Here is the next video, Episode #32: Body Panel Samples ->
https://youtu.be/hLIB1BC-HAESelect the Materials for the Final Body PanelsThe molds are now complete and ready to build the final body panels. Its time to select the reinforcement fabrics, resins, and core materials that will go into the body panels. There is a wide variety of materials that can be combined in limitless ways.
Reinforcement Fabric ChoicesChop Strand Fiberglass Mat As the name implies, chop strand is made up of small pieces of cut fiberglass fibers that are glued together with a polystyrene glue. Then polyester resin is applied to the chop strand mat, the polystyrene in the polyester resin breaks down the polystyrene glue and release the chop strands. Chop strand mat is not compatible with epoxy resin.
Chop strand mat is a good choice for molds that have many compound curves. It is easy to apply and builds thickness quickly. The downside of chop strand mat is that it absorbs a lot of resin, so the finished part tends to be heavy. For example, I used chop strand mat to build the mold and it weighs about 100lbs.
Fiberglass FabricFiberglass fabric is a single layer of long fiberglass threads woven together. There are many different weaves to choose from.
• 1 over 1 means that 1 horizontal thread is woven over/under 1 vertical thread. 1 over 1 weave is less expensive, but the weave restricts the fabric’s drapability over curved surfaces.
• 2 over 2, 4 over 4, etc. means that the weave skips over threads. For example, a 2 over 2 weaves the horizontal thread under vertical 2 threads and then over 2 threads. Likewise, it weaves the vertical thread under horizontal 2 threads and then over 2 threads. This looser weave allows the fiberglass threads more room to move, so they drape better over curved surfaces. The downside is that the fabric threads tend to unravel as you work with it.
S-Glass Fiberglass FabricS-Glass is specifically made for structural applications. The fiber is about 20% stronger than normal fiberglass. S-Glass is woven similar to normal fiberglass fabric. The downside is that it is 2x the cost of normal fiberglass fabric.
Biaxial Fabric
Biaxial fabric consists of two layers of fabric stitched together, with their fiber strands lying at +/- 45 degrees to the edges (instead of along the roll and across at 90 degrees). Biaxial builds strength and thickness quickly. The downside is that does not drape as well as single layer fabric. Biaxial is sometimes called “1708” fabric.
Carbon Fiber fabricAs the name implies, the fabric is made from long carbon threads that are woven together. Carbon fiber is much stronger than fiberglass and yields much stiffer parts. Carbon fiber fabric comes in a variety of different weaves, similar to fiberglass fiber.
Aramid Fabric (Kevlar)Aramid Fabrics are engineered to be impact and abrasion resistant. Aramid fabrics are used in bullet proof vests, front splitters, race car floors, etc. Aramid fabric comes in a variety of different weaves, similar to fiberglass fiber. Kevlar is a brand name for aramid fabric.
All of the above reinforming fabrics come is a variety of weights. The lighter weight fabrics drape better, but are not as strong as the heavier fabrics.
Resin ChoicesThere are three main types of resins: polyester, vinyl ester, and epoxy.
Polyester ResinPolyester resin is the most widely used type of resin. Polyester resin is UV resistant and least expensive. Polyester is not as strong/stiff as epoxy resin. Polyester resin has a shelf life of 6-12 months
Epoxy ResinEpoxy resin offers the highest strength, but is the most expensive resin. Epoxy resin will yellow with UV exposure if not coated with a UV resistant topcoat. If polyester gel coat is applied to epoxy, it will not bond. However, if epoxy is applied to polyester gelcoat, it will bond. Epoxy resin also has an exceptionally long shelf life of 24-36 months.
Vinyl Ester ResinsVinyl Ester Resin is often considered a cross between polyester and epoxy resin. The cost of Vinyl ester falls between polyester and epoxy. Vinyl ester resin is becoming more commonly used in high performance composites like racing, marine, and aerospace. Vinyl ester resin has a very short shelf life of only 3 months.
Core Material ChoicesA core material is a structural layer that is embedded between laminate layers. Core materials are optional, but they add significant strength to a laminate. The most common core materials are foam, wood, and honeycomb.
