Composite Tube Molding

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#1

I’m a new member on this site, but I’ve enjoyed reading the wealth of information that it’s members provide.

One topic that seems to come up over and over is the fabrication of composite tubing. This is a subject that I’ve studied at great length for my own needs. I’ve experimented with a variety of methods for producing structural composite tubes, but finally came up with a process that met a rather lengthy list of requirements. Having seen so many folks struggle with this topic, maybe my experience will help.

It’s certainly not a new idea, but I’ve found that using bladder molding is the absolute best method to produce high quality composite tubing for structural applications. It takes a good deal of front end design and fabrication work, but I haven’t found another method delivers as much control over the process and quality of the result.

It’ll take me some time to post up pics and info about the process I use, but I think some of you may get some value out of it. I can testify to the results as I use the tubing I’ve made in very demanding applications every day.

Ben

#2

I’m interested in seeing your methods Ben! Thanks for sharing.

#3

There are a number of reasons I use bladder molding:

I needed to produce tubes where I could accurately control the outside diameter of the finished tube. While certainly possible with post-finishing operations like centerless grinding, why not produce tubes the correct size in one step.

By altering the inflation tube (form, mandrel or whatever you prefer to call it) diameter, and some simple math with respect to wall thickness delivered by various layup schedules, I can control the inside diameter as well. Only on rare occasions to I need to control the ID, as most of the time I design fittings, furrels, etc. to match the resulting ID of a tube. Performance of the tube dictates the OD and layup schedule, so that’s priority number 1.

Fiber compaction and fiber-resin ratio. By laying up the reinforcements to a slightly smaller diameter than the mold and expanding them to the mold surface, there is less distortion of the fiber orientations than compressing them around a positive mandrel. With a mold that can allow excess resin/air to escape and even pressure throughout the laminate, I get full saturation of the reinforcements, even compaction and good control over fiber/resin ratio.

For tubes that require bonded fittings or joining, I can mold the tubes with peel ply under the laminate to provide a good surface for secondary bonding.

Surface finish. Always the same, always nice. No post finishing required.

#4

Like I said, there is a good deal of front end work to do.

A 2-piece mold will need to be produced for each diameter tube. This is alot more work than just polishing an aluminum tube for a male mold. You don’t have to miss out on that step though, I use polished aluminum or stainless tubing for the plug to create the mold.

Considerations:

Length of the mold. Obviously you’ll need a plug/mold that can accommodate the length of tubes you need to produce. There are some limitations, especially if you will be doing a wet layup of the reinforcements. I built my molds to produce the longest tube I anticipated needing, about 30" finished length. You also need to allow some extra length for ease of use and the fact that fabricating a raw tube long and trimming it to the finished length will be required. I added about 4" for this. You can mold any length tube up to the max. or even multiple shorter tubes at once.

Because this process is going to be involving a pressurized bladder, the mold cavity needs a few featured to accommodate this. One end will need to be closed and one end will need to capture the bladder, inflation tube, laminate so that when pressurized, it doesn’t squirt out of the mold. I’ll elaborate when I get the photos posted.

The mold will need to not only be able to operate with significant internal pressure, but do so without deflecting and altering the finished dimensions of the molded tubes. I built a fairly stout backbone behind my mold surfaces and use a series of evenly spaced steel clamps along the length of the mold while under pressure. I’ve found that only 30-40psi of pressure has been required for my tubing, and measured deflection of the mold at about 0.001" in the center.

#5

I’m listening

#6

I’m going to get some pictures prepared that will help as I proceed. May take a little time.

Ben

#7

So this is an example of the plug used for a 1.375" OD tube approx. 26" long.

The end caps for the aluminum plug are machined to close one end of the mold and provide a tapered neck with an open hole on the other. If you have a cnc lathe or radius cutting fixture for a manual lathe, the closed end cap should be rounded. I had neither at the time, so a taper works fine.

The diameter of the opening where the bladder will be inflated from requires some forward thought depending on the fitting used on the end of bladder inflation hardware that you’ll be using.

#8

Next step is to build a parting board to fabricate the first side of the mold. Most folks are familiar with this part of the process, so I won’t go into much detail. Feel free to ask if you would like a more thorough explanation (that goes for any part of the process).

#9

Not all of these photos are sequential from the fabrication of the same mold, but the process was the same for all the molds I built so if something looks like it was from a different project, it likely was.

Laying up the first half of the mold. I use Epoxy surface coat and epoxy resin to build my molds, but gelcoat and polyester, etc. would be fine. The actual reinforcements vary, but sufficient strength and stiffness should be considered along with building a “backbone” structure to support it. I made several different designs of a backbone, and the one pictured here isn’t the best. I’ll attach some photos later of what i found to work best.

