Question about "applying vacuum slowly"....

[Reidar77]

Ive bagged several hundred parts, infused just as many. Ive had relatively good success (more so with infusion).

The apply vacuum slowly part in almost every book Ive ever read has never really been explained to me. I understand it at its face value, but obviously it can have many meanings.

Previously I would apply vacuum slowly in a continuous flow. Maybe pausing a bit to make changes to the bag, but never enough to allow resin to flow.

On my latest part I applied vacuum in 5 degree increments, over 5 minute intervals, until I pulled 27". I messaged the bag after each vacuum change. Ill be demolding the part tomorrow, hoping to see a substantial change is cosmetics.

The logic Im using behind this is that at 5" for 5 minutes it will allow some gas and resin to move. Then the next 5" change will allow more pressure to be added to a relatively settled layup. And so on. What Im trying to do is move a little resin at a time vs. moving all the resin at once.

Am I headed in the right direction with this? Or can somebody shed some light on the whole applying vacuum slowly statement?

-Jason


Jason

 

[Compositepro]

I've not encountered the instruction to apply vacuum slowly although there are a couple of reasons that it maybe helpful.

One would be to allow adjustment of the bag while it is pulling down to avoid bridging.

Second is to minimize pinch-off of airflow out of the laminate. Trapped air between plies of tacky prepreg is one of the main causes of voids. Applying full vacuum quickly can cause the edges of laminates to become fully compacted before much air has been removed from the center of a laminate. An analogy would be sucking on a drinking straw too hard. Too much suction causes the straw to collapse and no fluid will flow through it, whereas low suction allows you to drain a cup.

 

[T2Raz]

One of the reasons to apply vacuum slowly is to reduce shock to your vacuum system especially if you have other parts under vacuum on the same system. Another reason is so you can carefully monitor the part while reducing the vacuum slowly which can eliminate fit errors such as pull-aways, bridging, bunching etc.

 

[GBor]

Is this a prepreg or a VARTM where you are drawing the resin from a bucket? If the former, then some of the issues listed above sound pretty reasonable. If the latter, clamp the hose from the bucket to draw your vacuum and then slowly release the clamp.

I agree that the slow vacuum would be helpful to avoid wrinkling, bridging, etc.