Flock Spraying
Introduction.
No, it has nothing to do with sheep or flocks of birds !. The flock we are talking about here is made up of short textile fibres, charged up to a hundred thousand volts and then fired like tiny arrows, into a surface covered with an adhesive.
The Flock.
Typically nylon or rayon fibres cut to specific lengths. Standard lengths are 0.5mm (for imitation velvet), 1, 2 and 3 mm for a short pile like finish. In more recent years fibres up to 5 mm long have been used to produce a carpet 'pile'. For the home user, with simple application equipment, the maximum useable fibre length is about 3mm long.
Fibres also come in different thickness measured in 'deniers'. They also available dyed in a wide range of colours, and you can get a sample / shade card from the manufacturer.
Electrostatics.
What makes flock spraying possible is the laws of electrostatics. One law says that we can give objects, such as fibres an electrical charge. The charge can either be positive or negative. Another law says that objects with like charges repel each other .... and objects with unlike charges attract each other. So a positively charged fibre would be repelled by a positively charged object. Also ..... a positively charged fibre would be madly attracted to a negatively charged object etc. So basically to make a fibre do what we want it to do we have to give it an electrical charge !. It can then fly over a distance of several inches ... or feet to an oppositely charged object.
Another interesting thing is that electrostatic attraction causes the fibre to fly like an arrow .... following the electrostatic rules of force. So when it hits the other oppositely charged body it is going to try and stick into it like a tiny arrow. Now if we 'fired' a fibre at a tin can, there is no way it could penetrate it, but as soon as it touches it .... it attains the same electrical charge as the tin can. Whoops ... remember 'like charges repel', so as soon as the fibre arrow touches the tin can, it attains the same charge ... is repelled and now wants to be attracted to an object with the opposite polarity. If you think about it the most probable place it gets attracted to ... is where it came from in the first place !. But .... as soon as it gets there and strikes that object ... it attains the same charge and is again repelled .... probably from where it last came from. So in practice our little fibre arrow fly's backwards and forwards between the two oppositely charged objects. Of course in practice there is not one tiny fibre arrow, but thousands .... or millions of them ..... shooting backwards and forwards between the two oppositely charged objects.
Electrostatic Flock Spraying guns.
An electrostatic flock spraying gun is very simple. Basically it is a small metal tray or container to hold the flock. All we need to do is apply a very high electrostatic voltage to the metal tray and instantly every fibre in the tray becomes charged ... and off they go !. Before we go any further I think we need to talk about electrostatic generators.
Electrostatic voltage generators.
To make the fibres fly well, we need to charge them up with a voltage of a hundred thousand volts !. Sounds dangerous doesn't it ?. However you generate voltages like this every day of the week. For example, when you comb your hair with a plastic comb, you generate voltages of this kind. If you had an ebonite comb and you combed fur with it, you would generate even higher voltages with it. So the question is why don't these voltages kill you ?. The answer is that they are electrostatic voltages that cannot produce sufficient current to harm you. Mains voltages of only 240 volts can kill you so stay away from them. Here we are only talking about electrostatic voltages ... which can cause sparks to jump across to your fingers, but they will not harm you. So how do we make an electrostatic generator ?.
There are many different types of electrostatic generator, but many work on the same principle as the combing of your hair, ie we rub two dissimilar material together ... and zap ... we get a spark !. If we have a spark ... then we have generated a voltage of several thousand volts !. OK lets build an electro-static flock spraying gun !. The biggest problem you are going to have, is to find the rubber band !.
A Practical DIY Electrostatic Flock spraying Gun.
Rubber bands are made from natural latex rubber. We need one about two inches wide and and about six to eight inches long ... that is the bad news. The good news is .... since they are made of natural latex rubber we can make our own, if we have to !. This rubber band is going to run between two plastic rollers, one of which is going to be rotated by a small electric motor. It could be a battery powered motor.
Rollers are interesting ... especially since rubber belts want to ride off them ... and stop everything working !. However during the industrial revolution ... it was noticed that rubber belts always ran on the highest point of the roller !. So to stop our belt trying to run off the rollers, all we need to do is make the rollers slightly barrel shaped !. The belt will then always self center itself on the roller. Having solved that problem we have to remember that between the two rollers we are going to have several hundreds of thousands of volts, so what ever the rollers are mounted on has to be a good insulator. A good choice is Perspex.
