Micro Scale Huey Project

Micro Scale Huey Project

Postby SamIam » Fri Mar 08, 2013 3:52 pm

OK, this is no were near done, but I've got to get some of this out of my system already.

I've seen quite a bit of interest online in kit bashing plastic model helicopter kits to fit an Blade mCPx helicopter. The process goes something like this. Get yourself a plastic model kit, than built it around an mCPx frame. Simple huh? Of course this results and a grossly over weight mCPx. It'll fly but just barely. Then there's all the hacking of plastic to get things to fit. Some conversions look good, others not so much. You can thin down the plastic kit by grinding out the inside to save weight, but this is tricky. At the end of the day a plastic model kit simply isn't a great 'scale' solution.

I'm going to take a different approach. Instead of using the kit directly, I'm going to make a detailed mould of the kit, then make my own scale shell for a fraction of the weight. Here's how it works.

It starts with a plastic model kit. In this case a 1:48 scale Heuy Hog. This is barely big enough for an mCPx (I also have a 1:35 kit which would be a better fit), but it's a great size for a smaller msrX or nano.

Huey Plastic Model Kit
mould_kit.jpg (82.57 KiB) Viewed 9813 times
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Re: Micro Scale Huey Project

Postby SamIam » Fri Mar 08, 2013 7:45 pm

The process is really pretty simple. First I picked up some liquid Latex from Michael's craft store. The latex was a little more expensive then I like to spend on my projects, but liquid latex is just so darn much fun. I later found liquid latex at Gail's Florist for much less. At Gail's it's sold as a 'rug backer' for applying to the back side of rugs. But it looks like the same tub and if anything, it's a little thicker which is a good thing here.

First I sliced up the plastic model kits so that the section ahead of the main rotor shaft is separate from the section behind. I wanted to be able to mould a font half and a back half that the mechanics could be built into without having to dismantle everything. Next I laid the two plastic body (back) halves on a flat cutting board. Some plasticine was used to fill holes and block the 'open end' at the front. Then the latex was carefully painted over the whole plastic piece.

You can almost make out the shape of a Huey under the latex.
Liquid latex painted on to plastic model kit parts
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Liquid latex is really easy to work with. It washes up with water, and doesn't really stick to much (except itself) so you don't have to mess with mould release or anything. It also picks up detail perfectly, but you have to be careful not to let any bubbles get stuck to the piece. When it's wet it's white (like white glue) and as it dries it turns yellow/clear. The active ingredient is ammonia (don't sniff it!) that air dries and once dry it can't be re-liquefied. Don't use water to thin it down, use more ammonia. Adding water to liquid latex will actually draw the ammonia out of it, making it harden even more.

The only draw back here is that you have to put it on in thin layers and you have to allow each layer to dry. So it's paint on the latex, then set it aside to dry over night, then another layer of latex, then another over night dry, etc, etc. So while it's incredibly easy to do and takes very little time, you do have to count on the process taking several days or more.

To make the mould a little stiffer, I laid drywall repair mesh (fibreglass screen with a sticky side - comes in a roll) on top of the latex half way through the layers and buried it in more latex.

The latex by itself isn't really 'stiff' enough to use as a proper mould, so once I think it's thick enough (and fully dry) I mixed up some plaster of paris and covered the latex in burlap pieces soaked in plaster. Just like a cast used to mend a broken bone. In fact, it would probably be just as easy to get plaster cast wrap and lay up a plaster case over the mould that way. The plaster will hold the shape of the latex mould (the latex mould will hold the detail).

Once you've got a completed mould, the fun stuff begins :)
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Re: Micro Scale Huey Project

Postby SamIam » Fri Mar 08, 2013 8:14 pm

Moulding parts is fun and easy. In a nutshell, you spray the inside of the mould with your final paint colour, then lay up tissue and some kind of bonding liquid (glue, varnish, futura floor wax, etc) then pop the whole thing out when it's dry.

Because it's latex (and lots of things stick to latex), start by lubing up the mould. I used good old Armor All (the stuff you use to make your car vinyl shine). It wets down the latex and seals it well. Just spray it on, and wipe it around with a clean cloth. Next I'll spray the inside of the mould with paint. Flat olive drab is a good colour choice. Make sure the paint gets into every nook and cranny. Since the latex is clear, it helps to hold it up to the sun (or a bright light). You'll see right away where the paint is to thin or didn't get into details. The smallest details will be picked up by the paint alone.

Let the paint dry thoroughly and try not to handle the mould too much. You don't want dry paint flaking out. Position the latex mould in it's plaster cast and you're ready to go.

I've tried a few different things for laying up the tissue shell. I can't say I have a favourite yet, and I think there's much more experimenting to do. Here's a few that I've tried:

- Many layers of tissue soaked down in a water based polyurethane varnish (can get heavy fast).
- Tissue soaked in Futura liquid floor wax (didn't bond as well as I'd hoped).
- Single big piece of tissue (didn't conform to details and the paint crumbled).
- Tissue with drywall mesh embedded (super strong, but the 'mesh pattern' came through the final finish).

I haven't tried watered down white glue and to be honest I've been impatient with the tissue layers. Things would probably work better if I just did one light layer at a time and allowed things to dry thoroughly between each layer.

