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ronvieky
ronvieky • Jun 2, 2009

Plastic Tube extrusion?

Hi. This is my second post in row, but a different topic this time.

I'm stuck at a problem. Coming from a mechanical background, plastics and polymers seems far too alien to me. Though i seem to understand the basic material properties, the processing, and extrusion mechanics are quite complicated, and i'm unable to obtain a model or try a mathematical formulae to understand its design considerations.

In the vast field of Plastic processing, my main concern is - Plastic Tubing.

The plastic is fed through the hopper, the screw compresses and melts the plastic under shear, the plastic exits through the die, it takes the form of the round oval shape- after passing through a particular apparatus for even mixing and melt pressure- and finally exits through a die-mandrel combination. The mandrel controls the inside diameter of the tube, while the die limits the OD of the tube. The spacing between both- die-mandrel gives the thickness of the tube.

After passing through the die- the tube passes through a water bath, for quenching and taking its final form. In some cases, it even passes through a small area of vacuum sizing, where the dimensions of the tube are controlled.

After passing a long channel of water bath- about 10m - 15m in length, it is pulled by a haul-off unit. I can't remember its name, but its basically a belt conveyor pulles arrangement, driven by an Ac motor, which in turn drives a set of gears, and finally a belt conveyor- which pulls the tube from the extruder side, and lets it fall freely at the other side.

The die and mandrel are designed to the approximate dimensions of the plastic tubing. But the speed of this puller - is used to fine tune the dimensions.

So even if i calculate the die- mandrel dimensions for the right size of tube- i find here- that the engineer here has been using the same set of die-mandrel for atleast 2-3 different sizes of tubings.

Can anyone tell me more about this? Is the Young's modulus here? I remember something about the young's modulus, but it was more about steel. Never read anything about plastic there.

Is there a way to calculate, the different speeds of the puller , or Haul-off unit, to get a set of different size tubings from a particular die-mandrel design?

Any reference sites from web would be greatly appreciated. Thanx.
zorif
zorif • Jun 8, 2009
hhhmmmm bit tricky.and i hope iam getting your question correctly.
i don't think the pulling process is controlling the dimensions of the tube.bcz u said after injecting the plastic the die will be passed thru water for quinching.so by the time it comes out of the water,the plastic will get settled and remain the size of die.(both OD&ID)
if the process is hot pulling(which can not be done with plastics) the dimessnions of the output might depend upon the speed and lag of pulling.
ronvieky
ronvieky • Jun 10, 2009
well, something is pulling the tube out of the Die isnt it? The haul-off unit or you can say the puller is pulling the plastic tube, and causing a strain in the longitudinal direction. The speed of puling the tube, and the speed of plastic extrusion - Screw RPM mut be matched through some complicated maths to get the right OD and ID.

Supprisingly, people do it with Hit and trial. But i dont want to do it with Hit and trial. i want to solve it and calculate the right Forumla, so that, all tube sizes, are calculated way beofre there's a die change. So that less time is wasted in hit-n trial.

Can u explain me, how longitudinal strain can cause the Tube OD and ID to cahnge. ? and what should be the Speed / rather force in the Long. direction to casue the Tube dimensions to change.
zorif
zorif • Jun 10, 2009
some thing is pulling the tube".this is is just an extraction process.the ID of the tube will held the shaft(inside core) bit tight.when the plastic come from HOT to COLD the pipe will shrink a little.so to remove the pipe from DIE u need some pulling arrangment.
i have seen some application where by change of pulling force and speed one can vary the size of outcome.but it will be only for OD not ID.
take an example of wire making.its a drawn product.depending up on the openeing in DIE and pulling speed you will decide the diameter of wire.

the best advice will be TRAIL and ERROR. some times even the greatest discoveries in the world are based on these 2 words. 😁
best of luck

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