GoKawiilWe’re all familiar with mixing red, yellow, and blue paint in various ratios to instantly make all kinds of colors. This works great for oils or watercolors, but fails when it comes to cans of spray paint. The paint droplets can’t be blended once they are aerosolized. Consequently, although spray cans are great for applying even coats of paint to large areas very quickly, spray-paint artists need a separate can for every color they want to use—until now.
Back in 2018, when I first saw professional spray artists lugging dozens to hundreds of cans to their work sites, I was inspired to start noodling on a solution. I’ve worked at Google X, Alphabet’s “moonshot factory,” as a hardware engineer, and I’m now building a startup in mechanical-design software. I’m no painter, but I know my way around mechatronics.
I wanted my solution to be inexpensive and simple enough to build as a DIY project and functional enough for an artist to use, without breaking their flow. So I began prototyping a system that combines base colors while they are still in pressurized form from off-the-shelf cans.
This new rotary pinch valve can be opened and closed in tens of milliseconds and prevents backpressure from clogging lines. James Provost
I tried a few approaches where pres-surized paint from the base-color cansfed through tubes into a mixing channel, before emerging from a spray head. To control the ratios, I decided to borrow a trick that would be familiar to anyone who’s ever had to control the bright-ness of an LED using a microcontroller: pulse-width modulation. Initially, I used electronically controlled solenoid valves to release the paint from the cans. The paint would flow into a mixing channel for a relative duration that corresponded to the ratio of the base colors required to make a given hue. However, this failed because different cans never have the same internal pressure. Whenever two valves were open at the same time, the pressure difference would make paint flow backward into the lower-pressure can.
As an alternative, I removed the mixing channel and tried making the paint pulses from each can sequentially converge into a tube so that no more than one valve would ever be open at a time. Surprisingly, this worked perfectly. The backflow was eliminated, and it turned out that the natural turbulence of the flow was sufficient to mix the paints. Let’s say you want to produce a clementine orange color. This requires yellow and red paint in a ratio of 1:2, so the yellow valve opens for a period of time, and then the red valve opens for twice as long. The system then keeps repeating this cycle of pulses in a rapid pace to instantly create the spray-paint color you want.
The theory is straightforward, but making this work in practice took quite a bit of experimentation. First, I had to determine the actual durations of pulses that would produce evenly mixed colors, not just their ratios. I also needed to work out the size of the tubing (too narrow and you’d get low spray force; too wide and you’d have paint accumulating in the tubes). Eventually I settled on a maximum pulse duration of 250 milliseconds and a tube diameter of 1 millimeter.
Inventing A New Valve
Even though the system worked, the solenoid valves I used constantly clogged up. Designed for water purifiers, the valves didn’t prevent paint from entering the mechanism, where the paint would harden. Moreover, when the valves were turned off, they could stop backflow only if the inlet remained pressurized. So disconnecting a paint can from the system would cause instant leaking. Other off-the-shelf valves I tried couldn’t cycle fast enough and were too expensive.
I had some spectacular failures along the way of the sort that only pressurized paint can provide.
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