bambam102

Hey guys,


Ive seen many different methods of building a sandwich composite that were a bit unusual, in particular, Lance Campbells use of wooden balsa blocks on his sr-71 project. A while ago I did a small experiment but instead of using blocks, I used spray foam. the insulation triple expanding type. In my small test I was so impressed with the mechanical bonding of the composite, and resultant strength, that im considering using it on a scratch project. has anyone ever tried this? any thoughts would be greatly appreciated

ByLoudDesign

Having used poured foam in the boat building to provide flotation in voids and etc.. I know that you have to be very careful with how much you spray in a confined area. It must have some place to expand to or the pressure will jack thing a part. In a small plane you could get distortion. Is it the "GREAT Stuff" product?

SCALECRAFT

My Feedback: (13)

What do you want to achieve by using foam in glass.

Steve

wyowindworks

How are you controlling the thickness of the spray foam to create a composite sandwich?

bambam102

It was a product very close to great stuff. In the small test I did, the intent was to create additional thickness and rigidity in the mold walls. I put down 3 layers of glass, then sprayed a layer of foam, sanded the foam to shape, then 3 more layers of heavier glass. In the end, I got a very strong quickly made sandwich. my test piece had very strong compound curvature too it and the foam was easy to use on these curves. again, the plan is to apply this technique to making the mold, not the actual fuselage part.

bambam102

This is what gave me the idea to try foam.

wyowindworks

Interesting.

ByLoudDesign

You might give soer though to strip planking the foam on to the surface, and then shape it and lay up th second surface!

SCALECRAFT

My Feedback: (13)

After making molds of various sizes and shapes, I noticed a few things that made things easier or a waste of time.

We concluded the following.

Even on you SR71 molds with the flat shape on the fuse

1)A rigid mold is good but does not insure a straight part once removed. Bulkheads play a bigger role in that along with jigging.

2) Some flexibility in the mold allows for the part to be removed easier by flexing the mold.

3) If you insist on stiff, using a wood or steel frame to support a mold is easier and less time consuming. Can be designed to hold the mold when you lay up a part as well.

4) With a part in the mold, very heavy and hard to handle. This would be a problem no matter what at your size.

Steve

If you were making a rod or something in that nature, I could see needing a super rigid mold, but for fuses. Not really.

SCALECRAFT

My Feedback: (13)

A pic of one of our early composite models. The mold is around 20 years old and is around 3/16" thick. Made from 100% "E" glass . 2 layers 4oz, 2 layers 6oz, 1 layer 20oz..



West Systems epoxy.

Steve

bambam102

beanbag filler? thats totally new to me, Ill have to check that out.

The planking added to the foam is an idea I was considering.

@SCALECRAFT, You make great points, I think that somewhere along the line about 15 years ago I heard a rule of thumb is that you want to go at least 3 layers of glass thickness of the mold beyond the thickness that you anticipate for the actual part that you will be pulling from that mold. I have used this rule of thumb ever since. and it has served me well for small parts. Now that Im planning a much larger part, Im looking for alternate ways to build up the mold thickness than endless glass layers. A little flex surely does aid in part extraction, but with a mold larger than 10 feet, I cant help but think my normal methods would have to be fortified . a little more thought required I guess.

I really appreciate the input and advice on this fellas.

wyowindworks

I guess it really depends on your expectations and the process used to fabricate the part. I use a lot of high pressure bladders in fuses to compact the layup, remove excess resin, and do the seaming all in one shot. This requires a stiff mold to prevent the mold from distorting and bulging between the bolt/clamp locations.

Also, if you are vacuum bagging into the mold to create your parts, necessary if the parts utilize a sandwich construction, then the mold must be able to the hold it's shape while in the bag.

If you do painted in the mold parts you want the seaming edge to be very tight and not step (align incorrectly). A stiff and stable mold becomes very important. Unreinforced flanges will begin to dent and permanently bow from the continual localized clamping pressure, especially if you use bolts. This prevents the molds from fully closing which then prevents the indexing from fully engaging. The end result is a wider seam that steps. The flashing on my parts mics from .002 to .004" thick. This makes trimming and part clean up go faster as well.

