Thin split-mould tube: how to make it faster & better inside quality?


Thin split-mould tube: how to make it faster & better inside quality?
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Gert Jan Pieterse
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Hi there,

My first composites project is a tube with bend in it. It is a rectangular shape with round corners, roughly 24 x 18 mm. The goal is to use those as tent poles for a lightweight mobile clinic for emergency medical teams.

So far I have made 2 parts with the vacuum bagging method, using the green 50 mm tubular release film from Easy Composites, as well as their other vacuum equipment. I have to say, for a first try it looks and feels awesome! (So whoever from EC is reading this: thanx). However, I'm not yet completely satisfied because:
- It takes a lot of time to fill a mould and prepare it for vacuum use,
- The inside of the tube is not very consistent. I do need to stick parts into the end and therefore a consistent inner face is very important for a good fit.

Here's a picture of the split mould with tubular film, you can see that the (very stiff) film makes it hard to properly close the aluminium mould.



I didn't have a ramp controlled or multi-stage oven, therefore the part did get quite some pinholes after 1.5 hours on 120 deg C. Sad I'll buy another temperature controller with ramp control so I can use the 'official' temperature ramp profile. I used XC110 205 gsm twill, then 2 layers XC130 UD, then another 205 gsm twill.



Inconsistent inner face, due to 'overlap' from the bottom mould into the upper mould:



Here are my own thoughts on how to improve the process and I'd love to hear from anyone with more experience whether I am on the right track!

- Faster production
Since putting down the layers (and getting all the protection sheets off them) took me more than 1 hour, I was thinking myself to skip the UD layers and instead try 2 layers: 205 gsm outside, 410 gsm inside. This gives less stiffness in the bending direction & decreased wall thickness, so I'll have to test the poles very well before using.

Also, to get rid of the vacuum preparation, I was thinking about using pressurized bladders. The shape is very easy so I can imagine that a standard silicon tube would work? Any experiences on what type of tube, wall thickness, release chemicals etc would work?? Or maybe even easier: an inner tube from a bicycle?

Using pressurized bladders would also mean that I can skip the XC110 prepreg and go for the 'autoclave' XC130 prepreg with shorter oven cure profile... than hopefully, with a simple ramp -> 120 deg C for 1 hour -> cooling, I can get pinhole-free outer face... Good idea?

- Consistent inside face
Since I need to insert parts into the tube, the inner face is quite important as well (Glueing not an option). I did try to make a roll-wrapped rectangle tube (with round corners), but that didn't stick very well to the mandrel. Also getting the shrink tape on right was quite difficult, it turned out all wrinkled Tongue For this split-mould method the outside face looks pretty good and consistent in dimensions, but the inside has the well-known inconsistent thickness due to overlap.

I was thinking myself to either put in a lot of extra layers at the end and than afterwards have a CNC mill clear the opening (adding extra expensive production steps) or try to place a few layers carefully on the right places in the bottom corners to act as 'dummy overlap layers'. Then the thickness would be bigger there as well. Any ideas on how to best tackle this issue?

- Mould optimisation
I did get quite some overflow resin into the bolt holes... making it very hard to remove the bolts and open the mould. Any best practises on how to protect the bolt holes on the split face between the mould halves?

Curious to hear what you all think, looking forward to read your feedback!

With kind regards, Gert Jan

Rosta Spicl
Rosta Spicl
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- Mould optimalization:
For AL split moulds I obviously use the filleting wax apllied on bolt thread and bolt head. During screwing it fills the hole, thread and contersink. Apply the release agent on all the flanges and screws very precisely.
- Consistent inside face
Big challenge...The pressurized bladder doesn't solve non-constant wall thickness. Due to overlap the plies you know. The pre-cured insert is a good idea. I use obviously the AL part, precisely machined inner flange, laminated in. You right, sometimes the insert didn't stick very well. The solution is use the adhesive film applied on the insert outer surface, or apply epoxy glue (for example RG 5 min fast glue works ok). If the part is pre-cured carbon insert, the contact surface has to be sanded very well or peel ply.
Presurized bladder offers much more better outer/inner surface quality, pinholes free, glossy (if the mould is glossy). If the autoclave is not available, this solution works pretty well.


Edited 3 Months Ago by Rosta Spicl
Gert Jan Pieterse
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Rosta Spicl - 6/29/2021 2:11:20 PM
- Mould optimalization:
For AL split moulds I obviously use the filleting wax apllied on bolt thread and bolt head. During screwing it fills the hole, thread and contersink. Apply the release agent on all the flanges and screws very precisely.
- Consistent inside face
Big challenge...The pressurized bladder doesn't solve non-constant wall thickness. Due to overlap the plies you know. The pre-cured insert is a good idea. I use obviously the AL part, precisely machined inner flange, laminated in. You right, sometimes the insert didn't stick very well. The solution is use the adhesive film applied on the insert outer surface, or apply epoxy glue (for example RG 5 min fast glue works ok). If the part is pre-cured carbon insert, the contact surface has to be sanded very well or peel ply.
Presurized bladder offers much more better outer/inner surface quality, pinholes free, glossy (if the mould is glossy). If the autoclave is not available, this solution works pretty well.



Thanx Rosta, that's some very valuable insights. Especially the wax on the threads, sounds very logically to do Smile

I'll see how I can add the insert. You mean that I could insert it directly during the assemby in the mould in one of the mould halves, or better after curing the tube? In that case, would epoxy glue fill the void due to the inconsistent wall thickness?

Nice, seems like the bladders are the way to go. Now I should start polishing the mould to ge a shiny product immediately haha.

Rosta Spicl
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Apply all the plies, put the insert into one half, close the mould.
Be carefull about the insert stiffness if presurized bladder. I go close to +8bar and the insert should be enought stiff due to deformations caused by pressure...depend on tolerance requests.
Another  proposal, to much complicated, in places of flanges laminate the wall a little bit thicker and machine the flange after curing process.


Edited 3 Months Ago by Rosta Spicl
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