I’m sure this is old news to many of you, but I thought I’d pass along a gem gleened from Dr. Frank Mitchell’s scratch-building workshop at the Nats last week: he mentioned the use of 2-part epoxy glue (the 1/2 hour curing type) for filling seams along fuselage halves and filling other areas. The stuff doesn’t shrink and doesn’t have the grit of other putties.
Well, I haven’t tried this yet; however, I’ve been using it to simulate fabric covered areas (on rudders,e.g.), and it’s working beautifully. The one drawback may be that it dries very hard, though not quite as hard as super glue. Also, it seems, it could probably be used as a casting material.
I’ve been using it for years, although I use mostly the 5 min. setting type. Back in the dark ages, before CA, epoxy was used extensively in model building. I don’t think it’s the best choice for seam filling on most kits, but it’s very useful for attaching resin or PE to plastic. I also use it for assembling & filling gaps on metal or resin figures. Very useful stuff. Every modeling bench should have it available.
Do you ever have a problem with the 5-minute stuff drying? I’ve heard that can be an issue, that the 30 minute stuff dries hard, but the 5-minute stuff hardly dries. Is this ever a problem? If not, I’ll drop my new best friend for the 5-minute epoxy.
I use the Quick-Cure Epoxy from Bob Smith Industries, which is intended for modeling use. As long as you mix it accurately, it will dry hard, although not as hard as fully cured CA. If you get careless with the mixing ratio you can have a problem, but I’d guess that’s true for the slower setting Epoxies as well.
The 5-minute stuff can feel a bit softer/more flexible than others; this is due to the curing agent: all the 5-minute epoxies I’ve bothered to look at have a mercaptan-based curing agent, while the 90-minute epoxies have an amine-based curing agent. I don’t have enough experience with them to definitively say which is “better” but the chemical components are almost always listed on the package these days.
Also keep in mind that the time denoted in the title are guidelines of working time, not actual time to cure. Cure time can be affected by the degree to which the two components are mixed, as well as the amount of adhesive present (smaller quantities take longer to cure)
I’ve used the 5 minute stuff for attaching metal figures together (90mm and above) and have had no problems with arms, legs, or torso sag. I would swear by it, but as soon as I do, something catastrophic will happen such as all my figures becoming quadrpilegics.
The ‘set’ times don’t always mean the epoxy is fully cured-I always wait overnight before handling-but I love it and keep 5 and 15 minutes versions on hand. I do HATE the smell-it’s like wampuss kitty.
I have extensively used the 5, 15, 30, 45 and 60 minute epoxies in both my RC modeling and plastic modeling. The time listed is the “pot life” or working time before the stuff sets up to the point you can no long do anything with it. The cure time or time to achieve a full strength runs from 1/2 hour for the 5 minute variety to 24-36 hours for the 60 minute variety. Most epoxies are mixed in a one to one ratio and varying that ratio does not change the cure time, it only makes them weaker if you deviate too much from the 1 to 1. Mixing several ounces at a time and being off a drop or two is not a problem. Mixing one drop of each component and having one drop bigger than the other is a problem, causing inconsistent and incomplete curing. Careful use of epoxies can provide great results in modeling.
If you don’t mix the 5 minute stuff properly, you can get a “tacky” finish, rather than a hard one. That’s no good to anyone, so the watchword is ACCURACY.
Both mercaptans and amines have their own unique smell. One form of mercaptan is the stinky stuff added to natural gas to help detect leaks. Many amines smell like fish.
Most consumer epoxies are mixed in a 1:1 ratio. I’ve dealt with several experimental and/or application specific systems that aren’t.
Smaller quantites take longer to cure, if they cure at all. For example, for one of the projects going on in my lab, I’ve developed this tray that’s held together with a very small amount of epoxy. We exceed the manufacturers recommended cure temperature by 10-20 C (18-36 F) and recommended cure time by 15-20 minutes, and the stuff only partially cures (we can tell by the color; it darkens to the fully cured color after sterilization).
The reason is that the curing process is an exothermic reaction, meaning it produces heat, and exothermic reactions are accelerated by increasing the temperature. In addition to energy (heat) taken from the environment, the reaction turns somewhat parasitic in that it also consumes/feeds off the energy produced in the cure process. By curing only small quanities at a time, the process isn’t nearly as efficient, and you end up with partial cure. (You can put things in an oven or heat it with a hair dryer/heat gun to complete the reaction, but plastic doesn’t react well to heat… do so at your own risk).
Hey CKF-thanks for the info-very interesting and I’ve heard of mercaptans and amines and their use in natural gas and also in Wampuss Kitties (aka skunks). But I do notice that small quantities don’t bond well so I use CA.
Yup, I’ve been allowing my epoxy to dry at least four or five hours. It gets pretty solid, and I was thinking of using it as a casting material.
The other use I discovered today: I’ve carved a propeller from balsa and smoothed the surface (to prepare for casting) by brush painting it with the epoxy and then sanding. The nice difference from superglue is that it doesn’t cure so quickly, so this sort of painting-on is possible.
Man, this stuff’s great.
On the other hand:
I’ll think twice about using it to cast the propeller. Since this will require only a tiny bit of material, I’ll stick with regular old casting resin.
By small as I mentioned above, I have a ring of epoxy ~5 mm in diameter 1mm wide, and less than .5 mm thick… if you were to roll it up into a ball, it might make a drop. We’re also using a somewhat exotic epoxy.
Partially cured just means not fully cured, but it can still come out dry to the touch. Obviously, if it’s still wet or tacky, it’s not done curing. If you want to make sure the cure advances as far as possible, you can stick the part in an oven… epoxies won’t melt on you, but I’m not sure how silicon/RTV reacts to heat.
If you can, be prepared to make two molds so that if the first doesn’t turn out, you can fall back on another plan (of course, that probably should be done in any case).