Rear Bumper Fabrication Project – Part 1

While this blog showed little activity throughout most of 2017, the fact is, there actually was a bit of progress being made on a somewhat ambitious endeavor I decided to undertake.  The following update spans several months of time in the project and MANY hours, but is condensed for easier reading.  As of this writing, we are working consistently on the project in hopes of moving the project forward with much more energy.  But first, a little background:

A few years back, I was on the hunt for a ready-made composite “tucked” rear bumper and in the market; there were a few commercial outfits that offered them.  So, I set to doing my “homework” and communicating with a number of builders who had experience ahead of mine, I was able to quickly eliminate two of the offerings for either fit issues or quality issues (or both).  That left a single source that talked a very good game and frequents most of the larger Mustang forums on the net (mostly to maintain a passive-aggressive advertisement campaign that is clearly frowned upon by most forum admins, but that’s another story).  Anyway, with some trepidation, I ordered a tucked fiberglass bumper from the outfit and was assured I would love it and that “countless” customers have installed them without a single problem.

The day came when my new bumper arrived and I headed straight out to fit it to the car and admire the end result.  Ya………never happening.  While the gel coat surface was pretty decent, the bumper fit like absolute CRAP.  So, trying to be the “good feedback” customer, I documented every issue with pictures and verbal description, along with measurements where needed and sent it over to the supplier.

Naturally, on review of my notes, the response was the usual “nobody else ever had a problem with it until now” and “there must be something wrong with your car” routine.  Given that data and Ford body assembly manuals and specifications don’t lie, I returned the garbage bumper for a refund and set off to thinking about alternatives.  At the end of it, the options that were out there really didn’t have the right “look” I was after anyway, so I guess there was little lost in the end but time.

Selecting the Fabrication Method

First on my mind was to just knuckle-up and chop and section a factory steel bumper and get on with it.  But, if you have ever done a chopped, sectioned and tucked steel bumper for a Mustang, you will understand my hesitancy in choosing that route if other viable alternatives exist.

After a number of “happy-hour” conversations with friends and colleagues in the “biz”, I was encouraged to build a bumper from scratch.  But instead of steel, go to the trouble of making a tooling-quality mold and fabricate the bumper from vacuum-bagged, resin infused epoxy/carbon fiber (or epoxy/fiberglass) laminate.

My first thought was; “Sure!  Let’s pick the one way to build a rear bumper that likely involved more work that working one up in steel!”  What a great idea!  As it turns out…….it was the perfect idea.  Will it be faster?  Nope.  Will it be less expensive?  Nope.  Do we have all of the tools to produce the mold?  Nope (but close).  Will it work to make a bumper exactly the way we want?  OH YES.  Will it allow us to make extra parts, exactly like the original, over and over again?  You betcha!  Are we just a wee bit crazy?  Beyond the shadow of a doubt!  So………..guess what we’re gunna do????

Building the Plug

On a complex shape like a rear bumper, the first order of business is to make a full scale model of the exact shape you want in a medium that can be easily shaped.  This model is often referred to as the “plug” or “buck” and in its finished form, will be a beautiful “A”-class surfaced model of the exact shape that the composite rear bumper will be.  From this plug, the tooling mold will be pulled and all finished parts will be molded in it.  This is a rather gross oversimplification, but you get the idea.

After countless hours walking my brain through the entire process I would use to create my unique, tucked rear bumper, I decided to construct the plug using a simple steel support structure with 2-part pourable, closed-cell urethane foam poured around it to perfectly match the plug form to the body.  This would allow the foam plug to perfectly index to the body every time so even the smallest details could be maintained and were repeatable each time the bumper model would need to be taken off the car for work.

After protecting the entire tail of the car with masking tape and waxed foil tape for easy release of the foam, a temporary box was fastened to the back of the car to contain the 2-part expanding foam we would use to produce the core of our bumper plug.  With an expansion ratio of roughly 30:1, we would need less than ½ gallon of foam base stock to complete the plug core.

A simple piece of 1/2" steel bar is used as the inner structure of the foam bumper plug for support and to provide a place where hard mounting points can be welded so the bumper can be easily mounted to the body.  Also, you can see the masking that has been applied to the rear bodywork to prevent the urethane foam from sticking to the body.

The steel support structure was bolted into place through the factory bumper bracket holes and the urethane foam components were mixed and poured into the temporary box and allowed to expand to match the body contours and cure overnight.  The next day, the box as cut away from the rigidly cured foam “blob” now stuck to the back of the car.  The mounting bolts were removed and the plug core was gently pulled away from the body, revealing a perfectly molded inner surface that registered very securely into the features of the rear bodywork.

A simple box structure was attached to the back of the car in order to keep the liquid, 2-part urethane foam from leaking and to provide an outside boundary to keep the foam against the body as it expands and cures.

Next, the plug core was bolted back on and the rough trimming of the shape began with very simply hand saws, an electric knife and Surform tools.  After about an hour, the bulk of the foam core was carved away and the very slightest vestiges of a new bumper shape were starting to emerge.  At this point, there would no longer be a need for the crude tools of rough shaping and all other work would involve very delicate sanding, measuring, sanding, measuring, sanding, and………measuring.

