How to Make a Resin Infused Carbon Fiber Part (VARTM)

How to Make a Resin Infused Carbon Fiber Part (VARTM)

When you typically think of resin infusion for carbon fiber parts, you probably think of a super complex process that you couldn’t possibly do in your garage.  This is only partially correct.  Resin Transfer Molding (RTM) is the traditional process of creating a resin infused part.  RTM requires a top and bottom mold with the layers of carbon fiber sandwiched in between, much like a compression molded plastic part.  In both instances resin (or plastic) is injected into this closed mold to fill the entire space while under pressure.

Vacuum Assisted Resin Transfer Molding (VARTM) on the other hand only requires a single tool surface.  This tool could be as simple as a waxed glass surface or as complex as a machined mold.  Instead of using a second mold on top of the layers of carbon fiber, a vacuum is pulled over the part and resin is “pulled” across the part with the same vacuum that is holding it under pressure.  This process can be completed in anyone’s garage with just $300 worth of equipment.  This picture blog will walk you through how to make high-gloss one-side-finished parts like the pros.

 

Step 1: Get the Equipment

Let’s be realistic, if you can’t buy it on Amazon, you probably won’t buy it.  So I’ve only included links to the products you need that can be bought on Amazon.  This doesn’t mean you can’t find better deals elsewhere, so please, shop around (McMaster-Carr is great)!

Most importantly, we need a pump.  You can check out my previous blog post on vacuum pumps or I can just skip ahead and tell you that this pump is the best.  You will also need a valve and some barbed tube fittings that can be screwed onto your pump.  You can pick those up at Home Depot or McMaster-Carr once you have sourced your pump.

Next, you need a resin catch pot so you can protect the pump you just bought.  This is a very important item that can’t be missed for resin infusion.  This is probably exactly why they charge you an arm and a leg for one of these things.  They are a very simple contraption so if you are feeling handy I’m sure you could make one from scratch for less than $50.  Either way, DON’T RUN THE VACUUM WITHOUT THIS SETUP BETWEEN YOUR PART AND THE VACUUM.

Best Resin Infusion Vacuum Pump

Resin Catch Pot for Infusion

Step 2: Purchasing the Consumables

I’m going to assume that you have some experience with carbon fiber and fiberglass if you are considering attempting resin infusion.  With that said, make sure you buy yourself the carbon fiber you need as well as some thin resin or infusion resin.  Despite their awful website, Composite Envisions sells some very cheap thin/infusion resin systems starting at $50 a gallon (this is about 1/3 the price you will see elsewhere).

Once you have those two key elements secured, you will need the following: 3/8” spiral tubing wrap, 3/8” plastic tubing, plastic t-connectors, tacky tape, green flow medium, peel ply, and vacuum bagging film.  Most of these are standard vacuum layup supplies that you should already have lying around if you have messed around with carbon fiber before.  The stuff special to infusion is the green flow medium and all the tubing.  All of the tubing can be purchased on Amazon or McMaster-Carr, but the green flow and other bagging supplies must be purchased from a composites shop online.  I recommend Composite Envisions again for low prices (they also sell all the tubing).

Spiral Wrap for an Infusion Layup

Step 3: Setup

Okay, time to get down to business.  I am going to be showing an infusion on a simple door sill mold, but I recommend starting with an infusion on a simple flat surface to test out your equipment.  I would also not try to infuse a distance further than 12 inches on the first go at it.  The picture below is of an infusion we completed for a flat sunroof panel plug.  This would be a good place to start:

Basic Resin Infusion Setup

Step 3a: Wax and Tacky Tape

No matter what you end up making, you always start off by waxing the mold!  Use just about any tooling wax and apply at least three layers for your first part.  Follow the instructions on the tooling wax.  I recommend Orca Shimmer or TR 104.  The surface of your mold should be glossy and smooth.  Whatever your mold surface looks like now is what your final part will look like when complete.  I also recommend laying down your tacky tape before waxing so that it sticks to the surface better.  You can then wax right up to it. Once waxed, DO NOT TOUCH THE SURFACE with your dirty fingers the oil on them can cause sticky spots.

Waxed Mold Surface

Step 3b: Carbon Fiber

With the mold waxed, you are then ready to lay down your dry carbon fiber.  This is the biggest benefit of resin infusion: no mess!  With a typical wet layup you have to manually squeegee the resin into the carbon fiber.  This results in a sticky mess and a distorted weave.  Through the VARTM process, you can lay the fibers exactly how you want them and never touch the resin.  Be sure to measure the total weight of the carbon fiber at this point so that you know how much resin will be required later.

