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US Army Characterized Continuous 3D Printed Carbon Fiber-Reinforced Thermoplastic Composite Parts

Geometry of Tensile Specimens.

A trio of researchers with the US Army Tank-Automotive Research Development, and Engineering Center (TARDEC) in Michigan recently published a study, titled “Characterization of Continuous Fiber-Reinforced Composite Materials Manufactured Via Fused Filament Fabrication,” that worked to characterize continuous carbon fiber-reinforced thermoplastic composite parts that were 3D printed on a Mark Two 3D printer.

The abstract reads, “The current work has focused on characterizing the tensile performance of continuous fiber reinforced specimens manufactured via Continuous Filament Fabrication (CFF). The specimens were tested in multiple orientations with and without continuous carbon fiber reinforcement. When comparing 0⁰ carbon fiber reinforced specimens to specimens without continuous reinforcement, the average yield strength, tensile strength, and elastic modulus increased by factors of 20X, 15X, and 240X, respectively. When comparing the results for specimens with 90⁰ oriented continuous reinforcement to the 0⁰ specimens, there was a 60% drop in yield strength, 62% drop in tensile strength, and 52% drop in elastic modulus. These results indicated that mechanical performance is reduced significantly when load is applied perpendicular to the fiber orientations. The adhesion between adjacent layers was tested by printed specimens standing vertically on the print bed. These specimens had the lowest strength of all specimens. The authors recommend follow on testing using rectangular specimens with bonded tabs per ASTM D3039-17 to reduced issues with fiber alignment that were encountered with the dog bone specimens.”

As most 3D printed parts are built from the bottom up, it’s not unusual for out-of-plane material properties to be weaker than in-plane ones. When in-plane printing occurs of continuous fibers, the completed parts can have increased stiffness and in-plane strength, but researchers don’t have a clear idea as to how continuous fiber reinforcements affect an as-manufactured part’s mechanical anisotropy.

“In order for design engineers to utilize continuous fiber-reinforced AM parts in structural applications, they will require the mechanical properties of these materials in three dimensions,” the researchers explained.

The researchers used the nylon-based thermoplastic Onyx by Markforged in their study, along with continuous carbon fiber tow coated with a binder material, and 3D printed several test specimens in order to gain a better understanding of how much of an influence the continuous carbon fiber reinforcement would be:

• Group 1: Onyx (in plane, Nylon/Carbon plastic): ID# 1-1, 1-2, 1-3
• Group 2: 0⁰ fibers (in-plane, aligned carbon fibers):: ID# 2-1, 2-2, 2-3
• Group 3: 90⁰ fibers (in-plane, perpendicular to carbon fibers): ID# 3-1, 3-2, 3-3
• Group 4: z direction (out-of plane, perpendicular to carbon fibers): ID# 4-1, 4-2, 4-3

To make analysis easier, the team only tested specimens with unidirectional fiber orientations. The pure Onyx specimens in the first group were 1.8 mm thick and used as a baseline, while the 0° specimens from Group 2 featured two 0.125 mm layers of Onyx on the roof and floor, along with two Onyx layers on the side walls; the rest was filled with carbon fiber that were “oriented longitudinally in the direction of pull for a tensile test.”

“Additional specimens 3-1, 3-2, and 3-3 were printed with fibers oriented perpendicular to the tensile pull direction. These specimens had the same thickness of Onyx on the roof, floor, and walls as the previous set of specimens,” the researchers explained. “It is noteworthy that for these specimens, since fibers were oriented perpendicular to the direction of tensile pull, the print head must turn corners within the gauge section, and therefore, the fiber orientation within the gauge section was not perfectly unidirectional.”

Schematic of specimens on print bed to show specimen placement and fiber orientation (where relevant).

The Group 4 specimens were 3D printed vertically, and were tested for adhesion evaluation between fiber-reinforced layers. Then, the researchers conducted Thermogravimetric Analysis (TGA) and Fourier Transform Infrared (FTIR) Analysis on the Onyx specimens in order to gain a better understanding of the material’s thermal characteristics; tensile testing was also conducted until total specimen failure.

“When comparing 0⁰ carbon fiber reinforced specimens to pure onyx specimens, the mechanical properties increased by orders of magnitude,” the researchers explained. “For example, the average yield strength, tensile strength, and elastic modulus increased by factors of 20X, 15X, and 240X, respectively. When comparing mechanical performance of the fiber-reinforced specimens to the Onyx material, the significant improvement in mechanical performance is consistent with traditional laminated composites, where unidirectional specimens have strength and stiffness orders of magnitude higher than a homogenous epoxy matrix material. When comparing the results for the 90⁰ specimens to the 0⁰ specimens, there was a 60% drop in yield strength, 62% drop in tensile strength, and 52% drop in elastic modulus. These results indicated that mechanical performance is reduced significantly when load is applied perpendicular to the fiber orientations. However, the relative drop in mechanical performance was not as significant as what is observed for many traditional unidirectional composites tested at 90⁰ orientation. The adhesion between adjacent layers was tested by printed specimens standing vertically on the print bed. These specimens had the lowest strength of all specimens.”

