Super Strong, Waterproof Foam Core

Super Strong, Waterproof Foam Core

Use the foam core from Readi-Board to create durable foam core sandwiches of paper and fiberglass. 

Abstract

Resurfacing Adams Readi-Board enhances its durability. This article compares the strength, added weight and cost of resurfacing Readi-Board with several different types of paper and fiberglass (FG). Results of this study were compiled and ranked according to cost, weight and strength. Data indicate that, depending on the material used to resurface the foam, the resulting foam core sandwich is stronger, holding up to 400% more weight. The conclusion is irrefutable, at least in my opinion, that the advantages of producing a model with resurfaced foam core has advantages, those of being waterproof and stronger, that far outweigh (pun intended) the disadvantages, those being preparation time and added expense and weight.

A Little History

When I started getting interested in building foam RC aircraft a few years ago, I started following Dave Powers. I really liked the ease of the build but I was shocked at the price of Depron. To get around the cost of Depron, I used fan fold foam insulation but it was almost twice as thick as the 6 mm Depron and had inconsistency in thickness. In order to get foam of the correct thickness, I built my own hot wire foam cutter that could produce foam sheets from the Corning EPS foam insulation (if you are interested in how to build your own hot wire cutter, write a comment and if there is enough interest, I’ll write a build article on hot wire foam cutting). What I discovered was that when EPS is sliced into thin sheets, it was really flimsy, so carbon fiber rods/tubes were needed to give some structure to the foam. Here is my Extra 300, using RCPowers plans. I replaced the flat wings with airfoils cut with my hot wire and the 6mm EPS foam was reinforced by resurfacing it with 0.73 oz/yd fiberglass. The pink wing is bare Corning EPS insulation, the blue is the same foam covered with FG, but I added a drop of cyan inkjet refill ink to the resin when laying it up.

I poked around on the web to see what others were trying. I was introduced to Readi-Board by RL Adams Plastics by watching Ed (ExperimentalAirlines) http://www.rcgroups.com/forums/showthread.php?t=1587275 build several planes and wings. I thought Ed was really creative and I was intrigued with the ease and strength. However, I was convinced (in my mind) that foam core board was really heavy compared to depron especially with an added layer of tape.

Fast forward a year when I discovered Flight Test. I watched a few FliteTest videos and was somewhat convinced that foam core board might be a viable material for scratch building, but, I maintained my mental argument that paper covering had a low strength to weight ratio. Regardless of my apprehensions, I began using it. Since then, my appreciation for Readi-Board has grown, as well as my discoveries of its inadequacies. I really like using Readi-Board to mockup a build and see if the plans fit. What I don’t like is that the models made from Readi-Board are not as durable and long lasting as I would like. I found Readi-Board has a tendency to warp after getting moist, and who doesn’t want to paint their plane or fly when it is humid, raining or there is dew on the grass?  I built a few FT aircraft and their wings or other flat parts warp due to the humidity or paint that I applied, even after oil-based Polyurethane treatment. http://flitetest.com/articles/minwax-on-your-plane

Additionally I didn’t like the way the paper peels from the edges or that if the Ready-Board got a slight bend, its structural integrity was toast.

I was really interested in Peter’s article on how to waterproof your plane - http://flitetest.com/articles/waterproofing-your-airplane because it seemed to have the solution to warping issue. So, I had to try it out. I drove to Walmart and picked up the supplies. I couldn’t find the short roll of brown craft paper featured in Peter’s article, maybe I was looking in the wrong section of Walmart, but they had some wider rolls of Duck brand “Kraft Paper”. Next to it was packing Paper (you know the stuff you are supposed to buy when packing up your expensive dishes for moving) which was a lot less expensive per square foot (sqft) than the Kraft paper. So I picked some of that up along with the polycrylic. The cost of the Kraft Paper turns out to be 5.7 cents per sqft, the Packing Paper 2 cents per sqft. I haven’t checked for other sources, but there may be less expensive options.

On the way home, I had a few questions.

• Does Kraft paper make the foam core stronger than with the original paper covering? 

