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Carbon Fiber Fender Fabrication with Downing Atlanta Composites

March 9, 2009

In the 27th day of the month of February in the year Twenty-0-Nine (as well as the Monday after that Friday) I went to the site of Downing Atlanta Inc..  Through someone I met through my “Creative Decisions and Design” class Teaching Assistantship, I had been put in contact with Jim Downing, a Georgia Tech graduate who heads up the operation.  Downing, Inc. houses Downing Atlanta Composites, Downing Atlanta Superchargers, and is also the home of Jim Downing’s SCCA race team. 

My master’s thesis, which I am currently working on at Georgia Tech, is focusing on fatigue in cross-ply carbon fiber/epoxy laminates.  However, I have had no personal experience with the manufacturing of composite parts (with the exception of ‘reading about it’ in a textbook).  Particularly with respect to composites, the method with which they are manufactured plays a large role in determining their eventual mechanical performance.  So, my ignorance of composite manufacturing methods would be a very large hole in any claim I could make about being an expert in composite materials–master’s degree or no master’s degree.  

Fortunately for me, the planets aligned themselves and the sun shone in my favor: Downing Composites has extensive experience in manufacturing composite parts, I was very interested in banishing my ignorance in this area, and Jim Downing had a need for an extra set of hands to prep some car body fenders for an upcoming race.  So it was that I spent a couple 11 hour days working alongside Jerry ‘Rabbit” Lambert–that unfathomable well of composite manufacturing experience–and Jeff, his youthful apprentice in The Black Art of Black Fibers.  All are great chaps to the last man and I owe them, big time, for this experience.   Particularly since they were all kind (and patient) enough to allow me to take pictures while we went through the process of making the fenders.  Since they allowed me to take photographs, I will record the process here for internet posterity.

Thanks guys!


The first day I was there we created the molds for the racecar fenders.  The second day I was there, we created the fenders using the previously made molds.  Unfortunately:

1) I missed seeing how they applied the base surfaces of gel coat to the “pattern” (the term will be explained shortly).  The purpose of the project was to get an additional 2″ of wheel clearance relative to an older set of fenders.  To accomplish this, the older fenders had been beefed up with Bondo to get the additional 2″ of fender height and then gel coat was applied to this beefed up fender…  The ‘beefed up fender’ served as the form and shape which the new composite fenders will take and this ‘base form’ in composite manufacturing is called the “plug” or “pattern.”  If the plug had had to be made from scratch (i.e., if no “older fender to apply Bondo on top of” existed), a plug would have to have been fabricated from a foam block or some other material which could be sculpted to the desired shape of the final part…  This shape would then have served as the pattern which gel coat would have been applied to.  (Bondo is a two-part putty: a polyester resin which, when mixed with a hardener, will set to a rock-hard consistency.  The user can apply the putty while it is still pliable, allow it to set, and then sand it to shape.  It is a very commonly used ‘plastic body filler’ which can be used to fill in or build up components which have been damaged or dented.  “Tooling gelcoat” will be explained later.)

2) I also missed the last few steps of the fender manufacturing process: adding the last sheet of carbon fiber cloth over the top of the honeycomb core.  Thus, I also never got to see the final product after removal from the mold :-(.  Jeff and Rabbit had also worked on the molds a little bit in my absence (on the Saturday and Sunday between the Friday and Monday which I worked with them).  However, the work I missed on those days did not include any steps which would have been crucial for including in a blog entry about ‘how carbon fiber fenders are made.’ 

DAY 1: Making the Molds

When I showed up on Monday morning, the first two things I saw sitting on a couple tables were a pair of these:




The white surface which you see is the outside surface of a gel-coat shell which had been applied to the ‘pattern’ of the older fender, as mentioned earlier.  In the picture above, the arched area in the shell is where the front left wheel of the race car is sitting.  I took a picture of a photograph of the previous year’s racecar, and it shows where these front fenders are in the overall body:


The fenders on the racecar in the image above are the “old fenders” which had Bondo applied to them to get the extra 2″ of space between the tire and inside surface of the fender.  Several days later, after reinforcing the gel coat shell, these “old fenders” would be removed from the mold.  To flash forward in time for the duration of one picture (for the sake of illustration) here are the Bondo’d “old fenders” which were later removed from the fully completed mold, but which are actually currently inside and supporting that white gel coat shell shown above:


Essentially, my ‘practical experience’ with making composites begins after the point at which gel coat had been applied to these fenders… I ‘started work’ on making the mold part-way into the process: I was now starting work at the point where the mold is continued to be built up from that gel coat shell shown above.  It is unfortunate that I did not get to see how they made applied the gel coat to the ‘old fenders.’  To make up for this gap in my experience, the best thing I could do was dig up information on how gel coat is applied to a pattern, list the information here, and supplement it with simple drawings to make up for not having photos to show the process.  By doing this, I am hopefully not misrepresenting the process by restating what others say while not experiencing it myself.  But hey–you do realize that this is the internet, don’t you?


