The Disc Manufacturing Process

[JHern]

We've had a number of interesting discussions about the disc manufacturing process in other threads, and while the quantity and diversity of information about plastic mold injection is steadily increasing here at DGR, it is scattered amongst numerous other topics. So I figured I would start a thread dedicated to the disc manufacturing process, to collect such information and discussion in a single place.

To start things off, I figured I would post this informative video about plastic injection molding that has been the dominant manufacturing process for discs ever since pie tins gave way to plastic...

 

[turso]

That's a damn huge Xerox machine. Interesting clip.

 

[CE-Leopard]

I used to work in engineering and did a lot of work with plastic parts, including injection mold design and construction. I've also done some non-disc related work with the guys at Quest AT-- I don't know if they are even around anymore, but it was a great learning experience for me just to talk to the guy making molds and learning about the various issues he encountered. I could probably answer a lot of questions if anyone has any.

 

[JHern]

I used to work in engineering and did a lot of work with plastic parts, including injection mold design and construction. I've also done some non-disc related work with the guys at Quest AT-- I don't know if they are even around anymore, but it was a great learning experience for me just to talk to the guy making molds and learning about the various issues he encountered. I could probably answer a lot of questions if anyone has any.

Cool! Here are a few questions, to begin:

1) What are the real kinds of plastics used for DX-type (incl. D, Pro-D, etc.), Pro-Type (incl. X, etc.), Champion-type (incl. Z, etc.), and Star-type (incl. ESP, etc.)? It seems that the early plastics were made from something common like some sort of blend of polyethylene or polypropylene, while the Z and later generation plastics contain polyurethane.

2) How do manufacturers account for plastic shrinkage when designing a mold? Do they use mathematical (e.g., finite element) models of the shrinkage, or do they just go by trial and error (and spending a lot of time and money at the machine shop)?

 

[keltik1]

 

[dgdave]

http://www.questat.com/#page-9&page=page-9

 

[CE-Leopard]

1. You're right. From my understanding, the cheaper DX discs are predominantly polyethylene and the Champion/Star discs are primarily Polyurethane. There is a continuum there, some plastics will have more of one than the other, or they might have some completely different portions like Hytrel or Nylon. But the two main ones are generally the basis for the blends, I believe. They are all proprietary blends and there are a million different additives they can use for weight, flexibility, color, and feel.

2. Generally for a specific plastic and temperature you will assume a constant shinkage rate around the whole mold. For example, I used to design parts in 3D and then blow them up by a set percentage, then use the blown up model to create a mold. There are exceptions, where you learn from experience that certain shapes might shrink more or less than the general rule. I imagine they learned a lot in the early runs at the disc companies.

Discs are so large that they have quite a bit of residual heat when they come out of the mold, and the final shape can be strongly influenced by the conditions under which they are cooled. I know for a while at gateway they were cooling discs in special racks to make sure they cooled to an extra-overstable shape. That's not generally practical, so that's one reason why you see so much variation from run to run of the same disc.

 

[JR]

I think that Pro is pure Hytrel and Stars are a polyurethane/Hytrel mix. Champ, E and Z and from feel and looks probably Opto too are straight polyurethane if you don't count additives.

 

[jubuttib]

http://www.questat.com/#page-9&page=page-9

That video, while interesting from a historical standpoint, is also absolutely horrible from a physics standpoint...

 

[CE-Leopard]

http://www.questat.com/#page-9&page=page-9

That video, while interesting from a historical standpoint, is also absolutely horrible from a physics standpoint...

Yea it doesn't seem like he has the best understanding of flight when you listen to him talk, but there's no denying that those original Infernos and Raging Infernos were bombers. He definitely had something figured out for a minute there. And they did some runs of discs in the same plastic as the first run Gateway Spirits, which is the closest to the old opaque CE that I've seen.

 

[JHern]

Plastic types

Much of what mikes919 and JR said is very similar to what I've heard from a variety of other sources. Unless anyone has additional information, the corresponding plastics (not including additives) appear to be:

DX/D-Plastic: Polyethylene
Pro/Elite X: Hytrel*
Champion/Z/E/Opto: Polyurethane
Star/ESP: Hytrel*-Polyurethane blend

*Hytrel is a registered trademark of DuPont. There are many different kinds of Hytrel (see http://www2.dupont.com/Plastics/en_US/Products/Hytrel/hytrel_amer_data_sheets.html for a list) Does anyone know which particular Hytrel is used by disc makers?

