This post contains background information for my Prototype Rails presentation in January 2004. It deals with the general problem of printing white using a laser printer with an emphasis on the hardware and how it needs to be modified.
The monitor/display screen on which you are reading this post is conceptually equivalent to printing color (including white) on a black piece of paper. Each address on the screen (indicating a picture element, or pixel) has three color values associated with it. They are called red, green, and blue and abbreviated as an ordered triplet (R,G,B). When R, G, and B are all zero (0,0,0) then no color is applied to that pixel and the black of the paper shows through, as in the body of the lettering of this text. When R, G, and B are all at their maximum value (255,255,255) then the full three colors are applied to that pixel; they mix together to make white. There are 16, 777,216 (R,G,B) combinations altogether and they make up the family of colors that your monitor/display screen can generate. In this scheme, white is generated as the combination of full red, green, and blue.
For technical (and historical) reasons, a color printer works a bit differently. The basic idea is predicated on the notion that the paper is white, so no facility for printing white is provided. Instead the emphasis is on accurate rendition of the color spectrum and getting a good rich black. Inside a color laser printer there are four toner cartridges; they contain very fine powders of four process colors. The color specification internal to the printer (more on this in the next post) for a pixel is a given amount of Cyan, Magenta, Yellow, and blacK and is abbreviated (CMYK). When the color laser printer prints out a pixel, it applies a mixture of each of the four process colors that is designed to reproduce the corresponding RGB color specified by the artwork. So the computer sends a string of (RGB) color values to the printer, and the printer decodes these into valve settings for the (CMYK) cartridges. If everything works properly, all of the colors on a computer screen can be rendered onto the printed page. Except for white! But hey, the page itself is white so we'll say the the absence of any color values (0,0,0) is white by default.
For those of us that want to print white on a clear film (i.e., the aspiring decal printers) using a laser printer, the absence of a process white is a problem. And that's a real limitation for those of us modeling steam-era freight cars - a lot of the lettering was white stencil paste over a darker color base coat. As far as I have been able to determine, there is no reasonably affordable OEM solution. A five-cartridge color laser printer (C,M,Y,K,W) would be really nice but there's really no market demand...
There is an aftermarket approach using laser printing technology that is moderately expensive, which I'm experimenting with to learn the tricks of the trade. The approach (as designed by the vendor) is to replace the 4 process color cartridges in the printer with 4 white toner cartridges, or more precisely with 4 empty reconditioned toner cartridges into which you pour white toner. The approach that I implemented was a slight modification. I wanted to have the capability to print gray as well as white, so I kept the black (K) cartridge and replaced the C, M, and Y cartridges with white.
Why an OKI printer?
The vendor specifies an OKI printer. I already had an HP color laser printer and just wanted some white cartridges for it. It took me a while to get over a mild huff and understand the reason for the OKI printer. It has to do with the way the toner cartridges and imaging drums are assembled.
Here's my HP color laser printer; compact, reliable, and easy on toner (click to enlarge):
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This is a nice little self-contained bit of technology, and it works well, but the problem is there's no easy way to take it apart, clean it out, put white toner inside the reconditioned assembly, and seal it back up. Everything is designed for a single use (or factory reconditioning) so there's no easy way to get a good supply of clean white toner inside one of the HP cartridges.
Here's the OKI printer. It's a lot bigger than the HP, and heavier. I originally thought this was an example of less elegant engineering, and it may be that, but its also an example of an assembly that is easier to get inside to the guts:
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These are the process color cartridges inside the top of the OKI printer. The one closest to the camera is the blacK (K) cartridge. The other three behind it are all clean, remanufactured cartridges into which I have poured white toner:
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The monitor/display screen on which you are reading this post is conceptually equivalent to printing color (including white) on a black piece of paper. Each address on the screen (indicating a picture element, or pixel) has three color values associated with it. They are called red, green, and blue and abbreviated as an ordered triplet (R,G,B). When R, G, and B are all zero (0,0,0) then no color is applied to that pixel and the black of the paper shows through, as in the body of the lettering of this text. When R, G, and B are all at their maximum value (255,255,255) then the full three colors are applied to that pixel; they mix together to make white. There are 16, 777,216 (R,G,B) combinations altogether and they make up the family of colors that your monitor/display screen can generate. In this scheme, white is generated as the combination of full red, green, and blue.
For technical (and historical) reasons, a color printer works a bit differently. The basic idea is predicated on the notion that the paper is white, so no facility for printing white is provided. Instead the emphasis is on accurate rendition of the color spectrum and getting a good rich black. Inside a color laser printer there are four toner cartridges; they contain very fine powders of four process colors. The color specification internal to the printer (more on this in the next post) for a pixel is a given amount of Cyan, Magenta, Yellow, and blacK and is abbreviated (CMYK). When the color laser printer prints out a pixel, it applies a mixture of each of the four process colors that is designed to reproduce the corresponding RGB color specified by the artwork. So the computer sends a string of (RGB) color values to the printer, and the printer decodes these into valve settings for the (CMYK) cartridges. If everything works properly, all of the colors on a computer screen can be rendered onto the printed page. Except for white! But hey, the page itself is white so we'll say the the absence of any color values (0,0,0) is white by default.
