In previous posts, we discussed the potential for 3D printed tooling to disrupt the composite tooling supply chain, provided examples of 3D printed composite tooling in industry, and walked through some of the tips and tricks to FDM composite tool design. In this post we will discuss post-processing of 3D printed composite tooling produced with FDM technology.
Traditional metal and FRP (fiber reinforced polymer) tooling typically requires post processing after the initial forming or milling processes. Post processing steps include assembly of backing or support structures, addition of inserts/bushings, and often times polishing to achieve the desired surface finish. FDM composite tooling, similar to traditional tooling technologies also typically requires some level of post-processing depending on the size, application, and complexity of the tool. The most common post-processing operation for FDM composite tooling is sealing.
As-built FDM composite tools have an inherent porosity (see Figure 1) and surface finish that is not acceptable for most composite part applications. Therefore a post-processing operation is typically necessary to achieve the desired tool performance.
Addressing Surface Finish:
A variety of methods can be utilized to improve the surface roughness of the FDM tool surface including manual abrasion, media blasting, tumbling, and skim-coat machining. The current best practice to meet surface-finish requirements (<64 µin Ra) and provide vacuum integrity is manual abrasion followed by application of an epoxy sealer, Figure 2. The recommended sanding and sealing process is fully detailed in Appendix B of the FDM for Composite Tooling Design Guide. However, the basic process consists of two thin coats of a high temperature epoxy sealer with a light sanding before each coat to maximize adhesion. After the second coat of epoxy, progressively finer grit sand paper (120-800) is used to achieve the desired surface roughness. It is important to note that the goal of this process is not to sand out the layer lines but rather to fill the low points in the surface and remove any large peaks. The result should be a net zero dimensional change in the surface geometry while providing a smooth surface with vacuum integrity.
As mentioned above, the inherent porosity in an FDM composite tool can be addressed with the application of an epoxy sealer. Although Stratasys typically utilizes a tooling epoxy known as BJB TC-1614, a wide range of epoxy resins can be used provided the selected material can withstand the required curing process.
Additional methods for addressing the need for surface preparation and sealing include the application of adhesive-backed FEP films, such as Tooltec® and Toolwright from Airtech International, Inc. FEP films are often a quicker and easier alternative to the more traditional epoxy sealer. However, they can be difficult to apply to compound contours and they are typically less robust than epoxy sealers, requiring replacement after just a few cycles. The unique properties of FEP films make them well suited to repair tooling and other situations, where only a few parts will be needed.
Similar to conventional tooling, FDM composite tooling typically requires some level of post-processing. Stratasys is continually evaluating alternative post-processing methods and sealers to enhance performance and simplify the finishing process. Skim coat machining is one alternative process that has shown some promise. Stay tuned for the next edition of the FDM composite tooling design guide for more information about post-processing, case studies, technical data, and tips and tricks to ensure success with FDM composite tooling.