Here is a potentially novel technique for making transparent water-clear
(very low optical diffusion) phototools with a laser printer and a
laminator. This technique should work with various gummed paper "laser
printer transfer papers" but in this case I use an inexpensive gummed
paper from a DIY faux postage-stamp making supplier in Colorado (Olathe
Post):
.
http://www.wcp-nm.com/osc/catalog/product_info.php?cPath=26&products_id=110I will refer to this as the "Olathe Post" paper: "55lb. English Finish
(matte) Dry-gummed Paper" 25 letter size sheets for USD$14.95 plus
postage. I think this is 90gsm. I found that with correct preparation,
it would hold the laser toner better than the Pulsar paper referred to
below, probably due to the matte finish of the gum. I understand the
Olathe Post people have been getting this particular paper for some time
and intend to continue to supply it in the future.
A water-clear, low diffusion, transparent, phototool would be useful for
exposing photoresist on PCBs, or photoresists for other purposes such as
making polymer printing blocks:
.
http://www.toray.com/business/products/ict/printing/pri_001.htmlor some other rubber stamp process, exposing photosensitive solder-mask,
exposing a photosensitive material for a screen-printing stencil or
perhaps for exposing photoresist for etching a solder paste stencil from
brass shim. There are many reasons why we might want a "phototool".
I think the ideal phototool:
1 - Has strong blacks, at the wavelengths of interest - typically near
UV (see message 30333).
2 - Is transparent to the wavelengths of interest.
3 - Has sharp edges and is capable of fine lines.
4 - Is water clear in the transparent parts - not diffused.
5 - Is accurate and stable in X and Y dimensions.
6 - Is robust and can be re-used.
7 - For double-sided PCBs, can be aligned with another one with the
PCB slipped inside it.
Point 4 is highly desirable, since there is often some distance between
the black material in the phototool and the layer of photoresist or
whatever which absorbs the light. With a diffused phototool, no matter
how parallel the rays of UV, the light will splay out in all directions
from the phototool itself, reducing the sharpness of what should be a
sharp-edge shadow.
The following discussion only makes sense with a relatively collimated
light source - such as a smallish source at some distance, so the rays
of light only diverge 5 or 10 degrees, or less. Please see my next
message for how I use a cheap 500 watt quartz halogen flood lamp at 60cm
or so. This is a reasonably collimated source of light, with plenty of
near UV. Using fluorescent lamps only centimetres away would lead to
very uncollimated light, so no matter how low the diffusion of the
phototool, we would get a fuzzy image.
As far as I know, the only transparent materials which can be used with
laser printers are plastic of some kind with diffused optical
properties. As far as I know this is due to the need to make the
surface a little rough, to hold the toner in place as it is transferred
from the drum onto the film, and must stay in place as it goes into the
fuser rollers.
A general problem with laser printer phototools is that their blacks are
not very black. However, maybe they will still have enough contrast for
many purposes, if the exposure is chosen carefully. My initial tests
with Riston show this to be the case. Riston seems to be quite high
contrast, so I have been able to get good sharp images using laser
printer toner on its own in the phototool.
Some laser printers can operate at 1200 DPI in both X and Y directions.
I use a now-obsolete Brother HL-5250DN, which has a true 1200 DPI mode.
The paper or film travels at half the speed of the normal 600 DPI mode.
(The mode is "1200 dpi", not "HQ 1200" - which I think is 600 vertical
and 1200 horizontal.) 1200DPI is 47.24 pixels per mm.
My HL-5250DN, when using an original Brother toner cart with its
original toner (not one of my refilled ones), is capable of producing
0.2mm lines with 0.2mm spaces between them. This is about 10 pixels.
(I assume the current model HL-5350DN has the same performance.) These
are inexpensive. In my experience it has been a thoroughly excellent
printer. I refill the toner carts and have been very happy with the
HL-5250DN since about 2006.
The first step is to print onto the gummed side of the above-mentioned
Olathe Post paper. This has a white, opaque, gummed layer. This is
hard to distinguish from the plain side, but when examined with an
eyepiece, it can be seen to be non-fibrous and to have some vertical
streaks in it, which don't seem to affect the results. This paper is
not as thick as the blue paper with transparent shiny yellow dextrin gum
of the Pulsar Toner Transfer Paper:
.
http://www.pcbfx.com/main_site/pages/products/transfer_paper.htmlThe Olathe Post paper travels more smoothly in my printer than the
Pulsar paper. It is my impression, at least with smaller (half Letter
~A5) sheets that the Pulsar paper, due to its stiffness, would flick its
tail in certain parts of the travel and so cause movement of parts of
the sheet which had unfused toner sitting on them, shaking the toner
from where it should be. Also, the Pulsar paper tended to have gaps in
the toner in some fine-line areas, which was not a problem with the
Olathe Post paper. (Pulsar sell a foil to fill these gaps when the
toner is used as a resist itself.)
I found the Olath Post paper was dry when it comes from the pack. This
may cause the toner not to bond to the gum, due to the gum being
crystalline hard. So before printing I suspend the sheet face down on a
wire rack 2cm or so above a tray of vaguely warm water for a few
minutes. This higher moisture content seems to ensure that the toner
will stick properly to the gum.
Next, I used the laminator described here:
.
http://tech.groups.yahoo.com/group/Homebrew_PCBs/message/30148Although I wired this for cooler operation for Riston lamination of
copper-clad PCB material, it still seems to be hot enough to operate
with ordinary laminating pouches. This means it is hot enough to melt
the glue on the inside surfaces of the two sheets of plastic. As far as
I know, the technique I describe here will work with any ordinary
document laminator.
To prepare the pouch for lamination, I cut a piece of ordinary 80gsm
white paper about the same size as the piece of Olathe Post paper.
The layers are:
1 - Top laminating pouch sheet with glue downwards.
2 - Olathe Post paper with gummed side up, with laser toner on top.
3 - 80gsm backing paper.
4 - Bottom laminating pouch sheet with glue upwards.
I run this through the laminator, cut around the paper edges, and
separate off the top two layers which are now bonded together, with the
previously amorphous glue particles now fused into a water-clear
transparent continuum with the top plastic sheet, going down to whatever
interface this glue has with the top surfaces of the toner and gum.
I soak this in warm water. With a minute or two I can peel off the
gummed paper, wash away the gum from the plastic sheet and dry the sheet.
I now have a clear plastic sheet, with a glue layer below, holding the
toner in place, with no gum or paper. However the surface of the glue
where there was no toner is not flat, so it has a rather diffused
optical property. This is due to the glue having solidified against the
matte surface of the gum.
The fix for this is to run it through the laminator again with a single
sheet of laminating pouch facing it with glue upwards. All the glue
melts together and I have no more amorphous glue or any exposed glue
surface. Where the toner is, I have:
1 - Top laminating pouch sheet.
2 - Melted glue from sheet above.
3 - Toner.
4 - Melted glue from sheet below.
5 - Bottom laminating pouch sheet.
Where there is no toner, I have:
1 - Top laminating pouch sheet.
2 - Melted glue from the sheets above and below
3 - Bottom laminating pouch sheet.
The glue seems to have about the same refractive index as the plastic
sheet, so there's no reflection at the interfaces.
While the blackness is not as good as a proper photographic film
phototool, I suspect it will be good enough for many purposes, including
especially Riston or probably other negative photoresist films.
In the future I will do further tests and present photos etc. at
.
http://www.firstpr.com.au/pcb-diy/I will post another message about a simpler technique for creating
phototools with reduced diffusion, by using oil.
- Robin