Why Is My Mould Screen Sagging, and How Do I Tighten It?

The screen on your mould has gone soft. It may droop in the middle when wet, ripple across the surface, or feel loose along one side.

Check it dry and wet. Some screens look fine until the pulp lands on them. Lay a straightedge across the frame, first with the screen dry and then with it wet. A gap under the straightedge means a sag.

The sheet may tell you first. One US patent on papermaking gear notes that a deformed mould makes it hard to control the thickness of a sheet, so look for sheets that are thick in one place and thin in another in the same position each time. The Papertrail adds that a supported screen allows more pressure when you couch the sheet off. If sheets stick to the screen when you couch, check the screen as well.

This guide covers Western-style moulds with a stapled screen. A bamboo su in a keta frame is a different system, and tightening it is a different job.

Change one thing at a time. Check each item before you pull a staple.

1. Set the frame on a flat table. If it rocks, the frame has warped. Fixing the screen will not help until the frame is true.
2. Check each corner with a square. Look for open joints.
3. Inspect the staples. Look for rust, lifted heads, and staples that have pulled out of the wood.
4. Check the tension side by side. Press along each edge. A slack side tells you where to work.
5. Lay a straightedge across the screen when wet. If it is taut along the edges but still bellies out in the middle, the screen needs support, not more tension.
6. Look for damage. Torn mesh or broken wires need a new screen, not a tighter one.

Take the mould out of use and let it dry

Separate the deckle and rinse off all pulp. Helen Hiebert notes that pulp is easiest to remove while it is still wet, so clean the mould right away, then let it dry out before you work on it.

Pull the staples along the slack side

Lift the staples along the loose edge with a flat screwdriver or staple lifter. Leave the other three sides in place. If the whole screen is slack, begin at the center of a long side.

Stretch the screen and staple from the center out

Pull the screen taut with one hand and set a staple at the center of the edge. May Babcock's instructions start with a staple at the center of each edge, then work outward from side to side. Mary Tasillo suggests a staple about every inch around the perimeter. Stretch as you go, and keep the screen flat.

Replace any rusted staples

One US patent on papermaking gear notes that staples and nails can rust after repeated use, contributing to uneven sheets. Set new staples beside the old holes, in sound wood.

Cover the edges

Trim loose ends and sharp wire. Cover the edges with duct tape, keeping the tape no wider than the frame so that it does not change the size of the sheet. The same patent notes that duct tape can wear out and come loose, so check it each session.

Add support under the screen if it still flexes

Both Tasillo and Babcock put a layer of hardware cloth directly against the frame, under the window screening. For larger moulds, Tasillo suggests gluing plastic egg crate, cut to size, directly beneath the screening. The Papertrail's ribbed moulds use ribs about 2 inches (5 cm) apart, held against the underside of the screen by a second frame, so the screen stays tight to the deckle. That frame screws off, so the back of the screen can be cleaned thoroughly.

Test the mould wet

Pull a practice sheet and check the straightedge again. Lay the deckle on top and see that it sits flat all the way around. Pull three sheets and compare them under a light before you get back to real work.

• Seal bare wood with two or three coats of polyurethane before you attach a screen, as Tasillo does.
• Clean the mould and deckle after use, let them dry out, and store them flat to prevent warping, as Hiebert advises.
• Do not spray a hose hard at the back of the screen. Helen Hiebert warns that water pressure can force fibre between mesh layers.
• Check the staples and tape before every session.
• Note repairs on your batch log, so you can see whether the same side keeps failing.

A frame that has twisted or cracked will keep failing, however well you staple it. One US patent on papermaking gear notes that wood exposed to repeated moisture tends to warp, twist, and crack. At that point, build or buy a new frame. Keep the screen if it is still in good shape.

A torn or heavily corroded screen should be replaced rather than tightened.