I use a sand/epoxy core in many of my molds. I heat the mold in a low temp oven prior to the layup. The epoxy and sand will hold the heat and stay around 100 degree for a couple hours. This will thin the resin when making the parts making the whole process lighter and faster to perform.....while your epoxy pot stays at room temperature.

Lastly, a mold made with some form of sandwich is simply more likely to hold it's shape for the long haul.

If you are painting the parts post molding or don't want to debulk the layup with some kind of assistance (vacuum bag, bladder, press, wrap) then the standard fiberglass back mold may work for you. The only molds that I build like that are small canopy molds...unless they will be vacuum bagged.

A nice mold that produces quality parts saves you time and money in the long run.

wyowindworks

Other mold sandwich cores:

SCALECRAFT

My Feedback: (13)

All good methods. However, for this type, specific to rc composite fuse or wing application, with free atmosphere lay ups, will not fail in its intended purpose.

A lighter lay up if vac bagged, infusion, or pressure mold , perhaps if an ounces scale is used, then yes technically lighter. But the extra materials for a few ounces, in our experience is not worth it.

Especially since we have developed composite configuration methods using only simply glass that yields stiffer fuses and wings without adding addition weight.

I see your point though on such a large mold, but I would have used a less labor intensive method.

Impressive project.

wyo, your molds look very nice.

Steve

wyowindworks

<style type="text/css">v\:* {behavior:url(#default#VML);}o\:* {behavior:url(#default#VML);}w\:* {behavior:url(#default#VML);}.shape {behavior:url(#default#VML);}</style><xml><w:worddocument><w:view></w:view><w:zoom></w:zoom><w:compatibility><w:breakwrappedtables></w:breakwrappedtables><w:snaptogridincell></w:snaptogridincell><w:wraptextwithpunct></w:wraptextwithpunct><w:useasianbreakrules></w:useasianbreakrules></w:compatibility><w:browserlevel></w:browserlevel></w:worddocument></xml><style type="text/css"> Style Definitions */table.MsoNormalTable{mso-style-name:"Table Normal";mso-tstyle-rowband-size:0;mso-tstyle-colband-size:0;mso-style-noshow:yes;mso-style-parent:"";mso-padding-alt:0in 5.4pt 0in 5.4pt;mso-para-margin:0in;mso-para-margin-bottom:.0001pt;mso-pagination:widow-orphan;font-size:10.0pt;font-family:"Times New Roman";}</style><xml><o:shapedefaults v:ext="edit" spidmax="1026"></o:shapedefaults></xml><xml><o:shapelayout v:ext="edit"><o:idmap v:ext="edit" data="1"></o:idmap></o:shapelayout></xml>Ounces? We must live in different worlds. In the DLG realm saving 10 grams can mean everything. A 40" long tooth pick fuse that weighs 50 grams goes for $120. If you can get the exact layup down to 42 grams they will pay $190. That's a $70 price difference on every part. After 50 parts you just banked an extra $3500. The extra 15 hours that you just put into the mold so you can use a 60 psi bladder just made $233 per hour for those 15 hours. If you do 100 fuse you make $466 per hour for those 15 hours.

I guess it depends on the realm in which you are working and the expectations of those who you are working for.

Wouldn't a guy be ecstatic if the fuses came out pre-primed. No sanding, pin hole filling, feathering or sanding down stepped seams, priming, sanding again, priming....just simply add the color. Put the base color on there and you could save half a pound or more on a 1/4 scale by having no primer or filler in the fuse. That's a serious labor savings (that a builder can provide for a fee). You put some investment into the mold and every part that comes out of it can bring you savings (or make you money). Even if you're not in business the amount of labor that is spent prepping a composite part for paint is huge and surprisingly heavy. In fact the prepping of the part for paint can be the most drawn out process in a composites build. If the part pops out painted or primed you just saved yourself some serious time, money, and weight. If the seam is wide or stepped then it has to fixed.....more sanding, filling, priming on EVERY part.

