Puretch Processing Instructions

Instructions were originally written for artists using the film for photo engraving prints but changes have been made for PCBs, illustrations show large copper plates for etching prints.

Processing Puretch is easy and simple. By following all of the steps and tips given here you should achieve foolproof results.

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 for etching.

Materials needed:
• yellow light bulb (bug light)
• metal polish
• degreaser: like RightStuff
• 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
• heating device: hair dryer or electric blower space heater
• clock with second hand

PREPARING THE BOARD A clean surface is VERY important for adhesion and all traces of cleaners or brighteners must be removed. If copper is oxidised, clean with a metal polish. Mechanically clean and degrease the plate with a cleaner such as RightStuff until the plate rinses grease free with a sheet of water. Thouroughly rinse this in tap water and dry with hair dryer. Laminate resist immediately to avoid oxidation of the plate.

LAMINATION
1. In a room lit with a yellow bug light, cut a piece of film larger than the plate or image. 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.

2. 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.

3. 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. 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.

4. Heating the film evenly is the best way 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 or a hair dryer at close range (180-200ºF). A board 8x10" will take approximately 5 minutes with a hair dryer on high heat at close range. 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.

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, 7 is the target. 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 .

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. A tub of 100% sodium carbonate or soda ash can be bought at a swimming pool supply store very cheaply.

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 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. Humidity should be less than 70%. Place in developer 1 minue 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.

CURING Harden the resist in sunlight or UV source until the resist turns a darker blue-purple. Re-heat the board surface with the hair dryer, just like drying the resist in the previous step. The board is now ready for conventional etching in ferric chloride or other etchants.

STRIPPING Exposed resist can be stripped in a tray with a stronger mix of soda ash:water, 100grams:1Litre. RightStuff will also remove resist. Use all materials at your own risk.

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