What does a RIP do for an Ink Jet Printer

There is a limit to what a desktop ink jet printer can reproduce, but RIP software and printer combinations that sell for less than US$800, can rival the screen printing positive output of imagesetters costing 10 times as much, 20 years ago.

Ink jet printers are designed to reproduce color prints on nice white paper, but screen makers need high contrast positive images on clear film with hard sharp edges and opacity to stop UV energy. This is not easy to do with transparent inks designed to create full color prints by overlapping and blending CMYK inks on coated white paper. To increase ink deposit, and to control the dot gain from all that ink - and convert vector image curves into bitmap images that can be printed, you need a software RIP.

Computer Files
Vector graphic programs like CorelDRAW and Adobe Illustrator, create images made from individual 2 dimensional 'objects' - points in space described with mathematical formulas that can only exist inside computer memory. These objects are combinations of an 'outline' or 'stroke' and individual properties assigned to each object like fill color. Because they are built from mathematical formulas, it is very easy to scale them, (change their size), with an instant change of one small part of the formula.

A vector image mathematical formula means that large areas can be described with very small files and if you want an image to be 6 centimeters or 6 meters at final size, the computer file is basically the same size and a much smaller file size than raster-based bitmaps from Photoshop or scanners.

Vector images are resolution independent because the resolution is not part of the file, as it is with a bitmap; the resolution of the final image is determined by the output device. The only way to make mathematical vector curves visible is to rasterize them - to convert them into a bitmap image that can be output on a monitor or printer.

The slang term RIP comes from 'raster image processor'. You need to convert the curves from vector graphics programs like CorelDRAW and Illustrator, to a bitmap so ink jet printers can reproduce them. Types of software RIPs In the days before computers separated art, photographs or the actual art were scanned with very expensive equipment and 'RIP' software converted the scan into CMYK positives that were made with a photographic imagesetter.

Most art today is created and separated in desktop computer graphics programs. If the artist uses spot colors to create the image, the image is already separated and ready to be output. These vector files use the Adobe Postscript language and only need to be translated into a language the printer understands. Modern RIP software can still make CMYK separations or just translate the Postscript output to increase ink deposit, and to calibrate and control halftone shapes from the increased deposit. Figure of Calibration Dialog Box

One of the real benefits of a RIP is to fine tune and calibrate the size of dots - so the finished print on your substrate, is the size the designer wanted. With all the steps and conversions an image takes before it gets to press, a 30% dot never starts out as a 30% dot. This is a fact of nature and has to be compensated for. This calibration is usually done with a densitometer, but can be done with a microscope or magnifier that can measure actual dot size.

Beware. Color management software to control photographs or posters printed in full color on different types of papers also control ink deposit and are called RIPs, but they are of no use to a screen maker because they lack the ability to shape Postscript halftones.

Inkjet Film History
In the late 1990's, when photographic film companies stopped making diffusion transfer film, someone discovered the 'Ink Jet Back Light Film' media setting on some of the DYE Epson inkjet printers like the 3000 and 1280. This setting increased ink deposit enough to stop UV energy to make screens. This saved and revolutionized the screen printing business as we searched for a computer solution to replace our process cameras and anemic desktop laser printers that didn't have enough UV density.

Some laser printers can achieve 2.0 density, but I regularly print 3.5-4.0 UV density images with a very inexpensive Epson R1800 printer. This is something a laser just can't do.

Newer Epson ink jet printers didn't have the 'Ink Jet Back Light Film' media setting and Epson dye or pigment ink rarely measures above 1.6 UV density using the Epson driver. Printers that don't need to make halftones also need a RIP to increase ink deposit even at 1440 dpi.

Ink Jet Heads
Epson printers have become the defacto standard for screen making positives and direct to garment printing. All three major manufacturers use variable sized droplets, but HP and Canon 'thermal" heads with a fixed chamber size, that heats and the ink forces it through he print head and onto the film. Thermal heads don't allow programmers to increase the droplet size so finished positives rarely achieve UV density greater more than 1.5 - 1.6.