OK lets stop and think about this for a moment. So far we have a bottom roller, driven by an electric motor .... which via the rubber belt drives the top roller .... from which we hope to get up to one hundred thousand volts to charge the fibres.
At the moment there is one element missing and that is to generate very high voltages we have to rub together two dissimilar materials. OK we need a bit of aluminum kitchen foil and some glue. What we do is to coat the BOTTOM roller surface with glue and then smooth on the aluminum kitchen foil. We only want it on the surface in contact with the rubber belt, not the sides of the roller. Now when the motor drives the bottom roller around, static electricity is generated between the two dissimilar materials ... ie rubber and metal foil. At the bottom roller the static voltage generated will only be a few hundred or thousand volts ..... But for every revolution of the bottom roller ... these voltages are 'stacked up' on the rising belt and the total voltage from the bottom of the belt to the top of the belt is the sum of the voltages generated .... and they add up to the required voltage.
There are only two elements missing now. One is that the bottom of the rubber belt must have a reference to ground. The other is that there must be some way of tapping off the very high voltage from the top of the belt. Since very high electrostatic voltages like sharp edges, we do this with a metal 'comb' on both the upper and lower roller. The lower comb is connect to ground and the upper comb connected to the tray that holds the fibres. The combs do not actually touch the rotating rubber belt, but should be as close as possible. Of course, all these components cannot float in mid air so we put them into a plastic case ... with a few other refinements. Nothing hidden, nothing secret ... what you see ... will work. If you would like more details just send me an email.
The target surface.
The surface to be covered with flock, could be anything and made of any material. Once hit by the fibre 'arrows' we have to make sure they stay where they hit. We do this by simply coating that surface with some sort of adhesive. This could vary between the extremes ..... wallpaper paste, paint ..... up to polyurethane and epoxy resin. How durable the finish is depends upon the durability of both the fibre and the adhesive keying that locks it onto the substrata. Any fibres that do not stick in the glue attain the same state of charge ... and are instantly repelled back to the highest opposite charged object ... ie the flock gun tray... where they attain the same charge as the tray and are immediately repelled back to the target etc., etc..
Generally the aim is for 10% of the length of the fibre to penetrate the adhesive. In practice, most methods work since they are never tested to the extreme. For example seasonal cards made in school by children are not subjected to the extreme wear conditions that are applied to carpets.
Over-spray.
In the electrostatic world anything that is not positively charged is the opposite. This means that if the charged cloud of charged fibres cannot find a definite target they will go where-ever they are welcome. This can include the walls, the ceiling and even you. So it is always a good idea to supply a default target, which will automatically collect over spray. A small earthed metal plate about 18" above and behind the flock tray will suffice. I think it advisable that any student using electrostatic flock spraying equipment should wear a mask and protective goggles.
I also advise that all over spray fibres be collected for possible resale to scenic modelers.
Preparing the gun before use.
1. Wear latex gloves when cleaning the inside of the machine. This is to avoid leaving acid fingerprints on the surface of the insulators .... which would prevent the build up of static electricity on the belt. If you have accidentally handled the belt without the gloves, wash the belt and then dry it thoroughly, before refitting it. Remove stray flock from surfaces with a soft brush.
2. Damp can also prevent the build up of static electricity. One solution is to fit a 20 watt bulb inside the gun case to dry out the air inside. You can switch it off when the gun is running properly.
3. To test whether the gun is ready, sprinkle a pinch of flock onto the flock tray and it should immediately jump off the tray and stick to the sweat on your fingers. Another way is to put a finger near the flock tray and a spark will jump towards the finger. The spark is completely harmless, but you may feel a tiny pinprick sensation. Also if you stand on a rubber mat and touch the flock tray .... your hair will stand on end and you will end up looking like Einstein !
4. Coat the surface to be flock sprayed with the adhesive of your choice. Hold it about six inches above the flock tray and switch on. You will see a cloud of flock fibres flying towards the target. Where-ever there is adhesive the fibres will penetrate it and stick in it. Over a period of a few seconds all of the target surface will be covered with a flock pile and there will not be any exposed space for new fibres to stick to. At this stage, switch off the machine and careful place the object somewhere safe for the adhesive to set. If you touch the flock before the glue has set, you will end up pushing the fibres flat.
