Layup on polystyrene

Excellent explanation, thanks again. I contacted Laszlo, and seems he really have good prices. I'm still wondering which foam would be better for weight saving a 1lbs EPS with stringer(s) or the 2.2 lbs Dow 60. It would be nice to estimate somehow how much fiberglass I need for either foam to achieve the same strength.

 

Have you run the numbers for how much weight is involved for the sail you are designing for each foam type?
Also- here is ACP composites page for supplies:

http://www.acp-composites.com/home.php?cat=4752

They are a good shop but don't get led into believing you need much gear to do this job-
The $5 dollar bag, some Mylars and maybe some breather cloth is about it from them. The pump I spoke of before is ample for this job.

I still have not posted my treatise on "vac or not".....
Whats up with that..?

 

Looking at the main element of the wing, the weight difference between the two foams (1 lbs vs. 2.2 lbs) is about 280 grams. If it allows me to use 2 layers of 4oz glass instead of a 6 oz and a 4oz, then I save about 130 grams in the laminate using a 50:50 glass-resin ratio for the calculations. On the other hand, if I go with the lighter foam, I'll have to use a stringer along the wing, so it may come out with the same weight at the end with lot more work.

It's all theoretical though, because, I still have no klew whether these layups are appropriate. I'm thinking that with the Dow 60 foam, a 4+6 oz layup should be more than enough, because there are numerous examples on the net using similar layup or even less glass on spyderfoam for building surfboards. It seems to me that the 2" thickness of my wing is also similar to a surfboard's thickness, and people stand and jump on those things...

To be on the safe side, I was also thinking that I could use spreaders and diamond stays like they do on multihulls with rotating masts. Of course it will also add weight and drag.

The second element of the wing, which is a NACA0012 foil with a 5.9" chord will be made from 1lbs EPS, because the load is much smaller on this. It's hinged to the main element at several locations, and there is no compression on it. It's actually good if it can flex a little bit.

I've looked at all the stuff I would need for vacuum bagging, and I have a question. If I use mylar sheets for the surface, it will prevent the resin from escaping the laminate, so the escape routes are only along the leading and trailing edge of the wing. Is that enough to get rid of excess resin? Wouldn't it be better to use that perforated release film intsead of the mylar? The description on the ACP website says the pin holes allow controlled removal of excess resin when vacuum bagging... Obviously there will be more sanding at the end, but I may end up with a lighter wing and better glass:resin ratio. No?

 

'Vac or not':

OK- In the broader world of hand layup in both repair and builds one is fighting to gain good adhesion to the substrate and between fabric layers while trying to keep resin content under control.
For many jobs the layup will act in specific ways to try and up the resin content as you work. This includes a 'spring' of the cloth from the surface (particularly in bends), some float of the fabric on resin, and some puffing up of the fabric as it absorbs resin (particularly true for Kevlar).

All of the above can make the part weigh more for no additional strength.

On other side of things, in the lack of sufficient resin the theatrics of the cloth can pull air into the layup creating pin holes that have to be watched as finishing proceeds and can produce voids if the layup springs off the substrate. Also- the heat from cure can cause out gassing from the structure which will cause voids.

While doing glass work some attention must also be paid to how fair the surface is if it must go to finish

The simplest way to combat all of these in hand layup is just working the fabric with the appropriate glass rollers and squeegees to work the fabric down and pull off excess resin. This works quite well and anyone doing much glass work is not giving it too much thought.

A more complex approach is to use a vac bag which will combat all of the above issues neatly for some trouble and expense.

I mostly just lay up by hand. When I can, I employ what I call a 'poor mans vac setup'. For this I just grab a handy garbage bag or tape and wrap the part down tight or cover & squeegee if a flat surface. Where it can be used, his technique is remarkably effective in keeping the glass from swelling up, springing from the surface or pulling air down into the layup.


In all the prior posts of mine I am presenting a technique I have used that is something of a hybrid vacuum approach.

This technique differs in several ways from the vacuum bagging and infusion approaches that are more common and really comes into its own for smallish weight critical parts.
It uses a impermeable thin plastic sheet in place of the more traditional 'peel ply' release fabrics.

1) The lay up is not done on the part itself- it is done on the carrier Mylar, then transferred and bonded to the core during the vac process.

2) The core is not wet out during layup.

3) There is no scavenging of excess resin off the layup during the vac bagging process.

The reason for the above is all about controlling resin content.
The way is works:

-You lay the fabric on the Mylar in the arrangement and layers as required by your build.

-You wet up the fabric fully and then pull off ALL the excess resin.

The last is important. What is means is that when placed on the foil all the resin there remains in the layup. Too much and the penalty is weight, too little (hard to achieve) could mean bonding issues.

-You place the Mylar 'carrier' over the foil, vac down and the fabric is pressed tightly to the core, bonding there when cured.