After the foam has expanded and cured, the temporary box is removed and the rough foam buck is rough trimmed to get rid of the extra material.

There really is no secret to establishing the final shape of the bumper.  In its simplest description, it’s pretty much the “artist’s eye” that guides the form.  I have had the image of what I wanted in a rear bumper clarified in my mind’s eye for several years, so I would often close my eyes and envision what I wanted and then open ‘em up and remove bits of foam that didn’t look like my mental image.

With simple sanding boards and Surform tools, the basic bumper shape is roughed-in.  The shape is roughly 1/2" to 3/4" larger than the final shape will be.

After rough shaping, the first stages of intermediate shaping can begin.  This takes the shape to with 1/8" or less of the final bumper profile.

Since the shape is essentially a mirror image from left to right, I made a series of almost 20 templates to document the final shape of the plug and allow me to replicate the shape from left to right as closely as possible.  The process to arrive at the final shape took roughly 80 hours of hand work before I was happy with the shape and fit of the base plug.  And once I had that shape completed and all of the templates made and massaged to fit perfectly, I removed the plug, placed spacers between the body mount and plug and reinstalled the plug again.  Naturally, this placed the entire plug surface proud of the body by the thickness of the spacers, so I had to once again shape the entire plug back to the desired shape.  However, since the templates were now fully complete, the process of “drawing down” the final plug shape was only about 8 hours.

Final shaping begins with lighter grit sandpaper boards and blocks and once a shape is defined, a template is made to ensure the shape is maintained and can be repeated later on.

The final foam bumper plug shape is established and checked many times with the templates to make absolutely sure the shape is correct.

“Skinning” the Plug

With the plug now in the exact shape I want, it needs to be protected from damage so the final bodywork and finishing can take place.  The foam plug surface is now very delicate and is easily damaged by even the slightest impact.  So, to protect the plug, a thin covering of an epoxy/fiberglass laminate must be applied to the plug to allow the shape to be preserved and protected and to provide a solid base on which body fillers, primers and paints can be applied to finish the plug.  The process of applying the protective laminate surface to the plug is called “skinning”.

This very light, 2-oz fiberglass veil cloth is used in the process of skinning the plug.  This material is very shear and when wet-out with epoxy resin, it virtually disappears into the laminate skin.

A layer of regular masking tape is placed around the perimeter of the bumper to help with releasing after the epoxy laminate has cured.

All of the fiberglass cloth is carefully cut ahead of time and numbered or lettered to aid in placement when the skinning process begins.  This saves huge amounts of time and ensure accuracy in the layup.

Quite simply, skinning the plug involves wetting the surface of the foam plug with a special epoxy laminating resin and embedding (laminating) a few thin layers of very light fiberglass cloth into the resin surface.  Special tools are used to roll out any air bubbles that may be trapped in the resin and cloth and the matrix is left to cure for several days to ensure complete hardening of the resin and stabilization of the plug.

The skinning process is a rather straightforward process of wetting out and filling the closed-cell foam surface with epoxy resin and embedding two layers of light fiberglass cloth into the resin.  Then all the air bubbles are rolled out of the matrix and the entire lot is left to cure overnight.

Trimming the Plug

Once the epoxy laminate “skin” has fully hardened, the plug is pulled from the body and any excess flash is trimmed off the plug back to the original shape.  Now, the bumper plug is much more stable and solid and it can be handled with confidence that no damage will occur and the shape will be maintained even when the bumper plug is not bolted to the body.

Prepping the Plug for Finishing

With the plug now fully skinned, the heavy masking was removed from the body and the plug was test fit to the body to begin to establish the scope of the finish work required.  In the “skinned” state, the plug is still pretty rough on the surface and will require very fine detail work to get all of the gaps perfected and the surfaces filled and finished to the perfect, class ”A” surface necessary to pull a mold from.  The bottom line is simple:  To get a perfect part, we need a perfect mold surface.  And to get a perfect mold surface, we need a perfect plug!  Needless to say there is a TON of work left on this plug to get where we need to go, but at this point, the shape is exactly what I want and it achieves the exact look I want in a tucked rear bumper.  When mocked up on the car with tail light bezels in place, the look is very clean and changes the look of the 70 Mustang tail substantially without looking odd, angular or “industrial.”

The skinned plug is now well protected and offers an excellent foundation to begin the final plug finishing process.  When completed, the plug will be a fully finished, "A"-Class surface from which the actual mold will be pulled.

This shot really captures the look of the bumper nicely.  This bumper shape is exactly what I have kept in my mind's eye for years and we will soon be able to reproduce it in either an epoxy/carbon composite or an epoxy/fiberglass composite.

Now, the process will slow down once again as we begin to refine the plug edges to achieve the desired gaps all the way around the surface and to begin final shaping of the license plate aperture.  Then we will move on to preparing the surfaces for final finishing on our way to mold making.  LOTS of work to go, but in the end, I think it will all be worth it!