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Q-talk 142 - Vacuum Pump Controller

by Mike Evans

Why did I feel I needed one? I’m no expert on vacuum pumps, so I bought a $75 wet pump from Harbor Freight. After reading up on it, I found out two things: 1) It will over-heat if run too long. 2) It goes through the lubricating oil fairly fast. So, if I wanted to maintain a vacuum on a part for the duration of a cure, that could be anywhere from 2 -6 hours. That’s not even considering the energy consumed (~300 W).

Step Under Vacuum.

I’ve been fascinated with vacuum forming for quite a while. So, when I started thinking about building some steps for my Tri-Q200, I had a small, irregular part that was very difficult to glass. With a vacuum on the laid up part, I can ensure that no glass comes away from a corner or edge and if I have a depression, the glass will be forced into it and stay until cured. One of the current aircraft manufacturers uses a slightly more advanced technique where the resin is sucked into the glass with vacuum. They claim it yields a lighter and stronger part than a conventional layup without vacuum bagging.

Rough Step After Cure.

Just as an example of how good it can work – I carved a small piece of foam (about 1.5” square and only 1/2” thick) and tried to wrap one ply of bid around it and then vacuum cure it. It started as a disaster. It was too small to “wrap” neatly, so I ended up wrapping it like it was a Christmas present! Amazingly, after the vacuum cure, all the edges were flat and it looked quite good.

Like any normal 21st century person, I got on the internet. I found this link that suggested the use of an old auto vacuum advance regulator. I went to my local store and told the guy what I wanted, and he said cars don’t have those anymore. No great surprise there. But we checked on some older cars, like a ’79 Chevy, and he found that car had one, and he even had it in stock. I also found some springs in the same store and even some plastic fittings, so I was on my way.

The basic concept is simple. The vacuum regulator is, of course, tied into the vacuum system. As the vacuum increases, a rod moves at the other end of the regulator. I simply arranged a micro switch which would be activated by the rod such that, when there was no vacuum, the switch would be “on”, providing power to the pump. When the vacuum increased, moving the rod to its limit, the switch would be activated to shut off the pump. The purpose of the spring is to “bias” the vacuum regulator to function within a range of vacuum, say from 14 to 20 in. of Hg. By reducing the spring tension, this range could be shifted to 5 – 12, etc. I have observed that a vacuum of only 10 inches is quite adequate for this purpose. By the way, the spring is strong enough that I had a hard time extending it even a little with my hands.

Depending on how good a seal I had, the pump would run on about a 30% duty cycle. However, I wanted to be able to run it even less. I realized that I didn’t need to keep a vacuum on the part ALL the time. If I pulled a vacuum once every 5 minutes, for example, that would still do the trick. I could not do that with this controller, though.

So, having been an electronic technician in a previous life, I decided to try building an electronic circuit which would give me more control. I won’t go into those details in this post, but I will say that I used two 555 timers, one to control the “on” time of the pump and the other to control the “off” time. The 110v interface is a solid state relay that is controlled with a 5-15 volt signal from the timers. The result is I can turn the pump on anywhere from 10 seconds to 2:30. The off time can vary from about 1 min. to around 10 min. Of course, at 10 minutes, the part is without vacuum most of the time, but each time the pump runs, the glass gets sucked back where it belongs and as the resin hardens, the glass relaxes less between “sucks”.

Sandwich materials ready to apply vacuum.

A few more details on the system: The pump has a capacity of 2.5 cu ft per minute, which seems adequate. I used 3/16” clear tubing (no reinforcement) which I picked up at Home Depot. I drew down a full 30” and nothing collapsed. I used a mason jar as a resin trap (I put it in a 1# coffee can to be safe) and my son’s 5 gallon soda keg as a vacuum reservoir (Since it is made to contain pressure, not vacuum, some modification must be made to the lid). Oh yes, Harbor Freight also had a big auto vacuum gauge for about $12 which works very nicely – easy to read. I had some plumbers putty lying around, so I’m using that as a sealant for the bag. Here’s a good primer on bagging. I haven’t been using all the materials Bertram recommends. I just put some peel ply around the piece and then some breather cloth over that. Of course the bag surrounds the whole mess. You will also want to put some breather cloth over the entrance to the vacuum line so that it doesn’t plug up with resin or plastic.

In case anyone is wondering if the full atmospheric pressure will crush the blue foam, I have tried it and no problem. Of course, the pressure is pretty equal all around the part, but it is still over 14# per Sq. inch max.

Here’s the basic component list:

  1. Vacuum pump 5/16 by 3/16 “ vinyl tubing ($6 for 20 ft)
  2. Automobile vacuum regulator (# on box V435 or M09201 (Auto parts store, about $15)
  3. Micro switch (110 v, 5A or more, single pole, double throw - SPDT or SPST)
  4. Strong spring
  5. Eye bolt & nut
  6. Vacuum reservoir (I have 5 gal)
  7. Vacuum gauge (optional – about $14 at harbor freight)
  8. Peel ply
  9. Breather cloth (almost anything that has some thickness will do)
  10. Bag sealant (Plumbers Putty – 14 oz for $2, reusable)
  11. Resin trap (optional – I have not had any issue with resin being sucked up.)