Rear Bumper Fabrication Project – Part 3: Process Development & Test Parts!

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

After all vacuum leaks were addressed, we were able to pull the vacuum bag down to full vacuum in preparation for the resin infusion.

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.

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.

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!

 

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.

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.

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.

Our second trial layup was improved by using smaller pieces of fiberglass fabric that allowed better conformability to the mold details.

Here, the peel ply, infusion mesh, and resin feed tube and fittings are in place for the second trial.

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.

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.

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.

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).

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!

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!

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.

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.

Rear Bumper Fabrication Project – Part 2

Another several weeks have passed and A LOT of progress has been made on this new bumper project.  As in all things composite, the “tooling” phase is without a doubt the most expensive, in both dollars and time, of the entire composite part manufacturing process.  Well, this one sure is living up to that standard as well!

We left off with a rough plug that was “skinned” in light fiberglass and epoxy resin and ready for the final shaping and finishing processes.  The shape of the bumper was starting to emerge quite nicely, but to get the plug to an “A” class surface and as perfect as I could make it would take a tremendous amount of time and patience.  Fortunately, as the project continued to emerge, the encouragement I started to get from other 70 Mustang owner was really starting to build.  I must admit, it’s very motivating when others see your work and want to put it on their cars too!  Even though this bumper is a 1970-model only fit, even the ‘69 guys were pushing to have me do a version for their unique requirements.  We’ll have to give that some serious consideration once this one is done.  Anyway, enough of that, let’s catch up!

Finishing the Plug

With a plug is “near-net” shape, the finishing process is very similar to many aspects of regular automotive bodywork.  The surface has to be filled and smoother to a surface that can accept a finish that can be polished to a very high standard.  However, there is a detail element involved that is much more complicated that is necessary to ensure symmetry and “flow” in the part so its finished state is visually appealing without any areas that draw specific attention to themselves for being “out of place”. 

The final plug shape and finish is essentially achieved with multiple filling and sanding phases using various fillers, glazing putty, guide coat and a mountain of handmade templates and station markers to ensure the left matches the right, the top matches the bottom, etc.

To get the best guide surfaces I could, I used a combination of different colored high-grade body fillers along with dry guide coat to give me as much visual feedback as possible on the finer surfaces.  However, the human eye is simply not good enough to rely on exclusively to ensure the complex shapes are identical from one side of the bumper to the other.  So the first order of business was to create templates from the foam pattern that would work as precision guides to get the finished plug surfaces to match.  Now, a dumb old bumper project would theoretically be pretty simple to nail a shape.  Well…….nope.  By the time the shape could be truly documented, we had made some 37 different templates to characterize ONE HALF of the bumper so we could get the other side to match.  By the time all was said and done, I had been sanding and fitting templates for almost 6 weeks before we could work on finishing the gaps in preparation for pattern coating.

After skinning with light fiberglass cloth, a "scratch-coat" of body filler is applied to the entire plug surface to form the foundation of the final shape.

For the initial heavy filling, Evercoat Rage Gold body filler (a shop favorite) was used for it's ease of application, and smooth sanding characteristics.

As the shape becomes more refined and filling come sin thinner and thinner layers, we switched to Evercoat Metal Glaze for the final detailed surfaces.

Clearly seen here, several layers of fillers are used in contrasting colors along with dry guide coat in the process of refining the shape of the bumper plug.

Station lines and markers are added to the surface to allow accurate location of templates used to gage the final shape.

Over 37 individual pattern templates were made to ensure the most perfect final shape that we could achieve.

The simplest features often turn out to be some of the most complex.  The license plate aperture is so simple in appearance but incorporates many intricate details and compound curves to get the look just right.  In areas like this, precision templates are a lifesaver!

The end cap fit-up is really where a custom fit rear bumper shines.  The flow into the natural curves of the Mustang body are key to a good looking finished product.

This kind of symmetry from one side to the other is only possible using many precision templates when making a plug by hand.

And that's it!  The final finished shape is set and ready for the finishing touches to be applied.

