Screen printing with Riso stencils

Screen printing with Riso stencils
Difficulty	Intermediate
Duration	20 min.
Tools	Scissors or craft knife Masking tape Medium mesh (~200 LPI) silkscreen (stretched on frame) (optional) chipboard frame (optional) Ink retardant Other screenprinting materials

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The stencils created by the Risograph can be used with a screenprinting setup to manually press ink through the stencil. This is an alternative way of making an image for silkscreening—rather than using an emulsion-based process or another form of stencil. It also allows non-Riso inks (such as opaque whites, metallics, and other additives) to be printed through Riso stencils, as well as printing on fabrics or other strange stocks.

Principles

Risograph stencils are usually inked immediately after the image on them is burned. The first print (the "proof print") is used to press them into the surface of the drum, which saturates them with ink. For this process though, clean/dry stencils must be generated—so the Riso has to be manipulated into making but not inking a stencil.

This technique is dependent on three manipulations of the duplicator and the material properties of the stencil it creates:

• How stencils are normally burned and inked when printing.
• How the risograph detects whether or not there is a stencil on the drum.
• The purpose of the rice paper mesh of the stencil material.

Normal stencil making & inking procedure

Since duplicators were originally marketed as office equipment, one of their basic metrics was "first-print-out" time, AKA how long it takes from hitting a print button to actually having a print in hand. Both the stencil making and stencil removal process on the Risograph are quite slow, to save time, they happen simultaneously. Here's the order of operations:

1. As soon as a new stencil is sent or scanned, the Risograph will immediately attempt to remove the existing stencil, and at the same time begin burning the new one.
2. The drum spins once while the clamp is flipped open, and the old stencil peeled off the spinning drum and fed into the disposal unit (it will later be crushed into the stack of disposed stencils).
3. Meanwhile, the MMUMaster Making Unit immediately starts feeding in stencil from the roll and burning the image, from the lead edge to the trailing edge.
4. The drum cannot "receive" the stencil until the old stencil is completely removed, so the Risograph checks if it can still detect a stencil on the drum, or if anything went wrong in the stencil-removal process.
5. If the removal process was successful, the drum waits with the clamp open until the lead edge of the stencil is fed in from the MMU, then closes to grab it.
6. At this point, the image is likely still being burned, so the whole system waits for it to complete. There is a storage compartment in the MMU that the stencil fills up as it is fed and burned.
7. Once the image is complete, the drum spins once more, while a sheet of paper is fed—this prints the "proof" sheet and presses the stencil onto the drum (the proof actually prints with slightly different settings than all other prints—it prints with greater pressure, and more slowly, to try and evenly press the stencil onto the drum mesh).
8. At the end of its spin, the stencil is cut off the roll, and the machine returns to its idle state, waiting for the printing operation.

How the Riso "sees" a stencil

In order for the machine to properly place stencils on drums (and detect when something has gone wrong) it has to be able to "see" if there is a stencil on the drum or not (step 4 above). This is facilitated through a reflective sensor above the drum and the strip of black rubber tape which runs along the length of the drum, just to the right of the clamp.

At a specific angle of rotation (right when the reflective sensor is pointing at that strip), the sensor shines a light onto the surface of the drum and looks for the reflection.

1. If there is a stencil on the drum, the light will hit the shiny white surface of the stencil, and the sensor will detect the reflection.
2. If there is no stencil on the drum, the light will instead hit that black rubber strip and get absorbed by it (the part name is "light absorber strip"), and no reflection will be seen.

Meshes and ink diffusion

To ensure even coverage, the drums are designed with a series of 4 meshes, which act to spread ink out evenly underneath the stencil. The drum body itself is a very coarse mesh with a honeycomb pattern of holes cut into it. The screen is two layers of square metal mesh, the inner layer is a medium mesh, and the outer layer is a finer mesh. And the last layer of mesh is the stencil itself.

The substrate of Risograph stencils is a sheet of rice paper, with a mesh of pulpy fibers—the image itself is burned into a thin plastic film (shiny, under light) deposited on the surface of this paper.

Procedure

Creating an uninked stencil

First you will need to create a blank (or "confidential") stencil to act as a barrier, sealing the ink inside the drum, then trick the Risograph so that it cannot remove this stencil (and believes it has already been removed), and finally burn the stencil of your image and remove it from the drum.

