silicon with glass fibers+viscosity

[niki29]

We have a major problem with silicone contains glass fibers.
Till now we have done severals trials and non of them has given good results. Base on our results we know that in this case the properties of extruded parts (before curing) and exrtrusion itself depends on rubber viscosity. The temperatures of extruding is below 50C (as recommended by producer). The hoses after extrusion are so soft that it is impossible to put them on mandrelas. we tried to increase viscosity to sertain level but we decrease the scorch time simultaneously...
Pre-curing is just a temporary solution; not to implement for serial production.
if you have any experience with that kind of rubber/problem;please give me some tips. any help welcomed.

 

[GrahamBennett]

Are the hoses reinforced in any way? Can you extrude directly onto a mandrel via a T-head extruder?

 

[niki29]

No, they are not reinforced and we don't extrude it via T-head extruder.
We have an ordinary extruder with a screw designed for silicon.lenght - to -diameter ratio is 10:1.

 

[GrahamBennett]

Thanks Niki.

After 31+ years in the rubber industry, one thing I have learnt is that you never ever change the compound to suit the process. You have to change the process to suit the compound otherwise you make too many compromises with the compound's properties.

Now, I have to say I have little experience with silicone processing but I do have direct experience of extruding very low viscosity EPDM compounds. I have always found these are best extruded directly onto a mandrelt hrough a T-head extruder. I would expect the same results from silicone extrusion.

You should also find that the glass fibres will align themselves with the direction of flow through the die if the die land is long enough.

 

[niki29]

Our lab tests had demonstrated that there must be connection. tests were done as follow: every 2-3 day we tried to extrude parts from the same batch of compound (every time we did MDR tests) and when the visc. was between 1,13 - 2; it was impossible to produce pipes. with visc. equal to 3 or aroud this value parts could be done.
Moreover we had sth what we called a "sponge" effect (the cross-section of pipe was full of porosities)for rubber of low visc.
Tell me, please, what if the effect of re-milling on viscosity?

 

[GrahamBennett]

Are you saying you use ML on the rheometer chart to determine viscosity? If this is the case you are wrong. Rheometer ML measures an elastic response and does not correlate with Mooney viscosity.

Porosity in extrudates can come from lack of feed (try feeding bigger strips or pellets), trapped air or moisture in the compound. If trapped air, try milling compound at 1:1 roll speeds rather than 1:1.15 or so. If trapped moisture, try mixing in about 5phr calcium oxide, which acts as a descicant.

Remilling silicone shouldn't affect the viscosity but, if you let the rubber get too hot, it will begin to scorch which will show up as an increase in viscosity.

 

[niki29]

thanks