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