Printing 4 color on 2 color machine.. need help.

Hello guys..
Im new in classic offset printing. i have 2 color KBA Rapida and i need to switch to this machine.. i know how everything work.. but.. i have problem with color density.. i have experience working on 4color watherless machine.. heid. di.. but it hard to get real color on 2color machine..
because i dont know how need to look first two color.. i dont have densitometer.. but even if i have.. i dont know will be usefull because i work with small number of copy.. 300, 500.. rarely 1000 (i know you will tell me now that these are not offset quantities... but...)
how to get good color on finish with 2color machine?

any help?

thx a lot.

 

You should at least get a densitometer and maybe a Matchprint proof.

 

You are going to waste alot of money on setup sheets, time and plates if you're regularly running these quantities in 4C on a 2C press. I'd highly recommend looking into a low-end digital press from Xerox, Canon, Ricoh or Konica Minolta. You can lease a decent one for around $400-500/month on a 60mo lease (and this will include a densitometer which you need either way). This will allow your to produce your jobs faster also save on the costs of your washup chemicals, blankets, towels, etc.

Alternatively, you can start with outsourcing to trade printers like 4Over, GotPrint, or someone local. You're going to lose money doing 300-500 offset.

 

Try to use a colorbar whenever possible and use your PMS book swatches of CMYK that should get you as fast as you can. Also see if you can buy a ink preseting program like presspercent. Please PM me to ask you about waterless printing

 

I'll second that. Color bars across the bottom of the sheet and a pms book.
Conventional press, conventional technique.
Might pay to run a test plate with some 25, 50 and 75% screens and simply adjust your image setter to 'close enough by eye'
I always find it easier to achieve both colour and fit if I print 1 up wherever possible, especially for short runs.

 

Keep all your spoils and use them to settle down the colour on the next job. That's a big help as well.

 

Is your water level high? Sometimes (especially with high gloss paper) water will stay on the paper and cause the second unit to wash out. When I print 4 color process on my 2 color I always run the darker color on the first unit and monitor the water closely.

 

Don't know if its still available but you used to be able to image what was called a color key. Color key was a 3M product and the preferred method for printing multiple pass 4 color process. Each individual color was imaged onto a sheet of thin acetate in the required color. Each color was created on the color key with the appropriate color created from dyes. Each of the four sheets of acetate were registered together to create a full 4 color image. All you would need to do is take a sheet of the stock your printing, and place it behind the color or colors you want to visually separate. This allows you to get a progressive view of the job as its happening.

 

Blast from the past Tom!

 

Definitely not viable quantities for offset, especially in 2 passes. A second hand Xerox C75 or similar would be much better. I bought mine off Ebay for £1500 and had it checked and installed by a Xerox engineer.

 

See my article on ‘Print to grey balance’

 

If you're determined to use that press and you have a digital background you need to know about ink tack. Tack is the ability of ink to split. Even if you are putting the darker ink on the first unit, you need to consider tack. High tack goes on first and lesser tack on top. If you are familiar with peanut butter and jelly sandwich. You have one piece of bread, if you put the jelly on the bread first the peanut butter will not stick to it to give you the proper density. That is wet ink on wet ink.

 

BUT if you fold and squeeze the bread between the knife and butter you can achieve your result! -though i doubt it can work with printing!-

 

Attached are the numbers you need to print to - you will need a densitometer that measures 'dot gain'.

 

Dan... you are assuming with your numbers that the prepress department has output plates that will work with them densities. I think that a better assumption would be, to error on the side of caution, and assume that the very fact that they are doing multiple passes on a 2 color press would indicate that PERHAPS their pre press department would not be that finely tuned.

 

It would take a good 1000 sheets to get a big old press like that settled down to the point where a denso would be any help anyway. Those sorts of machines are really not intended for short run work.