Carbon fiber stock, almost there.

General Composites Discussions Composites Talk
#1

Ok, I have been refining the process and materials used over the last year, I believe it’s close to going forward with full production but there are a few minor details that need to be addressed. Part; composite rifle stock using carbon fiber, kevlar, fiberglass, or carbon and fiberglass depending on weight requirements. Up to this point, all molds are laid up by hand using tooling gel coat and tooling resin, all polyester products primarily die to cost. The parts are laid up using epoxy resins, which is drastically reducing the cycle count of the polyester molds, so new more robust molds are top on the list.

The majority of the parts are laid up using a tinted epoxy base coat, so any micro bubbles or voids are not visible. When a clear carbon fiber part is laid up, a small amount of voids and air bubbles are present. I am sure they are in all the production parts, but only visible when the epoxy is left clear since there is no place to hide.

The layup method is as follows;
-a thin base clear or tinted epoxy brushed into the mold and allowed to tack. This eliminates print through and gives a much better surface finish
-layers of laminate placed in both sides of the mold (2 part closed mold), wet out by hand and rolled using bubble roller
-silicone bladder is inserted on one side of the mold, halves joined and bolted together
-mold is placed inside a vacuum bag, approximately 27 Hg vacuum is applied to the bag, bladder is then inflated starting at 10 psi ramping up to 40 psi, vacuum ran for 30 minutes. Bladder pressure remains at 40 psi for the cure cycle.

When the clear part is removed, there are areas which have trapped air voids or pin holes, which require repair thus increasing finish work. This brings me to the questions; do the final production molds need channels to allow the trapped air to evacuate? Current molds have about a 3 mil gap for flashing, which seems to allow the majority of trapped air and excess resin evacuate the part. Would increasing the bladder pressure potentially solve this?

Here is an image of the part

#2

I do a lot of bladder molded tubes and a few other more complex shapes and one thing that I do that may be helpful is to use peel ply between the bladder and laminate stack.

With some of my molds, shimming the flanges apart a few thou wasn’t enough to eliminate trapped air. If you think of processes like sand casting, air evacuation routes have to be placed in very strategic areas to ensure that all air can escape the cavity as the mold fills. Pretty much the same thing that’s happening with a bladder mold.

If it isn’t practical to have holes in your mold in the areas that frequently trap bubbles (and it usually isn’t practical to do this), I’ve found the next best thing is to allow the bubble to escape under the laminate rather than from the mold surface. An easy way to do this is to provide a flow media (either actual infusion flow media, or peel ply) between the reinforcements and the bladder. Try to place this material so it provides a path from areas of your mold that are most problematic to areas where the air can more easily escape, like the flange. For extremely problematic areas, you can use two or more layers of peel ply to ease the flow of air.

The peel ply surface in my case performs double duty, reducing trapped gas during inflating and leaving a good surface under the laminate for secondary bonding.

On a complex part like yours, it may be difficult or impossible to completely remove the peel ply from the underside of the cured part, so try to think ahead about removal or whether leaving it entombed under your part will be a problem.

#3

Peel ply is something I have not attempted, it may be worth a try. It may become problematic if the peel ply can not be removed, the hollow shells get filled and the epoxy filling compound needs to bond directly to the inside of the shell. I appreciate the insight, I’ll give it a try!

#4

I take it you are planning to manufacture these in some number?

If so, then why not just switch to prepreg now? I think you’d have a very easy time of this with prepreg. I"ve made similar parts with similar process using prepreg, and it was pretty straightforward.

If your mold can take more pressure, that may help. I think that you need more space for the air/resin to flow. If using/adding vents is a pain or problem, couldn’t you just put some breather bleeder down the part or in the areas that need more evacuation? Maybe use a perf or peel ply, then bleeder? Pulling peel ply might be a pain as Petey mentioned.

again… switching to prepreg would make this easier in many ways but then you have to think about the CTE of the molds which may change the dimensions of the cured part. Plus you’ll need the oven to cure. Plus new materials.

good looking part so far. What rifle is that for?

#5

Thank you for the compliment and input sammymatik. It is a part that is in limited production currently utilizing only one mold, about 10-15 units per month. I am ready to take the part to full production of approximately 40-60 parts per month. Obviously having all the small concerns ironed out and applied to the new mold will be important, but the design of the finished product is all complete. I have considered prepreg, cost and current mold design have been the limiting factor, as well as the retail cost of the part. Once the part is cured, the bladder can easily be removed by applying vacuum to collapse it. Any material that is inside the shell after layup will be nearly impossible to access or remove. I will try the peel ply and see if that solves the issue, as well as test to see if it can be removed from the shell.

I have been reluctant to apply more than 40 psi to the existing mold. I am confident the bladder will take higher pressures, but uncertain about the mold. I considered attempting casting a mold using either aluminium powder or corian sand, but both seem to be difficult to locate in the US. Second idea is to mill a mold deck out of some heavy 1 3/4" MDF solid core panels I have, and use either a casting resin or heavy shore D urethane to cast the detailed portion of the mold. I think this method will give greater flexibility to play with flashing gaps, relief channels, etc. I even considered attempting infusion with a modified mold design, not sure what the results would be but it could be promising? By designing vacuum and resin flow channels in the mold, as well as perimeter seals which could be either cast or milled into the design, this would also replace the need to vacuum bag the mold.

The stock is designed for Remington short and long action, depending on how the inletting is milled.

#6

Very cool. Yah your layup looks very good, little or no distortion in the weave.

the molds we use for bladder stuff from aluminium are pretty beefy. They work well and take over 150 PSI no issues. They get bolts through and are held by the press, so they have very uniform pressure keeping them shut. using a silicone pressure intensifier takes an even stronger mold, but gives autoclave type pressures or better.

If you have access to a CNC, the wood mold should be a good way to prototype it out on with a cheaper material. Then after you nail it down you can switch to metal. The advantage of metal is the durability and high temp, so you could switch to prepregs if you wanted to. Obviously this all eats into your profits.

I’m sure you could infuse if you really wanted to. The setup would be a bit different but, you’d probably not have to worry about the bubbles in that area since you would evacuate the air and degas the resin. I like infusion but, getting things setup can take a little bit of practice. Plus dry layup can be a bit of a chore but probably easier than wetlayup. If you design a mold as you say, the turn around time would be pretty quick and since all the locations for vacuum and intake would be there so you could have your helpers, if you have any, make a part like this pretty quickly. As with most parts, the tooling is a large part of the expense but also a great way to save time and money with a more efficient setup.