Vacuum Pressing

Vacuum Pressing

Raw Notes #

This is a temporary page containing raw, unedited notes on this subject. This page and the information here is incomplete, and may be inaccurate.

  • I started by vacuum pressing, and did so for a few years.
  • Dig up old picture gallery of this.
  • I use a dedicated vacuum pump designed to run 24/7. Simple, small, quiet, easily rebuilt, which we did once. I kept a backup on hand, too, just in case.
  • Mold differences.
  • Vacuum tape: the yellow is thicker and easier to get a good seal with. Notes on getting a good seal.
  • Keeping the tap clean during layup. Don’t remove the paper until you have the bag in place, and remove it in stages.
  • Connectors
  • Breather
  • Measuring pressure at the opposite end from the connection point.
  • Time
  • Heat options, hot box. Get old pics of the hot box I built.
  • Disposables and waste.
  • Surface finish
  • Voids
  • Total pressure: what a pump can really draw vs. “total vacuum”.
  • Pre-bending cores to fit the mold.
  • Keeping core alignment during layup and bag setup.
  • Dry run to fit all board pieces as well as breather, bag, fittings.
  • Pre-placing in-bag fittings and some breather.

This is the source for all things vacuum bagging: This is what you want for a thru-bag connector:

Old notes on vacuum vs pneumatic #

  • Pros and cons of each. You can build a fine snowboard or ski with vacuum molding, but for top quality results I really do believe you need to move to pneumatics. I have direct experience moving from vacuum molding to pneumatics and the results are striking.
  • Autoclaves: yikes!
  • Press pros: great pressure application; smooth top sheets; excellent squeeze out; excellent heat transfer.
  • Press cons: expensive to build vs. vacuum; molds must be beefy to take the pressure; requires air compressor; explosion hazard; compressor to maintain (though generally useful for other things);
  • Vacuum pros: low cost; easy to get started; molds are easier to construct.
  • Vacuum cons: poor heat transfer; consumables; uneven top sheets; generally poor squeeze out; vacuum pump to maintain (and not useful for other things);
  • How we made molds. Buying vacuum material at reasonable prices. Yellow vs. the normal vacuum tape. Decent pressure. Measuring pressure well. Heating with a warm box. Include the old vacuum molding layup pics, and any others we can find. Gast pumps (runs 24/7, quiet, cool, you can rebuild them if necessary, well worth the money.) Getting good pressure (wrinkles and such for a good seal, adding extra tape where necessary, keeping the tape area clean, etc.)

Post on vacuum vs. pneumatic #

Posted: Thu Jan 29, 2009 11:26 pm Post subject:

Brewster wanted me to elaborate on my apparent dislike of vacuum bagging based on my statement in another thread: “IMO, a pneumatic press is so, so, so much better than bagging, but bagging is a great way to get started on the cheap.”

I got started with vacuum bagging, and built boards that way for many years. It is a fantastic way to get started since it is cheap and flexible, and way easier to get started with than building a pneumatic press. And you can make some nice boards vacuum bagging. Lots of small builders have started this way. However, of all the small builders that I know that started vacuum bagging, all of them are using a pneumatic press now, as am I. Don’t get me wrong below… vacuum bagging isn’t all that bad, but you asked why I believe it is inferior to a pneumatic press, so that’s what I’ll explain now…

On the process of building:

  1. Vacuum bagging uses a lot of extra disposable materials over a press: bag material, breather fabric, sealant tape. Extra money, extra prep, extra fuss late in the layup cycle, extra trash at the end of it all. Sure, you can try to reuse the bag material, but it’s a pain to salvage and it’s easy to end up with a poor seal next time. To keep cost reasonable you can buy in bulk from excellent suppliers like Airtech International (, but now you’ve got even more big rolls of supplies in the shop.

  2. Heating the laminate evenly is far easier with a press than with vacuum bagging. With a press you can place a heat blanket in close contact with the top and bottom of the laminate, which is what you honestly need to produce a good, consistent board. With vacuum bagging you’re more limited. You can try to place blankets inside of the bag with the board… more hassle, more likelihood of screwing up your blankets, etc. You can heat from below with one blanket, and heat from above with another blanket outside of the bag, but that’s not as good. You can heat the top with infrared heaters, but again, not as even and good.