• How could the strength be measured? 

• Is the added cost of paper, polycrylic and time really worth it? 

• How much weight does it add? 

• I know that fiberglass (FG) is waterproof and strong. Would it be as effective as paper?

• What is the cost difference? 

So, I set out to answer these questions with an experimental design to measure cost, weight and strength. To measure the strength, I decided to use a similar method to that which we used in a high school physics projects. We made balsa wood bridges and determined the strength of the span by measuring the amount of weight that could be applied before it broke. 

The Readi-Board was prepared by cutting strips 5 cm wide and 50 cm long (same as the width of Readi-Board). The original weight was measured, 

the paper was peeled from both sides, 

the bare foam was weighed, the foam was recovered with different materials and then the final foam core sandwich was weighed and tested.

Here is a list of materials for coverings that I had on hand:

1. Kraft Paper (Walmart $4.27/75 sqft or $0.0569/sqft))

2. Packing Paper (Walmart $9.44/ 480 sqft or $0.0196/sqft)

3. FG  0.73 oz/yd (Thayer Craft $0.301/sqft)

4. FG 1.43 oz/yd (Thayer Craft $0.168/sqft)

5. FG 3.15 oz/yd (Thayer Craft $0.208/sqft)

6. Red Rosin Paper (I use as workbench covering Home Depot $11.97/501 sqft or $0.024/sqft)

7. Packaging Tape 1.6 mil 1.8”x100 yd, Walmart $0.039/sqft)

The Cost of Resurfacing Readi-Board Foam.

The following graph provides what it costs to cover both sides of a Readi-Board sheet with the different materials, including the cost of the Readi-Board:

Packing Paper adds 17 cents to the 20”x30” sheet while the 2 ply FG 0.73 oz/yd adds 5 dollars. The graph makes it very apparent that the addition of FG has a considerable cost impact - but most of it still costs less than Depron which runs around $3.95 for the same size sheet (20”x30”).

Recovering Readi-Board Foam with Paper

One of the fears I had with recovering foam core with paper is, will the paper warp when it dries? Most papers shrink when they dry due to properties of their fibers, that is the magic of covering a plane with tissue paper and misting it with water. The tissue shrinks tight to the plane’s framework.

The first recovering I tried was with the Kraft Paper. I followed the procedure provided by Peter, however, instead of using a box/square tube fuselage structure, I covered both sides of a flat piece of foam. I recall in the video that they only covered the fuselage and mentioned not to cover the wings with the craft paper. My first try at recovering Readi-Board seemed to go well. I applied polycrylic to the paper and foam and then smooth them together. I turned the foam over and repeat the procedure. After both sides were attached, I painted a top coat and hung it out to dry.

The Packing Paper was next… it worked real nice too. It feels much lighter than the Kraft paper but I didn’t find any indication of paper weight on the package. The Kraft paper feels a little heavier than standard 20 lb copy paper and the packing paper feels like less than half that of copy paper.

Next up was the Red Rosin paper from a local building supply. If you didn’t already know, rosin paper is commonly used in a wide variety of construction applications, including roofing, flooring, and as a general jobsite protective covering - it also comes in brown. I use it as a work bench covering - that is why you see it as the background in most of my photos. When I cut the Red Rosin paper, it rolled up. I lightly misted it with water and it laid flat, and then it was really easy to work with.

Like with the Kraft Paper, I painted a coat of polycrylic on the rosin paper and then the foam and smoothed them together and then painted more polycrylic on the outside surface.

In the morning, the results of the recovering were interesting. The Kraft Paper was severely warped, the Packing Paper and Red Rosin paper were straight as an arrow. On a second round, I tried misting the Kraft paper with water, like I did with the rosin paper, before applying Polycrylic, but that didn’t work either (far left Kraft paper sandwich below).

At this point in my model building hobby, I have covered a few models with very light fiberglass and I really liked the results. So I thought I would compare fiberglass to the paper coverings.

Recovering the Readi-Board Foam with Fiberglass.