5. When the adhesive has set, brush off any loose flock and admire your handy work !
Changing Flock during work.
It is possible to speed up the change of flock types as one works, by putting the flock into aluminum foil kitchen dishes. Just drop the foil container onto the flock tray and you will find that they work perfectly.
Remote flock applicators.
In some applications such as model making and art, we only want to apply flock to very small area's such as the bearskin helmet, on a toy soldiers head. It is possible to make up an adaptor using EHT cable to a small hand piece that can clip onto tiny flock containers or much larger Tupperware style boxes. It allows a much more natural style of application.
Selective masking
To get special effects we can use all of the selective masking tricks that are used in air brush painting.
John Kent June 2007
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5th March 2011
I received an Email from someone enquiring about flock guns, so I decided to post the reply here just incase it helps anyone else.
One question was where can one buy flock ?.
First flock is graded, by material, the common types being Rayon or nylon.
Flock is graded by the length of the fibre, typically 1mm, 2mm or 3mm. Fibres as long as 5mm have been used to make carpets.
Flock thickness is graded in Deniers. The denier is based on a natural standard: a single strand of silk is one denier. A 9,000 meter strand of silk weighs one gram.
Flock is available in a wide range of colours ....
I have not looked at this subject since 2007, so my information may be sadly out of date, but I seem to remember buying my flock from a company called Peels in UK although the two colour charts are marked ....
"CP. Cellusuede Products Inc.,
500 North Madison Street
Rockford
Illinois 61105"
Naturally no telephone number or email address. I think what happens at the moment is flock may be manufactured in China, bought in bulk and then broken down into kilo bags at wholesaler level in the west. Retailers then break down the bags into smaller amounts for the hobby market. Several retailers in UK sell flock, but not the full range
Flock guns.
The flock guns we are interested in are either of the electrostatic type or the type that uses an electronic circuit to generate the very high voltages required to charge up the flock fibres. I used to make electro-static flock guns and give them away to schools and all of them were based on the Van de Graff electrostatic generator, described above. The person who enquired about the gun's said that they had seen a DIY article on how to make a flock gun from an electronic fly swatter, so I thought that it would be interesting to have a look at this approach, so today I bought a new electronic fly swatter for £5 and it is now in pieces, waiting for divine inspiration !. The one I bought has the 'Dinomark' registered trade mark on it with a small picture of a friendly looking dinosaur.
The two halves of the handle are joined together by three screws and nuts. Unscrew these and you can remove the top half of the handle. Inside you will see a small electronic module and a battery holder for two AA cells.
There are two wires from the electronic PCB leading to the yellow 'bat', that contains the three electrode screens. The two outside screens have a mesh of about 10mm which allow the insect to pass through it, where it gets electrocuted by a spark from the central fine mesh electrode. Electrically the two outer screens are connected together, for one connection. The other connection is to the central screen. As I did not want to use the bat as it was, I cut the connections from the board to the bat in order to completely remove the bat. I then split the yellow plastic frame of the bat to extract the metal screens, as I will probably be using them as raw material later. Very important !. I have deliberately avoided touching the PCB or the inside of the case with my bare fingers. If I did that then the acids from my fingers would tend to stop the charge on the 'capacitor' building up in the first place
The three metal screens that were in the bat actually formed a capacitor, which was charged up to a high voltage by the electronic module in the handle, so somewhere in our new design, we are going to have to introduce a replacement capacitor. I also have a gut feeling that the output voltage of the electronic module is going to be extremely marginal, so I will have to take steps to get the best out of it.
The destruction so far !. I have removed the yellow 'bat' from the handle, after cutting the electrical connections to the bat. The two fine wires at top right of the PCB went to the two outside screens. The insulated brown wire went to the central screen.
Now I have a problem !. I don't actually need a flock gun as I have no use for it at the moment ... and I really do need to have an application in mind in order to proceed. For example does one need large applicators in order to cover large area's or are you simply going to use it to flock black busbies onto soldiers heads ?. Or maybe we should aim at something that will do both ?. It may well be that the low out put voltage of the electronic module may limit the distance we can shoot the flock, but there is only one way to find out !.
