Bagging/Infusion On The Cheap
Vacuum Infusion and Vacuum Bagging can be accomplished are not only reserved for industry and can be accomplished in a workshop setting. Relatively inexpensively as well. One of the major variables is the shape of the part trying to be built.
We will look at a flatstock for now. The desired outcome could be either a test panel or a piece needed for flat construction. The mold will be the difference between this and any other more complicated shape. The mold must be able to be sealed from the atmosphere and waxed for release of the finished part, so my favorite “mold” is either a flat sheet of steel or a sheet of plate glass depending upon desired surface finish. This must then be waxed up with some paste wax to enable part release.
I always like to start with creating the perimeter seal. The 1/8 or 3/16 inch thick by 1/2″ wide gray butyl tape is the best. This is basically sticky on both sides and has wax paper on one side. It is sold at infusion suppliers or online as well. My little secret is that it is also sold at building supply stores, as it is used on polebuilding construction projects when sheets of steel siding/roofing are sealed together with this stuff. So this is unrolled and pushed down onto the mold and the wax paper is left on and facing up. Then I like to use cover this with some 2 inch wide masking tape.
Now we can start the part construction by placing our layer of gelcoat if so desired. Once this is cured, it is followed one of two ways. For vacuum bagging, the catalyzed resin-wet layers of reinforcing materials (glass or carbon) are placed down, including any core material. Once this is satisfactory, the peel ply is placed over the laminate, followed by the breather material. This is typically quilting batting, and a couple of good layers will do well to hold the excess resin from the laminate. Then the bag will be installed and a vacuum will be pulled which will pull the resin up into the breather material.
For infusion, the gelcoat is followed by the dry layers of reinforcing glass or carbon and core. Then any resin runner strips are added with a separation from the laminate with peel ply. Two circuits are setup; one for vacuuming the air out, and one for bringing resin into the laminate. The Vacuum circuit is accomplished with spiral wire loom wrap around the perimeter of the part leading to a T connection which will poke thru the bag. A resin inlet T will also pass thru the bag and lead into the runner strip. Now a bag will be placed over the part and sealed down. The hoses need to be vacuum rated, and can be either pvc or polyethylene.
A resin trap will be needed between the vacuum pump and the laminate for both infusion and bagging. This can effectively be accomplished by using a PVC pipe sealed at both ends with endcaps. Then drill and tap plastic or brass fittings and seal well with pipe dope. The fittings need to be at the top so that the resin will drop out and into the pipe. The resin will cure in the pipe and can be disposed when solid and full.
The bagging operation is accomplished by using polyethylene plastic that is free from holes, and in the range of 4 to 6 mils of thickness. Thicker plastic can do the job, but can be difficult to work with. The plastic needs to be cut larger than the area to be covered, typically by around 20 percent. It needs to be placed on squarely, and pushed into the butyl tape without wrinkes to create a good seal. The excess plastic is artfully taken up with extra pieces of butyl tape in these troughs that are connected to the main tape line around the edge. This can be very frustrating and is difficult to describle. But the bag will be sealed to the mold to create an airtight cavity. The resin hose needs to be closed off with vice grips.
Now the vacuum will be turned on. For the vacuum bagged part, once the vacuum is on, the part is basically left to cure. For infusion, the laminate is left to the vacuum to remove air and moisture for 20 mins to 2 hrs depending upon size. Infusion requires at least 27.5 inches of mercury, and 29 would be awesome. Any leaks need to be identified and eliminated. This can be difficult, but is necessary.
Now we are ready to put resin into the laminate. The wet resin is mixed with catalyst in buckets where the resin hose from the part is placed. Resin hoses are opened up and the resin flows from the bucket into the part. Once the part is full, the resin line is clamped off and the part is left to cure under vacuum. I typically wait at least 12 hours before demolding the finished part.
Vacuum Infusion Processing
Resin infusion processing offers several advantages over traditional open mold processing techniques. All of the reinforcements and core materials are placed in the mold without resin, so care can be taken for close fit and proper orientation. And it is a lot cleaner without the sticky resin. Resin waste is typically lower because all of the resin is added at once. VOC’s are reduced as well, and are only emitted from the open mixing containers. There is much less worker interface with messy and stick resin on people and tools so cleanup materials and personal protection equipment expenses are reduced. The laminate itself is typically more consolidated, uniform, and visually pleasing.
One of the considerations that needs to be taken into account is that the ratios are different and the glass and resin are more compacted by the process. Using the same layup schedule would result in a thinner laminate that is lighter weight and uses less resin. One drawback to this is that the cross sectional area is less, usually resulting in a loss of stiffness. This can be regained by increasing the core thickness to compensate for that loss and to restore overall panel thickness.
Infusion processing does require specialized equipment, consumables, and materials. The resins need to have much lower viscosity(flow like water). The core and glass need to have holes and channels for the resin to flow. A bag needs to be created to cover and seal off the laminate to the mold with out ANY air leaks. The mold needs to be able to be sealed to the bag and be airtight. A high vacuum needs to be able to be drawn on the bagged mold to move the resin. The vacuum pump or venturi needs to be able to achieve a minimum of 28 inches of mercury.
The basic process is that the mold is gelcoated abd skinned with a good, low-shrink resin and chopped strand mat. Then layers of dry glass are added. Coring is placed nice and tight and glued if needed. And the final layers of glass are placed. During the process, any gaps in coring or glass will “bridge” where the vacuum doesnt pull them down, and solid resin will fill them. So good fit is very important. The the bag is placed and sealed after auxililary runner strips are added and resin feed hoses are placed. The perimeter vacuum channel is plumbed to the vacuum pump.
Air is removed from the laminate for a good amount of time before the resin lines are opened up and resin flows into the part (do not forget to catalyze it). This is part of the art of flowing resin to fill the whole laminate with resin before an area gets cutoff from the vacuum or the resin begins to harden. There are some tricks and practice is key.
Resin is stopped just before fill and hopefully resin doesnt run into the vacuum line too far, but that is what resin traps are for. The vacuum is left running as the part hardens and gets to full cure in a couple of hours.
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