This is why a RIP that increases ink deposit of the variable sized droplets is so important for screen printers. Epson printers with piezo heads can be programmed to increase droplet size, and are the only inkjet printers that RIP companies write drivers for.

Epson uses a method of ink ejection called piezoelectric. It is very simple to control the electrical signal to control drop size compared to controlling the heating of the ink to make it eject. To print, an electrical signal is sent from the printer to a ceramic printhead which will expand and then contract. The compression causes a drop of ink to be ejected through the nozzles and onto to the inkjet film. The piezo ejection method results in a smoother, more consistent droplet than HP or Canon thermal heads.

The easiest way to increase film opacity is to increase the printing resolution to 1440 dpi from 720 dpi. The mathematics of doubling the resolution, quadruples the number of dots in each square inch which helps the transparent black ink do its new job of stopping UV energy. Even with a RIP, you get better results with higher density, but, it takes much longer to print.

Ink Jet Films and Inks
Ink jet ink needs a top coating on polyester film to enhance ink adhesion, control dot gain, drying time, and moisture resistance. If there was no topcoat, the ink would smear and bleed like water drops on glass.

The coating for dye inks is called swellable, and they look like gelatin or capillary film coatings. They swell and absorb the 100% water-based liquid dye ink. Alas, if you get a drop of water on them, the ink can re-wet and BLEED like mascara in the rain. New Epson Claria dye inks are more bleed resistant and have a much longer resistance to UV fading compared to their last generation of dye inks. Dye inks do need more time before you can use them because the ink has to be absorbed into the swellable coating.

In November 2002, Epson stopped making the Epson Stylus 3000 as they focused on pigment ink inks to make photographic images last longer. Colorfast pigment inks for long lasting photographs were introduced in the late 1990's. Pigment inks need a micro porous coating (like a gravel driveway or ice cubes in a glass), so the pigment particles that can't be absorbed by a swellable coating (like poppyseeds on a bun), can get into the coating. You can use dye OR pigment ink with a micro porous coating and many people prefer the micro porous coating because films can be stacked right out of the printer without smearing. Stacking is not a good idea until you are sure the ink is actually dry. If you leave positives stacked after printing, the humectants in the ink can put a yellow halo around your black image and give the film a yellow cast.

Often, porous, nano porous or micro porous coatings are called water proof. If you lick your fingers and pinch a piece of film, you will make one side sticky as it absorbs the moisture with capillary action. Suppliers started selling this more expensive micro porous coating as "water proof" to those people that wanted to pay for a safer coating. This has always concerned me because all inkjet films are made to absorb water based inks. Since the inks for Epson printers are water-based, none of the inkjet films are waterproof - they should have been named "bleed-resistant".

The water based pigment ink is absorbed with capillary action and carries the pigment particles into the coating where they can hide under the surface, build up and stick to the sides of the coating particles when the water evaporates, just like any water based ink. This makes them resist bleeding because they are contained in the coating and APPEAR to be dry. This does not mean the ink is dry! It's just below the surface and can be pulled out of the coating with your vacuum frame it the ink is not allowed to dry.

Water can also be absorbed from the air and keep the ink moist and wet. Many people in high humidity conditions experience problems with ink that is pulled out of the film by the vacuum during stencil exposure and it stains the stencil and can't be used again.

The downside to pigment inks for screen printers is that the irregular shape of pigment particles has to be coated in a clear resin so it will flow smoothly through the inkjet heads. Combine this with the fact that pigment particles must comingle with nano porous particles means pigment ink must be printed thicker than 100% DYE ink that can flow smoothly and link up when it is absorbed by the swellable DYE ink coating.

Ten years ago, ink jet printers were primitive. We are only at the beginning of the age of inkjet printing as membrane switch companies are working on decoration and printing conductive inks. EPSON have introduced their 4 color Stylus Pro 4400 in Europe targeting sign makers. Making positives for screen making can only get better and better.