The key is to get the tissue 'into' the details so it gets a good bond with the paint. Otherwise the paint will crumble away when you pull the piece out. There's no doubt a balance between the layers of tissue and bonding. Once you've got the moulds made, it's only pennies worth of material and your time to try new things.

Here are two halves layered up in tissue and drywall mesh ready to be popped. The green tape was used to hold the latex mould edges down tight against the supporting plaster cast.

Two halves layered in tissue and drywall mesh
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Shazam! One highly detailed, reproduction of a plastic kit piece. See how neatly it comes out of the latex mould?

Moulded tissue reproduction
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And just look at the detail. I mean, really look at the detail! You'd never know how small this is from the picture.

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And now compare the weight. The original plastic kit part weighs 5.15 grams.

Original Plastic Kit Part
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The new tissue reproduction part weighs only 2.9 grams and it's an exact copy.

Moulded tissue part
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This is one of the first moulded pieces I made. I'm sure I could make it weigh a lot less. In fact, if this were just a 'skin' for a structural tail boom, I could probably make it thin enough to weigh next to nothing.
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Re: Micro Scale Huey Project

Postby SamIam » Fri Mar 08, 2013 9:02 pm

OK, now for more fun.

Another idea I have is the use of spray foam to add light weight strength to the structure. Here's my weapon of choice:

Great Stuff!
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There are all kinds of spray foam (even in the Great Stuff brand). This particular can is made for use with a user provided spray gun (a tube with a pistol grip and a trigger button). The regular foam cans with the applicator nozzle will plug up after the first use, so you pretty much have to use the whole can or it's wasted. The spray gun used with this can lets you shoot foam without 'corrupting' the can, so you can use a little or a lot (and let it sit for months).

Another feature of this particular can is it's low expansion and quick setting, so it won't blow the mould apart. Most spray foam keeps expanding for hours after you spray it and can cause all kinds of problems. This particular brand also stays soft and spongy when it's cured instead of turning hard (don't know if that's an advantage yet) and seems to maintain small bubbles. When spray foam gets used in large volumes the bubbles tend to pop and grow together until you're left with big air pockets instead of nice sturdy foam.

Prepare the moulds same as before. Lube up with Armor All, then spray a paint layer and lay up a tissue layer.

Now it gets interesting. Place the two mould halves together and wrap them tightly together. I used tensor tape to wrap them. Tensor tape is like a stretchy tensor bandage, only it 'sticks' to itself. I learned about this after my dog ended up at the vet's.

Prepared mould
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Once the mould is all wrapped up, I press the foam spray gun nozzle to a hole in the tip of the tail and let her rip. Foam will start bulging out the open end.

Filled mould
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A little curing time later and out it comes. Here's a really good look at both the latex mould and the plaster cast that supports it.

Mould and Plaster Cast
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And finally, the finished piece:

Finished piece
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Now, there were all kinds of problems with this piece. Problems that can be avoided.

First, I tried to use a single piece of tissue (lazy) in each mould half (yellow tissue). It didn't get into the crannies very well and didn't bond to the paint well in those locations. The paint stayed in the mould in those spots.

Second, I didn't fully wait for the tissue layer to dry (impatient). So it didn't hold shape like it should.

Third, I don't think I lubed the mould this time around either and it didn't want to pop out very easily. The tail fin got 'bent' while removing it and that left a crease in it.

But these are all problems that can be over come, and like I said, failure costs only pennies and time. Still, not bad for a first try.

I made my moulds halves separately on a flat surface with no key lock between the two halves. that was a mistake because I really have no way to match up each half accurately. My next set of moulds will be created over a one piece 'back end' with a parting line and interlocking keys so the two halves can be locked together.

Laying up tissue can be a bit of a pain. It's messy and sticky and then you've got all kinds of clean up afterwards. So I thought, what if I skip the tissue, and simply paint the mould and spray foam it. Nothing, and I mean nothing could be easier than that.

So here's another attempt, no tissue.

Paint and Foam - no tissue
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Now that's what I'm talking about! The detail came out great, no voids or break aways. The foam really grabbed the paint. The thinner parts of the tail in particular are perfect. There are some problems in the thicker parts. It seems that when the spray foam gets thicker, it just can't cure properly and the bubbles collapse. This come through the surface detail looking like hail damage (it almost looks like size appropriate battle damage).

I think there's an easy fix for this. Just don't let the foam get thick. My plan is to create an 'inner' mould to fill the main space so that there's never more than a 1/4" thickness cavity for the foam to fill. I think if the foam area can be kept thin, it will quickly cure through without collapsing into pock marks. And the final piece will have a hollow cavity which will no doubt be very useful for later construction.

But really, this method of creating a piece is so easy, once I get the technique down I'll have plenty of Huey parts to start flying.

My latex moulds still have good detail, but making them as separate flat sides was a mistake. The edge of the mould were the flange meets the piece was always thin and raw (the latex slipped under the edge of the kit part while I was making it). It doesn't reproduce a clean edge on the finished piece. And then there's the whole alignment of mould half issue. In other words it's time for new moulds.

My next set if moulds will be made from silicon (common bathroom silicon caulk), with proper interlocking halves.
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