Calibrating a manual screen-printing press is easy; the tough part is in motivating a person to calibrate said press. They think: “Don’t try to high-tech it, after all it’s a manual… we’ll just make another pass or flash every color.” A poorly-calibrated press is a problem but this blasé attitude about slowing production is the greater issue. Here, we’ll deal with both pathologies but, before you casually waste another minute, consider the cost of your time per minute.
The process of calibration will take between one and four hours to get it right. If your floor is level, the press is robust and was leveled in position, all platens seem to print the same and there is no best head for any one color, you may not need to at all or it will only take about an hour. Conversely, if those factors are only a dream, you may need a sidekick for half a shift to get the press to print properly. At any rate, please read the entire article before you begin the press calibration process.
The hardware store will have a small torpedo level which will be used throughout the calibration process. A magnetic level with a "v" cut into one edge or one side will most accurately gauge the plumb of the center shaft of the manual press. (All images courtesy HIX Corporation)
What to purchase?
Go to a hardware store for a 4 inch-long magnetic level with a V-block on one side to plumb the center shaft of your press. A post-level will be necessary to level the platens and screens on the press. You’ll also need a 24 inch bubble level and a wrench or two to adjust the elevator bolts which sit between the chassis of the press and the shop floor. The wrenches will also come in handy in adjusting the platens and screens to ensure each are on the right printing plane.
Find the flattest head on the press by following step 3. This will establish what will be used as platen #1.
1. Find/build a flat screen
Find a flat, stretched screen (without an image); check the underside of the frame and mesh to be sure there is no tape or debris of any sort on the surfaces. Then, gently place this frame on the glass of your exposure unit. See if the frame is flat by checking all four corners to see if any rock on the glass. When you find (or build) a flat frame, set it aside for later.
2. Plumb the center shaft
Put the 4 inch-long magnetic level on the center shaft and use the back rings to see if the center shaft of the press is vertically level. Check the shaft at positions 12, three, six and nine o’clock in order to verify your findings. If the backside rings on the level say you’re good to go on all positions, you can move on to the platens. If not, adjust the elevator bolts that seat the base of the press on the shop floor until the center shaft reads “plumb” (vertically level). Secure the elevator bolts.
3. Designate platen #1
Find the flattest platen on the press by laying the 24 inch bubble level on an edge on top of the platen. Look for light leaks between the edge of the level and the surface of the platen from corner to corner.
Place either the V-block or magnetic side of a torpedo level in contact with the center shaft of the press. It should read as level on all sides when the level is held in the vertical direction. Adjustments to plumb are made against the floor with leveling bolts and lock nuts on the base of the press.
If the level is stable on the platen, does not rock and there is a light leak at the center of the platen, this indicates the platen has a “dish” and should not be used for press calibration.
If the level is not stable on the platen and rocks like a see-saw or if there are light leaks at the ends of the edge of the level, this indicates the platen is “crowned” and should not be used for press calibration.
If the level is stable on the platen and there are no light leaks between the edge and the surface of the platen, this indicates it is flat in the direction measured. Check again from the opposite corners. If the level is firm without light leaks in the other direction, mark this platen #1 and use it only for press calibration.
4. Fine-tune for plumb
Place the post level on the center of platen #1 (that which was selected for calibration). This bubble level gauges front-to-rear and side-to-side at the same time. Whatever rotational position this platen is in, we’ll call 12 o’clock—put a mark on the chassis of the press to where, once you spin the platen, you will be able to return it to this spot again.
5. Level platen #1
By looking at the front-to-rear bubble, observe whether the platen is level front-to-rear. If not, level it by adjusting the bolts on the underside of the platen. But first, look at the side-to-side bubble. If the platen is not level side-to-side, it will need to be leveled as well.
It is critical to level platens side-to-side to avoid the operator compensating by “pushing a bit harder on the left side of board number x.” If this squeegee-pressure correction sounds familiar, be extra careful with this step.
6. Check platen to chassis
Use your wrenches to barely loosen the bolts securing the platen to the chassis. Look at the level to pick which direction—either front-to-rear or side-to-side—is the least level. Adjust it until it is close to level, then adjust in the other direction until it is also close to level. Go back to the first direction and fine-tune, then the opposite and fine-tune until the platen is level in both front-to-rear and side-to-side directions. Retighten the blots in order, bit by bit, around the platen. Snug the first one, then the second, then the third (and fourth if available). Go around again until all bolts that secure the platen are secured.
If more than one operator is printing inconsistently on a press, you may have conjured up a theory about one being taller to explain irregular prints. While you may be right, before investing in a soap box for the vertically-challenged operator, see if the platens are level front-to-rear.
7. Center shaft check
Once the platen is level you will want to rotate it from its 12 o’clock position to three o’clock and check again for front-to-rear level, taking care to use the backside rings every time. If the backside rings say okay for front-to-rear, you’re good-to-go. If not, find your magnetic level because the center shaft is not quite plumb.
8. Partner up; tighten elevator bolts
You’ll need a partner to complete this step. One person will go below the press to loosen and adjust the elevator bolts. The second party stays up top to check the level as platen #1 is spun from 12 to three o’clock, and back and again, etc. Once platen #1 is shown to be level in the front-to-rear and side-to-side directions you can quick-check the platen at 12, three, six and nine o’clock. Finally, tighten the elevator bolts for the last time.
9. All platens on the same plane
Spin platen #1 back to 12 o’clock. Lay the 24 inch level on its face on top of the platen, running front-to-rear and centered side-to-side. Move your flash cure unit over the top of the level and adjust its height until it rests gently on top of the level. Adjust the flash so it gently touches the top face of the level in both front-to-rear and side-to-side directions.