– Most information taken from this website as well as this one. 

Step 1: create a “plug” or “pattern.”  The pattern is the shape of the final composite part which you want to make.  It’s usually a solid object, and it is important to remember that the external dimensions of the pattern must be the external dimensions of the composite part which you plan to create

Many times while working with Rabbit and Jeff to build up the mold or the fender itself, I found myself getting confused because, as Rabbit says, “You have to think inside out and backwards.”  It is also important to note that the surface quality of the pattern will correspond to the surface quality of your composite part: if care is not taken to prepare the surface of the pattern, the surface of the final composite part will be equivalently shoddy (surface texture can also be improved by the next step: sealing the pattern with epoxy).  For the sake of illustration, we’ll say that the composite part we want to make is a conical shape and that the “wood pattern”–created to the dimensional specifications of the composite part you want to make–looks like the following (a cone is actually a good shape for the purposes of manufacturing composites because it has a draft to its sides which allows the pattern to be removed from the gel coat shell, as well as allowing the composite part to be removed from the mold):


Step 2: if the pattern was created out of a porous material, it must be sealed.  Wood and foam are common pattern-making materials and they are porous.  Patterns cannot be porous because the gel-coat inner surface of the mold, applied to the outside of the pattern is initially applied in a liquid form.  If the pattern is porous, the gel coat will be absorbed into the pattern and what you will have created in ths case is a pattern with a nice shiney gel coat–that cannot be removed.  Your pattern is ruined, and time is wasted.  The pattern and mold must be designed so that they can be removed from each other.  Sometimes this will require making a “parting line” in the mold. 

What can be used as a sealant?  Polyester or epoxy resins will work.  Obviously the thickness of a coat of epoxy, relative to the size of the pattern, is extremely thin.  Depending on the dimensional accuracy you want to achieve with your composite part, the thickness of that coat of epoxy sealant may or may not be accounted for when you are making your pattern.  I’ll represent the sealing of the theoretical “conical wood pattern” with a layer of theoretical “blue epoxy” whose thickness is severely exaggerated for the sake of illustration:


 Step 3: after allowing the sealant to cure, a wax must be applied on top of the sealant.  The wax will help prevent the gel coat from adhering to the epoxy surface of the pattern.  A Carnuba-based wax must be used (silicon-based wax will not work since it will not allow the PVA coating to stick to it–explained in the next step).  Several well-polished coats of wax should be applied with clean toweling, with the last coat allowed to stand for 24 hours before applying gel coat, to allow the volatiles to evaporate.  We’ll say the theoretical “wax” is light-brownish in color:


Step 4: Apply PVA (polyvinyl alcohol) to the waxed pattern.  PVA is a mold-release agent which gel coat epoxy is incapable of adhering to (Jeff did tell me this when I was working with him, and hey, it’s on the internet, so it must be true).  It is also conveniently water soluble so it is easily washed off of the gel coat shell later with water.  Regarding its application to the waxed pattern, I’ll just quote directly from a website I mentioned earlier: “Best applied using a spray gun using high air pressure (80 to 100 PSI) and low output of liquid. Apply several thin coats followed by a heavier wet coat of approximately 2-4 mils [read with a wet mil gauge]. If spray equipment is not available, acceptable results can be obtained using a poly foam brush, lightly wetted with PVA, and a delicate one-way brush stroke. Allow PVA to dry at least 30 minutes or until tack-free and glossy.”  Our theoretical layer of “PVA” is pink:


Step 5: Apply tooling gel coat.  It’s important to use tooling gel coat.  There are many types of gel coat, and many are used for cosmetic surface finishes, such as on the surface of boats to achieve an appealingly smooth and glossy finish.  Tooling gel coat, while it also is glossy and smooth to promote easy demolding, is also particurly tough–allowing it to withstand the rigors of demolding.  Tooling gel coat’s durability gives the mold the durability needed to withstand making many composite parts from the same mold.  Tooling gel coat of course can only be applied on surfaces which will allow the plug to be removed later.  Masking off areas with paper and tape to prevent key areas from being sprayed with gel coat can be done.  To quote one of the websites again:

Tooling Gel Coat will give the mold a hard, glossy and long lasting surface. Tooling gel coat is best applied with pressure pot spray equipment. The tooling gel coat should be applied in two smoothly sprayed coats of 20 wet mils per coat. Each coat should be developed with three spray passes. The first coat should be allowed to gel, (approx. 90 min.) before applying the second coat. An easy test to assure that the gel coat is properly gelled is to lightly touch the surface with your finger. If you leave a slight fingerprint and no gel coat sticks to your finger, the gel coat is ready for the second coat. This test will also apply to determine when the gel coat is ready for the first layer of fiberglass and resin. If spray equipment is not available, the gel coat can be applied with a brush. Brush the gel coat onto the plug with a very full brush load, brushing in one direction only. Keep the brush well loaded with gel coat. A wet mil gauge is very helpful in determining the 20 mil gel coat thickness when brushing or spraying the tooling gel coat. CAUTION: Do not allow tooling gel coat to cure completely as it may shrink and pull away from the plug. This means you should never leave tooling gel coat overnight or over a weekend without first laminating at least one layer of fiberglass. This is the most critical step.

Look!  It appears that our theoretical “gel coat” is a white surface layer! 


NOTE that only the sides of the pattern have been covered in gel coat.  This will allow the pattern to be removed later–the pattern is left in place, inside the gel coat shell, until fiberglass cloth and tooling compound have been applied to reinforce the gel coat shell.  This summarizes most of the work done prior to my arrival.  Note also:  if you look at the first picture in this post, you see there are some “skirts” or “flanges” around the edges of the shell.  These flanges serve as “overhangs” around the perimeter of the main composite piece.  They serve two purposes:

  1. After the composite part is made, the excess composite which was laid up on these flanges will be trimmed off, down to the true edge of the composite part.  It would be MUCH more difficult to lay up fabric if all the fabric edges had to line up exactly with the true edge of the composite piece…  It’s much easier to just trim off excess later. 
  2. The flanges serve as a surface to seal the vacuum bag down onto (explained in the second day of work).

These flanges had the approximate stiffness of poster-board material, and are affixed to the inside of the pattern with (green) tape.  That tape will be removed later, after the shell is fully reinforced: successive layers of glass laid up on these flanges will reinforce them.  I’m not sure what that ‘poster board’ material actually is…  I’m fairly sure it’s a material that can be waxed and PVA’d to promote successful mold release, because I do not think Jeff and Rabbit gel coated the inside of these flanges.

Also, the recommendation recently quoted from that website suggests that you need to apply a layer of glass to the gelcoat while it’s still tacky to prevent the gelcoat from pulling away from the pattern during curing.  I think the gel coat was allowed to cure in our case, but a material called flox (resin mixed with finely milled cotton fiber) was applied to the outer surface of the gel coat to promote adhesion of the glass, which we will next lay up on the gel coat.  So, there was some lost knowledge, and I only began to see these gaps in my knowledge of the process after I started writing this.  I hope to return to work to volunteer again at some point and I’ll clear up these questions if I do.


After the gel coat shell is made, it is time to reinforce it.  The gel coat shell, were it to be removed from the pattern at this point, is thin and flexible and, by itself, it cannot serve as a mold for building composite parts.  A mold for fabricating composites has to be resistant to thermally induced stresses during curing and must be able to withstand the rigors of part removal–this can be a forceful process of flexure and abrasion.  To reinforce the shell, it is beefed up with two things: fiberglass and a two-part plastic called “tooling compound.”  This is where I started helping Rabbit and Jeff out–putting fiberglass on the shell. 

First, fiberglass cloth pieces had to be cut from a roll of glass cloth.  They had rolls of material on a rack next to a cloth-cutting table with a measuring tape tacked to it.  Experience with building molds of this size allowed Rabbit to quickly cut out appropriately sized pieces of fiberglass cloth (they used coarse weave bidirectional fiberglass cloth to do this–sorry, didn’t get the specifications for it).  Rabbit had a notebook with cloth size measurements in it which would be sufficient for covering the surface of the fender shell.  Here’s the cloth cutting table and rolls of material:


With fiberglass cloth cut to size, the outside of the gel coat shell now has to be painted with epoxy using paint brushes.  This will make it easier for the fiberglass to be “wet out” with epoxy when the fiberglass is laid on top of the gelcoat.  First, resin and hardener have to be mixed together to create epoxy in its liquid form.  I’m not sure what brand of epoxy Jeff and Rabbit were using, but it was a two-part epoxy mixed in a ratio which depended on the application it was to be applied in…  For high heat areas (engine or radiator casing, for example), more hardener was needed.  For these fenders, four parts resin to one part hardener was appropriate.  A white cardboard tub is placed on a digital scale and resin and hardener are poured into it to the appropriate ratios:


The resin and hardener now have to be THOROUGHLY mixed (wood paint sticks were used), making sure to scrape the edges and bottom of the mixing container to get the two liquids to mix up.  Small batches are mixed, and the amount which is mixed is largely based on experience.  With epoxies, it’s important to mix in small volume batches (i.e., make up several buckets of small batches as opposed to one large bucket).  This is because a greater volume of epoxy will catalyze faster: the crosslinking which is occuring when the resin and hardener are mixed is an exothermic reaction.  A larger volume batch will retain more heat, which speeds up the cross-linking, which further limits the amount of time that the resin can be successfully applied to the part.  Here, Rabbit is painting the epoxy on the gel coat shell of one of the fenders:


Note that Rabbit is wearing long sleeves, pants, and two pairs of gloves: a cloth liner (this, largely for comfort) with a latex glove over it.  Hand lay-up of composites is a very chemical-intensive process, and these precautions must be taken to keep all of the chemicals off of your skin.  Jeff and I had short-sleeve shirts on, so we wore Super-Cool Arm Socks to keep the nashty shtuff off of our arms.  Jeff assures me that getting epoxy on your skin will cause it to break out, so the fashion faux-pas is worth it:


Once the gel coat has been well-painted with epoxy, it’s time to lay the first ply of glass cloth.  If a “zero-degree ply” is defined as having one of the fiber axes in the bidirectional weave along the length of the car, this first ply which is laid down is a “zero-degree ply.”  You can see that the edge of the cloth is being laid so that it is roughly along a line parallel to the length of the car:


If the cloth is hanging off of the edges of the gel coat shell, it is trimmed off.  These smaller pieces are used to cover the small areas of the form which were not able to be covered with the large pieces of cloth.  For complicated areas, such as the fender wheelwell ‘arm’–the thin piece running down behind the wheel–very small excess pieces of cloth can be used to build up the glass in that area…  The goal is to completely cover the surface with a ply which, roughly, has one of the fiber axes oriented at ‘zero degrees.’  Then, the paintbrushes are used to lightly dab and press the cloth down onto the gelcoat, ensuring a good ‘wet out’–meaning the glass cloth has fully absorbed the epoxy (this is visually evident, the glass cloth goes from having a white shiney appearance to a darker, wet saturated look–refer to the following image).  A back-and-forth painting motion with the brush is NOT used because this can pull the weave of the fibers out of alignment.  Next a “45 degree ply” is laid up on top of the “zero degree ply”: 


When arranging the different portions of cloth relative to each other, they should be laid down so that they overlap each other by a few inches to provide continuous glass coverage for that ply…  If adjacent pieces of cloth are laid up so that their edges abut each other perfectly, then a weak line in the ply will be created.  It is better to have them slightly overlap:


I was impressed by how easy it was to wrap a complex shape such as this with the glass fiber.  I thought laying fiber cloth on something like this would be like fighting to wrap an oddly shaped Christmas present in paper, but the cloth wraps around curved shapes and tucks into corner fillets very easily.  The same was true of the carbon which we laid up much later.  With enough dabbing with the brush, the glass cloth would lie down and wet out on any surface on this part, no matter how convoluted.  Dabbing the glass cloth down was a very important thing to do in this step in particular, actually; the glass had to be well seated in the corners, air pockets had to be removed, and glass ply wrinkles had to be smoothed out off the edges.  This was critical because these two initial glass cloth plies must be flat against the underlying gel coat–if voids exist in these two first layers of glass, the inner gel coat surface facing the fender will collapse inwards during vacuum bagging, and there will be blemishes on the final component.

Once the two glass plies were fully wetted and smoothed out, the brushes are cleaned out by placing them in a bucket of acetone.  The next step is applying tooling compound to the mold.  Tooling compound (they used Adtech EL 323 tooling compound) is a two-part plastic (blue resin, yellow hardener–mixes to make a uniformly green compound).  Tooling compound gives the mold the rigidity it needs to stand up to the abuse of demolding many composite parts from it.  It also exploits the bending moment equation familiar to engineers: Stress = (Moment)*(distance from neutral axis)/(Second Moment of Area)…  By placing the glass fiber sheets on the outside surfaces of the tooling compound, the strength of the glass fibers is placed farthest from the cross section’s centroid–for the strongest potential mold.  As mentioned, tooling compound is a two-part mix:


 While the tooling compound is mixed, another employee should prepare the tooling compound “rolling table.”  This is basically formed by taping parallel strips of 1/8″ foam core onto a table, and covering them over with the green plastic vacuum bagging material (which the tooling compound will not stick to)…  This creates an area where the compound can be rolled into flat sheets of uniform thickness with parallel surfaces–similar to rolling out cookie dough, but more precise:


The metal tube which is used to roll out the compound has to be continuously wetted with acetone.  The tooling compound’s tackiness causes it to adhere tenaciously to the metal pole if you don’t do this.  I never got the hang of how to roll out the compound without having it stick to the aluminum tube, so I was responsible for carrying the flattened sheets of tooling compound to the fender for arrangment and placement:


This is repeated until the entire fender is covered in tooling compound, except for the flanges:


 The last bit of work which we did on this day was placing a final layer of glass cloth over the tooling compound (again at the “zero degree” orientation).  More epoxy was made up and the tooling compound itself was painted heavily with it.  Another set of of pre-cut glass cloth was then arranged on top of the compound, being sure to extend the glass onto the flanges this time and to fully wet it out in all areas (by having the glass cloth of this layer extend onto the flanges again–as it had with the first two layers–the tooling compound is effectively sealed inside three plies of glass fiber cloth):


To assist in visualizing the work which was done in this first day to complete the mold, I’ll again employ the theoretical “conical part” which was used to illustrate the gel coat process earlier.  For clarity in this drawing, I won’t illustrate the layer of epoxy sealant, the coat of Carnuba wax, and the coat of PVA….  I’ll just show the gel coat adjacent to the pattern, as well as all the “reinforcement layers” which we applied while working today:


DAY 2: Making the Fenders with the Molds

The first day of work was a good 10.5 or 11 hour day.  This second day was about 12 hours of work.  It was on the second day that we started working on making the fenders themselves–as opposed to the mold needed to construct them in.  The second day’s work consisted of:

  • Cutting up and setting out the materials necessary for making the carbon fiber fenders (including the material for vacuum bagging).
  • Wetting out a primary layer of E-glass in the mold
  • Applying and wetting out the first layer of carbon fiber bidirectional weave
  • Performing the first vacuum-bagging process
  • Applying a reinforcing rim of carbon fiber around the edge of the fenders
  • Applying the “vail” which will soak up resin and seal the base of the honeycomb core to the carbon below it
  • Applying the pre-cut pieces of honeycomb and foam core into the fender
  • Performing a second vacuum-bagging process

I mentioned earlier that I worked with Jeff and Rabbit on a Friday and a Monday, and that they had done a little bit of work in my absense on Saturday and Sunday.  That work included (and is shown in the following image):

  • Removing the ‘old fender pattern’ from the inside of the cured mold (as suggested in the previous picture where the ‘wood pattern’ was removed to yield the ‘completed mold.’)
  • Flipped the molds over and mounted them in wooden support cradles to prevent them from deforming while carbon fiber is being laid up or is curing in them.
  • Reinforcing the flanges where necessary with strips of fiberglass and bolts
  • Applying a coat of Carnuba Wax and PVA to the inside surface of the gel coat, to prevent it from sticking to the composite part which we will be laying up.


Just as in the first day I worked at Downing Composites, the first order of business is cutting up and setting out all the material which will be needed for the day.  Today, the vacuum bagging process would be done twice, and thus a lot more material would need to be cut up during preparation.  Carbon fiber was first cut to length and again, Rabbit had experience with this size part before, so he had records telling him what sizes of cloth to cut up.  A neat trick which Rabbit showed me is, after having measured to where you want to cut across the carbon fiber cloth, you pull a single tow out of the weave (‘tow’ is another word for a strand of the “yarn” of carbon fiber–carbon fibers are incredibly small in diameter, and are grouped together into larger strands called ‘tows’ for weaving into the bidirectional carbon fiber cloth we are working with here)….  Removing one of the tows reveals a line that is straight across the cloth, relative to the weave.  Because the bidirectional weave is easily pulled out of alignment, if you just cut a straight line across the cloth without following the weave, you will end up severing many fibers, mid-strand, which are spanning the cloth.  This will weaken the edges of your cloth and your composite part.  Here is a picture of the carbon cloth with a single tow removed to guide the cutting process:


E-glass is actually going to be applied to the mold first in order to form the outer layer of the racecar fender (“think inside out and backwards”).  E-glass is used as the surface layer because the fender will be sanded and painted later.  The sanding process may damage the outermost ply, and rather than risk sanding into the carbon fiber by laying it up as the outermost layer, one ply of E-glass is put down to serve as a protective barrier.  E-glass cloth cut to appropriate lengths:


Peel-ply is cut to approprite lengths:


What is peel-ply?  When carbon fiber is wetted out with epoxy and the epoxy is allowed to cure, the surface will take on a very shiny, smooth texture if it is just left exposed to the air.  This shiny surface can actually be a hassle when a composite manufacturer wants to bond something to it…  The shiny, smooth surface will have to be roughened up with sandpaper before something can be successfully bonded to it.  Peel-ply saves you that headache.  By applying peel-ply to the wetted out carbon fiber surface before it cures, the a smooth, shiny surface is prevented from occurring during the cure process.  The peel-ply is microscopically rough and porous, and after the curing process is completed it can be pulled off of the cured carbon/epoxy surface to reveal a matte finish, not a shiny finish.  The matter finish is microscopically rough, just as the peel-ply is.  This rough impression left in the epoxy by the peel-ply enables a composite manufacturer to bond something to it immediately with more epoxy.  As a contrast between the “shiny” and “matte” surfaces which results from not using peel ply / using peel-ply, look at this close-up image of one edge of an oil cooler air scoop:


Perf (‘perforated’) sheet is also cut to length.  Perf sheet is a blue plastic film which has regularly spaced holes in it.  These holes allow resin to be sucked from the wetted out carbon fiber cloth, through the peel-ply, through the perf sheet itself, where it is then trapped in the bleeder sheet.  Perf sheet:


Green vacuum bag plastic is cut to length and so is bleeder sheet (‘vacuum bag’ is kind of a misleading term–it’s actually not a ‘bag’ at all, it’s actually just plastic rolled sheet which is twisted and looped and folded to fit onto an oddly shaped part such as this fender…  it’s then sealed to the flanges on the fender mold with sticky tack to form the vacuum seal).  The bleeder sheet is a blanket-like material which serves to absorb the excess resin which the vacuum bagging procedure removes from the carbon fiber cloth.  Bleeder sheet and vacuum bag:


The materials are stacked in the order in which they will be used, as shown below.  Also shown is a roll of yellow “sticky tack.”  Sticky tack is very similar to the gummy adhesive material which is used to affix posters to walls.  Its purpose here will be to form a seal between the vacuum bag and the mold.


Honeycomb / foam core will be epoxied between two layers of carbon fiber in the fender layup process.  This is done for the same reason that the tooling compound was used in building the mold: by having the carbon fiber cloth placed far from the neutral axis, the fender will have much greater resistance to bending deflection (except honeycomb core is much lighter than tooling compound!).  Here is an example of this layup which I saw in a ducting hole from an older fender:


So, honeycomb and foam cores need to be prepared and set aside for layup later.  These cores are traced out (with a Sharpie) to the dimensions specified by templates which correspond to the shape of the fender itself.  The core is cut out using a utility knife.


The vacuum valves are also set out.  These are two-part metal valves.  The upper half connects to the vacuum hose, which will provide the vacuum to remove excess resin from the system.  The lower half is taped to a piece of bleeder sheet inside the vacuum bag.  To connect the two pieces, a hole is punctured in the bag where the lower half of the valve is placed on the fender layup inside the bag, and the upper half is twist-locked to the metal base through the bag…  An o-ring on the periphery of the vacuum valve seals this interface to maintain the vacuum.  Vacuum valves:


Since the mold had already been prepped with Carnuba wax and PVA by Rabbit and Jeff the day before, we applied resin to the inner mold surface with paint brushes.  This time, a small amount of black pigment is added to the resin to blend the resin in with the carbon fiber (which will be applied later).  As mentioned earlier, the first set of cloth which will be applied (which will form the outer surface of the racecar fender) is E-Glass.  Here, Rabbit and Jeff are laying the E-glass into the mold and cutting it to shape:


Just like before, a dabbing motion must be used to wet out the cloth with epoxy; this will prevent the fibers from being pulled out of alignment in the bidirectional weave:


After the E-glass is thoroughly wetted out, the first layer of carbon fiber is laid out on top of the E-glass.  It too is wetted out with the black epoxy and it is trimmed to size, so that the green tape on the flanges is still exposed (this rim of green tape ensures that room will be left on the flange for the sticky tack seal which must be placed on the flange to form the vacuum bag seal):