How many parts does it take to save you extra time and money on the back end when it comes to finishing? In my experience it doesn’t take very many. If you put the time and money in early you get to save time and money on every part that comes out of the mold. The more parts you make the more time and money that you save. The cheapest mold is not the one that cost the least (labor + materials). Its the one that produces the best quality part with the least amount of labor including finishing (unless you are only making a few parts...in that case you didn't need a mold).

Does a standard glass backed mold work? Yes! Is it viable? Yes! Do I respect those who invest the time to make a standard glassed back mold? You bet!!! Is it the most economical for creating 25 seamed and painted parts? In my experience it's not.

I'm really not trying to be critical. I'm just living on the other side with parts that pop out painted or primed and ready for paint and seams that are only .003" wide with no steps. I think everybody should join me!

Anyway....you can see that I have opinions on the subject.

bambam102

@wyowindworks.. Those all sound like excellent options, but I think they apply to parts pulled from the mold, and not the mold itself. you mention the weights of the layup and primed parts, correct me if I am misunderstanding you. you also mention the use of sand... can you expound on that? it is another method that Im unaware of.

@STEVE Im in total agreement. lightening methods is something I have never botherd to apply to molds, only the parts that come from those molds.

The balsa block isnt my technique, it was just one that made me consider alternate methods of shoring up very large molds. actually my thinking is that almost any material(within reason) that can provide strong mechanical adhesion in the matrix, may be useful if one wants to build up a certain thickness without it being pure glass.

now when it comes to building the parts, which is another topic altogether, Im much more conventional. I just wanted to get some thoughts, even if only speculation on using this foam material in a large mold, as Ive already tried it in a small mold.

Bamford

wyowindworks

The options that I listed in post #13 are for molds not for parts.

Many cores used when making molds are really just some type of filler. Poraver, bean bag filler, sand, even rice can be used when mixed with resin. You use just enough resin to get the filler to stick together. I use sand in areas that will be undergoing large compression forces like the clamping flange for example. A practice that will significantly improve a mold is do a reiforced flange mold with a full length indexing key. Essentially half the glass to be used is put down, then a sand/epoxy slurry is pour about about 3/4" thick around the flange (dam required). The sand sinks and the excess resin floats to the surface. You then use this excess resin to wet out the second halve of the layup that goes on top of it. This sandwiched flange prevents the mold from developing deformation near the bolting/clamping locations. The fulll length key prevents the mold from ever stepping. If you use a full length key but don't reinforce the flange the mold can still step because the flanges will deform from the constant localized clamping presure. The reinforced flange mold is my absolute bare minimun when making a mold that includes a seam. These can also handle moderate inflation bladder pressures (30 psi) depending on the internal surface area.

You can mix epoxy and sand together in a really dry packable consistency as well. Lay down some glass, then pack on the epoxy/sand, then more glass. These molds are very hardy, hold heat, are super stable but very heavy.

The lightest filler that can still handle high clamping forces is tooling compounds or tooling dough.

The bean bag filler works great but cannot handle higher clamping forces around the flange.

The downside to steel frames is the expansion differential if you are heat curing in the mold (yeilds the best surface finish on painted parts). You typically need some kind of interface between the mold and the steel. If the mold is large then you need a flexible interface between the steel and the mold. We did a canoe mold that utilized a steel frame. We put the steel frame on with silicone before the mold had fully cured. The epoxy mold shrunk just a little bit and caused the whole thing to bow. We had to cut the steel frame in multiple locations and reweld to remove the stress. Lesson learned.

Another option is the egg crate backing (see photo). The downside to these is handling a clamping technique on molds that get clamped together. Holes in the side rails can work well. I still use 2mm coremat on molds like this. The coremat allows me to use less resin and still get adequate thickness with less labor. 20 ounces of glass, 2 mm Coremat, 20 ounes of glass makes a pretty rigid structure that if faster to construct than laminating just glass to 1/4" thick. On large surfaces the the thickness is really important as it can bow between the ribs on the egg crate.

