Someone has asked (see “world’s smallest blimp” thread) me to share my envelope fabrication methods. So I have started this thread. I hope others will feel free to chip in as well with their own techniques.

The subject is covers much ground, so it will take some time and many postings to cover. I shall start with some clarification on materials.

“Mylar” is a DuPont’s trade name for its polyester film. There are other trade names, such as Melinex, for the same stuff. It is a NON heat sealable plastic with good gas retention, strength and stiffness, so makes good envelopes. It is available from many model suppliers in silver and clear, and in thicknesses down to 2 microns.

The term “mylar” is also often used for the stuff foil balloons are made from. This is actually polyamide film (“nylon” to use the usual trade name), laminated with a heat sealable layer (polyethene, I think). It is available in silver or white, but as far as I know only in 25 micron thickness. This makes it of limited value if for really small blimps. I actually use the stuff only for outdoor models. However polyamide has better gas retention and is tougher than polyester, although more flexible, making it a better envelope material.

Straight polyamide film does exist. I have used some 15 micron for a few envelopes; it has proved very durable and better than polyester. However, it is very hard to get; I have had to go begging to suppliers.

There is another complication with polyester. Some films have their surface treated, either chemically or electrically, to improve adhesion of inks. This also improves glue adhesion. As polyester can’t be welded, it must be glued, this is important. 12 micron and thicker, as it is used by the packaging industry, often is treated. Thinner stuff usually isn’t. All the thinner polyester I have tried from various suppliers isn’t. 12 micron polyester supplied by Tony Avak almost certainly is, it has very good adhesion which makes it good envelope material. I’m surprised Tony hasn’t shown up in this forum, but anyway he sells the stuff in 10 foot lengths. Send him an e-mail at avakar@erols.com.

To avoid confusion, I shall refer to genuine Mylar as “polyester”, and the foil balloon stuff as “laminated nylon”. I will concentrate on polyester envelopes, as that is what I use most for indoor blimps, and is the most difficult to join.

Thanks Alan,
I really appreciate your willingness to share your techniques and building experience.
I am particularly happy that your building in Australia, as you would know it is often difficult to get materials etc hear that are more easily available in North America or Europe.

Nicholas.

*looks thoughtful* are there heat activated adhesives that work with common emergency mylar blanket material? *I think only one side of this material is *coated* so, if I were to apply the heat activated adhesive that side, would it work? and what irons would work for making seams? o.O.. *has to build a custom bag for the microblimp*

Gluing Polyester Films

The only reliable way to join polyester film is gluing. I have always used contact adhesives, although silicone sealant and even cyano can work, but silicone is heavy and takes ages to cure, while cyano is not flexible, so the join cracks easily.

The best contact adhesive I have found is UHU Power Stick (NOT UHU office glue). This comes in a solid form, you merely wipe it across the 2 surfaces, let it dry (a minute or less), then press them together. A little heat and pressure from a heat iron at a medium setting afterward improves the joint (not hot enough to shrink the plastic).

Before applying glue make sure both surfaces are clean, particularly of any oily stuff. The best cleaning agent is MEK (methyl ethyl ketone). But make sure you do it in a well ventilated area and wear rubber gloves. It is a known carcinogen (like nearly everything else these days!). Alternatively, use methylated spirits. You’ll still get quite a reasonable join with normally handled surfaces, provided you haven’t put sweaty hands or oily substances on them.

Another method, courtesy of Tony Avak, is using fusible web. In Australia we call this hemming tape, available from dressmaking stores, according to Tony it is Therm O Web or Heatbond Ultrahold in USA. This works very well on 12 micron polyester, but it seems to be useless on any thinner grades I’ve tried. I’m pretty sure this has to do with surface treatment (see post #1).

If you want the envelope to stand up against any internal pressure, all seams must be lap joints, as shown below, not cusped joins, like on foil balloons. This will work with laminated nylon (foil balloons) because the heat sealed join is much stronger than glue, but glued joins have much lower peel strength. Also, the cusped joint leaves an ugly protruding seem, to my mind.

Another point: with metalised film, glue on the non-metalised side. This is the less shiny side. If still not sure, scrape or corner on each side with a razor blade; the metal with scrape off the metalised side leaving it clear.

