Molded Carbon Fiber Leading Edges

Molded carbon fiber leading edges are used on many models airplanes because they give the wing great torsional stiffness and weigh very little.  These leading edges are also called D-boxes. When glued to a wing spar the closed tube looks like the letter D.


Simple tools are needed to mold these carbon fiber leading edges.  The form is similar to a vacuum bagged wing except that the aft edge of the form must be at least ¼” thick. The airfoil can be cut off mid chord or the trailing edge of the airfoil can be thickened.  The chord of the form should be at least ½” more than the intended chord of the molded leading edge.
The leading edge form must be waxed with several coats of high temperature mold release wax.   High temperature wax buffs out more easily and if the carbon epoxy lay up is thick the wax will not fail due to temperature build up from a thick layer of epoxy.    An extruded aluminum bar 1/8” x 1 ½” about 6” longer than the form is used along the aft edge of the form.  The bar creates an air passage along the form and pulls the vacuum bag tight around the leading edge of the form.
Start by  folding a couple of inches of bag sealant around the vacuum line two inches from the end.  Tape the line to the forward edge of the metal bar at one end. Add a strip of bag sealant on the top and bottom of the metal bar along the aft edge. 
On the bottom side of the metal bar add sealant to close the ends of the seal on both ends.  The end seals extend beyond the metal bar and seal the vacuum line also.
Cut a sheet of vacuum bag film that will wrap around the form and seal to the metal bar on both sides.  Lay the film out and seal the bottom side of the bar down along one edge.  Add bag sealant on top of the bar on both ends, the seals extend to close the bag up to the leading edge of the form.  Do not remove the backing paper until you are ready to close the bag.  Add some breather fabric around the end of the vacuum line.  
The molded shell is made from a bias cut carbon fabric.  Lay out the fabric with narrow masking tape along the edges,  this stabilizes the fabric to control distortion.  The distance between the tape lines should be ½” wider than the final dimension around the molded leading edge.  The carbon fabric is wet out and molded between two layers of peel ply.  You can use nylon vacuum bagging film on both sides, but I find that outer surface quality is better if a paper peel ply is used on that surface.
Coverall release on paper Paper peel ply is made by painting the plastic side of freezer paper with a coat of Coverall mold release.  The mold release will need to dry for several hours before it can be used.  Both peel ply layers must be at least 3” larger than the carbon fabric on all edges. 
Stack up the three layers (paper peel ply,carbon fabric and nylon bagging film). Lift the top bag film layer and pour a line of laminating epoxy down the middle of the fabric.  Pro-Set or MGS epoxy is recommended for carbon fiber parts.  I also recommend that you use a West Systems Notched Spreader to spread the epoxy.  Of course I use the edge with no notches.  This tool is made from hard plastic so you can apply lots of pressure to remove all the excess epoxy from the fabric.  Start spreading the epoxy under the plastic film. Push the excess epoxy out of the fabric.  If you push the epoxy parallel to the carbon tows in the fabric you will reduce the distortion in the fabric weave.  Push the epoxy at least an inch from the fabric that way you will be able to see additional epoxy as it is pushed out of the fabric.  Flip the stack over and push the epoxy out from the paper side.  Flip over to the plastic side and continue to push epoxy out of the stack increasing the pressure until you can not see additional epoxy being removed from the fabric. 
Cut the excess epoxy and peel ply away around the edges of the fabric. Leave ¼” of peel ply around the edges.  Mark the center of the fabric on both ends.  On the bottom of the form line up the marks with the leading edge and tape the paper peel ply in place so it will not slip when you pull the bag around the leading edge.
Put the form in the vacuum bag against the metal bar, pull the film tight around the leading edge and stick it to the upper side of the metal bar starting at the center of the form.  Keep pulling the film tight and sticking it to the bar a few inches at a time out to one end then go from the center to the other end. 
Seal up the ends of the bag and start the vacuum pump.  There will be a few leaks in the bag but squeezing the seal at the leaks will seal the bag. 
After the epoxy cures cut the bag open and pull the shell off the form, trim the ¼” of excess peel ply around the fabric then peel the layers off the molded shell.  Remember you will need to wash the Coverall film off of the carbon shell with water.   The shell is now ready to trim to size and glue to the spar as the leading edge for your model.


Prices are in US dollars and subject to change without notice.