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Jan 21, 2010, 11:00 PM
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VARTM - Resin Infusion Exercise


Overview of VARTM and Objectives

The acronym VARTM stands for Vacuum Assisted Resin Transfer Molding, or, in short, resin infusion molding.

The theory behind resin infusion is that dry laminates can be placed in a mold, allowing unlimited setup time, then liquid resin is inserted in the laminate by means of vacuum pressure.

The primary benefit of infusion is, of course, the unlimited setup time of the dry materials inside the mold. As there is no hand lamination involved, infusion is also "cleaner" and more "shop friendly" than other techniques.

Lastly, but most importantly, the use of VARTM allows the composite engineer to specify the resin/fiber ratio in the laminate. A volume of resin can be calculated to perfectly match the reinforcements used, providing lighter, stronger parts with fiber/resin ratios as high as 70/30.

{In the field of advanced composites, lamination techniques can be ranked as follows:
-Hand Lamination (fiber/resin ratio usually 50/50 or worse)
-Hand Lamination, then Vacuum Bagging (fiber/resin ratio of 50/50 or 60/40)
-VARTM or Resin Infusion (fiber/resin ratio of 60/40 and up to 70/30)
}

Now that we have a very basic "theory" of VARTM well understood, let's continue to the design specifications for this exercise.

Purpose of the exercise: Learn basic infusion techniques through the molding of a flat laminate in a specially constructed mold. Compare the properties of the three laminates that will be created - the first with a 50/50 fiber/resin ratio, the second with a ratio of 60/40 and the last with a ratio of 70/30. All laminates will have identical reinforcements. The difference between the three laminates will be the f/r ratio.

Why do we want to experiment with different fiber/resin ratios? In advanced composites, the properties of the finished laminate are fiber-dominant or fiber-oriented. That is, the properties of the laminate most resemble those of the reinforcing fibers. Fibers give the laminate properties like strength and stiffness, while the resin provides properties like resistance to solvents and toughness. Back on topic - pure resin is brittle. An ideal laminate should contain enough resin to bind and consolidate the fibers in the laminate, but still be well below the level required for the laminate's properties to be fiber-oriented. Thus, we do not want the laminate to be too resin-rich, but we also want to keep from starving the fibers of resin. The fiber/resin ratio is a delicate balance between the properties of fibers and matrix materials.

All the practical steps of resin infusion will be shown throughout the exercise. Some aspects of infusion will be explained in advance - others will be explained as the exercise progresses, and still others will be explained during the conclusion of the exercise.
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Jan 22, 2010, 12:54 PM
Composites Kid
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Practical Aspects of VARTM


During the process of resin infusion, the liquid resin needs to travel through the reinforcements until the entire laminate is infused with resin. Obviously, resin will always want to follow the path of least resistance.

One important aspect to be considered in infusion is the resistance of the materials to infusion. Some materials, especially tight weaves, are difficult to infuse because of the resistance they offer to the flowing resin. Others, such as continuous strand mats and stitched reinforcements, are excellent flow media, as they allow the resin to flow easily through the laminate.

Also, the viscosity of the resin affects the speed of the infusion and the ease with which bubbles can be eliminated by the flowing resin. For this exercise, we will be using thinned polyester resin to allow for a fast and clean infusion.

In large, often expensive layups, artificial flow media, such as perforated foam cores, channeled foam cores, vinyl web flow media and filter jackets are used to facilitate the travel of the resin. Flow media may be necessary for some complex shapes. For this exercise, we will be treating the reinforcement (fiberglass mat) as a flow media, as it offers excellent flow characteristics.

In figure 1, we can appreciate that if resin is allowed to enter the laminate at one specific point, the resin will want to follow the path of least resistance to the vacuum inlet, leaving most of the laminate untouched.

The solution to this is to use a flow media or a means of making sure the resin spreads through the entire laminate. A very easy method of doing this is to use spiral wrap and t-fittings. Figure 2 shows how using spiral wrap and t-fittings on the resin line or inlet and the vacuum inlet allows the resin to spread evenly throughout the entire laminate.

Figure 3 shows a mold nearly identical to the one that will be made during this exercise. The shape of the mold allows resin to flow cleanly from the resin feed to the vacuum inlet.

