Fiberglass Mold Making and Fabrication How To – Project Blog

After the resin cures, I like to let the new mold sit untouched on the plug for a few days.  Letting the mold sit for a few days prior to de-molding helps prevent warping of the mold.  In this case, my plug is of a long, thin fender and has a much greater chance of undergoing a noticeable warp then some of my other projects so I am more careful at this stage then usual. 

De-molding the fender becomes an easy task.  The PVA and wax layers were not breached and as soon as I began to pull one corner of the fender plug loose the entire mold separated from the plug.  Just as planned – so far.

The next step in a project like this is to clean up the new mold of any defects or sharp edges around the flange areas.  I begin by using a rag and water to remove the PVA from the inside of the mold.  The result of my cleaning reveals a perfect molding surface.  Had I found any defects, I would have filled them with bondo, sanded them smooth and then thoroughly waxed over them before molding any parts.

In order to make my first parts with this mold I begin by applying several (four) coats of mold release wax to my mold’s surface.  When the wax has dried and has been polished to a smooth shiny finish, I apply two coats of PVA with a foam brush.  On larger projects I will use my air gun but this project didn’t warrant that much hassle.  In this case, I applied a light PVA coat, allowed it to fully dry, then applied a second coat for extra protection.  With both the PVA and the wax applications I made sure to cover any areas of my mold, including the flanges that were formed over the parting plane.  This will insure that my parts don’t get stuck on the mold.

My most recent fiberglass project is a set of fenders for my beach cruiser. I know that there are already fenders available on the market, but I really didn’t care for them. In addition, I do enjoy building my own things when I can and it just so happened that this would be one of those projects.

To start with a project like this, I first needed to decide how I would get the shape and size of a fender that I was looking for. I want to have a fender with a round contour that would provide better coverage of the tire than what store bought fenders provide.

Initially I considered obtaining the right shape for the fenders by covering an inflated tire on the rim with foam weather stripping and then using that as the foundation on which to build a fender. The more I thought about this the more problems I was imagining.

Then one day I was walking by the 99 cent store and I saw those round foam pool toys that the kids use, I think they are called noodles. I knew right away that I was 99 cents away from the perfect base on which to build a plug for my fenders.

Once I got that foam roll home I pulled a wheel off my bike, removed the tire and proceeded to tape that foam roll to my rim. It fit the channel perfectly, and was the right height and width. I wanted my fender to cover about one half of the diameter of my tires so I used enough of the foam to accomplish this.

With the foam roll secured to the rim, I decided to cover the foam with a layer of masking tape. I did this to protect the rim and because I wasn’t entirely sure what the fiberglass resin would do to the foam, or if it would release cleanly from the foam.

At this point it is time to think about applying the fiberglass resin and mat. I began by building a simple cardboard stand to hold the rim steady while the materials are applied. Once I was happy with the stand I covered the entire rim and tape covered foam with PVA release agent.

Now I am ready to build a fender plug.

Happy Glassing

Steve

www.fiberglassmoldmanual.com

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Fiberglass Mat Fiberglass Cloth

To begin laying up a fender, I mix about a half a cup of gelcoat with the recommended amount of hardener.  Once this is thoroughly mixed, I use a disposable chip brush to paint the gelcoat into the mold.  I try to get a heavy layer of consistent thickness.  This gelcoat needs to cure to a tackiness before I apply the fiberglass mat and resin.  Usually this takes about 45 minutes. 

While I am waiting for the gelcoat to cure a bit, I begin to prepare for the next steps of this process by tearing 1 ½ ounce mat into small pieces that will be easy to lay into my mold.  Since this is a fender, it has compound curves – one that goes side to side and one that goes front to back.  In my experience, smaller pieces of mat are easier to work with in a mold like this.  Just FYI, my pieces of mat are approximately 4” X 4”.

Now that the gelcoat has cured to a tack, I mix an 8 ounce cup of resin and catalyst (as directed by the manufacturer) and begin to work my way around the inside of the mold with resin and mat.  I soak a chip brush with resin and use it to blot the resin onto the mat over my freshly applied gelcoat.  Since the gelcoat is still tacky to the touch, it holds the mat in place while I soak it with resin.  I work my way from one end of the fender to the other and then back again (two layers of mat and resin) being careful to make sure that the mat has no air trapped underneath and that the mat overhangs the mold by an inch or so.

