plastic glue/joint strength

[jef955]

I have been working on an Estes D region tomahawk for the last week or so - it's pretty much done. My only issue is that the plastic to plastic joints seem to have very little strength. Plastic to body tube seems OK. I am using Testors plastic model cement from the tube - as Estes recommended. When I gave a light wiggle of each fin 2 days ago - I could hear them cracking in the joint :y: Today I went down each one with a "fillet" of their liquid cement, and it does'nt seem much better. I seem to remember someone suggesting a plastic cement thats melts/welds the plastic itself together and provide a much stronger joint, but I can't seem to find it (of course). I used this same tube on an Estes Blue Ninja about 2 weeks ago, and it seems to have worked fine - I assume it's a different plastic on this model. Any suggestions or ideas ? Thank you all in advance for your help or suggestions.

 

[w9ya]

Go here for the straight story on what kto use on this kit;

< https://www.rocketryplanet.com/content/view/2051/38/ >
https://www.rocketryplanet.com/content/view/2051/38/

They CAN be bonded as if they were molded from one piece witht he right glue. And this particular kit, with the stock fins and fin can, has been flown to mach 1.6 !!

 

[Micromeister]

If your worried about fin strength. you can always use a little 5 or 30 minute epoxy fillets on your plastic to plastic or plastic to cardboard joints. a very small radius fillet will greatly increase the strength without adding to much to the visual appearance of the joint.
Hope this helps.

 

[w9ya]

All that is needed is a proper bonding of the fins with a proper application of the right stuff. And sometimes epoxy is not as strong as other glues depending on the material. As far as adding epoxy fillets to gain strength....well ya gotta get the glues to actually attach to the material. In this case, the material will glue just fine without epoxy using the technique outlined in the link I posted above.

Locally our club has gained alot of experience with this particular model and glue strength in flight. Specifically you can't get anything bigger than a j-class motor in that frame no matter how hard you try. So the limit is around 60- some odd g's, lower than mach 2, and about 2 miles high.

How do I know this ? Been done here locally. See here;

< https://arsabq.org/LaunchReports/Contests.htm >
https://arsabq.org/LaunchReports/Contests.htm

(Look at the tomahawk contest info and results. We still have two more launch dates to beat Tony L.'s flight.)

None of the successful flights used epoxy fillets. ALL of the above flights used glues that actually 'melted' the plastic parts into one assembly. We did have flights using more orthodox glues and techniques which are NOT listed in the results linked above. These flights had failures in the fin assembly and were disqualified.

 

[troj]

None of the successful flights used epoxy fillets. ALL of the above flights used glues that actually 'melted' the plastic parts into one assembly. We did have flights using more orthodox glues and techniques which are NOT listed in the results linked above. These flights had failures in the fin assembly and were disqualified.

I'm not surprised -- epoxy is the wrong adhesive (solvent, really) for plastic.

-Kevin

 

[Micromeister]

You're both correct epoxy isn't the correct adhesive for mounting the stryene fins to the styrene fincan. That wasn't the question ask by the author. He ask about adding a little strength to the solvent welded fin joints.

While Testors liquid Plastic cement and tube cements are probably the weakest of the solvent welding agents out there, it can work with careful application. Personally I wouldn't use the junk but most people don't have easy access to Methylene chloride (MC), they'd have to settle for one of the weaker solvent such as Plasticstructs weld, Evergreens product, 7R or some of the other weaker solvents.
All that said once the fins have been solvent welded and allowed to completely set, a fine radius 5 or 30 minute epoxy can and does add a lot of strain relief strength to the somewhat brittle solvent welded joints.

Hope this clears up the suggestion for the readers other then the thread author.

 

[w9ya]

I do not understand why someone would want to use two glues when a simple application of the right glue will work better. Besides unlike what you suggest, the proper glue *IS* readily available at well stocked hobby shops and presumably elsewhere. ALL three r/c hobby shops in my town carry it as regular stock.

ALSO; I wrote earlier that epoxy would not work well if used to fillet with Testors being used for the fin-root-2-fin-can joint as you suggested once before and re-iterated immediately above. I stand by my statement based on my club's collective experience with this particular model over the past year. It will fail, and faill before the much simplier method I linked. I am sorry that you cannot accept that. ..... OH WELL !!