At this point you may be asking yourself - how does the resin enter the bag and how do you keep resin outside of your pump? Obviously, stray resin inside your expensive vacuum pump is not a good idea. Figure 4 shows the use of a resin cup, with provisions to clamp off the resin line once the infusion is finished (to avoid harmful bubbles entering the laminate), and the use of a resin trap. A resin trap can be as simple as a vertical section of PVC pipe sealed at both ends, with the line from the bag entering at a lower point than the pump line. This allows for stray resin to stay at the bottom of the trap and never enter the pump.

As of today, I am starting the construction of a plug, which will be used to make a mold nearly identical to that in Figure 3.
Jan 22, 2010, 09:24 PM
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Plug Finished - Mold ASAP


As of now, the plug is finished. I first cut out a rectangle of 2.5mm balsa, then beveled the edges a centimeter back on an angle of about 45 degrees.

I then sealed this plug using wood sealer. This stuff is brownish-amber, has a strange consistency almost similar to that of nasal mucus (), and looks like it all came out of a very large nose .

The trick with this wood sealer is to thin it to the point where you would be able to spray it with a detail gun, then paint it on using a very soft brush. The wood will soak up the first two coats very fast. Let the sealer dry to the almost tack-free stage (2 minutes), then brush on another coat. I brushed on a total of four coats, leaving the last coat to dry for 10 minutes. I then sanded the plug with 220 grit and a foam sanding pad.

The plug was then painted with some purple acrylic paint I had left over from the last time I did some airbrushing. After this was well cured, I sanded the entire plug with 220 grit. I then mixed up some pinhole filler (piroxiline primer + talcum powder) and slathered it on with a broad squeegee.

When the filler was dry, I sanded it using 220, than 360 grit. All depressions and pinholes were effectively faired.

The plug was finally polished with 3M Imperial polishing paste. The one that has a subtle smell like bananas mixed with peaches

The "parting board", which will then become the flange of the mold, was made from 4mm MDF, cut to size on a table saw.

The MDF was sealed using four coats of wood sealer applied with a broad squeegee. Then, the sealer was sanded lightly and a coat of piroxiline primer was brushed on. Tomorrow I will sand this coat and polish the surface with 360, 400 and then 600 grit sandpaper.
Last edited by Alex.Schweig; Jan 25, 2010 at 07:59 PM.
Jan 27, 2010, 09:25 PM
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Comparison - Infusion Time of Two Common Non-Woven Reinforcements


The two laminates that will be made in this exercise will be used to compare the resistance to infusion of two common nonwoven reinforcements - Style 450 chopped strand mat and 3/4oz continuous strand mat.

For now, the "infusion inside a mold" exercise is on hold until this small exercise is complete.

The two 15x15cm laminates produced in this exercise will be made using a 30x30cm glass plate as the tool. The bag will be sealed to the plate using double-sided tape. A vacuum line will be placed at one end of the laminate and the resin feed line at the other end.

The tubing used for this exercise is 1/2cm aquarium tubing, with the appropriate T-fittings and valves.

The first laminate to be infused consists of a 15x15cm, single layer (8 grams) of Style 450 chopped strand mat. The second laminate to be infused consists of a 15x15cm square of 3/4oz continuous strand mat, with enough layers stacked up for the square to weigh 8 grams. As of now, these reinforcements have already been prepared.

Note that the laminates will NOT be infused together. The first will be infused, then removed from the tool after four hours under vacuum. The tool will be cleaned, waxed again and the appropriate tubing will be prepared for the next laminate (the resin line and its fittings are not re-usable).

These two infusions will be made at a 50/50 fiber-resin ratio. The entire infusion will be timed for both laminates.

Now, on to the practical aspects of the exercise. The vacuum system to be used is able to pull up to 26" Hg of vacuum. This is enough vacuum to insure a good infusion as well as provide considerable pressure on the laminate.

The bag material is thick PE clear drop plastic, sold at hardware stores. This plastic bag material was chosen because it is strong and because it will not need to be waxed before use - resin will NOT stick to it at all.

A small resin trap will be placed between the bag and the pump. The resin trap is simply a vertical section of PVC pipe with endcaps. The line from the bag enters near the bottom of the section and the line to the pump exits at the top of the section. This way, any resin that gets into the vacuum line will stay in the resin trap and the pump can be operated safely.
Last edited by Alex.Schweig; Jan 27, 2010 at 09:37 PM.


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