Detailed views of fracture surface of specimen 1-1, showing fiber breakage, fiber pullout, and matrix cracking.

The researchers determined that the materials used in this study have a high degree of mechanical anisotropy, and that others need to consider the 3D anisotropic mechanical properties when they are used in structural applications.

In addition, the team also determined that the traditional dog bone-shaped tensile bars they used for the study were not the best choice for specimens manufactured using CFF, mainly because of “the unique fiber placement process and local variations in fiber angle around the curved radii,” and recommend that other researchers use rectangular specimens with bonded tabs, per the ASTM D3039-17 Standard Test Method for Tensile Properties of Polymer Matrix Composite Materials.

Co-authors of the paper are Robert J. Hart, PhD, Evan G. Patton, and Oleg Sapunkov.

Discuss this research and other 3D printing topics at 3DPrintBoard.com or share your thoughts below.

Markforged celebrates over 100 Global shipments of Metal X 3D printer

Boston 3D printer manufacturer Markforged has announced the completion of over 100 worldwide shipments of its Metal X system. The announcement comes almost two years since the product was announced at CES 2017, and eight months into the machine’s commercial availability. Greg Mark, CEO and founder of Markforged, comments, “Demand for affordable, strong, and safe metal 3D printing […]

Desktop Metal and Markforged reach agreement over industrial espionage claims

On October 2nd 2018, Bostonian 3D printer manufacturers Desktop Metal and Markforged settled a legal case, and a subsequent countersuit, relating to claims of IP infringement, trade secret misappropriation and industrial espionage. According to a joint press release from the companies involved, “Both Markforged and Desktop Metal acknowledge that neither company, nor the individuals named in […]

Comparing FDM 3D Printed Parts with Carbon Nanotubes, Continuous Carbon Fiber and Short Carbon Fiber

Fused deposition modeling, or FDM, 3D printing has several advantages – thermoplastics can be used, which are easy to handle and are strong and durable enough to be used for producing both prototypes and practical parts. Additionally, FDM 3D printers use a simple mechanism to melt and extrude resin that doesn’t need expensive parts, like lasers, which makes the machines less expensive. But, the technology does not always provide enough strength for mechanical parts.

That’s why additional materials with good mechanical properties, such as carbon nanotubes (CNT) and fiber reinforced composites, are often added to improve strength; depending on the length, carbon fiber can also be divided up into both short and continuous fiber. A group of researchers from Doshisha University and Kyoraku Co., Ltd., both in Japan, recently published a study, titled “Comparison of strength of 3D printing objects using short fiber and continuous long fiber,” that compared the usefulness and strength of objects 3D printed with short carbon fiber, continuous carbon fiber, and multi-wall carbon nanotube (MWCNT).

The abstract reads, “In this research, composite materials were used to improve the strength of FDM 3D printed objects. The nanocomposites made from polylactic acid as matrix and multi-wall carbon nanotube as filler, short carbon fiber reinforced composite and continuous carbon fiber reinforced composite were prepared, and tensile test was carried out. As a result, the continuous fiber reinforced material exhibited tensile strength of about 7 times and elastic modulus about 5 times that of the other two materials. The strength was greatly improved by using the continuous fiber. The fracture surface after the test was observed using a scanning electron microscope. The result of observation shows that adhesion between the laminated layers and the relationship between the fiber and the matrix are bad, and improving these are necessary to increase strength. Comparing those materials, it is possible to improve the strength in some degree by using short fiber while maintaining ease of printing. On the other hand, by using continuous fiber it can be achieved significant strength improvement while printing was complicated.”

The fracture surface of PLA/MWCNT

To make their PLA/MWCNT nanocomposite, the researchers used polylactic material as a matrix, with MWCNT as a filler, and formed the material into a 1.75 mm filament. They used commercial ONYX, carbon fiber, and NYLON materials from Markforged to 3D print tensile test pieces from continuous carbon fiber reinforced thermoplastic (continuous CFRTP) and short carbon fiber reinforced thermoplastic (short CFRTP).

“The specimen shape is different due to the limitation by the performance of the 3D printer,” the researchers wrote in the paper. “For PLA/MWCNT, smaller one was chosen to avoid warp and print quickly. The PLA/MWCNT has three outer walls and fills inside alternately at 45 degrees and -45 degrees.”

