Bad accuracy in prepreg moulding parts
Bad accuracy in prepreg moulding parts
(OP)
Hi there,
We are building some carbon fiber reinforced epoxi parts. Some "boxes" such:
We have ordered precise moulds (20 microns), steel in the outer parts, aluminium (7000 series) for the inner-mould-mandrels.
200 microns prepreg (Fiber/fabric: Torayca M46J / plain weave, 43% of epoxy E022, Cured ply thickness: 0.180mm) was selected.
We wrap the inner-inner-moulds-mandrels with the pre-preg (after thoughtfully clean them and apply release agent) :
Place them in the outer mould:
Close everything:
(note there are dowel pins for both inner and outer moulds). We have 5 screws in a length of 300mm and we tighten with a torque of 85N*m
and start curing in the oven
Then we take it out and wrap the whole thing (which is already soft) with an outer layer:
Press very strong again and back to the oven.
We expect (from the moulds dimensions) a wall thicknesses of 0.3mm everywhere. But we are getting both bigger walls and pockets. The total outer dimmension is up to 700microns bigger. We have remeasured the moulds and they are fine, there is something wrong with the procedure.
Does anybody know what we are doing wrong?
thanks
regards,
We are building some carbon fiber reinforced epoxi parts. Some "boxes" such:
We have ordered precise moulds (20 microns), steel in the outer parts, aluminium (7000 series) for the inner-mould-mandrels.
200 microns prepreg (Fiber/fabric: Torayca M46J / plain weave, 43% of epoxy E022, Cured ply thickness: 0.180mm) was selected.
We wrap the inner-inner-moulds-mandrels with the pre-preg (after thoughtfully clean them and apply release agent) :
Place them in the outer mould:
Close everything:
(note there are dowel pins for both inner and outer moulds). We have 5 screws in a length of 300mm and we tighten with a torque of 85N*m
and start curing in the oven
Then we take it out and wrap the whole thing (which is already soft) with an outer layer:
Press very strong again and back to the oven.
We expect (from the moulds dimensions) a wall thicknesses of 0.3mm everywhere. But we are getting both bigger walls and pockets. The total outer dimmension is up to 700microns bigger. We have remeasured the moulds and they are fine, there is something wrong with the procedure.
Does anybody know what we are doing wrong?
thanks
regards,
RE: Bad accuracy in prepreg moulding parts
RE: Bad accuracy in prepreg moulding parts
Here, in microns, the expansion of every part:
Do I have to take away these 72 microns from the machined mould?
The problem we are having is that the width of pockets/holes is not constant along the depth even though the mandrels have good parallelity/planarity
Is it possible there are problems with the resin is not flowing? The length of the parts is 300mm (~1 foot) and we are pressing two times 180 microns to 300microns
thanks
RE: Bad accuracy in prepreg moulding parts
Prepreg is a mixture of solid fabric with fluid resin. Within a very limited range of thickness (a few thousandths of an inch) the fabric does act like a compressible spring. Are the splices butted or overlapped? It appears you have overlaps, which would completely change the prepreg thickness in these areas, and fabric will not flow like a fluid.
Your tooling design philosophy seems to be flawed from the start, and not capable of achieving the tolerances you are expecting. One fix might be to place spacers between the bars at the ends where there is no prepreg.
RE: Bad accuracy in prepreg moulding parts
B.E.
You are judged not by what you know, but by what you can do.
RE: Bad accuracy in prepreg moulding parts
I have not included information about the position of the inner bars; we have positioning end-caps on both ends with dowel pins for the position of each individual part. That's the reason why we have the holes on the bars (two in the corner for pins, on in the middle to allow convection of the hot air inside them)
pics:
Outer moulds closed
There are no prepeg overlaps when wrapping the bars, butted joint (which never faces another butted joint).
The setup is preheated at 90ºC (194ºF) (without the final outer prepeg layer) but with the positioning endcaps. Then everything opened again, the outer layer is put in and back to the oven at 120ºC (248ºF) for 3 hours. Is it possible that the moulds (the total part - core) never get completely at the final temperature and this creates the difference in dimensions?
We thought that the mixture Steal (316) with aluminium (7075) would be good to press better the prepeg. What would be the recommended material for this case?
thanks
RE: Bad accuracy in prepreg moulding parts
This is an old problem first faced by people who cast materials for a living. To compensate for this they made devices called expansion rules which accounted for the different dimensions of hot versus cold materials.
B.E.
You are judged not by what you know, but by what you can do.
RE: Bad accuracy in prepreg moulding parts
The resin will also expand with temperature offsetting what the fiber does, increasing the final size a little more.
I expect the rest is that the steel plates bow from the pressure applied by the heated aluminum. If the centers are wider than the faces then that is likely the cause.
Performing an structural deflection analysis might clarify what is happening.