How much vacuum are you using
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I just let the pump pull it’s maximum, which will depend on your altitude and the quality of your pump. At my altitude and with my pump I think it’s somewhere around 25 inHg. There are check valves, vacuum switches, and vacuum accumulators that can be connected so you don’t have to leave the pump running while the epoxy cures.
rharshberger
Well-Known Member
iirc from my Dad's fiberglass shop, we rarely pulled more than 20"Hg for most bagging operations, however the advantage to pulling a higher vacuum is that the final composite will be closer to the ideal cloth to resin ration, and compressed as much as possible. The difference between 20" and 29"/30" is about 600 to 700 psi clamping force on a 1 sq ft area (had to look it up), at 29" the clamping force is over 1 ton. Personally I use 20" Hg on all my rocketry projects that are vac bagged and it has worked very well for me.
DaveW6DPS
Well-Known Member
As much as I can get. I use a 200 cubic inch chamber to help maintain vacuum after pumping down.
Thanks for the replies, I used to use an old air compressor pump and washing machine motor... i would just let it run and it would pull about 24" with a good seal....recently i snagged a Gast pump and ordered a vacuum switch...the Gast pulls almost 27" and I'll have to see what the differential on the switch is....should be quieter....i was just curious what other people were using
I'm considering a chamber as well, I'll try it and see how it does without....I do fins 1 side at a time on a glass plate so I can get a pretty good seal...
DaveW6DPS
Well-Known Member
It doesn't have to be too big or too complicated. I use a 10 inch piece of 2.5 inch ID ABS pipe with a tee tapped into one end cap.
That sounds simple enough,...do you have any kind of check valve between the tank and the pump?
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No need to go overboard with flat layups. Anything over 10 PSI is fine; I usually shoot for something like 12. (14.7 PSI is atmospheric pressure at sea level).
stantonjtroy
Well-Known Member
I have to agree with John. I've been working with composites for over 34 years (20 professionally). Typically I shoot for 12"-15". Not a problem all of the time but drawing too much vac can cause the resin/epoxy system to cold boil. Typically this will just add to the pin hole count come filling time but worth mentioning.
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If you can get it to boil then I suggest vacuuming the epoxy before application to get the air (and any moisture) out. I'm presuming no thinning was done. Thinning in even rather small quantity with solvents tends to destroy the physical properties of the epoxy system. Isobutyl alcohol does the least damage on average, IIRC, but I recommend avoiding it. If the viscosity is not what you want, use a different system.
The vapor pressure of epoxy is such that these sorts of pumps are not going to boil off epoxy. I've worked down to around 10^-9 Torr (plasma physics experiments when I was an undergrad) and we're not talking that sort of pressure range here. You might also want to vacuum your core to pull moisture and any other volatiles out before applying epoxy, if you plan to pull a decent vacuum while bagging.
https://www.eng-tips.com/viewthread.cfm?qid=423344
FWIW, I've also done bladder molding, where strength to weight ratio was critical. For that I typically run about 45psi and have been quite a bit higher. Yes I can easily tell the difference in results. I settled on about 45psi as that was somewhere around the point of diminishing returns. Besides, higher pressure increases how far through the walls the mold fragments will go if it lets loose. But anyway these are pressures well beyond what you'll get with any vacuum system. Vacuum can't produce as much compression as you'd like, if your core can take it.
Gerald
The vapor pressure of epoxy is such that these sorts of pumps are not going to boil off epoxy. I've worked down to around 10^-9 Torr (plasma physics experiments when I was an undergrad) and we're not talking that sort of pressure range here. You might also want to vacuum your core to pull moisture and any other volatiles out before applying epoxy, if you plan to pull a decent vacuum while bagging.
https://www.eng-tips.com/viewthread.cfm?qid=423344
FWIW, I've also done bladder molding, where strength to weight ratio was critical. For that I typically run about 45psi and have been quite a bit higher. Yes I can easily tell the difference in results. I settled on about 45psi as that was somewhere around the point of diminishing returns. Besides, higher pressure increases how far through the walls the mold fragments will go if it lets loose. But anyway these are pressures well beyond what you'll get with any vacuum system. Vacuum can't produce as much compression as you'd like, if your core can take it.
Gerald
I've never liked thinning epoxy either...the only times I've done it was when glassing bare plywood to help it soak in a bit better...this time I think I may try hitting it with a thin coat mixed with fast hardener, then wait a bit and coat it with a slow hardener mix....lay the fabric on...then wet it out and bag it....the reason I asked what vacuum people were using is I do have a vacuum switch now and can actually control the amount of vacuum.....
