bleed and feed

Discussion in 'Fiberglass and Composite Boat Building' started by Tungsten, Dec 19, 2013.

  1. Jim Caldwell
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    Jim Caldwell Senior Member

    Probably because I was worried about a useable part and let the resin flow longer.
     
  2. Steve W
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    Steve W Senior Member

    I just got done with my first two actual infused parts for a customer, they are port and starboard mirror images of each other about 13ft2 each, I weighed them after they were all trimmed up and there was only maybe 1/2 lb difference if that, weighed on a bathroom scale so not that accurate but near enough. I expected the second part to be heavier because I felt the first was a little drier than i would like so we shutoff the vacuum on the second part as soon as it was fully infused so that it would keep clamping pressure but would not continue to draw resin out of the part. We had very little leak down and only dropped to 24hg by the time exotherm was complete using VE resin so I think it worked to a degree but still looks drier than im used to with vacuum bagging. I think im going to do another test panel without the core with the same layup so I can get a good handle on glass/resin ratio so i will know what it looks like.

    Steve.
     
  3. groper
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    groper Senior Member

    shutting off the vacuum will not "dry out" the laminate. Its actually a risky practice...

    What happens is the laminate relaxes as the vacuum pressure drops, and it will draw in air through whereever is leaks in. This will create dryish looking patches and pinholes...

    Leave the vacuum on same pressure whilst it cures, and you should have a high quality, pinhole free laminate...
     
  4. Tungsten
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    Tungsten Senior Member

    Most of my tests have been just glass,slowing things down and allowing the part to fully wet out before clamp off has made some very clear samples,much better then any of my wet bagging attempts.I've always left vacuum on since Ve resin cures so fast it isn't an issue.

    Sometimes the timing is just right and the resin kicks as its flowing through the peelply break,about an inch before the vac.
     
  5. Steve W
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    Steve W Senior Member

    Thanks for that Groper, I took your advice on the second part and held the mesh back 2" and used extra strips to feed the corners and it worked well,when we shut the vacuum of we did monitor the gauge and it did not go below 24hg before max exotherm. The end result was pretty much the same as the first, really quite nice parts but I would like to see it a bit more resin rich. Im not sure what I have to do to achieve this.

    Steve.
     
  6. groper
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    groper Senior Member

    You cant produce resin rich parts with infusion - thats the beauty of it. This begs me to ask you why you want to waste more resin, and waste more money, and produce heavier parts, just because you would like to? Are you getting pinholes or some other imperfection? If so, adding more resin is not the answer...

    The way in which you can control the vibre volume fraction is via the vacuum pressure BEFORE AND DURING infusion - not afterwards. This alters the compaction of the laminate.

    For example, if i infuse at 99.999% vacuum (very near absolute vacuum) i get laminates with about a 70% fiber volume fraction. even with such low resin content, there is no pinholes or any other imperfection, the laminate looks like a pane of window glass. If instead i open the vacuum bleed valve a little, and only pull down to 80% vacuum before infusing, the fiber volume fraction drops to around 64-65%.

    I use epoxy, so very strong vacuum pressure is not an issue for me as the is no volatiles in the resin which can be boiled off at high vacuum levels. As you are using styrene resins, you will have to approach things diffferently. I would suggest bringing up your vacuum to as close to absolute as possible to remove any moisture first. You need to get above 29inches at sealevel at normal room temps before this will happen. Then after an hour or so, depends on the moisture content of your layup, you will need to back it off down below the vapour pressure @ ambient temp of styrene. Once at this point and allowed some time to stabilize, proceed with the resin inlet.

    This target vacuum pressure (gauge pressure) varies depending on your altitude - which is why its better from now on to discuss vacuum pressures in terms of absolute vacuum pressure - NOT gauge pressure. This means specifying the vacuum in terms of pressure above absolute zero. Gauge pressure is the pressure below ambient - which can vary depending on atmospheric weather conditions and altitude. So whilst your gauge might read the same every time you infuse, your not being consistent as the weather is always changing from day to day. Absolute pressure gauge is important piece of gear that professionals use. If your close to the boiling point of styrene, the day to day variation in atmospherics could put you over the edge and cause some boiling of the styrene which can produce voids in your laminate.

