Author Topic: ??? WHITE SKYWAY WITH WIERD CIRCLES ON IT, IN THE MOULDING ??? (Read 3500 times)

bmxbob · « **on:** July 12, 2008, 12:00 AM »

just got this from a guy that i used to ride with bitd, so it could quite easily have been mine in the past anyway its an alloy hub front skway but it has wierd circles all over

thought it was just dirt but they are in the moulding ?

anyone have some the same or know of the weird circles ?

pics tomorrow cos im gannin to me bed

tara

toys19 · « **Reply #1 on:** July 12, 2008, 12:55 AM »

Bob, here is my 1 in a million chance to actually use my masters degree in Materials Science to provide you with an answer, I'll try not to be boring..
This is to do with a phenomena known as polymer crystallisation ('scuse the spelling as I've had a few tonight). Its not like crystals that you see in new age shops, its to do with the polymer (posh name for plastic) long chains orienting themselves in an orderly fashion. Although its known as crystallinity its really officially called semi-crystalline as only some regions in the plastic have this orderly arrangement. Crystalline just means ordered. In plastics this is often seen as little round circles, called spherulites and if you look closely the circles will have little spokes in them, this is the very long chains of the plastic (or polymer) arranging themselves in circles, this happens during cooling from the molten plastic the wheels re made from. The chains line up and spread out around a centre making small spherical shapes. These spheres show as circles on the surface becasue when its made they are pushed hard against the walls of the mould tool used to make the wheels. Injection moulding is carried out under massive pressures.
Semi crystalline regions contribute to the toughness (or tuffness) of the material and are basically good news for the bmx wheel designers of the the late 70's . The fact that you can see them may be to do with a few things. It could be that the plastic is shrinking with old age and any surface or undersurface structures are being exaggerated, the ridges will pick up dust and dirt and make them more visible. Or maybe the surface has not experienced any wear (through rubbing with cream cleaner for example ) and the tiny ridges are there from new. I'm going to have a look at my tuffs in the morning and see if they are there. I reckon its probably more likely to be seen on lighter coloured ones..
So if a wheel is kept in the mould at a high temp (some fraction of the melting temp of the plastic , maybe 1/2) the long chains of the plastic are mobile, they have thermal energy and can move around and then they are likely to arrange themselves in ordered ways.
The number and size of the spherulties present will depend on manufacturing conditions such as temperature, pressure and time held at those temps or pressures, and will be variable across any production run, so they may not be as obvious in some tuff wheels.
Anyway if you clean the wheels the dirt will wash away and the circles will be less obvious.
Tuffs are made from a type of nylon (also known as polyamide, dupont trade name is Zytel) where semi crystallinity is common. These can be reinforced with glass fibres, glass balls or carbon fibres. A type of carbon is called graphite and graphite tuff wheels have carbon fibres in them to make them super stiff, although technically the carbon fibres are not the graphite from of carbon, it has become common usage, even in industry, to call carbon fibres graphite.
There will be an exam at the end of the lesson...

bmxbob · « **Reply #2 on:** July 12, 2008, 07:43 AM »

toys19 · « **Reply #3 on:** July 12, 2008, 07:47 AM »

Lets see the piccies then?

daz bmx it! · « **Reply #4 on:** July 12, 2008, 01:20 PM »

A picture tells a thousand words?....unless your Luke

toys19 · « **Reply #5 on:** July 12, 2008, 01:23 PM »

Yeah sorry it was about one in the morning and I was pissed, makes me chatty...

bmxbob · « **Reply #6 on:** July 12, 2008, 01:59 PM »

sorry bout the wait

here ya go, take a look

it is grubby but the circles are regular and in the mould ?

any ideas ?

bmxbob · « **Reply #7 on:** July 12, 2008, 04:24 PM »

owt special ?
or is normal ?

Bob_Acid · « **Reply #8 on:** July 12, 2008, 04:29 PM »

Quote from: jeff spicoli on July 12, 2008, 02:00 PM

Quote from: toys19 on July 12, 2008, 12:55 AM
Bob, here is my 1 in a million chance to actually use my masters degree in Materials Science to provide you with an answer, I'll try not to be boring..
This is to do with a phenomena known as polymer crystallisation ('scuse the spelling as I've had a few tonight). Its not like crystals that you see in new age shops, its to do with the polymer (posh name for plastic) long chains orienting themselves in an orderly fashion. Although its known as crystallinity its really officially called semi-crystalline as only some regions in the plastic have this orderly arrangement. Crystalline just means ordered. In plastics this is often seen as little round circles, called spherulites and if you look closely the circles will have little spokes in them, this is the very long chains of the plastic (or polymer) arranging themselves in circles, this happens during cooling from the molten plastic the wheels re made from. The chains line up and spread out around a centre making small spherical shapes. These spheres show as circles on the surface becasue when its made they are pushed hard against the walls of the mould tool used to make the wheels. Injection moulding is carried out under massive pressures.
Semi crystalline regions contribute to the toughness (or tuffness) of the material and are basically good news for the bmx wheel designers of the the late 70's . The fact that you can see them may be to do with a few things. It could be that the plastic is shrinking with old age and any surface or undersurface structures are being exaggerated, the ridges will pick up dust and dirt and make them more visible. Or maybe the surface has not experienced any wear (through rubbing with cream cleaner for example ) and the tiny ridges are there from new. I'm going to have a look at my tuffs in the morning and see if they are there. I reckon its probably more likely to be seen on lighter coloured ones..
So if a wheel is kept in the mould at a high temp (some fraction of the melting temp of the plastic , maybe 1/2) the long chains of the plastic are mobile, they have thermal energy and can move around and then they are likely to arrange themselves in ordered ways.
The number and size of the spherulties present will depend on manufacturing conditions such as temperature, pressure and time held at those temps or pressures, and will be variable across any production run, so they may not be as obvious in some tuff wheels.
Anyway if you clean the wheels the dirt will wash away and the circles will be less obvious.
Tuffs are made from a type of nylon (also known as polyamide, dupont trade name is Zytel) where semi crystallinity is common. These can be reinforced with glass fibres, glass balls or carbon fibres. A type of carbon is called graphite and graphite tuff wheels have carbon fibres in them to make them super stiff, although technically the carbon fibres are not the graphite from of carbon, it has become common usage, even in industry, to call carbon fibres graphite.
There will be an exam at the end of the lesson...

