Author Topic: Automatic paint mixing.  (Read 816 times)

Online S. Heslop

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Automatic paint mixing.
« on: September 01, 2018, 01:04:43 PM »
Been posting about this in another thread but thought I should make a dedicated one.

I've got a vague idea that I could set up some stepper motors to depress syringes to dispense small amounts of primary coloured paints in small and hopefully accurate quantities to closely match a colour selected from an image on a computer. With the aim of making mixing colours for acrylic or oil painting a simpler process.

Going with cyan, magenta, yellow, black, and white since that should give the widest range of colours possible from a handful of primaries.

(these are blunt needes, not much risk of accidentally injecting myself)


So trying to match this target colour with 60% cyan, 20% magenta, 60% yellow, and 0% black by the CMYK colourspace.



Dispensed .60ml cyan, .20magenta, .60 yellow, and .40 white. Or as close as I can get.



It's somewhat close. To the right is adding 20% more black at a time to 60% to match the darker shade in that earlier image.


So there's a few things I sort of understand that I need to explain, or at least state that i'm aware of. I'm aware that the RGB display gamut doesn't entirely overlap with the gamut of CMYK, and i'm aware that displays are emissive where as paints are reflective. Or additive and subtractive. And that ambient light (as well as the colour balance between cameras and displays) will make it very difficult to get an exact match. Even jpeg compression shifted the shade of that colour swatch a bit.

The other thing is that CMYK in printing works by overlapping dots of somewhat tranlucent inks, and the white is from the background. That's why im working on the assumption that the amount of white to mix in will be whatever percentage is left over from only the highest percentage of the other colours.

What i'm having a hard time getting my head around is the idea that these volumes might need to be proportional to each other in some way. And perhaps different pigment levels in each paint might affect the ideal proportions. I think more tests are in order, but i'm thinking about how I could try get some actual data to calibrate stuff with.


Mechanically i'm also considering if there's a better way to dispense. The plastic syringes tend to flex a bit and trapped air bubbles up at the plunger end don't help either. Ideally i'd want something that'd really jet the paint out to minimize strings and droplets hanging on the tips of the nozzles.

Offline tom osselton

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Re: Automatic paint mixing.
« Reply #1 on: September 01, 2018, 04:12:43 PM »
You could try going to a  paint store and ask if it would be possible to get a copy of their formulas. Somewhere around the house I have some transparent Mylar that is coloured in 10% to 100% tones of each primary colour made to be stacked to achieve the desired colour it can be altered by the tone of the paper itís held against.

Online S. Heslop

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Re: Automatic paint mixing.
« Reply #2 on: September 01, 2018, 04:30:41 PM »
I'd kind of assumed paint store mixers were all automatic now. I was looking at paint mixing machines online and trying to find instruction manuals but couldn't find anything that seemed useful. Could be worth the trip though.

Offline BillTodd

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Re: Automatic paint mixing.
« Reply #3 on: September 01, 2018, 05:57:31 PM »
interesting lecture wbout colour perception

Bill

Offline seadog

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Re: Automatic paint mixing.
« Reply #4 on: September 01, 2018, 06:57:41 PM »

Offline Joules

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Re: Automatic paint mixing.
« Reply #5 on: September 02, 2018, 01:49:36 AM »
How about some form of colour feedback as your batch is mixed.

https://www.amazon.co.uk/Adafruit-TCS34725-Color-Sensor-Filter/dp/B00OKCRU5M/ref=sr_1_7?ie=UTF8&qid=1535867119&sr=8-7&keywords=adafruit+sensor

Maybe even copy existing colours, no idea how accurate the sensor is.
Just get doing and make swarf, you can decide what its going to be later.   :thumbup:

Offline Will_D

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Re: Automatic paint mixing.
« Reply #6 on: September 02, 2018, 04:36:53 AM »
Surely the most accurate method to dispense paint would be by weighing the amount delivered. Try to integrate a digital scale into your system!
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Online S. Heslop

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Re: Automatic paint mixing.
« Reply #7 on: September 02, 2018, 12:32:39 PM »
How about some form of colour feedback as your batch is mixed.

https://www.amazon.co.uk/Adafruit-TCS34725-Color-Sensor-Filter/dp/B00OKCRU5M/ref=sr_1_7?ie=UTF8&qid=1535867119&sr=8-7&keywords=adafruit+sensor

Maybe even copy existing colours, no idea how accurate the sensor is.

