hoses shrinking 1

[niki29]

our production has a problem with extruded rubber hoses.
they are built with two kind of EPDM separated with poliester string. the hoses shrink about 8%.
I was assured by rubber's producent that the compound may shrink but only to 3%, poliester to 2%.
I check the process of extrusion (speed, pressure) and everything seems to be ok with the process.
what can I change to reduce the shrinking?
I'll be grateful for all tips.

 

[GrahamBennett]

Niki, are the hoses shrinking before or after cure? What are the set temperatures, head pressure, screw speed, screw type, haul-off speed on your extruder? Are your hoses knitted or braided with polyester?

 

[niki29]

The hoses shrink after extrusion.The extrusion parametres are as follow:
temperature 70C for all zones, screw speed 10m/min, haul-off speed 9m/min, the screw is 130cm long and the diameter is 90mm.
the poliester is braided with the innner rubber layer and covered with the external one.

 

[GrahamBennett]

Nikki, everything you quoted looks reasonably OK but I would recommend increasing the head and die temperatures to around 90-95°C.

I suspect you are stretching the hoses by pulling them out of the line faster than the rubber is exiting the dies. The hoses are then shrinking back to compensate for this. If this is the case, there are two things you can try:

1. Increase the screw speed slightly - this could result in the need for new dies because of increased gauges.
2. Decrease the haul-off speed slightly - again this could result in the need for new dies for the same reason and a slight reduction in production rates.

Graham.

 

[niki29]

ok, I try.
one thing bothers me also: I've just compared MDR results for inner and external EPDM and the curves runs in different way. The external one has shorter T90 (time after 90% curing) than the inner one. May it cause additional stress?
and one more thing if I rise the temperature, what with the pressure? It will also change (it'll rise I suppose. will it affect to extrusion proccess or the extruded parts?
thanks graham

 

[GrahamBennett]

Increasing the head/die temperatures should cause the head pressure to drop slightly as the viscosity will decrease causing an increase in flow rates.

The inner liner of the hose should be somewhat faster curing than the outer to compensate for heating up the mandrels to cure temperature.

 

[niki29]

I have change the speed of screw, but it seems that it works only with increasing temperature. The shrinking is within 2-3% and it's smaller than before. Thank you.

 

[GrahamBennett]

Great. Thanks for letting me know the outcome of your tests.