Infusion-Test Panel and Fuselage

Ran across an interesting Youtube video demonstrating an epoxy resin infusion process on some test panels and fuselage.  It is interesting how everybody has their own terminology and technique for resin infusion.  There is definitely more than one way to get the job done.

They use an interesting layup, including lots of the Soric material.  I have used this before, and it is a good material to infuse with.  Made by a company called Lantor, it is a non-woven polyester material that acts as a core material.  It appears that the folks in the video are using the SF grade Soric, which comes in several thicknesses.

An advantage of using Soric as a core is that it flows resin very well for infusion.  It is easy to cut and handles well.

Disadvantages also abound.  One of them is the possibility of print-thru on the surface of the laminate.  Another is the negative effect on the structural properties of the laminate.  This non-woven material does not have much crush resistance such as a balsa or foam material.  A serious issue that I have found is the higher risk of delamination.  Like any core, this material works by separating the two skin layers to create a sort of “I beam” effect.  The problem is that this material is not inherently strong within itself.   Though it does become saturated with resin during a proper infusion, it is not nearly as strong as glass or carbon fiber reinforcement.

As the video demonstrates, a proper resin infusion can look easy.  With proper materials, practice, and knowledge it can be.

Wind Blades

The new composites application that everybody is discussing is composites wind blades.  The large, three-bladed wind generators have been around for a few decades, mostly in Europe.  The U.S. has been catching on in the last couple of years as a way to make cleaner electricity.  These windmills are very tall, and have blades that are 100 to 400 feet long, depending upon output rating and location.

The wind blades use glass carbon fiber, resin, and coring to make a long, stiff and lightweight blade that will attach to the hub of the windmill.  These blades are very long, requiring huge manufacturing facilities to make them.  The transportation of these blades is important as well, as they require specialized trucks and trailers to handle such large pieces.  Large cranse are required to lift them into place at the job site.  They are relatively heavy, and must be lifted fairly high, requiring a significant lift capacity.

Resin infusion with epoxy resins is the normal manufacturing technique of which I am aware.  They use compsite molds that have a constantly changing surface shape due to the complex geometry of the blade.  The holy grail for these blades is to make longer blades at lower weight.

This application again demonstrates the advantages of composites.  Complex geometry, high strength to weight ratio, and impact resistance are important aspects of wind blades.

There are several manufacturers of the wind blades in the U.S.  MFG is a specialty composites molder that has been around for ages and is in the wind blade market.  Vestas is another company with operations in the U.S., along with LM Glasfiber, as well as others.

The Importance of Testing

The testing of properties for Composites laminates and structures can be tricky. Much of this is due to the complex base of ingredients and makeup of the laminate. When we are putting together a combination of resins, reinforcements, coring, fasteners, curing agents, fasteners, etc. we can radically affect the properties of the FRP structure. This can be beneficial and disadvantageous.

ASTM test specifications can be used to determine properties such as stiffness, chemical resistance, impact strength, etc. There are several designed specifically for FRP laminates. These can be very informative, especially when developing and comparing potential laminate designs.

House tests can also be developed to test for more specific properties on test panels and the overall structure. A flat and square test speciment may test out great for certain properties. When it is put into actual application, it may have differenet features which make it stronger or weaker than the flat test panel. Things such as holes, notches, square corners, curves, and other geometric shapes have a wide range of effects. They may increase localized thickness from overlap joints, may cause stress concentrators, or may have thin areas because of an area being difficult to reach.

One of the projects I worked on recently involved the conversion of the processing from hand layup to resin infusion processing. For product liability we wanted to prove that we tested the laminates for strength, and doing an overall structure test was out of the question. But testing flat and square infusion panels would just give us a number. So we setup a baseline with the same size flat and square test panels made with the old laminate schedule using hand layup. This old laminate schedule had been very suitable in its application. We just needed to ensure that the new infused laminate schedule exceeded the strength of the original hand laid laminate schedule.

We setup this direct comparison between laminates by removing the variables of the part shape itself. We also added some safety factor to ensure for any other variables that may be involved. Our supplier tested the panels for us, giving us official ASTM results. We worked through a few different scenarios with the new laminate schedule, received test results, and made our decision. Future inquiries into our engineering work can be backed up with technical testing data.

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.