1. Place a drum into the Risograph.
2. Make a blank stencil (i.e. one with no image burned into it):
   • You can use the "confidential" function if your machine has it, by pressing the confidential button and then hitting START when prompted.
   • Otherwise you can send a blank file or scan an empty, clean scanner bed.
3. Wait for the Riso to print its blank proof sheet, then remove the drum from the riso.
4. Manually release the lead edge of the stencil from under the clamp. To open the clamp:
   • On and newer machines, press down on the orange or blue rectangular lever on the top of the drum at the front of the clamp.
   • On RPRN machines, press down on the white wheels at the front and back of the clamp.
   • On machines, lift up the metal tab at the front edge of the clamp bar. Be sure to close the clamp again after removing the lead edge from it.
5. Carefully cut off the uninked lead strip of the stencil, exposing the rubber strip.

Be careful not to cut into the metal mesh screen underneath the paper stencil! Once damaged, the screen cannot be repaired, it must be replaced.

6. Tape the stencil down along the full length of the drum, with the tape covering as little of the black rubber strip as possible while still securing the stencil.
   • If the cut edge of the stencil comes up during this process, it will likely partially tear the stencil off the drum, and you'll have to start over.
   • If the tape covers too much of the rubber strip, it will be seen by the reflective sensor, and the Riso will know the stencil is still on the drum.
7. Put the drum back in the Risograph.
8. Make a stencil with your actual image—it will get wrapped around the barrier stencil that you taped to the drum.
9. Carefully remove the drum (your new stencil isn't stuck to the drum with any ink, so it will be flapping loosely around the drum).
10. Open the clamp again and remove your uninked stencil

To reset the drum, you'll need to remove the taped on barrier stencil by hand (and it's best practice to seal up the drum with another stencil, so that it doesn't dry out).

Preparing the screen

In order to pull ink through the paper stencil, it needs to be stretched into a rigid surface. There are two basic methods for this:

1. Tape the stencil to the outside/printing surface of a pre-stretched silkscreen (probably around 200 LPI mesh, to simulate the metal mesh drum screen), with the shiny side facing out. When printing ink will pillow under the stencil slightly—you can reduce this effect by cutting the excess stencil away around your image before taping it down.
2. Glue or tape the stencil into a flat chipboard frame (this is similar to the screens created with the Print Gocco).

Either way, you'll want to stretch the stencil taut across the frame, in the same way you would stretch a canvas.

1. Tack the centers of opposite sides in place each with a small piece of tape, with the stencil stretched tight between them.
2. Tack the remaining two sides in the same way.
3. Tack all four corners, pulling each tight as it is taped.
4. Tape fully around the stencil, sealing up the edges.
5. If using a silkscreen, mask out the rest of the screen with additional tape.

Printing

The actually printing process just follows existing screenprinting methods—using a rubber squeegee to pull or push ink through the stencil. However ink in the paper stencil will dry much faster than it would in the regular fabric mesh of a silkscreen. To remedy this, an "ink retardant" can be mixed into the ink to increase its open time (but this will also make it slower to dry). It's important that the screen is always left flooded and there is not too long a delay between prints.

Theoretical modifications and alternatives

A number of theories have been proposed for alternatives to this method, or further optimizations:

• Permanently modifying a "dry drum" on which to make screenprinting stencils. If a drum can be tricked to function dry, it wouldn't require taping down a barrier screen for this process—the operator would just load up the dry drum whenever they wanted to make an uninked screen. A drum could be modified in this way by being fully cleaned out, and then short circuiting the ink sensor so that the drum always detects that it has enough ink.
• Forcing a stencil removal error—this would make it so that the stencil never even reaches the drum—after burning it would be left in the storage area of the MMU (and generate an A34-218 error). It could be either removed manually from the MMU and cut off by hand, or test mode could be used to feed it into the empty drum cavity after removing the drum and cut it at the correct length.
• Using this process to create screens for the Print Gocco, by pre-producing reusable frames and burning stencils 4–6 up. Screens made in this way would be printable using the original Gocco hardware instead of squeegeeing through them.