If using laminated nylon, making seams is easy. Just follow instructions supplied (if you get it from West Coast Blimps).

Spinbotz: I am not familiar with the emergency blanket material, I don't know.

well, emergency blanket is mylar of some kind.. but I am trying to find out whether there's a glue that will work with it... o.o..

Here's a link to a double sided tape which MIGHT work for making a balloon with, it's doublesided, and you'd have to cut it so it's thinner and can follow the edge of a gore to create a seam. http://www.wingsandwheels.com/page28.htm btw, I don't know if this tape (double sided, so you can do lap seams) will work for emergency blankets, but from what I do know of emergency blankets is, that they are a form of metallized mylar, *much thicker width wise than the mylar Alan used for his airship bag and suitable only for far larger blimps prolly well over twice the size of Alan's micro blimp.

Using Sticky Tape

I have long since given up using sticky tape to make envelopes because:
1. It is heavy. The backing is usually 12-25 micron. If making a bag from 4 micron film, the tape will weight more than the gores.
2. It doesn’t last. Commercial sticky tape falls apart after about a year. Aviation/military tapes are better of course, if you want to pay the extra.
3. All plastic shrinks and expands with changing humidity. If the tape is different material or thickness to the envelope, it will shrink differently, causing wrinkles, which become leak passages. Taped envelopes progressively start to leak.

Envelope Design
Now I’ll tackle the design of the envelope. I have always built traditional “ellipsoidal” airship envelopes, ie with a circular cross-section and elliptical or streamlined profile. There are many other possibilities, such as “saucer” shaped, but I won’t be covering those.

The traditional method for airship envelopes is to make them from identical longitudinal panels, called “gores”, stretching from nose to tail. The seams are then like lines of longitude, with nose and tail being the poles. The number of gores can be anything from 2 up, I usually use 8. The more gores, the smoother the envelope will look when inflated and pressurised. With few gores, the seams will “wrinkle” as the flat gores try to accommodate to the double curvature of an ellipsoidal form. The number of gores required to prevent this depends on the size and shape of the envelope, film thickness, and pressure you inflate it to; there is no simple formula. Of course it’s no problem, it just doesn’t look good. Also it won’t look good if you are not very accurate in measuring and cutting your gores, any inaccuracy is multiplied by the number of gores around the circumference. The photos below shows the result when I didn’t take enough care in gore cutting, you get “puckering” along the seams.

To design the gores, draw a half profile of what you want the final envelope to look like (full size, if you can). Then at intervals along this curve, measure the arc length from the nose (L) and the radius to the centreline (R). Then transfer these to the gore pattern as shown in the diagram below, making the gore width 2πR/N, where N is the number of gores. Join up these lines with a smooth curve, and that is your gore pattern. Of course, you can do this computationally (as I do now), by setting up an equation or lookup table for the profile, computing gore width and length, and printing off the pattern (if your printer can handle the size).

The pattern should be drawn up on cardboard or plastic sheet (eg acetate or ABS) and cut out. This is the critical step that must be done accurately. It is best done with a modelling knife, cutting along a spline of metal strip or similar bent to the curve and pinned down at intervals, close enough so that it does not shift from the pressure of the knife blade pressed against it. In my latest model, I used two 3mm square Perspex strips, bent around pins at the measured points and then glued down, so that the Perspex strips themselves became the pattern. This was quite successful. Then use this pattern to cut out the gores from the envelope material. Use a sharp blade, preferably a new one.

Joining the Gores

As I said before, I always use lap seams on polyester film envelopes. If you put two gores side by side, you can see the edges curve away from each other. So how do you make a lap seam?

You need to make a building tool, as shown in the pic below. The profile of the tool is the same as the profile of your envelope, although it need not be accurate; merely the radius of curvature has to be at least as small as your actual envelope profile at each point. Thus one tool can be used for a number of similar envelopes. Cover the top with a strip of 1 mm ply or veneer to give a smooth surface. Then put on double sided sticky tape, and cover this with “post-it” tape, paper tape with the same adhesive as post-it notes. I use because the adhesive is weak, and you can peel the envelope off the tool without damaging it. This works fine down to 4 micron film. I tried with 2 micron, but even post-it is too strong for that, the film stretches and tears as you try to pull it off.

