DGR Homework: parting line height and relative stabilities

[turso]

So its not so much the dome, it's the PLH that makes the diff?

I've have three Kraits, all diffrent dome heights, one was really 'domey' and flew like a meat hook, I heated it up, but took out too much dome, made it slightly concave, flew worse then a Momba, very understable.
Could reducing dome lower PLH? or is it a combo that determines stability? I actually think maybe the the 'dome' has more effect then the PLH?

Playing with heat and plastic can do all sorts of things to the disc. If it has changed a lot in its stability, the PLH and/or even the wing shape might have changed.

[turso]

In my opinion I always go domey means more glide and tiny bit more over stable. Flat is less glide and tiny bit under stable. But these are very small minor differences. The plh is the big thing. But for me usually the domey discs for whatever reason seem to have a lower parting line. So I think most of the time ( this isn't treacly a rule of thumb to many variances) they go hand in hand.

Thoughts?

I wouldn't and couldn't give a definite statement of domes and PLH going hand in hand before testing around 500 discs of each mold to get a proper sample size. Personally I have seen no correlation with PLH and dome height, or any correlation in stability change between domey and non domey discs.

[JR]

There is a correlation of dome height and PLH especially in flat discs with too little plastic in the flight plate thin flat topped Star Katanas. Those can't be domed up easily or at all with hot water treatment because the Star plastic won't stretch infinitely and having so little plastic stretches almost imperceptibly leaving a hard and early fading disc very different from the rest of the Katana variants.

[jubuttib]

Domeyness affects glide, first and foremost, and to an extent speed (bigger cross section, slows down faster, can give the impression of added overstability because the fade starts earlier, but the reason is different from real overstability). There have been some trends that have held quite true, flat ES-DD2s are always more overstable than domey ones, domey Kings and Swords are almost always more overstable than flat ones (Latitude on the whole seems to be in the domey -> more stable/overstable, but not every time), flat Firebirds tend to be the most overstable ones, etc. etc., but it's all mold, plastic, manufacturer and even run dependent. For example many people hold that the older flat Champ TeeBirds are more overstable and better than the others, and the domey ones flip easier and aren't as good, but at least part of one run of domey TeeBirds flew like a damn speed 7 Firebird.

If you want to figure out stability look at the PLH, the dome is never a consistent indication of stability.

[pask2155]

Just a random question with all of this... But each plh needs to be compared to the other discs of the same mold... Correct? It's not that you can grab any disc and judge it... You need to compare it to other discs of the same mold?

[dgdave]

Yup

[JHern]

Thermal contraction and stress is the key to all of this business. As the plastic cools during the molding process, it shrinks, but not uniformly, causing deformation such as lifting of the parting line, doming, etc.. Every plastic has a different thermal expansivity as well as variations in rheology, resulting in a variety of different final shapes after the disc cools.

From what I've read on the web, the earliest discs were made with cheap polyethylene- and/or polypropylene-based plastics. These have a thermal expansivity between 1E-4/K and 2E-4/K, which is actually fairly high for a solid. Now they add other rubbery compounds to DX and Pro, but I think these also have a relatively large thermal expansion. Champion plastic, on the other hand, is made with a high proportion of polyurethane-based plastic, which has a smaller thermal expansion coefficient, around 5E-5/K (or roughly 2-4X smaller than DX).

The upper part of the rim is connected to the flight plate, while the lower part of the rim isn't braced. This causes the rim to shrink more on the bottom, than the top, which bends the nose down and lowers the PLH. The greater the thermal expansivity, the more the PLH will be depressed. This is probably why polyurethane-rich plastics produce more over-stable flight characteristics in the final product than high thermal expansivity plastics.

If the entire rim shrinks to a large degree, and the flight plate is not overly stiff, then you can produce a domed top. This effect is exaggerated if the flight plate plastic contracts less than the rim, which is probably usually the case since the flight plate cools more quickly since it is thinner. Dominess is correlated with the amount of upper rim shrinkage, while the PLH is correlated with the relative degree of upper vs lower rim shrinkage, so these are typically 2 different things although one can imagine ways that they might become correlated (but they certainly don't have to be).

The consistency and speed of the molding process also affect things. If you try to mold discs too quickly, they won't cool as much and will tend to have artifacts that slower produced discs do not. And any other compounds added for color, etc., will give rise to differences. So there are many factors at work. If a disc manufacturer wishes to make a consistent disc, then polyurethane-rich blends, slow production and long cooling times, and using the exact same plastic blend, are all necessary.

PS...Check out the bubbles-in-flight-plate blizzards, vs the bubbles-in-rim blizzards for some of the most extreme variety out there...

[jubuttib]

Yup, which makes it all the more amazing how consistent MVP can be after they've tuned the process for whatever disc they're running. The first runs usually have the most variance because they're not used to molding the disc yet (though usually still less than many other manufacturers), but subsequent runs are usually bang on with what they intended.

