Print Flattening Third Generation: Arbitrary Sizes Revisited

Earlier this year I revisited this subject as my darkroom printing had changed to using odd sized papers based upon a new aesthetic of smaller prints with wider borders. Since I found myself cutting down larger standard paper to custom sizes my flattening solutions based on standard paper sizes (8x10", 12x16", and 16x20")  were not up to the job  (Click the link above to see that solution and find links to the original versions.) 

The fixture I made earlier this year works well and has offered the needed flexibility. It is however not simple to fabricate and requires woodworking tools like a router and skills that are not available to everyone. At the time I considered using magnets and metal sheeting but with a quick search on the web I couldn't find the sheet material I wanted in the quantity (1 sheet) and size I needed. Last week I looked again in eBay and found a vendor who sole 600x400mm (23 1/2"x15 3/4") sheets of perforated mild steel that is galvanized (very close to 20"x16"  my largest paper size). I chose perforated as I always thought this would speed up the drying process with water able to escape from the back of the print. My perspex/acrylic or plywood solutions are particularly bad at this.  

Blog Post Cover Photo

I went with galvanized mild steel over stainless as stainless steel can be non magnetic which would defeat the purpose. Galvanized of course helps with water resistance in terms of corrosion. I would have strongly considered mild steel and painted it as I ended up painting the galvanized perforated sheet anyway as the cut edges are not galvanized and I am concerned about the zinc staining the paper. Finally the steel is 1mm thickness with 3mm holes on a 3 mm pitch. (I found this in the UK here on eBay...)

Example Perforated Sheet 

Metal Specifications

• 1mm thickness
• Mild Steel
• Galvanized
• 3mm holes with 3mm pitch
•  600x400mm (23 1/2"x15 3/4") Size
• Cost £15.34 including shipping

Magnets

You want Neodymium rare earth magnets as they are very strong for their size. They are relatively cheap as well. I started out not knowing what shape or size I wanted. There are a few choices. Mostly of them are discs or rectangles of various dimensions. I started out with 10mm diameter disks 3mm thick that are countersunk to take a screw. 

Countersunk Magnets 10mmx3mm

My plan was to drill 10mm holes in strip of wood at intervals and secure them with screws. What I learned is you need to be sure you have the right size screws as one that fit tended to protrude from the top of the magnet. The magnets are also very brittle and will break with very little effort.  

This print drying methods depends on keeping the edges of the wet paper clamped evenly and tightly enough that as the paper dries and shrinks the paper is pulled taught. Too much clamping strength can cause the paper to tear and too little results in less flatness or puckering. Fortunately there is quite a wide range of clamping for that gives good results.

As a result of these considerations my initial attempt left out the screws and I mounted the magnets with the countersunk side down to maximize the magnetic contact. I resolved to glue them in place with epoxy though for the time being they fit snugly enough in the holes that they don't need glue for now. I started out mounting them at 30mm intervals as I only bought 20 in the first batch. 

Prototype Strip
(Overall length 20cm or about 8" 1.8cm width with magnets at 3cm intervals)

I have bought more magnets and have gone with 10mm x 3mm discs but without a hole or countersink. These should be more powerful and I will glue them in the same fashion. If the clamps are not powerful enough I can double the number of magnets by gluing at 15mm intervals. (50 of these 10mm x 3mm disc magnets costs £9.89.)

Solid Disc Magnet

I also bought some 25mm x 10mm x 3mm rectangular magnets as an alternative. These should be more powerful and as a result might need fewer of them. (10 of these cost £9.98)

Rectangular Magnets

Mounting Concepts 

There are many ways to make these strips. The important characteristics is they should distribute the force evenly along the strip so the paper is clamped consistently along the entire edge. Consequently the magnets pictured above should be flush with the wood surface or slightly below. In the previous version I had good success with placing strips of wet-dry sandpaper on the edges of the clamps strips to improve the grip but also promote the movement of water vapor from the edge. In this case the magnets will be slight below the surface of the sandpaper. Of course the further magnets are from the metal plate the less strength of pull they will have. 

In order to optimize the magnetic strength against the metal plate the magnets should be mounted in alternate polarities N-S-N-S-N-S etc. This will allow the flux lines to reinforce each other. Making the poles orientate in the same direction will still work but the flux lines between the magnets will tend to cancel each other weakening the attraction to the plate. With a 30mm spacing as I have chosen this effect is probably fairly small. 

I assume that drilling the 10mm holes is accessible to anyone who wants to construct this. A brad point drill bit is best as it cuts the cleanest hole. 

An alternative might be to build a slotted strip. Most DIY places will sell small strips of wood.  Below is a simple drawing of the concept. All the pieces can be glued together.  I plan to try this with the rectangular magnets when they arrive. 

Slot-based clamp

First Attempt...

Here is the first attempt to use the clamp. I chose a discarded print and soaked it in water before clamping on the perforated sheet. 

New System in Action

For two edges I have retained my Perspex strips and bulldog clips from the first generation versions. These could be magnet strips as well. The wooden strips are the two sizes of the prototype magnet strips I made. The clamps are easy to apply and adjust/remove. They are 18mm tall so it is a simple matter to rock them along the long edge to get them loose. 

Once I have dialed in the force then I will make a variety of sizes just like the prior version. A variety of 200mm (8"), 100mm (4"), 50mm (2"), and 25mm (1") versions should allow me to tackle any print as long as it is in 25mm (1") dimension increments. 

Result!

Well my first print was Ilford MGFB Classic paper. It is probably the worst for curl and the results was great. 

Dried Print

The print dried as flat as I expected, and in less time than the other versions due to the perforations in the plate. In the humid weather I find a slight curl is typical as the paper side expands more than the emulsion side of the paper. This has never been a problem when mounting or framing. You will note that the paper where the magnet clamps held the paper down have some yellowish staining. This is due to the wood making direct contact with the wet paper and pulling some of the wood colors into the paper. I did not waterproof these prototypes but will now give them a coating of polyurethane varnish in the final version. 

I think the forces of the clamp are good so now I will make more when my new magnets arrive.   

Update: Completed System and Further Notes

The new magnets have arrived and I have been able to complete the flattening fixture. It continues to perform as expected. 

The new magnets are solid 10mm x 3mm discs and are in fact stronger. I have modified the wooden strips the magnets are mounted in to include a 4mm through-hole behind each magnet. This allows me to push the magnets out again if needed or they become skewed. 

In the end I found the magnets press-fit tightly enough in the 10mm holes I drilled that no adhesive was needed. I used a wood block and mallet to gently tap the magnets flush with the wood surface. The through-holes allowed me to adjust his if I tapped them in too deeply or they became skewed.

The next step was to cover the magnets in polyurethane varnish.  I noticed with my first test print that the magnets had left a stain (see below). This is despite the claim they are corrosion resistant. Ideally the magnets never touch the paper but that is difficult to guarantee hence the precaution. 

Gray stains on the edge of the paper due to the magnets corroding. 
(Yellow stain due to wood.) 

A couple of the blocks I have already added the sandpaper strips. All blocks I made from 18mm thick plywood scraps as the aesthetics are not important to me. The variety of lengths allow me to adapt to most paper sizes. 

The panoply of different clamping blocks.

Here are the new blocks in use. 

New blocks in use.
(Note 4mm holes to allow magnets to be pushed out or adjusted.)

New flattened test print