Bladder Molding Advice 1

[ArturoJones]

I am currently involved in an effort to develop new molds for bladder molded tubular products and I am looking for some advice on tooling and process design. I have experience and a reasonable understanding (I think) of autoclave molding, film infusion, as well as the liquid molding technologies however the bladder molding is new to me.
We have done some preliminary development work using molds which were given to us and so far the results have been wanting. We are following what I would believe to be a typical bladder molding process: roll-wrapping pre-preg on to silicon bladders, extracting the mandrel, placing the pre-form in the mold, mold goes in the heated press, bladder is pressure cycled and then left pressurized at 90psi. The parts have the right outer shape and are well compacted, that is to say there are no issues with fibre bridging, however porosity is prevalent in the parts.

It seems to me that strategies for eliminating porosity are based (in the most general sense) on two philosophies: a) Use vacuum to remove all the air from the laminate before the resin flows (vacuum infusion / semi-preg approaches) b) Keep the resin (as opposed to the fibre bed) pressure high, particularly when near gelation (autoclave / RTM approach). Our current moldset has no provision for applying vacuum to the part. Secondly, the molds have very large flash cavities which offer essentially no restriction to resin flow. It seems that because of this, increasing the bladder pressure just bleeds more resin from the part without actually generating any resin pressure.

With that preamble out of the way, here are my specific questions:
1) Is vacuum necessarily applied to the mold cavity in successful bladder molding operations?
2) What measures are usually taken in order to restrict resin flow, and hence to facilitate higher resin pressures during processing? It would seem to me that smaller flash cavities and / or restrictions between the part and the cavity would help. Is this on the right track?
3) How critical, if at all, is the timing of pressure application? Is there any advantage to applying the pressure stepwise, or perhaps waiting until the resin viscosity has started to climb before applying the full process pressure?
4) Our bladder pressure of 90 psi was rather arbitrarily inspired by autoclave cure cycle recipes. Are there significant advantages to be gained by going to higher pressures?

Obviously any other process advice would be welcome as well. Thanks in advance.

 

[Compositepro]

You need to determine if your porosity is due to trapped air or excessive bleed. It sounds like trapped air but it is possible you suffer from both issues. Trapped air will show as larger voids where fibers are not compacted together. The air pressure is keeping the fibers or resin from filling the void. Excess resin bleed shows as highly compacted fibers without a good coating of resin everywhere, or as porosity between filaments in the tows.
More pressure will lessen the size of trapped air bubbles but vacuum is more effective unless you use hundreds of psi or more. However, vacuum must be properly applied to be effective. If there isn't a good physical path through the prepreg air cannot get to the vacuum.

Staging the resin viscosity prior to applying pressure is a very common practice to prevent excess resin bleed. This technique though is a time consuming process in Production and can be very touchy and inconsistent. It has to be optimized through trial and error for different part sizes and geometry and the reactivity of different batches of prepreg can vary enough to cause problems. You will have a time window where presure must be applied. If you want the window to be large for a robust process your cure time will generally have to be long and slow.

 

[Dawson1]

I am also working on bladder molding and seeing a similar phenomena. I am new to the bladder process, and have managed to get beautiful parts with a regular cure resin system. I am now trying a "bladder molding" resin system with a faster cure cycle, and getting voids between the fiber intersections (3k Plain Weave). I have played with times and temps and have seen no improvement.

I do not pull a vacuum, and as I said, the previous parts are perfect (to aid in answering the original post). I have found that pressure above 90psi does help our particular product (90 psi makes a nice part, but our straightness improves amazingly at 140psi).

I would appreciate any further input on the voiding issue. Based on the earlier response, I will try a lower pressure scenario with the new resin, but that doesn't seem to be what's going on for me. I will pay more attention to weights and specs to help evaluate resin loss.

Thanks,
Dawson

 

[Compositepro]

I don't see how lower pressure will help. If you don't pull any vacuum you will trap some air and more pressure will help reduce it's volume. If your problem is excess bleed you may have to develop better sealing methods to stop the resin bleed or stage the resin to increase its viscosity prior to applying any pressure.
Without any seals the resin should be highly viscous or at a soft rubbery stage when pressure is applied. You can do studies on a hot plate to dtermine the right time and temperature. The staging temperature might need to be at a temperature lower than the cure temp if the resin has a short gel time.

 

[MechanicalChef]

Similar to #3 - waiting for the resin to gel has had large implications for us.

During a material screening process for an out of autoclave cured material, we combatted terrible porosity. The autoclave trial ran fine (2 hours @350), but the vacuum-cure failed. The issue was traced back to the specs, and them not calling out a long enough gel-time. Eventually, it took 4 hours at 260 + a post-cure of 350 for 2 hours.