For the continuous CFRTP, carbon fibers oriented in the load direction were 3D printed in the center, while the outside was covered with either neat resin or short fiber reinforced composite; this last was used to 3D print the short CFRTP in the same manner as the PLA composite had been fabricate.

The researchers completed a tensile test on the pieces, and used a scanning electron microscope to observe images of the specimen’s fracture surface. They also looked at their stress and strain.

“In PLA/MWCNT, the stress increased almost linearly until fracture,” the paper explained. “The breaking strain was about 1 ~ 2%, and no stress reduction was occurred. Compared with neat PLA, the elastic modulus was not greatly improved but the tensile strength was improved and increased by 48% when 1wt% of MWCNT is added. In that case, the tensile strength was 53 MPa and the Young’s modulus was 3 GPa. Until 1 wt%, the tensile strength was improved as more CNT is added, but strength was decreased when 3wt% was added. It is because the aggregation of MWCNT. The aggregations are considered to act as internal defects of the material.”

Aggregates and voids

When more MWCNT was added, the number of aggregates increased. The researchers found that the relationship between the fiber and the matrix, along with adhesion between the laminated layers, was not good – when these are improved, the strength will increase. Significant strength improvements can be achieved by using continuous fiber, but the 3D printing process is complicated, and it’s necessary to use modified equipment, such as a special nozzle. But short fiber is easier to print, and still offers some degree of improved strength.

“The short CFRTP and PLA/MWCNT are inferior in mechanical properties compared to the continuous. But they can be printed with conventional 3D printers without special modifying,” the researchers explained. “Especially the nanocomposites demonstrate its effect by adding a small amount. The mass concentration of fiber was 35.7 wt% for continuous CFRTP and 14.3 wt% for short CFRTP, but MWCNT was 3wt% or less. Generally, the smaller the amount of reinforcement, the more easy to print. In fact the PLA/MWCNT nanocomposite can be printed with commercially available 3D printer without special modified in this study. Continuous fiber and short fiber material should each have merits and demerits and should be used properly.”

The broken specimen (continuous CFRTP)

Co-authors of the paper are T. Isobe, T. Tanaka, T. Nomura, and R. Yuasa.

Discuss this and other 3D printing topics at 3DPrintBoard.com or share your thoughts below.

News Digest: Tridues, GoPrint3D, Admatec, Weerg, Xjet, FIT, Polygonica and more

This edition of our 3D printing news digest Sliced contains, UK distribution rights for 3D printers, expansion of FIT, a trade secrets war, Polygonica update, ANSYS VRXPERIENCE and full-color 3D printing services. Desktop Metal partners with Trideus for distribution Desktop Metal, a Boston based manufacturer of 3D printers, has partnered with Belgian AM solutions provider, […]

Star Rapid’s Gordon Styles Talks About 3D Printing In Metal and China

Gordon Styles is a true industry veteran. Having transformed a manufacturing company into one of the UK’s leading 3D Printing service bureaus Gordon sold the company in 2000. That move would have already made him a very experienced 3D printing executive who was manufacturing using 3D printing years before most had even heard of the technology. He more than cemented his status however when he dropped everything and moved to Hong Kong to start Star Rapid. That company later relocated to Zhongshan and there grew to be a 3D printing and CNC service that now employs over 300 people. One of the very first in the country Gordon grew his business in China and now is increasingly serving the world from there. Star Rapid has injection molding, CNC, metal printing, vacuum casting, assembly services and a range of 3D printing technologies in-house. The company makes painstakingly hand-painted prototypes, series of a few hundred housings or tens of thousands of parts for customers all over the world. We interviewed Gordon to talk about metal printing and what it is like to work in China.

Rapid prototyping and prototype manufacturing.

For what applications do I choose what metal 3D printing technology?

Metal 3D printing is ideal for products or parts that need to be high-strength yet low-weight. This is the reason the technology has become so popular within the aviation, aerospace, automotive and medical sectors. Metal 3D printing is also an option if a design is so complex that it cannot be manufactured using any other process. However, it should not be viewed as a “rescue process” for poor designs as 3D CAD models have to meet specific design criteria for a successful build.

It’s important for a designer to work closely with a design engineer to first identify if metal 3D printing is the ideal manufacturing process for that part. Metal 3D printing requires very precise design considerations that differ greatly from plastic 3D printing. Also, it would be a mistake to think a part designed for CNC machining could be “run through” a metal 3D printer. Often, if a part can be designed using another process such as CNC machining, then that process should be utilized as it is more cost-effective.

Why is metal 3D printing so difficult?

Metal 3D printing uses a high-powered laser to melt and fuse successive layers of powdered metal into three-dimensional solid parts. While there are many advantages to using this process, such as building more complex and feature-rich components, it can be a difficult process overall. Designing for metal 3D printing requires very specific requirements when it comes to gaps, holes, part orientation, supports and more.

