The Directionality of Plastic

Does plastic have grain or directionality?

Looking back a bit, there was someone from one forum who disagreed with me as to whether or not the LED modules that came with the MG Exia IM can be disassembled or not in order to replace the LEDs with that of another color. He said they can't be disassembled, vehemently stating it as a note (because I was supposedly being ignored), whereas I demonstrated that it can be disassembled, with pictures.

Sadly, for one other guy in that same forum, this is one of those cases where one contest what I say in the context of his "expertise."  I'm not saying he doesn't know anything or what he said was wrong, nor do I contest what he has said in the context of thermoplastics, but, I'm saying he doesn't know what I know through experience, and I simply won't say anything here or anywhere just because I feel like it. When I say something, you can be sure it's not hogwash.

I am currently reworking the Wings of the Angelus, and this gave me an opportunity to prove my point and that I am not spreading misinformation or making things up.  The issue came up when I posted something about the sanding process to remove seams (i.e., my presanding method).  What I said was "treat the direction of the seamline as if it was grain of wood,"  and the guy came in, guns ablaze, with his knowledge of thermoplastic and degree in engineering and that plastic has no grain.  To quote him:

"Really sorry to have to say this, but this post is filled with misinformation. As a person who has created injection molding and studied material properties for an engineering degree I can happily say this without any caveats."

I have engineering background as well and familiar with materials and structures.  I don't mention that fact often because it doesn't really matter in most cases in my line of work (I'm a Graphic Designer and Creative Consultant by profession).  In this case, however, the guy had to qualify what he was saying with his degree and basically said he was an expert and I was lying.

Plastic has directionality, if not grain (though by definition, grain indicates directionality so his point is moot), as a result of the direction they flow into while being extruded as well as how they settle into the molds. This directionality is obvious with those uneven tones that form around a gate. The reason why you see gates at odd places, sometimes smack in the middle of a part, is to prevent that directionality allowing the plastic to spread into different directions instead of one.

This directionality is more obvious with sheet plastic, like plaplates.

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The example above is a 0.5mm thick high impact sheet, also known as polystyrene sheet or plaplates (the more expensive branded type) to most. The green arrows indicate the folds I made, whereas the red indicates the directionality. Since the piece is small, note that even though I folded the sheet in one particular angle 90 degrees of the other, the sheet still broke at that red line. Is thist an indication of grain or directionality, or was I spreading misinformation just because a certain thermoplastic "engineer" didn't know this rather obvious property of plastic? 

I reiterate, I am not saying what he said with regards to thermoplastic was wrong.  What I am saying is he doesn't know what I know, based on my experience and, as demonstrated here.

Here's another example, bent in multiple directions. You can clearly see the directionality of the plastic here by looking at how the ridges formed close to breaking point.

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I exploit this directionality when doing scratch builds and reinforcing fixes.  Logically, I sand in the direction of the length of the part, or along the seamline because it's easier to sand that way.

What annoyed me and prompted this blog post (as well as reply to that subject in the forum) was his patronizing tone and basically calling me a liar.

Hi MatX, looks like you're up for an argument. I'm not. I'm glad you have a lot of experience with gundam, but its a bit world out there, and thermosetting plastic has particular properties which have little in common with your descriptions. If you don't actually know about something... you shouldn't just make it up, you're just misinforming people who come here for help. Best of luck.

Copied and pasted verbatim.  He called me a liar, then, wished me luck.  He wasn't up for an argument, but started one.  It is a big world out there, and my profession exposes me to more than just Gundams.  I could have taken what he said with a grain of salt, pun intended, but I believe he should study the actual plastic more than just the process he got a degree on.

Directionality of Plastics Part II


  1. Interesting post. Though this brings up relevant which is, how you test the directionality of the plastic without bending it like you did? And what about curved surfaces? I guess those have to have directionality as well, but in that case, should we send along the curve or the direction of the plastic. Just wondering.

  2. One can't really test for directionality using any other method, but, for molded thermoplastics especially curved surfaces, it's always safer to assume it's along the line of the longer curve. Most modern kits, specifically the PS armors, have more panel lines inside than out. I believe those are placed there to increase the strength of the molded plastic rather than for show.

    Also, plastic can be molded from one gate, but, why do we have molded parts injected in two to four places? The logic behind that to distribute the directionality in several places instead of one. Polystyrene Sheets (plaplates) are perfect examples of this directionality, since they're fed in one direction and thus the plastic's molecules orient themselves in that direction as well.

    Now, the issue came up because the "expert" thought I said to sand it along the grain of the plastic, when I said no such thing. Regardless, in my experience, I always sand in one direction as much as I can, and that's along or parallel the seam or longest edge (even with curved surfaces). Since I also do progressive sanding, any deep scratches can be eliminated by the process and the primer should cover the rest.

  3. Hmmm... I guess the text books doesn't tell degree holders everything. And sometimes, not everything in the text book is correct - there might be little details and assumptions that were made to simplify our macro view of the "big world out there"...


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