thats what I was gonna say

I've seen it before, give them a good scrub and see what happens.

lazychubs · « **Reply #9 on:** July 12, 2008, 04:36 PM »

looks like little stickers
as i did my green ones bitd with the circle repair ones from school

PHIPPSIO · « **Reply #10 on:** July 12, 2008, 04:36 PM »

I Have a faded blue set with the exact same appearance. I just thought some joker had messed around with them bitd as some kind of decoration......shows what i know about material science

spesh · « **Reply #11 on:** July 12, 2008, 06:23 PM »

i had the same circles on my pro class rims when i cleaned them they turned out to be holes!!!

ps im only joking but that is strange

toys19 · « **Reply #12 on:** July 12, 2008, 08:29 PM »

Ummmmmm, to be totally honest after seeing this pictures, I am less sure that this is polymer crystallinity, which shows how much I know. These circles look more regular than most of the spherulites I have seen before. I'm not saying it isnt, but it does look like someone stuck a load of stickers on. Or it could be some surface feature of the mould, we should ask Ed Ferri of coloured tuffs to see if he has an opinion.
The lesson is, never trust a scientist....

CD17 · « **Reply #13 on:** July 12, 2008, 08:42 PM »

I think this is the surface of the mould, maybe this is where the plastic is injected in to the mould through a load of these holes $:-\$

Waxintaxin · « **Reply #14 on:** July 13, 2008, 11:05 AM »

stickers, just clean it off with a scotch bright pad and some kitchen cleaner or are you saying there indented ?

toys19 · « **Reply #15 on:** July 13, 2008, 11:36 AM »

It's not where the plastic is injected in, that is called a gate, these can be found on the inside edges, see photo below.

If you are sure they are marks in the part from the mould tool and not the results of the application of stickers then its possible is that these are ejector pin marks (See piccy below). There a few present on all of my tuff's. An ejector pin is used to push the wheel out of the mould tool after it has cooled, the mould tool will be in 2 halves (the line were the two halves join is also shown). There are 20 ejector pin marks on my tuff wheels, one in each corner of each section. Yours seem to be much bigger and have many many more, so I dunno if its this either.

Also it could be that these are marks from, copper cooling/heating inserts put into the mould tool. Thse are used to control the cooling rate of the part after it has been injected, which inturn will control the formation of semi crystalline regions and effect the toughness and stiffness of the wheel. But I have never ever seen this many witness marks from cooling inserts in a mould tool..

Mould tools wear over time and are replaced, also manufacturing techniques change rapidly so during the lifetime of tuff manufacturing the method will probably have been fiddled with. The earliest wheels will have been made from a mould tool that will have been experimented with an awful lot, so they may have tried changing the position of ejector pins or cooling inserts....

Are you really sure its not from stickers??? I remember some awful stick on reflectors that were about that size people used to stick them everywhere - I think they were the result sof some school road safety campaigns...

Whatever it is, I'm sure they will clean off so let us know how it turns out.

firebird4130 · « **Reply #16 on:** July 13, 2008, 11:52 AM »

I had a white skyway front wheel, had the same type of circles but all over the inside of the rim ,so the tyre covered them I thought it must be some kind of factory fault

toys19 · « **Reply #17 on:** July 13, 2008, 12:10 PM »

I reckon its defo heating or cooling inserts, once they had sussed out they needed them on the next tool they made they probably moved them to under the tyre so you cannot see them.

bmxbob · « **Reply #18 on:** July 14, 2008, 08:35 PM »

cleaned up mags

it was just sticker marks

but there is an indented circle at the each end of every spoke, ejector pin marks

these are the ones i was looking at and thought the rest were the same

anyway they cleaned up ok, now all i need is a rear white and a front yellow Tuff II

toys19 · « **Reply #19 on:** July 14, 2008, 08:46 PM »

Quality! It's funny how the stickers seemed the same size as the ejector pin marks! The funny thing is I sent this thread to mate of mine an old injection moulding guru who has beind squishing platics for 50 odd years. He's now retired and he was all up for asking you to send him the wheels so he could tell us exactly what it was... (He's even more of geek than me)

bmxbob · « **Reply #20 on:** July 14, 2008, 10:05 PM »

Doh !

well at least it gave us someting to talk about for a couple of days

lazychubs · « **Reply #21 on:** July 14, 2008, 10:21 PM »

so i was right lol
cleaned up ok tho

billstup · « **Reply #22 on:** July 14, 2008, 10:22 PM »

They`ve come up nice Bob, give the yellow one a spray of wd40 then a wipe over and the fading should dissapear, and orphan front tuffs of all colours come up quite often on ebay, but orphan rears are a lot rarer