I was curious to know how much weight and strength would be added when the foam was resurfaced with FG. I have stock of 3 different weights of FG cloth, 0.73 oz/yd, 1.43 oz/yd and 3.15 oz/yd (Thayercraft.com is the best deal for FG cloth). Just so you can get an idea of how light this FG cloth is, when I say oz/yd that is ounce per square yard - 0.73 oz/yd is 20.7 g/yd. The fiberglass cloth was cut and laid on the foam. Epoxy resin, from US Composites, was applied and the wetting promoted with a plastic squeegee, although I have used popsicle sticks, playing cards, and plastic gift cards. After a smooth layer of epoxy resin was applied, the excess resin was squeegeed off. I overlayed one side and let it cure overnight before applying FG to the opposite side. The FG surfaces were finished by trimming the excess and sanding the corners with 120 grit sandpaper.

The following picture shows the degree of curve produced from the Kraft Paper, but not from the Packing Paper or FG. All I can say is WOW> I like the flat stuff.

At this point I thought the cost comparison wasn’t truly fair (apples to oranges) because I used “expensive” epoxy resin on the FG and the inexpensive polycrylic on the paper, right? Surprise, surprise, surprise!!! The 8 oz polycrylic was $8.74 or $1.09 per oz whereas the US Composite 635 Thin Epoxy runs $0.45 per oz (I buy it by the ½ gallon from US Composites). I couldn’t believe it, epoxy resin is half the price of polycrylic. With this information I just had to go back and create more foam sandwiches with paper using epoxy resin as the adhesive.

I tried using epoxy resin to attach the Red Rosin and packing paper to the foam. The rosin paper didn’t adhere at all, or rather, it adhered only where there seemed to be more epoxy. The paper seemed soaked up the resin leaving none to adhere the paper to the foam. I did not press it down during the cure, so I don’t know if that would help. The packing paper worked great!

The foam sandwiches were finished by cutting off the excess material with a hobby knife and sanding the edges.

How much weight was added?

The Readi-Board weighed in at 7.56 g and any resurfacing material, except tape alone, was heavier. What you can’t see in these data is the fact that roughly ½ (53%) the weight of Readi-Board is the paper - before peeling, the strips weighted 7.56 g and 3.56 g after.

How Strong are the new Foam Sandwiches? 

Here are a few pictures of the weight being added to the foam core “bridge”. Reading left to right from top to bottom, Kraft paper with no weight, Kraft paper with 42 g, Red Rosin paper with 480 g, FG 0.73 oz/yd with 230 g, FG 3.15 oz/yd with 890g, Readi-Board with 230 g broken, 2 ply FG 1.43 oz/yd with 1.192 kg (2.63 lbs). The bottom two pictures are the 2 ply FG 1.43 oz/yd broken and with the weights laid out. 2.63 pounds is a lot of weight to hold up across a 50 cm span. 

Here is the data from the strength test with the final number being the amount of weight on the span when the structure failed.

This demonstrates that the Kraft Paper (at least the stuff I purchased) is the weakest. Resurfacing the foam with Packing Paper produced a sandwich that held the same amount of weight as the unaltered Readi-Board. It is a little heavier but the bonus is - it is waterproof. The big surprise for me was that the Red Rosin paper really held up better than I predicted. The FG covering was noticeably stronger and provided greater rigidity to the foam than any of the papers, but it is not as pliable as paper so the building and joining techniques need to adjusted if you use this type of foam sandwich.

A note about the failures. In general, regardless of the surface material, the foam sandwiches would break when the deflection was between 3 cm and 5 cm (see supplemental data at end of the article). The breaking, in all cases, was due to the compression surface (top) folding. The extension surface (bottom), in all cases, showed no signs of damage.

The Strength of the Paper/Fiberglass-Foam Bond.

One of the complaints about Adams Readi-Board is the ease of which the paper peels from the foam. Some people have asked on the forums, why doesn’t Adams Plastics use a better glue? I’m not so sure the paper is “glued” to the foam. Rather, I think the paper is as used as a container or “form” for the foam in the manufacturing process. So, the question before us is, will the new foam core sandwiches suffer from the same inadequacy? 