Once this mat and resin cure to a gel-like state (this takes about ½ hour of 45 minutes) you can safely trim the overhanging extra mat with a razor knife.  If the resin pulls or is too sticky to cut, check back in another 15 minutes and try it again.  This is the best way to clean up your parts of excess fiberglass.

After trimming, I allow the part to sit in the mold for a day or so to let it fully cure.  Once fully cured, I pop the part from the mold using a couple of plastic putty spreaders.  I simply use my fingers to pick at the part until I get slight separation from the mold then I push the plastic spreader between the part and the mold and work my way around until the part pops out.  Since I used adequate release wax and two coats of PVA, this part easily separated from the mold.  One fender down, one to go!  

My New Beach Cruiser With Fiberglass Fenders Installed

I just finished building a new mold for a 1/10 scale radio controlled car. One of the great things about making radio controlled car bodies out of fiberglass is the detail and the undercuts that can be formed using multi-piece molds. The realism is much greater than that which can be found in a vacuum formed body.

Multi-piece molds are always necessary for building three dimensional objects – whether those objects are hobby related or not. With the exception of very simple molds, multi-piece molds are almost always required or at least preferable. The obvious reasons for the use of a multi-piece mold are driven by the ease of extracting your parts from that mold. Molds that are very deep are difficult to lay-up parts in and can be very difficult to pull your parts out of.

The draft angle of your plug is a major determining factor that will determine the need for a multi-piece mold. Smaller parts may require no draft angle while larger parts that are deeper than 12 inches will require a draft angle of up to 5 degrees.

Automotive parts other than hood scoops, fender flairs, spoilers, etc are often done with one piece molds. Larger parts, such as doors, hoods, entire bodies and the like are almost always done as two piece molds.

I was looking at my first and main Fiberglass Mold Manual in which I outline the procedures for building a simple one piece mold. The funny thing about this manual is that the project that I follow in this manual ultimately required 3 separate one piece molds. A project requiring 3 separate one piece molds can also be labeled as a multi-piece mold.

Multi-piece molds do give many benefits to the builder, but there are a few drawbacks. Multi-piece molds do produce a flash line that must be dealt with. Flash lines require filling, sanding and refinishing. Flash lines are created by imperfections in the edge of the mold and in the general alignment of the pieces of the mold. In my opinion flash lines are a minor trade off for the many benefits of a multi-piece mold.

Happy Glassing

Steve

www.fiberglassmoldmanual.com

Now that the foundation for my plug is laid, I can start building a fender. At this point I will just take a moment to regress.

When I was younger, I am sure that this plug would have been built from a much more difficult process than what I have outlined. I am not sure how I would have done this but it wouldn’t have been easy – I probably would have used a template of the rim, done complicated measurements of the total tire diameter and so forth. Building plugs for projects like this are much easier and more likely to provide a nice product if you can use the base object as the foundation for your plug. In this case, I used the actual rim itself. In one of my manuals, I actually used the hood of my jeep as the foundation for building a plug for the hood scoop. This method of plug building takes much of the guess work out of the building process.

Getting back to the plug, I begin building the plug by laying a layer of 1 ½ ounce mat and resin over the foam. Once this cures, I follow with a plastic spreader and bondo to smooth out the surface of the fender. Once the bondo is cured, I smooth out the surface with my electric sander with a course (100 grit) sandpaper to smooth it out. Like with any plug build, I go through many bondo applications followed by sanding to smooth out the surface.

As with many plugs, just about the time that I get this thing smoothed out I see a problem with it. From a side view, it appears as though the center section of the fender, when viewed from the side, isn’t as thick as the ends of the fender. In order to check this, I drilled a hole in the end of a 1/8 inch, inch wide by 36 inch long piece of aluminum stock. The hole is just large enough to slip onto the hub of my rim. I then clamp an L bracket onto the stock aluminum and slid it down until it makes contact with the fender at what I perceive to be the highest point. As I sweep the L bracket around the top of the fender from one end to the other it becomes obvious that not only is my center section low, but I have many uneven points along the surface of the plug.

Fixing these uneven points is pretty easy. I now have a way to measure the outer diameter of the fender and I can use this tool to obtain consistency in the outer diameter.

