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A long time since i did this.....

Laying up a GRP cowl

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Tim Ballinger17/01/2020 15:01:47
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Last year I used a new GRP cowl when renovating an old Cessna 172 designed by Eric Fearnly.

I can not remember where the original came from back in 1973 but I did make a mould from it and layed up a spare just in case. That is what i used in last years refurb. I had kept the mould as well as the spare.

A fellow modeller spotted all this is also renovating his old Cessna 172. He has asked if I could make him one from my old mould. Now it is over 40 years since i used this skill set but as the weather is shall we say not conducive to flying atm . I agreed to give it a go. Maybe foolhardy given there are may of you out there who are well skilled in this area but I also said I would record my attempt in a blog. So here we go.....

Tim Ballinger17/01/2020 15:25:42
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799 forum posts
290 photos

Firstly here is the mould.

It has suffered a few dings over the years which I have attempted to fill then polished with carnauba wax. I’m hoping any remaining imperfections will polish/sand out of the final cowl or cowls depending on how many attempts I need to get one that is acceptable.

71193d2b-acf0-4ae1-9445-961859adadb3.jpeg

I am going to use a chemical release agent viz just wax and the first coat has been applied in the above pic. Dries very quickly but is almost invisible so you just hope it is all covered ( applied with soft cloth) . Instructions suggest 6 coats for a new mould so I will give it another 5. The release agent is meant to be multi use, hmmm.

Next pic shows most of the kit I have assembled for this task..

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Tim Ballinger17/01/2020 18:43:10
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799 forum posts
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I am anticipating using 3 layers of cloth so I have already cut the pieces I think I will need.

473e3944-6380-4bc4-8c51-ec723012e433.jpegI should be able to lay three layers of cloth in 1 go as I bought epoxy laminating resin withe slow hardener which gives between 90 and 120 min pot or Working time viz the 15 mins you get with the standard or fast cure.

Next step is to brush on the gel coat. I am going to use what is shown as epoxy compatible polyester gel coat. According to the blurb this allows the epoxy laminate to laid from about 3 hours after applying the gel coat up to 24 hours afterwards so some leeway.

Accurate Mixing ratios are required so it will be out with the digital scales to orchestrate the mixing. My method of adding the catalyst will be by pipettes of which I have many and are throw away items.

Only the gel coat has the Nasty styrene content so it is on with a mask as well as gloves for that bit - windows open too.

Hope to tackle the gel coat in the morning.

Edited By Tim Ballinger on 17/01/2020 18:44:33

McG 696917/01/2020 18:58:00
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It just doesn't seem that you need much help, Tim.

Just don't make the gelcoat to thick as it is quite heavy and can become brittle and crack if applied too thick.

Start with your layers of glass tissue when the gelcoat is still tacky.

One thing to remember is that an overdose of resin doesn't give any additional strength, only extra weight...

You should still be able to see the wave of the tissue between the layers you apply.

Cheers

Chris

kevin b17/01/2020 19:24:34
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May i suggest laying each layer of cloth up separately and use 15 min catalyst.

If you try and layer it up in one go with a slow cure you will either end up very tired from pulling the resin back up the mould for hours, or with a cowl that is very thick at the front and very thin at the rear. That is unless you have a machine to keep rotating it until the resin goes off.

I have experience !

crook

Tim Ballinger17/01/2020 19:36:30
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799 forum posts
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Thanks for the comments both. I went for slow cure because I am forever having to rush jobs when using 15 min cure. However, even after being frugal with the resin, If cowl no 1 with slow cure gives me the trouble you suggest it might then I will order up a bottle of fast catalyst ( resin is the same whichever) and go for cowl no 2 !

Thanks guys and any other tips more than welcome.

Tim

McG 696917/01/2020 20:38:17
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Tim,

As Kevin B suggested, don't do your 3 or more layers at once, the only thing you can obtain is a build up of heat which is a total 'not done' at all.

Wait at least for some 'half-curing' time between them, but that can be tricky with 'fast' resin hardener.

Prepare small quantities of resin at once and repeat afterwards if needed.

Patience is your friend with composites, young man.

Cheers

Chris

Allan Bennett17/01/2020 20:47:16
1733 forum posts
53 photos

I think the only thing I don't do as you've described Tim is I don't use a gel coat. I don't see the need for it, as the quality of the finish is determined by the final painting. I use West System 105 resin with slow hardener, and use 50g cloth for my first (outer) layer followed by one or more layers of 200g cloth. I lay them all in one session, and haven't been aware of any problem with gravity-induced bulges. I stipple the resin into the cloth with a 1/2" brush layer by layer, and have never had the need to squeegee the excess off with a credit card, as I often read about in forums.

