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Quote from: toys19 on July 12, 2008, 12:55 AMBob, 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
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...
