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!