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KOSTAS
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On my first try to produce a composite part a came across this unexpected issue .I 3d printed a fork core for a bike frame i wrapped the core to CF and then vacuum bag it.But when i measured the dried part the fork legs were closer(25mm-1') they previously were !!! here my photos    why this happened ?How can i over come it ?Should i alter my design in order to compensate for the shrinkage ?Should i build a support to hold the fork legs in the vacuum bag ?Thanks in advance!
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Steve Broad
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The actual process of pulling the air out of the vacuum bag contracts the bag and can tend to pull flexible parts out of shape. I have solved this by adding support struts on the outside of the bag and held them in place whilst pulling the vacuum. For example, placing a strut between the ends of your forks on the outside of the bag. Make the ends of the supports as large as possible in order to reduce the risk of marking the carbon (unlikely as there won't be a lot of pressure pulling the ends together).
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KOSTAS
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Group: Forum Members
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+xThe actual process of pulling the air out of the vacuum bag contracts the bag and can tend to pull flexible parts out of shape. I have solved this by adding support struts on the outside of the bag and held them in place whilst pulling the vacuum. For example, placing a strut between the ends of your forks on the outside of the bag. Make the ends of the supports as large as possible in order to reduce the risk of marking the carbon (unlikely as there won't be a lot of pressure pulling the ends together). thank you steve for your idea , i will try to build something like a support structure . is this phenomenon encountered in other cases ? for example in a closed triangular shape , can vacuum send my designs out of specs? How can i predict such a behavior ? thank you
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Steve Broad
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+x+xThe actual process of pulling the air out of the vacuum bag contracts the bag and can tend to pull flexible parts out of shape. I have solved this by adding support struts on the outside of the bag and held them in place whilst pulling the vacuum. For example, placing a strut between the ends of your forks on the outside of the bag. Make the ends of the supports as large as possible in order to reduce the risk of marking the carbon (unlikely as there won't be a lot of pressure pulling the ends together). thank you steve for your idea , i will try to build something like a support structure . is this phenomenon encountered in other cases ? for example in a closed triangular shape , can vacuum send my designs out of specs? How can i predict such a behavior ? thank you Difficult to predict, but as you are exerting over 14lbs of pressure over every square inch of your mould with a wrinkled vacuum bag there is bound to be some pulling and pushing going on. Any flexible mould is liable to be distorted so you have to either design the mould to have little or no flex or add support at the bagging stage. I have experienced this problem when adding ribs to flat sheets. Even after carefully positioning the breather and bag the ribs can still be bent out of vertical.over a couple of hours in the oven or 20 or so hours under vacuum. However, this may not sure the problem entirely. For example, when I popped my race race front clip (2 layers of 210gm carbon with additional material around the edges) from the mould after 3 hours in the oven, the rear of the wings flexed in a tad. Not an issue but it happens.
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KOSTAS
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This issue really bothers me a lot , cause i could not predict it and i could not explain it .After some study i have a theory , i cant say its the cause but might be an explanation. I printed a U shape with thin walls that was not airtight, that means that the vacuum pump pulled air from the inside of the core also.Thus making my mold behave like a bourdons tube  the working principal of many pressure and vacuum gauges .When you apply pressure to the curved tube the tube expands, when you drop the pressure it contracts.Initially i thought that i should print my core airtight so that air from the inside of the mold not to be able to escape under vacuum , but in that case this might bring the opposite result and make the u shape expand this time!! Of course i could print my mold solid but this is my last choice since i s a big waste of printing time and material . i will try first the added support you method you suggested just my thoughts thanks for the feedback anyway 
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oekmont
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If you close the print, the're is no force in the core itself when you pull vacuum. The pressure outside the bag is 1 atmosphere, as is the pressure in the core. The pressure between core and bag is zero, and this is why the bag is forced against the core, and the core against the bag with equal strength. An open structure however has to be able to take the forces of the vacuum without collapsing.
But this is not you problem. Your part did shrink to this, but this should be immeasurable small for you. Your problem is that you placed the bag flat over the part. The way across your part, following it's contours is longer than the way across the part without following the contours at all. And as the bag was evacuated, and came to a point where it had to move inward to fill in the space, it was already to tight, to slide over your part and compensate the additional length. So the bag had to stretch to get into the corners. And by doing so, it created an equal force, which pulled the arms of your wishbone inwards.
And this is why you should never put the bag on without wrinkles. Theoretically not even on a flat sheet.
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Steve Broad
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+xIf you close the print, the're is no force in the core itself when you pull vacuum. The pressure outside the bag is 1 atmosphere, as is the pressure in the core. The pressure between core and bag is zero, and this is why the bag is forced against the core, and the core against the bag with equal strength. An open structure however has to be able to take the forces of the vacuum without collapsing. But this is not you problem. Your part did shrink to this, but this should be immeasurable small for you. Your problem is that you placed the bag flat over the part. The way across your part, following it's contours is longer than the way across the part without following the contours at all. And as the bag was evacuated, and came to a point where it had to move inward to fill in the space, it was already to tight, to slide over your part and compensate the additional length. So the bag had to stretch to get into the corners. And by doing so, it created an equal force, which pulled the arms of your wishbone inwards. And this is why you should never put the bag on without wrinkles. Theoretically not even on a flat sheet. That's a better way of explaining it than mine :-) However, I would still would place a bar of some sort between the ends of the fork as you evacuate.
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oekmont
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Absolutely. The bar will keep the ends at the right distance. Than you only have to worry about bending the two arms. But this should be less of a problem.
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f1rob
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Dosent matter how big you make your bag or how careful you vac down it will never pull down symmetrically, there will always be resistance from one point or another
If your bagging somthing that can move or flex then it will !
Brake drums on the f1 cars are made with lots of printed formers now.
What they tend to do is on the open end they print it with 6mm holes
You then have a plate with pins on the correct centres that you push into the holes then bag the whole lot up
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KOSTAS
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Thank all of you so much for your fast and though insight , this is such a good place to be and learn, i spend quite a time here reading As you may see from my picture i have bought and use a tubular vacuum bag 460 mm wide . Since these consumables need some processing time and are not readily available to the place i live the faster way to get a bigger bag is to slice open the bag i have and connect two pieces .It would be rally easier for me to see a picture of a big bag with wriggles in order to understand how to strategically place my item in the bag.Sorry but i have zero experience .
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