Gapping the Plug for Final Shape

As a last step before applying the pattern coating, we needed to perfect the body to bumper gaps to precisely match the shape of the stock ’70 Mustang body contours.  This is not as easy a job as I had imagined and it required a rather creative solution to applying body filler to a gap while minimizing the work to create a precision edge.  The solution that I settled on was to use fluted plastic sign board as a slightly compressible backer that would allow me to install the bumper plug to the body with the sign board sandwiched in the gap so I could apply filler directly into the gap and against the sign board.  This allowed for a very precise and consistent gap to be shaped with a minimal amount of filler material and very little sanding to get the desired shape.  With the exception of the end caps which had to be gapped by hand, the gaps were very consistent and quite easy to achieve using this unique technique.

To get the gap edges of the plug perfect, we used fluted sign board as a barrier to allow the plug edges to be filled with body filler to the precise shape of the body as translated through the sign board.  This technique worked flawlessly and allowed very precise gaps to be established in very short order.

With the bumper plug installed over the sign board barriers, body filler was simply applied to the gap edges as deep into the gaps as possible and allowed to dry.

Most of the excess body filler was removed before it set.  This is a good shot of how the lower gap edges looked after filling.

after the body filler had set into the gaps overnight, the sign board barriers were removed, revealing gaps that were perfectly set and requiring only a minimum of sanding to establish their final shape.

The end cap gaps were done by hand as the sign board was not flexible enough to conform to the tight curves in these areas.

Pattern Coating:  The Key to the “A-Class” Finish

A lot of composite pattern makers will apply standard epoxy automotive primer to a plug at this stage and sand it out to 800 or 1000 for the final finish.  This is certainly viable for many molded parts that will be finished along with the other bodywork on a car.  However, if there is any chance you decide to mold a carbon fiber part that will be displayed “raw”, the plug surface, and consequently the mold surface, must be polished to the surface quality you expect in the finished part.  The reason is that the mold surface itself will be precisely duplicated in the final part, so the better the plug surface is, the better the finished part will be and the less finish coating or polish work will be required on the final part.

As we have decided that we may offer copies of this bumper to other customers, it was decided to produce a polished, tooling-quality mold that would allow a very high quality surface in the part so minimal prep work would be required to fit.  Generally speaking, all composite parts used on older muscle cars will require a little bit of work to get them to fit each individual car properly.  That is usually because the composite part is actually much more consistent than the cars they are intended to fit.  As much as we would like to believe our old cars are exactly the same from car to car, the truth is they are sometimes unimaginably different (sometimes as much as ½” in the rear bumper area on a 69-70 Mustang!!!).  However, starting with a part at a higher grade of finish is always better than the alternative.

This leads us to the application of “pattern coating”.  Pattern coating is quite simply the composites version of an automotive high-build polyester primer.  However, true pattern coating has a unique chemistry that allows it to be wet sanded well out past 2000 grit if necessary and then polished to a true “A-Class” finish. That can be molded directly.  This is relatively “new” technology that has many valuable attributes in making composite parts.

I use a product called Duratec and spray it using a dedicated HVLP gel-coat spray gun with a 2.0mm tip.  A medium-heavy wet coat of about 40 mils is applied and allowed to cure overnight.  Then the bumper plug is reinstalled on the body and dry sanded with 220 paper to flatten and shape this “scratch coat” to the desired shape.  This first coat is designed to fill and level any imperfections left from the final plug shaping and seal all of the various materials, leaving a homogeneous surface that can be prepared for molding.

Once the first coat has been sanded, an additional 2-3 coats are applied, wet sanding to finish between each coat with 400 grit paper, until the surface is smooth and without any voids, pinholes or imperfections.  The final coat is then wet sanded in stages out to 1500 grit and the entire plug is then polished to a full gloss with Meguiar’s #105 & 205 compound and polish.  After a thorough cleaning with standard wax and grease remover, the plug is finished and ready for re-installation and mold construction.

