Photopolymer bases provide a foundation to make thin photopolymer plates type high. When mounted with the proper plate material, they should print like any other letterpress form. That said, there are some special considerations when registering the base depending on what type of press you use.
Lockup and Registration
Bases are locked up like any letterpress form, with quoin keys, furniture, leading, and/or reglets. The printable area is the full size of the base, so in other words you could conceivably print a full 9 inch by 12 inch image on a 9 inch by 12 inch base. Try to avoid setting the quoins too tightly, because bases have a tendency to buckle upwards, and don’t typically need much pressure to stay put; bases don’t usually “work up” as you might experience with handset type. There are some other lockup considerations based on which type of press you use, cylinder or platen.
Cylinder Press Lockup and Registration
On cylinder presses, you can use a base that almost fills up the press bed. You will want to leave room for quoins to hold the base in place, so with a 14×20 Vandercook press bed you could effectively use a 13×19 base.
Platen Press Lockup and Registration
On most platen presses, you’ll typically want your base quite a bit smaller than the chase. The main reason is that larger bases restrict you from positioning gauge pins easily. Additionally, platen presses (other than Heidelberg Windmills) are not able to effectively print forms more than half the size of their chase size. For instance, a 10×15 Chandler and Price struggles with typical forms larger than 5 in x 7.5 in; the impression as well as image saturation tend to be more uneven the larger the form. So often a 6 in x 9 in area works well for a 10×15 C&P, as it allows more flexibility in positioning gauge pins. It’s possible to use a larger base on a C&P (and in fact many printers do), but if this size is not necessary, a smaller base gives more flexibility. It is vitally important to make sure your gauge pins are well clear of the base to avoid smashing them or your base when the press closes to print.
Platen presses have a “sweet spot” in the bottom center of their platen where the impression is heaviest. You will want to position your artwork on your base so that any artwork with heavy coverage or extra heavy impression is in the bottom middle.
Heidelberg Windmill Lockup and Registration
Although Heidelberg Windmills are platen presses, they don’t have the same constraints as other platen presses because of their unique design. The lay gauges and grippers on Windmills don’t interfere with the placement of the base, and the press is built to effectively print larger areas than other platen presses. A 10×15 Windmill easily accommodates a 9×12 base and a 13×18 chase accommodates a 12×16 base.
Photopolymer bases work equally well with gripper registration and with lay gauge registration. Bear in mind that if a lay gauge pin hits the surface of the plate, it can mark the stock that you’re printing on.
Unless you have a very tiny base, don’t plan on running with a frisket. The arms of the frisket are thicker than photopolymer plates and will smash the base.
Affixing your plate on a smooth, even surface is key to achieve quality printing. Keep your base smooth and clean, and avoid cutting on or otherwise scoring or denting your base. Clean with type wash or other quick-evaporating solvent to prevent ink buildup.
Magnetic Base Usage
Although magnetic bases are designed for steel-backed photopolymer plates, it is possible to print plastic-backed photopolymer in some circumstances. You will need to make sure that the combined height of the base, the plate, the adhesive, and any underlay material is type high. When using a base with inlaid magnets, attach the plastic-backed plate to the magnetic surface of the base. When using a base with a magnetic sheet surface, you should flip the base over so that the magnetic surface is attached to the press bed; this allows you to use the harder metal surface on the bottom of the base to mount the plate. Don’t ever flip over a magnetic base with inlaid magnets or you will have a very hard time removing it from the press bed.
Non-Magnetic Base Usage
Although these aluminum bases are designed specifically for photopolymer plates, it is possible to tape down a steel-backed plate to the surface for printing. You will need to make sure that the combined height of the base, the plate, the adhesive, and any underlay material is type high. Tape the edges down with Scotch tape; the typical photopolymer adhesive will be too aggressive and will not remove easily.