SAMPE Journal - November/December 2016 - 24

Feature Article
Table 1. CTE for FDM materials and common traditional tooling materials.

Accuracy / Tolerances
FDM is capable of producing parts (tools) with
accuracies of ± 0.09 mm (0.0035 inch) or ± 0.04 mm /
mm (0.0015 inch / inch), whichever is greater. It should
be noted that all accuracies are geometry dependent
(primarily due to the thermal nature of the process).
Additional information on machine accuracy can be
found on the Stratasys website (including a white paper
on the topic). For development of the Design Guide,
accuracy data was compiled for various representative
tool geometries, both before and after thermal cycling -
refer to those sections for additional details.
For composite parts that require greater accuracy than
what can be achieved directly from the FDM machine,
production of "near-net" shape tools, combined with
skim-coat machining is a viable option. Additional
development work is underway on this topic and will be
provided in subsequent versions of the Design Guide.
Process Parameters and Tool Design
Fabrication process and cure cycle parameters,
particularly cure pressure and vacuum bagging method,
impact the design of FDM composite tools and
specifically the "style" of the tool. FDM composite tools
are primarily classified as "shell" style or "sparse build"
(cellular) tools. The basic differences are shown in Figure
2.
Shell style tools are effective for most applications,
can withstand autoclave pressures exceeding 0.7 MPa,
and are conducive to both surface and envelope vacuum
bagging methods. And for many tool geometries, they
are the most cost-efficient design as they minimize
material usage and build time. Sparse build tools tend
to have greater overall rigidity and stability and some
geometries dictate the use of such a construction, as is
demonstrated in more detail in the Design Guide. Sparse
build tools can also be either surface or envelope bagged.
When envelope bagging is used, additional guidelines
regarding construction parameters (e.g. sparse build
spacing) should be followed to avoid damaging the tool.
24

Figure 2. FDM tool construction styles.

Tool Preparation (Post-Processing)
The FDM process inherently produces some level
of internal porosity due to physical limitations of the
extruded material beads, as can be seen in Figure 3,
which shows the cross-section of tool paths (extruded
material) for an example build layer. The process also
produces perceptible build layers, which vary based on
the shape of the part and the layer thickness. As a result,
to ensure high quality surface finish and vacuum integrity,
post-processing of FDM tools is typically required.
In preparation for use, tools are abraded to smooth
out build lines, sealed, and then undergo a final polish,
resulting in surface finishes consistent with typical
industry requirements. Although requirements do vary
somewhat across industries, typically a finish of 1.6
µm (64 µinches), Ra is considered acceptable. Using
a standard process with progressively finer abrasive

Figure 3. Cross-section of FDM tool paths for an example
build layer to illustrate the inherent occurrence of porosity
(from Stratasys Insight software).
SAMPE Journal, Volume 52, No. 6, November/December 2016
Table of Contents for the Digital Edition of SAMPE Journal - November/December 2016