The only materials that could be easily “peeled” from the foam was the heaviest FG (3.15 oz/yd), and the 2 ply 1.43 oz/yd FG. It appeared that the reason they could be peeled away from the foam was because the FG and epoxy layer was stronger than the foam. In other words, when the FG was peeled off the foam, there was a thin layer of foam still adhering to the FG as can be seen below.  

All the papers and other weights of FG broke or ripped indicating that the covering was weaker than the bonding glue/foam interface. From left to right, FG 0.73 oz/yd, FG 3.15 oz/yd, rosin paper with polycrylic, packing paper with epoxy resin, and kraft paper with polycrylic.

I hope this is not misleading in any way. Be aware that the surface material, any that I tried, did eventually peel from the foam when enough evenly applied force was applied. Here is a photo of a “T” joint that was subjected to extreme stress. The FG pulled away from the foam, just like the paper pulls away from the foam in the Readi-Board.

So, the surface material, whether FG or paper, still peels from the foam, or the foam separates from the foam. Resurfacing the Readi-Board will not be the complete solution I was hoping for. For those important joints, it seems that the foam-hot glue-foam joint is still stronger and more durable than having a layer of paper or FG in between. Still, the best joints are created from sliding one piece of foam core into a slot in another piece. 

Final Comparison. 

Well, if you made it this far in the article, you deserve a dish of ice cream and a warm towel on your head. I have presented a lot of information, but you may be still asking… so who won? In order to demonstrate this, I performed some heavy duty reverse polar, anti-integration calculations:

(not really, it just sounded good). I just graphed the strength to weight ratios and the strength to cost ratios and then gave them a rank based on the % each sandwich measured up to the highest performer.

The Strength to Weight Ratio was calculated by dividing the weight the foam sandwich supported by the weight of the foam sandwich.

It was not surprising to me that the 2 ply FG had the highest strength to weight ratio because it held 1.192 kilos, but what surprised me was that rosin paper ranked right up with the FG. I was also stunned that the Readi-Board+tape held up so well. The tape added a lot of strength and is relatively light and inexpensive. Kudos to Ed at experimentalairlines and others for using tape on the Readi-Board! Tape on the foam alone is only good if you want to make circular structures because it bends very gracefully and has no strength whatsoever. It sagged under its own weight and touched the table with the first weight added, and it didn’t break.

Another way of looking at this information is to graph the % increase in weight of the material with the % increase in weight supported.

Graphed like this, it is easy to see that Kraft paper added 83% of the weight of the original Readi-Board and decreased the weight it could support by 61%. Don’t get confused here, the Foam+Tape is the Readi-Board foam with packing tape used to recover. As suspected, the general trend is for the heavier material to hold more weight, but there are a couple of outliers. Packing Paper with Polycrylic did not increase the strength and was 14% heavier than Readi-board, but remember, it is waterproof. The Packing Paper adhered with epoxy resin had a 22% increase in weight and could hold 43% more weight, and it too is waterproof. On the far right, the 2 ply FG 1.43 oz/yd held 5 times the weight of Ready-Board and only doubled the weight of the material, and yes, it is waterproof. Did I say these were waterproof?

The Strength to Cost ratio was calculated by dividing the weight the foam sandwich held by the cost of the foam sandwich. This data is for all you cheepos’ out there, like me, who want to know the best cost value.

When ranked this way, the Rosin paper came out on top because it is relatively strong and very inexpensive with the runner up being the Readi-board and tape. Another noteworthy observation is the comparison of packing paper with polycrylic and epoxy resin. Since the epoxy resin/packing paper combination held nearly 100 g more than the polycrylic/packing paper combo, it ranked higher - seems the epoxy resin adds more strength than the polycrylic.

Rank Order 

The rank order was calculated based on the % that each sandwich reached the strongest and the rank values from strength to weight and strength to cost were added together. Out of 2 as the highest possible rank, here are the results.

