shringage after vaccum

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shringage after vaccum

Author	Message
Steve Broad	Steve Broad posted 7 Years Ago HOT Post Details
Group: Forum Members Posts: 408, Visits: 4.1K	+x KOSTAS - 9/4/2018 2:37:29 PM +x Steve Broad - 9/3/2018 7:55:18 PM 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). 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	KOSTAS posted 7 Years Ago HOT Post Details
Group: Forum Members Posts: 30, Visits: 244	+x Steve Broad - 9/3/2018 7:55:18 PM 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). 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
	Reply Like 3
Steve Broad	Steve Broad posted 7 Years Ago HOT Post Details
Group: Forum Members Posts: 408, Visits: 4.1K	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	KOSTAS posted 7 Years Ago HOT Topic Details
Group: Forum Members Posts: 30, Visits: 244	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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