To use, as shown in the third diagram, stick two gores side-by-side on the post-it tape. Start from the nose or tail, and stretch the gore as you stick it on, so it goes on without wrinkles. Tightly curved areas near the nose or tail will need more stretch. You must pull it longitudinally, and at the same time pull it sideways to prevent longitudinal wrinkles. It takes practice.

As with gore cutting, the accuracy here is the key to a nice smooth envelope. Rule a line down the middle of the post-it tape (using a flexible straight-edge) as a guide to pulling on the first gore. When pulling on the second gore, the edges of the two gores should line up with no gap or overlap. They should also line up length-wise. Put marks on the edges of the gores at intervals, to line them up when pulling onto the tool (you can do this by putting holes along the edges of the gore pattern, and marking through these as you cut the gore).

When you are satisfied the gores are lined up, clean the surface and wipe on a thin layer of UHU glue. Cut a seam tape from the same envelope material, and clean this and wipe on glue. When the glue is dry (not tacky), complete the join by laying this seam tape over the gores. Start from one end, working it down while keeping it stretched by pulling tangentially on the free end. Remember once it touches the gores you can’t pull it off and try again, the only way to redo it is to remove the entire tape seam using MEK or similar solvent, working it under the seam tape with a brush, and then cleaning up and starting again. After the seam tape is down, go over it with heat iron (with sock) at medium heat. Leave it a few minutes to cool down, then peel the entire assembly off the tool, leaving the post-it tape behind. You have finished one seam. The same post-it tape should last for quite a few seams before it loses its stickiness and needs to be replaced.

(edit april 2013) One thing I should add, the thickness of the tool, and thus the width of the seam, need only be up to 25 mm, even for quite large envelopes (1 meter diameter). For small envelopes, it can be less. For my smallest blimps (150 mm diameter), a 10 mm seam is sufficient.

The Final Seam

The previous post described how to make the seams, all except the last one. You now have an envelope with a gaping split from nose to tail. I have used 3 methods for the final seam.

1. Place the envelope on the tool, but with the centreline of a gore, rather than a seam, over the post-it tape. Now lay another post-it tape over that, sticky side down. Lay double-sided tape over that, stuck down to the tool beyond the ends of the envelope. Make sure it is pulled tight, so that it will not lift when you are pulling on the gores for the final seam. Then lay a post-it tape over that, sticky up. This then forms a surface just like the tool, on which you can make the final seam. Then peel the finished envelope off. It will have a 3-layer tape strip inside it, which you extract from the nose or tail. This is a fiddly job. A useful aid is a long strip of wood with a wedged end that you can use to prise the envelope off the tape all the way along.
2. Wrap the envelope around the tool, with suitable cloth padding to protect the envelope material, and make the final seam, leaving an unfinished gap long enough to extract the tool. Then finish the seam using a smaller tool, which can be extracted out the nose or tail hole. With short fat envelopes you may need 2 extra tools, doing it in 2 stages.
3. This method will put all the seams on the inside of the envelope. Start at the nose (or tail) and pull one end gore onto the tool to about mid-length. Then bring the envelope over the top and pull on the other end gore, just like making a normal seam. This will leave room to work inside the envelope to make the seam. Then pull on more of the 2 end gores, making the seam in stages. You will have to start peeling the envelope off at the nose to make “slack” to enable you to pull the gores on as you get near the tail. At the last stage you will be working through the tail hole. When complete, peel off the finished envelope, which will have nothing inside.

I haven’t explicitly said it before, but of course the gores would not go all the way to the nose and tail. Cut the sharp ends off so that when assembled there is a small hole at each end. Glue discs of envelope material on each end to complete the envelope.

Also consider where to put the inflation tube. I usually put it at the tail, so the tail is sealed with a disc of heavier material (about 50 micron) with a tube secured in it. For the smallest blimps, I use a plastic drinking straw, but on anything bigger than 60 litres a bigger tube is required, or it will take ages to inflate or deflate. A tapered plug seals the tube. Also consider using a non-return valve, so the envelope will hold pressure after inflating while you fumble the plug into the hole. Most foil balloons come with a non-return valve; cut it out and fit it on the inside end of the inflation tube. Then to deflate the envelope you will have to insert a smaller tube up the inflation tube to keep the valve open.

