Comparing lamination techniques; Contact, Wet Bag, Infusion verse Pre-preg Autoclave
Contact lamination is always a messy business; the laminators wearing discardable cloths, Tyvek suits, charcoal filtered respirators and rubber gloves. Fabric sheets and shaped partial parts are placed in the mold and wet out with catalyzed resin by hand using rollers, squeegees and brushes. Additional resin will be catalyzed several times through any given lamination; the time pressure of the part starting to set before lamination is complete drives an energetic process. The best hand laminators using all fabric fibers may achieve a 45% fabric ratio, but the number is usually closer to 40%. Aerospace components are seldom manufactured in open molds. The potential for inclusions the vector structural failure is too great and the fabric/resin ratio is unacceptable. Discardables include rollers per part and brushes and cloths weekly.
Vacuum bagging, also called wet bagging, was formerly used in aerospace lamination. Outer layers are often contact laminated into the mold. Successive layers, often including foam cores and their partial “covers” are contact laminated behind the initial layer. A perforated ply and absorbent blanket are placed over the part’s inside before vacuum is applied. The vacuum forces excess resin through the perforated ply into the blanket, which is discarded after the part gels and sets. Problems with vacuum bagging include those of contact lamination, the time pressure and health problems of working with catalyzed resin, the inclusion of some contact lamination in the finished part and variability of weight, fiber to resin ratio and voids inclusion due to early catalyzation of resin and incomplete vacuum. When things go well fabric/resin rations run 53/47%. Discardables are the byproducts of both contact lamination, rollers, brushes and vacuum construction, inner ply, blanket and bag.
Vacuum Infusion, also named Closed Cavity Vacuum Infusion [ccvi], or Vacuum Assisted Resin Transfer Molding [vartm], is a technique used in the manufacture of aero-space components. Multiple fiber sheets and shaped pieces are precisely placed in the mold and vacuum applied. Negative, vacuum, pressure pulls a measured amount of resin through the fabric components and resinating the part. The single vacuum “pull” and closed resin application greatly reduce resin and processing material waste and reduces styrene emissions by 90%. Error is also reduced because the resin is not catalyzed until after the parts are placed and vacuum is stable. Discardables approximate thos of the second half of vacuum bagging, peel ply, flow medium and bag.
PrePreg Vacuum lamination further improves fiber to resin ratios by employing a heat catalyzed resin impregnated fiber which runs ~ 58% fiber to 42% resin ratio. Again, sheets and shaped pieces are placed in a mold, then vacuum bagged in an autoclave, the heat, ~600dgF, catalyzing the resin. Epoxy resin is easily formulated for heat catalyzation, a recent development allows VE to heat set as well.
Other than the cost of prepreg fabric, the significant problem with heat set epoxy is the cost of the autoclave, which can run to $1M for a canoe/kayak sized unit. A tertiary issue is the need to oven cure epoxy parts before use. Medium size ship builders use a lower temp epoxy formulation that sets in room-sized ovens. Discardables include resinated scrap and the bag.
For those who like numbers: Hand, or contact, laminated Placid boatworks SpitFire hulls weight 16.5# with tanks installed. Vacuum infused hulls weigh 14.5#, both with gel coated outer surfaces. With CCVI, our 12’ SpitFire fabric content went up 24oz, resin use down 48oz, and total part weight down 24oz. Our fiber to resin ratio improved from 45/55 to 56/44. CCVI parts are of higher and more uniform quality, lighter, stiffer and stronger than contact laminates or traditional vacuum bagging ar reduced cost compared to the latter. PrePreg Autoclaved parts are better yet if the buyer will pay for more expensive equipment and materials. CEW
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