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I built 2 - 4" x 12" chambers, I'm not sure it's completely sealed yet but it runs for about 20 sec every 5 minutes....thats without a Mac valve or a checkvalve....I'm sure it'll be more often with a bag
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It doesn't have to be all that much more leak with a bag. It all depends on you seal, your connections, your breather, etc.
There is much more to properly setting up a bag than just throwing a bag around something and pulling a vacuum. The latter just results in rather uneven vacuum.
If, for example, you are bagging some plywood fins with composites but the fins are not attached to an airframe (to make it easier for this example):
1) Prepair mylars with wax as a mold release.
2) Paint topcoat color on waxed mylars. Use several barely misted coats. DO NOT TRY TO FLOW IT OUT!
3) Let paint dry though it doesn't have to be cured dry.
4) Roll laminating resin (like MGS, Resin Research, etc, not epoxy glue and not thinned epoxy) onto the mylar paint side. Note, put your very well mixed epoxy as a puddle on one side of a plastic picnic plate. It makes it easy to pick up just enough to dampen your foam paint roller, and the thin layer of epoxy prevents it from kicking off quickly (epoxy is exothermic).
5) Lay on dry layer of fine weave fiberglass, like the 1.2 or 1.6oz stuff you can get from various suppliers. You want a tight fine weave.
6) Roll the dry fabric onto the damp layer of epoxy. The epoxy wicks up from the bottom. Therefore there is NO trapped air. Note, when rolling, start from the center and roll in the direction of the fibers. DO NOT ROLL IN FROM THE EDGE! That's how you pick up your fabric and roll it onto your roller! That leads to having a bad day! Once the fabric is wet out then you don't need to be as concerned about the rolling direction, but, never roll from outside to inside!
7) Roll some more epoxy on top that fabric layer.
8) Place dry layer of fabric on top of this stack. Repeat steps 6 and 7 until you have all your layers there.
9) Place a layer of paper towels on top of fabric stack on mylar.
10) Put wax paper on top of paper towels.
11) Put a layer of breather material on top of wax paper. Breather can be fabric, layers of paper towels, cotton batting fabric, etc. It is to provide a vacuum path all around the part. The breather extends beyond the part in all directions.
12) Flip the part over, and repeat steps 10 and 11.
13) Slide into vacuum bag.
14) Put rope in bag along a long edge of the breather, overlapping. Even fold the breather over the rope. Run the rope to end where the vacuum connection will go.
15) Seal the bag.
16) Draw at least a few psi vacuum for at least two minutes to perhaps 5 minutes.
17) Take part out of the bag. Unfold layers down to the now epoxy soaked paper towels. Discard those paper towels. If the fabric layup still looks wet rather than slightly damp, repeat this drydown process steps 9 through 16. When done the appearance of the damp fabric must be uniform over the whole surface.
18) Dampen your core with rolled on epoxy. NOT dripping, NOT running, just dampened.
19) Roll additional epoxy along the perimeter inch. You can make that region wet but again, not runny drippy. Depending on how thirsty your wood happens to be, you might have to roll on epoxy more than once. Don't be in a hurry! Part of the reason we are using laminating resin is to give us the time to do it right! IF you don't have at least an hour working time, you are using the wrong epoxy system or are working at too high a temperature. Each 10 degrees C increase in temp halves your working time.
20) Put your fabric stack with mylar onto the core, fabric down. Typically you'll have two of these you are doing at the same time; one goes on each side of the fin. So you get a mylar sandwich with a fin inside.
21) Put little strips of masking tape at spots along the edge of the mylars, connecting them together so stuff doesn't slide around on you.
22) Put this sandwitch back in the bag, surrounded by paper towels, wax paper, breather fabric, etc, just like before. Seal up the bag.
23) Draw the full vacuum progressively. Check for leaks, and address any now before it is too late.
24) Put bag under vacuum into your cure box. Initial curing temperature should likely be about 120F and no more than that. If you are using West Systems epoxy, you are screwed. Don't use it. It isn't aerospace epoxy; the properties are all wrong. It is excellent epoxy for applying fabric to wooden boats which is a rather different application, pun intended.
25) Leave it there 24 hours.
26) Pull it all apart to get to your part.
27) Put your part back in the hot box, and use the post-cure heat cycle recommended by the manufacturer in the technical data sheets.
28) When your part is cool again, pull it out, clean up the edges, and you have your already painted fin with a perfect finish.
Gerald
There is much more to properly setting up a bag than just throwing a bag around something and pulling a vacuum. The latter just results in rather uneven vacuum.