Flock dispensers.
In some applications it may be required to change flock colour several times during an operation. Rather than have to clean the gun out on every colour change a simpler method is to store different colours in separate dispensing cassettes. To change colours you simply remove one colour flock dispensing cassette and replace it on the handle with a cassette containing flock of the new colour. One method of doing this is to use small' 'Tupperware' boxes as cassettes. The inside of the box has to be conductive to allow the flock to be charged up. one method of doing this it to line the bottom of the box with self adhesive aluminum tape, which you can purchase from Poundland. Obviously the method you use to click the cassettes on and off the handle must also provide and electrical connection from the voltage generator to the foil lined flock cassette. As to the actual size of the cassette, it really depends upon the application. For small jobs it could be the size of a soap box or even much smaller, for applying fine detail.
To allow the flock to escape we have to cut a square hole in the lid of the box and cover the hole with the fine mesh screen, we removed from the yellow bat. To fix it onto the plastic lid you can use rivets, glue, screws etc.. Of course when you turn the box on it's side all the flock will drop to the bottom of the box and be distributed from there. The solution if to make up some slotted strips of material, that are as deep as the box and when assembled turn the interior into a lot of smaller compartments, in which the flock will get trapped evenly ....
At the lower end of the scale, consider a small scoop dispenser about the size of a small tea spoon, that clips onto the handle for fine work ?. Different jobs require different solutions. The handle approach does have another advantage in that flock tends to 'clump' together, so it often helps to give the dispenser a sharp rap it loosen it up.
The electrical cable between the voltage generator and the handle has to be capable of handling the high voltages employed. One easy way of making such a cable is to use the flexible plastic pipe that home brewers use to siphon beer, wine etc. Simply run you electrical wire up to hole in the pipe and it will give adequate insulation.
Flock guns general.
My starting point was the Van de Graff electrostatic generator. So lets have a look at that first.
http://www.mos.org/sln/toe/VDG_works.mov
As you can see the classic Van de Graaff generator is extremely simple in construction, basically consisting of a rotating belt on which static electricity charges are stacked up. A practical generator can be even simpler. If you look at the animations you can see that the charging voltage is applied to the bottom metal comb, which means it requires an external energising voltage of around 500 volts. However by modifying the bottom roller we can make it generate it's own energising voltage, simply by covering the bottom roller with self adhesive aluminum foil tape. Now when the roller rotates it causes the metal tape to rub against the latex belt and a static electricity voltage is generated and placed onto the bottom of the belt. As the belt moves upwards, these charges stack up on each other producing a very high voltage at the top metal comb.
Another simplification is that we do not need to use a sphere at the top, it will also work with a flat plate ! . OK that is all we need to know to design a Van de Graaff generator for home use !.
The case.
We need to house our generator inside a case, to protect it from dust and flock. Since the generator is in a case, it makes it easier to dry out any residual humidity, which would stop the charge building up. It also keeps fingers off the working parts !. I used to use a plastic waste paper bin for my housings, which not only worked well but also looked good !. The first thing to do is cut the bottom off about 4.0 cm high.
Then flip it vertical.
The piece that you cut off will now be the lid for the finished machine. That leaves a hollow plastic cylinder ...
Next we need to cut out two identical aluminum discs that will just fit into the cylinder....
The top aluminum disc will be fixed into position with silicone filler (from the inside) about 4 cm down from the top of the cylinder. This will form the conductive tray that holds the flock. The bottom disk is to be made removable and will be the base on which we will build our voltage generator assembly.
The machine 'lid'.
A circular hole is cut in the op of the lid, leaving a rim of about 25mm. A circular disc of fine mesh covers this hole, through which the flock will fly. The lid is secured to the case body either with magnets or small toggle catches. The purpose of this lid is to stop the flock falling out of the gun when inverted. Again a honey comb separators in the flock dispenser chamber will help provide a uniform coverage and stop the flock falling to the bottom.
The Motor.
