Does bleeder use is important?

[avior]

During my last stage I was surprised to notice that bagging sequence learned at school (prepreg layup – oven/autoclave cure) is not exactly the same that is used in all aeronautic shops. More, now I saw a few of the same parts bagged without bleeder. Does it means that bleeder use is optional? What is the bleeders function? Also I found that the same part made in different shops is covered with different breather material thickness . . . Does breather thickness influence the cured part quality (thickness, resin content, mechanical properties, etc.)?

 

[PanelGuy]

Each part's requirements can be different depending on technique, geometry and material. Some shapes and sizes do not require bleeders, where it is a must on others.

Bleeders help ensure the entire part is under vacuum evenly. Closer to the port it is easier, the farther away, harder. these materials are thrown away at the end and so only add cost, your economies are better if not needed. As well, bleeders may provide the ability to remove excess resin from the layup, particularly wet bag lay-up. In these cases varying the bleeder over a lay up change your performance factors drastically.

I have used your basic gauze type bleeders, the heavy batting bleeders as well as simple bubble wrap (by ththe way, bubble orientation is very important). It just depends on what you need to accomplish with the bag and what your costs are.

 

[SWComposites]

Bleeder material removes excess resin from the part. Breather material is used to allow air and volatile removal under vacuum. The two purposes are very different even though sometimes the same material is used for both purposes. The use if bleeders is highly dependent on the specific prepreg material. Some materials are designed to use bleeders during the cure cycle to ensure a void free part and acheive desired resin content (these are mostly very old prepreg systems); many materials are designed such that they do not require a bleeder. Use of the wrong type/amount of bleeder (or bleeder when it is not appropriate) can affect part thickenss, quality, properties, etc.