SAMPE Journal - November/December 2021 - 66

FEATURE / PRINT ORIENTATIONS
hoppers through flexible tubing via vacuum. Other
high-throughput LFAM material technologies will
share similarly temperature-dependent or timedependent
process constraints and will likewise
benefit from hardware and software advances that
can provide additional process design freedom.
Main Applications
Due to the size, utilities requirements and costbarrier,
LFAM technology is most used in industrial
applications. High throughput rates are achieved
partially by low-resolution deposition which can
result in excess weight compared to traditional
manufacturing techniques. Ideal applications are
primarily industrial tooling, particularly when
risk of engineering change is high and tooling lead
times are critical.
Architectural facades and other stylistic or
artistic applications have also been found as many
structural and dimensional requirements are often
relaxed.
EXPERIMENTATION
Two critical process parameters exist that must
be addressed during the engineering of an
LFAM process. Through experimental study and
industrial trials, physical limitations are observed
and used to produce manufacturing engineering
design guidelines.
Previous Layer Temperature
Controlling for the previous layer temperature
(PLT) is the most important consideration for
LFAM process planning. An upper and lower
PLT threshold exists and defines an acceptable
temperature range. The rate of additionally
concurrently
continuous
cooling
to
ambient
temperature imposes a time-dependent constraint
on
this processing window. Heated build
volumes are impractical with LFAM systems after
considering energy efficiency, equipment wear and
various safety and access issues.
The PLT is influenced by many aspects of
the part geometry as well as the material choice.
Contributors include wall thickness, polymer melt
temperature, material heat capacity and influence
from additives or fiber fill type.
The upper threshold is defined by the
temperature at which an in-process print operation
remains so pliable or viscous that it fails to be selfsupporting
resulting in slump. The lower threshold
is defined by the PLT that unacceptably weakens
the previous layer polymeric bond strength. This
can result in degraded part tensile strength or more
catastrophic fissures or interlayer delamination
66 | SAMPE JOURNAL | NOVEMBER/DECEMBER 2021
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Temperature
Group
Room Temperature
Heat Assisted
Elevated Cure
Processing
Range (ºC)
20 - 40
40 - 140
140 - 210
Tooling Application
Assembly Fixtures, Trim Tooling,
Concrete Formwork, Transportation
Fixtures, Inspection Fixtures
OOA/OOO Layup, CFRP Tooling
Patterns, Fiberglass, Adhesive Bonding,
Thermoforming
Automated Composite Layup Tooling,
Hand Layup Cure Tools, Carbon/Epoxy
Composites.
Table 1. Temperature use application categories for LFAM industrial tooling.
due to internal stresses that develop as the
printed body differentially approaches ambient
temperature.
With a material and process sufficiently
characterized, a PLT between the upper and lower
thresholds is targeted. This temperature target is
used to derive a minimum layer time which in-turn
drives the deposition head velocity (programmed
feedrate) in direct proportion to individual layer
path length.
Geometry Overhang Limit
The geometry overhang angle limit produces
another common LFAM process restriction. The
ability to print any double curvature surface relies
on the ability to deposit layers at some degree of
offset without direct vertical stacking. Overhanging
features can be found in any printed object that is
not simply extruded along a fixed vector normal to
the build plate.
Known colloquially as a '45-Degree Rule' the
contributing factors to determining this acceptable
limit are complex. The PLT and the fully dynamic
Figure 2. Experimental print cooling data between upper and lower
processing thresholds for Previous Layer Temperature.
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