CW Blog

A novel carbon fiber product designed and sized to improve the performance of sheet molding compound (SMC) has been commercialized by Zoltek Corp., a Toray Group company (Bridgeton, Mo., U.S.). The patented, pre-spread fiber, called PX35 KS, is scored as the tow band is produced, converting a 50K tow product into 3K sub-bundles, which, in turn, are spooled and supplied as a multi-end tow. Once the pre-scored tow is fed into a chopper unit and the tow’s fiber length is cut, it falls apart into the 3K sub-bundles as it is compounded into the SMC dough. Reportedly this improves fiber handling, dispersion and wetout, and provides a more uniform fiber bed that improves the flow of the composite during compression molding.

“The benefit of improved dispersion is that it allows manufacturers to use a lower-cost carbon fiber while achieving the higher properties typically seen by a smaller tow,” notes Christopher Thomas, Zoltek automotive business director-Americas. “You essentially pay for the cost of 50K [automotive-quality tow], but you get the performance of 3K [aerospace-quality tow],” adds Tobias Potyra, Zoltek automotive business director-Europe.

Fabrication of large aerostructures using composite materials typically requires a mold made with a metallic material with a low coefficient of thermal expansion (CTE) so as to minimize distortion of the part during autoclave cure. Invar is, understandably, the material of choice in aerospace manufacturing, thanks mainly to a CTE of about 1.0 × 10⁻⁶ K⁻¹. Such dimensional stability comes at a price, however. Invar used for composites fabrication currently runs about $13/kg (compared to about $1/kg for carbon steel).

At this price, material must be used conservatively, and only for the manufacture of tools that will be used to fabricate production parts and structures. There is, however, much developmental manufacturing activity that takes place before that production tool is created. Thus there is a demand for a low-cost, autoclave-capable material with short-run durability for prototyping and low-rate production applications. Large-format additive manufacturing (AM) is in a position to fill that need.

It may go without saying, but the continued viability of composites in commercial aircraft programs depends on reducing costs and improving production rates. Automation of production processes, such as automated fiber placement (AFP) and automated tape laying (ATL), improves speed and efficiency, but the need for careful, often manual, visual inspection has been a common time constraint. Just how much time is added to the overall production time by manual inspection? Accounts vary from 30 percent on the low end to more than 60 percent, as reported in a paper presented several years ago by Robert Harper of Fives Cincinnati (Hebron, Ky., U.S.) and Allen Halbritter of Boeing (Chicago, Ill., U.S.). So, though variable, the time penalty is significant. 

In recent years, numerous companies have been working to develop automated inspection technologies that can keep up with automated production. Such efforts include inspection systems from companies like Electroimpact Inc. (Mukilteo, Wash., U.S.), Danobat Composites (Elgoibar, Spain), Apodius (Aachen, Germany), FACC AG (Ried im Innkreis, Austria) and Fives.

The commercial aerospace industry is poised for some exciting changes. As the industry prepares for a new round of major program launches, including a potential Boeing NMA (New Midsize Airplane), the question of where and how composites will be applied is on entire composites industry’s mind.

CW recently released a special edition, in collaboration with Additive Manufacturing and Modern Machine Shop, dedicated to the advanced materials and process that will likely play a role in Next-Gen Aerospace.

Whenever a composites manufacturer thinks about fabricating a part or structure, he or she, understandably, focuses on the basic, raw ingredients of a composite application: resin, fiber, tooling, processing, finishing. Absent from these considerations most of the time is fiber sizing — the chemistry applied to fibers to facilitate handling, minimize abrasion, promote resin/filament adhesion or convey some other property to the fiber to help optimize mechanical properties in the final part.

Sizing is easily overlooked because it is typically applied by the fiber manufacturer using chemistry that is often tightly controlled. Further, sizing is a material that is most noticeable by the handling, processing and performance headaches that arise in its absence. When it is present, on the other hand, it performs with rare recognition.