It’s
been a long time between blog updates again, but I will blame social media (and
getting married!) as a major contributor to this condition as I have posted
numerous quickie updates to our Instagram and Facebook project pages as
progress has been made on the rear bumper.
The lure of easy, on-the-fly updates has really put a damper on conventional
blogging and somewhere, I need to find a better balance. Also, I have been getting more and more video
requests and I continue to contemplate that with more seriousness. In the meantime, let’s get caught up!
In
Part 2, we left off with a polished mold that was ready to begin the process of
molding our first test parts. I was
under no allusions whatsoever that I would sacrifice a few test pieces to learn
the intricacies of molding such a complex part.
However, a few of the more important lessons learned along the way had
more to do with choices in materials options than technique. More on that in a bit.
For
the first test parts, it was decided to target a finished part thickness of
0.125” using a composite laminate matrix of fiberglass cloth, Soric infusion
core material, and infusion grade epoxy resin.
This combination laminate would be produced using the vacuum resin
transfer method of molding and results in a compression-molded part of near
perfect resin to laminate ratio with very little waste and almost no odor or
mess. Under a near perfect vacuum, the
laminate stack is under about 7 TONS(!) of compression force over the surface
of the mold before the infusion resin was introduced!
Like
anything scratch built, there are numerous ways to approach the creation of a
composite part like this. Rules are
rarely “hard-and-fast” and it is likely that several attempts and techniques will
be needed to arrive at the best overall manufacturing solution. Our little bumper project has certainly been
no different. Along the way, we have made
some rather unexpected materials discoveries along with a number of small (and
some not-so-small) setup and process details that make huge differences in the
quality of the finished part. And of
course, we have had a few failures along the way as evidence of our steep
learning curve.
Molding the First Part –
Success in Failure
Someone
once said: “Success is a terrible teacher.”
The longer I live; I have come to understand this concept more and
more. If we were immediately successful
at everything we attempt, there would be no growth. No improvement. No evolution of a craft. We would be effectively stagnant.
With
this idea firmly in mind, the transition into our first molding trials was much
easier as we were predisposed to accept the inevitable failures as our most
valuable teachers. In this, the evolution
of the molding process and techniques has been quick to develop and our part
quality has improved very quickly as a result.
But as I have committed in the past, I will share the “ugly” so anyone
who follows this work will know just what was involved should they ever
endeavor to do something like this themselves.
Like
most endeavors, research into the processes and materials involved is a place
that demands discipline and time. Around
our shop, I am most often accused of “overthinking” most things. However, I am reasonably confident that do so
has saved untold thousands of dollars in wasted materials on many aspects of
our projects. However, it may be true as
well, that I have cost considerable time in my rather slow evaluation
process. In any case, I am sure the
argument will rage long after I’ve been sent to the shed and handed the
“noodling” off to someone else.
As
it goes, our research into our desired molding process had many aspects pinned
down with certainty. For example, we
knew exactly what laminate components we would use (e.g., epoxy resin,
reinforcements, layup order, vacuum pump and resin trap setup, general vacuum
bagging configuration, etc.). These
details are relatively easy to confirm as there is a large database of
experience out there to tap for this type of thing. However, we discovered that there are equally
numerous “intricacies” in this realm that seem to be left to your own
discovery. This became disturbingly
obvious on our first attempt to mold our first bumper sample.
From
the beginning, we committed to using the best available materials we could get
our hands on to avoid having issues as much as possible. In that, we chose the top-of-the-line vacuum
bagging materials based on recommendations from respected suppliers and seemingly
objective reviews by users. This ended
up being our first and most critical mistake.
Our
first mold preparation and composite stack layup went particularly well and we
were able to get into the vacuum bag assembly phase with everything looking
textbook perfect. The vacuum bag
assembly went well from all aspects until we reached the point where the actual
vacuum bag material came into play. We
chose a bag material with the highest available stretch properties (Stretchlon
200) and was advertised as being compatible with all typical infusion
resins available (particularly epoxy resins).
From the off, we had difficulty chasing down vacuum leaks that were
extremely small but had to be fixed to allow a proper cure. This involved many hours of careful leak
detection effort before we could achieve full vacuum integrity and could move
on to the infusion process. This should
have been our first warning with this particular bagging film as later
investigation showed numerous “micro-perforations” in the film as it stretched
to conform to the part.
However,
there was one unexpected reaction that doomed our first try in a major way. Specifically, when the moment came to
introduce epoxy resin into the mold under vacuum, the bagging film adversely reacted
with the epoxy infusion resin, turning into a rather greasy sheet of cling wrap
that lost all physical integrity at every single location where an even
moderate feature was positioned. In the course
of about 20-30 seconds, about a dozen massive holes opened up in the vacuum bagging
film and the entire mold was fouled after being roughly 75% through the infusion. At this point, there was really no choice but
to let it cure off and hope there was no damage to the mold.