The idea is you'd be doing the mixing by hand, and the machine just dispenses. Something that mixes too would need to have the whole chamber purged every time you want a new colour. And it turns out artists paints are pretty expensive!

Offline Joules

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Re: Automatic paint mixing.
« Reply #8 on: September 02, 2018, 01:05:49 PM »
It was more the feedback could be used to refine the dispensing.  A sample on a piece of clear tape, or mix within a glass dish.  It would also tell you if the mix was consistent...  errr maybe...  or near match to a sample colour.  Create colour swatches etc with defined values, that would change from paint to paint that are used as your primaryís.
Just get doing and make swarf, you can decide what its going to be later.   :thumbup:

Offline JerryNotts

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Re: Automatic paint mixing.
« Reply #9 on: September 02, 2018, 03:23:50 PM »
I nearly jumped out of my skin when I read the title of your thread. In a sort of dissolute way I have worked for nearly 50 years in paint laboratories.

One of the paintmaker's long term problems has been colour matching. I have had my share of frustration. I don't know whether I should follow this thread  or not.

My own colour perception is fairly adequate, certainly enough to frustrate any one who apporoaches me with an Ishihari book.

It's not just getting the colour that needs work.

Jerrry

Offline tom osselton

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Re: Automatic paint mixing.
« Reply #10 on: September 02, 2018, 03:38:12 PM »
They made us use densitometers at work trying to get the pictures the same on their two presses as well as cut back on ink they gave you the colour breakdown in %.

Online S. Heslop

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Re: Automatic paint mixing.
« Reply #11 on: September 02, 2018, 07:36:34 PM »
It was more the feedback could be used to refine the dispensing.  A sample on a piece of clear tape, or mix within a glass dish.  It would also tell you if the mix was consistent...  errr maybe...  or near match to a sample colour.  Create colour swatches etc with defined values, that would change from paint to paint that are used as your primaryís.

Oh right. I do think trying to get some actual data would be the next best step. I wonder how accurate those colour sensors are. I was reading about them a bit ago, can't remember why, but they have an array of red green and blue filters over a bunch of light sensors if I remember right. Wonder if that'd affect how the colour is read.

Surely the most accurate method to dispense paint would be by weighing the amount delivered. Try to integrate a digital scale into your system!

For small amounts i'd need an analytical balance. And I have exactly that, but it's display is toast and replacements are £120. I've been sitting on it since replacing the display is a bit on the expensive side, and since it's broken i'd be hesitant to try sell it. If I was smarter i'd try reading the display data and seeing if I could output that to a different and cheaper display.

I still think stepper motors on syringes would be the easiest way to dispense. I've just been looking at the world of solder paste dispensing syringes. They seem alot sturdier than the medical type.

I nearly jumped out of my skin when I read the title of your thread. In a sort of dissolute way I have worked for nearly 50 years in paint laboratories.

One of the paintmaker's long term problems has been colour matching. I have had my share of frustration. I don't know whether I should follow this thread  or not.

My own colour perception is fairly adequate, certainly enough to frustrate any one who apporoaches me with an Ishihari book.

It's not just getting the colour that needs work.

Jerrry

This does seem to be a much more complicated topic than i'd anticipated. Matters aren't helped by everyone seeming to avoid answering the questions when asked on other forums.

That said I feel exact paint matching isn't so important for art. It'd be a big deal if youre touching up a wall or your car where you're seeing the two shades against each other in differing light, but for an entire painting they'd only need to be consistent to themselves. But it'd be nice if its accurate too.

Offline rowbare

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Re: Automatic paint mixing.
« Reply #12 on: September 04, 2018, 10:41:02 AM »
Mechanically i'm also considering if there's a better way to dispense. The plastic syringes tend to flex a bit and trapped air bubbles up at the plunger end don't help either. Ideally i'd want something that'd really jet the paint out to minimize strings and droplets hanging on the tips of the nozzles.
Stepper driven peristaltic pumps would be a good way to go. They are easy enough to DIY or you can get pump heads on eBay or Banggood for cheap. With a stepper driven pump, you could calibrate them to steps/ml or mg. You can also get small motor driven peristaltic pumps that might do the job. These you would calibrate by time ml or mg per second.

