Rapid Prototyping and Experimental Design

How to fabricate sandwich panels, part 1.


Moving diagonally from the lower left corner to the upper right are the elements required to bag a sandwich panel: plastic vacuum bag, black mastic sealing tape, glass bagging table, lower-surface peel ply, the carbon sandwich panel itself, upper-surface peel ply, pink perforated ply, and finally, bleeder/breather ply.

Up to now, this series has mostly focused on testing in order to validate material performance and manufacturing approaches. Although we’ll continue to feature articles on testing, the next several articles will focus on using what we’ve learned to shed light on our fabrication processes. This month we’ll start with what is ostensibly the easiest composite component you can make, a flat sandwich panel.

As noted in previous columns, flat panels are readily available from companies like Aerospace Composite Products. But they are simple and fun to make, and a relatively easy intro to making molded, vacuum-bagged parts. Flat sandwich panels are light and stiff, making them ideal for internal structures like wing ribs and bulkheads. Or they can be taped together to make fuel tanks or other storage compartments.

Panel materials prepped for layup, from left to right: four layers of 6-ounce plain weave carbon fiber, one sheet of H45 Divinycell (hidden under carbon), one sheet perf ply, two sheets peel ply, and one sheet bleeder/breather.

The first step is to choose your facing and core materials. We often use plain-weave 5.7-ounce (aka 6-ounce) carbon fiber (Hexcel 282) since it is inexpensive and handles nicely; it’s stable and with careful handling, the warp/weave will stay straight and perpendicular. Also popular is 2×2 twill (Hexcel 284). It has the finished appearance people associate with Class A carbon parts and drapes nicely, but is a little less stable so requires more care in handling. There are innumerable other fabrics out there of various weaves and weights, more or less exotic. Scroll through the offerings of companies like ACP, Soller Composites or Fibre Glast to see what’s available. As a side note, some of the hybrid fabrics look pretty cool, but be aware that working with Kevlar can be challenging since it can be difficult to wet out and does not cut/sand nicely.

Haas CNC engineer Sam Kaplan uses a rotary cutter to cut out the carbon.

Sam weighs the carbon, using a pair of inspection mirrors to read the screen.

For core materials, you are likely looking at foam, honeycomb, or end-grain balsa. Balsa tends to be popular in impact-resistant applications like boat hulls and has other advantages/disadvantages. Honeycomb, which we’ve discussed in previous articles, can be challenging to work with, so be prepared to experiment to get best results. Foam is the easiest core material to work with. The two most commonly available foams for homebuilders are Rohacell and Divinycell. Rohacell has higher material properties but is considerably more expensive; Divinycell is more pliant and seems to be more damage tolerant. Most suppliers offer various densities; look for something with 3- to 6-pound density (i.e., Divinycell H45, 60, 80, 100, or Rohacell IG51, 71, 110).

Any number of laminating resins will work fine. Popular brands are Jeffco, Aeropoxy, E-Z Poxy, Pro-Set, and MGS to name a few. I personally use MGS 285 for critical applications such as wing skins and spars; it has good high-temp properties (when post-cured), wets out easily and is low odor, and is certified for glider production in Germany, which adds a level of quality assurance. It is also one of the more expensive systems. For less critical applications I like Jeffco since it is significantly cheaper. Based on forum posts, EZ builders hold E-Z Poxy (but only with 287 hardener) in high regards as a fuel-proof fuel tank epoxy (epoxies in general and E-Z Poxy in particular will be a subject of a future column). It’s nice to experiment with different systems, but once you start making big (read: expensive) parts, it’s best to use a system that you are familiar with so you know what to expect in terms of pot life, wet out characteristics, etc. Note that not all epoxies are intended for laminating.

Waves in the fabric can be straightened by pushing down on opposite sides of the fabric and pressing outward.

heap but powerful used lab pump is capable of pulling 99+% vacuum. Pump breathes through the black handle, so the drink can stops any oil mist from contaminating the area. Barely visible on the left is a small clip fan that cools the pump during operation.

For this article we will be making a 24×24-inch flat panel. The layup schedule is 2-core-2: two layers of 6-ounce 282, -inch core of H45 Divinycell, topped by two more layers of 282. To overview: the bottom facesheet (2 layers of carbon) will be wet out and laid on a sheet of peel ply on a waxed glass table. We’ll then place down the core, followed by the top facesheet of carbon. This is followed by another layer of peel ply, then perforated ply, and finally a layer of bleeder/breather cloth. The complete assembly will be vacuum bagged to the table and allowed to cure. After cure, the panel can be carefully peeled from the glass and will be ready to use.