The first coat of Duratec pattern coating was applied by brush as an experiment.  While it worked fine, it left a surface that required far too much effort to sand to shape.  All subsequent coats were sprayed with an HVLP gel-coat gun with much smoother results and a lot less effort to sand.

Back on the car and the bumper plug pattern coating was ready for initial sanding with 220 grit paper.  This gets the shape established very quickly in preparation for the additional "finishing" coats to come.

Dry sanding starts with 220 grit on the first coat of pattern coating, and progresses to 400 grit dry on the subsequent coats.  From there it's all wet sanding operations out to 1500 grit.

Wet sanding of the final coat of patter coating starts at 400 grit as seen here.

Once the plug is wet sanded out to 800 grit, a dull sheen starts to emerge and you can really start to appreciate the final shape of the bumper contours.

At 1000 grit wet sanded, the surface has a satin shine to it.

The final 1500 grit wet sanded finish is really where things get exciting.  From here, the polishing phases will bring the surface to a very high standard.

Polished to a "A" class finish is the key to getting a premium mold surface.  The reflections in the plug surface show just how nice the surface has turned out.

The corner details really pop with the polished plug standing out against the primed body panels.

Making the Mold

To ensure as accurate a mold as possible, we will lay up the bumper mold directly on the car.  This ensures the most support to every area of the mold and ensures the best fit and finish of the mold.  Naturally, this can be a very nervous endeavor as many valuable surfaces of the car are at risk if there is any issue in building the mold or separating it from the body.

The first and arguably most critical step, in preparing the molding surface is to mask the entire rear section of the car in a layer of masking tape. This provides the ultimate barrier to any of the mold making materials and ensures there is always a sacrificial layer between the mold and the sheet metal.  On top of the masking tape, a layer of aluminum “foil” tape (as used in the HVAC industry) is applied to the body surfaces where the mold will be laid up.  This tape has a natural self-releasing characteristic to it and works very well in “on-body” molding like this.  It readily accepts a thin layer of release wax or liquid releasing agent and will sacrifice its bond from the underlying masking tape rather easily if necessary.

Then the bumper plug is carefully mounted on the body, ensuring the fit is precise and alignment is exact.  This is the last time the plug will be fitted before the mold is made, so extra care at this stage is important.  Once installed, the bumper plug can be cleaned once again with grease and wax remover and all of the gaps filled with filleting wax and smoothed.  Filleting wax is a special beeswax-based formulation made especially for composite molding.  Each gap between the bumper plug and the body is carefully filled and smoothed to allow a clean blend between the surfaces that will ensure a clean mold transition and avoid mechanically locking the mold to the body or plug.

Next, the entire surrounding surfaces of the car are masked off and tooling gel coat is sprayed onto the surface to create the foundation of the mold surface.  Tooling gel coat is a very thick liquid polyester resin that is difficult to spray without a proper gel coat gun.  However, it is essential that gel coat is sprayed rather than brushed to ensure the absolute highest quality mold surfaces.  Two to three coats is usually required to get the proper film thickness and once the last coat has cured to a firm tack, the first “lock layer” of fiberglass is applied to the mold area using a high-grade isophthalic polyester resin and few layers of light fiberglass cloth and allowed to cure overnight.  This light cloth lock layer prevents the heavier chopped strand mat (CSM) from printing through into the gel coat layer and reinforces the gel coat in the process.

Over the next several days, an additional 7-10 layers of CSM is added to the entire mold surface to build thickness and strength into the mold.  We were looking for a mold thickness between .250” and .375”.  On larger parts like a full body panel, this thickness would need to be about 0.500” to provide proper structural integrity for accurate parts reproduction.  After the final layers of fiberglass mat are applied, they are allowed to cure for several days to ensure complete integration of all layers.

At this point, the mold will look rather ugly with fiberglass “hair” creeping from around the edges in places and drips of resin covering the masking paper on the body and drop cloth/paper covering the floor.  The mold will be very rigid to the touch and inevitably, the creeping fear that it will never come off will start to invade your brain (ask me how I know!).