As for scales, you can get cheap mg electronic scales quite cheaply.

Good luck with the project and keep us posted!
bob

Offline JHovel

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Re: Automatic paint mixing.
« Reply #13 on: September 04, 2018, 10:54:35 AM »
I think you are on the right track.
There are plenty of stepper-driven syringes in use in hospitals for all sorts of things including pain killer drugs, even epidurally. So the accuracy of even very small doses of liquids can be controlled well that way.
For this purpose, I actually think you don't even need 5 pumps, you could swap syringes in the same pump, since it won't get contaminated. You just have to load the syringes in the same order each time  :nrocks:
Cheers,
Joe

Online S. Heslop

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Re: Automatic paint mixing.
« Reply #14 on: September 04, 2018, 04:42:30 PM »
I did think about peristaltic pumps but i'm not sure how you'd handle the non linear flow rate. There's probably a way to map it. Plus for viscous fluids I wonder if there's a chance the tube could sort of... collapse. I assume they work on the elasticity of the tube returning it to roundish to draw more fluid in at the start of the stroke, or from pressure on the reservoir to force it in. It seems like alot to go wrong, or change over time, or with different paint thicknesses.

Yeah I think syringes seem most promising. The main issue is how i'd handle (and eliminate) stringing. I've got some stuff to read on that but I think what might work is retracting or suck back. That at the end of a dispensing stroke the stepper can pull the plunger back a touch to hopefully break the flow sharply and maybe pull whatevers hanging on the end of the nozzle back in.

This might be another slow project though. I've been looking at finally building that big 3d printer first since some arrangements I had fell through, but ones that've freed up a bit of money I want to try spend before it slowly disappears on all the Good Stuff I shouldn't be buying.

Offline Joules

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Re: Automatic paint mixing.
« Reply #15 on: September 04, 2018, 05:03:22 PM »
Youíre getting into the territory of the 3D printer using retraction.  Do you still plan on having 5 nozzles ?

How about using a wide section of wiper blade on a servo to squeegee the syringe ends.  The wiper sweeps the same colour area each time so no cross contamination.  Maybe stagger the syringes in a line if you drive each one individually.  I was trying to think up a way of using one stepper and cycling the syringes as mentioned earlier, the wiper would also move and use the same section per colour.

Interesting project, can see 3D printing being very helpful here.  Look up gattling guns for rotating your syringe prior to extrusion of paint, or Geneva mechanisms.  Now youíre at two steppers and a servo.

In fact with two Geneva mechanisms, one could rotate the paint and the other do the wipe per colour, now we do away with one servo as well.
Just get doing and make swarf, you can decide what its going to be later.   :thumbup:

Offline Joules

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Re: Automatic paint mixing.
« Reply #16 on: September 04, 2018, 06:20:00 PM »


I particularly like the above mechanism as it can do the index and wipe in one action.  The indexing lever for the wipe ring could raise the wiper a fraction so all the other wipers miss the other syringes.
Just get doing and make swarf, you can decide what its going to be later.   :thumbup:

Online S. Heslop

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Re: Automatic paint mixing.
« Reply #17 on: September 04, 2018, 06:35:32 PM »
I had also thought about a carousel. More than anything I figure it'd be slow as each of the 5 colours is indexed and the stepper resets it's position for each plunger. And you'd miss out on the ability to draw paint back.

It would look very cool though, and having one stepper motor would make it easy to make it a fairly chunky one. I also wonder if you could come up with some sort of mechanism so at the end of the stepper motor screw's travel it engages the indexing mechanism, meaning you'd only need one motor. Plus it'd make the dispensing tips easier to access and reduce the need for any kinds of pipes routing the syringes to the tips.

Wipers could be an option. But I think disposable dispensing tips might be better way to deal with dry paint building up on the nozzles. I'm thinking though that instead of a shutter, borrowing more ideas from 3d printing, some kind of silicone rubber condom that sits over the nozzles that can serve as both a seal to stop them drying out when the device is placed on its stand as well as be easy to clean off if dry paint builds up on it. I guess my biggest concern with strings of paint is that they might affect the dispensing accuracy if you've got a considerable amount of paint sometimes dangling and sometimes falling off.


