Dumb Question
 

Hello all,

I apologize if this has been covered ad nauseum in the forums already, but after having searched, I can't find anything that answers what I'm looking for.

I understand that some layups in a female mold employ vacuum bagging. I am at a total loss as to how this occurs. My feeble mind tells me that the bag would just stretch taught over the top of the mold, and not go down inside of it to reach the layup. What am I missing here? I would like to try my hand at scratch building a pattern airplane for next year, and it will have a composite nose and canopy, and I need to make them as light and as strong as possible, and I would imagine that a vacuum bag would help achieve this. Also, I imagine that I would need to let this layup cure in an oven of some sort, or am I thinking too much?

Thank you in advance for your replies.

Regards,

Ryan Smith

RE: Dumb Question
 

First of all, the bagging material used is fairly elastic. Full vacuum will result in a pressure of app 14.5#/sq in. more than enough to force the bag material into the complete mold cavity.

My advice is to not bother with the bagging process for the relatively small parts you intend to mold. Carefull application of cloth and resin will give you the required result. There is no need to cure in an oven if the correct epoxy is used.

Ed S

RE: Dumb Question
 

Most of the common epoxies "we" use are designed as room temperature cure...They will A stage cure at "room temp" (as specified by the manufacturer ie: 70-90F or whatever)...

Post curing may be an option but not mandatory for many room temp epoxies (check the manufacturer's specs)...I would not bother post curing a non structural component such as a cowl or canopy...Post curing involves subjecting the part to an elevated temperature after the A stage cure is complete...I think I've seen a recommended post cure for West systems on their website which is something like 4 hours @ 120 degrees (don't quote me on that I'm going off memory)...It's defiantly not a requirement though for a room temp laminating resin...

There are hi-temp epoxies available which do require post curing to achieve their maximum mechanical properties or an A Stage cure...I don't think many people have much use for that type stuff in the R/C world...The Jet guys may use a little?? I think there stuff is more in the form of hi-temp adhesives rather than laminating resins...I believe the hot stuff on their planes is typically shielded so the composite airframes don't need to be "Hi Temp"...Plus the temps they are seeing are actually too hi for most hi temp epoxy laminating resins...I think that's why they use heat shields or ceramic paints in those areas but I'm no expert on the subject...

I also agree with Ed in that there won't be much gained by vacuum bagging the cowl or canopy...However if you are interested in acquiring the necessary equipment and learning the process for future projects then by all means go for it...Nothing wrong with bagging those parts but once again it's not mandatory and would be an added expense...I vacuum bag most of my stuff because I already have the equipment but even then I don't bag everything...Like Ed says if you just watch your resin to cloth ratio and do a nice job working the cloth you can actually make a nice wet lay-up part...

If you were going to vacuum bag a cowl or something deep like that you would probably want to use either pleats or an envelope bag so that you have enough excess bag material to get down to the nosering so to speak...While it's true most bagging films do stretch it defiantly has it's limits...You want enough bag there to do the job and avoid "bridging" (stretching taunt and not contacting the surface)...

In our backshops at work we have these re-usable vacuum bag tables for working on smaller doors and panels...The vacuum tables have a material on them that stretches like crazy!! It's not practical for our purposes at home as they are very expensive but it's pretty amazing how that material will conform to a part...Us guys working on planes in the hanger just use the same common bagging stuff you see in the links below...

Here are a couple of decent links you may find interesting:
http://catalogue.airtech.lu/product....t_id=355〈=EN
http://www.fibreglast.com/category/V...Peel_Ply_Tapes
http://www.torrtech.com/Pages/index.html

Good Luck with your project!!

Chuck

Edit: BTW..There are no dumb questions...Just dumb answers... ;)

RE: Dumb Question
 

Thanks for the replies guys!