Things to Consider in Additive Manufacturing using Continuous Fiber

The ability to purely additively produce complex parts with high strength and low weight, without expensive layup tools or autoclaves, was increasingly sought after. This has created a niche of opportunity for innovations among the machine tool building startups, who are challenging the status quo and aiming for the multi-billion euro rapid large-scale thermoplastic composites market.  

Figure 1. Continuous Fiber Filament Extrusion 2.5D (Source: Markforged)

Reality

Many in the 3D printing industry will argue that large-scale continuous fiber 3D printing already exists in the form of continuous fiber extruders. For example, fused filament fabrication (FFF) 3D printers have a mechanism that introduces the continuous fiber in the molten thermoplastic stream, embedding it in the printed part. 

Other processes use UV-catalyzed thermoset resins, mixing their continuous fiber reinforcement with the resin right at the tool point, and initiating the snap-cure resin with UV radiation in the process.

However, any composites engineer who has worked in the Aerospace or Formula-1 industry can notice that the composites produced this way has a limitation of 2.5d motion and lacks the critical layer-to-layer bonding.  These technologies can produce larger FDM-style prints with less thermal distortion.

Figure 2. Large-scale Fiber-reinforced 3D printed part (Source: AMFG)

Vision

A True Continuous Fiber Placement/Printing is a platform that leverages the benefits of FDM and Placement technologies that do not require layup tools, vacuum bagging,  autoclave, or other secondary processes. The platform enables agile accessible composites production, accelerating advanced composites manufacturing for all industries and manufacturers alike.

Physics-first principle approach

Any structural component's main function is to allow smooth stress flow to enable the transfer of loads and moments from one end to another. Continuous fibers are the most efficient load-carrying members, acting as smooth channels for stresses to flow, without any breakage. The matrix material joining the fibers ensures that all the stress flows are equally distributed among these fiber channels. To build the most effective composites structure needs to make sure the stress flow is continuous, smooth, and well-aligned without any breakage. To achieve this you want to have:

1. Well distributed fiber along the stress flow direction

2. Optimal Fiber Volume Fraction (FVF) ratio

3. No porosity in the matrix

4. Bent fiber has zero stress flow

Figure 3. Different characteristics of 3D printing (Source: FormLabs)

Attributes of solution

1. Use pre-impregnated tapes, that ensure distributed fiber

2. Compact the tapes to get the maximum number of fiber-packed

3. Heat & press to achieve a minimum porosity

4. Place fiber under tension to get them straight

Figure 4. Attributes of solution

The above solution results in a compact AFP solution. Now to address the other aspect of True Continuous Fiber Placement, it has to create shape without a given mold and should not require post-processing

1. No mold: Print a mold/substrate shape with FFF type printing.

2. No Autoclave: Use thermoplastic in-situ consolidation

Video 1. Automated Thermoplastics Fiber Placement with Laser (Credit: The University of Texas at Austin) 

Solution

A 6D motion platform capable of mounting AFP and FFF head interchangeably, creating a True Continuous Fiber Placement/Printing solution.

Building the solution

To build the most capital-efficient true continuous fiber placement and printing solution, you need to identify:

1. 6D Motion platform of choice (KUKA, ABB, FANUC, etc.)

There are many different brands of industrial robots to choose from, and they are all becoming more and more accessible every day. The key benefit of these standard platforms is they are available worldwide and are highly scalable.

2. Thermoplastic reinforced fiber placement head

Addcomposites provides thermoplastic-compatible fiber placement tool heads that can be integrated into the latest robotic platform. One-day installations are possible and available for select robotic platforms.

Figure 5. Example of a 3D laminate produced by the Single-tow Automated Fiber Placement (AFP)

3. FDM extruder

With the open-source movement in this area, pallet extruders are available from many suppliers around the world, so finding one that suits your needs is easy and affordable.

Addcomposites has the answer

Integrating all the solutions is also a big challenge, as each system has its safety and communication protocols that need to be met. At Addcomposites, we have experts at your disposal to focus on your problems at hand. Besides that, the opening access to AFP programming and simulation software (AddPath) will enable everybody to plan using all these systems together. Our automation expert can be contacted to help connect all these systems for seamless communication on their personal or work computers, enabling digital composites additive manufacturing from home or the office based on the cloud-based license. 

Video 2. Thermoplastic Layup by Addcomposites' Single-tow Fiber Placement (AFP-XS) with Humm3®