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Thread: Midtone Machines - Automatic Film Processing Machine

  1. #31

    Re: Midtone Machines - Automatic Film Processing Machine

    That is fair enough - I was going to ask what kind of volume production you were expecting. I have used heat resistant epoxy-resins from Smooth on (Epoxacast HT) there is a good store there in NZ too can't recall name.

    Just note that the temperature would still need to be a bit higher than process temperature but if you are preheating then it should be OK. I have used the pads to heat up a Bakelite rondinax from room temperature it needed to get up to 90degC to reach around 38 internally - at that point the plastic was starting to gas and sustain some surface marring.

    The contacts themselves serve other purposes such as tank identification for process selection/amount of rolls etc so you may need to design around fail-safe processing prevent over/under filling etc. If casting embedding a stainless pad on the underside of the tank should be quite easy to achieve so perhaps give it a try? That resin is quite expensive I was only using it for small machinable parts, entire tanks this size will be $$$, if you use a pad you could get away with lesser resins methinks. The pad itself would not have to not have to share any electrical signal, just be a transfer heating element say from a silicone heater.

    Just food for thought, good luck with it.
    The Container Lab
    www.thecontainerlab.com/blog

  2. #32

    Re: Midtone Machines - Automatic Film Processing Machine

    I got a few quotes for representative parts in the early days both for vacuum resin casting and injection molding (offshore manufacturers). The cross-over point where injection molding made sense when including tooling costs was about 350 parts. Of course another difference is the injection mold tooling cost up-front is massive (probably would have been well over $100k for this project) while the vacuum resin cast tool cost is split over every ~10 parts, so there's much less risk and capital investment required up-front.

    That said if there ends up being a lot of interest I could potentially go injection molding which could potentially bring the cost down a bit.

    I can see how having to heat the outside of the tank to actually get significant heat transfer into the liquid would require quite a high temperature, but in this case I'm effectively stopping heat transfer out of the tank by maintaining the outside of the tank at a slightly higher temperature than the liquid. As for knowing what tank is fitted that's a good point, I might potentially be able to do that with some non-contact method (maybe a magnet) to tell the machine if a 4 or 6 roll tank has been inserted. If there is an overflow it will get caught by a spill tray at least.

  3. #33

    Re: Midtone Machines - Automatic Film Processing Machine

    Sorry I meant the external heating pads on the tank will still need to be well over process temperature = poor transfer coefficient through the drum. Unless you preheat the drum itself or have more heating surface area (eg wrap it around the whole tank) Would just have to test and try it wouldn't drop that much anyway.

    The main thing is if you don't want any contacts on the drum is it going to be a mostly open loop system? Or figure a way to get around that.
    The Container Lab
    www.thecontainerlab.com/blog

  4. #34

    Re: Midtone Machines - Automatic Film Processing Machine

    The heating pad under the drum will be closed-loop controlled, I'll probably have to do some experimentation to work out how much higher than the process temperature it will have to be to account for heat loss and poor heat transfer through the plastic. The heating of the fluid in the develop tank will be essentially open loop in that the exact temperature of the fluid won't be measured but the temperature of the outside of the tank will be measured and controlled. I imagine I'll have to control this to a few degrees higher than the desired process temperature to maintain correct fluid temperature once losses are taken into account.

    The heating of the chemicals prior to going into the develop tank will however be fully closed loop as both the heater and the temperature sensor will be immersed in the fluid, so the develop tank heater only has to maintain the correct temperature for a few minutes, not get it there in the first place.

    All of that said and done, if during testing I find that I can't keep adequate temperature control of the fluid in the develop tank during the process then I can always add in a sensor to close that loop, I just think that I can avoid it which has definite benefits to cost and reliability.

  5. #35
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    Re: Midtone Machines - Automatic Film Processing Machine

    In rotating the spindle, do you have any provision for avoiding standing flow patterns? The obvious one would be periodically reversing the spin, but even that needs testing with respect to different chemical processes, reversing intervals, and core-and-reel configurations to assure that it's effective.

  6. #36

    Re: Midtone Machines - Automatic Film Processing Machine

    Quote Originally Posted by Oren Grad View Post
    In rotating the spindle, do you have any provision for avoiding standing flow patterns? The obvious one would be periodically reversing the spin, but even that needs testing with respect to different chemical processes, reversing intervals, and core-and-reel configurations to assure that it's effective.
    Yes the plan is to periodically reverse the direction of rotation, there is also quite a bit of axial clearance for the spindle which will result in some displacement during agitation. I'm not convinced yet that I'll need to do anything more as I think the environment inside the tank will already be quite turbulent but if I found that I had to do something during testing there's plenty of scope for that.

  7. #37

    Re: Midtone Machines - Automatic Film Processing Machine

    Quote Originally Posted by AndrewBurns View Post
    I just think that I can avoid it which has definite benefits to cost and reliability.
    I really wouldn't worry too much, I think the problem with the Phototherms was that the control and SW was so rudimentary with no overrides or fail-safes (say if the tank couldn't be identified via magnets or probe failed due to too much resistance). The SS models worked towards this and improved flexibility with custom programs etc but still used the same hardware.
    The Container Lab
    www.thecontainerlab.com/blog

  8. #38

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    Re: Midtone Machines - Automatic Film Processing Machine

    I would be very concerned about all the valves. Seals going bad etc.

  9. #39

    Re: Midtone Machines - Automatic Film Processing Machine

    Quote Originally Posted by Duolab123 View Post
    I would be very concerned about all the valves. Seals going bad etc.
    For a variety of reasons I'm currently going to use pinch valves, essentially the valve is a section of silicone tube that is squished to prevent flow. It's a fairly common technique in medical/chemical applications where pressures are very low and chemical compatibility and a lack of trapped volume or places where contamination can collect is important.

    I feel they're good for this application for the following reasons:

    - Based on the design I need a valve that positively blocks flow in both directions, common direct acting solenoid valves tend to only really block flow in one direction and will pop open when pressurised in the reverse direction, which I can't have in this case
    - Pinch valves are quite simple and there is no material exposed to the process fluid other than the tube material
    - They don't trap any fluid volume or present any cavities where deposits could form, which is good for minimising potential chemical contamination

    The tube itself should be good for several hundred thousand cycles (potentially ~ 1 million rolls of film) and replacement should be easy and cheap.

  10. #40
    Peter De Smidt's Avatar
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    Re: Midtone Machines - Automatic Film Processing Machine

    Peristaltic pumps? I used them with salt-water aquariums for years. The maintenance boils down to replacing the tubing every few years.
    “You often feel tired, not because you've done too much, but because you've done too little of what sparks a light in you.”
    ― Alexander Den Heijer, Nothing you don't already know

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