Influence of pressure in Injection molding of LSR (Liquid Silicon Rubber)

[Lookid]

Hello everyone, I'm new here. I'm a student in mechanical engineering and I'm actually working on the Injection Molding process for LSR.

As you probably know, for silicone, the matter is cold injected and the mold is heated. So when the mold is completely filled, the temperature of silicone rises and silicone tries to swell but it's not possible, so the internal pressure rises. And here I don't know how to answer this question :

Does this pressure (caused by swelling) has an influence on the curing time ?
If yes, is it by an elevation of temperature ? A faster crosslinking due to this pressure ?

Thanks for your answers

[Suhas]

Hello,
All the best for your studies and education. Do well.
I think your question is a little to theoretical and with almost zero experience in silicones I only have one comment: The part design and the flow are both very important and to understand your question, you will need to do some structured research. In thermoplastics, we do something called a viscosity study during which the pressure either increases or decreases with injection speed - and that increase or decrease is part design and runner diameter dependent. So I have no answer when someone asks me 'Does it increase? or Does it decrease'?
All the best,
Suhas

[chrisprocess]

Hello everyone, I'm new here. I'm a student in mechanical engineering and I'm actually working on the Injection Molding process for LSR.

As you probably know, for silicone, the matter is cold injected and the mold is heated. So when the mold is completely filled, the temperature of silicone rises and silicone tries to swell but it's not possible, so the internal pressure rises. And here I don't know how to answer this question :

Does this pressure (caused by swelling) has an influence on the curing time ?
If yes, is it by an elevation of temperature ? A faster crosslinking due to this pressure ?

Thanks for your answers


I have 0 experience molding silicon, but based on your assumption pressure and temperature are manipulated by volume, right?
I'd suspect a skilled molder could make any of the scenarios possible

Difficult to generalize this question. It would likely be on a case by case basis - speed of injection, runner type, gate dimensions, balanced fill, shear etc. etc.

In the end good documentation from prototype to production will be your answer.

[Lookid]

Thanks Suhas and chrisprocess for your answers. I think I didn't express myself well, cause it's not that complicated in my point of view, so i'll picture it in another way :

Take a piston-cylinder system and heat it up to 150°C for example, put silicone in it instead of air, and press ! That way, the piston-cylinder acts as a mold.
- Case 1 : you apply a force F1 on the piston, the silicone cures at a pressure P1
- Case 2 : you apply a force F2>F1 on the piston, the pressure of silicone is now P2>P1
Is the silicone gonna be cure in a shorter time in Case 2, or it doesn't make any differences ?

Well, actually I should be able to try this in real life, and that should answer my question ! Thank you for your help :)

[chrisprocess]

You've increased pressure. Two things can happen 1) temperature may increase (to some degree) or 2) volume of silicon will decrease

Probably one over the other depending on rate in which pressure is added and specific material properties. In the experiment you described above we can assume the volume would be less due to added pressure. So no, it would not take longer to cure because volume changed, not temperature. However I'm not sure how comparable this experiment would be to real silicon molding. But let us know what you find out!!

In general, I would think if temperature is increased it will take longer to cure.

Here's an illustration from Borealis website how flow distance affects pressure and shrinkage
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