There are few important advantages to this technique which I will group into two camps; Ease and Weight.

Ease

-Light fabrics can be fussy. They tend to pull out of bias and cling to everything including you while you are trying to do a precise layup.
-The cores can be fragile and tough to maneuver while laying up especially if small.
-Both sides of the foil are done in one swing
-Out of the bag: you are very nearly facing a completely finished product. No fairing, filling or fussing.
-Form integrity is easily maintained as per prior posts.

Weight

-By wetting out the fabric on the carrier, you can control resin precisely in the finished product.
One leaves just enough resin to provide a bond to the core and a glassy smooth surface. There is no excess in the 'sponge' of the cloth or a fully soaked core surface.
Nothing is squeezing out as there is just enough to do the job no more.

-By nature of a smooth finished core out of the bag, nothing is added to gain a fair surface.


Now all of this may seem too fussy but if weight is a part of the design you are trying to control, this technique does so very well.
The savings are great if used on projects of the appropriate scale. It goes with out saying that there is no need to bother on just about any full scale structure. On your small gal however... One has to do the numbers and see whats up.


A couple of things occurred to me.
It could be you are really above the scale where this approach is most useful and just using a standard vacuum bag or hand layup would be recommended:

Your raw foil core may not be particularly flexible.
The weight gains may not be sufficient to be a full argument though the approach still will gain the other ease and fairing benefits.

One thing to think about if the core is sufficiently rigid it could be placed on a couple of pins in the ends and swung horizontally with the trailing edge down. It will be easy to wrap right around with light cloth. The fabric can be pulled quite tight with squeegees and a gloved hand. Then I would use the 'poor mans vac' setup which will leave you with a quite smooth surface. Either placed into your core "molds" or not. If the numbers you do reveal that the sail can afford it, a microlight bog can even be trowed over (mostly sanded off later) in prep for a nice paint job.

I still must caution; while I do plenty of glass work, I am at most a 5 gal pail of resin per job sort of guy. There are some here who do far more builds who might be able to chase this down better.

So there you have it. Lots of words for a simple question still not answered.


I remain interested in the design approach for the spar if you can crunch some numbers for the sail.
Also keep a tapered CF tube spar on the table perhaps- it might be do the trick.

And finally:

I think you need to not follow this example- I believe your foil is a different animal than that slab of a surf board.
We really need someone who can do wing skin load calculations to go much further with this..

 

That's exactly how I imagined doing it from the beginning. Especially because I'll have small rods sticking out from the wing by the time I laminate it. Those will be glued into the core and reinforced first.

I'd be interested why you think there might be higher loads on the wing than on a surfboard with a person standing and jumping on it. But yes, I'd really appreciate if someone could come up with some rough calculation on skin loads and layup schedule.

How flexible is this Dow 60 foam? Would it be damaged if the skin allowed some flexing of the wing? Will it have to be completely rigid to maintain the integrity of the core?

Thanks again for the rest. I think I'm convinced now that vac bagging would be useful. I just have to figure out how to create vacuum. I've seen a few ideas on the net to convert bicycle pumps and small compressors. The one you suggested is a manual pump, right? I'm afraid that for bigger projects that might involve too much pumping. I was also thinking of using my pressure washer. I could build a water tank from PVC pipes, fill up with water. Connect one end with a hose to the inlet of the pressure washer and the other end to the vac bag (no water in that hose of course). Since I have a pressure washer I only need to spend a few bucks on PVC pipes to test it.

 

What I mean is that for the same length, the foil of yours will see high skin loads if an equal force is applied.
I am not saying that you should be thinking even heavier skins though- I think building the sail as a beam to support same load as surf board is too high of spec..

I like the Mityvac pump it is manual and works like a charm. It does fine for projects of the size you are speaking of.

The 60 is flexible enough that no damage in the range one would anticipate.

Remember that the foam is offered as #40 and #100 in addition to the #60. Laszlo will have some other foams as well though I would start with a design analysis and let this direct towards the optimal core/spar/skin.

 

If & it's a big if, you want to do without vacuum bagging, this short article offers an option.

http://duckworksmagazine.com/03/r/articles/glass/bottom.htm

Also, Low Density Rigid Polyisocyanurate Foam (PIR) Insulation Boards as manufactured by Celotex or Kingspan for insulation purposes, is a much better core than polystyrene and in the UK is available as "seconds".

http://www.aandainsulationservices.com/

 

Just a crazy idea here I had last winter insulating windows. What about a heat shrink material? You can get all types of that stuff for all kinds of apps. With a good foam cutting setup/jig, some foam, and a hair dryer, you could build a workable sail. That foam, all by itself, can be very rigid. Plus, no matter how sophisticated you make it, you are still gonna have a 'fragile' wing that you need to plan on rebuilding...often I would think.

I wonder if this shrink wrap could be utilized as a vacuum bagging alternative.