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David Paule 01-08-2020 06:50 PM

Epoxy and Microballoon Practice
 
It's been a while since I worked with epoxy and microballoons and foam, and since I had enough stuff to work with, I decided to make a practice kit.



Starting at the far back, is a roll of fiberglass cloth which I didn't use today. Just in front of that is a bag of microballoons, left over from the sailboat I had the first decade of this century. Then the tall can is some epoxy resin, Rhino or Jeffco 1307 LV, and to its right is a quart of the associated medium hardener. At the right of the hardener is my mixing cup and the two pieces of scrap foam that I glued together with the microballoon and epoxy mix.

In the foreground we can see the gram scale and the syringes I used to meter out the resin and hardener.

This is the first time I'd used this resin system. Steve Smith recommended it, and so far, based on this one trial, I like it. It's available from Aircraft Spruce. Previously I'd mostly used the West Systems epoxy.

The gram scale measures and reads down to 1/100 of a gram. The closest I could get to the goal weight was about .03 grams and that took some doing. Frankly, I don't think that precision was necessary, but that's a guess; I don't really know.

The mix had the consistency of peanut butter. I could have made it drier and if I repeat the experiment, I probably will.

I'll let this cure completely and then carve and shape it and sand it to see how it feels.

Dave

Taltruda 01-08-2020 10:43 PM

My mixing method
 
I have the same epoxy and like it. I think the lancair guys use it.. anyway I thought I would share my mixing method.. I have an amazon scale, and little squirt bottles that come in a three pack from harbor freight. I place a little disposable plastic cup on the scale, tare it back to zero, then add epoxy (10 grams) tare back to zero, then hardener (2.2 grams). If I need more, I increase it at the same 100:22 ratio by weight. I keep the metal cans in the fridge in an attempt to prolong their shelf life. It?s super clean with no spilling. I haven?t thought about syringes, but do they leak in between uses?

David Paule 01-09-2020 10:24 AM

A scale that reads in 1/10 grams is definitely easier to use than one that goes to 1/100. For larger quantities I'll use that.

The syringes definitely leak. You can see a few drip marks on the paper. I've stored them in separate zip-lock bags and will probably change to your containers, thanks, or something else.

Dave

DanH 01-10-2020 07:08 AM

Quote:

Originally Posted by David Paule (Post 1398781)
A scale that reads in 1/10 grams is definitely easier to use than one that goes to 1/100. For larger quantities I'll use that.

The syringes definitely leak. You can see a few drip marks on the paper. I've stored them in separate zip-lock bags and will probably change to your containers, thanks, or something else.

Ditch the syringes Dave. Get something like wide mouth condiment squirt bottles for hardener, and a 1 qt pour bottle for resin...something easy to refill from the can.

An electronic scale is great, but can be a lot like switching from steam to an EFIS...we begin to worry about small values because it displays small increments.

I've been mixing epoxies on an old 0-500 gram beam scale since the mid 80's. I think my father-in-law salvaged it from someone's trash can.

Quote:

Originally Posted by Taltruda (Post 1398695)
I keep the metal cans in the fridge in an attempt to prolong their shelf life.

Don't do that. Viscosity is too high until it warms to shop temperature, and cold cans will condense water. With epoxy, capped cans and bottles last a long time on the bench.

You really want the shop, the work, and the resin at about 75F.

David Paule 01-13-2020 06:48 PM

Templates
 
I made some templates to transfer the correct fair shape to the foam, once I have it glued up. These are thin plywood. The lines at the left are where the panel goes.

The bottom template is the center one. The others will be used on both sides of the mold for symmetry. Sorry for the background. The color and hue are really too close to the wood for a good photo.



Right now three of the five slabs of foam are glued on with epoxy and micro balloons.

If you've guessed that I'm creeping along, you've guessed right. These days I'm barely spending time at it.

Dave

David Paule 01-24-2020 08:36 PM

Foam Fun
 
The fiberglass canopy fairing needs to fit the skin, and there were clecos poking out of it, holding the front top on. Time to screw it down. On the RV-3B, this skin is attached with a lot of #8 screws. The side screw holes got dimpled for #8 screw heads as you'd expect. The instrument panel flange wasn't wide enough for #8 dimples so I decided to use NAS8702U1 8-32 screws instead. These screws are really good: the material is 160 ksi A286, strong enough to survive a number of installations and removals, corrosion resistant and very tough - cracks don't propagate easily in that material. It's the material that many spacecraft screws are made of.

This particular part number has a #8 body and a #6 head. I had to get a special dimple die for that, and Cleaveland came through.

In this photo, the instrument panel screw, on the right, is in a #6 dimple as it should be. But the screws on the bottom (the side of the fuselage) are #8 dimples and these screws just don't fit. I included one of the standard #8 screws for comparison. Since this is merely to temporarily replace some clecos, the use of the wrong screws here is okay.



The foam stack finally got micro'd together.



Using a Japanese back-cutting saw, more commonly used for fine woodwork, the corners came off easily.



And then I started in with my old Stanley Surform wood rasp. This was slower but before I ran out of time, I did make some progress.



Lots more to go.

Dave

David Paule 01-26-2020 08:26 PM

Foam, continued
 
I'd noticed that the foam block was sliding side to side as I carved on it. Not surprisingly, since the bottom was flat and resting on the fuselage longerons. Its shape prevented fore/aft motion and rotation but nothing kept it from shifting laterally.

Two small foam blocks took care of that. This photo shows the bottom with the foam blocks in place. We're looking at the bottom and the back of it. I'd brought it into the house for the glue to set up, the house being warmer than the shop.



The addition worked.

I continued to shape the foam and it's a bit closer than this photo suggests, but not much. I'm using a Surform file and expect that in a day or so, I'll switch to a 36 grit longboard that I made.



The other side is about the same.

Dave

AlpineYoda 01-27-2020 12:16 AM

For cutting foam, there are ?hot knives? (a hot wire knife) that are commonly used for other applications. I suggest you check out any google search for model railroading and foam blocks. Foam insulation blocks are used a lot for landscaping contours in model railroading and the hot cutters avoid dulling normal knives and don?t create lots of dangerous foam dust like a normal knife does.

Model railroader magazine has had lots and lots of references to cutting foam over time.

David Paule 01-27-2020 11:02 AM

Well, thanks, always good to learn about another tool. I knew about hot wire cutting for airframe parts and even have a hot wire cutter that I've used in the past. Since this is a compound curve, I didn't think it would be appropriate here.

As it is, I expect to be done with the gross foam removal shortly using the tools at hand:

Saw for the rough trim,
Surform for the coarse shaping,
36 grit longboard for the close shaping.

Dave

wirejock 01-27-2020 11:27 AM

Hot knife
 
Used to use one of those to cut Radio Control wing foam cores. Pretty cool.


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