 

[troj]

ALSO; I wrote earlier that epoxy would not work well if used to fillet with Testors being used for the fin-root-2-fin-can joint as you suggested once before and re-iterated immediately above. I stand by my statement based on my club's collective experience with this particular model over the past year. It will fail, and faill before the much simplier method I linked. I am sorry that you cannot accept that. ..... OH WELL !!

I agree whole-heartedly.

Epoxy doesn't stick to plastic for squat; it will pop out of the joint relatively easily.

-Kevin

 

[davel]

For gluing plastic to non-plastic, consider a MMA (Methyl Methacrylic Adhesive) such as Devcon "Plastic Welder".
https://www.devcon.com/products/products.cfm?familyid=182
This stuff acts almost like an epoxy with a built-in plastic solvent.

 

[w9ya]

Good to know. Thanks for the information.

The original question was about the fins to fin can. These six parts are all made of the same plastic.

 

[powderburner]

Have any of you guys (the ones who are fans of "miracle" solvents and adhesives) ever done any pull-testing or peel-testing to see how completely the plastic-cardboard joints hold together? (Like, maybe on something that is already ruined [due to core-sampling?] and is "available" for a post-mortem?)

I can understand using the solvents on plastic-plastic joints, but I have a really hard time putting my complete trust in those plastic-cardboard splices.

And in my experience, those Testor's glues (in any color of tube) are a joke.

 

[w9ya]

We all agree that the Testors glues are poor. (Even the guy that thinks you can add epoxy to the plastic to plastic testors junction to improve it.)

As for what to use on plastic to cardboard, that depends on the nature of the plastic and so forth. I have had great results with one kind of glue to then experience poor results when using the same glue yet with another kind of plastic.

 

[troj]

I have had great results with one kind of glue to then experience poor results when using the same glue yet with another kind of plastic.

That's the challenge (and joy) of dealing with plastic -- an adhesive that works well on one may work poorly (or not at all) on another.

Case in point, I have a rocket I made from pill bottles, and had to find an adhesive that worked on them. Model cement for styrene models did about as much as epoxy and yellow glue -- it'd just sit on top of it in a blob.

I found some exotics that would do the trick, for about $150 / unit (something like 1/4 oz). Too rich for my blood!

I finally found a Devcon product (Plastix, if I remember correctly) that worked well with this particular plastic -- I've had to repair bottles, but never a glue joint. But it's worthless on other plastics.

-Kevin

 

[Micromeister]

I do not understand why someone would want to use two glues when a simple application of the right glue will work better. Besides unlike what you suggest, the proper glue *IS* readily available at well stocked hobby shops and presumably elsewhere. ALL three r/c hobby shops in my town carry it as regular stock.

ALSO; I wrote earlier that epoxy would not work well if used to fillet with Testors being used for the fin-root-2-fin-can joint as you suggested once before and re-iterated immediately above. I stand by my statement based on my club's collective experience with this particular model over the past year. It will fail, and faill before the much simplier method I linked. I am sorry that you cannot accept that. ..... OH WELL !!

Simply because the thread author ask for ways to strengthen the weak solvent welded styrene joints HE is experiencing. That your club has had good luck with 7R in your limited experience is fine. In the example sighted the author is not happy with the Bond He has achieved.

You and your club has a years experience with this model, thats great, I have more then 20 years experience flying PMC and Odd-Rocs used all kinds of different Plastics and plastic to cardborad combinations and adhesives of all kinds. I've yet to see a good fillet made using a solvent welding agent of anykind. Even forming Liquid plastics made up of the base material desolved in the solvent being used just doesn't really add any strength to styrene welded joints. I'm sorry your not understanding what I'm trying to pass on here. We are not aruging weather one of the decent solvent welders is the best method for attaching styrene to styrene, that's been well established. We are arguing about the advantage of adding a small radius, epoxy fillet to those same fins, as a strain relief buffer.
Kevin, You couldn't be any more incorrect about epoxy adhesion to most plastics and Polycarbonates for that matter. As Long as the surface is prepaired it holds with a death grip.
I have Lots of strange plastic models with polycarbonate, and styrene fins that have been flying for DECADES without failure one. Stand by our statements all you wish, but don't dis things your not familiar with or simply don't understand. I'll stand by my year of proven preformance as well.
Hope all this helps the thread author with his problem.