That completes my envelope making process. I had no more in mind to say, so somebody else will have to make some replies to take this thread any further.

Wow, this whole thread is amazing. Huge amounts of useful information, thankyou for sharing it all!

That's the first time I've had the whole mylar/metallised mylar/laminated nylon thing sorted out. I've got a large roll of 23 micron melinex here that I was assured would be heat-sealable, only of course it isn't, and I've also got some 'mylar' which heat seals very easily which must of course be the laminated nylon. I've terribly confused some poor sales bloke at a local company, he was sending me samples of actual mylar, which explains why I couldn't get that to heat seal when other 'mylar' I could get from a different company did.

Right, now to work out where I can get UHU Power Stick in the UK at short notice.

Alan, scince I am horrible at math I have a couple of questions. If L is 20" and R is 10 " and I want to use 8 panels what would 2TT R/N BE? Of course these numbers are ficticious and onlyt for example. I am looking to make a Hindinburg of about 16 feet long.

sherm, I saw your first envelope thread. I notice you have used lap seams. Did you use a tool similar to the one I have described? I'd be surprised if UHU power stick was not easily available in UK, in hardware or even newsagents. Your leaks problem makes me think I should say something about leaks, they are such a pain. I'll post something soon.

papajohn, that symbol in the formula is pi (3.14159...). So in your example:

2 pi R/N = 2*3.14159*10/8 = 7.854"

I hope that helps

Originally Posted by Alan Sherwood

sherm, I saw your first envelope thread. I notice you have used lap seams. Did you use a tool similar to the one I have described? I'd be surprised if UHU power stick was not easily available in UK, in hardware or even newsagents. Your leaks problem makes me think I should say something about leaks, they are such a pain. I'll post something soon.

UHU power stick is not easily available, it seems, I've been doing some research today. Nowhere on the high street has it, and I've found a few odd places online but they all want several days for delivery and I'm now very very short on time. (I'm making these blimps for a university project and there's only three weeks of it left; I started trying to make them about two months ago, but there's been so many problems with materials and determining sizes and all the the rest of it that I still haven't got anything operational!) Post-it tape also doesn't seem to be available anywhere I can find, online or in shops.

This morning I've tried using hemming tape to join pieces of the 23 micron clear melinex I've got. Unfortunately it did the same as all other methods of joining that stuff I've tried, it seemed to give a lovely strong joint straight away but then once it's been left for 20 minutes, the joint just falls apart. I wonder if this is the result of some sort of coating but the company that I got it from (a plastics specialist) claimed it was uncoated. I used a wok (handy curved thing...) and some double-sided sticky tape and strips cut from post-it notes - I can see how that works well to hold the seam nice and straight, and it seems odd that the post-it glue is still there after applying an iron only a couple of layers above but that seems to be the case, so I'm not complaining.

I hadn't read this thread when I was making that envelope I've just done, but I like the shaped jig idea. I was using taped seams for strength - this thing is for indoor testing use but it's going to have to get moved around from place to place and handled a fair amount so I figured it'd be better stronger, plus who wants bits sticking out? I was using a leather-working block I've got, a steel block with a gently curved top, covered with a couple of thicknesses of linen to take the heat, and just pulling the seam into shape over it one length at a time. I think this has resulted in too many creases hence the leaks. I've been going back over the seams with an iron as best I can but I think there might just be too many small holes and I might have to abandon this envelope. I've already got pieces cut for another and I don't want to have to abandon those - I'm thinking I might try using the jig idea and see if I can make one crease-free enough to hold.

I'm just working out what else to even try. The alternative is to go back to cusped joints and have them sticking out but I think they'll be vulnerable to creasing too, and it means starting all over again with patterns and buying more mylar and all the rest. I wish I hadn't done 12 gores now too but it seemed a sensible number at the time!

Alan,

I hope you realise that your effort to share with others is deeply appreciated.

You may event think about organising seminars on building a blimp, and sell/export by Fedex/UPS the necessary hardware like polyester, glue and tools... This is what KAP (kite aerial pics) guys are doing very succesfully... I live in Mexico and specialised hardware is very difficult to find, so I like purchasing all necessary - and proven - things from the same seller...

I love this thread, the hand made drawings and I am improving my English with lots of new technical words...

Thanks again Alan !