Step 1: Prepare Your Work Surface

1. I lay up and bag my smaller parts on a 40×60-inch sheet of -inch tempered glass. You can find tempered glass sheets quite cheap on Craigs-list. Clean the glass and wax twice. Use a Sharpie and draw a 30×30-inch square on the glass. This is your “bag box” where you will bag the panel. Apply mastic tape over the Sharpie lines. Do not remove the wax paper backing from the tape at this time.

2. Cut a square of plastic sheeting (Lowe’s aviation grade 4-6 mil) 36×36 inches. Remove the wax paper from the top strip of mastic only, center the plastic sheeting over the box, and seal to the top mastic. Roll the plastic up under itself to the top of the box to get it out of the way. The reason for rolling the plastic under itself, rather than over itself, is so that if epoxy drips on the plastic while you are working, it will not contaminate the side of the plastic that will seal against the mastic (wet plastic is hard to seal).

3. Cut another sheet of plastic approximately 3 feet wide and 12 feet long (or four sheets each 3 feet square). You will lay up the carbon on this plastic and use it to transfer to the “bag box.”

Epoxy is squeegeed on the carbon. Pour the epoxy around the center and squeegee out to the edges.

This picture shows the lower-surface peel ply, two lower-surface layers of carbon (hidden under blue foam core), foam core, and two upper-surface layers of carbon.

Step 2: Prepare Your Layup Materials

1. Cut four layers of carbon fiber 24×24 inches. You can orient the cloth fibers vertically/horizontally (0/90 degrees) or at a bias (45 degrees), or both. The only requirement is that the stack be balanced. That is to say, if your bottom layer is oriented at 0 degrees and the next layer is 45 degrees, place your foam core followed by a 45-degree layer, then a 0-degree layer. Unbalanced layup schedules are prone to warping. Cutting with an Olfa-type rotary cutter is easiest and disturbs the weave less than scissor cutting. Rotary cutters work great on carbon, but your mileage may vary on other materials. Rotary cutters need a pliable surface underneath to work well, such as a PVC cutting mat. White 1/8-inch panel board (available in 4×8-foot sheets from Lowe’s) works well as a cheap cutting mat. Wear gloves when handling carbon to avoid contaminating it with oil/dirt/sweat from your skin.

2. Cut one sheet of core 24×24 inches. It is generally not necessary to prep the core other than blowing with an air gun (clean, dry air—no oil!) to remove dust from processing. If you live in a damp environment, you may wish to place the core and fabrics in a dry environment for a day or two prior to layup to reduce any absorbed moisture. Kevlar in particular can absorb significant amounts of moisture.

3. Cut one or two sheets of peel ply 25×25 inches. There are probably as many varieties of peel ply as carbon fiber. Both the 1.8-ounce and 2.97-ounce Dacron sold by Aircraft Spruce work fine. If you want peel ply on both sides of your panel, cut two sheets. If you want a glossy surface on one side, cut only one sheet of peel ply and place it on the top of the laminate stack; omit the sheet on the glass side. Peel ply is removed from the final part and provides a textured surface for bonding.

4. Cut one sheet of perforated ply 26×26 inches. Aircraft Spruce E2760 works great. Peel and perf are cut oversize in order to prevent direct contact between the carbon and bleeder plies. Peel and perf both remove easily from carbon; bleeder will bond to it and be very difficult, if not impossible, to remove. The perf acts as a release between the bleeder and peel, and also throttles the absorption of resin into the bleeder.

5. Cut one sheet of breather/bleeder 24×24 inches. Aircraft Spruce 4.5 ounce or 10 ounce both work fine. For small layups/low viscosity resins/high shop temps/strong vacuum pressures, 4.5-ounce bleeder may become completely saturated, which precludes its ability to act as a breather. In these cases, I typically use 10-ounce bleeder, but care must be taken not to overbleed. See previous articles discussing this issue. Also cut two strips approximately 2×5 inches. These will be placed under the vacuum pad and serve as an air channel to the panel.

6. Prep one or two squeegees for wetting out the fabric.

7. Prep resin plus hardener equal to approximately 130% of the weight of the cloth (in this case, approximately 13.2 ounces or 150% if wetting out core—see below). I personally do not trust ratio pumps and weigh out my resin using a gram scale. This is not as quick as using ratio pumps so for larger projects, I have to pre-weigh all my resin ahead of time. Be sure to use unwaxed mix cups. Tongue depressors make cheap mix sticks.

8. Make sure your vacuum pump system is ready to go. Vacuum pumps can be found relatively cheap on Craigslist. I have a $100 Harbor Freight pump, but that is a third-level backup to my two good pumps. Hard polyethylene (3/8 inch) with push couplings is my plumbing of choice, but there are many tubing options. Just choose something that won’t collapse under vacuum. I tee off to two Aircraft Spruce Vacuum valves (p/n 8112) via two AQD 500TF quick disconnects. They are a little pricey but worth it when doing lots of parts.

Peel ply is smoothed on with a squeegee.