I also have no doubt that the cheaper plastics shrink more, if you take a look at some DX, Pro and R-Pro discs they're pretty consistently smaller than the same discs in premium plastic (the diameter can be off by a millimeter or two easily, and rim widths also have been a millimeter narrower in some cases). One extreme example is the Comet, which is rated by the PDGA to be 21.8 cm in diameter, but especially in Z it bloody well isn't. 21.8 cm would be one mm larger than your regular large diameter mid, and exactly the same as an Axis for example, but when you actually measure it it's 8 mm larger in diameter. The old plastics they used back then surely shrinked more than the modern Z plastic, assuming they haven't changed the mold.

[JHern]

Indeed, I think MVP uses a very high proportion of low thermal expansivity polyurethane in the flight plate, and they seem to have a better grasp on molding technology and quality control, in general. The double-mold they do is pretty fancy stuff, they definitely show off their skills!

I've also noticed that Champion/Z/polyurethane-rich plastics in general seem to be more consistent than other plastics, which is also what you'd expect from a smaller thermal expansivity.

I'm thinking about running a finite element model of the shrinkage, just to get an idea of how much the shrinkage varies as thermal expansivity and plastic rigidity is varied, as well as how much the PLH and dome are changed. Should be pretty easy to set-up.

[keltik]

are you doing the FEA on the mold or the disc itself. If you have access to SolidWorks you can do a mold flow analysis on the tooling itself. I think you would have to know the exact chemistry of the plastic to do a thermal expansion analysis. and for that you would probably need a mass spectrometer. and then you'd still need to know the molecular structure of the compound in both its fluid and solid states. Then you could possibly say how the atoms/molecules reorganize themselves after a thermal event.

[JHern]

...If you have access to SolidWorks you can do a mold flow analysis on the tooling itself.

I don't have access to the mold. I would only model a generic putter, a generic mid-range, and a generic driver. I would make up the shape, based on typical forms of each variety. Maybe I would pick a few from the Discraft profiles posted on their website.

I think you would have to know the exact chemistry of the plastic to do a thermal expansion analysis. and for that you would probably need a mass spectrometer. and then you'd still need to know the molecular structure of the compound in both its fluid and solid states. Then you could possibly say how the atoms/molecules reorganize themselves after a thermal event.

The point isn't to simulate the process, rather, the idea is to model the process. There's a difference. In modeling, I would vary 3 parameters: the total thermal expansion/contraction (expansivity times temperature change, a dimensionless parameter), and a suitable pair of elastic parameters (e.g., the bulk and shear moduli). I would step through combinations of these parameters over the range of variation exhibited by all plastic types, and I would use the results to find out how PLH, dome, etc., varies systematically as these physical parameters are varied. The results could later be applied to any particular plastic, by simply looking at the portion of parameter space that is relevant to that plastic.

[JHern]

I talked to some guys who used to mold discs for Innova, it was interesting, especially following up on this discussion. They confirm that the final shape of the discs are produced by shrinking out of the mold. At least when they were working at Innova, the disc would cool for 30-40 seconds in the mold, and then it would open up and the disc would pop out (though they said that some plastics would get stuck, such as CE). Sometimes, when they weren't busy, they would "cook" a disc, and leave it in the mold to cool down for 30 minutes before taking it out, in which case the disc would assume a shape very similar to the mold, itself. One of the guys had a Firebird that was cooked this way, it had a very flat top and a ridiculously high PLH. He said it hooks extremely hard, so much so that he can only throw it 200 ft max.

[jubuttib]

I talked to some guys who used to mold discs for Innova, it was interesting, especially following up on this discussion. They confirm that the final shape of the discs are produced by shrinking out of the mold. At least when they were working at Innova, the disc would cool for 30-40 seconds in the mold, and then it would open up and the disc would pop out (though they said that some plastics would get stuck, such as CE). Sometimes, when they weren't busy, they would "cook" a disc, and leave it in the mold to cool down for 30 minutes before taking it out, in which case the disc would assume a shape very similar to the mold, itself. One of the guys had a Firebird that was cooked this way, it had a very flat top and a ridiculously high PLH. He said it hooks extremely hard, so much so that he can only throw it 200 ft max.

.... Iiiiiiinteresting... This could actually be a plausible explanation to why so many of Innova's CFR runs are A) more consistent and B) more overstable (not all, but many), and production runs are A) all over the place and B) on average less stable. If they're pushing them out for a big production run they don't have any extra time to let the discs cool in the mold, whereas when doing a smaller run for CFR they can be more careful and let the discs sit a bit longer...

Just a thought, purely hypothetical.

[JHern]

I talked to some guys who used to mold discs for Innova...

.... Iiiiiiinteresting... This could actually be a plausible explanation to why so many of Innova's CFR runs are A) more consistent and B) more overstable (not all, but many), and production runs are A) all over the place and B) on average less stable. If they're pushing them out for a big production run they don't have any extra time to let the discs cool in the mold, whereas when doing a smaller run for CFR they can be more careful and let the discs sit a bit longer...

Just a thought, purely hypothetical.

Makes perfect sense to me. I'll see these guys again tomorrow, I'll ask them...

[discspeed]

^^^^This is what the guys at MVP told me when I showed them my big D cfr PD and the Z NukeOS...They said both discs looked like they cooled in the mold.