McG 696917/01/2020 21:22:31
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3369 forum posts
1287 photos

@ Allan,

You can go on with composites without gelcoat for sure.

But gelcoat is a lot stronger/harder - read 'impact free' - than normal epoxy or polyester resin at the final top surface.

As for using 50 up to 200gr tissue, I imagine you're building parts for full size aircraft, not even Ultralights...

Allow me to wonder how much one of your cowls would weight?

For our - say up to 1:6 scale toys - all we do need is a few layers of 25gr/m² tissue... maybe one layer of 50gr at the end, if you're getting a bit paranoid...

Cheers

Chris

Tim Ballinger18/01/2020 08:47:05
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799 forum posts
290 photos

Thanks to you all for the comments. There are indeed many pitfalls when working with composites and I sense we have all experienced one or more of them. As for this project I will be starting with a 20g/m light cloth and depending how it conforms I intend to repeat with the same weight before finishing with a heavier 200g/m.

With the slow cure and the essentially quite thin ( probably around 2mm ) overall lay i am not expecting any huge heat build up. I am hoping the extra time the hour pot life gives me will allow me to stipple only the minimum necessary resin into the cloth but we shall see. As I say if it does not work out as I anticipate I will modify the process and have another go.

I will report back on the results , good or bad!

Tim

Piers Bowlan18/01/2020 10:37:12
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2168 forum posts
53 photos

I built a GRP cowl for a Model Designs Bearcat years ago from the original ABS cowl. I made the mould and like Kevin said, the epoxy did pool somewhat at the front of the cowl making it a little thick. As it was weight at nose of the aircraft rather than the tail I was not too bothered and it was very strong. I thought that if I need to make another cowl I would try turning it up side down to cure, so allowing any excess epoxy to drip out the bottom which was going to be trimmed off once cured. My original cowl never got broken however (too strong) so I never got the chance to test the theory!

Edited By Piers Bowlan on 18/01/2020 10:38:18

Tim Ballinger18/01/2020 11:03:38
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799 forum posts
290 photos

Well gel coat is on. Biggest issue is seeing that everywhere is covered. You really only need about a 20g mix but to make sure I mixed the correct ratio (2% by weight) I started with 100g of resin and added the requisite 2g by pipette.

You can not see anything in the pic really except for the excess resin in the tray alongside. (It’s shallow to stop any heat build up and premature curing) .Started to go thick bang on it’s quoted 15 min pot life.)

Plan is start the lay up in around 4 hours time. Should be tough enough but still a bit sticky. Instructions reckon adhesion is excellent even after 24 hrs. 
Piers I hear your thought about upside down but I reckon I would be asking for laminates to sag if I tried that so I’ll stay the right way up.

8df4e244-c877-4190-b2f8-9a6885b337e3.jpeg

Edited By Tim Ballinger on 18/01/2020 11:07:54

Piers Bowlan18/01/2020 12:45:00
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2168 forum posts
53 photos

That was my only concern, sagging, or the glass cloth separating from the mould. I recon it would be OK doing one layer of the lay up at a time but the only way to be sure is to try it with a test piece, not your precious mould.

Tim Ballinger18/01/2020 14:53:55
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799 forum posts
290 photos

It took me 80 minutes to lay up 3 layers of cloth and having mixed up 78 g of resin at 30% ratio I have about 40g unused. After 100 mins the remainder is just starting to thicken .

It was so much easier having all that time to work with. You could actually spend time working the resin into the cloth without having to spoon it on in a race to beat it curing in the middle of the job.( slight exaggeration but those that have tried this will know what I mean).

I used 2 layers of 20g and 1 of 200g cloth as planned. I used large oval shapes to cover the bottom or nose of the cowl and then just overlapping rectangles to go round the sides. A couple of small discs of 200g cloth right at the front to make sure of strength where it will be drilled out for the prop shaft.

Currently no pools of resin and no heat but then it is only just starting the long cure.

Of course I have 30 hours to wait before I find out if it even comes out of the mould let alone what sort of finish I will get.

Any way this is what it currently looks like.

894cfbc6-7f29-41d8-87a7-144952a38bf1.jpeg638e4b7c-4994-4b66-8f56-f02eed918c7d.jpeg

Allan Bennett18/01/2020 21:26:37
1733 forum posts
53 photos
Posted by McG 6969 on 17/01/2020 21:22:31:

@ Allan,

. . . . As for using 50 up to 200gr tissue, I imagine you're building parts for full size aircraft, not even Ultralights...

