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BAAM Design Guidelines
What is BAAM?
Big Area Additive Manufacturing (BAAM) is an industrial scale, additive manufacturing system designed for production and manufacturing environments. Additive manufacturing, commonly called 3-D printing, is the process of creating a three dimensional part from a virtual model. A digital model is imported into a “slicing” software as a STL file. The model is then separated or sliced into layers, which creates g-code to define the toolpath.
BAAM currently has three different nozzle sizes: 0.01 inch, 0.20 inch, and 0.30 inch. The 0.20 inch nozzle is typically used in applications where a smoother surface finish is needed, and the 0.30 is used when a higher mass flow rate is needed. The nozzle size determines the “bead size,” which ultimately defines the minimum width of your part. The 0.20 and the 0.30 inch nozzle has a bead width of 0.22 inches and 0.34 inches, respectively. When designing the width of a part, the designer should make the part’s width a multiple of bead width. For example, if a part required a 0.50 inch wall thickness, the designer can make the 3-D printed wall thickness either 0.44 inches with two beads from the 0.20 nozzle, or he or she can make the wall 0.68 inches with two beads from the 0.30 nozzle. The layer height also varies based on the bead used. The 0.20 inch nozzle has a layer height of 0.10 inches, and the 0.30 inch nozzle has a layer height of 0.16 inches.
Because 3-D printing builds parts by depositing a layer of material on top of a previous layer, “gaps” and “overhangs” have to be supported by a previous layer of material. Figure 1 shows an example of a gap.
The archway illustrated in the figure shows two columns with a space, or gap, between them. Depending on the size of this gap, the bead could need a support structure to keep the bead from sagging or falling off when spanning this void. If the distance between the two columns is less than 1.50 inches, then the extruded bead will be able to span the gap without additional support. If the distance is greater than 1.50 inches, than an additional support structure is needed. Support structures on BAAM are made with the same material as the part. However, a layer of powder is applied where the support structure meets the part. This powder keeps the two layers from bonding together.
Overhangs occur when one layer protrudes further than the previous layer. Like gaps, overhangs can cause the printed bead to sag if the feature is too extreme. The angle between the ground and the final layer should be 60 degrees or higher, as shown in the following figure. Like gaps, support structures can be added when the angle is less than 60 degrees.
Insets, Infills, and Skins
Additive Manufacturing has the ability to add as much or as little structural rigidity to a part as needed. Parts are given a number of “insets,” or perimeter beads. The space between these insets is filled with an “infill.” The slicing software for BAAM allows the designer choose the number of inset beads, the infill pattern, and the percentage of infill. Therefore, when designing a part, the operator does not have to include the infill pattern in the 3-D model. A typical part has a 2 bead inset with a 75% grid infill. Figure 4 shows example of different infill patterns.
Infill patterns help reduce the weight of a part while maintaining structural rigidity by sparsely filling the shape. In some applications, like tools and molds, you may want a solid layer for the bottom and top surface instead of a sparse pattern. A “skin” is a slicing software setting that is used to apply any number of solid layers to top and/or bottom surfaces of a part.
Some applications with thin walls require a part to have a 1-bead inset. When designing a part with a 1-bead wall, design the part as a solid objective and set perimeters to 1 bead in the slicing software
High Temperature Polymers
Special blends of Polyphenylsulfide (PPS), Polyphenylsulfone (PPSU), and Polyetherimide (Ultem) were developed on BAAM to make tooling and molds for autoclave environments of 350ºF and 100 psi. These high temperature polymers are currently restricted to a mid-scale geometry of approximately 6ft of length x 4ft of width x 6ft of height. This equates to a layer time of 5 minutes or less. Layer times that exceed 5 minutes will cause poor layer-to-layer bonding, which could result in poorer mechanical properties and crack formations between layers.
High temperature polymers must also be processed with a nitrogen or argon purge flowing into the extruder. The inert gas prevents the polymer undergoing thermal oxidation during extrusion.
The beginning and ending of a layer should be placed on a sacrificial section or an area that is not the working surface on a mold. The starts and stops of a layer tend to have a rougher texture, which could be undesirable for a final part.
Coefficient of thermal expansion (CTE)
- ABS with 13% CF
- Parallel to Extrusion 9.85 µm/m⁰C
- Perpendicular to Extrusion 119.8 µm/m⁰C
- ABS ………. 75 μm/m°C
- PC ………… 70 μm/m°C
- Al ………….. 22 μm/m°C
- Invar ……… 1.5 μm/m°C
- Pyrex ………. 4 μm/m°C
- Graphite ….. 2-6μm/m°C
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