Puretch processing guide written by Jennifer Page, Cape Fear Press. All rights reserved ©2001-2015. Translation Hugo Bos
Puretch Processing Instructions
Puretch is the thinnest photopolymer film available. It is a non-toxic, biodegradeable, aqueous developing, negative working resist ideal for high resolution positives producing a durable resist that can be etched. It is no longer necessary to pre-thin the resist for etching fine halftones. Pre-thinning photopolymer film exposes the emulsion to oxygen, degrading it and leaving it less sensitive to UV light and yielding unpredictable results.
Puretch cannot be used for "Intaglio-Type". The photopolymer layer is too thin to allow enough ink in it to create a good black.
If you want a photopolymer resist that you can consistently etch fine halftones without pre-thinning and pouring half of your resist down the drain, this is it.
- yellow light bulb (bug light)
- Ajax, whiting or Rottenstone
- sharp mat knife blades
- paper towels
- tape and push-pins
- spray mister bottle
- distilled water
- 100% sodium carbonate (pool supply store)
- squeegee with firm sharp blade
- photo tray and soft nylon sponge or small pump-up garden sprayer
- heating device: space heater fan, hot plate or hair dryer
- clock with second hand
PREPARING THE PLATE.
A clean plate is VERY important for adhesion and all traces of cleaners or brighteners must be removed. Mechanically clean and degrease the plate with a cleaner such as whiting, Ajax or Rottenstone until the plate rinses grease free with a sheet of water. Thouroughly rinse this in tap water and dry with paper towels. When using copper, brighten the plate to remove oxidation with a brightening solution of water, vinegar and salt. Always mechanically clean plate after brightening. Laminate resist immediately to avoid oxidation of the plate. Note: It is not necessary to abrade plates for adhering Puretch, mirror finished plates can be used if they are properly degreased..
- In a room lit with a yellow bug light, cut a piece of film larger than the plate or image. If the artwork is a small portion of the plate, a piece just bigger than the art can be cut. The film can be tacked down, dull side up, at the corners on a bulletin board or Homasote. Tack the film down first, then trim it to size. see photos 1 and 2. Tip: Unroll film with roll parallel to your shoulders, not perpendicular to you. This will help in lying film down later.
- The film is sanwiched between two layers of protective mylar. Remove the dull mylar by picking at the surface near a corner with one of the tacks. Re-tack that corner (see photo 3) and with the film still tacked down, peel the liner away, tearing it out from under the tacks and discard it.
- With a spray bottle of distilled water (only), mist the surface of your film evenly. see photo 4 Mist plate lightly also. Remove the tacks nearest to you and grab those corners. Pull wet film out from under the two other tacks. Bring film over to plate, with the plate hanging off the edge of table. see photo 5 Align bottom edge of film a little lower than edge of plate. Then lay the film on the moistened plate, try to let it lie down progressively leaving no bubbles. If there are wrinkles anywhere, lift film there quickly to remove them. Lift and move plate fully onto table top. Spray the top of the film lightly to lubricate the squeegee and squeegee all the bubbles out starting from the center outwards in all directions, light pressure first, then firm. From the top side, trim edges of film with a sharp blade. Squeegee more if necessary. For smaller pieces mist plate and apply film by hand in the position desired and squeegee. The squeegee method is the best way to avoid bubbles and wrinkles during lamination. Tip: Keep your squeegee table very clean to avoid contaminates from seeping under edge of film, contaminates can cause film not to adhere at edges of plate during developing.
- Heating the film evenly is the best way to get a good adhesion and remove the moisture from the film. Without heating the plate and film you would have to wait hours for it to dry on it’s own before exposing your plate. Copper quickly conducts heat so you can do this any number of ways. You can use a space heater fan in a drying cabinet (reccommended), a hair dryer at close range, or hot plate on low for 1-2 minutes (180-200ºF). Larger plates will require slightly longer heating times than smaller plates. Example: an 8x10" plate will require 5-6 minutes with a hair dryer on high heat at close range. Prop plate up on spacers so table will not absorb all the heat. With a drying cabinet and electric space heater fan heating times can range from 5-15 minutes depending on plate size and wattage. Rotate plate for even heating. Overheating with any method will cause small bubbles and pinholes to form. FYI The photo circuit board industry wet laminates the film with a hot roll laminator at 250ºF at 2-5 feet/minute and the plate exiting the laminator at around 150-160ºF. Ask us about laminators..