If, for example, you are bagging some plywood fins with composites but the fins are not attached to an airframe (to make it easier for this example):
1) Prepair mylars with wax as a mold release.
2) Paint topcoat color on waxed mylars. Use several barely misted coats. DO NOT TRY TO FLOW IT OUT!
3) Let paint dry though it doesn't have to be cured dry.
4) Roll laminating resin (like MGS, Resin Research, etc, not epoxy glue and not thinned epoxy) onto the mylar paint side. Note, put your very well mixed epoxy as a puddle on one side of a plastic picnic plate. It makes it easy to pick up just enough to dampen your foam paint roller, and the thin layer of epoxy prevents it from kicking off quickly (epoxy is exothermic).
5) Lay on dry layer of fine weave fiberglass, like the 1.2 or 1.6oz stuff you can get from various suppliers. You want a tight fine weave.
6) Roll the dry fabric onto the damp layer of epoxy. The epoxy wicks up from the bottom. Therefore there is NO trapped air. Note, when rolling, start from the center and roll in the direction of the fibers. DO NOT ROLL IN FROM THE EDGE! That's how you pick up your fabric and roll it onto your roller! That leads to having a bad day! Once the fabric is wet out then you don't need to be as concerned about the rolling direction, but, never roll from outside to inside!
7) Roll some more epoxy on top that fabric layer.
8) Place dry layer of fabric on top of this stack. Repeat steps 6 and 7 until you have all your layers there.
9) Place a layer of paper towels on top of fabric stack on mylar.
10) Put wax paper on top of paper towels.
11) Put a layer of breather material on top of wax paper. Breather can be fabric, layers of paper towels, cotton batting fabric, etc. It is to provide a vacuum path all around the part. The breather extends beyond the part in all directions.
12) Flip the part over, and repeat steps 10 and 11.
13) Slide into vacuum bag.
14) Put rope in bag along a long edge of the breather, overlapping. Even fold the breather over the rope. Run the rope to end where the vacuum connection will go.
15) Seal the bag.
16) Draw at least a few psi vacuum for at least two minutes to perhaps 5 minutes.
17) Take part out of the bag. Unfold layers down to the now epoxy soaked paper towels. Discard those paper towels. If the fabric layup still looks wet rather than slightly damp, repeat this drydown process steps 9 through 16. When done the appearance of the damp fabric must be uniform over the whole surface.
18) Dampen your core with rolled on epoxy. NOT dripping, NOT running, just dampened.
19) Roll additional epoxy along the perimeter inch. You can make that region wet but again, not runny drippy. Depending on how thirsty your wood happens to be, you might have to roll on epoxy more than once. Don't be in a hurry! Part of the reason we are using laminating resin is to give us the time to do it right! IF you don't have at least an hour working time, you are using the wrong epoxy system or are working at too high a temperature. Each 10 degrees C increase in temp halves your working time.
20) Put your fabric stack with mylar onto the core, fabric down. Typically you'll have two of these you are doing at the same time; one goes on each side of the fin. So you get a mylar sandwich with a fin inside.
21) Put little strips of masking tape at spots along the edge of the mylars, connecting them together so stuff doesn't slide around on you.
22) Put this sandwitch back in the bag, surrounded by paper towels, wax paper, breather fabric, etc, just like before. Seal up the bag.
23) Draw the full vacuum progressively. Check for leaks, and address any now before it is too late.
24) Put bag under vacuum into your cure box. Initial curing temperature should likely be about 120F and no more than that. If you are using West Systems epoxy, you are screwed. Don't use it. It isn't aerospace epoxy; the properties are all wrong. It is excellent epoxy for applying fabric to wooden boats which is a rather different application, pun intended.
25) Leave it there 24 hours.
26) Pull it all apart to get to your part.
27) Put your part back in the hot box, and use the post-cure heat cycle recommended by the manufacturer in the technical data sheets.
28) When your part is cool again, pull it out, clean up the edges, and you have your already painted fin with a perfect finish.
Gerald
I have to say, I'm very happy with my pump now....I had a problem with the vacuum switch causing the motor to " chatter" a bit at shutdown...so I added a $13 programmable delay timer relay....the vacuum switch flutters a bit as it hits the level of vacuum I've set it for, rapidly switching the motor on and off until the switch completely shuts off. The timer relay allows me to set an amount of time after the vacuum switch shuts off before the power to the motor is interrupted....I have the delay set for 250ms and probably don't need anywhere near that much....but it runs great now.....holds a vacuum for about 1 hour then runs a few seconds...much nicer than running continuously...
