Bad accuracy in prepreg moulding parts

drodrig · Oct 18, 2016

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,

Compositepro · Oct 18, 2016

Have you accounted for the thermal expansion of the aluminum and steel? The part dimensions are set by the size of the mold at cure temperature, not room temperature. M46J is a high modulus fiber with a negative longitudinal CTE. M46J composite will have very close to zero CTE while the Aluminum will expand and contract much more.

drodrig · Oct 19, 2016

We cure at 120 degrees C (248ºF) in the final baking during 3 hours.

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

Compositepro · Oct 19, 2016

Yes, you need to account for this growth. How are you keeping the spacing between the bars? Are you expecting the prepreg to maintain uniform spacing of the bars? If so, how can you expect the bars to control the dimensions of the prepreg? The fact that the aluminum bars expand more than the steel enclosure is what allows you to get the mold closed at room temperature.

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.

berkshire · Oct 19, 2016

It would appear that you are using the pre-preg itself to control the wall thickness to get this, and the part right, you have to remove the amount the mold expands, the 72 microns, plus the expansion of the outer mold , As Composite Pro said you need to control the locations of the individual bars. You already have holes in the ends of the bars , an endplate with taper dowels would do this. It would also appear that you are using the differential expansion of the Aluminum versus the stainless steel to provide the compression for the pre-preg . This is not an ideal solution but it can be made to work. you would have to pre heat the mold to seat the endplates before the pre-preg gelled.
B.E.

You are judged not by what you know, but by what you can do.

drodrig · Oct 20, 2016

Hi there,

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

berkshire · Oct 20, 2016

If you are controlling the position of the inner pieces with dowel pins then you have the positional accururecy problem handled , now the remaining problem is that your final dimension is controlled by the temperature of the mold, so you have to machine the mold to be dimensionally correct at the final curing temperature.
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.

3DDave · Nov 4, 2016

You have 60 extra microns (want 300, install 360) at 5 locations to start, so the oversize is 300 microns from that. Then another 288 microns from the aluminum expansion, so almost 600 accounted for.

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.

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