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Contributor
psofiagida
Posts: 18
Registered: ‎07-03-2013

End of packing/holding - cavity pressure

296 Views, 4 Replies
02-01-2014 10:21 PM

Hi all,

 

guys i am stuck over here. Cound anyone help me plz ?

My part has been 100% filled at 1 sec. I used a constant packing profile of: 0 sec - 50 MPa

                                                                                                                                        4 sec - 50 MPa

                                                                                                                                        0 sec - 0 MPa


However, the part's weight remain constant after 3.5 sec. So instead of using a packing time of 3.5 sec, i extended it for 0.5 sec more. Which is enough, considering that my cycle time is 11 sec. Moreover, for tpacking = 3.5 sec, some negative values of sink marks appeared (-0.0026) and some small values of sink marks very close to the gate area. My problem is that when i checked the logs, the clamp force decayed to zero at the end of packing (5 sec). Clamp force actually is the cavity pressure, am i not right ? In what way can this affect my analysis ? One thing i already detected, is that shrinkage at 5 sec and at the end of cycle is THE SAME.

 

thanks.

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Employee
Alex.Bakharev
Posts: 44
Registered: ‎04-10-2013

Re: End of packing/holding - cavity pressure

02-02-2014 03:42 PM in reply to: psofiagida

Yes, you are right, the clamp force is the product of the average cavity pressure and the projection of the cavity on the partyng surface. Zero clamp force means zero cavity pressure.

 

You can produce clamp forces time plots or cavity pressures time traces for any node via gui, do not need to read the log for it.

 

Anyway the cavity pressure going to zero near the end of the packing time is quite normal. Do not see anything particular wrong with it. Cavity pressure going to zero during packing means that the gates freeze before. Possibly you can save on electricity by making packing time a little bit shorter as packing after freezing the gates off does not help the molding.

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Contributor
psofiagida
Posts: 18
Registered: ‎07-03-2013

Re: End of packing/holding - cavity pressure

02-03-2014 08:16 AM in reply to: Alex.Bakharev

thanks.

 

what is pretty weird though, that in the manuals it says that shrinkage remains constant after the part mass stops changing. How is this even possible ? At some time during packing-holding the part weight will stop increasing, and then cooling begins. Shrinkage occurs all this time. For example, my part weight is 37.4 gr and stops increasing at 3.5 sec. This is the reason i put 4 sec. But according to the manual, shrinkage after t = 3.5 sec, should remain the same till the cycle ends. Have i not uderstood sth well or... ?

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Employee
Alex.Bakharev
Posts: 44
Registered: ‎04-10-2013

Re: End of packing/holding - cavity pressure

02-03-2014 03:45 PM in reply to: psofiagida

Well, after the mass stopped increasing the material stops coming to the cavity (gate freezes). If we assume that after cooling material ends up at the same state then the average final shrinkage would not change whatever you do after the gate freeze.

 

There are some factors that change this assuption though:

  • If the gates are frozen but the cavity is still has some molten material then it can flow redistributing shrinkage within the cavity
  • The final state of the material depends upon crystallisation and crystallization might depend upon the molding condition
  • The viscoelastic effects redistribute shrinkage from inplane to the thickness directions.

After the gates are frozen the injection unit cannot have any effect on whatever is doing in the cavity. You can influence it by the cooling time though.

 

If I remember correctly crystallisation and viscoelasticity at packing are only provided in the scandium (tech preview) line of products and may require additional material data. Flow in the moltem core on the late packing and early cooling stages are obviously supported by all versions of the code.

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Distinguished Contributor
mayur_dhumal
Posts: 164
Registered: ‎05-05-2011

Re: End of packing/holding - cavity pressure

03-03-2014 03:43 AM in reply to: psofiagida

Hi,

 

Practically you cannot decay your cavity pressure in just '0' sec.

so 3rd line of your packing should say  2 sec - 0 MPa.

 

Secondly, Negative sink mark is due to overpacking, you need to reduce packing time or pressure.

 

Thirdly, If your cavity pressure does not get zero at the end of the cycle i.e. Clamp tonnage should have a Zero tonnage value to eject the part, other wise injection molding machine will not allow you to eject the part.

 

Regards,

Mayur.

 

 

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