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Hollow moulding - a LOW TECH method…

For those interested in the use of composite materials
Jolly Roger
Posts: 519
Joined: 30 May 2015, 20:35
Location: Sutton Bank, North Yorkshire

Hollow moulding - a LOW TECH method…

Post by Jolly Roger »

I really like hollow moulded flying surfaces on model aircraft.

They have that lovely solid look, are commendably strong and light, and can be more robust than wooden structures which is handy if your models have to deal with the rough and tumble of slope flight. Maybe it’s just me but whenever I pick up a traditional wooden structure, I’m worried I’ll poke my finger through thin ply sheeting or dent some balsa. I am clumsy like that.

On the other hand, composite models can be a heap of work if you do them properly…making plugs for every part and then making female moulds off them. It’s only really worth it if you plan a production run.

So I’ve been pondering for a few years how to make a hollow moulded item without all the expensive equipment and in a fraction of the time.

When I smashed the back end of my fave 4m semi scale glider last Summer, I decided I’d have a go making a hollow-moulded tail fin/rudder.

It’s actually gone really well…
Last edited by Jolly Roger on 24 Feb 2022, 23:42, edited 1 time in total.
Jolly Roger
Posts: 519
Joined: 30 May 2015, 20:35
Location: Sutton Bank, North Yorkshire

Re: Hollow moulding - a LOW TECH method…

Post by Jolly Roger »

It’s Monday morning and I sketch out a new fin shape on some 2” blue foam, checking that the surface area matched the original fin, but going for a more trendy fin shape with less sweep and higher aspect ratio.
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Jolly Roger
Posts: 519
Joined: 30 May 2015, 20:35
Location: Sutton Bank, North Yorkshire

Re: Hollow moulding - a LOW TECH method…

Post by Jolly Roger »

Off to the computer to print out a top and bottom profile. NACA 0009 thickened to 10%. Glue onto a scrap of 6mm MDF and then 2 mins of scroll saw therapy.
This is fun. I am making stuff, seeing fast results and it’s all going well.
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Jolly Roger
Posts: 519
Joined: 30 May 2015, 20:35
Location: Sutton Bank, North Yorkshire

Re: Hollow moulding - a LOW TECH method…

Post by Jolly Roger »

Cut the outline of the foam block, draw a datum around the edge, and glue on the profiles.
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Jolly Roger
Posts: 519
Joined: 30 May 2015, 20:35
Location: Sutton Bank, North Yorkshire

Re: Hollow moulding - a LOW TECH method…

Post by Jolly Roger »

And now just to show you how low tech I can go, I cut around one side of the templates using a SAW. I could have used a hot wire cutter. But I’d have to make one first. And I’m in a hurry.
2 mins later I’m sanding the last few mm down to the templates using my long permagrit sanding block. It’s surprisingly easy. Especially with the low early morning sun casting helpful shadow across the surface.
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Jolly Roger
Posts: 519
Joined: 30 May 2015, 20:35
Location: Sutton Bank, North Yorkshire

Re: Hollow moulding - a LOW TECH method…

Post by Jolly Roger »

I survey my progress, check my watch and I’ve been in the workshop just over an hour.
Often I can spend 2 hours in there and have essentially nothing to show for my time, so this is brave new territory for me.

I tape over the blue foam with parcel tape, so that I can remove my fin skin later (remember, this is supposed to be hollow moulded, so I don’t want a sold cored structure, no, that would be cheating).
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Jolly Roger
Posts: 519
Joined: 30 May 2015, 20:35
Location: Sutton Bank, North Yorkshire

Re: Hollow moulding - a LOW TECH method…

Post by Jolly Roger »

Now to decide on a lay up for the first skin.

I did a lot of experimenting over Christmas with different cloth types and weights, and I learned a heap. Which is a euphemistic way of saying I made many, many mistakes.

Anyway, I had best results with 160gsm carbon/Kevlar as the main structural layer, with a top layer of 100gsm glass twill, to seal in the Kevlar and give some thickness to work up a good surface finish.

I cut out the cloths, waxed the parcel tape, and got ready to mix my first batch of resin…
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Jolly Roger
Posts: 519
Joined: 30 May 2015, 20:35
Location: Sutton Bank, North Yorkshire

Re: Hollow moulding - a LOW TECH method…

Post by Jolly Roger »

Fabric layer onto the parcel tape, epoxy screened on with an old credit card, peel ply laid on top.

By the way, Terry White introduced me to peel ply a few years ago and it’s a perfect example of something I’d previously scoffed at as being unnecessary complication and waste of money, but which I would now never be without. When removed after the resin has set, it takes away most of the excess epoxy, reducing finished weight, and gives a textured surface perfect for adding further composites.

With the first skin laid up, I could have popped it into a vacuum bag, but then this wouldn’t qualify as a low tech method.
Also, I scoff at vacuum systems as being an unnecessary complication and waste of money. :D
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Last edited by Jolly Roger on 24 Feb 2022, 23:32, edited 1 time in total.
Jolly Roger
Posts: 519
Joined: 30 May 2015, 20:35
Location: Sutton Bank, North Yorkshire

Re: Hollow moulding - a LOW TECH method…

Post by Jolly Roger »

Tuesday morning I enter the workshop early, but tentatively. Will the plain sailing of the first day run aground on the rocks of reality?

Nope - the first skin is pretty much perfect! Hurray! I made a thing!

Straight on with the second fin skin.

Out with my saw and cut off the other side of the foam block, then sand down to the template.

Now, just for the laughs, I decide I will make the rudder with a living hinge (thanks to the encouragement of others on this site to try this method).

I hesitate for a moment, troubled by the fact that the rudder, previously a very symmetrical thing, will be very asymmetrically hinged on one side. But no, this is ok. I fully expect this whole attempt at hollow moulding to fail at some stage, so I may as well take a risk with this new-fangled hinging method.

So the layup is 160gsm carbon/kevlar again, then a 4cm wide strip of peel ply to act as the hinge, then 100gsm glass on top.

Epoxy gets mixed and off we go…
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Last edited by Jolly Roger on 24 Feb 2022, 23:47, edited 2 times in total.
Jolly Roger
Posts: 519
Joined: 30 May 2015, 20:35
Location: Sutton Bank, North Yorkshire

Re: Hollow moulding - a LOW TECH method…

Post by Jolly Roger »

While the second skin goes off, I realise I need to figure out how to attach the tailplane and fit servos to make bits wiggle.

I laminate a tailplane seat from 3 layers of 1/8” ply faced with carbon that happen to be on the workbench.
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