Carbon Fiber on a Mold

Step 3c: Peel Ply

After laying down your carbon fiber, you must put down a porous release cloth or peel ply.  This is a very important step unless you want your green flow permanently stuck to your part.  I recommend using 4 to 6 layers here so that you can absorb extra resin and ensure an easy release of the green flow later.

Step 3d: Flow Medium

On top of the peel ply goes the green flow medium.  This helps the resin flow across the surface and absorb into the carbon fiber.  Without this layer, your resin would never start moving.  It is important to place the green flow only in the locations you want resin to flow.  If it goes off of the part, the resin will also go off the part and that is unnecessary.  Also, do note that some flow mediums are directional meaning that resin will only flow in one direction across the surface; placing it wrong will ruin your part.

Step 3e: Resin & Vacuum Ports

Unlike a traditional (+$50) vacuum port, spiral tubing and T-connectors are used for an infusion.  Before cutting any tube, you need to plan how the resin will flow.  There are few tricks to this.  Typically, you want your resin to travel the shortest distance possible so as to ensure the entire part gets infused.  For instance, on a 12” by 30” part you would want to run tubing down both the long edges and infuse across the 12” direction.  However, there is an exception to this.  If your part is very narrow, you can have issues with infusing too quickly.  The resin will reach the other side of the part before it has had a chance to soak into the part.  In special cases like this you may have to infuse the long direction, but wrap the tubes up the sides like a horse shoe in order to help the resin reach the far end.  Pro tip: If your part is large in both directions, consider setting up vacuum lines around all the edges and running the resin into a port in the center.  If you do this, be careful that you build up enough release material on the part so that the excess resin and port can be later removed. 

These resin ports are very simple to make and entail cutting a length of spiral tubing long enough to reach the far ends of your part.  Then fold it in half and insert a plastic T-connector into the tube like the picture below shows:

T-Spiral Tube Wrap Setup for RTM

It is then important to wrap the tube in peel ply.  I like to cut a hole in the middle of a sheet of peel ply, insert the T-Connector head through and then wrap the tube up with it.  The cloth is not going to want to stay in place, but a little bit of masking tape will help.  Wrap some tape in the middle around the T and around the ends.  Try not to use more tape than needed as this will restrict the resin flow.  This wrap accomplishes two things: it ensures the resin port will be able to release from the part and that the vacuum bagging won’t get sucked into the spiral tubing and cause a rip.  Notice the tape placement in this picture:

Final Wrapped Tube for Resin Infusion Port

Last, these tubes must be placed onto the mold.  When able, I recommend placing the spiral wrap tubes off the part, but still on the green flow medium.  This allows the resin to easily flow into the part, but isn’t directly on the part which can be difficult to remove when cured and cause heavy resin build up on the part.

Step 3f: Vacuum Bagging

The final layer is your vacuum bagging.  This will be applied over the top of all of the other layers and stuck down to the tacky tape just as a traditional layup would be completed.  Ideally, avoid creases, wrinkles, and pleats on the part itself.  Any of these will cause the resin to flow more easily and could create a path of low resistance that will adversely affect where the other resin flows.

In order to make an airtight seal around the T-connectors, I recommend wrapping a piece of tacky tape around the top of the T under the vacuum bag, cutting a hole in the bagging, sliding the T through, and then wrapping an additional layer of tacky tape around the top.  Be sure to check here for leaks when you pull vacuum.  At this point you are almost ready to pull vacuum and suck resin!

Vacuum bagged setup for RTM

Step 3g: Vacuum, Catch Pot, and Tubing

The last step of the setup process is to run the vacuum and resin lines.  At this point, you should know which port is your “in” and which is your “out” based on the previous steps of setup.  Connect a short length of tubing (1-2ft) to your “in” port and bend the last 3 three inches over on itself and clamp off with a vise grip.  On the other side connect another short length from the “out” port to the catch pot and a final longer length of tubing from the catch pot to the vacuum pump.  It is best to place the vacuum on the ground or on a solid non-flammable surface to avoid vibrations and potential fire hazards.  The diagram below shows a simple infusion setup for a part made on a flat mold surface.