To smooth out the surface of my fender, I lay a bead of bondo in the center along the length of the plug. While the bondo is still able to be spread, I drag the L bracket which is still attached to the hub through the aluminum stock along the surface of the plug at a level that represents the highest point of the plug. The L bracket levels the surface of the plug creating consistent thickness of the plug.

After this application of bondo cures, I can begin spreading bondo over the surface again to smooth it out. This time I am careful to use the newly laid central ridge of bondo as a guide.

Happy Glassing

Steve

www.fiberglassmoldmanual.com

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Fiberglass Mat Fiberglass Cloth

I let this mold cure on the plug for almost a week before demolding. This mold popped off its parting board with little effort. Likewise, the mold separated from the plugs with ease. The process for removing the mold from the plug is simple. I use a half dozed small plastic putty knives that I insert between the mold and the parting plane.

Once I have one putty knife between the mold and the parting plane, I simply insert another putty knife at the edge of the separation that occurred as a result of the first knife. This is repeated time and time again as I work my way around the perimeter of the mold. Once you get started, this is an easy process.

At this point the mold is inspected for any flaws, repaired and then prepped for its first part pulls.

Making parts with this mold is very similar to making the mold itself. I begin by applying five coats of mold release wax to the inside of the mold. Once the mold is waxed, I can apply a coat of PVA to the inside of the mold with my spray gun. After the PVA dries, I can do my first parts lay-up. This is part that I really enjoy – I am very close to seeing the results of my efforts and planning.

Happy Glassing

Steve

www.fiberglassmoldmanual.com

I have written blog entries in the past in which I have shared my experiences with new techniques and new products that are available for construction with composites. This is another one of those.

Whenever I am building the surface of a part that is being made of chopped mat, I end up with a fairly course surface that will need a final finishing.

I have employed various means to smooth out a fiberglass surface. Some of these finishing methods include the following:

For some applications I use veil over the final layer of chopped mat. This method hides most of the course fibers of the chopped mat without adding an excessive amount of extra weight to the final product. In fact, when I add a final veil covering I very rarely have to add any extra resin to the finish. Usually I can use a small roller to press the veil onto the surface of the still uncured mat / resin underneath it forcing the resin to seep into the veil and provide adequate saturation on the surface. The individual fibers of veil are much finer than those of chopped mat so the final surface is smoother.

For other applications I mix a putty-like consistency combination of resin and cabosil that I use to smooth over the surface. This has been one of my favorite methods for finishing a fiberglass surface. The cabosil / resin mixture can be made as thick or thin as your project requires. Once this mixture is spread over the surface and cures, it can be sanded smooth and finished.

Sometimes I will simply skin the surface with bondo. I like bondo because it sands so easily and cures so fast. You can smooth out a surface in a hurry with bondo. The drawbacks to using bondo are that it is easily chipped / damaged and it almost cures too fast. I have wasted a good amount of bondo over the years by mixing more than I could use in its working period.

The other day I went to my local fiberglass supply store and asked for a ¼ pound of cabosil. The gentleman that helped me asked if I wanted something that would sand a little easier. I said “sure” and he suggested microballoons. The microballoons mixed into a nice white paste and spread easily on the surface of my project. It also sanded easily as promised. In addition to easy sanding, it also seems durable as it passed the hammer tap test that I performed on the tip to the bow of my boat project. It didn’t hit it full force but I hit it hard enough to crack bondo and the microballoons didn’t crack. I was impressed.

In the beginning of this article I said that I was writing about a new product. Microballoons are not “new”, they are simply new to me. Once again, we tend to get comfortable with methods and materials that are known to us and close ourselves off to other products and procedures that are available.

Fiberglass Deep Vee Radio Controlled Boat

Happy Glassing

Steve

www.fiberglassmoldmanual.com

There are several ways to join fiberglass halves. Fuselages, boat hulls and car bodies usually require joining of at least two pieces in order for the project to reach a point of completion.

With some applications, a sleeve can be built into the plug with will allow the second part to slip into which will result in a nice joint that can be epoxied together. An application that comes to mind in this circumstance would be the deck of a model boat to the hull. This technique needs to be planned well in advance since it requires integration into your plug. This technique also is only applicable to those projects that won’t be visually hampered by a visible seem or joint on the final product.

