The Durst RCP20 (and its larger brethren) continues to be a popular roller transport processor, despite the machines having been manufactured in the 1970s-1980s. Many of these by now are affected by problems with damaged rollers. In this blog, I’ll report a first hack-attempt at reconditioning them. Not tested yet, but I think I’m on to something.
First, let’s have a look at the problem at hand. What you generally notice, is that the machine doesn’t run very smoothly anymore. It somehow ‘bumps’ or ‘hiccups’ every now and again, even though it appears to still work just fine otherwise. Then, when cleaning the roller racks (which should be done after every use!), you hit upon something like this:
The image shows the rollers removed from the developer rack. Note how they’re warped and bulging in places. It generally doesn’t take much time before the problem gets out of hand entirely:
This also gives away the cause of the problem, and to properly describe it, let’s have a look at the construction of the roller itself. This becomes evident if we strip away the black rubber lining that constitutes the outside of the roller. Inside, we see this:
The black rubber surface is in fact a hollow tube that is formed or pressed around a pale grey, plastic/polymer lining. In the photo above, it’s partly removed (due to progressive damage and a little additional prying in my behalf) to expose a rusting layer underneath. This is a bit puzzling, because the rust does not appear to extend to the end of the central rod. How come? This becomes clear when we strip away a little more of the grey lining:
The metal central rod in fact consists of two parts. An inner rod that appears to be made of a hard, durable stainless steel. There’s a second layer of softer steel around this, and this is evidently not stainless at all. Apparently, the grey plastic that normally extends about 10mm past this rust-prone steel lining should prevent chemistry from ever getting to the rusting part. But give it enough time (roughly 45 years in this case) and sufficiently aggressive chemistry, and sooner or later, the soft steel lining around the central rod starts to rust. Since rust tends to expands the metal substantially (it starts to flake and press outward), the layers on top of this rusting rod start to expand and ultimately crack, too.
So what to do? There’s a couple of scenarios to play with. They all have their pros and cons.
The first I explored was to simply hand it over to a professional. I contacted a business that would re-line these rods with a new poly-urethane finish. Mind you, this was before I did a proper diagnosis and under the assumption it was a simple rubber lining on top of a metal rod. They quoted a price of €45 excl. VAT for this. Per rod.
Given that there are 8 rods to a rack and 2 racks to a machine, this would imply >€700 excl. VAT to recondition all the rods in a machine. In hindsight, it’s probably more, because this does not include any allowance for the cleaning & de-rusting and possible replacement of the actual steel parts. One could DIY that bit, of course. Still, it’s more than I’d like to spend on the project, even though I understand that these machines now sell for €1k and upwards (which, frankly, I find ridiculous – but this blog isn’t about market dynamics and price elasticity).
A more reasonable option I read about was some guy who apparently did the repair himself. He ‘simply’ pressed the black rubber surface away from the rod using some kind of setup with a vise, removed the rust from the metal, applied some rust converter/primer and then put back the original lining. I never saw any photos or read more details about this process. Apparently it can be done, although I personally am not equipped to do this. If you have a well-equipped workshop, I’d really consider this, because it would be the most straightforward method. Provided that it works.
A better-documented approach appeared a about a year ago online, in the form of this YouTube video (please note: not my work!) There’s some preciously brief discussion about it here on Photrio, but really, the video itself is quite self-explanatory. This seemed appealing enough and I tried for a few nights to source a combination of rods and tubes that would work. No cigar (as of yet), although evidently it can be done. And this option is still quite attractive, since it effectively makes a brand-new roller from potentially affordable parts.
Since that video, I didn’t spend much thought on it for about a year, until I noticed one of the rollers on my ‘daily driver’ RCP20 was totally wasted (bloated, cracked – the works). I had already swapped out most of the rollers with a spare machine I have, so I have one working machine, and a parts donor with a whole slew of rollers in pretty abysmal condition. It’s a ticking time bomb, so I’ve restarted thinking about somehow repairing the rollers.
So here’s the feeble attempt I’ve done, mostly to come to grips with how the materials work and what suitable options for a more durable/realistic repair would be, that can actually be repeated N times to fix all the rollers I have.
First, I stripped away part of the grey lining to expose the affected rusting steel so I could remove all of the rust. I did this by brushing it off and then putting the rusting end in a jar of hot citric acid solution (a spoonful to a glass; it’s not critical), and then left it to sit for a day or so. A toothbrush took care of the remnants, leaving a badly pitted but otherwise OK iron surface. I then gave that a couple of layers of spray-on anti-rust primer. The result is as shown:
Now, I had to replace the grey plastic liner that I chipped off. I decided to 3D print a fitting replacement, so I took some measurements, fired up Fusion and drew a hollow tube with a tapered end:
Printing it took less than half an hour, after which I tried to fit it in place. My 3D printing process is pretty much uncontrolled and the inner diameter ended up much too narrow, so I had to heat up the part to fit it in place. That’s not good, because I’m pretty sure I must have damaged the anti-rust coating. In a future attempt, I think I’ll use a clamshell approach consisting of two parts that clamp around the central rod, and are then glued together. Anyway, the prototype now looks like this:
Now came the tricky bit, and sadly, I didn’t take any photos. When removing the outer black rubber lining, the easiest way I figured it could be done was simply cutting it off. So I made a single lengthwise cut and peeled off the rubber. Worked fine, but of course, the problem is, how to get it back together again? I ended up buying some contact glue fit for “all materials” (hey, if the package says so, it should be true) and glued the seam together. It didn’t look too great, but it has held together so far. By the way, the reason I simply cut the rubber is because I had in mind to replace the rubber with a 3D printed lining altogether. I later decided that I wanted to try put the original rubber back, because as such, it was undamaged apart from the crack on one end where the rust expansion had occurred.
The tricky bit was to get the black rubber back on the whole thing, but this turned out to be really easy after applying some silicone spray along the length of the rod. Will the silicone spray damage the rubber? I really wouldn’t know, honestly. Let’s hope it won’t be too bad.
I don’t really trust the glued seam on the black rubber lining. With the 3D-printed replacement in mind, I had already ordered some heat-shrink tubing ahead of time, and I decided to apply that over the entire length of the rod. This should make for a new (and replaceable, if the need arises) surface. I guess/hope that the added thickness of the heat-shrink stuff will be within the tolerances of the rack, but that’s a bit of an uncertainty for now. The end result looks like this:
Doesn’t look too bad, and not too great at the same time, either. Will it work? I haven’t tried yet. I don’t want to take apart the rack of my working RCP20 just yet. Instead, I’d like to work on some more options and then put together a spare rack I can test with. Honestly, even if this particular one will work, I don’t expect it to last as long as the original. It has mostly served as a test bed for better repair/reconditioning approaches, and dismantling this one roller has helped me to better understand the problem, which should also help in preventing it in repaired or replaced rollers.
So, this is one of those “I’ll be back” moments. I’ll revisit this project at some point, although it’s on low priority for now since I have a working machine (in fact, two – I also have a 30cm Durst Printo in working condition).
If I’m going to do a repair along similar lines, I think this is what I’ll do differently:
- Make the 3D printed lining a two-part clamshell type so as not to damage the anti-rust coating on the metal rod.
- Maybe work on a 3D printed replacement of the black rubber as well, and line that with heat-shrink tubing, so as to end up at the same outside diameter as the original rod.
- Make the 3D printed parts out of something more chemically resistant than the PLA I used for this prototype (for all I know it may basically dissolve over time in the alkaline developer!)
- If I’m to do more rollers this way, I’ll probably consider sending out the 3D printing work to get it done in better quality (material- and tolerance-wise) than I can do here at home.
Writing this blog also inspired me to have another look at the option of replacing the rods with an assembly as outlined in the YouTube video linked to earlier. Honestly, this would be a superior method. I’ll just have to source the required materials.