Prevention of Slurring
By Fred Williams
Editor & Publisher, Type & Press
Published Fall 1977
Causes of slurring or blurring of printed characters can be mystifying at times, causing a pressman to spend considerable time improvising various remedies to eliminate this blemish. If the offending area is viewed through a magnifying glass the source of the trouble can usually be pinpointed and counteractive measures be tried.
Under this glass slurs which show an excess of ink deposited on one side of the letters or lines, may be traced to rollers hitting form too hard because they are too large in diameter for the roller trucks, in which case trucks or tracks should be adjusted by wrapping trucks with surgical tape or attaching strips of cardboard to roller tracks. Oversized waterlogged rollers often can be returned to normal size by subjecting them to very mild heat. Rollers should not have a diameter more than 1/16" larger than trucks. Keep a caliper handy and measure roller diameters often.
If a slur appears as if ink had been wiped from type it is probable that rollers are dried out and have shrank. These rollers, being of a smaller diameter than tracks, skid, wiping the ink from parts of the form, creating an "outline" or "halo" effect. New rollers are needed but a sheet of galvanized metal under the type form may raise it sufficiently to prevent skidding. Sliding rollers also can be eliminated by locking up type high bearers on the sides of the form. Cleaning of the roller tracks will also be beneficial. Old rollers can have tack restored by wiping with weak 1ye water.
This same "halo" effect may occur from using an ink too soft, the ink being squeezed out from underneath the type when it is pressed into the paper.
Probably the most prevalent cause of slurring is malfunctioning grippers. Grippers must hold the paper firm and flat against tympan to prevent all movement of the sheet, during impression and to strip the stock from the sticky type form when these two surfaces separate. If the topsheet is taut and the paper lies flat upon it, being so held during impression and if form is pressed against it firmly without any side motion and then leaves the paper clean, there will be no blurring at this stage of the printing.
If the slur appears along the left side of the form only, it may be caused by the impracticability of positioning gripper close to left margin because of left guide pin. If a spring type guide pin is substituted, gripper can be placed to close right over guide pin. This variety of pin will spring back up, undamaged, after impression.
Grippers should strike on paper margins, as close to printing area as possible. For added stripping power, fingers may be slipped over grippers, to enable them to reach into open areas of form.
It a slur occurs when the sheet leaves the form it is probably because the grippers do not have a sufficient strength to pull sheet away all at once, the paper pulling or partly peeling off with a dragging motion, causing a slur.
To eliminate, run string or rubber bands between grippers, being careful they do not contact type form during impression.
A metal, crossbar between top of grippers will permit fingers to strip paper from top of form.
As the chase recedes from the impression point, sheet should start to leave top of form first, and stripping devices should be set accordingly.
If a slur appears along the upper portion of the sheet only, it is an indication that grippers are biting sheet too had at the bottom and not at top. Remedy by bending the grippers so they hold sheet with equal pressure both top and bottom.
To prevent sheets from wrapping around rollers when running a job containing heavy solids, glue a piece of cork on top, aide of gripper and a strip of sandpaper on the underside at a point where it touches the sheet. When gripper depresses, the cork will be squeezed against the furniture in chase, pressing the sandpaper against the sheet, assisting in the stripping of the stock from type.
Stripping of Sheets
To further assist in stripping, a double piece of string may be run between gripper bar and cross string, making sure it does not contact printed area. A piece of 1/2 inch cork attached to string will help in stripping operation.
Curled paper which must be flattened between the form and platen before impression can be responsible for a slur. This is noticeable when printing near the gummed flaps of envelopes. To correct, flatten stock by rolling or curling in the direction opposite the curl and setting grippers and fingers in close to hold sheet flat. Stock stored in plastic will be kept dry and flat until needed.
A springy form or chase which gives under impression can cause slurring. To detect, note if form or chase moves during impression. Too tight a lock-up may cause form to "belly up."
Slurs Along Borders
Slurs along the edges of a border may be caused by air trapped within the border as press closes. Compression forces air out between border and paper, producing a slur. Remedy by drilling holes through border just under printing surface, allowing the air to escape.
If paper sticks to form in spite of grippers and cross stripping devices being employed, the fault may lie with the use of an ink that is too tacky. Use a reducer to reduce stickiness of ink used.
Baggy, loose packing will cause slurring as the form must press out a cushion of air between the topsheet and packing before it can make a firm contact with the sheet to be printed.
If the packing cannot be pulled tight by clipping the corners of the topsheet, both it and the topsheet should be replaced.
If the press bed and platen are not parallel it can be the cause of slurring on large forms, being emphasized on presses with bearings that are worn. Remedy: adjust the platen screws.
To adjust platen screws, lock up a mint foundry cap "M" in each corner of chase and one in the middle. Take impression and then adjust screws so all letters print with identical pressure.
Other less common causes of slurring include: improper makeready, form locked too high in chase, loose saddle springs, rubber rollers not soft enough to follow contours of the form and lumps or waves in topsheet.