Masking the body surfaces starts with standard masking tape and is followed up with aluminum HVAC "foil" tape. Note the tail light openings have been covered in thin sheet metal panels to allow a smooth molding surface all the way across the back of the panel.

Once the foil tape is down, it provides an excellent barrier to the polyester molding resin and has excellent self-releasing properties with even small amounts of release was or chemical release agent.

Special filleting wax is used to fill all of the gaps around the bumper plug before molding.  This wax is a beeswax based material that softens to a moldable consistency with the warmth of your hand and is easily smoothed and shaped.

Filleting wax is carefully smoothed into all of the gaps around the plug to ensure a leak-free and smooth fillet.

Another view of the filleting wax used on the outboard gaps of the bumper plug.

A small piece of fluted sign board was used to fill behind the license plate recess to provide a smooth molding surface and filleting wax was used to seal and radius the inside corners prior to molding.

The final step before applying gel coat is to mask the entire rear of the car from overspray.

Gel coat should only be applied by spraying using a special gel-coat spray gun.  We used a 2.5mm tip in our gel coat gun and the product sprayed beautifully.

The finished mold included about 10 different layers of fiberglass reinforcement to achieve the desired finished mold thickness of between 0.250-.375".

Pulling and Trimming the Mold

If all of the proper molding steps and precautions have been strictly followed, this phase of the mold making process stands to be very rewarding.  Also, if ever there was a time to NOT get impatient and in a hurry, it’s now!

We started the de-molding process by removing all of the masking paper from around the mold and most of the surrounding masking tape.  Then, using a few flexible plastic wedges, the mold was gently pried off of the cast aluminum end caps and allowed to sit for a few minutes.  This mild separation pressure started to allow the mold to release from around the complex ends with audible light “cracking” sounds.  Next, a bit of air pressure was directed between the lower valence and the mold which allowed a very substantial separation of the entire lower mold edge.  At this point, the mold was simply lifted free of the car with more light “crackling” noises until is lay in our hands as a perfect female mold of our bumper plug.  No mold damage, no plug damage and no body damage!

Immediately, all of the residual filleting wax was scraped off the mold with a Popsicle stick and the mold was cleaned with wax and grease remover to remove the bulk of the mold release and filleting wax residue that remained.  Then, the flange surfaces were trimmed using an air-powered body saw and the mold edges were sanded smooth with a DA sander and 120 grit paper.

With careful forethought and preparation, the mold should pull cleanly from the body with a little help and patience.  Our mold pulled cleanly from the body with very little force, leaving the bodywork undamaged and the mold in near-perfect shape. 

After trimming and smoothing the edges, the mold is ready for final finishing.

Finishing the Mold

In similar fashion to the plug itself, the mold requires a little finish work to ensure the best molding surface condition is achieved prior to making the first part.  Sometimes, there are slight imperfections (a grain of dust or stray mold release residue, etc.) that causes a small blemish on the surfaces well.  So, using the same technique of wet sanding (starting with 800 grit then out to 2000), we condition the mold surface for final polishing, once again using the Meguiar’s #105 & 205 polishes to bring the surface up to a mirror gloss.  The flanges themselves are also hit with compound and polish to smooth any small snags that may hurt the vacuum bagging materials or snag skin during the molding process.

Small blemishes and textures can be easily wet sanded out of the mold surface using the same techniques used to finish the pattern coating on the plug surface.  This small correction has been wet sanded in 800 grit and is now ready for final finishing and polish.  

The surface correction has been finished and polished out and is now absolutely invisible.

The finished mold ready for the first trial part to be molded.

And that’s it!  While this post rounds out several months of work, we are finally at the point where we can start planning the first molded part.  We have all of our vacuum infusion molding equipment in place and we are well on our way to having the exact reinforcement schedule figured out along with the resin selection.  The target layup will be a 0.125” thick part with a hexcel core material in an epoxy/fiberglass composite.  This should result in a very strong and light finished bumper that can be finished in the same method and standard as the rest of the body.  Part 3 should be fun!