This graph indicates which sandwich is strongest and most cost effective. The top winners are 

1. 2 ply FG at 1.43 oz/yd (Winner)

2. Rosin paper (Runner up)

3. Readi-Board plus tape (3rd place)

4. FG 3.15 oz/yd (Thick and strong)

5. Packing paper/epoxy resin combo. (Most nearly matches Readi-Board)

I want to point out the 2nd and 5th place finishers. Why? Because, they are paper, relatively inexpensive, stronger than Readi-Board, and can be manipulated like Readi-Board when cutting out plans and building (and they are waterproof). The FG sandwiches may need to be joined with different techniques - see below if you still want more info.

Conclusions

Using Adams Readi-Board to build models has several advantages, it is relatively strong, inexpensive, easy to use, easy to glue and readily available (at least where I live). However, from my experience, Readi-Board has a few things that I don’t like. I dislike the fact that flat panels warp if they get damp from the air, grass or paint, the paper peels from the foam relatively easily and once bent, the strength of the material is compromised. One slight bend that slightly creases the paper covering and the structure of the sandwich is severely weakened, to a point that threatens to cause an in flight structural failure. To me that means it is great for building a prototype, but is not great for durable long-lasting planes. To achieve the durability in my models, I will recover my Readi-Board foam.

I have tried to be objective in this study. If you want to make a model that is more durable and long-lasting, I think it is productive and well worth the time to resurface Ready-Board. The resulting foam sandwiches are stronger, waterproof and they don’t peel as easy, however there is weight penalty when resurfacing. Another benefit of resurfacing Readi-Board is that the builder can choose how much strength to add to which portions of the aircraft. For example, the square tube fuselage and airfoil wing are pretty strong by themselves due to their overall shape and therefore they don’t require heavy FG whereas single foam surfaces, such as rudder and elevators, may require heavier FG.

Things I learned:

• Epoxy Resin is not all that expensive when compared to other adhesives.

• Multiple layers of FG are more than double the strength of a single layer.

• If you want to resurface Readi-Board and you don’t want to get into the fiberglass, then packing paper is a better option than Kraft Paper.It is pretty simple and adds strength and waterproofing. 

• Readi-Board and tape are a great combination.

Building with FG Sandwich board

You should be asking, will FG sandwiches build like Readi-Board? Cutting? Folding? Gluing? 

To answer that completely, I will need to post another article. However, I don’t want to just leave you hanging, so here is the spoiler. 

1. FG foam sandwiches cuts like Readi-Board, but the skin is a bit tougher. You will go through a lot of blades… so check out how to sharpen your hobby knife. 

2. FG doesn’t like bending into sharp angles like the paper on Readi-Board. If you are going to use FG sandwich board, you will have to learn new techniques similar to those you would use to join Depron, or any other non-surfaced foam. One thing that I have learned about FG - when it breaks, the edges are sharp and lined with tiny micro shards of glass. FG slivers are really a pain to dig out of your skin because you can’t see them. Usually you find out the next day that you have one or several glass slivers when you find red welts on your skin. My advices is for you to sand all the edges and perhaps wear gloves until that is done. 

3. Adhesives, however are the same as using Readi-Board. Hot glue and epoxy are my favorite choices. 

by HilldaFlyer

November 2014

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Supplementary data (in case you haven’t had enough already - time to get some more ice cream). During the strength test, every time I laid additional weight on the foam core sandwich, I waited 2 seconds and then recorded the span’s deflection. This data provides additional information, like

• Most of the foam core sandwiches were flat, starting at a height of 8.1 to 8.2 cm (except for Kraft paper which started with a 2 cm sag (10 cm).

• Most of the sandwiches broke after deflecting 2 to 4 cm indicating this is the point when the compression of the foam/surface was compromised.

• FT 3.15 oz/yd and 2 ply FG 1.43 oz/yd behaved similarly, but the 2 ply was stronger. It is well known when working with FG or carbon fiber (as well as other things like plywood) that many thin layers is stronger than one thick one.

There were 4 foam sandwiches that broke after deflecting 4 or more cm, Readi-Board, FG 0.73 oz/yd, 2 ply FG 0.73 oz/yd, 2 ply FG 1.43 oz/yd.