You should test for exposure times with a step scale made in the same manner of your halftone positives and expose it for various times on one plate. If you have a 21 step Stouffer transmission scale, the last visible step after developing should be somewhere between step 6-9, use #6 for higher resolution positives. A vacuum frame is reccommended for any high resolution image. Place halftone or art wirh ink or emulsion side to the plate in vacuum frame. Expose plate with a UV light source (a clear point light- metal halide bulb, arc, or sun - not flourecents). The distance of the light source shoud be at least 1.5 x the diagonal of the plate. Example: The NuArc N1000 with 1000 watt Mercury Vapor (light is 2’ from frame) will expose a Stouffer at step #6 in about 5 seconds. Puretch is a very fast exposing photopolymer.
Note: It is best to establish exposure time first with the Stouffer then expose halftone step test and etch that for best etch times. If you don't have a Stouffer, etching and printing of the halftone step test is reccommended to be absolutely sure of exposure. If small dots are not fully developing out and etching, the exposure is probably too long..
The developer is an aqueous solution of 1% sodium carbonate. Weigh 10g of soda ash and dissolve in a small amount of hot water, then add room temperature water to make 1 Litre. OR using a liquid medicine measurer, 1 1/8 fluid oz. of powder will make 1 gallon of developer. A tub of 100% sodium carbonate or soda ash can be bought at a swimming pool supply store very cheaply..
Spray development (at 30 psi.) is the industrial method and yields sharper edges of resist than tray developing. The sprayer strips unexposed resist easier without abrading exposed resist. Fill sprayer with developer and pump to get full pressure. Set the nozzle to a smooth fan pattern. In a room with a bug light, carefully peel the protective mylar from one corner of plate, using a piece of tape to lift the corner. Make sure the mylar lifts without the resist. In one swift move, peel sheet from plate. If the plate was cleaned properly and heated to remove all moisture, it should remove easily, leaving the resist in place. If it does not, it may need longer heating times or humidity in shop may be very high. For large plates prop plate up in sink with a plexglass "backsplash", for smaller plates, lay flat in sink. Spray plate immediately after exposure, moving fan over entire image. You may spray at 5-10 second intervals, spraying at least half the total time. The image should develop at 60% of the total development time and the remaing 40% of time cleans the unexposed copper (about 50-60 seconds total). Rinse well (with a sprayer is preferred) with cool tap water while gently wiping residue from plate with hand or sponge. If tap water is soft (alkaline), rinse plate, then spray plate with a solution of water and distilled vinegar, 3:1 (to harden water and halt development) then rinse well. Most water is hard so this is usually not necessary. Dry with high pressure air (reccomended) or gently wipe dry with paper towels or dry and also blow dry with hot hair dryer. Advantages of the spray method are the developer is always fresh and ready, development is very visible and plates can be developed without having to deal with bulky trays. Different sprayers may vary, higher pressure sprayers will develop quicker, these times here work with small, inexpensive garden sprayers like the one pictured.
In a room with a bug light, pour room temperature developing solution in a tray. Carefully peel the protective mylar from one corner of plate, using a piece of tape to lift the corner. Make sure the mylar lifts without the resist. In one swift move, peel sheet from plate. If the plate was cleaned properly and heated to remove all moisture, it should remove easily, leaving the resist in place. If it does not, it may need longer heating times or humidity in shop may be very high. Place in developer immediately after exposure. Periodically and gently wipe entire plate with a soft nylon sponge to aid in removing unexposed resist. Plate should appear visually developed at about 35 seconds, develop 15-25 more seconds (about 50-60 seconds total), agitating tray, without sponge to remove resist residue. Rinse well (with a sprayer is preferred) with cool tap water while gently wiping residue from plate with hand or sponge. If tap water is soft (alkaline), rinse plate, then spray plate with a solution of water and distilled vinegar, 3:1 (to harden water and halt development) then rinse well. Most water is hard so this is usually not necessary. Dry with high pressure air (reccomended) or gently wipe dry with paper towels or dry and also blow dry with hot hair dryer.
developing tip: Spray development, spray rinse, and high pressure air drying is the preferred method especially for 1440 dpi greyscales or anything very fine. Skip using your hand, sponge or paper towels, this way nothing mechanical touches the plate and it prevents unhardened resist from being wiped into and clogging very small holes in the resist. It is the best way to completely rinse unexposed resist. See this page for more info
Harden the resist in sunlight or UV source until the resist turns a darker blue-purple. The plate is now ready for conventional etching in ferric chloride or other etchants. Puretch is durable enough to proof image at least once and continue etching, use care when inking the plate for this. Keep in mind, printing with the resist on will hold slightly more ink than with the resist stripped and will yield a contrastier image. .
Exposed resist can be stripped in a tray with a stronger mix of soda ash:water, 100grams:1Litre. Household oven cleaner is a great quick stripper that is non-solvent, not non-toxic. Oven cleaner contains lye, read all labels before using and do not use on aluminum. Use all materials at your own risk..
1. marking Homasote board for plate size.