The motor only has to drive the latex belt on the pulleys, but a certain amount of tension has to be applied to the belt to stop it 'flapping' as it rotates. I used induction motors that were originally intended for mains fans and tape recorders, which had a speed of about 1400 RPM, but it is not critical. It does make life easier if the motor spindle is as long as the bottom roller. The motor is fixed to the base disc using 'L' brackets which your local hardware store will sell.
The Rollers.
The top one has to be free to rotate on it's 6.0mm diameter spindle. The bottom roller is fixed to the motor spindle. I used 1" Perspex rod to make both rollers, but other plastics would also be suitable. Flat belts tend to run off pulleys. The trick is to give the roller a small radius, to make it barrel shaped. Belts always ride to the highest point of the pulley and hence by using a barrel shaped roller, the belt becomes self centering on the pulley.
The bottom roller needs to be covered in aluminum foil, this can either be self adhesive tape, or foil glued onto the roller. Trim the foil at the ends of the roller so that it does not touch the spindle.
The combs.
High voltages like points and sharp edges, so the edge of the lower comb that ALMOST touches the belt, needs to either have a knife edge or like a convention comb, lots of sharp points. Now with the combs we have a compromise situation, as high voltages do not like air gaps, so we need to adjust the comb to be as close as possible to the rotating belt ... but if it is too close it will start chewing up the belt !. Since the belt will follow the curve of the roller, the comb should be shaped to follow that curve.
The bottom comb is electrically connected to the bottom aluminum base disc, so in profile it will look Z shaped and bolted to the base
Since we need to be able to adjust the tension of the latex belt by moving the upper spindle up or down, the upper comb must also be adjustable to set the comb to belt clearance. If you fit a light compression spring to the upper comb adjustment screw it will touch the upper aluminum disc and make good electrical contact with it, without having to worry about making or breaking connections when removing the case from the base.
The case heater.
One problem with any electrostatic generator is humidity. The solution is to install a small 20 watt light bulb inside the case to drive any humidity out, and then you can switch it off for normal use. How do you know when the heater has done it's job ?. Well usually it takes only a couple of minutes, but a quick check is to switch on the gun and drop a pinch of flock on the flock plate. If all is well, it will instantly disappear !. Which leads us onto targets.
Targets.
When the gun has warmed up it will shoot fibres at a very high rate, whether you meant it to or not !. With a flock dispenser plate charged up to 100.000 volts just about ANYTHING else in the universe is of the opposite polarity and off our little fibre arrows will fly !. Usually the closest object to the gun is your own body so it pays to think about what you are going to do in advance and if possible build in a few safe guards. One, is to always give the gun a target to shoot at, if you forget what you are doing. It could be something as simple as a small earthed metal plate hung on the wall with a container under it to collect any stray flock flying around.
High voltages like to take the short route and one way to make sure that the flock goes to your piece of work and not the ceiling is to make the work as 'earthy' as possible. For example if you are flock spraying a glass bottle, which is not a particularly attractive body, you can make it more so by dropping an earthed metal rod into it before you spray. For rubber hand puppets etc, stuff a bit of earthed aluminum foil into the target to be flock sprayed.
Safety.
Normally we expect a fibre to penetrate the glue for about ten percent of it's length. Accidents do happen, so it is wise to use goggles and a mask, particularly in darkened rooms where you cannot see the flock cloud.
Some adhesives are inflammable and it is possible for sparks from the gun to ignite them.
Assembly Sketches.
Cannot draw I'm afraid, but here are some crude sketches that show how I used to make Van de Graff generators .....
Notes.
The lower comb plate (purple), Electric motor (green) and Perspex support plate are mounted to the base using 'L' brackets (blue).
The upper and lower comb knife edges should be shaped to follow the curve of the belt on the barrel shaped rollers.
The bottom roller is covered with aluminum foil.
The spring on the top comb plate makes an electrical connection to the upper aluminum flock charging plate.
The lid with the screen top is fitted to the top of the case using toggle catches or magnets.
The top roller spindle hole in the Perspex support plate is slotted to allow belt tension to be adjusted.
The green screw on top of the top comb allows the clearance between the comb and the belt to be adjusted.
Fit the 20 watt heater bulb somewhere on the base plate away from the belt.
Electrical wiring is not shown.
Side view
.
Notes.
