Plastic bag tech inflation/deflation mechanism

David Fefferman

I’ll be using this (https://www.stanwinstonschool.com/tutorials/make-a-monster-plastic-bag-technology) plastic bag technique to make a fleshy, pulsating wall in a haunt that I’m building. Essentially, a person will be melting into the wall from the neck-up, and I’d like to make the organic, fleshy pieces on the wall inflate and deflate to the rhythm of a heartbeat.
I’m wondering if anyone has a good technique to automate the inflation/deflation of these bags? I was thinking an inflatable bed pump, but I don’t know how I could automatically switch the flow of air. Hook it up to a servo that physically flips a switch? Seems overly complicated. I’m open to ideas!

Chris Ellerby

Hi David,

That sounds like a great idea.  One option would be to divide parts of it into sections that would have the heartbeat effect so you limit the total volume of air that needs to change pressure to get the effect.  For example having some veins/tumors on the wall that do the pulsating while the rest of the wall remains at a steady pressure.

For a good pulsating effect you want to keep a base line of pressure in the volume, then rhythmically increase/drop the pressure by a set amount.   To me it sounds like a large piston might be a good candidate. 

Lets say you have a volume of air that's being kept pressurized by a constantly running fan. much like what's used on bouncy houses.  If you had a large piston attached to that same volume, like a plunger in a big PVC tube, when ever that plunger cycles up/down it would increase/decrease the inflated volume.  This could be automated on a rotating crank shaft, much like the pistons in a car.   You would likely need to add a one way valve coming off the fan, so when new air is introduced to the volume by the piston it does not escape back out the fan intake.  You can easily make a one way valve just sing a light weight flap that can only open in one direction.

I would do a small scale mockup to see if that effect works.  And also do a lil math to find out how big the piston would need to be in relation to the total volume of the envelope to make sue it's possible.  I would suggest not having the air volume too high (make it somewhat flat) so it does not require a massive amount of air to pulsate it.

Pneumatics area another option.  You could use a solenoid valve to give quick bursts of air to the volume, which might achieve the effect.  It would be somewhat loud, so you may need to have the valve hidden some distance from the effect, and it would require some electronics to automate.  Does your haunt already use pneumatic systems?

Another option, if that person in the wall is an actor and not a prop, would be to have them operate the effect manually using a bellows style foot pump.  You can use a dummy lower body on the outside, or just have their lower body already melted into the wall to obscure the leg operating the pump.  Manual control is often the easiest solution.

/Chris

David Fefferman

Hey, thanks for the input Chris! All excellent ideas. I think the “external lung” piston inflating the walls to their maximum is the most viable. I can do some digging, but offhand, do you know of any crankshaft-like electric motors with enough torque and speed control to gently plunge a multiple-foot piston? Maybe I could rig something to a power drill at lowest speed..? Heck, the mechanism itself might end up being a neat prop to put on display as an iron lung keeping a creature in a neighboring room “alive.”  
I do have pneumatics for another experience in the haunt, but the quick-burst nature might not give me the pulsating look I’m aiming for. 
I wasn’t planning to have any actors in this room but, on your recommendation, I’m going to pick up a bellows style foot pump to experiment with. At the very least, it should help me flesh out a prototype. 4ish hours of pumping might get a little tiresome, even if I routed the pump to an adjacent, actor-occupied room. Plus, I’d likely need a few of them to get the air volume necessary to really make the room pulsate.

David Fefferman

Just a little update, here. Spoke with an electrical engineer friend and he’s pretty sure that air pumps (like air mattress pumps) inverse airflow based on polarity. By using an arduino controller, he thinks we can flip polarity at a set interval to get a blow-then-suck action to make the walls pulsate.
Given the complexity of hooking up an external electronic control to something like an air piston, the air pump just seems like a more direct, less complex solution with fewer points of failure. Going to meet up with him in the coming weeks and give this a go! 

Chris Ellerby

How big the mechanism needs to be would depend on the volume of air in the envelope for your wall, and how much that volume needs to change to get the effect you desire.  That's part of why I was suggesting limiting the segments that need to pulsate to a smaller volume so they can update more rapidly and with more movement.

One popular DC motor used a lot in haunted attractions is automotive windshield wiper motors. 

I also wonder if there are other ways to get the pulsating effect mechanically (something moving behind the plastic) rather than requiring the air volume to change, which is somewhat complicated.   It's an interesting challenge!

/Chris