Spraying the inside of a mixing box

darkroom_rookie · Feb 19, 2012

Is it possible to replace polystyrene/styrofoam sheets in a 4x5 mixing box with spray paint in order to achieve a larger diffusion surface? The enlarger in question is a Fujimoto 450M-C and what I basically want to do is modify an extra 35mm mixing box I obtained recently. All mixing boxes for this enlarger are the same, as the only difference is the thickness of the polystyrene sheets and size of the bottom opal sheet. I've tested a 2mm polycarbonate opal sheet and it can withstand the heat, so it's easy to replace the bottom diffusor with a 6 1/8" x 5 3/8" piece, which would give a more even illumination for a 120 WA lens. So instead of using a 5-6mm polystyrene, perhaps the sides of the box can simply be sprayed with thermally resistant white paint intented to be baked after the final coat. Could it withstand the temperature?

Ian Grant · Feb 19, 2012

Durst mixing boxes use polished aluminium, and do a good job. I saw some reflective gold material similar to what's used on flash umbrellas in a store last week, it's also available as silver, that would work, it's heat resistant.

You could use 4 mirrors, or 4 pieces of polished aluminium

Ian

darkroom_rookie · Feb 19, 2012

Ian,

Should I look for a specific type of aluminium (apart from the surface finish)? What type of glue is heat-resistant?

d_rookie

Ian Grant · Feb 19, 2012

You can buy aluminium with a polished finish, it's a while since I bought any but in the UK there's someone selling smaller bits on Ebay, I was buying 8ft x 4ft sheets a few years ago so a different proposition.

Once cured Epoxy is heat resistant.

Ian

octofish · Feb 19, 2012

I did a similar thing where I had to reline the inside of a cracked, yellowed mixing box in a Meopta meochrom color head. I used a high quality artists acrylic in titanium white and applied a few coats of it using a brush. It seemed to work pretty well. I don't know if the performance is as good as what the mixing box would've been new, but it was definitely better than the degraded surface. I got at least a couple of stops more light. If I did it again, I would've used a better brush (or spray) and made sure the coat was perfectly smooth, as it was it was pretty rough. Not sure how much difference that would've made though.

As far as heat resistance goes, it's only relatively early days. I did a stack of printing with it straight after and everything seemed fine, but I haven't used it for a while. If it degrades, it's not going to be hugely difficult to do it again though.

If you find any more suitable paint, I'd love to know actually.

ic-racer · Feb 19, 2012

What are you trying to do? Are you trying to make a 4x5 mixing box from a smaller one? If they indeed are all the same except for the lower diffusion plate, what would be the purpose of altering the internal diffusion material?

If you do alter the internal diffusion material you will have to do some testing. Although the Durst mixing boxes are lined with shiny aluminum, the ones I have use a diffuser both before and after the reflective aluminum box. Also, the lower diffuser is thicker in the middle.

darkroom_rookie · Feb 20, 2012

ic-racer,

They are indeed all the same, housed in identical metal boxes. Actually, it looks like there was only one designed (35mm) because of the presence of an aluminium light tube, cca. 1 1/4" long and 5/8" in diameter, behind the UV filter inside the box. I wouldn't expect this tube to help much with the diffusion of light. There's only one diffuser at the bottom, high-grade opal poycarbonate and thicker in the middle. 6x9 and 4x5" boxes have thinner styrofoam and have a larger diffusion sheet, around 5 3/4 x 5". 6x9 box just adds a piece of paper (!) with a 78x100mm cut in the middle. I did find styrene material 1.5mm thick very similar to one used for plastic drinking cups, but am worried about its resistance to heat.

L Gebhardt · Feb 20, 2012

The Ilford 500 mixing boxes just use white paper board as the liner Some 2 ply matboard would work well. They also have a layer of aluminum right at the bottom. Lot's of options, but I think you will just need to try a few to see which gives you the most even light with your enlarger.

Mark Fisher · Feb 20, 2012

They designed the enlarger to have very diffused light. Polished aluminum is a very specular surface. I would not glue anything permanently to the box without first establishing how even the light is. White paint would be a better bet. Polished aluminum may work well in a Durst, but that is the way it was designed. If you want to try the aluminum, try taping on aluminum foil. It is fairly reflective without being mirror-like. BTW, Styrofoam is really common in enlargers. I've seen it in Leitz enlargers with zero degradation. The only advantage I'd see to aluminum is perhaps faster times.

darkroom_rookie · Feb 21, 2012

I'll go with reflective white paint for the start and try to see if the housing heats noticeably more than with polystyrene. Lexan polycarbonate sheet that has replaced the 35mm diffusor withstood the temperature so far, hopefully it won't start softening or even melting without styrofoam... Thanks to everyone for the replies.

DREW WILEY · Feb 21, 2012

The problem with aluminum or stainless sheet is that the mirror box has to be appropriately designed
for it, since this kind of material reflects in a more specular manner than the styrofoam did. If you use paint there is a significant problem with yellowing and possible bonding failure over time. You need a paint with a maximum amount of quality white pigment. The very best is almost pure barium
sulfate, but at something like $200 per half pint, you'll probably not opt for it. But you can find things
in the art store with a high mixture of barium and titanium dioxide. But it won't be in a spray can. Test under a heat lamp for heat tolerance (you don't need a fire in your enlarger, or darkened areas!).

darkroom_rookie · Feb 21, 2012

Drew,

Does this kind of paint adhere well to a fine-matted steel plate? I believe this particular mixing box is made of that type of metal. Also, should I test it under the same halogen bulb that the enlarger uses (EJL 200W) or any heat lamp?

Spraying the inside of a mixing box

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