Puretch™Processing Instructions by Cape Fear Press
Please read all directions because they differ from other photopolymer processing methods. Processing Puretch is easy and simple. By following all of the steps and tips given here you should achieve foolproof results. Instructions were updated Feb 2011.
Puretch™ is the thinnest photopolymer film available. It is a 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.
PREPARING THE PLATE Plate cleaning and lamination need to occur back-to-back so prepare to laminate right away. Rubber gloves are recommended for polishing and degreasing. A clean plate is VERY important for adhesion and all traces of cleaners must be removed. Pour distilled water into a clean photo tray at least 1/4 inch deep. Mix up a medium strong solution of TSP and water into a wide mouth plastic tub or glass jar. Mechanically clean the entire plate with a metal polish and leave the dirty polish in place. Dip your nylon brush in the TSP solution and work the polish loose on the plate. Also make sure to wash off any polish that creeps around to the back of the plate. Rinse and continue to wash the plate surface with the TSP brush until the plate is fully degreased and the front rinses with a solid sheet of water and does not bead up. Thouroughly rinse plate (front and back) in cold tap water, wipe off excess water with a paper towel (do not let it dry) and immediately place clean plate in the tray of distilled water. Laminate resist immediately to avoid oxidation of the plate. Plates can still oxidize in water if left for some time. If laminating several plates at a time, refresh the distilled water for every 2-3 plates to ensure cleanliness. Note: It is not necessary to abrade plates with sandpaper for good adhesion as is recommended for other photopolymers. The higher the polish, the better the resolution and print quality will be.
2. The film is sanwiched between two layers of protective mylar. Remove the dull mylar by scraping at the surface near a corner with one of the tacks. Re-tack that corner and with the film still tacked down, peel the liner away, tearing it out from under the tacks and discard it.
3. Remove plate from tray and place wet plate on the table with some of the plate hanging off the edge of the table. Remove the tacks holding the film nearest to you and grab those corners of the film. Pull film out from under the two other tacks. Bring film over to plate and with the plate hanging off the edge of table, 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. Wet your squeegee to lubricate it or sprinkle film surface with a little water and squeegee all the water 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. 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.
PLATE DRYING Your relative humidity in the darkroom should be 60% or less. Plates can be dried overnight and in the dark without any heating. Do not cover the laminated plates with anything so they will receive good ventilation. For quicker processing the film can be evenly heated to get a good adhesion and remove the moisture from the film. Copper quickly conducts heat so you can do this any number of ways. You can use a space heater fan in a drying cabinet (recommended), 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. It's easier to simply dry large plates overnight without heating. 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.
EXPOSURE 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 with the 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 #7 in about 13 seconds. Puretch is a very fast exposing photopolymer.
DEVELOPER 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. 1 & 3/4 level teaspoons using quality kitchen measuring spoons also equals 10 grams.
SPRAY DEVELOPMENT Spray development (at 30 psi.) is the industrial method and yields sharper edges of resist than tray developing. A breathing mask is recommended with a high pressure sprayer to avoid breathing developer and undeveloped resist. 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 and dried properly, 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 too high. Relative humidity should ideally be 60% or less and problems begin occurring at around 70%+ humidity. 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. 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 tap water is hard so this is usually not necessary. Quickly blot plate with paper towels 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.
TRAY DEVELOPMENT 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 and dried properly 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 too high. Relative humidity should ideally be 60% or less and problems begin occurring at around 70%+ humidity. Place plate in developer immediately after exposure. Evenly and gently brush entire plate with a soft natural bristle brush to aid in removing unexposed resist. Plate should appear visually developed at about 35 seconds, develop 15-25 more seconds (about 60 seconds total), agitating with brush to remove resist residue. Rinse well with cool tap water while gently brushing residue from plate with the brush. 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 tap water is hard so this is usually not necessary. Quickly blot plate with paper towels and also blow dry with hot hair dryer.
CURING 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. Etch as soon as possible to prevent plate oxidation. Puretch is durable enough to proof image at least once and continue etching, use care when inking the plate for this and water based etching inks work best for proofing the plate at this time since they clean up with just soap and water. Proofing with oil based ink will be more difficult to fully clean so you can continue with a clean etch. Keep in mind, printing with the resist still on will hold slightly more ink than with the resist stripped and will yield a contrastier image than with it stripped.
ETCHING Just prior to etching, briefly pre-wet plate in Puretch devloper in a tray or wet a paper towel with some developer and wipe over the entire developed plate. Rinse the developer and shake off excess water. Immediately submerge plate in etchant and agitate with feather. This will cause the etchant to begin biting evenly.
STRIPPING Exposed resist can be stripped in a tray with a stronger mix of soda ash:water or TSP, Tri Sodium Phosphate. Read all labels and wear gloves.
SAFETY Wear gloves when degreasing, developing and stripping Puretch and wear a dust mask when using a sprayer. Puretch is a very safe resist when used properly. Use all materials at your own risk.