10. Designate platen #2
Check the flatness of platen #2 (the next platen) as you did the first platen in step three. If this platen is flat as prescribed you can proceed. If it is not flat, remove platen #1 and put it on position two. In either case, we will refer to this platen in the following steps as #2.
Some higher-end presses have adjustable beams to hold the platen that are typically preset at the factory. If not, be sure to level these support arms before dealing with the platens.
11. Adjust head #2
Slide the level out from between the flash and platen and spin the platens to align platen #2 under the flash unit. See if you can fit the level without forcing it between the platen and the flash like a feeler gauge. If it fits, observe to see how much clearance exists between the top face of the level and the underside of the flash. This is the distance you will need to raise platen #2 so it ends up on the same plane as platen #1. If the level does not gently fit between platen and flash, platen #2 will need to be lowered.
Digital levels are sexy and worth the investment, but if you learn how to accurately read a bubble level, they are virtually just as accurate. See “Reading a Bubble Level” (above) for instructions.
12. Raise/lower platen #2
Spin platen #2 out from under the flash and center the post level on its surface. Now comes the tricky part—adjust to level either by raising or lowering the platen. If there was clearance between the flash and the face of the level, #2 platen should be raised as it is leveled. If the level won’t fit between the flash and the platen, platen #2 should be lowered while being leveling.
13. Finish platen #2, repeat for others
Alternatively check level and distance by spinning platen #2 either in or under from the flash, using the 24 inch level on its face and the post level centered on the platen’s surface. Once the platen #2 is leveled and the feeler gauge level fits gently between the platen and the flash with the same fit as with platen #1, platen #2 is done. Repeat this by alternating; leveling, raising or lowering for all other platens.
14. Adjusting the screens to level
Get the screen frame from step one and clamp it in any head position, marking this head as screen #1 (s#1). Bring platen #1 to its home position at 12 o’ clock. Spin screen s#1 over the top of platen #1 and lower the screen into print position.
Just as platens need to be level side-to-side, each of the screen holders must be parallel side-to-side and front-to-rear to each of the platens.
15. Create off-contact distance
The press should be adjusted so there is a slight gap of 1/8 inch between the screen s#1 in printing position and the #1 platen surface for screens with tension levels above 25 N/cm². For screens with tension levels below 25 N/cm², the gap should be 3/16 inch. You can use coins or washers stacked atop each other until they are 1/8 inch (or 3/16 inch) thick.
After all heads are level side-to-side, platen #1 should be leveled front-to-rear, then continue in succession with all other heads.
16. Level off-contact
Lift the screen s#1 and place the washers on the center of platen #1. Then, gently lower the screen into position atop the washers and put the post level on the center of screen s#1. The goal is for screen s#1 to be level at the same time it gently rests on the top of the coins/washers. Observe the top of the screen as it is lowered onto the washers to be sure they do not protrude through the top surface of the screen mesh.
17. Finish screen s#1; repeat for other heads
Alternately adjust level front-to-rear and side-to-side while maintaining contact between the underside of the screen mesh and the washers. Once screen s#1 is level in both directions while it is resting gently on the washers, it is finished. Repeat this process using the same screen frame with all other heads.
18. Don’t forget the flash
Reset the position of your flash according to the manufacturer’s instructions. Bear in mind that, if your flash is a solid panel, it will need to be parallel about 1/2 inch from the platens at the lowest temperature you can deal with. If you have lamps, don’t put the flash closer than the centers of the lamp placements. For example, if the lamps are on 2 inch centers, the flash shouldn’t be closer than 2 inches. Make certain the flash remains level anywhere on the floor.
Off-contact distance is critical and usually under estimated. It can only be established once all platens are on a single plane and all heads are on a parallel plane above the platens. At this time, use the specs in step 16 to establish the optimal distance and use shim stock to adjust for a high-or low-tensioned screen.
In addition to the torpedo level shown here, using three bubble levels will make the calibration process shorter and more accurate. The three are the torpedo level with a V-block or magnetic side, the post level for simultaneous, biaxial measurements and the 24” shop level for testing platen flatness.
Done and done
At this point the center shaft is plumb, and all of the platens are level and on the same plane. In addition, all of the screens are level when they are in print position and the flash is level to the platens. When this is the case, the press will print with less pressure, more consistently from platen-to-platen and with less deflection.
If production requires a screen of unusually-low tension, it may help to put a shim stock between the shelf of the screen holder and the screen frame in order to raise its off-contact distance so all screens print similarly and registration will be facilitated.
Yes, this is a time-consuming process. But, whatever time is invested in calibrating a press will be recouped in the first few days as it speeds the print process. Presses should only need to be calibrated once in a while, but check parallelism at the end of the first week, then again at the end of the first, second and third months. At that time, it may make sense to tune up the calibration. But, it will take far less time this time around and will keep the press running at profitable speeds.
Reading a Bubble Level
Digital levels have earned popularity not because they are inherently more accurate than bubble levels, but because the average user does not know how to use accurately use the latter. Here’s how:
• Lay the magnetic level on any platen to where the bubble is visible.
• Now move until your eyes are on the same plane as the height of the level and perpendicular, directly in front of the level.
• The bubble (hopefully) is somewhere near the two black rings.
• Look again at the rings with both eyes open but shift your focus to the rings on the backside of the ampoule. The shift in focus will make you far more discriminating, when the bubble is between the two rings on the backside of the ampoule, it is level.