PS: Powder Sorry I missed your question. Really its a matter of identifing the types of plastics and/or substrates you bonding together or bonding to. You'd have to be more specific about What plastic to what cardboard joint your talking about. In most cases, kit manufacturers try to make the mating parts of the same material, if your talking about things like the Tomahawk where TTW fins are used, the bond is between the fincan and fin with the rootedge only semi-attached to where it sits against the motor mount. It's sort of like adding a bump on the side of the tube to "help" keep the root edge from sliding around but it's really not a bond. This is the reason the Thead author is hearing the creeking when he wiggles the fins. There really is NO bond between the fin and MMT body tube, The Bond is at the fincan.

 

[w9ya]

Well sir, my compliments on a throughly engaging rant. And I hereby and herein declare you to be the winner. I am sure you will also be happy that I declare any actual flight experience, under high-g and high-speed, to extreme altitudes, and with this exact model to be of no relevance whatsoever because you say so. Epoxy rules and joints of plastic are simply not good enough regardless of how fast or how many g's they successfully work at.

It is also critically important to ruin a scale model with epoxy fillets because you say that is the correct thing to do. I am glad you have shown me the light concerning this. Gosh to think I have been so wrong about this up until now.

I'll just throw out my flight notebook and come to you next time for your vast experience. I also hope you will be judging my next scale model entry because now that I am using epoxy fillets, I will need a sympathetic judge.


This discussion about the superiority of epoxy leaves me wondering if my time isn't wasted addressing the obvious. To anyone still bothering to read this now completely silly thread, rest assured that epoxy is not always THE answer. Essentially it is an almost universally useful solution but often a sub-optimum solution. Good strength measurement studies, easily available on the web bear this out. If I am not mistaken, they have even been linked and discussed on TRF in the past year or so.


And somehow I am sure there will be a rebuttal of some sort awaiting me tomorrow. At this point I have already conceeded to the superior rants herein, so I will not be wasting any more of my time to consider an answer.

 

[w9ya]

I said I wasn't going to reply to anything else and I won't be. But here's some humor for ya all;

Today, just a short while ago, our expert's self-proclaimed let's "add a (filleted) strain relief buffer" of epoxy to 'plastic welding' construction said that using a welding sort of plastic joint for construction and ONLY using epoxy for the plastic to wood parts gives him rockets that (are) "....still flying well with no regrets."

I'm not kidding, look here;

https://www.rocketryforum.com/showthread.php?p=29426#post29426

(Real life is often more entertaining than what we can dream up.)

 

[RangerStl]

OK, this disagreement has been interesting, but it has officially crossed the line into name-calling. I was asked to step in so I am.

w9ya, if the question asker had used the right glue from the start, things would be better. Yes, you are correct. Given the opportunity to do over, epoxy is not the better choice.

However...

Micro, given the circumstances and the fact that the fin is already Testors tube-glued on, the only option in this case might be to use epoxy fillets to strengthen a weak joint.

We must choose the solution that best fits the problem.

Lastly, there is no excuse for name-calling because nothing in this thread should be fought over. A warning has been issued and infractions will follow if this does not stay a mild disagreement.

N

 

[RangerStl]

As a P.S.

There's a lot of editing of these confrontational posts going on right now.

For anyone who thinks they can edit or delete a post and claim they never said something... ALL versions of ALL posts are saved on the database and we can pull them up and compare them at ANY time. :eyepop:

So, I strongly suggest that any time anyone wishes to say something that they carefully consider the repercussions before hitting the "post" button.

Thank you.

 

[w9ya]

Since the original message clearly stated the testor's glue joint (presumably root to fin can) was cracking, the fin would have easily come off. So starting over WAS a very real and easy to do consideration. I did not state this as I thought it was obvious. Mea culpa.

