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First Plastics, Then Metals and Now Composites

By Kevin Ayers Consulting Editor, SME Media

Impossible Objects LLC, a Chicago-based company, has brought to market a new composites material manufacturing technology known as Composite-Based Additive Manufacturing (CBAM) 3D technology which produces Carbon Fiber Reinforced Plastic (CFRP) or Polymer Matrix Composite (PMC) parts. Robert Swartz, founder of the CBAM process, derived the concept from the 2D printing industry. Swartz said that when you print a book, each page in the book is different and 2D printing technology can print, stack, and collate reliably at very high speeds.

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Aurora Flight Sciences had problems with SLS mounting clips breaking on drones. The SLS clips were replaced with parts made of CBAM, which is stronger and stiffer.

The CBAM process starts with a CAD file, electronically slices the file into layers, and uses the bitmap layer data to print the image onto the composite sheets. The process uses nonwoven composite sheets as the build material. The sheets contain long fiber lengths and have a random fiber orientation throughout the sheet.

 

The process starts by conveying a composite sheet to a print bed and uses the CAD slice layer data to drive inkjet heads to deposit a water-like wetting fluid onto the sheet surface to create the layer image. The sheet with the wetted image continues moving to a powdering station where an overhead trough floods the composite sheet unselectively with a uniform thickness of thermoplastic plastic powder.

Next, the powdered sheet is conveyed to a depowdering station where the thermoplastic sticks to the wetted sheet region. Loose thermoplastic material on the sheet is blown off and recycled for future builds. A high level of recycling and material reuse is possible since the thermoplastic materials are deposited and removed at room temperature, causing no changes to the chemistry of the thermoplastic material.

Printing then Pressing

The printed sheet is then automatically aligned and stacked onto a tooling plate until all sheets that make up the CAD file are printed. The tool with all the printed sheets is then moved to a heated press where the selected thermoplastic material is raised to its melting point and then mechanically compressed to final part design height. In the mechanical pressing process, the thermoplastic material flows between the long fibers in the nonwoven sheet to form the composite part(s), eliminating the need for discrete fabrication tooling, multiple fabrication steps, and skilled hands-on labor.   

Part removal is the final step. This is accomplished by bead blasting the build block to retrieve the composite parts. The bead blaster shears the fibers where there is no thermoplastic material, making a sustainable process where the nonwoven fibers can be recycled.

The CBAM process has significant build flexibility due to the wide range of composite and thermoplastic build materials available to make PMC or CFRP parts. The system  can handle carbon fiber, fiberglass and Kevlar composite sheet material and a number of thermoplastic matrix materials. Some of the matrix thermoplastic materials used to date include HDPE, nylon 6, nylon 12 and PEEK materials.

Composite parts built with the CBAM process have around 20% fiber by volume where the fiber volume fraction is controlled by the compression and calculated by the following formula.

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A = Areal weight of fabric sheet

ρ = density of fiber

h = layer thickness

Impossible Objects has current efforts to continue to raise the carbon volume fraction and feels that it can achieve near 40% volume fraction for much stronger composite parts in the future.

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The advantages of CBAM parts are that they have similar strength-to-weight ratios as aluminum alloys and are 50% lighter compared to aluminum parts. The material Ashby chart shows how the CBAM materials with a 20% volume fraction compare to polymeric and metallic material properties. As the carbon volume fraction continues to rise, the red circle will move up the strength Y axis, raising uniformly the CBAM flexural material and specific strength lines.

Mechanical Strength

A closer look at the CBAM part materials shows significant mechanical strength properties compared to conventional polymeric additive processes. One main contributing factor for the increased strength performance is provided by the long fibers in the nonwoven fiber sheet. Most conventional fiber reinforced plastics have short fiber typically less than 50 μm in fiber length. The standard CBAM material with the long fiber is 2× to nearly 10× stronger compared to conventional additive polymeric processes. CBAM material has the highest tensile modulus, while twice as strong as another additive material with the closest modulus. 

A recent study performed by a Fortune 500 company evaluated the feature detail and surface finish of a demonstrator article and found that the CBAM parts  have very good surface finish feature and sharp edge detail, geometrical tolerances, capability to produce thin walls, and did not have part warping compared to other conventional additive manufacturing processes.

A number of post finishing capabilities such as machining, mass finishing and hand sanding can be utilized to finish CBAM parts to the desired surface finish. CBAM parts can be finished to a surface finish Ra of 20 micro inches.

The challenge for any new manufacturing material and process is to meet the technical and business case requirements for a specific application. How does one identify the appropriate applications that can utilize the improved strength, temperature resistance, and light weight CBAM material properties? One low-risk approach is to evaluate different types of assembly and fabrication tooling needed to make a product.

In most cases, tooling is one of the first technology insertion points where businesses can generate equivalent tooling performance with significant savings in cost and scheduling using additive manufacturing.

Recent CBAM examples of CFRP tooling families include complex shaped composites lay-up tools and caul plates, high-temperature tools with operating temperatures around 250°C, metal forming tools, and higher strength assembly fixtures.   

Tool Testing

In 2016, Oak Ridge National Lab Manufacturing Demonstration Facility (MDF) performed testing of CBAM autoclave composite tools made with carbon fiber and PEEK matrix. These tools were cycled up to 150°C under extreme pressures up to 1000 psi (6.89 MPa). They maintained very good dimensional part control, autoclave survivability and achieved a lower Coefficient of Thermal Expansion (CTE) compared to a standard metal composites lay-up tool.

Some of the first industry CBAM part adopters are from the unmanned systems, robotics, and the electronics industries. The material properties along with similar strength to weight ratio and 50% lighter weight structures compared to aluminum allow for greater payloads and extended operating times—key product performance benefits for an unmanned system.    

Aurora Flight Sciences, a leader in the development and manufacture of advanced unmanned systems, utilized the Selective Laser Sintering (SLS) 3D printing process to manufacture nylon mounting clips that attach the rear horizontal stabilator to body of the vehicle.

These SLS mounting clips would frequently break when the vehicle would make a hard landing. It was due to the lack of SLS material strength properties and stiffness. To resolve this matter, the SLS parts were replaced with CBAM made with a carbon fiber with nylon 12 matrix material.

Another emerging CBAM application family is for electronic enclosure cases. Customers are finding improved electromagnetic interference protection as well as lighter weight assemblies for enclosure cases made with the CBAM process.

Some of the emerging applications that utilize the design and build freedom of the composite-based additive manufacturing technology are to make integrated, monolithic structures like drone airfoils. They illustrate how internal structure such as core and stiffeners can be integrated into a single piece. This allows the designer to vary feature detail and part wall thicknesses to optimize structure for strength, weight, and ultimate performance.

What will be interesting to witness in the future is the use of different material build combinations enabled by the CBAM technology. The use of unidirectional carbon fiber materials will significantly increase part strength and stiffness by 3× to 6×, increase a larger number of thermoplastic matrix material options, and increase the potential to utilize other matrix materials. The ability to use materials like thermoset epoxies and powdered metal alloys to produce either thermoset parts or Metal Matrix Composite (MMC) parts will open exciting new opportunities.    

Besides expanding the range of material build combinations, CBAM technology will continue to scale up to make larger part sizes and utilize robust printing technologies to achieve build speeds similar to traditional manufacturing methods. The result will be to further advance the technology to exciting new composite applications in other industries.

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