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Elongation/shrinkage
| Elongation & shrinkage | |
|---|---|
| Difficulty | Beginner |
| Duration | 10 min. |
| Tools | Ruler |
Based on the way stencils are made in the risograph, if certain test modes are not properly calibrated, printed images can be elongated or shrunk by 0–3 mm. This comes from two places:
- The speed of the write roller when making a stencil—if it is faster than it should be, the image will be elongated; if it is slower, it will be shrunk. This is a physical property of the changing properties of the depleting stencil roll (so it changes over time, based on how many stencils remain on the roll). On two-drum machines and full-featured one-drum machines, compensation for this often happens automatically, but on older and lower model-number machines, this will regularly need to be calibrated for 1:1 printing.
- The speed of the scanner head (when scanning)—if the head moves too fast, the image will be shrunk; if it is too slow, it will be elongated. This is a more stable setting, so it rarely needs calibration (and should only be calibrated after correcting elongation/shrinkage in the write roller).
Overview
Calibration can be performed if the dimensions of the printed image do not match the dimensions of the original. What needs to be calibrated depends on how the machine is used on a regular basis.
- If the stencils are made both from the scanner bed and sending files from a computer then both the write roller speed and scanner head need to be calibrated.
- If either the scanner bed is used exclusively or files are sent from a computer exclusively, then only the write roller speed needs calibration (though both can be calibrated if desired).
On older and lower model-number machines however, pursuing perfect calibration can be a wild goose chase. It's only really necessary when miscalibration is detected between layers, or when perfect scale to the sheet is needed (to match fold lines, for example—as in a magic zine).
Procedure
Both calibrations can be made by comparing a measured (printed) distance against a goal (original) distance, along the left/right axis of printing (along the feed direction of the paper).
Printing and measuring
The means of making a print and measuring it are different depending on whether the calibration is for master making (to calibrate the write roller speed) or scanning (to calibrate the scanner head speed).
Master making
- Make a crossed lines test pattern from test mode (with paper in the feed tray and a drum in the riso).
  |
RPRN | Z+Any machine released with or after the RZ line, i.e. RZ/RV/EZ/EV/SF/SE and MZ/ME/MF/MH machines. |
|---|---|---|
119 |
51 |
81
|
- The resultant grid will either have (relatively) square cels (for machines with square aspect ratio, i.e. 300 × 300 or 600 × 600) or rectangular cels (for machines with RISO's 600e rectangular aspect ratio; 600 × 300).
- For square cels, measure 20 cels across in the feed direction, this is the measured distance. Then measure 20 cels up/down (perpendicular to the feed direction), this is the goal distance.
- For rectangular cels, measure 20 cels across in the feed direction, this is the measured distance. Then measure only 10 cels up/down (perpendicular to the feed direction), this is the goal distance.
Scanning
Make sure to make the scanner bed calibration only after the master making calibration or if the scanner bed is used exclusively .
- Scan an original with a lengthwise line or other known distance between points. This is the goal distance.
- Measure the distance on the resultant print. This is the measured distance.
Changing settings
On newer Z+Any machine released with or after the RZ line, i.e. RZ/RV/EZ/EV/SF/SE and MZ/ME/MF/MH machines. machines these adjustments are made in test mode, while in older machines they are made with rotary DIP switches on one of the circuit boards in the back of the machine.
Test mode calibration (RP/RN/Z+)
The settings changes are a compensation that's based on the percentage (in thousandths) of scaling desired—with positive values elongating the image and negative value shrinking it. There's a simple formula to determine the adjustment:
Rounded to the nearest number.
This is how much the relevant test mode must be changed by—so it's a relative value not an absolute one.
The test modes to be adjusted are:
| RPRN | Z+Any machine released with or after the RZ line, i.e. RZ/RV/EZ/EV/SF/SE and MZ/ME/MF/MH machines. | |
|---|---|---|
| Write roller speed | 287
|
547
|
| Scanner head speed | 182
|
382
|
The result from the equation should be added to the current value for the new, adjusted value.
For example if the goal is 150 mm and the measured is 152 mm, the calculated change would be -13. If the current value is
+5, the new value should be-8.
Rotary switch calibration (GR/RP)
On older machines, the adjustment(s) are made by changing the setting on a small rotary switch on the Image Processing PCBThe circuit board in charge of scanning an image, dithering it, and sending it to the MMU. behind the back panel of the riso. These small switches are adjusted with a Phillips #0 screwdriver, changing which value (0–F) that the arrow on the switch points to.
- On 3770 and machines the switch will be labeled FB (flat-bed).
- On other models the switch will be labeled SLA (scanning length adjustment).
The adjustment is fairly coarse (not fractions of a percentage as in the newer machines) and based on the length of elongation/shrinkage in millimeters.
| 0–4 | 5 | 6 | 7 | 8 | 9 | A | B | C | D–F |
|---|---|---|---|---|---|---|---|---|---|
| (do not use) | +3 | +2 | +1 | 0 | -1 | -2 | -3 | -4 | (do not use) |
| Elongation | Shrinkage | ||||||||