Vacuum bag approach

Context:

• Venturi (mainly cause I had no idea what this was when I came in):

  • a device that connects to compressed air (left side in the picture below, labeled “air supply” and uses it to create a vacuum through the bottom nozzle.

  • good video: https://www.youtube.com/watch?v=hLZkPFrQCDk&ab_channel=PhysicsDemos

  • Open

    

• In the pre-COVID first stage layup of the bottom panel, a single venturi and vacuum pump were used. However, these two in conjunction were not enough to hold vacuum on the bottom panel.

• Holding vacuum is important for curing carbon fiber as constant pressure on the prepreg is needed for the resin to flow and “wet out” the fibers. Without it, surface layers would become resin starved and in turn, much weaker.

The Approach

Use a bunch of venturi’s to “beat” the rate of air leakage. In theory, the air being pulled out and the air entering the bag via leaks will even out at some point and rest at constant pressure so long as the venturi's stay on and the vacuum bag is not compromised.

A total of 4 venturi’s and 1 vacuum pump were used. The 4 venturi’s came from Formula Electric, Rocketry, Watsub, and Formula 1.

Setup

When it came to the placement of the air inlets in the vacuum bag, there were three factors that played a part in the final placement.

Firstly, certain areas of the bottom panel layup required higher pressure than others. For example, the sides of the hood (blue) needed the most pressure as it was perpendicular to the ground, meaning gravity was constantly trying to peel the Nomex off the bottom panel.

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Secondly, the strength of the vacuum sources (venturi’s and pumps) varied. The strength of the vacuum sources are as follows: vacuum pump, Watsub, Formula 1, Rocketry, Formula E.

The last factor was avoiding choke points. Typically for other layups, it's been common practice to only have one inlet, which works well when you only have one vacuum source. However, when you have multiple sources, if you were to lead all of the sources into the single inlet, you limit the strength of vacuum you can pull; the limiting factor being the cross-section of the air inlet.

Final Layout

(1) Vacuum pump. (2) Watsub. (3) - Formula 1. (4) - Formula E. (5) - Rocketry.

Results

With the 4 venturi’s running, and the vacuum pump sitting idle, the bag was able to hold a constant pressure. Unfortunately, we were unable to measure as the gauge was faulty. However, I was able to get this picture of the bag, to give a general idea.

For context, the bag itself was difficult to move along the bottom panel with friction from your finger.