At work when we do wet layups, we let the whetted cloth hang in a room until it hits B-Stage. Why not just wet the cloth on the mold? What are the advantages/disadvantages to wetting on mold?
Thanks,
Aaron
I like to wet in the mold. I think it’s more difficult to work the air out pre-wet fabrics. I’ve also found that layups done with pre-wet fabrics tend to have more porosity problems even when vacuum bagged. I problem solved an issue for a long boom section for a UAV company. The only thing that I did differently was wet the layup in the mold rather than pre-wet it. My section past the test while theirs failed. I simply think mine performed better because it had less porosity.
I also am not convince that pre-wetting in most scenarios is any faster since the fabricator has to work harder to work the air out of the laminate. The advantage to a B-staged cut is that it’s rather sticky and can be worked into corners easier because it stays put. Obviously, you are using some high temp resins which make fabric saturation more difficult due to the viscosity…it might be faster to pre-saturate the fabrics with the thick stuff.
I will say that wetting in the mold and working pre-wet cuts is a slightly different skill set. If you are familiar with one you may find the other rather frustrating.
In my experience pre wetting gives more pinholes. Pre wetting is nice when you have bigger objects. few peple wetting out the fibres, and a few people placing them. For my urrent project I pre wet too. massive fibreglass rods, al UD’s. prewet the whole piece, fold together and place. Better then wetting out 40 layers…
Thank You guys for the responses.
Is it possible that if you need very precise resin/cloth ratios, pre-wetting serves you better?
Fiber to resin ratios are controlled by compaction and the processes ability to remove the excess resin as the layup is compacted to a thinner state. The question is how much resin does it take to end up with a void-less laminate for a specific compaction level and process. If the pre-wet fabric is dry then you need higher compaction levels and an efficient processes to end up with void-less laminates. If the pre-wet fabric is too dry you may end up with many voids even if processed at higher pressures. The resin to fiber ratio my look great if you look at it from the perspective of weight but the laminate may be weak and brittle because of it’s porosity. If you are pre-wetting it can be difficult to know how much resin you need.
A common location for voids with carbon is in between the weave intersections. Prepregs can suffer from this same problem unless processed and high pressures in an autoclave. When wetting in the mold each successive layer absorbs the excess resin from previous layer. Ideally you end up with an unbulked layup that is relatively void-less. You then use some compaction technique to compress the laminate to a thinner state and absorb/expel the unneeded excess resin. This is a guarantee that the laminate has a low porosity level.
It’s quite possible to weigh the dry fabric before the layup, weigh the part after layup (cured), mic the thickness, and calculate the porosity % of the laminate. This could be used to discover the porosity level of your layups.
for high fibre volume with wet layup I pre wet the fibre. Easier where possible is to use a vacuum bag, pressure, or other systems. When done right they give a better (surface)quality, need less attention whith the layup, and a better resin/fibre ratio (depending on what you need offcourse)
Why don’t work with infusion? Better partquality and the same work than vacuum bagging.
To be honest I am not sure what infusion is. The majority of the stuff at work is with prepregs. I thought vacuum bagging was the cheapest, most effective way to go without an autoclave in my garage.