It's fun thinking up goofy mechanisms. What if there's a carousel and the stepper motor's connection to the syringe plungers is a kind of comb shape that fits around the plunger, or something connected to the plunger, so it doesn't need to move as much to line up with each syringe and can do the retract at the end of the stroke. What if it can push all the syringes at once but 5 sorts of clutches or pawls disconnects each syringe from the stepper as it reaches the correct amount.  What if compressed air pushes the plungers like in solder paste dispensers, but there's some kind of encoder to read the position of the plunger rather than trying to measure the flow... But by the end of the day I think a £10 stepper on each syringe would probably be the easiest to get working, and still somewhat affordable.  I just wonder if they'd have enough power to really jet the paint out through a small restriction.

Offline Joules

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Re: Automatic paint mixing.
« Reply #18 on: September 04, 2018, 07:37:30 PM »
I think the question becomes, how much is the minimum paint to mix per batch.  The larger the amount, the smaller the error in paint at the nozzle.  The Arduino should track syringe position so it can line up on each one and know when one will need replacing.  I estimate a complete set of five colours extruded and indexed in under 30 seconds, depending on volume extruded.  Perhaps lift the carousel off and seal the ends in one go when not in use.  Certainly mix a few colours each session, than just one or two.  Nema 17 steppers would be man enough for the job, they manage to force plastic through fine nozzles.  Using the carousel you can deposit paint into a small screw top tube for mixing and storage prior to use.

Another problem with retraction would be, drawing air into the nozzle would promote quicker drying.  Look at inkjet printers and how they purge prior to printing.  All that expensive ink wasted, as printers didnít used to purge like they do now.
« Last Edit: September 05, 2018, 03:21:01 AM by Joules »
Just get doing and make swarf, you can decide what its going to be later.   :thumbup:

Offline PK

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Re: Automatic paint mixing.
« Reply #19 on: September 05, 2018, 05:07:36 AM »
There was recently a thread on solder paste dispensing in which micro-pumping using either a solenoid or piezo element was used to pump tiny qtys.


I wonder if that approach could have any application here...


Back to syringes, but still on the solder paste theme.... http://www.applecture.com/electronic-dispenser-for-manual-solder-paste-application-to-pcb-for-reflow-soldering-or-hot-air-soldering-technics-53341

PK

Offline Joules

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Re: Automatic paint mixing.
« Reply #20 on: September 05, 2018, 07:58:02 AM »
Love the motion of the machine, thatís the way to build a 3D printer.  Being able to move the nozzle independent of the bed for step over, just use the table for layer height.  Wide belts for drive....   Sorry wrong thread.

 :drool:
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Offline Chilliphil

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Re: Automatic paint mixing.
« Reply #21 on: September 05, 2018, 09:42:56 AM »
Can't help with the electronics side, but to get proportions have you looked a good old fashioned Pantone chart? If I recall there may be a CMYK version which will give you mix proportions for a visually matched colour. Printers get remarkably good at matching a Pantone sample against your target colour.

Online S. Heslop

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Re: Automatic paint mixing.
« Reply #22 on: September 05, 2018, 11:05:26 AM »
I did find those solder paste jet printers. I thought I mentioned them but I might've forgot to... I couldn't find much on how they actually work, just the implications that there's some fancy fluid dynamics going on to eject a droplet of solder paste. Cool machines though.

I've been looking at this solder paste dispenser. It's somewhat similar to the one linked. https://www.youtube.com/watch?v=cU9yRKr6NlM

Also i'm still not entirely sure what Pantone is. All I find when looking for it are big tables of predefined colours. It seems to be a fairly proprietary system.


Oh I was thinking when going to sleep that I should say I appreciate all the suggestions even if i'm dismissing alot of them. I'm enjoying thinking about this.

Offline Chilliphil

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Re: Automatic paint mixing.
« Reply #23 on: September 05, 2018, 01:01:23 PM »

Also i'm still not entirely sure what Pantone is. All I find when looking for it are big tables of predefined colours. It seems to be a fairly proprietary system.

[/quote]

Sorry I should more accurately have said Pantone guide. When I owned a printing business we had a swatch guide book containing hundreds or thousands of colour samples with the proportions of CMYK or mixes of readily available spot colours to mix to get your desired target colour. These books are not cheap (check out Ebay for example) but they are the standard colour referencing system in the print / design world. On reflection this might be far more complicated than you need but if it's of interest check out Pantone's website.