FoamClosed Cell foam is a light weight core material that is easy to cut, trim, and shape. It is used extensively in aircraft and performance automotive structures. Vinyl foam is commonly used and can be thermoformed with a heat gun. Polyisocyanurate is another common foam core material.
HoneycombHoneycomb is a panel of beehive like cells nested together. The honeycomb structure is naturally strong and leaves 90-99% open space in the panel. Nomex, aluminum, and paper are common honeycomb materials. Nomex honeycomb is used primarily for structural aerospace applications. Special layup techniques are needed to ensure the open space in the honeycomb doesn’t fill with resin.
Attachment:
32 6 honeycomb.jpg
WoodEnd-grain balsa is a widely used core material. It is light and less expensive than foam or honeycomb. It is easy to cut and trim. Plywood is also commonly used, but is significantly heavier. The downside is that wood may absorb water and rot over time.
Attachment:
32 7 Balsa.jpg
What is the Best Combination of Fabric, Resin, and Core?With all these choices, what is the best combination of fabric, core, and resin for your project? That all depends on the priorities of your project. The selection for this project is based upon the following priorities:
1. Strength - The first priority is to build sufficient strength into the panels. The panels need to be strong enough to withstand minor impacts, wind resistance at highway speeds, and reduce the chance of any cracking.
2. Weight - The overall weight goal for this vehicle is just 600lbs, so the second priority is to minimize the weight of the body panels as well.
3. Cost - This is a home project, so I need to keep my expenses in line. However, I’m willing to spend a little extra money to achieve priorities #1 and #2.
4. Time – I’m a workforce of 1, and my time is limited. I need to streamline the lay-up process where I can. I would like to mold the final body panels within just a few weeks.
Find your own SolutionNow it’s time to discover the layup schedule that will achieve your priorities. Its good to start by educating yourself by reading books and viewing YouTube videos on composite layups. Some good resources are:
• FiberGlast.com website has an excellent Learning Center with practical guidance
• Composite Material Fabrication Handbook #1 by John Wanberg, ISBN: 978-1929133765
• Composite Material Fabrication Handbook #2 by John Wanberg, ISBN: 978-1929133932
• Composite Material Fabrication Handbook #3 by John Wanberg, ISBN: 978-1935828662
• How to Fabricate Automotive Fiberglass & Carbon Fiber Parts Paperback by Dan Burrill & Jeffrey Zurschmeide, ISBN: 978-1613253663
These resources will provide a general sense of available materials and how to combine them into a quality composite layup. However, they won’t give you a sense of judgement about how strong these materials are in real life. For that, you need to build some samples and test them.
Choose Challenging areas of the Mold for the SamplesThe first step is to choose the most challenging contours of your mold for your samples. For example:
• Areas of the panel that will be weakest. These are area that are essentially flat panels with no compound curves. If your sample is strong enough in in these areas, it will be plenty strong in the compound curved areas. You may need to add core materials to strengthen weak areas.
• Areas with sharp curves or complex shapes. It can be a challenge for the fabric to lay down tight into sharp curves or complex shapes. The fabrics weave pattern and weight effect its drapability. You may have to use a different more flexible fabric in the areas that have sharp curves or complex shapes.
Choose Sample FabricsChoose the reinforcement fabrics based upon the project’s priorities. If light weight is important, choose stronger materials like carbon fiber. If weight is not important, fiberglass chop strand mat or biaxial may be a good choice.
Choose Sample ResinIf light weight is important, try epoxy resin. If low cost is important, polyester resin may be a good choice.
Build the Sample in Several Steps- Mask around the sections you want to build samples of. Then apply wax, PVA, or release agent to ensure the sample doesn’t stick to the mold.
- Start with samples that will probably be to light to satisfy the strength you need. Release the samples from the mold and test their strength to gain a sense of judgement. Then add layers in several steps until you achieve your strength goals.
- In some cases, you may need to try several combinations of fabric, resin, or core material to meet your strength goals.
Samples for the Tiger 700 ProjectThe main body side panel offers both the weakest area and the sharpest contours. I masked around the sample area and applied wax and PVA.