Note: mikes919 suggested that Nylon might also be used in some of these plastic blends.

Another question: When the disc makers use a blend, do they blend it themselves? If so, how well homogenized is the mix before it goes into the hopper?

______________________________________________________________________________

Shrinkage

...Generally for a specific plastic and temperature you will assume a constant shinkage rate around the whole mold. For example, I used to design parts in 3D and then blow them up by a set percentage, then use the blown up model to create a mold. There are exceptions, where you learn from experience that certain shapes might shrink more or less than the general rule.

Discs appear to be one of these exceptions, owing to their asymmetry, causing the wing to droop upon cooling, thus lowering the PLH (Parting Line Height). A couple of guys I talked to who used to mold discs for Innova showed me a Firebird that they let cool in the mold for ~30 minutes, so that its shape stayed very similar to that of the mold. The top was very flat, the PLH was very high, and the disc is so over-stable and hooks so strongly that even the biggest of arms can only throw it 250'. They also said that the dome of a disc is produced entirely during shrinkage.

What this information implies is that the mold itself must have a more over-stable shape than the desired shape of the disc it produces, since shrinkage lowers the PLH. Mechanically, this makes sense since the flight plate partly braces the upper rim, while the underside of the rim is free to shrink inward. The differential shrinkage at the top vs the bottom of the rim then determines the final PLH.

The degree of upper rim shrinkage should change depending on whether the flight plate gives way easily (e.g., if it domes up easily) or if it is more rigid. In other words, whether the flight plate is a strong or weak brace against radial contraction. I think this explains why the early Blizzards (with the bubbles in the flight plate) turned out to be so over-stable: the bubbles made the flight plate so flexible that it could no longer brace the upper part of the rim against shrinkage, causing the flight plate to dome up instead of resisting radial contraction, so that the upper vs lower rim differential shrinkage was more uniform, thus producing a higher PLH than would a stronger flight plate.

Also, different plastics will shrink to different degrees, because they have different thermal expansivity and elasticity. When I looked at thermal expansion coefficients, the typical value for polyethylene was significantly greater than that of polyurethane. Therefore shrinkage of DX-type discs will be greater than for Champion-type discs. Thus the under-stabilizing effects of shrinkage will be greater for DX than for Champion, which would explain why Champion plastic discs tend to be more over-stable than DX plastic versions from the same mold. Of course, thermal expansivity isn't the only property that matters, it is actually the temperature change upon cooling times the thermal expansivity, and the operating temperature of different plastics is also probably different. Also, the elasticity of the plastic also matters, as well as the glass transition temperature. The only way to sort out all these competing effects is to do some models of the shrinkage process.

...I imagine they learned a lot in the early runs at the disc companies...

I recall throwing some of the first discs produced by small start ups (e.g., the Medusa from Skyquest), and they were often very under-stable (more than the designer probably intended). Perhaps this is because they forgot to consider the effect that uneven shrinkage has on the stability characteristics of the disc, particularly the lowering of PLH.

Discs are so large that they have quite a bit of residual heat when they come out of the mold, and the final shape can be strongly influenced by the conditions under which they are cooled. I know for a while at gateway they were cooling discs in special racks to make sure they cooled to an extra-overstable shape. That's not generally practical, so that's one reason why you see so much variation from run to run of the same disc.

No doubt, while a faster rate of production is more cost-effective, it also gives rise to greater shrinkage artifacts and opens the door to stronger variations from run-to-run. The only sure-fire way to make a consistent disc is to allow it to anneal completely in the mold, but that costs a lot of money since the production rate drops significantly. These kinds of trade-offs could also be modeled and made quantitative, I wonder if any of the disc makers have done anything this sophisticated (such a thing is straightforward for a scientist or engineer).

 

[JHern]

An interesting disc golf talk show with the Rico Bros and Murder Mike, about Legacy discs and the travails of starting their own disc company.

http://recordings.talkshoe.com/TC-112399/TS-702388.mp3

Bamba explains that they use a plastics blending supplier, and they go back-and-forth with them to come up with blends that work with a given mold and process. Makes sense, it is easier and probably far cheaper to tweak the plastic than the mold, since the mold is the most expensive part of the process.

