Why is Plastic Model Breaking, How Best to Replace Parts

May 19, 2009 10:38 AM

Do unassembled plastic model kits become brittle or warped over time? Is there a preferable medium for building replacement parts? I'm assembling MPC's The Strange Changing Vampire (1970s) and pieces are breaking. I think it is styrene. One piece just didn't fit; I tried heating with a hair dryer (ineffective) and hot water (ruined the piece). I can replace it but the ones that broke are moving parts, essential to the function of the model. How-Not-Tu-Do-Torial in progress.
posted by rahnefan to sports, hobbies, & recreation (13 comments total)

Depending on how it was stored (and I'm betting a model kit from the 70s has seen a ton of temperature extremes in storage) yes, the styrene can warp. I've seen model parts stored in an attic over time melt. I would think brittleness can also set-in over time.

Take care to cut the pieces from the tree with a sharp Xacto knife, and not simply try to twist the pieces off.
posted by Thorzdad at 10:57 AM on May 19

the only cranky old plastic models i've built have been cranky old japanese robot models, and my experience is not that the plastic degrades, but that it was kind of crap in the first place. sometimes you gotta sand down a piece or fill gaps with testors model putty.

as to how to replace your broken part: what is the part shaped like, how is it broken?
posted by beefetish at 11:04 AM on May 19

Have to go home and make a pic to show you...
posted by rahnefan at 11:59 AM on May 19

when you do, i will have suggestions for you!
posted by beefetish at 12:07 PM on May 19

So I have this ancient half-assembled Airfix 32scale De Dietrich car kit right beside me. The newspaper that I used as a drip-catcher is from 1970. It always lived in the same cabinet, in a normal room at temperatures no higher than, say, 86 F.
There is no warpage in the parts, apart from one bit that I crammed into the box. (Checking with a bunch of other car models that my dad and my uncle assembled in the early sixties: no warpage either, but bits broken off).

I'm now bending a non-model section of the kit and yes, it is slightly more brittle than a new kit would be, snaps very easily.

So no (seconding beefetish): I believe that the part in your kit that didn't fit never was good. And yes: now they would snap more easily than originally. Heating a part is not a good idea in any case since they would most likely shrink more than anything else.
posted by Namlit at 12:46 PM on May 19

Wow, a volunteer standing by and an actual test performed on my behalf. My first question posed and already I love this place.

Please refer to this PDF of the instructions: http://www.mkdavis.net/scv/instructions.pdf

If you are unfamiliar with this model, basically the mattress flips over every time you close the casket lid and reopen it, so that the casket's tenant appears to change back and forth from vampire to skeleton. I'm making a web feature all about it but now that it's broken, there'll be a delay.

Please look at page 1 - step 2. There's a part 8VA and part 8VB. They fit together to form a 2-sided mattress; there are prongs that stick out from the foot ends. These together form sort of a tumbler for the key that winds this model (see page 4, step 9). The model can be hand wound, so if I can't fix that, it's still not a party pooper. But they also stick through parts 10V and V11 (please see page 2, step 5), and part 12V goes around them (and should be glued to them).

What snapped are parts 12V and those darn prongs that form the tumbler/axel thing. Basically the mattress now just has a stub on the end of it; it is still long enough to fit through 10V and V11. I'm no engineer but it seems like a lot of responsibility to put on those 4 small flimsy prongs, and without 12V, you cannot accomplish the trick. It keeps the vampire/skele from constantly spinning. I have an idea or two for doing it differently but would prefer to get advice from experts first.

Thank you friends.
posted by rahnefan at 2:20 PM on May 19

Sorry -- link: http://www.mkdavis.net/scv/instructions.pdf
posted by rahnefan at 2:21 PM on May 19

here is what i would do, as a veteran of finicky model robot assembly:

1. get yourself some putty, modeling cement (the viscous, toxic crap sold under the name Testor's Cement for Plastics or the like), and cyanoacrylate glue (superglue or its ilk). also some sandpaper (150 wet/dry) and electrical tape.

2. use the electrical tape to gently and carefully tape the prongs back onto 8VA and 8VA. "test fit" the parts to make sure that the prongs are pointing in the right direction and fit through everything properly. when you can live with the position the prongs are in:

3. very, very carefully use the modeling cement to glue the prongs back onto parts 8VA and 8VB. press them together hard so the plastic melts together. let it set overnight. if the model cement has beaded up in a weird way, sand it off.

4. i'm guessing 12V snapped cleanly in two or something like it. glue it back together with the cement in the manner of step 3.

5. assemble model as usual, using CA glue to affix the 8V parts to 12V. defend your efforts vigorously to your girlfriend, who thinks you are an enormous dork
posted by beefetish at 2:55 PM on May 19

Hum, I guess you can try to make 12V from sheet styrene, with a bunch of small sharp files and a bit of patience, and a drill of the right thickness for the center hole.
For the prongs, one would probably need to drill a little bit into the, hum, mattress, and re-insert some custom made-solid round piece of plastic that has the front cuts to form the prongs. Now if I understand right, there's a problem with making a suitable hole for the wire in the center. A good hobby store will have all: super fine drills, super fine saws and so on. And you'll probably have some scrap plastic from the kit, buy another one otherwise. But this is going to be a bit of a fiddle.

If you cut or drill into this material, always use the lowest speed possible, like in: a hand drill, otherwise stuff gets too hot. Good luck.
posted by Namlit at 2:58 PM on May 19

Thank you both. I'll try as much of that as possible; but half the prongs are missing, and the casket is glued together so tightly I don't think I can get the mattress out. What kind of putty do you suggest? I hope they have a good hobby store around here...

Since I know the shapes I need though, would it be a bad idea to sculpt one solid part that is both 12V and the prongs, and cement that to the mattress? Is polymer clay or two-part bonding putty a bad choice for that?

12V is about 1/16" thick.

LOL -- probably there is a better word for it than mattress.
posted by rahnefan at 6:13 PM on May 19

Forgot to say, I already tried cementing 12V and let it sit overnight. It only snapped again when I tried to put it back on the next day. There are other pieces of this that have snapped also, so I'm really thinking this model is very brittle.
posted by rahnefan at 6:18 PM on May 19

did you use CA glue or modeling cement when you reattached 12V? CA glue dries to a crust and adheres two parts together - modeling cement melts the two parts together, making for a far stronger bond.

i feel your pain, man.
posted by beefetish at 11:39 AM on May 20

The tube said it was modeling cement for plastic IIRC. It seems to have bonded in some places. But that 12V part is small and I'm thinking time has done its damage. I ain't done yet however!
posted by rahnefan at 12:28 PM on May 20