Allow me to wonder how much one of your cowls would weight?

For our - say up to 1:6 scale toys - all we do need is a few layers of 25gr/m² tissue... maybe one layer of 50gr at the end, if you're getting a bit paranoid...

Cheers

Chris

I can't imagine anything keeping its shape with only "a few layers" of 25gr tissue. I've just checked my Flair Magnatilla cowl, and it weighs 68 grammes painted. With electric models one often needs more weight up front anyway.

McG 696919/01/2020 11:24:44
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3369 forum posts
1287 photos
Posted by Allan Bennett on 18/01/2020 21:26:37:

I can't imagine anything keeping its shape with only "a few layers" of 25gr tissue. I've just checked my Flair Magnatilla cowl, and it weighs 68 grammes painted. With electric models one often needs more weight up front anyway.

It depends of course of the dimensions and the compound curves of the cowl, Adrian.

Ballerina's cowl is 17cm long by 10cm wide.

Glass tissue > 4 layers of 25gr + 1 partial layer of 48gr at the 'spinner' area + 2 bandages (2 cm wide) at the back (sides and bottom) for the mounting screws.

The weight out of the mould was just under 14gr...

Adding 2 glass laminated ply bits to mount the exhausts + build up filler + primer brought the cowl just above 19gr.

1420_cowl_primedweight_900.jpg

1413_cowl_thickness_900.jpg

At the thinnest spot the thickness is 0,57mm, while the average thickness is 0,7mm.

Perfectly possible thus

Cheers

Chris

Tim Ballinger19/01/2020 18:34:02
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799 forum posts
290 photos

I’ve become a real fan of slow cure, not only 90 min pot time but no obvious heat at all in its 30 hour cure time. At least that’s how long I had left it when I released it from the mould.

I am actually pretty pleased with the result . It weighs in at 32g or 1.12 oz ( 15cmx15cmx12cm) and feels rigid enough to me, certainly pretty much the same as the original.

Surface finish I am also pleased with , its picked up a few whitish spots from the bits of filler I used on the mould but that’s ok. I can see a trapped air bubble in the nose but it’s in the solidest part so would be ok.

That said I am going to make another. Reason? I commented it was difficult to see where I had painted on the gel coat and I obviously got a bit thin on one corner right on the edge of the mould and as a result there is about a 2cmx 1 cm bit missing it’s gel coat. Again not a real show stopper but my personal mission was to do the best I could so I will have another go. I am wandering if putting some pigment in the gel coat might help ensure a more even coverage.
Chris your cowl looks to have colour , did you use a coloured gel coat?

Anyway pictures below including a close up of the offending bit of missing gel coat.


3d59ec27-a768-49e4-ada4-ca8df0dda190.jpeg65e14f7f-8319-445f-9e25-b4efe4526666.jpeg794e771e-d1ce-47da-bf29-bed7581040ef.jpeg

McG 696919/01/2020 19:02:46
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3369 forum posts
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Super good result there, Tim.

I thought your gel coat was pigment coloured in the can as mine does. In the case yours is not - and it obviously isn't - I would suggest adding a few drops of resin or epoxy pigment. A lot easier to visualize your spreading.

I'm adding a few drops of black to my epoxy as well, which is the grey that you can notice in my pics.

... is the glass of wine in yours simply a 'cry out of Victory??? cocktail

Cheers

Chris

Tim Ballinger19/01/2020 19:38:38
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799 forum posts
290 photos

Chris,

Thanks for the kind words. The wine is indeed mine, a reward for walking round part of a very,very muddy half marathon at Doynton near bath. My other half ran the full distance , mad woman so I just do my own walk for an hour or so,.

I thought about buying some pigment when I bought the gel coat so I will revisit and colour before my next attempt.

Thanks Tim

Paul Whitwell20/01/2020 13:15:56
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4 forum posts

Hi Tim,

Thank you for taking me through the mold process - one which I am very new to. To everyone following this post, I am the other 'modeller' to whom Tim refers in his opening.

I am actually just about to start building this model, having purchased the plan from MAP some 40+ years ago .....there seemed no point in rushing into it . My plan showed a wooden front end construction but the idea of a molded version greatly appeals.

I see things have changed considerably since I was last modelling (there's the internet for one), so it's also encouraging to see how much help & support there is out there. For me, your photos speak a thousand head scratching sessions, so thank you very much not only your time, but also for posting this.

Paul

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