To print high quality images and be as productive as possible, our equipment has to be in peak form. Screen printing equipment has special requirements.
I print caps, and the first maintenance items I look at are the screen and squeegee. The condition of these two items will decline faster than other aspects of printing equipment that I also monitor. Some of these other aspects will surprise anyone who prints, regardless of whether you print caps, shirts, sweats, signs or other items, but first let's look at cap screens and squeegees.
Monitor Touch Screen
Screen
Cap screens are different from shirt screens, because one side must be a thin bar to print the image close to the bill of the cap. The image might be as close as 1/16" to the inside of the thin bar rather than out in the center of the screen like a T-shirt screen.
When the tension on a screen is minimal, the mesh might shift out of registration during printing and the ink probably will be driven down into the fabric. The color of the fabric can then be seen. So a flash cure and second print are required. That double work can be avoided with tight screens and by taking other steps.
Low tension screens cause ink to build up under the screen. Then the screens need to be cleaned periodically during a print run. The tension will be even lower close to the thin side of the frame, which is the image area, than in other areas of the screen. A tip off to this low tension at the bottom of the image area is a greater amount of ink being deposited.
One of the most important steps to producing top quality images and being productive is using high tension screens. Tension is more important for cap screens than shirt screens, because materials used to make caps, particularly foam front caps, are softer than T-shirts, signs and many other items
Stretch and glue screens, that is, those where the mesh is glued to the frame, probably will need to be re-stretched and glued before a job of any size. A retensionable frame can be used to tighten the mesh without removing the mesh. An aluminum tubular frame will hold more tension than wood.
If the frame is not retensionable, then the thin bar should be steel rather than aluminum, or worse, wood, Tension can be increased by deflecting the thin bar side of the screen in towards the center of the screen before attaching the mesh. Steel should we wiped down with mineral spirits or alcohol to remove the oil so super glue will hold the mesh under tension even through the bar is only 1/8" thick. Once the glue is dry on all four sides, then mesh can be laid over the flat side of the thin bar and glued to the flat side with super glue. (possible exhibit is photo showing deflection of steel bar on cap frame with quick clamp)
Getting mesh tension to a level just short of breaking the mesh is a critical maintenance step to avoid problems that affect quality and productivity. The screen and squeegee are like two blades of scissors that must be sharp to cut ink off and deposit that ink on the surface of a cap.
Squeegee
Cap squeegees are also uniquely different from shirt squeegees. Typically only 4" long, cap squeegee blades will deflect more than shirt squeegee blades, and particularly at the corners. When the image is being printed by a soft corner of the squeegee blade a different quantity of ink will be deposited than by the center of the blade.
Selecting a stiffer blade or triple durometer blade like 75-95-75 rather than the 70 durometer blade frequently used for shirts will promote quality and productivity. Changing the blade in the handle might be the best option. Selecting a squeegee with an aluminum handle with bolts through the handle, but not the blade, or wood handle with the blade glued into the handle will provide better support to the blade than a wood handle with bolts through the blade. Bolts that go through a blade are pressure points that will warp a blade over time.
A squeegee should be stood straight up on a flat surface like glass to see if the blade is straight. If there is light under the blade, then the blade will need to be sharpened. If there are nicks in the blade, or the cutting edge is not square and sharp, then the blade should be sharpened. Rounded edges will push ink through the mesh rather than cut the ink off so ink can be laid on the surface of the fabric. Once a blade is sharp, the square edges should be polished with a homemade polishing device before each job to keep the edges sharp. (possible exhibit)
When printing has been completed, ink should be wiped off immediately. If ink is left on the blade for days, the blades over time will become hard and the material will chip. If chemicals are used on the blade, the blade should be flushed with water before storing the squeegee. Squeegees should be stored without resting on the blade, because that would warp the blade.
Level
There are two parts to level: the screen and the platen. A pitch between the two could create a parallax and distorted image. A pitch also may leave more ink in some areas of the printed image than others.
Platens can be checked by rotating them under an independent stationary object. We use a flash dryer that is not plugged in. We tape paper to the flash dryer so we can see if all the platens are at the same elevation and just touching the edge of the paper when the platen is passing under the paper. This test also assures the person printing that the flash dryer will be at a uniform off-contact distance.
To check the level of the screen, we put a piece of plexiglass 1/8" thick and larger than the platen on the platen. We check the screen against glass, which we assume is absolutely flat, and then put the screen in the press. The mesh must be flush against the plexiglass while the color arm of the press is resting on the off-contact bolt, or we have a screen problem. Once we pass this test, we can register on-contact with the positive on top of the plexiglass for best quality color registration, but then remove the plexiglass for a uniform off-contact printing condition.(possible exhibit)
Flexing
Push down on the platen. If the platen is not as solid as a concrete floor, the platen should be supported to prevent color registration problems. Then try to move the channel clamp that holds the screen laterally. If the two registration points on the sides of the color arm are snug against the color arm, then look for movement of the color arm towards the center of the press to correct. Try to rotate the channel clamp. Any rotation must be addressed for repeatability of print quality.
Rotation
On multi-color machines a bearing of some kind is being used. Bearings to work properly must be loaded, that is, have pressure on them. So if the screens do not rotate freely, then there may be too much pressure, or there could be foreign material in the bearing. Spray adhesive and lint can be the culprit. We do not use spray adhesive, and so we don't get accumulations of lint. A bearing can be cleaned with WD-40 or similar lubricant.
Springs & Seals
A spring where the coils all touch adjoining coils passes visual inspection. If the coils do not touch, the spring has been over extended, and the only option is replacement. The manufacturer should have designed the press so over extension is not a possibility.
The hidden risk with springs is that they were heat treated by the spring manufacturer. Heat treating makes springs brittle and subject to fracture, and therefore dangerous. If the springs have been heat treated, cover the springs or tie the coils together to prevent bits of steel flying through the air at high velocity.
Some presses use air cylinders like used with some hatch back cars and exposure units. There is a seal where the piston rod enters the housing, and over time the seal wears. Then the air cylinder will not support the weight of the screen and color arm. When the seal is worn, the air cylinder needs to be replaced.
Platen Size
I get a lot of telephone calls about this problem. Caps are constructed in a wide variety of sizes just as different size shirts can be purchased. When we print youth shirts, we don't use a full size adult platen, because we know that would stretch the fabric out of shape causing the image to appear distorted when the shirt is removed from the platen.
Over the years cap styles have moved from the large foam fronts to the low profile that fits snugly against the head. Old cap press equipment might be designed for the larger caps. More frequently, the problem is the cap. A constructed or fused buckram cap is intended for embroidery, and not screen printing.
Flexible front caps will conform to the cap platen better. If stiff front caps must be used, then adding a pad the size of the image to the surface of the platen may solve the problem. The pad can be cut from a mouse pad, but be sure to raise the screen height a similar distance by inserting a ruler between the bottom flange of the channel clamp and screen in order to maintain the off-contact distance.
Conclusion
Quality printing and high productivity require attention to details, and some of those details are unique to cap printing. However, each issue is also worth consideration when printing shirts, sweats, signs or any item.
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