HOW TO: Make a Carbon Fiber Part (Part 6: vacuum bagging CF in mold)

HOW TO: Vacuum formed Carbon Fiber Part for $200 (Part 1)
HOW TO: Make a Carbon Fiber Part (Part 2: making a clay mold)
HOW TO: Make a Carbon Fiber Part (Part 3: making a fiberglass mold)
HOW TO: Make a Carbon Fiber Part (Part 4: making the budget vacuum bag)
HOW TO: Make a Carbon Fiber Part (Part 5: mold prep and cutting CF)

HOW TO: Make a Carbon Fiber Part (Part 6: vacuum bagging CF in mold)
HOW TO: Make a Carbon Fiber Part (Part 7: two part piece and final steps)

Next is set up. The $60 shipped vacuum pump I use has decent reviews and is the most mass produced pump you can find. It's one weakness is that it probably isn't meant to run 12+ hours so even though it has a little fan, it gets burning hot after an hour or so... just like your sport bike would if you let it just idle. I didn't realize this at first so the clear air compressor oil that comes with it ended up burnt and I had to change it out... luckily the little pump didn't burn up. So now when I run the pump I run it outside and have a floor fan pointed at it just a few inches away turned up on high while it runs. Now over heating is not an issue and it runs for 12+ hours straight no prob... I have it run outside the garage door.

It draws 3cubic feet per minute (CFM) but "they" say 1 or 2 cfm is adequate. However, the lower cfm you go, the better you have to fully make sure your system is perfectly air tight. So far, despite all the clay etc, I have never got a vacuum bag to be 100% leak proof. thankfully the maximum negative pressure anything can be on earth is -29 in.hg... which is actually zero in reality because it's impossible to be less than zero pressure... all -29 means is you're overcoming the +29 pressure that exists on earth at sea level. 

Little vacuum pumps like this can pull maybe -28 maximum. So whether you have a little pump like this or 700 horse power v8 vacuum pump make no real difference if both were used on the same vacuum bag set up. The only difference would be if you had horrible leaks the big pump could hold more of a negative pressure than a 3cfm pump. That's the basic understanding I have of these pumps, but all I'm saying is don't waste your money getting a fancy air pump. However no mater what project you are doing a good rule of thumb is "your pump may be too weak if you cannot draw down to full vacuum within 5-8 minutes of turning on the pump"

Now on to the ever popular topic of how much epoxy resin to use for a given amount of Carbon fiber. I'll be the first to say I'm not an expert. I'm going off my thorough testing and instructions I received with the Max Clear resin I bought. There are only debates about the exact ratio of carbon fiber to resin ratio's that you should use and how to infuse them. Some people set up their vacuum bag dry and draw the resin through one end, through the CF, and turn off the resin infusion once it reaches the other side... Other's like myself understand that requires infusion mesh and extra steps to achieve the same end product. So I'll talk only about the breather cloth method.

Then there is the debate about do you calculate your resin based on "fiber weight fraction" (wf) vs "fiber volume fraction" (vf)... 

Resin fabric ration - Composites Central

The instructions you get with the Max Clear resin (which I am VERY happy with) say to use 65% carbon fiber to 35% resin by weight (equals 100%). I measure in ounces just because that's what most people do. However, I found the instructions UNDER estimate the amount of resin you need for a given vacuum bag job. This may be due to the fact they assume your set up will use exactly the ideal amount of resin to result in the perfect ratio of Carbon Fiber to Resin that is achieved in a correctly done vacuum bag set up like this. 

That is: Whether you calculate the perfect amount of resin or slop on an entire jug of mixed up resin on to your Carbon fiber before putting it in the vacuum bag... you're going to end up with the same CF to resin ratio of about (65%/35% in the end product (assuming you have everything set up right as described here). (some claim 60% is the max)

The problem with the manufacturer's instructions pouring out just enough resin (65/35) because that's how much resin ends up staying in the part is neglecting to account unintentional loss... that is, some resin is going to stay in the bottom of the cups you mix it up in, some resin is going to saturate into the bleeder cloth, etc. I'm surprised at this, but I'm sure it's so they can't be accused of recommending customers to use too much resin. 

The evidence of their under calculated resin is evident in the example on the left of the picture where I weighed out 65% worth of carbon fiber to a measly measured amount of 35% resin that barely filled the bottom of the cup.

Through this how to project I learned the correct ratio by purposefully using a little too much resin: 39% fabric to 61% resin which I calculated to be 2.23 ounces by subtracting the dry weight of my bleeder cloth before the vacuum bagging to the weight of the bleeder cloth after the job which had cured resin in it. 

The ideal ratio I now use is 57% Carbon fiber fabric to 43% resin. 

This ratio is only applicable to Max Clear 1618 epoxy resin because I tried it with the thicker viscosity of Max Clear HP "high performance" and you need more resin than 57/43 and even then the resin is too thick to use in vacuum bagging easily because it didn't migrate evenly as 1618 does. Meaning the HP didn't distribute under the mechanical pressure that is created under full vacuum... the part wasn't ruined. But the HP is best used for hobbyists that do free lay up projects with epoxy resin such as encasing bottle caps or other knick knacks in a layer of clear resin on a table top. 

