Thursday, November 11, 2010

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:

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.


  1. This was very informative and quite educational. It's nice to know that much of the internal detail inside the Gunpla armors are more for just show, and perform a structural purpose. I guess this explains why most Gunpla kits are quite sturdy, even though the entire kit is technically hollow.

    And I'm also glad for an explanation for gate locations and parts layout on a runner. Kudos to Bandai for trying to find a balance between molding efficiency and builder convenience.

    I also like how you incorporate your knowledge of directionality to a sanding method. I've been doing it from time to time without realizing what makes it effective.

    All this might be meaningless info to a typical modeler, but it's very intriguing, and helpful to keep in mind.


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