For those of us that want to print white on a clear film (i.e., the aspiring decal printers) using a laser printer, the absence of a process white is a problem. And that's a real limitation for those of us modeling steam-era freight cars - a lot of the lettering was white stencil paste over a darker color base coat. As far as I have been able to determine, there is no reasonably affordable OEM solution. A five-cartridge color laser printer (C,M,Y,K,W) would be really nice but there's really no market demand...
There is an aftermarket approach using laser printing technology that is moderately expensive, which I'm experimenting with to learn the tricks of the trade. The approach (as designed by the vendor) is to replace the 4 process color cartridges in the printer with 4 white toner cartridges, or more precisely with 4 empty reconditioned toner cartridges into which you pour white toner. The approach that I implemented was a slight modification. I wanted to have the capability to print gray as well as white, so I kept the black (K) cartridge and replaced the C, M, and Y cartridges with white.
Why an OKI printer?
The vendor specifies an OKI printer. I already had an HP color laser printer and just wanted some white cartridges for it. It took me a while to get over a mild huff and understand the reason for the OKI printer. It has to do with the way the toner cartridges and imaging drums are assembled.
Here's my HP color laser printer; compact, reliable, and easy on toner (click to enlarge):
Inside the printer, in a sliding drawer, the four process color cartridges are arranged (K, C, M, and Y) in order from front to back:
This is the C cartridge upside down. The bigger blue roller in front is actually an imaging drum. This gets hit by the laser beam and makes sticky spots for the toner grab onto, and then the toner is transferred onto the paper and fused. Behind the big blue roller is a smaller roller which is actually colored by the cyan (a shade of blue) toner. On the top of the cartridge (which is at the bottom in this photo because the cartridge is upside down) is a reservoir of this cyan powder. The powder falls down out of the reservoir onto the smaller roller, which is contact with the imaging drum, and the powder is transferred to the imaging drum and then onto the paper.
This is a nice little self-contained bit of technology, and it works well, but the problem is there's no easy way to take it apart, clean it out, put white toner inside the reconditioned assembly, and seal it back up. Everything is designed for a single use (or factory reconditioning) so there's no easy way to get a good supply of clean white toner inside one of the HP cartridges.
Here's the OKI printer. It's a lot bigger than the HP, and heavier. I originally thought this was an example of less elegant engineering, and it may be that, but its also an example of an assembly that is easier to get inside to the guts:
These are the process color cartridges inside the top of the OKI printer. The one closest to the camera is the blacK (K) cartridge. The other three behind it are all clean, remanufactured cartridges into which I have poured white toner:
Like the HP design, the OKI design combines toner storage with an imaging drum into the printer cartridge assembly. Unlike the HP, it is a two piece assembly that is designed to easily come apart (by the user - NOT in some factory). In this photo, I've taken the cartridge apart into its two components:
The component at the lower left is a regular OKI toner cartridge (which is upside down in this photo). It doesn't matter whether it contains C, M, Y, or K because the only role it plays in the converted printer is to close an electrical sensor to indicate that it is present (the printer won't print without this piece installed). Note that the opening where the toner would normally come out is sealed with white tape.
The component at the upper right is the imaging drum (which is rightside up in this photo). As designed, the toner falls out of the cartridge and into that long narrow trough (which in this photo is filled with white powder, but would normally get filled with C, M, Y, or K powder from the reservoir). From that long narrow trough, the powder falls down onto the imaging drum where it is transferred to the paper and later fused. Since the toner container is sealed with tape, no color toner can get down and into the trough. Instead, the operator pours the fine white powder into that long narrow trough by hand, puts the sealed toner container back on top of the imaging drum, and installs the whole thing into one of the "white" slots in the printer (i.e., any slot except for black, the way I am set up).
So the OKI design offers a real advantage if you want to print white. You can get clean, remanufactured imaging drums that have no toner inside them, and fill up the trough with the powder. The process of refilling takes about 5 minutes and can be repeated hundreds of times. Easy access, no drilling holes into the cartridge, etc. The taped toner container on top seals the whole thing shut. The way I am set up is with one black printing cartridge in the K slot and three white cartridges in the C, M, and Y slots. Whenever I run out of black, I can just get a new black toner and replace the old one. If I ever run out of white I can just remove the toner container from the imaging drum and pour more white into the trough, then seal the whole thing back up again. Simple and easy.
Charles Hostetler
Goshen, Ind.