|Article 5 - Vacuum bagging the Hull Shell|
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In last months article we described the completion of the male hull mould, in this article we follow the building, and vacuum bagging of a hull shell.
Vacuum bagging is a process in which the fibreglass, resin and foam core materials are placed loosely on the hull mould, and then forced into the shape of the mould using atmospheric pressure until the resin has set.
Step 1 - Preparing the mould: The mould surface has to be coated to stop the fibreglass and resin sticking to it. After a quick cut and polish, the mould was coated in 8 layers of release wax. A final safety layer of liquid release agent "safelease" is added just before the mould is used.
Step 2 - Preparing the vacuum bag: A vacuum bag is constructed from a large sheet of heavy-duty plastic sized to fit over the mould surface. A special, double-sided vacuum tape is applied around the perimeter of the sheet. This tape is used to stick the sheet to the perimeter of the mould to form an airtight seal, or "bag". A strip of masking tape is used around the perimeter of the mould to keep the surface clean for when the vacuum bag is attached.
Step 3 - Cutting the foam core: The hull shell is constructed from a sandwich of 10mm Herex foam between layers of Fibreglass. The foam is extremely light weight at 80g per m2, and quite tough. Sheets of foam are placed on the hull surface, marked roughly with a marker pen and cut out with a Stanley knife or jigsaw. To ensure that the foam can be "pushed" into the mould's shape by the vacuum bag, without air pockets being formed holes are made in the foam core. The holes are spaced about 25mm apart allowing the trapped air to escape. The holes also allow excess resin and bog to escape from under the foam.
Step 4 - Laying up the inside hull skin: With a male mould the inside skin is applied first. Because the inside skin will be a mirror image of the glassy mould surface a layer of peel ply is added to areas where internal frames will be attached. This roughens the inside surface so the frames can be glued in place without having to sand and possibly damage the thin inside fibreglass skin. Peel Ply is a form of Nylon Taffeta material that does not stick to resin, and can therefore be removed when the fibreglass has cured.
Epoxy resin is poured onto the mould surface (you really hope you have enough wax on the surface when doing this) and spread out. The peel ply is then placed on the mould in the frame locations and where the false floor, and any other structures will be attached.
A single layer of 200gm fibreglass (pre-cut earlier) is then placed onto the mould and "wet out" with the resin already on the surface. Plastic spreaders and metal rollers are used to push the fibreglass onto the mould and remove any air bubbles and excess resin. It is important to control the weight at this stage, the aim is for an approximately 1 to 1 ratio of resin to glass cloth, i.e. for each square metre of 200gm fibreglass, 200gm of resin is used.
Step 5 - Attaching the topsides foam: After the inside skin has been given a few hours to tack off, the topsides foam is attached. The foam is firstly laid flat on the floor and masking taped together. A very thin layer of thick epoxy filler mix is then spread onto the foam surface to prime it, and to fill the slightly porous surface. The epoxy filler is then applied evenly to the fibreglass skin on the mould using a notched spreader. This filler mix is used to glue the foam core to the inside skin. Around 200g m2 of filler are used in this process.
The masking tape is then removed from the mould taking any spilled resin/filler with it. Two layers of plastic shade netting are laid over the top of the foam to allow a path for the air to be sucked out of the bag. The double sided tape is then used to stick the vacuum sheet down around the mould perimeter. A vacuum hose is placed through a sealed opening in one end of the bag. The hose is attached to a plastic conduit that has been perforated with holes to allow the air to be removed from the bag.
After the vacuum pump is turned on and any leaks are sealed up, all the air is sucked out from the bag. This causes a vacuum to exist between the mould surface and the bag. Atmospheric pressure, around 97.4 kPa, pushes the foam onto the glass, and forces any excess filler out through the breather holes in the foam core. The pump is left on over night until the filler has set.
A specialised laminating epoxy resin supplied by Adhesive Technologies is used during construction. The resin ADR246 / ADH28 is stronger than normal resins, and has better heat tolerance. To cure the resin correctly requires an elevated curing temperature. Our hulls were cured at between 50-60o over night, while be vacuumed down, in an "oven" constructed from sheets of 50mm polystyrene.
Step 6 - Attaching the bottom: After the topsides have cured, the bottom is vacuumed down in a similar manner to the topsides. The major difference in attaching the bottom is that the foam needs to be pre-bent using a hot air gun in the front sections to match the tight curves. The gunwale foam is also attached during this stage.
Step 7 - Preparing the foam for glassing: After all the foam has been attached to the inside skin the outer skin is attached. The foam surface is long board sanded with 40 grit paper to remove any excess filler that came through the foam breather holes, and to remove any imperfections in the final shape. The better the foam surface is made at this stage, the less fairing work, and added filler weight needed after glassing. Also to minimise fairing work the foam is routered out where the fibreglass layers overlap. The foam is also routered out along the transom, and around the centre-case to allow an extra layer of glass to be used to strengthen those areas.
Step 8 - Glassing the outer skin: The outer skin is made up of two layers of 200gm fibreglass. The foam surface is primed with filler to fill the imperfections in the foam surface, and to minimise the weight of resin needed to wet out the fibreglass skin. The two skins are then applied to the foam ensuring that the cloth overlaps occur in the routered channels. The resin is again cured at a high temperature.
Step 9 - Removing the shell from the mould: After the hull has cured sufficiently it can be removed from the mould. While still on the mould a cradle is made which will hold the unsupported shell in shape until framing and the transom can be added. The hull shell is "broken off" the mould by working plastic wedges between the mould and inside skin releasing the shell along the gunwales and transom. The shell is then slowly worked off the mould by lifting at the gunwale and transom until it completely breaks free.
Once released the shell is taken off the mould and placed in the cradle for fitting the frames. More on this process next month.
Special thanks to Grant Burke at Flow Solutions, Paul Hakes at High Modulus, and Grant Beck at Adhesive Technologies for the assistance and technical input for our first vacuum bagging project.
We have now made three hull shells, over three weekends. Two of these have been taken back to Wellington and Taranaki for finishing with one remaining in Auckland. For current progress details have a look at the class website, www.javelins.org.