Another variation of the same principal. This one doesn’t use the shallow taper to capture the end of the bladder. You can see in one of the pics that the bladder has to be stretched out over the fitting which allows the end to pull back in around the flat face. The collar then captures the bladder between the two surfaces.
Works just as good, easier to machine.
At this point, it’s time to start molding some tubes. If you’ve done the math correctly, you should be able to apply the materials you may require under the tube (release film, peel ply, etc), apply the reinforcing fabrics (wrapped tightly around the bladder and wet out) and still be able to place the assembly into one half of the mold with a little room to spare.
Place the second half of the mold over the first and install the locating pins/bolts to maintain alignment. Space your clamps evenly along the length of the mold and snug them down.
Connect the end fitting to a compressed air source and begin ramping up the pressure. This requires some experimentation but for most of my tubes I increase the pressure about 5psi every few minutes up to about 35-40psi max.
One thing to be aware of: the excess resin and the air between the bladder and the mold needs to escape in order for the laminate to evenly get compressed against the mold surface. This means that some resin should be squeezing out between the mold flanges and dripping all over your bench. This is why I keep my alignment bolt locations outside the area of the mold that makes up the length of the tube. If you have alignment bolts in the path of this excess resin, you’re gonna have problems.
It also means that mold release should be applied to the flanges of the mold as well as the cavity itself. a very thin film of resin should be evenly left on the flange after the mold is opened.
If the mold is closed too tightly, this excess air/resin cannot escape and the process will fail.
Also, at the pressures used, you’re dealing with a fairly large amount of force inside the mold. woodworking clamps or some other light duty clamp WILL NOT be sufficient to ensure safe pressurization of the mold.
A failure during pressurized molding could range from best case; the bladder bulges out between the flanges, pops and your part is ruined or worst case; the clamps fail and the rapid loss of support results in catastrophic failure of the mold.
Think ahead, over-engineer and work slowly. I guess that goes for just about everything.
After curing, you should separate the mold halves to find something that look like this.
Great write up. Thanks for sharing your work.
Here’s a link to a member’s bladder molding operation that I found very interesting: http://vimeo.com/35648020
Looks nice, thanks for sharing, but was there a reason why you didnt just CNC your self a female aluminum mould? The time, cost, quality and accuracy would be superior IMO.
Roger,
That video shows the variety of parts that can be bladder molded. I was involved in model aviation for a number of years, plenty of good opportunities to fabricate composite parts.
hojo,
CNC molds would definitely be nice to work with. At the time I built these molds I didn’t have a mill large enough to produce them that way. The cost to outsource that project for non-commercial use molds would have been a limiting factor. My main goal in my thread was to demonstrate how it can be done inexpensively for non-commercial fabricators.
I do have access to the CNC equipment now, so maybe I’ll do some aluminum molds some day.
Great write up, thanks for the effort. Looks like you’re making some nice tubes, what applications are you using them in?
I used the carbon fiber tubes to build all of my wheelchair frames.
The first project was a tennis chair, then my everyday chair, then a racing chair (which required few tubes and more unique components).
I built them with traditional tube/lug construction, except the lugs were individually layed up around the bonded tube joints.
Careful design of the tubes themselves, the geometries of the frames and the assembly process has yielded rather impressive results.
My frames are a fraction of the weight of comparable aluminum or titanium frames, extremely rigid, not prone to weld fatigue (a common failure mode in metal frames), highly scratch and impact resistant, and pretty darn cool looking IMO.
I’ve been abusing these chairs every day for years and haven’t had a single durability issue. At this point, I’m confident they will last much longer than I will.
There are a few variations on the bladder molding process that I’ll post up showing how I was able to mold some of the curved tube sections that are used in those wheelchairs.
First, here’s a few photos of some crude destructive testing I did on my 1" OD carbon fiber tubes.
The tube I tested to failure was one that went bad during molding due to a leaking bladder. I figured if a faulty tube still demonstrated sufficient bending strength, a successful tube would be ok.
I did some calibration of my small arbor press with a scale and then some aluminum tubing. Using a 50lb. fish scale to pull the handle at a specific distance from the pivot and measuring the resulting force on the scale (had to reduce the force on the scale with a little offset bar due to limited range of the scale) gives the mechanical advantage of the press. A little beam theory math, then applied sufficient force to the carbon tube as to cause it to fracture.
