Care and Use:
Photopolymer plates are very sensitive to UV light until after exposure, development, and re-exposure. Unexposed plates should be covered with a light blocking mask.
KM 43 plates have a thinner emulsion and expose more quickly than Solarplates or the new KM 83 plates.
For future reference it is helpful to make a guide of approximate times for each process in regards to specific films and drawing materials. The last two pages of this manual has a chart for this purpose.
When exposing a separation to the plate always place the separation face down, with the drawing materials or printed ink in direct contact with the emulsion of the plate. If the separation is placed face up, the print will be the reverse of how it was made on the separation.
Clean the exposure unit. Make sure the glass is completely clean and free of scratches, fingerprints, or foreign material. Use glass cleaner and lint free paper towels. Scratches and objects in the glass may interfere and appear in the plate and print.
Make sure separations are free of lint, dust, hair or foreign material.
Exposure unit vs. Solar exposure
An exposure unit with a vacuum frame will give the most accurate, and repeatable results. Ideally, the exposure unit should have a single-point light source. Units with tube lights or multiple light sources may undercut the film and cause definition loss. This is especially detrimental in relief printing. KM plates should be exposed using an exposure unit.
In lieu of a vacuum frame and exposure unit, Solarplates and ?KM83? can be exposed in the sun. Exposing in the sun depends on the climate of the region, weather of the day, and the limitations of daylight hours. A heavy piece of plate glass is acceptable for good film contact. Keep emulsion covered with a light blocking mask before and after exposure until the plate can be developed. Expose in an open area away from objects that may reflect light.
Determining the exposure time
There are many variables in producing a quality photopolymer plate. These include: light source, exposure time, bulb temperature and age, film contact, developer temperature, film transparency, and opacity of drawing material for both digital and hand-drawn separations. A change in any of these variables affect the end product.
Test plates and prints are highly recommended to find the most accurate exposure time, especially for the intaglio process.
Inspect the separation being exposed. Hold it up to the light or check it on a light table.
Identify areas that are drawn with the least opaque material or need to retain the most detail. It is likely not all information will be retained and an average exposure may need to be found.
Cut a long test strip of plate (see Section I C).
Identify parts of the film that include the widest range of information, preferably including a range of materials, flats and delicate marks. Use this section when making a step wedge test or test strip.
Step wedge tests
A step wedge test is an array of exposure times on a strip of plate. It may be done in increasing increments for a broad range, or small increments for a narrow range.
Example of step wedge exposure ranges:
Broad range in seconds, 8 steps for a total of 360 seconds or 6 minute range.
10 20 30 40 50 60 70 80
Narrow range in seconds, 9 steps for a total of 120 seconds or 2 minute range.
5 5 10 10 15 15 15 15 15 15
Start off with a broad range to achieve a rough estimate of where to begin, especially when using Solarplates or KM83 for relief purpose, as the exposure times may be long. Use these results to narrow the range until a correct exposure time is achieved. It may be necessary to perform this test several times.
It is important to keep track of the cumulative increments in relation to the plate.
Alternatively, a single exposure time can be exposed to a small test strip. This strip is exposed, developed, and subsequent strip exposures increased or decreased based on the results. This method allows for the same section of drawing material to be exposed and analyzed.
The following covers the procedure for making a step wedge test on an exposure unit for both the intaglio and relief processes.
Step wedge test: intaglio process
Exposing a photopolymer plate for the intaglio process requires a two-part exposure: one for the positive separation (either digital or hand-drawn) and one for the aquatint screen, which adds texture to large flat areas and tone. First find the correct exposure time using the positive separation.
Take care to not damage the finished separation, especially if using hand-drawn materials. It may be beneficial to make a separate small film with similar marks to test.
If using KM plates, remove the protective cover at this point.
Proceed only in safe light conditions.
Place photopolymer plate face up in the vacuum frame. Place the positive separation face down in direct contact with the emulsion.
With everything in place turn on the vacuum. The film should be smooth and flat against the plate without any bubbles or wrinkles. 20 in Hg or 10 psi is adequate for good contact.
A vacuum of approximately 20 in Hg (10 psi) is generally adequate.
Having good contact is crucial.
Place a mask on top of the glass over the plate area. The mask can be anything that blocks UV light (rubylith, goldenrod, black paper, a sheet of cardboard, etc.). Increments can be marked or estimated. A dark room clock, watch, or stopwatch will be necessary. Set the timer and begin the exposure.
Shift the mask over as the increments dictate. The mask allows control over the amount of UV sections of the plate receive. Make sure that each section has some material being exposed. If exposing on an exposure unit set the timer, expose, shift the mask and repeat as necessary.
When the exposure is finished, develop the plate (see Section V). The emulsion will still be soft so be careful when handling.
Keep in mind that results are cumulative. Refer to the previous charts for the broad and narrow step wedge range to calculate times.
Choose the correct exposure time or find the value range that appears to be the closest representation of the film. If necessary, repeat the step wedge test in a smaller range of exposure times closer to this recently determined value.
Once a correct exposure time is found, another test with the addition of the aquatint screen should be conducted.
If the separation has a range of lights and darks, expose the aquatint screen for the same amount of time as the positive plate.
If the separation has mostly light or delicate marks, cut the aquatint exposure time in half.
If the separation has large areas of black or flats, increase the aquatint screen exposure time to one and a half times the exposure of the positive separation.
Place the aquatint screen face down against the emulsion of the plate.
The aquatint screen can be exposed either before or after the separation.
On a new strip of plate, place the positive separation face down on the emulsion and expose for the correct time. Remove the separation and place the aquatint screen face down (matte side against emulsion). Expose a second time using the notes above as a guide.
When the exposure is finished, develop the plate (see Section V).
The only way to accurately asses the plate is to print it in black ink and compare the printed results to the separation (see Section VI).
Further adjustments may be necessary:
If too much polymer washes away and the image appears too dark lengthen the exposure time.
If not enough polymer washes away and the image appears too light shorten the exposure time.
Once the small test print appears satisfactory expose and develop a full size plate.
Step wedge test: relief process
Solarplates are preferred for the relief process because of their thicker emulsion. KM plates may also be used, but printing will be more difficult.
Detail of a relief plate in both KM (upper) and Solarplate (lower).
Finding the correct exposure for the relief process is similar to the intaglio process, except for a few minor differences: a negative separation is used, no aquatint screen is needed, and the developing time is longer.
A negative separation should look like the inverse of the desired print. Keep in mind the negative separation results in a positive printed image for the relief process.
Example of a negative separation.
Example of the printed result from the above negative separation.
When exposing the negative separation, the materials need to be dense and opaque. If there are pinholes or the materials aren’t opaque enough UV light will pass through and harden areas that shouldn’t print.
To determine the correct exposure, refer to the broad step wedge exposure range (see Section IV D). The exposure for the relief process can be quite long, assuming the materials are opaque, so a broad range is best to start with.
Follow the same steps for the intaglio step wedge test steps 1 to 7. An aquatint screen is not necessary. Print the test strip for accurate results. A variety of times will likely work well, the main difference is in the developing time, covered in the next section.
Once the small test print appears satisfactory expose and develop a full size plate.