Friday, 16 May 2014

Creating a screen printing UV exposure unit from acrylic nail art lamps

Some of the nerds have recently acquired space at the Boiler Room Studios in Hove - somewhere big enough to do some soldering, laser cutting, and - hopefully - some screen printing!

At the heart of any screen printing is a UV exposure unit. This allows an image (whether hand-drawn, printed, or photocopied) to be transferred onto the silkscreen for printing. By applying a photo-sensitive film to the screen, and using the image as a photo-resist (similar to how we use car paint as an etching mask for our PCBs) the UV exposure unit creates a hard, thin stencil on the silkscreen.

As well as using ultra-violet light to create an image on a screen for silkscreen printing, it will also be really useful for exposing and hardening off UV curable solder-mask paint. So we get two different uses from the same bit of kit!

In our new studio space we have a large office desk, which forms the base of our screen-printing exposure unit. Simply put, we're cutting an A3 sized hole into the desktop (in the larger, left-hand side of the desk) and covering this with glass. Under this glass, we'll have a tray holding a number of ultra-violet (UV) lights.

 In fact, we'll probably add a few plain while bulbs (or maybe some LEDs) and the whole thing could also be used as a lightbox to help with alignment and for tracing images. A real three-in-one-bit of kit!

We've read reports of people having success using acrylic nail "drying" lamps for photo-resist processing, as each unit has four 9W flourescent UV bulbs - removing the need to create complex balast-balanced circuits to get a bare UN bulb to light up. As they cost about a tenner each on eBay, and because we're aiming for a larger-than-A4 exposing area, we bought three.

They're pretty simple devices, which means they'll be easily hackable! Each one has a timer button, which runs the lamps for 2 minutes (120 seconds) exactly. They also have a flick-switch to make them always on. The units are surprisingly large. The photos on the internet don't really have much to give an indication as to their actual size. For some reason (maybe the price) we were expecting them to be physically smaller.

Each unit comes with 4 x 9W UV bulbs, marked as 365nM (although the instructions specifically say that the UV light is safe to look at, but recommends avoiding prolonged exposure to the eyes) has a removeable bottom plate and a cheap, plastic enclosure.

A few screws hold the top and bottom parts together. After opening, care needs to be taken in case the wires connecting the base to the lid are snagged (some of the wires were wrapped around each other on this unit).

The components on the lid are simply a push-button switch and a toggle switch, which can easily be re-sited elsewhere. The lamp holders are part of the moulded plastic enclosure and are not removeable - we're going to need to cut the lamp holder parts out of the plastic to keep them intact and make them reuseable.

As we have three of these, to make a single UV exposure unit, the centre-pole and the "always on" pole of the toggle switch can be shorted together (so each device is always switched on when plugged in) and the 120 second start button simply unplugged from the board. We're powering all three from the same single power supply (a single lead with a 240v mains plug), but keeping the individual fuse on each unit, just in case.

This single power supply will be switched via a relay, which allows us to create a PIC-based timer module to set the exposure time (for all 12 lamps together) in minutes and seconds, rather than have to switch each bank of 4 lamps individually.

We've already built a timer module (for an earlier LED-based UV exposure unit which worked, but was not very big) but that's slightly more than we need for this project. A simple relay switch with 7-segment countdown timer should suffice for this, since we're only having the one setting (full power). Which means we need to get all three of our individual units working from a single power supply.

The first job is to make a tray into which we can mount our UV bulbs and electronics. At BuildBrighton, this means grabbing the first bit of left-over sheet ply and some 1" x 2" battens and knocking together a simple tray.

We made sure that the height of the sides is high enough for the height of the UV lamp holders, chopped up some strips of pine that were lying around and screwed the whole thing together (no nails were harmed in the making of this wooden tray!).

Next, we needed to get the UV controllers out of the nail drying units. UV flourescent bulbs are not as straightforward as regular lamps or LEDs - you can't just put power to them and expect them to switch on. Each bulb needs complicated ballast control and to be balanced properly, so we just lifted the circuitry that came with the nail drying machines out of the enclosure and made a few simple modifications.

After putting a probe on the 3-way switch (with the unit unplugged from the mains of course!) we discovered that in the middle (off) position the  incoming (blue) neutral line is disconnected, and it simply shorts this to either the orange wire (always-on mode) or the black (120 second timer mode) as the switch is toggled.

Since we're going to control the on/off times of the entire unit (once it's built) we removed the switch and shorted the incoming "neutral" line from the 240V to the orange wire (so that the device acts as if it is in the always-on position). The other part shown in the photo above (connected to the incoming, brown "live" wire) is a fuse holder with a 3A quick-blow fuse fitted. We decided to keep this in place as an extra safety precaution (messing about with 240V mains can be both scary and dangerous, so always treat it with respect!)

One problem with the nail units is the use of cheap, vac-formed lamp holders, integrated into the large white housing. Grumpy Paul showed how to cut these free properly - not just going at them with a hacksaw as originally proposed, but to score around each lamp holder with a sharp blade, and them snapping them free from the surrounding material.

To make sure we were along the right tracks, we connected just two (of the three) modified devices together, using a bank of screw terminals fixed to some strip/vero board, and fired it up....

Success! All eight bulbs lit up first time. Things already look very promising! By simply repeating the whole process for the third and final nail drying unit, we got all 12 bulbs connected and running from a single 240V power supply:

Now for some quick maths: each bulb is rated at 9W (these are flourescent bulbs after all, which are much less power-hungry than traditional tungsten filament bulbs). So we have 12 x 9W = 108W (plus perhaps a few mW for the logic control on each of the boards which is insignificant in the grand scheme of things). Not much more than a single, bright, filament-based light-bulb.

Since power = current * voltage, we can re-arrange this formula to get current = power / voltage. Which means our unit is likely to draw 108/240 = 0.45 amps.

Even using the single plug from just one of the units, and a single fuse for the entire set of three controllers, we're well under the 3A rating of the quick-blow fuse (and the fuse in the mains plug). So no further modification is required - we should be just fine running our new UV exposure unit from a single socket, exactly as it it (ok, maybe we need to reposition a few of those lamps!)

Being able to switch the entire set of UV bulbs on and off from a singe power source means we can now get on with building our own PIC-controlled timer/relay unit, two switch the entire thing on and off, and allow us to set an exposure time to the nearest second. Perhaps that's for another post......

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