 

[bcr123psu]

FWIW, I think Steve is going to take that link down (if he hasn't already) once he has the full show edited.

 

[DiscGolfEcho]

As an industrial designer, I design products and as disc golf nuts, I wish I could do my own disc ( the only thing missing is the cash to do the machined mold! I guess China might come to help there).

Here is some videos, I found on youtube about disc golf disc manufacturing:







Side note on the last video about Vibram manufacturing process, it's compression molding compare to injection molding.

Also, I work with a special technical plasctic often called UHMW for Ultra-high-molecular-weight polyethylene and I always wonder if we could use the ultra resistant plastic as it can resist abrasion (its self lubricate), resist high and low temps and can be formed like Vibram disc (compression molding).

 

[PMantle1]

Ah, forged vs. cast

 

[JHern]

As an industrial designer, I design products and as disc golf nuts, I wish I could do my own disc ( the only thing missing is the cash to do the machined mold! I guess China might come to help there)...Also, I work with a special technical plasctic often called UHMW for Ultra-high-molecular-weight polyethylene and I always wonder if we could use the ultra resistant plastic as it can resist abrasion (its self lubricate), resist high and low temps and can be formed like Vibram disc (compression molding).

Sounds interesting, what does it feel like? Tacky? Is it significantly more dense?

Side note on the last video about Vibram manufacturing process, it's compression molding compare to injection molding.

Ah, forged vs. cast

Or maybe cut? I could also imagine a process of cutting a disc from a chunk of plastic with the right kind of initial geometry to avoid shrinkage artifacts and asymmetric residual stresses. Perhaps it could even be annealed somehow? That would probably be the most reliable and consistent method, although it might be a tad more expensive and time intensive.

For example, think about molding a lens-shape axi-symmetric chunk of plastic with mirror symmetry about the mid-plane, and set it aside for gradual cooling while still at elevated temperatures. The symmetry would translate into symmetric shrinkage after cooling out of the mold, reducing artifacts. Then cut a cylindrical chunk out of one side, to leave a flight plate and rim, and finish it with precision cutting tools to shape the nose. Et voila.

 

[JR]

Side note to a side note. That is the reason why Vibram needs to use so much lube and the chemical properties of the lube are probably such that it is hard to remove it quickly and why their grip improves a lot after using citrus wash or some such agent and warm water with lots of rinsing and rubbing. The difference in grip is major with this treatment.

JHern the method you suggest sounds expensive and time intensive from a commercial point of view. Production rate might me too slow to viable for anyone else bu a hobbyist that does not need to make a living out of making discs.

 

[JHern]

Here is an interesting discussion between Dave M and Dave D at the PDGA forum...

I'm not Dave D but have an opinion on why CE type plastic was discontinued by the polymer manufacturer (not by the molder Dave D at innova).
Typically injection molding companies in the US ( in order to make profit and be competitive) would want to be able to run parts as fast as possible, That particular type of TPU material took very long to set up. A disc made with current TPU ( champion, star, Evolution) usually takes about 40-60 seconds to be molded and demolded, where it is then set on a table for an additional 5 -6 minutes to cool.

We ran a similar style of plastic ( which we called nike spirits) that took nearly 3 minutes to set up,,,, meaning that at the end of an 8 hour shift instead of having 500 or so discs you would only have 160 ( thats if they were all good parts), and this particular type of urethane also produced lots of short shots ( bad parts) because it was also extremely difficult to mold through the .070 thick flight plate into a 1" triangular rim because of low flow rates. I heard from one innova molder that most of the CE discs were red, most likely do to the fact that the red colorant probably helped increase flow rates .

Since this material was so hard to mold ( not just by disc makers but most who used it) and often took up to 3 times as long to make a part, the polymer manufacturing company probably changed their formulation to be more competitive with the fast changing world of TPU development.

All in all there are about 40 different TPUs that could be used alone or blended together to produce HIGH END golf discs. I'd say the current Star discs are every bit as durable as the CE stuff and in my opinion are better engineered through formulations by Dave D to meet the requirements needed to achieve the right flex, surface feel (and whats most important,,,, )flight characteristic, for each specific model its molded.

does this sound accurate dave D?

