mullenc525

i have been using cheap bondo polyester resin to make fg cowls and vents etc for my airplane. however, i have had to use about 8 layers of 1.5 oz on these parts to get enough stiffness. I hear of glider fuselages made with 2 layers of 2 oz cloth, how are these stiff enough? is epoxy resin stiffer than polyester? can any epoxy be used (as long as it is thinned)? is epoxy heavier than polyester strength wise/layer wise? thanks a lot

Darrinc

My Feedback: (5)

Polyester is not a structural resin, epoxy is...
A low viscosity laminating epoxy should be used for all structural parts. You should be aiming for a 38% resin to cloth weight ratio and sealing the lay-up to prevent bleed.

Regards

vypr

My Feedback: (6)

The only reason I've ever found to use polyester is when you are doing something massive and want to save $$$. For everything I've ever done for RC I've used epoxy.

canadianjosh

Polyester is not a structrual resin, and epoxy is. So if you need more stiffness use epoxy.

buttuh

Ditto on everyone's comments. I cheaped out using polyester but I've found the light. I still use polyester gelcoat for mold surfaces but I'm phasing that out also.

RCspidey

Plus, I would add that thinning epoxy is never done if you wish to retain strength. It's OK for i.e. brushed on epoxy fuel proofing, but never thin it to wet glass cloth. There are different formulations for different purposes, and you need to be sure to buy something suitable for wetting out glass cloth when laminating. The main point is that you want to maintain 100% solids in your resin, never any volatiles.

Hope this helps & Best Regards,

Spidey

Al Stein

Where are you guys getting this??!?!!!

Yes, polyester is as much a structural resin as epoxy. For the 1950s, 1960s, and early 1970s it was about all we had and the fiberglass boats and planes from those days (both models and person-carrying) are still holding up just fine... including the little 14' racing sailboat I have out back and the 36/600 R/C boat in my garage.

The strength and stiffness of our fiber reinforced plastic (FRP) composites -- whether they be carbon fiber, kevlar or other aramid, or good old glass fiber -- the strength and stiffness come from the fiber. The role of the resin is to bind the fibers to one another and to the substrate, if there is one.

If you want reliable bottom line advice on this, it looks like you need to go to the sites of the composite suppliers... they post a wealth of reference and how-to information.

Jimbonk

I think the Problem here is that people are getting confused or misusing the term "Structural Adhesive " . Polyester is a poor Strut. Adhes. Epoxy is a good one.

Getting back to the original thread, I think you need to try a heavier cloth, 1.5 oz is pretty light stuff. I'm assuming you are using 1.5 oz because you are making small parts with small curves and you cannot get a 4 oz cloth to lay in the corners. ACP ( and probably CST too ) sells a 3 oz "Crowfoot Weave" cloth made for small work. It's a very "limp/soft weave" cloth. It sounds like it would be perfect for your purpose. Another alternative is to switch to Epoxy and use a layer of CF Mat with the glass. Even one layer of CF mat will make a huge difference in stiffness.

Jim B

RCspidey

Hi Al;

Yes, I agree with you that polyester resin has been used as a structural building block for many years.

The only differences I have been able to see between polyester and epoxy resins are (1) epoxy has less odor than polyester while curing, but epoxy can cause allergic reactions that compound with usage (I am not one of the affected, thank goodness) and (2) polyester resin is porous and you cannot completely cover a wooden assembly for fear of dry rot. Epoxy, using at least 2 decent coats, will be impervious to moisture intrusion, and you can seal all surfaces at will.

After having used both, I lean heavily toward epoxy resins, they have been improved on a great deal since their intro in the early 60's - - Today you can get very clear, very thin mixes, without the old "amber" coloration, and they afford great wetting properties and cured strength. Cool stuff.

Hope this helps,
Spidey

Mike James

I'm an "epoxy guy", having used both, and buy West Systems stuff by the gallon. Still, I always have a quart can of polyester around, in case I want to make something small, and do it quick... like maybe a 1/2" long part.

I'm highly interested though, in some of the new developments in polyurethanes, including some at this site. http://www.freemansupply.com/

Check out their videos... Very cool. Some of these fast-cure plastics can definitely rival thermoplastics, and I will be trying some of them on upcoming projects.