Elongation & shrinkage

Elongation & shrinkage
Difficulty	Beginner
Duration	10 min.
Tools	Ruler

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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:

1. 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.
2. 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 (this is often measured between registration or crop marks). 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 the master making unit (to calibrate the write roller speed) or the scanner bed (to calibrate the scanner head speed).

Master making

1. Make a crossed lines test pattern from test mode (with paper in the feed tray and a drum in the riso).

Crossed lines test modes

	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

2. 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.

Scanner bed

Make sure to make the scanner bed calibration only after the master making calibration or if the scanner bed is used exclusively .

1. Scan an original with a lengthwise line or other known distance between points. This is the goal distance.
2. Measure the distance on the resultant print. This is the measured distance.

Crop marks shortcut

On either of these calibrations, the measurement can be made between crop marks (or any other known distances) on a stencil already made on the drum. It can be done in the middle of a job, just by comparing the original (goal distance) to the print (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.

Z+ calibration

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 how much to change the value by.

$$({\frac {goal}{measured}}-1)*1000$$

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 ${\textstyle -13}$. If the current value is +5, the new value should be -8.

$$({\frac {150}{152}}-1)*1000\approx -13$$

$$5+(-13)=-8$$