Best Composite Properties

[quicksail]

Hello everyone,

I have a couple of general questions about composites and the different methods used to layup. What yields a better layup prepreg vacuum layup or vacuum infusion layup? To me infusion stills seems the best for getting void free high fiber to resin ratio layups. I have a few other questions but is there a good book out there that answers such questions? Most books I see are reasonably out of date as composites have come a long way in the past few years.

Also, to-date what method yields the best mechainical properties while giving the lightest structure possible (hull/deck)? I figure thick core and carbon but what about layup? Is heat more important then pressure in the layup process to avoid voids? I am looking for general guidelines and common sense look at composites.

Any answer would be most helpful, inquiring minds want to know.

Cheers

[Russ]

These are two distinctly different processes. Prepreg is typically used to accurately control the amount of resin in a laminate. The resin pre-catalized with heat activated catylist, and it is stored in a cooler. The layers are built up and bagged for vacuum. Most generally aircraft / aerospace parts are made this way. It is not unusual for the part to be bagged, vacuumed, and pressured in a heated autoclave. This produces a 70 / 30 glass / resin laminate. Believe me there are no voids. This is a costly process but the best.
INFUSION is where you lay in the fiberglas, foam, wood etc. and bag, then vacuum. The resin is then infused (sucked ) into the bundle from specific points that progress as the wet front moves toward the vacuum source. The catalyst is already mixed in so accuracy of set time is a must. This process has been around for 30 years but only became popular when HAPS standards made styrene emissions from open molding a no no.
Does this give you a basic overview? ---------- Russ

[Buckle]

Prepreging is far superior than infusion be it under a flexible tooling (i.e. a vac bag) or not (RTM-lite). However prepregs are used mainly for advanced composites structures where weight its critical such as in aerospace applications.

Like Russ said, prepregs are heat activated and in order to achieve maximum mechanical properties, the laminate needs to be post cured. Typically post curing of epoxies can result in 30% high mechanical properties. To summaries, the higher the extend of cure, the stronger the structure.

In terrms of vacuum, prepregs can typically be excerted to 8-13bar of pressure. With infusion, the maximum vacuum to be excerted onto a lamiante is 1 bar. For the record, 1 bar = 10 tonnes per square meter). Therefore prepregs can have higher Vf (volume fractions - glass to resin contents) due to the force excerted over the area curing cure. At this presure, voids are absolute minimal (<1%).

[Buckle]

Not being bias to prepregs, prepregs are great if you can aford them. Personally I'm an infusion man. Infusion is great. Its cheap and cheerful, and any monkey can to it with a bit of knowhow/training.

To summarise, if you can aford to use prepregs, use them, however only a few people can aford them to use them on a daily basis. Cost of materials isn't the only factor which needs to be considered. You also need to consider.

Cost of Quality plugs (especially for prepreg)
Cost Quality prepreg tooling (post curing requires expantion of tooling to be similar to laminate to avoid distoration).
Cost of plant equipment.
Energy for operate plant equipment (i.e. autoclave/oven).
etc
etc.