So as I said before I intentionally errored on the side of using too much resin knowing it would result in telling me the correct ratio.

So with all resins you have two liquids to mix together in certain amounts (this is 2:1) to activate the resin to start curing or hardening. Air is not required because it's curing not drying (like paint). This mix takes over an hour before it even starts curing to the point it could be considered less fluid like... that's why it's great for vacuum forming.

Anyway, in order to calculate how much resin you need:

1.) weigh your Carbon Fiber cloth. Let's Say we have 10 ounces

2.) Use the Carbon fiber to Resin ratio to calculate how much mixed up resin you need. Let's say you also want to use 57% carbon fiber to 43% resin (total always has to equal 100). So take the wt of your cloth and divide it by the percentage of fabric ratio you want .57

10/.57=17.5

3.) So now to calculate how much mixed resin you will need just multiply the number you got in step two that is 17.5 by the percentage of resin your ratio demands .43

17.5x.43=7.5 (you can also get this number by simply subtracting your CF weight from set two's number 17.5-10=7.5)

4.) Now you need to calculate the resin to hardner mix of your resin... this Max Clear 1618 product is a 2:1 ratio of "Part A" to "Part B". We know we need a total of 7.5 ounces of mixed resin. To calculate Part A multiply 7.5 by .66 = 4.95 ounces of Part A. Part B will be 7.5x.33=2.5 ounces (or 7.5-4.95=2.55 ounces)

Don't forget to zero the scale for the weight of your cup.

Make sure you don't put too much Part B in because when you get towards the end of your supply you'll short yourself the required amount of part B to the Part A you have left.

I use a drill with a larger "wood bit" with the sharp spade at the end broke off as a way to mix the resin for about 30 seconds moving about the cup at speed. The instructions say to then pour the contents into a second cup as to avoid using any unmixed resin... however, they may also assume we're not using a power tool to mix either... Some advise against doing this because "it instills air bubbles" however I haven't seen that. Also there's the fact the resin takes hours and hours to cure and under pressure the bubbles are surely resolved.

Now that the resin is mixed and your mold has the 3 layers or so of dried carnuaba wax laid down as a mold release you can apply some resin carefully to your mold with a soft brush. You don't want to be too forceful when spreading it around as to not disturb the hardened wax layer below too much. It's amazing how the palpable ridges of the dried carnuaba wax that feel much like the contours of a stretched piece of leather don't translate on to the final part. 

(I did not know about the carnuaba wax mold release in this how too so don't be concerned that you don't see it in the following photos)

I was generous in the loose application of the mixed resin knowing that the excess would be drawn up through the layers of the carbon fiber. It's far better to apply a little too much resin on the bottom of the lay up then the top. However, don't worry to much, the vacuum bag is good at dispersing the resin evenly and extracting the excess.

Then apply the layer of CF that you want to be visualized and make sure the weave is in the direction you like displayed. You can press your CF carefully into the mold... if it's a very sharp corner you could cut the CF to conform to the mold... but I didn't

Apply a layer of resin to the top of each subsequent layer you put down if you have resin left... If you remember from earlier I was purposefully using too much resin in my 39/61 ratio to calculate the appropriate ratio.

second layer that was 12k CF

third layer that was also 12k CF

Then apply the "peel ply" alternative that is perforated wax paper we made earlier. You don't have to put this right in the middle of your part, it just has to be on the edge of it with the breather cloth so it can draw the excess resin into it like if you brought a dry towel to the edge of a puddle of spilled milk it could draw much of the puddle into the cloth eventually by only touching the edge. Put this hypothetical situation into a vacuum bag and you'd nearly all of it.

Then your breather cloth goes on top of the perforated wax paper... I used a piece of fleece that seemed absorbent... you could use a dozen sheets of paper towel or whatever.

When you go to put it in the bag it's very hard to slide it in without pulling the wetted out CF from the mold... this problem created a flaw in my part that I was mostly able to cut out... the solution is to place strips of wax paper or plastic sheeting on the CF before putting it in the bag as to hold down the edge of the CF so it doesn't cling to the inside of the bag as you slide it in.... or you could make the opening to your bag larger.

Seal up the bag with the slotted suction tube inside. 

Turn on the pump and the fan you may have pointing at your pump... then as the bag is collapsing make sure it has contact fully in the contours of the mold... that is why it was important that the top of your bag has more plastic sheeting than the bottom layer.

This is an example of the air pocket that can occur if you don't push the bagging into the contours as it's sucking down... if you wait until this point under full suction than all you're going to do is tear your bag if you push into it... no worries if this turns up... just clamp the hose or turn off your pump until you can lift up the top layer and re-position it in. 

So here's a progression of the excess resin being absorbed into the breather cloth through our alternative peel ply. This is occurring over several hours and you can see the dry white cloth turning more wet and a gray color.