Fortunately,
when the resin had fully cured, the failed part pulled from the mold (albeit
with fairly considerable effort) and gave us an opportunity to inspect the
surfaces that did see full saturation and we were thrilled with the
results. With this result in hand and
the experience well documented, more “noodling” ensued in the quest for root
cause of the bagging film failure. It
was discovered that others had experienced similar issues with resin
compatibility with this particular bagging film despite of the manufacture’s insistence
that resin intolerance was not an issue.
After considerable research, we found a different bagging film (Stretchlon
800, same manufacturer) that ended up working well with our resin choice and
was much easier to seal. MAJOR lesson
learned.
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| Mold preparation is the first step in any composite layup. Since we had a very nice, Class "A" mold surface, we were able to apply chemical mold release to the mold and begin our layup for our first part trial. |
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| In our first layup stack, we used three different types of fiberglass cloth. here, two identical patterns are being cut from a roll of fiberglass fabric. |
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| We included a hexcel core material called Lator Soric in the layup. This material build thickness into the part while allowing fast resin infusion through the honeycomb channels. When the part is fully cured, the remaining hexcel structure is an excellent strength enhancement. |
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| Here is a closeup of the Soric core material. The honeycomb channels made into the material allow resin to flow more freely through the material, providing faster and more complete wet-out of the entire reinforcement stack. |
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| While not as "drapable" as fiberglass fabric, the Soric is quite conformable and worked well in the bumper mold. Here, we have trial fit the Soric over the bumper buck to see how well it would conform to the mold cavity. |
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| In this shot, all of the fiberglass fabric and Soric core material is in place and the mold is ready for the polyester fabric peel ply to be added. |
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here, the peel ply layer has been applied over the laminate stack. Peel
ply is a fabric barrier between the laminate stack and bagging
consumables. When peeled away from the cured part, it leaves a textured
surface that requires no sanding before bonding additional elements (like
mounting hardware, etc.
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| The red screen in this shot is the plastic infusion mesh that is placed on top of the peel ply layer. This mesh allows even distribution of vacuum across the entire mold surface and allows the resin to flow through the media evenly across the entire part during the resin infusion process. |
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| This is the completed first mold trial with the vacuum bag pulled down under full vacuum and ready for leak check. It tool far too many hours to chase down every micro-leak we encountered using this particular bagging film and that was our first indicator that things would soon unravel on this attempt. |
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| After all vacuum leaks were addressed, we were able to pull the vacuum bag down to full vacuum in preparation for the resin infusion. |
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| We used an excellent epoxy infusion resin formulation from Composite Envisions. This resin is extremely easy to work with and has a viscosity that is perfect for the resin infusion process we use. Unfortunately, our first trial was spoiled by a bagging film choice that did not live up to its advertised capability on a number of critical levels. |
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| In all its ugliness, you can see in this shot how the first trial was on it's way to success before the bagging film failure. In only about 35 seconds, almost 75% of the laminate was infused before the resin flow stopped. |
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| A closer inspection showing the transparency of the laminate stack and where the infusion line stopped. For reference, there are three full layers of fiberglass fabric between the camera and the Lantor Soric hexcell core material. Top shelf materials are key! |
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| Up close inspection shows massive bubbles in several different layers of the laminate. This occurred immediately when vacuum was lost due to the failure of the bagging film. However, it is interesting to note how the facing layers of fiberglass fabric can be seen within the laminate ahead of the Soric core material. |
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| Even as a failed, incomplete part, the strength of the cured composite matrix cannot be denied. This is my fat, well-over-200-pound ass standing on the failed first trial part with it straight on the shop floor and the deflection in the part still keeps the edges comfortably off the floor at the lower edges by a solid 5/8" to 3/4". This was eye-opening to say the least! |
Molding the Second Part –
Success Can be Bittersweet
In
preparation for our second molding trial, we collected all of our notes and
decided to apply all of what we learned (a good thing) and a few theoretical
remedies we thought might help (not always a good thing).
One
very helpful change was to modify the vacuum infusion fittings that allow resin
and vacuum to be applied to the laminate through the vacuum bag. This allowed better vacuum distribution prior
to introducing resin into the bag and similarly, it allowed for better resin
transfer into the mold during the infusion process. Go team!
Next,
we reversed the pathway the resin would flow across the mold to allow the mold
to saturate more evenly. This was
another great move as the improvement in resin flow was obvious. Score!
Our
third improvement involved the method of laying up the fiberglass fabric into
the mold. We used smaller, more tailored
pieces to assemble the fabric stack and this allowed for easier conformability
to the more intricate mold surfaces and made for an overall easier and quicker
layup. We’re on fire now!
The
final (and nearly fatal) idea was to not rely entirely on the chemical release
agent to prevent the part from sticking to the mold. Based on our first de-molding attempt, it was
surmised that the release agent wasn’t quite up to the task and perhaps the
application of an extra release mechanism might be a worthwhile step. So, we waxed the mold using the same release
wax we used to pull the mold off the car based on its demonstrated success. Bad idea!
As
it turned out, the mold infused beautifully and we were quite ecstatic that out
new bagging film was on point and the resin flow through the laminate stack was
much improved. For that moment, we were
ROLLING!
