Electroimpact has developed a continuous fiber-reinforced thermoplastic 3D printing system with 6 degrees of freedom, which can realize rapid moldless manufacturing of aerospace composite parts and new design freedoms that are impossible with existing manufacturing methods.
Since the American company entered the market with the first commercial continuous fiber 3D printer Mark One in 2014, engineers around the world have dreamed of a system that can 3D print large continuous fiber reinforced parts.
However, it is a troubling challenge to completely produce complex parts that are several times stronger than most metals by means of additive manufacturing. This is due to the density of plastics and the lack of expensive layup molds and hot pressing. tank.
At present, several experienced and established companies and startups have entered this field, trying to get a share of the broader multi-billion dollar additive manufacturing market.
In recent years, Electroimpact has been working with partners in the aerospace field to develop this technology to meet the application needs of OEMs.
Many people in the aviation industry believe that the 3D printing of large continuous fiber composite parts already exists in the form of automatic fiber placement (AFP) and automatic tape placement (ATL), both of which are through subsequent re-laying Add material layers to produce composite parts.
However, these technologies require a layup mold with the same shape as the finished part in order to place the material on it.
In fact, the continuous fiber 3D printer that people really want does not require the use of layup molds and vacuum bags, nor does it require a lot of auxiliary equipment such as autoclaves or secondary processing steps. On the contrary, it is just an ordinary manufacturing platform. , And eliminate as many constraints as possible in the system so that the end user can create a variety of parts and shapes that cannot be achieved with traditional methods.
The result is a new composite design that was previously unimaginable, including a more integrated structure, that is, reducing the number of fasteners and the adhesive used for assembly connections.
The basic principle
Any high-quality composite material component has three basic characteristics, regardless of the fiber and matrix material selected.
The three basic characteristics are: fiber volume content (that is, the ratio of fiber to matrix material), porosity and fiber straightness.
Electroimpact is not the only company pursuing a continuous fiber 3D printing system. In response to the challenge, other companies have adopted several completely different methods. Some of them have tried to combine traditional fused filament manufacturing (FFF) 3D printers with Mechanical devices for introducing continuous fibers into the molten thermoplastic stream are combined to embed the fibers in the printed part.
Other companies choose to use UV-catalyzed thermosetting resins to mix their continuous fiber reinforced materials and resins at the processing point, and then use UV radiation to initiate rapid curing of the resin during processing.
These co-extrusion processes attempt to integrate more steps into a single system.
First of all, it is difficult to achieve the uniform fiber percentage (50%+) required by the aviation level. The processing point of the system deposition material must be accelerated and decelerated as it passes through the programmed processing path of the component in space. Therefore, the process of injecting liquid or molten matrix material into the fiber must also be accelerated and decelerated simultaneously with the processing point.
Any experienced composite material manufacturer knows that the most uniform material will be obtained when the entire process is in a stable state and is not affected by large transients.
When this method is used to process high fiber content composite materials, it is difficult to evenly impregnate the fibers, which often leads to dry fiber patches and poor fiber distribution in the matrix material. Therefore, the impregnation step is best done offline on a dedicated production equipment that runs stably. This equipment is specially developed for the production of high-quality prepreg materials with uniform fiber distribution and fiber percentage.
The second problem is the porosity, the pores cannot bear the load.
By vacuum curing the laminated structure in the autoclave and applying several atmospheric pressures at the same time, the main structure of the composite material formed by the autoclave can have a porosity of less than 1%, thus meeting the gold standard of aviation level.
Although the applied force is huge, it can eliminate almost all pores in the laminated structure.
For non-autoclave material systems that are seeking more aerospace applications, the porosity that can usually be achieved is less than 3%. This type of material does not require an expensive autoclave, but still requires a vacuum to cure the laminated structure to eliminate pores, and also requires a curing oven. In short, if the laminated structure cannot be cured in some way, such a system will never achieve the low porosity necessary for high-quality parts.
Finally, the physical process of depositing continuous fibers requires deposition under a certain tension.
If this process needs to push the fibers at any point when the matrix material is softened, it will cause the fibers to form bundles. Fibers that are not straight cannot bear the load until they are straightened.
Therefore, for composite parts, bundled fibers mean that the load can only be borne by the much lower strength matrix material.
SCRAM technology
Electroimpact is developing a new technology based on the old technology, called SCRAM, also known as "upgradable composite robotic additive manufacturing."
This is a system that integrates an FFF 3D printer and a thermoplastic AFP machine. The system consists of a precision-operated robot, a rotating construction platform and a temperature-controlled construction room.
The end effector carries multiple material systems to print soluble support materials (tooling), continuous fiber ribbons and chopped fiber materials. Every printing starts with the robot depositing supporting materials onto the build platform. Subsequently, the robot automatically switches to print continuous fiber reinforced materials and chopped fiber reinforced materials to produce parts.
This continuous fiber is deposited by in-situ consolidation, where the strip is laser welded to the substrate and compacted in the process.
The continuous fiber reinforced composite material obtained in this way can reach the porosity level that can be achieved by the non-autoclave process.
The inclusion of the chopped fiber material system is a supplement to the continuous fiber reinforced material system.
Generally, continuous fibers with high fiber volume content will introduce geometric constraints that have not been touched by the omnidirectional FFF process. Compared with the characteristics that can be produced by simply using continuous fiber tape, the characteristics produced are much more complicated.
In these cases, designers can use chopped fiber materials to obtain desired characteristics. Once printing is complete, the support material dissolves, leaving only the finished part. Since the material system is completely thermoplastic, there is no need to use autoclaves or furnaces to solidify the parts.
Unlike traditional FFF 3D printing, the SCRAM process uses a true six-axis machining path to produce parts. Most of the additive manufacturing systems such as FFF, SLA and SLS are so-called 2.5D, that is, flat 2D layers are stacked one after another to form a 3D shape.
In contrast, SCRAM is a true 3D process. The end effector deposits materials in a true six-dimensional space of freedom. This is particularly important for depositing continuous fibers. It can ensure that the orientation of the fibers is adapted to the load path, and from A quasi-isotropic stack is obtained on the build platform.
challenge
The development of this complex technology faces four major challenges, involving material systems, printing hardware, control systems, and component programming.
Many companies are trying to develop this technology. Although some companies have made impressive progress in dealing with two or three of the four major challenges mentioned above, no company seems to be able to conquer all four major challenges at the same time.
First, the material system itself is the most basic challenge.
Although there are many polymers to choose from, once extreme requirements are increased, such as high temperature use, chemical resistance, smoke and toxicity requirements, etc., there are few materials to choose from. Moreover, their processing is also extremely challenging. Secondly, since it is the fiber that bears most of the load, it is desirable for the volume content of the fiber to be as high as possible, and to ensure reliable deposition of the material and achieve good bonding. For all these reasons, after many trials, Electroimpact decided to use a thermoplastic based on PAEK and a fiber content of 50% to 60%.
Electroimpact's approach is not to solve all these four challenging problems on its own, but to focus on its core strengths, namely hardware and control systems. In cooperation with industry peers who are most suitable for creating material systems and CAM software, the integrated system developed by the company has made significant progress in responding to the four major challenges of the appeal and demonstrated unprecedented capabilities.
Future
Although there is still a lot of work to be done in improving the maturity of SCRAM technology, its development speed is indeed very fast, and the enthusiasm and interest in it are driving its continuous progress. At present, the use of SCRAM technology has produced part shapes that could not be manufactured before.
The focus of the next development is to enhance its industrial production capacity and comprehensively improve its performance indicators. It is undeniable that continuous fiber reinforced 3D printing with true six degrees of freedom has arrived.