Details of heated aluminum mold design for bladder molding

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#1

I am having aluminum molds machined and plan to do my part with prepreg and bladders. Before I spend the money to have molds machined I want to make sure I have the details of the mold design worked out.

My part looks like this (inlet/outlet are 65mm):

For the bladders I am planning to lay sheet wax into both halves (laminate thickness), then use smooth on’s brush on vacuum bagging silicone.

One of the questions I have, is there a preferred method for attaching air fittings to the bladder? It looks like this will have an impact on mold design so I need to figure it out before I have anything machined. I am looking for a reliable connection that won’t put unnecessary wear and tear on the bladder and that I won’t need to spend a lot of time futzing with on each part.

Bladder forms gasket:

Big rubber plug:

Fitting wrapped with rubber band:

For heating elements I am planning to use cartridge heaters with a loose press fit into the mold. I expect the the mold will be close to 20lb. 6 heaters press fit into each mold half like this:

If I use 100w heaters I will have a total of 1200w to heat 20lb of aluminum to a target cure temp of 250F. Does this seem reasonable?

Lastly, the thermocouple. I am planning to use a short screw in thermocouple and design the hole so that when the thermocouple is tight, the end is making contact with the mold surface. Is there a better method?

#2

that should be plenty of heaters, I use 400 watts to bring a 20x2.5x5" mold to 310F, screw in thermocouple works well too. careful on the silicone, some types actually bleed gas through at certain pressures and stretching.

#3

Thanks for the follow up. I contacted smooth on to find out if they have any experience using that rubber for bladders. I also contacted a company that makes custom latex bladders but they didn’t seem interested in giving any clear indication of pricing, just that I would need to machine my own mandrel and I would get production pricing after an initial higher cost run.

With prepreg bladder molding, is it neccesary to crack the mold open during the initial inflation for air to escape? I am thinking maybe do the initial tightening with a thin piece of shim stock near the bolt holes to make a very small release path, pull the shim stock, and then fully tighten for cure.

The biggest question I have left is fittings for the bladder to air source. I like the simplicity of the top example with a plate to sandwich the gasket- however, I can imagine that would quickly wear out bladders and I am trying to design this workflow for relatively high volume.

#4

i discovered that the silicone tubing I was using bled air by inflating it in the sink, you could probably do that with your silicone bladder. my experience using latex is that it seems less permeable, but doesn’t nicely uniformly inflate the way synthetics do

#5

you wouldn’t crack the mold, you would design in the vents. I’ve used molds that have vent holes and channels to allow air/resin to travel some. With out vacuum, i doubt that air is going to really move very far. As with an autoclave, your applying such high pressures that the air is compressed and thus void free. On a part like yours, since it’s not a structural piece, surface quality is your main concern. So long as your mold is polished and shiny, the pieces should come out beautiful.

you’d paint on the silicone in each half then put the mold together to make the bladder? Why not forgo the bladders and just use vacuum? Would be a heck of a lot easier and cheaper in the long run. the end user wouldn’t know the difference.

these are your miata ducts? I thought about making one for myself… didn’t you post some files at one point?

#6

Yeah, these are my miata ducts. The previous designs were built around 3mm wall thickness required for 3d printed ABS. I made some 1mm wall carbon versions and found excessive clearance so I did a clean sheet redesign to better use the space:
http://i.imgur.com/tqe8cMW.png

The flattened section of the duct is only about 1.4" tall, manipulating a bagging stack in that space is tricky and adds consumables. If I was vacuum bagging them inside the split mold I would still want to do something like a silicone vac bag since getting a traditional bag in a tight space would be such a pain in the ass. I like the idea of bladders since the workflow is so simple. I will probably precut a large quantity of prepreg pieces and then each part will just be laying in prepreg, putting the bladder in place, closing the mold, and then ramping up bladder pressure.

The miata track car audience is really cost sensitive so I am trying to design a workflow that allows for minimal per unit materials cost and labor, which will allow me to make them relatively inexpensive.

I am planning to put a vent on the opposite end of the tube where there won’t be carbon. Maybe a 1/8" hole that I pad with breather fabric to keep the bladder from rupturing. I would imagine that vents placed anywhere along the actual part layup will print through?

#7

Thats a good improvement from the old version.

as for the bag, it’s a two piece mold so, you just put the bag in and then close the mold as you said. the amount of bag you’d use would be very small. Sounds like you’d need to do some costing out but, unless your gonna make a large run of these, i don’t think an rvb or bladder would price out bag materials? I was just suggesting it as it would alleviate some issues with bladder molding. A bladder is ultimately better but not sure if cost was more an issue, i’m not sure that your bladders would cost? If you make them yourself they should be ok so long as you don’t break them every other part. The ones i’ve used, made from dipped latex, were good but could break fairly easily.

the vent on the end should be fine. Yes any vents along he length would print through. But, I don’t think there is much of an issue when you run 70+ PSI on a part that’s not going to really be taking large structural loads. The most important thing, I would think, would be that the finish is perfect as possible?

I think your gonna be fine with the setup your designing. Look forward to seeing the final products.

#8

Yeah, the part is minimally structural. There are a few people wanting to use them for intercooler plumbing on turbo cars but I think if I up the thickness to something like 1.5mm it ought to be strong enough.

#9

6061 or 7075? 7075 is a lot more, but materials cost on this is a small part of the total cost of machining. Am I going to have a miserable time hand sanding 7075?

#10

How did polishing the 7075 go? I have tinkered with the latex bladders but bought off the shelf closed end tubes for a kidney shaped tube profile. I am very interested to know how the smooth on bladder went, and how the silicone worked with the prepreg.

My aluminum came out pretty good.
JGW

#11

Any updates? Interesting topic…