Since you are building large molds you may find "Advanced Composite Mold Making" by John J. Morena to be helpful. You will pay over $80 for it new. I found a copy used (google) for around $20.

The weights that I quoted where based on techniques that a robust mold will allow you to perform. A simple glass backed mold has it's limitations when it comes to debulking techniques (bladders, presses, vacuum bag) especially if the parts are to be seamed in the mold. The techniques used to process the parts often dictates the requirements of the mold.

I remeber seeing that SR-71 project eons ago. I was and still am quite impressed!

SCALECRAFT

My Feedback: (13)

wyo

Your opinion is welcome on an open public form. I'm always willing to learn.

With all due respect, I however, in this specific application, do not agree on one of your points. The primed or painted in the mold. Why??

First, for me personally, I can created a perfect resin finish on a fuse/wing with no paint in mold. My seams are perfectly aligned and very fine. Not much work on finishing, plus its fun for me. Its the best part to pick a scheme and finish your model. Its sets you apart. Plus, not much weight gain when I do it. Really.

Also I notice people always ask for where they can get a color match to the pre-painted fuse/wing. If you paint it, you have the paint.

On an SR71, the one on display at the Smithsonian near Dulles airport, Virginia, is flat black. If your mold is a high gloss finish (should be for ease of part release) all the colors one puts into the mold will have a sheen to them. Flat clear could work after you pull it out, maybe. But seams will still need to be cleaned up and feathered in in this case

Also, in this application, a fuse/wing that is not primed/painted can make it way easier to put in formers, servo trays, bulkheads, wing ribs, spars, ect...

Also, it shows the quality of the layup since you can see through it.

I don't calculate the cost of this as a business. We do it for ourselves only for fun.

I gather that bambam102 is also doing it for his own use. If not, he will soon find with China in the picture,he may sell a few but, it will not make much money for him.

I figure he doing it for the love of the model and composite configuration. ??

Steve

SCALECRAFT

My Feedback: (13)

By the way what is DLG?

Steve

wyowindworks

Discus launch glider. The competition class is F3K. They get chucked by hand to over 200 ft, have a 60" span and typically weigh around 10 ounces. Once chucked the pilot uses up drafts to complete a group of tasks to get points. In good conditions you can fly for hours with one throw.

Here is a fun DLG video

wyowindworks

After you demold the part don't you have to sand it to 400 to prep for paint? Do you really never open up micro pin holes when you sand it? Do you really only shot it once with primer and go right to paint?

With a primed part out of the the mold you wet sand the whole thing with 400 grit and paint. Thats it.

A part that comes painted from the mold IS LIGHTER because no primer is used or neccessary. The added weight of a paint job done in the mold is .015 to .02 grams per square inch. Most out of the mold paint jobs that I've done weigh .04 to .045 grams per square inch and is higher than .06 if a clear coat is used. From a production point of view the whole process is faster. If the mold is pristine the paint comes out pristine. No need to worry about dust, runs, orange peel, and other paint blem issues. Perfect mold equals perfect paint everytime.

If you like to paint post molding more power to you!!! I dislike it and avoid it at all costs.....although I don't build scalies.

I'll agree that a scale ship won't want a glossy paint. For you this wouldn't work. F3F, F3K, F3J, F5D, Quickie 400, Q40, pattern, dynamic soaring gliders, and almost all sport flyers like them smooth and shiny.

You speak from your realm and I speak from mine.....I have no idea what realm the currect project from bambam102 is?

SCALECRAFT

My Feedback: (13)

wyo. No pin holes and wet sand with 600, etching primer. One shot, yes.

In our warbirds we don't measure in grams. More in ounces.

bambam your model is a large Sr 71, correct?

Steve

SCALECRAFT

My Feedback: (13)

This model of ours painted. does not even warrant a weight check.

Steve

wyowindworks

Looks really nice!

saramos

Hi Steve,

I have a couple of questions, if you don't mind. What's the specs on the carbon tape? Is it layed between glass layers? Also, how many layers and what weight glass is used for the fuse and wing?

Thanks!

Scott