The Directionality of Plastics Part II

Note:  Read this ONLY if you are interested in trivial matters because this is a long one.



David D, from Canada, asked:

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.

(This was one of the comments from DoP Part I)

To which I replied:

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.

Pardon the color coded text, as I do not want to be misquoted nor misquote anyone.

This subject is quite trivial since most modelers don't really care about stuff like this.  Let's just say I prattle on with regards to this because I simply cannot stand people who accuse me of spreading misinformation because what I know goes beyond what they know or have read from a book.  As such, what I know based on actual observation is being challenged by what another knows academically, so, please do note that what I said above is an educated guess on my part based on observation, in this case, how gates are positioned on molded plastic parts.

To wit:

From Chapter 7, Page 147 "Thermoplastic Mold Design," Bayer Material Science website, specifically on Sprues, Runners and Gates and how they are located into the design of a mold.

Gate position can have a direct impact on part moldability, performance, appearance, and cost. The location of the gate determines the filling pattern and maximum material flow length. Ideally the gate would be positioned to balance filling and minimize flow length, typically near the center of the part or at strategic intervals for multi-gated parts. Often these best gate locations for filling are unacceptable for other reasons. For example, they might result in unsightly gate marks or weld lines in cosmetic areas, or increase mold construction costs. Cavity layout restrictions and mechanisms in the mold such as slides or lifters may also restrict gating to less-than-ideal locations. The best gate position is often a compromise between molding ease and efficiency, part performance and appearance, and mold design feasibility. The Design Engineering Services Group at Bayer Corporation has the experience and resources to assist you in choosing the optimum gate locations.

Gate position determines the filling pattern and resulting flow orientation. Plastics typically exhibit greater strength in the flow direction. Glass-fiber-filled plastics can often withstand more than twice the level of applied stress in the flow direction as in the cross-flow direction. Keep this in mind when choosing gate locations for parts subjected to mechanical loads. When feasible:

• Position gates to direct filling in the direction of applied stress and strain.


Emphasis mine.  Now, even without Thermoplastic Injection Molding background, or a degree in thermoplastic injection for that matter, why do I know this fact?  Does that mean I was pulling things out of thin air, or does molded thermoplastic indeed have directionality?


I'm not an expert on the matter, but working on and with molded and sheet plastics gave me a lot of time to observe their properties.  Looking at gated parts on runners, for example, modelers often complain why such gates are placed at such awkward locations.  Gates are located not to annoy modelers and hobbyists.  Those gates are placed there to exploit the inherent directionality of plastics as they are injected into molds (aside from balancing the distribution of the plastic being injected). 
 
The above text explains just that, and it came from reliable and credible source. Something a so-called "expert" should know off the bat.



Additional resources:

http://www.thefreelibrary.com/INJECTION+MOLDING+AND+DESIGN+WITH+PLASTIC-a078050031

http://www.custompartnet.com/wu/InjectionMolding

The second resource has illustrations and is more concise, and practically confirms most of the presumptions I've had before, like rib (reinforcement) placements in parts that require them.

Black Red, and Gold!

  
The expert who didn't want to argue is still arguing about the difference of injection molded plastics and sheet plastic when I've reiterated it had no bearing in the discussion, and the one who claims to be ignoring me and my posts is checking out my blog.

Kewl.  I've always known that guy was too insecure for his own good.  I don't often talk about issues like these on my blog, not that I don't consider them any less important, but there are times that even I run out of patience or get annoyed with the very same type of people.  I'm still human after all.

Anyways, Murphy's starting to rear his ugly head as I enter the final leg of the build and, I realized the grainyness of the metallics are quite reduced if I don't layer them with black or another color.
Here's the shield of the Angelus.  The gold stands out quite nicely against the black and red.  The "Stringy" texture is done by scribbling lines using Technical pen over Flat Black.  The India ink dries semi gloss.  It's one of those techniques I discovered quite by accident back in the days I was drawing/coloring heavily on paper (there weren't computers or Adobe for that matter).  I'd often use ink over flat black poster color, and sometimes, through an overshoot of dexterity or being too careful, I'd overshoot my target line and into the flat black.  Later I realized it could be used to my benefit, as you see now.

I made a few mistakes removing the mask and the red edges are quite uneven.  I might be able to fix those with clear red enamel later on.

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.







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.



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

Misconceptions about Paints...

I was posting very frequently in several forums a while back and the most common question I get when it comes to painting is "What paint do you use?"  My usual response was "I use Bosny and/or RJ London paints, and they are basically Acrylic Epoxy Lacquer Paints.  In the modeling arena, people only know either Lacquer, Enamel, or Acrylic paints, and seeing a paint type like Acrylic Epoxy Lacquer was a bit of a stretch.  In one such forum, one even told me to make up my mind if I was using either Lacquer or Acrylic because that person couldn't quite accept having both in the same paint (even after showing the label), and another one even said that "Epoxies" are two part glues that can't be added in spray can paints. One thought that it might just be regional naming conventions because I was living in the Philippines.  To quote him verbatim:

"Maybe It has to do with your country's use of english, As the definition of lacquer is a mixture of natural resin and pigment ( modern day we use chemicals). Comes from the Portuguese word lac, and they used it long ago as an alternative to tempera.  And I have never seen the term epoxy used like that.
 

Again I am not trying to tell you your wrong, as it seems like our countries must use the words differently."

The thing is, Bosny and RJ London are European formulas manufactured in Thailand.

Definitions-wise:

"1673, from Fr. lacre "a kind of sealing wax," from Port. lacre, unexplained variant of lacca "resinous substance," from Arabic lakk, from Pers. lak (see lac). The verb meaning "to cover or coat with laqueur" is from 1688."

From Dictionary.com:

–noun

1.

a protective coating consisting of a resin, cellulose ester, or both, dissolved in a volatile solvent, sometimes with pigment added.

2.

any of various resinous varnishes, esp. a resinous varnish obtained from a Japanese tree, Rhus verniciflua, used to produce a highly polished, lustrous surface on wood or the like.

It's a common misconception that the term "lacquer" corresponds directly to paint (or a paint type). Lacquer is actually a type of varnish (the other type is polyurethane, like Future, and even some acrylic varnishes) that is mixed with paints or pigments (mostly with automotive spray paints) that give finishes their lovely shine. 

Epoxy, on the other hand, is mixed with paint (or pigment) to increase its bond, its quick-drying property and overall strength. Epoxy paints are more resistant to light scratches. Hence, these types of paints actually don't need to be top coated unless you want extra protection (or that uber shine).  Epoxy is a term often associated with glue, but in itself is a resin-based "hardener."  That's why we call those two part glues as "epoxy" glues; one is the binder/glue, the other is the hardener/epoxy.  Also, epoxy can't cure sealed under pressure, and only when it get exposed to air or a certain temperature range does it start to harden.

Acrylic, in turn, is a base for pigments, much like how some paints have enamel and latex bases.  Acrylic and latex paints are water soluble and therefore can be thinned with plain  water.  Enamels are not, and require enamel thinners.

So, a Lacquer Acrylic Epoxy paint has 3 components; an acrylic-based pigment, epoxy to harden it, and lacquer to give it shine.