Edge of combs nearest belt, filed to a knife edge.
Bottom aluminum comb bolted direct to the base plate.
Support brackets not shown.
An actual Perspex support plate
Made about 1980 !. It is 200mm long x 50.0mm wide x 6.25 mm thick. The slotted hole is for the top roller spindle and can be adjusted up and down, to tension the belt. The two small holes above it are for the upper comb plate. This one still has to have a hole cut for the motor spindle and screw holes for the lower mounting brackets.
The belt.
I had my belts made by a UK company that specialised in latex sheeting products. They cut the belt material from a soft and flexible white latex rubber sheet and made a glued lap joint. This unfortunately stiffened the belt at this point as well as making it thicker, so the comb clearance had to be increased. Eventually I got them to make a diagonal lap joint that was less 'clunky' in operation.
When you have made or obtained your belt, wash it in detergent and water, rinse it and NEVER touch it with your bare fingers again !. Likewise when you have made you gun, disassemble it, wash each part with detergent, rinse and let dry and never touch the inside of the gun with bare fingers again !. If you obey this simple rule it will avoid a whole world of frustration and unhappiness !.
Making Flock !.
It appears that it is getting difficult to obtain fibres in lengths longer than 1.0 mm which will only produce a suede effect rather than an actual 'pile'. Also shooting longer fibres does produce problems. For example if you want to shoot 3mm long fibres, these will have a mass three times that of the 1.0mm fibre and require a corresponding increase in power to drive the fibres, 10% of their length into the adhesive or base material. So in short the lesson is to 'use enough gun!', in the first place, which means moving away from the 2000 volt fly swatters to the 100,000 volt + Van de Graaff style electrostatic generator.
How would I make flock ?. Well, commercially it is chopped to length from a bunch of fibres. I personally would look for something that already has fibres longer that I required, it could be a flock sprayed paper, a fur like fabric or perhaps a carpet type material .... or as one young lady suggested a real fur !. No cruelty is involved ... just buy an electric hair trimmer and practice on the neighbours cat !.
A few thoughts about perfection.
Flock spraying is an industrial process and as such has been developed to produce consistent and uniform finishes. In the natural world, nothing is the same and that is what makes nature unique. So if we seek to reproduce something from nature we have to capture the lack of perfection, ie the redeeming features that make any natural object unique and consequently, of interest.
Sunday 6th March 2011
The second thing I did this morning was look up the 'Tribo-electric series'. It is basically a chart that shows how positive or negative a material can get if subjected to friction. This is relevant in our case because if we consider the belt material and roller material for the Van de Graaff machine we want the voltage difference to be as great as possible. You can find three charts below ...
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The third thing I did was complete the fly swatter generator. Basically you have two wires, one for ground and the other is the 'high' voltage output. The one that went to the outer screens of the swatter I used as my ground wire. Now there is not much that one can do to improve the original idea, except that I thought it important to prevent any flock entering the handle. My other problem was that I wasn't sure what kind of flock dispensing system would be best for flock spraying puppet heads. So I decided to fit a 3mm banana plug type socket on the end of the 'gun' so that any kind of dispenser could be plugged in. Having done that the handle now looks like this ....
The first test was to measure the length of spark I could get from this, both with and without a capacitor and the best I could get was about 1.0 mm which I knew was not going to be good enough for the application. I had terminated both cables with needles, because I wanted to do the 'needle voltage test'. This relies on the fact that the distance a spark can jump between two electrodes is proportional to the applied voltage. So by measuring the maximum length we make the spark will in reverse tell us the applied voltage. Now I have never measured the output voltage of the fly swatters, but they are reported to produce voltages in the range 1500 volts to 2000 volts. So let us now have a look at a spark length ' voltage chart and see what we were supposed to get !.
We will have to interpolate at the lower end of the needle curve, but it looks as if we got exactly what we should expect from this graph, which is not a lot !. How does the swatter gun work then ?. From the video we can see that it does work, but only just. For the fibres to 'fly', that is end on in alignment with the electrostatic lines of force, the alignment takes time and energy. Now consider what you see in the video .... fibres can only exit the bottom of the tea strainer if they are already vertical !. Even so as the charge on each fibre reduces with distance, a lot of the fibres are striking the glue at other than right angles. This suggests two things to me. First is that a higher charging voltage is required, to get them vertical and the second is that because the fibres are hitting the target with a low velocity, they are not penetrating the adhesive deep enough, to 'root' them securely. To qualify this, the gun has to be quite close to the target surface for anything to happen.