Attachment:
32 8 masked mold.jpg
Sample #1: The first sample used 2 layers of 6oz carbon fiber on the flat weak area and 3 layers of 6oz S-Glass on the sharp contour. The S-Glass was laid at a 45-degree angle to improve its drapability. The laminate was bonded with polyester resin.
Once cured, the sharp curve seemed to drape OK and had sufficient strength (Success!). However, the flat area with carbon fiber was too flexible.
Attachment:
32 9 sample 1.jpg
Sample #2:Since the flat areas in Sample #1 was far to weak, Sample #2 focused on adding strength to that area. In this sample, I used the same 2 layers of 6oz carbon fiber, but added a large panel of 1/8” vinyl foam core material. A heat gun was used to bend the core material to match the shape of the mold.
Attachment:
32 10 Sample 2.jpg
The edges of the vinyl core were sanded down, so the top layer of carbon fiber would follow the contour of the bevel and reduce any air gap in the laminate.
Attachment:
32 11 Bevel edge on core foam.jpg
Although the shape of the vinyl core sheet was bent quite accurately to the shape of the mold, it still lifted during when the polyester resin was applied. This caused several defects in the final lamination. Even with these defects, the panel was significantly stronger.
Attachment:
32 12 sample 2 complete.jpg
Sample #1b: I then added 2 thick layers of gelcoat to the outside surface of Sample #1. This added a small amount of strength, but not nearly enough.
Using the lessons learned from Sample #2, I decided to add 2 ribs of vinyl foam core to Sample #1. The ribs were made of the same 1/8” vinyl foam core, but were only about 2” wide. I also cut scores into the ribs about every 1” along its length. The scores allowed the foam rib to naturally follow the curve of the mold.
Attachment:
32 13 ribs.jpg
I then encapsulated the ribs with 2 layers of 6oz S-Glass and polyester resin. This added significant strength, but not enough.
Attachment:
32 14 sample 1b complete.jpg
Sample #3: I now have a good sense of how strong 2 layers of 6oz (12oz total) of carbon fiber is, so let’s compare it to other fabrics.
I made a sample of 12oz of S-Glass and another sample of 12oz normal fiberglass on the same contour of the mold. Both samples were far more flexible than carbon fiber. The S-Glass is supposed to be 20% stronger than normal fiberglass, but I saw almost no difference in strength.
Attachment:
32 15 s glass vs normal fiberglass.jpg
Sample #4: All my samples up to this point have been laminated with polyester resin. Epoxy resin is stronger but more expensive.
Sample 1b has been the strongest experiment so far, so I decided to replicate it, but with epoxy resin.
Success! Sample 4 was significantly stronger than Sample 1b.
The following photos show the difference in strength. A 30lb weight was placed on both panels. The polyester panel on the right fully collapsed, while the epoxy panel on the left barely deflected.
Attachment:
32 16a Sample 4 Polyester resin.jpg
Attachment:
32 16b Sample 4 epoxy resin.jpg
Lessons LearnedSample #4 will be the layup schedule used in the final body panels. However, Sample #4 would produce a body that weighs 40-45lbs. The goal is 30lbs. I will modify the layup as follows to reduce weight:
• I will reduce the gelcoat from 2 thick layers to 1 layer.
• I will reduce the inner layers from 2 layers of 6oz fiberglass to 2 layers of 4oz fiberglass.
Other general lessons learned:
• Stiffness and drapability conclusion
o The carbon fiber is much stronger than S-Glass or normal fiberglass.
o The S-Glass is supposed to be 20% stronger that normal fiberglass, but I experienced almost no difference in strength.
o The vinyl foam core ribs added significant stiffness. Scoring the ribs is an easier and more consistent way of ensuring a tight laminate.
o Using a 6oz fiberglass fabric on a 45 degree angle will drape along the sharp curves of the mold.
• Cost conclusions
o I decided not to use S-Glass since it is 2x the cost of normal fiberglass.
o I decided to use Second Quality carbon fiber fabric. Second Quality carbon fiber has cosmetic defects that don’t impact its strength. First quality Carbon fiber is 6x the cost of normal fiberglass. Second Quality carbon fiber is just 3x the cost of normal fiberglass.