Most of the time, in order to obtain a clean joint that can be filled, sanded and finished in a way that produces no visible joint on the final product, you will need access to the inside of the joined pieces in order to produce a clean union. The best way to do this is to align the halves and use masking tape to hold the pieces together. I like to use thin strips of resin saturated fiberglass mat that I can lay across the inside of the seem. Depending on the strength of the bond that my project requires, I like to have about ½ inch of mat overlapping each side of the seem. Once this cures, your joint is complete. The only thing left to do at this point is finish the exterior.

Another method that I really like for joining fiberglass halves that don’t have to be water tight or incredibly strong is by using what I like to call “chemical applesauce”. “Chemical Applesauce” is a mixture of resin and cabosil that has been catalyzed with MEKP that has the consistency of applesauce. I use this mixture by taping the entire outer surface of the seem, making sure that the pieces have proper alignment. I then use a gloved finger to apply this mixture to the seem line from the inside. I use my finger to push and work the mixture into the joint. This makes an excellent filler as well. Once the mixture has cured, I can remove the masking tape and I am left with a strong, clean joint that requires only minimal sanding and finishing.

These are some of my favorite ways to join two halves of a fiberglass project. I do have a challenging project on the table now that will require special consideration when joining. I will keep you posted on my progress.

Fiberglass AH-1G Cobra

Happy Glassing

Steve

www.fiberglassmoldmanual.com

After the PVA dries, I brush in a nice healthy layer of gelcoat. Once the gelcoat cures to a tack, I begin to lay resin and mat. Since this is a fairly large item, I do two layers of 1 ½ ounce mat to provide a little strength without the part getting too heavy.

Once this fiberglass gels, I trim the excess with a sharp utility knife and let the mold cure to its full hardness. Since I will be joining the two halves of the helicopter, I don’t pull the pieces from the mold until I am ready to join them. This is because any shrinkage / warping will wreak havoc with my attempts to line up the two halves of this copter during the joining process.

Removing the parts from this mold is done just as described previously. I slip a putty knife between the mold and the part. Using several putty knives, I work the perimeter of the mold to loosen the piece. I progressively dig the putty knives deeper and deeper until my parts pop out of the mold.

Once the parts have been removed, I can clean / flatten the edges with a sanding block, align the two halves and use masking tape to hold them together and seal the seam. To finish this project I catalyze about 4 oz of resin and pour it along the inside of the seam that divides the two halves of this copter. The resin is poured into the fuselage through the opening where the exhaust port would be. I then tilt the copter forward and backward to cover the seam with resin. Once this cures, I can remove the tape and deal with filling any defects in the seam.

With this post, the helicopter project is done.

Happy Glassing

Steve

Fiberglass Huey Model

www.fiberglassmoldmanual.com

Preparing the plug is the next stage of this build. I begin by mounting the two halves of the plug to a piece of inch thick MDF board. I attach the plugs by using screws from the back side of the MDF. Some people glue or epoxy their plugs to the parting plane. I prefer to use screws because I can remove the plug from the MDF after the mold is finished with out destroying it.

Once I have the plugs securely attached to the parting plane, I need to go around the base of the plugs looking for gaps between the board and the plugs. Any gaps are filled with clay. This is just a matter of forcing the clay into the gaps in order to prevent resin from getting under the plug. When the gaps are filled and all excess clay is removed, I can apply mold release wax to the plugs and the MDF parting plane. I like both McGuire’s and Part-All mold release waxes. It is important to apply at least five coats to the plugs in order to ensure adequate coverage. The plugs should be polished to a shiny smooth finish.

PVA is now sprayed onto the plugs and the parting board. The PVA is an excellent barrier between the mold and the plug. I like to spray a coat, let it dry and then re-apply. Applying two coats of PVA can result in lost detail but that is not a concern for me with this project. An additional coat of PVA will ensure an easy release when I de-mold the plugs.

I apply the PVA with my spray gun at about 60 psi and a distance of 8 to 12 inches. PVA is cheap and valuable at the same time. This set of mirror image plugs is now ready for molding.

For the molding process I need my air compressor and dump gun along with: tooling gelcoat, fiberglass resin and chopped mat, MEKP, rubber gloves, disposable brushes and a respirator or fume mask.

Happy Glassing

Steve

www.fiberglassmoldmanual.com