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In my opinion the best way to obtain a truly black image using a laser printed image is to run your laser printed transparency through a laminator with a piece of "toner reactive foil". This is the same stuff that some people use when making toner transfer PCBs to cover the transferred toner to get better results when etching the board.
This material is not entirely opaque when held up to a bright light but when combined with the toner makes for a really opaque negative. I have used the black pigment type with good results. Although it appears to be the same as the foil used for "hot foil stamping" in the printing industry, I have seen some claims that the composition is slightly different. I have not seen the various metallic colored "toner reactive foils" but based on what I saw years ago with the gold and silver "hot foil stamping" material I would suggest using pigment colors as the gold and silver colors were not quite as opaque.
Black is probably the best choice however, red should work as well and could possibly even work better given that the red should block the ultraviolet light, possibly to a greater extent than the more neutral black.
I'm pretty sure any cheap laminator will work adequately. You probably do not need a high-end laminator as you would for doing toner transfer to a PCB.
You can find this material on Amazon or eBay as well as from Pulsar - the people that sell the green foil for use with PCB toner transfer and white foil for labeling PCBs. The section of their website that sells materials for making decals sells other colors including black. (https://colorfoils.com/buy-now.html)
You may still end up with a few random pinholes (from dust in your printer etc.) which can be touched up with a Sharpie or, better yet, a red or black "opaque pen" made for touching up film negatives in the printing / graphic arts industry such as : http://www.gwjcompany.com/index.php?main_page=product_info&products_id=6 or http://www.gwjcompany.com/index.php?main_page=product_info&products_id=4
Craig
Chris,
The pdfs look fine on the screen, but this is not theissue. We had a discussion here some months ago about thedensity of laser printed artwork. It turns out that thelaser printed output is a series of small particles thatare fused to the surface and each other. Although they arefused, the result is not a continuous line, but a porousone that allows light to leak through. I have seen twowork arounds. The first is to make two transparencies andstack them, banking on the fact that the pores will mostlynot line up. The other is to use a spray that increasesthe density of the print by chemically melting and fusingthe particles.
There are a number of commercially available sprays. Do aGoogle search on "laser toner darkener." The discussionhere also mentioned spraying with clear paint, such asKrylon. I tried it with either Krylon or Rustoleum clearspray. The transparencies became much more dense. I didn'ttry making any boards, I just was experimenting to see ifthe artwork showed improved density. I did note oneproblem with this approach. It was difficult to get theright amount of spray on the artwork without puddling andcausing the traces to distort. I suppose that withpractice my technique would improve.
Harvey