The starting ram position

[tkitazawa]

Hi, does anyone know the correct way to input the starting ram position for the absolute ram profile?

Below is the detail.

・starting ram position should be

A)the feeding position

B)the feeding position + the distance for suckback amount

It supposed to be A) I assume. However, I found someone claiming B) because of the following reason.

Since the pressure on the plastic at the ram position after suckback can be considered as 0[MPa] (technically not, but it means it's negligible) and there is no interface to set the backpressure for Moldflow, had better set the
starting ram position as B).

I could not disagree with the point supporting B). I'm not sure about the assumption that they say the pressure there is negligible, but seems right. If anyone knows about the mechanism about the pressure on the plastic and
the ram of the injection molding machine, please let me know about it.

Thanks,
tkitazawa

[bernor_mf · Accepted solution]

Hi @tkitazawa ,
noticed this was posted a while ago.
Maybe already sorted out by you.

Anyway,
the suck back, or the decompression, is to offload the melt after dozing, and to make check ring to close.
And also to avoid drooling of melt at nozzle.

Now, if doing a decompression it does not add more molten material even if it creates a longer distance to travel for screw.
When a screw decompresses, it is not supposed to turn, adding molten material in front of screw.

So if you have a certain doze or shot volume in Moldflow is should be :

・starting ram position should be
A)the feeding position

So, right: A), as you have reasoned.

If you use:
B)the feeding position + the distance for suckback amount
it will add molten plastic volume in Moldflow.

Depending on how you switch over this might cause not reaching switch over point,
or leaving a larger cushion than intended.

So I disagree on B) as in Moldflow adding distance of decompression adds melt volume to shot.

Hope this helps.
Regards,
Berndt

[PascalGosset · Accepted solution]

Hello,

When I try to compare real and simulated pressures, I often meet this problem.

Look at this photo I made of a machine screen:

The suckback is visible at start of the pressure curve.

The speed curve shows the ram advance during 0.2 s while the pressure stays zero.

To compare with the simulated pressure, I have to cut the first 0.2 s and

take it into account to set the filling time.

Like this:

The pressure increasing versus time is not terrific.

But the pressure at switchover is very good !

This rarely happens.

I hope this will help you.

[tkitazawa]

Hi, bernor_mf,

Thanks for the comment. I understood your reasoning very well. Appreciate it.