I measured about 1000lb. of force applied to the center of a 12" span before the tube finally fractured. The amount of deflection in the tube prior to fracture was likely the result of poor fiber compaction from the low molding pressure due to leaks in the bladder.
It is a perfect tutorial you made here…
Thank you from the bottom of my heart
My pleasure. Hopefully it saves someone all the experimenting I went through.
The really cool (at least I think so) part is the way I molded curved tubing sections. I needed some 90 degree and 45 (or 135) degree tubes with integrated furrels to join various tubes.
For the most part, the process is identical to the straight tubes except the problem of having a rigid internal tube to apply the reinforcements prior to placing in the mold. I needed something rigid enough to hold everything in place and still be flexible enough to snake out of the cured laminate. This took a bit of head scratching, but after doing some machining I realized that the Loc-Line coolant nozzles on my mill were the perfect solution. For those not familiar with that product, its a series of hollow molded plastic knuckles that snap together and can articulate around the ball and socket connections between them.
Various sizes are available and you can snap together as many as you need to get a certain length.
Once the tube has cured, a firm pull on the end and they’ll follow each other out of the part. Pretty slick.
Here’s a few photos of them. The reduced diameter ends of the curved tubes were molded to fit inside the straight tubes and leave sufficient bond gap for the structural epoxy used to joint them.
Here’s a few of the mold construction for the 1" OD, 90 degree curved tube on a 3" radius.
Here you can see how they mate up with the straight tubes. The straight tubes for this application are molded with peel ply between the bladder and laminate to provide a good internal surface for bonding. The ends of the curved section are molded to allow a bond gap of about 0.01" (recommended by the mfr. of the structural epoxy used for assembly) and sufficient length to provide a specific bond surface area.
Obviously you need to know the ID of the straight tubes you’ll be molding prior to building the mold for the sections that need bonded inside them.
Can we get some more info on the bladders? Nice work by the way.
Very nice!
Can this setup be used for prepreg ?
I mean gel coat ant resin that u use can handle temperatures about 150 celsius?
Great job and thanks for sharing
The bladders I use now are from Piercan, USA. They offer a variety of latex bladder tubes in a wide range of sizes. When I was trying to source bladders, they were about the only place I found that offered stock sizes (rather than hiring someone to produce custom bladder tubes).
I’ve had very good success using these bladders, though I’m certain a more appropriate option exists. A few things I had to contend with. The latex is pretty thick, so I had to buy several sizes close to what I thought I’d need and choose the best. The cured laminate doesn’t stick to the bladder very well, but removing them does take some effort. If you carefully remove them, they are reusable for several cycles (actually quite a few cycles).
Another thing to consider is that if you’ve calculated your material thicknesses with fairly close tolerances, then your bladder need to just fit over the inflation tube. When I was using welded plastic bladders, I had to build them a bit oversize and once assembled to the inflation tube/end fitting I pulled a very low vacuum under the bladder while laying the reinforcements.
With the latex bladders, I was able to order a size that fit perfectly and didn’t require that last step.
I’ve done some work with molding silicones recently and I think making custom silicone bladders would be worth a look.
For the curved molds, I’ve also gotten away with using a section of a bicycle inner tube as the bladder. Requires sealing the second open end with another little custom machined fitting, but works good and releases from the cured laminate pretty easy.
I don’t see why this method couldn’t be used for prepregs. You’re mold would need to be constructed out of high temp tolerant materials and probably have some provision for heating.
Bladder molding like this is done all the time in the bicycle industry with machined aluminum molds and prepregs. Hard to justify the cost for personal use, but if I had plenty of money, I’d definitely want to do it that way.
Since there’s no bleeder, what are you doing to control the fiber ratio? And how are you not getting a lot of pinholes since there’s essentially no place for the resin to bleed?
The mold deflects just enough under pressure to allow air and excess resin to escape between the flanges. Most of the time I spiral wrap a strip of peel ply under the laminate and I did some experimenting with bleeder wrapped around the bladder at the ends of the tube.
I had some minor pinhole problems early on, but they come out very nearly if not completely flawless now.
The combination of a more rigid mold, better clamps, ramping the pressure and a higher overall pressure was likely the cure for surface finish issues. I think ramping the bladder pressure over time is the most important factor of those for eliminating pinholes.