Another challenge can be the actual print time. The longer it takes to print a metal part, the more likely it is to fail. Long print times create more internal stress due to repeated heating and cooling that occurs during the metal 3D printing process. Longer prints with larger volumes run the risk of distorting and internal stresses cannot be relieved during metal 3D printing.

Understanding how to design for this process is key to creating a perfect part the first time. However, the technology is still relatively new so the design-for-manufacturing knowledge needed hasn’t yet become widespread and commonplace among the design and engineering communities. With more interest and training on metal 3D printing, the process will become less difficult over time.

How can I in advance see if my part will work in metal?

There are some software platforms available that can help simulate if a part can be metal 3D printed. However, it is important to have an open line of communication with your manufacturer. At Star Rapid, we work closely with prospective and existing customers to determine the design considerations that will help ensure a part doesn’t fail when it is being built. Given the expertise of our engineers, we can also help customers easily move from prototyping to new product introduction, then low-volume production.

You’ve operated service bureaus in China and the UK, what are the differences?

When starting a business in China, it was always a top priority to build a culture based around quality. As such, today we have a large portion of our employee base dedicated to quality control and quality assurance. This department utilizes the latest high-tech technologies to ensure materials coming into our shop and leaving our shop are of the highest quality. Positive material identification is taken very seriously; we have a FARO laser scanner, Zeiss Comet LED3 2 optical scanner and a Nikon CMM. Other tools we use include an optical emissions spectrometer and X-ray fluorescence gun, both are used to test metals coming into the factory. For plastics, we have a PolyMax hand-held analyzer.

Another difference is that, now, China is doing a lot more than the UK to support small to medium sized companies, as well as investing in initiatives aimed at upgrading its manufacturing and design sectors. The Made in China 2025 initiative is one such example, with the goal of making manufacturing more high-tech and move it up the value-added manufacturing chain by firmly latching onto the concept of Industry 4.0.

Are you thinking of using clusters of desktop machines in your service bureau for polymers?

At Star Rapid, we would never use FDM as a solution, so that excludes most of the desktop printers. Currently, we are looking at MarkForged for making inserts for silicone tools for vacuum casting. Outside of the MarkForded printers, we don’t have any plans for desktop printers.

Of the new 3D printing technologies which one are you the most excited by?

Carbon 3D is really exciting because it really has hit a price level that allows for structurally sound PU parts in low volume production.

Day Two of 3D printing at IMTS, smart factories and preparing for serial production

Self-driving 3D printed vehicles, beer-pouring robots, robots 3D printing, whirlyball playing robots – ok I made that last one up but automation – together with additive manufacturing – is clearly a dominant theme at IMTS 2018. 3D printing and automation Automation is self-evidently a technology that does not stand in isolation – there must be […]

3D Printing in Metal: Sign Up Before September 6 & Save

3D Printing in Metal, our three-week online class, starts September 18. The online class mixes live instructor-led sessions with online discussions and personalized help from course facilitators. By the end of the class, you’ll learn the key principles of metal additive manufacturing.

The best part? If you sign up by September 6 you’ll receive 23 percent off!

Sign up by September 6 for a discount on 3D Printing in Metal, starting September 18

In the class, you’ll hear from thought leaders like Mark Norfolk, President at Fabrisonic, who will present his session, Solid State 3D Printing of Metals. In Mark’s session, participants will:

Learn about the difference between fusion based AM and solid state based AM processes
Discuss and show examples of printing dissimilar metals
Discuss the build and qualification of heat exchangers for NASA JPL
Discuss and show examples of embedding electronics into metal parts

Mark Norfolk, President at Fabrisonic, class speaker

Want to learn more? Take a look at all sessions here. You can also check out our other online classes, Trends & Innovations In Dentistry and 3D Printing with Polymers, starting September 25 and October 16, respectively.

The class is presented by Markforged

Markforged cleared of IP infringement, Desktop Metal awaits countersuit

Markforged, the manufacturer of Mark series carbon fiber 3D printers and the Metal X, has been cleared of patent infringement allegations claimed by rival company Desktop Metal in March 2018. The case was brought in front of a 12-person federal jury in Boston on Monday 9th July 2018. After three weeks on trial, the jury […]

3D printing news Sliced GE Additive, Toll Group, EnvisionTEC, Markforged

Today in the Sliced 3D printing news digest, we take a look inside Canada’s first Marine Additive Manufacturing Center of Excellence; study colorful 3D printing; explore 3D printed castles; and conduct some heavy lifting. GE Additive, Toll Group, Xometry, EnvisionTec, Markforged and more all feature below. Read on to find out more. Getting educated A study conducted […]