The spark length is a good indication of how much voltage any system is giving out. It is also a good indication of whether a gun has dried out properly during the gun pre-heat, or that a fault condition has occurred, ie a dirty belt, or stray flock building up on the outside of the machine's case.
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Practical Van de Graaff generators, links and video's.
http://www.3bscientific.co.uk/rubber-belt-for-van-de-graaff-generator-u15301,p_83_671_612_1162.html
http://www.linux-host.org/energy/svande.htm
http://www.youtube.com/watch?v=QCWdFMO6yBk&feature=related
http://www.youtube.com/watch?v=lPBYYXNmKuk&feature=related
http://www.youtube.com/watch?v=UoCLmcsN0Gc&feature=related
http://www.youtube.com/watch?v=2rSYcKtBxZw&NR=1
http://www.youtube.com/watch?v=I2G0IdTWGQU&playnext=1&list=PL30C5EE0BDDE63E47
http://www.youtube.com/watch?v=O6gbbKwJAhQ&feature=related
http://www.youtube.com/watch?v=4GwK6zfaEt4&feature=related
http://www.youtube.com/watch?v=OyS5bZx1fns&feature=related
Conclusions.
As you can see, it is possible to make your own Van de Graff machine, with some simple tools and save yourself a small fortune in the process. For flock spraying you do not need a large machine, because 30,000 - 40,000 volts is more than adequate for shooting flock. If you use a low voltage DC motor, or a 'universal' motor then you can control the belt speed and hence the output voltage of the machine. Low voltage motor speeds can be controlled by pulse width modulating the supply voltage. Universal motors can have their speed varied over a wide range by the use of a 'drill speed controller' adaptor.
If you want a starting point for your design, I would start with the belt and make it about 50.0mm wide and laying flat, about 180.0mm - 200mm long. That will make your rollers about 25.0 mm in diameter and about 60.0mm wide. The actual dimensions are not critical. As you saw in the video's, for science tricks most Van de Graaff generators are made quite a bit larger than this, but too much voltage will simply spray your flock all over you and the room, on the other hand you only need to slow the motor in order to lower the voltage generated. One excuse for using higher voltages is when using thicker, or longer fibres, specifically 3.0mm and longer. The actual voltage produced is proportional to the length of the belt and the belt speed (as well as the materials used to make the belt and rollers. I turned my rollers from 1" transparent Perspex rod (Acrylic). But most plastics will do the job (see the tribo-electric series list).
To make the curve on the rollers, I made a simple attachment to mount on a lathe, although you could do the same thing with an electric drill.
The arm is pivoted at the rear and when the bottom end of the arm is moved to the right or the left, it describes an arc. The lathe chuck / motor / electric drill is switched on with the yellow lathe tool centered on the roller. Slowly moving the arm left or right will cause the curve to be cut on the roller. You can then sand the roller smooth, with it still rotating, if required. Do not polish the bottom roller, as a rough surface will be a better 'key' for the aluminum foil adhesive.
I have mentioned before that although the classical Van de Graaff machine uses a metal sphere as the top collector, I used flat plates with mine and they worked fine. If you think about it, you need your generator as a tool and the last thing you want is an open design that is going to covered in stray flock, or one that you can get unexpected shocks from, so it would be sensible to contain the whole generator in a case. For remote applicators, ie the lunch box on a stick, I made a second replacement 'lid' into which the remote EHT cable plugged into. You may have seen in the video's that some seem to pulse, in operation. This is caused by the metal sphere (or top plate) charging up and when it gets to the ionisation voltage of the spark gap, the spark is produced, then the sphere has to charge up again before the next spark. To shorten the charging process you can make the belt a little wider, but a 2" wide belt worked fine for me. Of course when flock spraying we will be using lower voltages that under normal conditions will not cause a spark and the top plate will stay charged during operation.
To be continued .....
(Clumping, tinting with an air brush etc.).