SAMPE Journal - November/December 2021 - 80

FEATURE / LARGE SCALE ADDITIVE MANUFACTURING
DUAL MATERIAL SYSTEM DESIGN
Design Considerations
There are several approaches for designing a
dual material system for large scale AM systems.
However, there are several constraints and
considerations that must be accounted for in a
successful design. The primary considerations can
be summarized as:
* Gantry system weight capacity: The BAAM gantry
weight capacity was set to a limit of weight of 135 kg (300 lb).
This limitation prevents excess inertia during gantry travel. Inertia
concerns stem from rapid directional changes during sharp turns
which could cause the extruder to shift or move under its own
inertia. The standard deposition speed during printing is 0.28
m/second (11inches/second). The weight limitation prevents the
addition of a second extruder to the gantry since the current
extruder weights approximately 90 kg (200 lb).
* Space limitation: Additional capabilities typically occupy
additional space. Since the maximum build volume of the
printer needs to remain the same, the optimal design should
occupy the minimal volume. Therefore, the new device
should either be compact or added above the extrusion
deposition line where build volume is not affected. Adding
a second extruder would require a large space either on the
same gantry or by adding a second gantry system to printer.
* Material switching and purging: Material that dwells
in the extruder for prolonged periods of time can lead to
problems such as polymer degradation and oxidation. As
an example, it has been observed that foam materials tend
to degrade when held at temperature for a prolonged time,
which could lead to decreased mechanical performance and
poor print quality (color changes and expansion decreases).
In the case of adding a second extruder, material would
need to reside in the extruder for the duration of the printing
period of the first extruder. In this case, it would likely be
necessary to purge the entire volume of the extruder in order
to maintain consistent material properties. This would result
in a significant amount of wasted material and excessive
layer times. An ideal system would be capable of switching
materials quickly with minimal dwell time in the extruder.
* Material type and processing conditions: The
compatibility of the feedstock materials being used will
be dictated by their chemical composition & processing
conditions. A dual extruder system would allow for a wide
variety of materials to be printed since they would not
be directly blended or required to flow through the same
extruder. If the materials were combined near the nozzle exit, a
co-axial extrusion system could be used to create a core-shell
structure where material A is extruded as an inner core while
Material B coats the circumferential of Material A15
. Although
this system could deposit two materials at the same time &
switch quickly, the material combinations would be limited to
those with compatible chemistries that would enable bonding.
* Deposition rate: BAAM currently extrudes at 45 kg/hr (100
lb/hr) when depositing carbon fiber reinforced acrylonitrile
butadiene styrene (CF/ABS). In order to maintain high deposition
rates, the dryers & blenders must be able to process materials
at a rate greater than the maximum deposition rate.
Design and Fabrication
In order to successfully develop a dual material
system for the BAAM system, several modifications
have been made. First, in order to comply with the
weight constrains, a dual hopper with a sliding
mechanism was designed, see Figure 1. In this
system, the feedstock materials can be switched
from Material A to Material B at the entrance
of the extruder. This allowed for multi-material
extrusion through a standard single screw extruder
and avoided the need for an additional extruder
(i.e. extra 90 kg). To reduce weight, all tubes and
pivot brackets were fabricated using aluminum,
and high strength steel was used in high stress
locations. A sliding mechanism was attached at
the entrance of the extruder to allow for material
valves to open and close when switching material.
The sliding mechanism consists of three primary
components: the pivot mechanism, the piston, and
maintenance/alignment features (Figure 1).
Switching material during a build involves
pushing or sliding a metal plate to open or close a
feed line as seen in Figure 1. The sliding mechanism
consists of a pneumatic driven piston that controls
a pivot arm that is attached to the feed tubes. The
pivot arm transfers the force from the piston to
slide the feed line between the two hoppers (Figure
1). A thin profile flexible mount piston that requires
0.689 MPa (100 psi) driving pressure to deliver a
force of 547 N (123lb) was used. The piston force
must be sufficient to either move aside or shear
through pellets that may be blocking the flow path
to prevent jamming of the feed tubes. Gravity fed
pellets are desired to be as close to a vertical path
as possible to prevent bridging in the feed tubes.
Small changes in the hopper angle are desired to
minimize bridging in the feed tubes because the
Figure 1. Design for dual material switching mechanism integrated to the
BAAM system; a) CAD drawing showing the dual hopper/feeding tubes,
sliding mechanism, and controlling piton, and b) Cross section at the extruder
entrance showing alignment and dust removal features.
80 | SAMPE JOURNAL | NOVEMBER/DECEMBER 2021
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SAMPE Journal - November/December 2021

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