For the quality of the board, the issues come down to: final top sheet quality, voids, and compression of the glass layers.

  1. Most top sheet material is somewhat rigid, but not that rigid. Uneven distribution of resin under the top sheet and in the top layer of glass translates into an uneven surface finish on the top sheet. Compare the top sheets of a bagged board and a pressed board. One will be irregular, the other will be super-flat. You can mitigate this by precuring the top sheet with a light weave of glass and a careful spread of resin, but now your back into the extra hassle zone.

  2. People assume that if you’re vacuum bagging that there will be no voids in your laminate, no air pockets. Wrong. It’s easy to have voids even when drawing “a vacuum”. You can draw a vacuum out of a small jar, right? The jar doesn’t collapse. It’s rigid enough to withstand the force. You can end up in the same situation vacuum bagging a board. If your core wont bend into the mold completely at ~12psi then drawing a vacuum isn’t gonna bend it in there. You’ll have a void. Maybe it will fill with resin and you won’t notice the weakness, or maybe it won’t and it will have a partial atmosphere in there (because, I assure you, you have not drawn a perfect vacuum.) But the void will be there. It’s really easy to end up with fine air pockets in your laminate vacuum bagging, too, even when your core does bend into the mold. I’ve got more than a few boards with clear top sheets to prove this.

  3. The PSI you get on the board with vacuum bagging without an autoclave is quite low. Measure how many in/Hg you’re drawing from the opposite end of the layup. You’ll find you’re not getting a full atmosphere. I used to get 25.5 in/Hg max with a nice, continuous duty Gast vacuum pump. 1 atmosphere is 29.92. Do the math and you’ll find that translates to about 12.5psi, assuming I’m at sea level and the weather is nice. If I’d drawn a perfect vacuum, I’d still have only gotten, what? 14.7psi or so, again assuming nice weather. The effective PSI you can get on the board with a pneumatic press is significantly better. The net result is you don’t get as good squeeze-out with a vacuum bag, and you end up with a thicker, heavier laminate.

The way you correct these issues is with more pressure, and the easiest (and I believe safest) way to get this is with a pneumatic press. You also get a nice, smooth aluminum plate over the top of your board with a press, which helps impart that nice smooth surface finish. This is quite difficult with a bag.

Now, you can build an autoclave and increase your pressure when vacuum bagging. I’ve never done that, so I can’t give you first-hand experience like I have above with a normal bagging process. However, I’d imagine it adds to the hassle factor. Now you have the added step of getting the bagged board into the autoclave and pressurize the thing. Now you’ve got a pump and a compressor going. Great, two points of mechanical failure now, in addition to the extra materials and the more difficult heat transfer. This is getting better why?

The difference in my boards when I moved to a press was night and day. Thinner, lighter boards with significantly more consistent and improved physical properties.

So, that’s why I like a press better than vacuum bagging. Again, don’t get me wrong, vacuum bagging isn’t horrible, and you will produce some nice results with a bag. I’d would simply assert that you can produce better results with a press.

One final note: cap construction often gets tied up with vacuum bagging, since it’s darn hard to do cap on a small scale with a press, but it’s easy with vacuum bagging. A word of caution: it is very, very hard to get a good pinch of the top sheet material over a cap with the pressure you get from normal vacuum bagging. This means you end up with voids right over the edge, right where you don’t want that weakness. I highly recommend sidewall construction, especially if you are vacuum bagging.

Good luck!

Followup: No, no voids when pressing, and I have made the same board shape, same construction, and same clear top sheet with a bag and a press. There are voids in the bagged board, and none in the pressed board. The glass layers are about 40% thinner (measured from the cut edge, above and below the sidewall) in the pressed board. Part of this is the better pressure. Another part, I believe, is the heating. The heated epoxy flows like water, so you get more even distribution and better squeeze out, even at the same pressure.

Last modified Saturday March 23, 2019, at 20:32:35 UTC

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