The
next day would prove to be extraordinarily bittersweet. The mold was stripped of the bagging
materials rather quickly and we were rewarded with the early indications of an
absolutely bubble-free and uniform composite thickness of about 0.137”. However, when it came time to pull the part
from the mold, the bottom of our program dropped right out.
The
“sweet” part was that the bumper we ultimately produced was near perfect after
trimming. Even though it is technically “useable”,
its total thickness is a bit too thin for our liking and we will need a thicker
laminate in the final part. But the endgame
here was that the mold and part it produced, along with the bulk of the vacuum
resin infusion process was an overall success.
The
“bitter” came about as a result of some simple errors during our mold surface
preparation. As it turns out, in
preparing the mold surfaces for the second trial, we cleaned the mold a bit too
well and essentially removed the “base” of the chemical release agent from the
flanges of the mold. Then, it was
discovered that the vacuum bagging sealant tape was placed about ½” further
outboard from the original molding position leaving a thin strip of exposed
mold surface without enough mold release.
This was compounded by the discovery that the petroleum solvents in the release
wax we chose to add had reacted negatively with the water-based chemical mold
release, rendering both agents much less effective than either would have been
by itself.
The
end result was a part that was even MORE difficult to remove from the mold and
due to the adhesions that occurred around the outer mold flange where no mold
release was present, the gel coat was damaged as the part was pried from the
mold. Fortunately, however, the polished
mold detail surface was not damaged other than to have a very light etching,
similar to water spots, permanently affixed to the mold surface.
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| This simple shot is very telling relative to a failure in our second molding trial. The mold was cleaned very thoroughly just prior to this picture being taken and this effectively removed most of the chemical release agent base we had established from the first trial. The masking tape indicated that we moved the bagging tape about 1/2" further out on the flange edges. The areas under the masking tape would eventually have too little release agent applied and this allowed resin to adhere to the mold surface during curing. |
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| Our second trial layup was improved by using smaller pieces of fiberglass fabric that allowed better conformability to the mold details. |
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| Here, the peel ply, infusion mesh, and resin feed tube and fittings are in place for the second trial. |
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| The new vacuum bag film worked very well on the second trial and was much quicker to verify perfect vacuum than the first bag film selection that plagued our first molding trial attempt. The photo above shows the second mold setup ready to be infused with epoxy resin. |
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| This picture shows the mold about ten seconds after the infusion was started. The resin front is just visible as a darker, edge across the bottom edge of the mold and as a larger dark spot in the center around the resin inlet point. The resin supply cup can be seen just at the bottom of this image. |
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| A close-up of the mold after being completely infused with resin. The composite materials are fully saturated with absolutely no bubbles. This method of construction achieve a near-perfect laminate to resin ratio and ensures the lightest and strongest part possible. |
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| This shot shows the second mold trial completely infused with resin with both the feed line and vacuum line clamped off for curing. After 24 hours, the mold was opened and the part removed (albeit with a few new issues we discovered as covered in the text above). |
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| Trail 1 & 2 together for comparison. The lower sample (Trial 2) is in rough trim and looks very much like the bumper we want! While technically two failed attempts, the evolution is undeniably positive. |
What Now?
With
a host of additional notes collected and many, many problems solved, the next
steps are rather clear and somewhat laborious.
First, the mold flanges are being repaired to restore the smooth molding
surfaces. This requires careful sanding
and preparation of the flanges to allow additional gel coat to be applied to
restore the surface and allow it to be sanded and re-polished.
Second,
the mold detail surfaces will be stripped and the entire mold wet sanded and
polished, starting with 800 grit and progressing all the way out to 2000 grit
and then polished to a fresh “Class A” surface.
This will restore the entire mold to “as new” condition and we will get
on with our third molding attempt while incorporating all of our lessons
learned.
The
biggest differences we will make are a move from epoxy resin to vinyl ester
resin. This will allow the use of a much
wider variety of fiberglass materials (like CSM) and is much cheaper and more
impact resistant than epoxy. It is also
less “reactive” than epoxy which should help in overall materials compatibility
as well. Also, we will be modifying our
laminate stack to produce a thicker overall part that will be easier to work
with in terms of body fit and it will be stiffer and more temperature stable in
the elements. And finally, we will stick
to the chemical release agent exclusively and quit trying to outsmart
ourselves. Fingers crossed!
Until
next time!
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| Here is the bumper pulled from the second trial in rough trim. Even after weeks of trial fitting to the body, the part fits the mold perfectly! |
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| We have been using the second trial bumper sample as a crude masking device to protect the mold surfaces while we begin repairs to the mold flanges that resulted from our mold release faux pa during the second trial. |
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| Here is where we are today: The damaged mold flanges have been sanded smooth and the voids that were created in the gel coat have been filled with fresh tooling gel coat and allowed to cure. We will finish these flanges with 800 grit paper and them proceed to wet sand and polish the entire mold to return it to Class "A" finish specs before we attempt another pull. |