Now, the process for creating a vacuum bag begins.  After the glass and carbon have been laid up in the mold, the fender needs to be vacuum bagged and cured so that these layers of fabric are cured before honeycomb is applied to it.  The first layer in a vacuum bag is the brown peel-ply…  It is laid up such that it covers all of the wetted out carbon fiber which is visible and it is then pressed down by hand.  All you are trying to do here is make sure that the peel-ply is completely in contact with the wetted-out carbon fiber (you don’t want any of those constrasting shiny/matte areas which were shown earlier!).  Here, Rabbit and Jeff are laying up the peel-ply:


The next step in creating a vacuum bag is laying up the blue perf sheet (tabs of tape are used to hold the perf sheet and bleeder sheet in place):


Then, a bleeder sheet must be laid up to absorb the excess epoxy; you can see here that small bits of green tape were used to secure the perf sheet since it doesn’t adhere to the resin or stay in one place like the peel-ply did:


After removing the green tape on the flange around the peripher of the mold, the sticky tack is unwound and pressed down onto the flange around the whole length of the rim of the fender.  The brown paper backing is left in place for now.  In this image, you can also see that small bits of green tape are used to hold the bleeder sheet in place, just like the perf sheet was:


Two vacuum valve bases are affixed to folded pieces of bleeder sheet with tape and then taped down on ‘opposite ends’ of the fender.  One of these is shown here:


Now, the green vacuum bag sheet is brought over and set over the fender.  The sheet has to be pressed down to fill in the wheelwell cavity, and care must be taken to not puncture the bag.  Don’t unnecessarily crease the bag.  Then, the brown paper backing of the sticky tack can be removed and the bag can be pressed to the sticky tack to form a seal, this is shown here:


At sharp corners in the mold, bunch up the bag and bring it together to form a loop in the bag whose base is at a point on the sticky tack.  Then, tear off a spare piece of sticky tack, and seal the loop itself by flattening the loop out with the piece of sticky tack between it.  Be sure that the top of the loop is sealed off with sticky tack, and that the base of the loop is sealed shut as well.  A couple ‘sealed loops’ are illustrated here:


Now, the vacuum bag must be pierced near where the metal base of the vacuum valve is located inside the vacuum bag.  The upper half of the vacuum valve must be twist-locked onto the base to form a seal through the bag, with the valve base:


The layup and mold is now ready to be taken into the walk-in heating room (doesn’t get much hotter than 140 degrees F) to have the vacuum hoses connected to it so it can cure and have excess resin pulled from the glass and carbon into the bleeder sheet.  These are the doors into Downing’s walk-in heating room:


The mold and layup is set on a table in the heating room and the hoses are readied for plugging into the vacuum valves:


There are bolts around the periphery of some of the flanges and they must be sealed with clay to prevent leaking around them:


Then the vacuum hose fitting can be plugged into the vacuum valves on the bag:


The bag will begin to lose its volume as air is removed from the system.  As this happens, the bag must be pushed into any deep pockets in the mold to prevent areas where the bag does not compress flat against the bleeder sheet–to do this the part is “sticked” in order to force the bag into the corners, fillets, and pockets of the mold (being careful not to puncture the bag!).  A rectangular stick with well-rounded edges enables this:


After two hours, the mold can be removed from the heater, as the epoxy has sufficiently cured to carry out the next step.  Below, a picture of the mold is shown after it has been removed from the heating room with the vacuum bagging material removed–note the flat matte finish of the carbon fiber from the peel-ply!


There are four more steps remaining in the work we completed this day:

  • Applying a ‘reinforcing rim’ of heavy-duty carbon fiber cloth around the perimiter of the fenders.
  • Applying the ‘vail’ material to soak epoxy into the base of the core to seal it to the carbon below it.
  • Applying the honeycomb and foam cores.
  • Vacuum bagging and curing the part again.

So, the first step which remains to be done is reinforcing the edges of the fender with a high tow-count, 2″ strip of carbon cloth (sorry, I don’t have the tow count for this bidirectional weave, but I can qualitatively say that the cloth was much thicker than the cloth already laid up into the fender, which has already been cured).  To do this, 2″ strips are cut from a roll of carbon fiber cloth and they are impregnated with epoxy on a paper covered table–they are not wetted out when they are on the fender itself.  Due to the thickness of this cloth, these pieces have to be thoroughly painted on one side with epoxy, flipped over and painted again:


The carbon surface of the fender must also be painted again with black pigmented epoxy:


Then the strips are laid down and cut to fit around the perimeter of the fender, and dabbed down with the brush:


Next, a material called “vail” must be applied to the wetted out carbon surfaces of the fender in all the areas where the honeycomb core cut-outs will be placed.  What is vail?  Vail is a highly absorbent, thin, semi-transparent fabric which–not surprisingly?–looks like a wedding veil.  Due to the fact that it easily absorbs epoxy, when it is laid out on a carbon fiber surface which has been painted with it, it becomes saturated with the epoxy.  When the honeycomb core is applied on top of the saturated vail, the vail ensures every part of the honeycomb base in contact with it will be bonded by epoxy.  The carbon which was vacuum bagged and painted with epoxy already is very flat…  The minute disparities in the flatness of the base of the honeycomb would cause parts of the base to not bond to this flat carbon fiber surface if the vail were not present (leading to delaminations)…  The fabric vail fills those disparities with a resin-rich bond.  Black fabric vail being laid on top of the fender:


After dabbing down the vail to make sure it’s thoroughly soaked, the pre-cut honeycomb and foam cores are placed into the appropriate areas in the fender.  More vail must be applied wherever necessary, to make sure all parts of the core sheets have vail lying under them…  Note that foam core–not honeycomb core–is placed in the area directly above the wheel.  Rocks or debris can be kicked up into the wheelwell and this demands a stronger reinforcement than honeycomb could provide for this area:


Next, the part is vacuum bagged again, with some minor modifications of the bagging process done already; discrepancies between the two processes are listed here and pictured below:

  1. Peel-ply on all exposed areas of wetted out carbon (or vail)–same as before.
  2. Strips of perf sheet are placed on top of the honeycomb as shown–it is not necessary to cover all of the honeycomb with it, or even all of the peel-ply.
  3. Bleeder sheet channels are laid over this–this allows air to travel up through the honeycomb, into the bleeder sheet channel, then out to the vacuum hose.
  4. Valves are located on top of the bleeder sheet channels, as before.
  5. Vacuum bag and sticky tack procedure repeated, just as before.
  6. Molds are taken into the heating room and connected to the vacuum hoses, as before.  Two hours to cure.


This completes the work which I did with Downing Composites.  To summarize what we laid up today, I’ll again employ the theoretical “conical composite part” which I’ve used many times before.  This theoretical conical part does not include the final, inner layer of carbon which was added the day after I had to stop working with Jeff and Rabbit.


It was at this point that our work for the second day ended.  The next day, after I departed, the parts would be removed from the vacuum bag and the final layer of carbon would be applied.  I suspect that the final steps to complete this carbon fiber fender would be:

  1. Apply vail to the areas where the core had been applied (I suspect that a layer of vail would have to be used–I can only imagine that, without it, the core would delaminate from the next layer which is to be applied: carbon cloth).
  2. Cut carbon fiber cloth pieces and wet them out on a table (I know for a fact that another layer of carbon of fiber was to be applied, and Jeff told me that wetting the carbon cloth out on a table before application to the fender was mandatory–otherwise the epoxy you brush on would just run right through the carbon and fill up the cells of the core).
  3. Apply the wetted out carbon fiber cloth pieces to the fender, being sure to ‘seal the core in’ by painting this new layer of carbon down onto the the ‘2″ thick-cloth reinforced rim’ applied earlier (similar to the way in which the tooling compound in the mold had been ‘sealed inside’ with several layers of glass cloth).
  4. Final layer of E-glass (maybe?  the inside of these parts probably do not need to be painted, and therefore might not require a layer of E-glass for sanding/painting protection).
  5. Sand and paint the fender, do any drilling or bonding for the purpose of making the fender able to be mounted onto the rest of the racecar body.


So that finishes the summary, as well as what will hopefully be the longest blog post I ever make.  Golly that was one humdinger of a huge post…  I know I learned something through the experience, and I hope I was able to convey it effectively enough so that you could too.  And again, I’d like to thank the guys at Downing Composites!

– Justin Ketterer

6 Comments leave one →
  1. Jean-Francois A permalink
    December 2, 2010 11:21 am

    So all your details are pretty great and very nice to see. I also do many part by infusion method.. but i wonder when the vacuum is pull and you see your resin taking into the bleeder.. How much vacuum is use ? 20inHg.. 26inHg ? .
    Thanks a lot

    • justinketterer permalink*
      December 3, 2010 1:48 am

      Honestly, I have no idea how much vacuum Downing was running. Sorry!

  2. September 21, 2011 2:30 am

    Thank you from the information

    • justinketterer permalink*
      September 21, 2011 11:22 am

      You’re welcome!

  3. david permalink
    January 18, 2013 4:45 am

    great info Justin! usually infusion requires a full 30inHg, or “full vacuum”. I usually pull 30in even when doing wet layup.


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