    Just like vacuum bagging, if the laminate is allowed to relax after its been infused (vacuum pressure is reduced) it will destroy the quality of the laminate very quickly. Air flows back into the bag through the vacuum port as the vacuum pressure is lost. The loss in pressure has to be equalized from somewhere - so even if your bag is sealed perfect, it comes back in through the outlet. If nothing enters the bag- the vacuum pressure has to remain the same! So reducing vacuum pressure after infusion, is the same as deliberately creating a leak!

    The only way you can get away with it, is to keep the reduction in vacuum below 200mbar - as this is the relaxation pressure exerted by an e-glass laminate. That is, it wont spring back and suck air into the laminate itself, with less than 200mbar change, instead it will only suck air into the bag cavity above the laminate and reduce pressure that way.

    Anyways, there is no good reason to do it - despite many people are doing it.
     
  7. Tungsten
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    Tungsten Senior Member

    On my first attempts I had the resin bucket level with the part,this relaxed the bag around the inlet area around half way into the infusion so there was extra resin there.After I clamped off this resin moved its way to the vac making a more resin rich part.I now lower the bucket before opening and I don't get the bag relaxing anymore.So there appears to be a fine line there about at what level the bucket is at.
     
  8. Steve W
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    Steve W Senior Member

    Groper, lot of good info there to digest. I am at 1430ft above sea level so I can only pull a bit under 29hg, to be clear we didn't let any air into the system, we shut the vacuum off and it never dropped below 24 which is less than the 200 mbar before exotherm was complete. I just inspected the parts more thoroughly a few hours ago and there are no pinholes, they are nice parts, they just look a little drier than the original test panel which I think is close to perfect. On reflection I think the only difference is that the test panel was done on glass with the mat side of the 1708 up while the actual parts were done on a melamine table which is nowhere near as shiny and also the mat side was against the table and the peel ply as they are sacrificial when prepping for paint. The parts are 12.11 ft2 and weigh approximately 29.5lbs each.

    Steve.
     
  9. Steve W
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    Steve W Senior Member

    Tungsten, so far we have been keeping the bucket below the part but not on the floor because even though I can get the room, the materials and mold table to 77F or more its always a lot cooler at the floor and will cool the resin down pretty quick, on our first part i didn't think about this and it took a lot longer to reach exotherm, the second part we sat the bucket a foot off the floor and a much faster gel.

    Steve.
     
  10. Tungsten
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    Tungsten Senior Member

    I had never done the conversion from Mbar to inHg,200Mbar=5.9Hg!I guess I have no reason to panic over a 2-3Hg loss then.
     
  11. groper
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    groper Senior Member

    Completely depends where the loss comes from :idea:

    If the reduction in pressure comes through the outlet - via opening a bleed valve to reduce vacuum - then the air entering the bag changes the pressure over the area where no liquid is present. This might be between the outlet and the resin brake for example. less than 200mbar change, no relaxation change in volume, no harm done to the laminate. But if there is a leak somewhere under the laminate, through the tool, or elsewhere where the air can be dragged across the laminate, then it will damage the laminate in this area. So if the leak occurs through the inlet (hose comes out of the bucket or non vacuum tight seal at the through bag connection) with a center feed, perimeter vacuum arrangement, it will relax the vacuum across most of the entire panel as the air enters and tries to migrate across the laminate to the vacuum outlet. Big problem...
     
  12. Tungsten
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    Tungsten Senior Member

    So true gropher so true.I now have a leak detector and I'm getting pretty good with the butyl tape and pleats.Hopefully leaks are a thing of the past.
     
  13. Tungsten
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    Tungsten Senior Member

    On the topic of release film,I've never tried it yet.I have someP3 that I used for wet bagging.Do I need to add holes to it?Can I just randomly poke more with an all?
     
  14. groper
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    groper Senior Member

    P3 is no good... It won't allow enough resin to pass through it. I use p16 the holes are about 5-10mm apart.

    Wet bagging requires you to limit the resin which is removed from the laminate , the p3 film helps to do that. For infusion, you don't want a restriction, it's only there to help strip the flow media. If you use infusion scored core materials, then you don't need any release film or flow media...
     

  15. Tungsten
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    Tungsten Senior Member

    Well unfortunately all my foam is plain so consumables it is.I'm trying to be weight sencitive so I could try some grooves.Don't need holes I'm just doing single side.

    Little V grooves with a router and straight edge could work.Chichen wire is often talked about I could just rig up a press maybe.

    The channels don't need to be that big do they?
     
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