Resin Infusion Diagram for Simple VARTM

 

Step 4: Pull Vacuum

Prior to starting your resin flow, it is crucial to check that a perfect vacuum can be pulled.  With the vise grip clamping down the resin input tube, slowly pull vacuum.  A simple ball valve on your pump allows you to control how fast you pull the vacuum.  It is important to go slow here in order to ensure that the spiral wrapped tubes end up in the correct placement.  Make sure the input is off the carbon part, but on release cloth and at least touching the green flow medium.  For the output, make sure it too is off the carbon part and on release cloth, but it is best to have it removed from the flow medium.  Leaving a space here will slow the resin once it reaches the end of the part so it doesn’t start filling the output too quickly leaving a dry part behind.

Once you have pulled a full vacuum and everything is properly aligned, double check to make sure there are no leaks.  Run your fingers around the tacky tape and give extra attention to any pleats.  Under vacuum, your part should look like the following picture and be ready for infusion!

Resin Infusion Layup Under Vacuum

Step 5: Weigh Out the Resin

At this point you will need to calculate the required resin.  This will be based on the specific mix ratio of the resin you purchased.  You will need the weight of the carbon you measured before, plus a few extra factors to determine the required weight.  The release cloth and flow medium both absorb quite a bit of resin.  We account for an additional 700 grams per square meter of resin to cover this layer.  Also, the spiral tubes and inlet tubes require a good bit of extra resin.  A good number to start with is 100 grams of extra resin to account for these.  No matter, once the infusion has begun, you can always kink the inlet tube and make some more resin before it runs out.  It is very important to never run out during the infusion!

Step 6: Infuse the Resin

It is now time to actually start the infusion! With your resin mixed in a large cup/container, remove the vise grip while still holding the tube kinked.  Submerge the entire end of the tube into the resin (wearing latex gloves) and try to fill the kinked end of the tube with resin before opening it up.  Failing to do so will introduce a bubble of air into the part (despite what many experts will say, this will not ruin your part, but do avoid it as best as possible).  The resin will quickly move up the tube, into the part and along the spiral tubing.  It will then slowly move into the part.  You will quickly be able to tell where you put too much tape, or didn’t get the tube onto the green flow, or where you have an extra pleat on the part.  All of these things will affect how the resin flows across the part, but at this point there is nothing to do but watch and wait.  This part is pretty mesmerizing to watch; I guarantee you will enjoy this part of every infusion you do.  Here are a few pictures of our resin infusion:

Resin Infusion Process

Once the resin has reached across the entire part and is starting to get sucked into the “out” tube you are good to re-clamp the resin “in” tube.  Doing this should put the part back into full vacuum.  Resin will continue to infuse through the carbon and excess will continue to be pulled from the part and into the catch pot.  Be careful once again to not allow any air into the part while clamping off the tube with the vise grips.

As easy as that, your layup is complete! You now must wait for the part to gel (typically 2-4 hours, depending on the resin) while maintaining vacuum pressure.  After this time, you can turn off the vacuum and let the part cure for an additional 12 hours or however long your resin system recommends.  Some infusion resins are extremely slow curing and may require additional heat or up to 24 hours in the mold.

Step 6: Removing the Part

After allowing your part to cure you are ready to open it up.  I hope you get the feeling of Christmas morning like we do every time we pull a part from the mold! Start by snapping off the resin inlet and outlet tubes; the resin should be brittle by now and they should snap right off.  Next, remove the tacky tape from around the edges, pulling the vacuum bag off as you go.  Assuming the mold was properly waxed, you should be able to pop your part right out!   There will be a bit more work to remove all the bagging materials from the backside, but this is very similar to the debagging process of a wet layup.

Final Carbon Fiber Part made with VARTM

If everything worked out accordingly, you should flip it over to reveal a beautiful high gloss part!  Of course, this isn’t a perfect world and the infusion process isn’t something that is easily mastered on the first go.  You will likely find areas with porosity or a dry spot at the far end of the layup.  All of these issues can be addressed in your next infusion by adjusting how you layout your tubes, how much resin you use, and how much time you allow it to infuse.  I recommend taking pictures as the resin infuses across the surface to refer back to those later when trouble shooting dry spots and porosity.  Often, dry spots are the result of an infusion that was too long (slow) and the resin never reached the end. Porosity is often the result of an infusion that went too fast, resulting in too little resin on the part because it skimmed across the top layers but never absorbed into the lower carbon fiber.

I hope this walk through has provided some valuable insight and that you will be able to start infusing your own parts soon! If you have any questions, feel free to reach out to gabe@commonfibers.com for advice.

And as a thanks for reading, use the code BLOGGER11 to get 25% off your next order on our sister site!

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