 

[RangerStl]

w9ya,

My question regarding this issue is, since "tube glue" is already all over the joint, how can one go about using the good stuff to weld the joint together? Will this all have to be cleaned off? I admit, I've never tried going back to fix something like this.

After I found the liquid welder glues like Methylene Chloride, I simply moved forward from there.

This might be a situation where a surgical removal is necessary, cleaning of the old glue, then a re-do.

Epoxy would work for a while, but I'm concerned that if the joint flexed at all, that it would pop off. It sounds like the joint is kinda rough already.

jef955, can you pop the fin and remove the old glue with an X-acto and some patience? I like Tenax 7-R glue.

 

[w9ya]

If I recall correctly, the fins are not touching the mmt and any cracking of the root fin joint effectively becomes a removable fin. I believe this is true even when up-scaling the mmt to 29mm. And any Testors based fillets should not cause additional problems removing the fin in as much as they should be worthless for strength.

The original message stated that the noted problem was with the fins (root joint), and specifically not the tube ("seems o.k.").

 

[AKPilot]

Gents, lets move on and quite inciting one another. There's more than one way to skin a cat.

There's a good article in the last Fine Scale Modeler magazine that lays everything out.

And please respect individual differences and/or preferences . . .

 

[w9ya]

Huh ? Rangerstl just asked me some questions concerning this, which I answered. He is your moderator right ? i.e. Should I ignore his questions ?

(I don't get it....)

 

[AKPilot]

My request was general in nature, to everyone . . .

 

[w9ya]

Huh ? ... "everyone" includes me, and since my last messages are answering your moderator, I am left with the impression I am not suppose to answer him. I am also left to wonder if your moderator was being sincere about wanting answers at all.

 

[AKPilot]

Ask him . . .

 

[w9ya]

LOL.....o.k I got it; ask your moderator if he really wants his questions answered, and even if he answers yes, do NOT answer him.

Glad we cleared that up !! (See that wasn't hard at all.)

 

[RangerStl]

AK, we got things covered. We still need to answer the question.

Maybe we should wait until jef955 comes back and tells us what kind of state his fin can is in before we try to solve the whole problem.

Is it just me, or does the state of someone's "fin can" sound too personal for a public forum? Silly me. :cyclops:



N

 

[jef955]

This post certainly seems to have gotten a good bit of attention ! I appreciate all the info, it seems a lot of you have good ideas to pass around. As for current status - the fins are still attached. I decided to leave them attached and not risk breaking them off and starting over, and risking damage to them. I did run 2 additional coats of the (I now know dreaded) Testors glue. This time I used the (borrowed) Testors liquid cement, and it seems to have given a considerabley stronger bond than the initial tube cement. The fins seem tight, and the joint much stronger so I think I am just going to leave it for now, since it still looks good and I seem to have a decent, good looking joint. I have since gone to Hobby Works and purchased a bottle of Tenax Can anyone tell me what type of plastic they use on the fins/can on the Estes Tomohawk and the Blue Ninja ? I got much better strength out of the Testors tube cement on the Blue Ninja, so I'm just trying to put a name to a "look" so I can pick the right glue for the job from the get go next time. Thanks again.

 

[RangerStl]

Jef955;

You'll find that the "best way to do something" is always a good discussion stimulator around here. :cyclops:

I'm not smart enough to identify most plastics by look or feel, but they definitely all respond to cements a little differently. The Testors liquid cement you used, was it in the bottle with the brush in the cap or the plastic dispenser bottle with metal tube tip?

The bottle stuff is Methyl Ethyl Ketone, a substance that shouldn't be taken lightly and always used with good ventilation, has been shown in the State of California to make trees grow upside-down, cause puppies to meow, turn your nose hairs purple etc. etc. etc.

Seriously, though... it is bad for you. The stuff in the bottle with the metal tip... I dunno what that is. It seems the tube glue goop is melted and smoothed out into your fillet and the liquid glue caused it to be better bonded to the plastic fin can. So you may be home-free now. Not the best bond, but good for most casual flying. It might even stand some 24mm composite reloads.

For body tube to plastic fin can, I like to rough up the plastic with med grit sandpaper and use a little epoxy. :2: