tooling material

[JeniaL]

we have to make several skins for helicopter engine covers. since it's not a serial production it's only a prototype at this stage we've decided to use MDF treated with resin as tooling material. porosity is our problem now. we can't figure out how to get rid of that.
curing process divided into 3 sections. pre-cure at room temperature under the vacuum of 28 HG inch then cure in a oven at 125 deg Celsius and then free standing post cure at 177 deg Celsius. the vacuum at all stages 28 HG inch.
Do you guys have any experience with MDF as tooling material? Any ideas how to get rid of porosity?

materials used for skins are Cycom 5320-1 prepreg and Cycom surface master 905 as first and last ply. staking 16 plies. compaction every two plies at 28 HG inch for 5 minutes minimum.

the thermal survey of a tool looks ok.

looking for your help.

Cheers.

 

[Compositepro]

Are you describing the fabrication and curing of the tool or describing curing a part on your PDF tool?

Before you can do any sort of vacuum bag process you must have a completely sealed tool face. Otherwise, vacuum will simply pull air though what you are trying to cure. To seal the MDF you must use multiple coats of a room temp curing epoxy. Any solvents or temperature or pressure changes should be avoided as this tends to pop bubbles that are covered by the dealer coating, causing pinholes in the coating.

An alternative is to envelope bag the MDF tool with FEP film (using vacuum). The FEP film becomes your tool surface.

The MDF should be prebaked at cure temperature before final machining and kept dry thereafter.

 

[JeniaL]

i'm describing curing process of a part on MDF tool. tool sealed from all sides with epoxy resin. before we start lay-up we hold a tool under vacuum for 12 hours.
we are doing some test panels on aluminum tool to see if the problem is MDF. i don't believe that porosity comes because of MDF tool.

 

[blakmax]

I'd look at your procedures for handling and storing your pre-preg material. Epoxies absorb moisture from the atmosphere if exposed to high humidity or for extended periods. When heated, the moisture boils off to form bubbles. There is a common perception that applying high vacuum (28in.) draws out the voids. This is not the case. What happens is that the low pressure encourages more moisture to evolve and worse yet, the void expands under the low pressure. It is the resin that flows to make room for the void, not the void which is drawn out.
Methods to minimise voids are outlined in 8.

http://www.adhesionassociates.com/papers/Managing Micro-Voiding of Adhesive Bonds.pdf

Regards

Blakmax

 

[Compositepro]

You do not explicitly say that you are doing non-autoclave cure but I assume so. This is far more sensitive to numerous factors than autoclave cure. Sealing all sides of MDF tool is a bad idea. MDF is highly porous and pressure will be created inside the tool by heating. Can you do autoclave cure? It would certainly help a lot.

You say precure at room temp. You must mean debulk. 5320 is an edge breathing material. It is very important not to pinch-off the edges during debulk or cure. Are you carefully following every detail of the Cytec bagging procedure?

Is the any pattern to the voiding, like mostly in center, with clean edges?

 

[JeniaL]

thanks for the response guys. yea it's OOA. i'm using room temperature compaction for 16 hours and not debulking. vacuum bagging procedure according to cycom data sheet. i don't want to run autoclave. we had to seal tool all around because of vacuum loss.
we are doing several test panels using MDF and ALU tooling. before i'm going into curing cycle investigation i want to see if i get porosity with ALU tooling. i'm gonna post NDI results in a few days.

 

[JeniaL]

so both test panels failed. but it's good cause now i know it's not because MDF tooling. we are playing with aid materials and edge dam. will see if this helps. guy from Bell suggested to trim edges every three plies so at the end of lay up all plies match the same EOP.

 

[HPedraza]

Jenial:
We made a wood tool (Mahogany). And when we made a vacuum bag process and the mold is working pretty good.
I saw an MDF tool, but unfortunately when we aplly the vacuum the mold was broken. So, we don't use MDF for tool.
Another option is Renshape, covered by an aluminum laminate. Also works fine, however, the life time of renshape is very limited and is some expensive.
Hope this information will be helpful.

 

[JeniaL]

thanks for the Renshape.
it's a prototype project and i'm not going to use tools more that twice. we already proved to our customer that porosity comes not because of MDF tool. CYCOM 5320-1 is PITA material however to get good results some actions must be taken before lay-up with MDF tooling. it's good to apply vacuum for several hours before lay-up.