Perf is laid on top of peel ply…

…followed by breather

Step 3: Lay Up the Panel

1. Wear safety gear. Mix your epoxy well, approximately 1-2 minutes, ensuring to scrape the sides and bottoms of the cup to avoid unmixed resin.

2. If you want peel ply on both sides of your panel, you will need to squeegee a layer of peel ply directly onto the glass first. Do not use transfer plastic, as the peel ply will wrinkle. Epoxy works its way slowly through peel ply, so be patient and squeegee from the center outward. Avoid wrinkles.

3. Pour approximately one-fourth of the resin on your first layer of carbon fiber and spread evenly with the squeegee. Use the minimal amount of squeegeeing necessary to distribute the resin. Excessive squeegeeing is unnecessary and only disturbs the weave.

4. Trim the plastic sheet approximately two inches around the perimeter of the carbon. Leaving the carbon on the plastic sheet, transfer the plastic and carbon to the bagging box, invert, and lay it wet side down, centered in the box. Carefully peel away the transfer plastic.

5. Repeat steps 3 and 4 for the second layer.

6. Place the core on the second layer. For vacuum bagged panels, it is not necessary to wet out the bottom face. Depending on the situation, I sometimes wet out the top face, sometimes not—it depends on application, part size, resin viscosity, etc. (This is where doing some test layups helps). If you wet out the top face, do so before placing the panel on the carbon.

7. Repeat steps 3 and 4 for the top two layers of carbon.

Plastic is sealed to the mastic around the edges. Don’t forget to install the vac valve pads before sealing up the bag!

If you end up with excess plastic as you seal, just add a pleat and seal with mastic.

Step 4: Bag the Panel

1. Center and place peel ply on top of the panel. Try to avoid wrinkles—smooth down with a squeegee if necessary.

2. Center and place perf ply.

3. Center and place breather/bleeder ply.

4. Place the bottom pads of the 8112 vac valve at opposite corners of panel (do not place them on the panel itself unless you want an impression of the valve on your final part). Place one end of the 2×5-inch breather strips under the vac valve, the other end on the panel. (For smaller parts, a single vacuum source is fine.)

5. Remove wax paper from the remaining edges of the mastic and seal the plastic to the mastic. The last edge will probably have some excess plastic, in which case you’ll need to make a dart from mastic to take up the excess.

6. Make a small incision in the plastic over the 8112 valve and screw in the top of the valve. Attach the AQD 500TF and pull vacuum.

7. Check the bag for a good seal and monitor your vacuum gauge. For a small panel like this, you should be able to pull 28 inches Hg relatively easily. Generally speaking, higher vacuum is better as long as you avoid overbleeding.

Wait for the panel to cure. I sometimes drop a heater (a cardboard box with hair dryers) on top of a layup to accelerate the cure rate, but this needs to be done carefully since excessive heat will lower the resin viscosity, and you can end up overbleeding the part. Best to let the part reach a gel stage and then slowly add heat.

I let my vacuum pump run the entire time (you’ll obviously need a 100% duty cycle pump, and I run a fan on my pump to keep it cool). There is no reason to run partial vacuum or use a pressure switch unless you want a low-pressure bag for some reason. After unbagging, you can immediately remove the bleeder/perf/peel. If you plan to bond the panel, it’s best to leave the peel ply on the part until just before bonding. This ensures a high-energy, clean bonding surface. The caveat here is that you do need to remove the peel ply before bonding. Failure to do so will likely result in a failed bond.

Panel under vacuum. Regularly spaced dots indicate a good layup. No dots indicates a dry layup. Complete saturation indicates an overly wet layup and will not be able to distribute vacuum pressure evenly. Note darts at both ends of the right edge seal to take up excess plastic. Note also the digital vacuum gauge. The pressure reading in microns is 29,866. This is equal to 28.7 inches Hg, or about 96.1% vacuum.

Finished panel, ready for trimming. Panel will be used for coupon testing, so relevant parameters such as date/time/temperature/humidity of fabrication, fiber/resin ratio, etc. are recorded before/after the layup and taped to the panel.

If you want to calculate your fiber/resin ratio, be sure to weigh the carbon and core before layup, and the finished panel after cure and before trimming. Subtracting carbon and core weight from the finished panel weight will give you the weight of resin in the panel. Once you have the weight of the resin, you can easily determine the weight ratio of fiber to resin (this has been discussed in previous articles).

Tools clean up well with white vinegar if cleaned before the resin cures.

Next time we’ll go a bit further and discuss bagging a wing panel on a mold. See you then!

Eric Stewart is designing and building the SR-1, a speed plane for setting records in the FAI c-1a/0 category (takeoff weight less than 661 pounds, including pilot and fuel). You can see more at facebook.com/TheSR1Project, including additional photos and videos of the subjects in this series of articles.


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