I wish I could get a 40 sec cycle. Ours are more like 50-80 seconds. That particular plastic didn't take much more than 60 seconds, so it wasn't that, but I'm sure they had some reason or just couldn't make it anymore. We have manufacturer's changes in materials all the time that we don't want changed. Nothing we can do about it other than shop around for materials. Most of the slight mold changes we make are an effort to keep our discs flying or molding the same with the changing plastics. Also, there are many more urethane plastics than 40. When you add all the variations, it's well over 400. I know one manufacturer who has over 40, so it's a maze. The sales people you talk to trying to get suitable plastic don't know what they are talking about most of the time so the only way to find plastic is to get samples and test it. You can't trust the numbers or the sales people.

And you're right about the mixing to get the right flex, feel, and flight. Rarely do we use just one type of plastic for a run.

We have the ability to get custom made plastics and it still comes out wrong most of the time. A bit frustrating.

Here is a news blurb about Gateway's plastic supply...
http://www.plasticstoday.com/imm/articles/drive-show-putt-dough

Here is another story from Alliance, the supplier of Discraft plastic...
http://www.apstpe.com/TPU-makes-fast-growing-sport-the-Ultimate

Of course the above are plastics-supplier articles, so they don't talk about consistency issues.

Anyways, plastic supply is everything in the injection molding game, but plastic supply does not appear to be very reliable, at least according to all the information I've seen. This is the primary underlying reason for variations in disc runs. And when the plastic varies so much that you have to cut an entirely different mold to get an acceptable result for a new plastic supply, it is a big expense for the disc maker. If I were going into this business, I wouldn't use any supplier unless they could guarantee a supply of exactly the same plastic for decades.

I wonder how difficult it would be to set up a plastics synthesis operation, specifically tailored to disc production? If you could make your own plastic in house, you could have extremely precise control over the process, ensuring consistent results forever. Of course, it looks as if the best plastics are coming from BASF, 3M, etc., and are probably protected by patents up the wazoo, so one might be in danger of stumbling on legal obstacles (but maybe a settlement could be worked out, royalty payments made, and everybody's still happy?).

 

[JHern]

JHern the method you suggest sounds expensive and time intensive from a commercial point-of-view. Production rate might be too slow to be viable for anyone else but a hobbyist that does not need to make a living out of making discs.

Maybe, maybe not. There are precision robotic cutting tools out there that are quite amazing, although carving out a cavity in TPU might be tougher than usual...who knows? Technology can be pretty awesome and unexpected, especially in precision cutting. So a big down payment on the robotic cutting tool, which can handle a similar or shorter cycle time as the injection molded lens blank. I think it would be fun if it were possible to make custom discs using a method like this, since it would be much easier to fine tune and alter the flight characteristics, while the CAD input interfaced with a finite element solver could be programmed to calculate all the PDGA measures to ensure it falls within the specs. But to do this, we would have to start thinking "outside the mold" so to say.

 

[JR]

Or finding a suitable option machine wise. TPU is tough. They CNC steel but it takes months to make large parts and probably a day or more for a disc sized thing if it is complex in shape. But why not use it for softer plastics that is medium to base grade plastics? That might be viable manufacturing time wise and financially spread over enough discs. Which in a time of brick and mortar shops not having room for all manufacturers let alone discs in the market is getting harder and harder. Each year there are more than one new disc manufacturer. I'm not sure if the growth in players leaves enough demand for new companies. I think Innova reaps the benefits of growth mostly with Discraft gaining a little and the others probably not so much or in the case of European companies local sales.

One problem is supply of suitable plastic blocks and the excess carved out plastic. It costs to manufacture and recycle. Ideally you would return the ground/cut material to the supplier of the blocks who would make new blocks from that material.

Money talks and you can get large quantities of plastics cheaper per pound and if you had unlimited money you could buy years worth of plastic and custom manufacture it to perfection or reject the batch. It is a contract issue. Working together with the plastic manufacturer in an iterative process until the material is good enough is theoretically possible but it would probably cost big time. Then there is the added complexity of not only needing to deal with the plastic supplier but also the block manufacturer. It sounds more difficult than it might be worth. Let alone the pricing.