#10

Note that on the end that will be open, the mold is only built slightly past the end of the taper on the plug. A mold flange as wide as shown was way more than needed for later designs, but it’s nice to have a bit extra to trim for a nice clean edge.

#11

With one half of the mold layed up, remove the parting board and prep for laying up the second half.

#12

I drilled some alignment holes through the mold before separating the halves from the plug. Initially, I thought that a series of bolts along the flanges would be sufficient for closing the mold when pressurized, but that turned out to be incorrect. The reason requires a rather lengthy discussion, so for now I’ll just say that I use the four holes in the corners with shoulder bolts for alignment only and the clamps (pictured later) do the job of holding the halves together under pressure.

Not the PVA still on the mold in this pic. I hate that stuff and have since abandoned it for Frekote release products.

#13

A little deflection testing proved that bolting the flange wasn’t sufficient. Clamping force needed to be directed over the center of the tube to prevent bulging of the mold cavity.

#14

Building a backbone like this one was the best solution I ended up with. All are bonded to the mold laminate with thickened epoxy adhesive.

#15

Much less deflection using clamps rather than flange bolts.

#16

With the mold completed, it will need to be prepped with your favorite mold release. Ive use them waxed and PVA’d, but use semi-permanent release exclusively now. Not only do I not need to prep the mold for each use, but I don’t have to contend with the film thickness of the PVA altering the final OD of the molded tube. More consistent surface finish as well.

I’ll detail the bladder components and hardware shortly.

Ben

#17

Nothing particularly complicated about this process so far, but the remaining steps can be a little challenging.

In order to prepare the rest of the require materials, some decisions about the tube you’ll be creating have to be made. A bladder will have to be built or sourced from a supplier. A mandrel (I’ll call it an inflation tube) will have to be made, and a fitting will need to be machined to bring together the inflation tube, bladder and air fitting to connect to the air source.

Considerations for the tube:

I won’t get into the selection of materials and layup schedule as that is highly variable and depends on the intended use of the final tube. The important variable for this discussion is the resulting wall thickness of the laminate.

You’ll also need to determine the wall thickness of the bladder and any release films, peel plys you may require.

Working backwards from the diameter of the final tube, subtract the thickness of these materials to get the diameter of the inflation tube you’ll need to produce. This tube just provides a rigid form to wrap the reinforcements around prior to placing in the mold. Allow some extra clearance so you do not have to “stuff” the laminate into the mold cavity, increasing the chance of pinching something between the mold halves when closed.

The inflation tube is then drilled at regular intervals providing even distribution of the air under the bladder.

Sometimes you get lucky and a readily available tube is available in the correct diameter. In these pics, a piece of 3/4" PVC pipe was just right. For other tubes, I ordered some thin wall aluminum tubing.

#18

Reading this as you post. This is great stuff

#19

The bladder itself and sealing it to the air fitting was the most difficult thing I encountered.

The basic idea is that you have a machined fitting that will be captured in the taper on the open end of the mold. This fitting is attached to the inflation tube. The bladder is then slid over the inflation tube and the end sealed to the fitting in some air tight fashion. the laminate is wrapped around the bladder, wetted out, placed in the mold and compressed air is introduced to inflate the bladder and force the laminate against the mold surface.

My first attempts were welding my own bladders from stretchlon bag film and sealing the open end to the fitting with o-rings. While I eventually did get this process to work pretty well, it was difficult, time consuming and didn’t yield 100% success.

The problems I encountered with this method were ensuring a perfect seal on the welded seams of the bladder and machining the fitting so that an o-ring could seal the bladder to the fitting.

This picture shows what that method looked like.

#20

Although I did successfully mold plenty of tubes this way, a much easier and much more reliable method was needed.

After studying the problem for some time, I decided that sourcing premade latex bladders was the best option (silicone would work great too I’m sure).

With a more appropriate bladder, sealing them to the fitting was the next challenge. What I came up with has proved to be very effective though did require some fancy machining of new fittings. I made several varieties of the same idea, all seem to work great.

Essentially what you see here is a 2 piece fitting. The bladder slides over the shallow tapered section of the fitting and a collar slides on the fitting from the other side. The inside of that collar has an internal taper to match the fitting and the outside matches the ID of the mold and the short taper on the back end to catch the matching feature in the mold. A washer and nut secure the collar, capture and seal the end of the bladder.

Ignore the lack of and inflation tube in this particular photo, I’ll explain what this fitting is used for later.