities [
1
,
2
]. During injection molding, a feedstock is injected into the mold to form a
so-called green part, in which a metal powder (solids loading between 50 and 67 vol% [
3
]) is
bound within a polymer matrix [
4
]. After molding the green part, the polymer components
are successively removed in a debinding step, followed by a final heat treatment to sinter the
remaining metal powder into a nearly full dense metal part [
1
]. The additive manufacturing
(AM) of green parts can thus lead to time and cost savings in metal injection molding when
it comes to functional prototypes, custom-made or complex parts with hollow structures
since no molds are required [
5
]. For this field of application, piston-based material extrusion
(PEX) was introduced as a new complementary AM process for MIM users by combining
the main advantages of the polymer-based AM processes fused filament fabrication (FFF)
and fused granular fabrication (FGF) [6].
Due to its ease of operation and low machine costs, FFF is already well studied for
printing green parts from powder-binder formulations similar to MIM feedstock [
7
–
12
].
However, typical MIM feedstock formulations must be adapted to filament requirements
such as a sufficient flexibility for spooling by adding, for instance, elastomers [
13
] or
amorphous polyolefins [
14
]. To keep changes to debinding and sintering as low as possible,
the use of highly filled filaments is thus not preferable for the intended complementary
green part production [
6
]. Screw-based extrusion, on the other hand, is suitable for this
purpose, as it allows conventional MIM feedstock to be processed [
5
,
15
–
18
]. Yet, machine
Materials 2022, 15, 351. https://doi.org/10.3390/ma15010351 https://www.mdpi.com/journal/materials
