Bombardier Learjet 85- OoA prepreg and infusion process
At the end of the year we have thought it would be interesting to review one of the most interesting discussions we have had in our activity on linkedin. It was at the Composites Group. The news that Bombardier was using dry fiber and Resin Transfer infusion on the Learjet 85 wing box started an enriching discussion about the technical difficulties and possibilities of resin infusion processes and out of autoclave curing.
Bombardier Learjet – composite
“Bombardier unveils OoA composites process for Learjet. Wingskins and spars for the plane are manufactured in Belfast using an in-autoclave resin transfer infusion (RTI) process. The fuselage and autoclave are manufactured in Querétaro, Mexico, via an out-of-autoclave (OOA) prepreg process.”
“ It makes use of composites not only to reduce airframe weight and increase fuel economy, but also to significantly reduce the part count because composites have enabled Learjet to produce large, integrated structures.”- Composites World
There was one main problem that faced Bombardier with the Fuselage and that was the addition of honeycomb core. This caused problems such as porosity, laminar indications and dry spots. The solution was to completely remove core and increase the monolithic laminate.
There were additional problems such as the factory’s high altitude. Querétaro Mexico, where Bombardier molds the Learjet 85 fuselage with out-of-autoclave prepreg, is more than 6,000 ft/1,829m above sea level. Full vacuum pressure at this height is 22% lower than at sea level, which is quite significant for OOA cured systems.
This was one of the “challenges” that Pierre Harter referred to in his SAMPE presentation when describing their M&P experiences on the Learjet 85 program.
The fuselage is female moulded and an internal bladder is used. It is made of a prepreg composite and then cured in an oven . It has long curing cycle time, 16 hours for some larger parts. Bombardier’s idea was to save money and not have the higher costs of maintaining an autoclave.
RTI infusion is used to mold the Wing box in Ireland. It has already been a success with RTI infusion on the CSeries Wing.
Most of the infused resins are oven cured under vacuum only pressure and therefore eliminate autoclaves, but that is not always the case. In particular, Bombardier’s process (called Resin Transfer Infusion or RTI) performs the infusion with vacuum only but actually cures the resin in an autoclave (presumably at elevated pressure). They get the advantages of 2:1 material cost reduction (vs. slit prepreg tape for AFP) and layup labor reduction because the dry fabric forms are wider and thicker than prepreg tape and are faster to layup. But they still cure in an autoclave.
Boeing and Airbus use other processes. We made a post about various infusion processes : http://tinyurl.com/infusionmethods.
As other examples mention: Airbus uses vacuum infusion to make the Aft Pressure Bulkhead of the A380 fuselage and Boeing does the same on the 787 – this is primary structure. All the 787 wing trailing edge surfaces, flaps and ailerons, are made the same way, and the Airbus A400 Cargo Door, another large part subject to pressure loads, is also made with infusion.
The RTI process at Bombardier uses dry carbon fiber noncrimp fabric (NCF) that can be quickly cut to size on flatbed machinery. Efficiently layed up, the NCF fabric incorporates a binder that facilitates preforming on a male tool, followed by infusion in a female tool. The tool is preheated, the part is bagged and Cycom 890 RTM resin, supplied by Cytec , is injected during autoclave cure. Stringers are cocured with the upper and lower wingskins. The process/material combination is certified by the U.S. Federal Aviation Admin. (FAA) for the application.
Out of Autoclave
The cost of autoclaves has always been an issue. They increase efficiency of heat transfer obtaining a lower void content. The cure cycle is shorter in an autoclave because of the density of the air.
Oven curing cycle can be 3 – 8 hours, in an autoclave it can be less than 2 hours. The problem with oven cure is that it is difficult to transfer the heat from the air . One option of improvement is to increase the airflow a lot, or increase the air density so it carries more heat or both.
Actually when compared to autoclave, pressure helps achieve high quality laminate not by increasing fiber volume but by reducing/eliminating voids and porosity, which can significantly reduce resin-dominated properties. While all strive to create parts with fiber dominated failure modes, that’s not always the case in all applications. In some conditions it is unavoidable (impact, for example) and you are better off with a void-free, porosity-free laminate.
A lot of work for OoA is being done in infusion processes , the premise is that you can also make a void-free, porosity-free laminate using vacuum only if your infusion resin and process works well. Users of infusion processes like CAPRI, VAP, VARTM, SCRIMP, etc. feel they can produce a good laminate using only vacuum pressure while RTI users feel more comfortable relying on higher pressure to achieve good laminate quality. If you use an autoclave, then applying pressure after the infusion segment of the process will further provide a reduction in void fraction.
Bombardier in his OoA prepreg ,method had also had to evaluate breathing methods, bulk and debulk cycles, dwell times and rheology to achieve the desired void content. Curing the part too quickly generated many small voids in the composite. Combating porosity required perfecting the management of resin viscosity over time, and was achieved only after extensive trial and error. Bombardier eventually developed an OOA manufacturing process that produced voids of less than or near 1 percent.
Self heated tooling
Getting back to the topic of long cure times in convection ovens, relying only on the energy provided by the air within the oven, residency times will be longer. By directly heating the tool as an adjunct to using the convection oven, “in oven” cure times can be signficantly reduced. Further if vacuum is maintained during the cure definitely the resin fraction can improve and the void fraction reduced.
Many companies practicing infusion use self-heated or directly heated tooling. In some industries, like wind turbine blades, the molds are so large (up to 75 meters or about 250 feet long) it is impractical to transport molds in and out of an oven (although some firms do that). Easier to build the heating system directly into the mold. That eliminates heat transfer problems and
provides benefits such as better thermal control, ability to tailor heat sources to the local mold heat capacity and section thickness, faster heatup rate, shorter cycle, etc.
Typically self-heated tools are used for high rate production and for large tools, but there’s no reason they can’t be used elsewhere, depending upon the economics.
Thanks to all the people whom contributions are making infused composites gain ground in aeronautics, and specially for their contribution to this discussion to:
Zachie de Beer (CEO at Aerodyne), Rick Pauer (CCP Composites Market Manager), David Maass (President of Flightware-consultant), David Lewis (Lead Composites Design Engineer and Production support engineer at Bombardier Aerospace), Joe Ouellette (CTO Acrolab Ltd. and Process Consultant), Andrew Butcher (Project Engineer for Pratt & Whitney Canada: Composites, F7X MRB Leader, Stress Analysis: onsite at Aircelle, Toulouse), Narasingha Satapathy (Aircraft/Helicopter structural design consultant at Tata Consultancy Services), Dick Blackmore (Founder at Carbon Composite Technologies), Paul Martin (design, development of extreme performance lightweight structures), Andy Mackay (composite technician, aviation, marine, automotive)
We also cited an article from CompositesWorld
Bombardier unveils OOA composites process for Learjet 85 – Pierre Harter (Bombardier Business Aircraft, Montréal, Quebec, Canada) – Composites World