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Overmolding - deformation: which is more reliable?

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Message 1 of 4
teopado89
541 Views, 3 Replies

Overmolding - deformation: which is more reliable?

Good evening everyone,


i was thinking about overmolding process and i have this doubt:

 

if i'm not wrong, overmolding filling and packing start immediately after first shot filling and packing (in moldflow analysis);


But in a rotative mold there is a "setup" time (i don't know if it's correct to call in this way) where the mold rotates to change the cavity, and during this time the first shot continues freezing and becomes more and more stiff.

 

1) Wouldn't be more correct considering the first shot like an insert to better calculate warpage of the second shot (because second shot flows over a stiffer layer) ?

Or do you think it's correct to consider as "first shot packing time" the time between switchover untill gate freeze (so real packing) on first shot + "setup time" to rotate the mold ? (setup time will not overpack the material because the gate is frozen, but maybe first shot will continue freezing during this waiting = mold rotation).

 

2) About first shot i think the deformation result will be more correct in the overmolding analysis respect the single first shot analysis because the first shot is re-melt. Right?

 

What do you do in an analysis like this?

 

Please help me with your experience.

Thanks a lot and enjoy holidays^^

3 REPLIES 3
Message 2 of 4
teopado89
in reply to: teopado89

Excuse me, i hava a doubt about analysis menu:

 

is it still available the warpage analysis in moldflow 2014? because i remember if i'm not wrong there were in preceding releases (at university years ago i found it, if i'm not wrong), but today looking for it in the menu i don't find.

And i have a full license.

 

May you help me please? thanks

Message 3 of 4
maxmarautodesk
in reply to: teopado89

Hi teopado89,

 

I am also interested in the overmolding topic and am hoping that I can help you. First, I would like to refer you to the Autodesk Moldflow help about an overmolding analysis:

http://help.autodesk.com/view/MFIA/2015/ENU/?guid=GUID-C7C89010-0F08-4564-9ADC-3C823661C201

 

Some considerations for your questions:

 

1) I think the answer depends a bit on how your mold transfer exactly works in reality. Plenty of technology variations are available, such as rotating the whole mold, working with shutters, movable elements, and retractable cores, shifting only inserts or cores, picking up the plastic part of the first shot by a robot and placing it insert-like into another cavity for the first shot, etc.

Besides the latter, I would assume that the way of modeling the process by Moldflow (Fill+Pack of first shot with immediate subsequent Fill+Pack of the second shot) is suitable in general. I think so, because your packing should not stop before the gate is frozen (=no material flow possible anymore). If this is the case, you should rethink your actual process settings. I assume that the needed packing time is much larger than the time necessary for the transfer of the inserts or mold. Anyways, if you want to account for the transfer time, I would just add it on top of the packing time. "Overpacking" in the software will not cost you anything or change the part weight, the part will just cool in the meantime.

One question arises that Autodesk can hopefully answer: What is the temperature that Moldflow assumes for the material of the first shot? Is it the remaining temperature of the previous shot (which could then be tuned by adding time for cooling (=mold/insert rotation) to the packing time or is it the mold temperature of the second shot?

 

2) I believe the help section should already answer your question. But to summarize, I think the overmolding study gives you better results, compared to

i) the single shot study of the second component with the first component being an insert. Re-melting the first component is important for the adhesion/bonding between the two components. Given sufficient bond strength, the first component will provide some mechanical boundary conditions and constrains (residual stress, shrinkage, etc.) from the first shot. By using the overmolding study, this will be accounted for by the software. I imagine that you could however model the first shot as an insert with some boundary conditions in Moldflow, but it will be difficult to estimate the right stress and strain values and distributions for your insert.Consequently, I think first shot as insert and then overmolding study of the second shot is not very handy.

ii) a pure single shot study of the second or first component. In this case, you have no mechanical interaction between the two components. As a consequence, the warpage results will not be correct.

 

One more thing that came to my mind:

The overmolding should give you the better results. However, if your molding step is very delicate, perhaps you want to account for the cooling and mold also in the simulation instead of having a simple fill+pack analysis of the part only. There, you might run into trouble. I do not think it is possible to model also the mold and cooling of both shots. This seems not very handy to me with most likely overlapping channel and mold elements of both shots. But please do correct me, if I am wrong.

 

Best regards,
Max

Message 4 of 4
teopado89
in reply to: maxmarautodesk

Thanks maxmarautodesk for your complete answer!

 

" Anyways, if you want to account for the transfer time, I would just add it on top of the packing time."

May you explain me this? Do you mean i have to create a packing profile (in overmolding process settings) where for example if my mold takes 5 seconds to rotate to the next injection position, i have to create a profile where pressure is costant zero for the first 5 seconds at the beginning, and then i set the real profile?

Maybe i'm wrong, but i don't think it's correct because in this way we are putting the setup time between overmolding filling and overmolding packing, and in reality it's between first shot packing and overmolding filling.

 

"One question arises that Autodesk can hopefully answer: What is the temperature that Moldflow assumes for the material of the first shot? Is it the remaining temperature of the previous shot (which could then be tuned by adding time for cooling (=mold/insert rotation) to the packing time or is it the mold temperature of the second shot?"

This is a very interesting question. I have a related question to add: is first shot temperature considered the same during overmolding, or will is increase a bit during second injection, and later it cools together with the second shot? (maybe the answer is in the link you gave me, sorry i'll read tomorrow)

 

I do not think it is possible to model also the mold and cooling of both shots. This seems not very handy to me with most likely overlapping channel and mold elements of both shots. 

 

In this case i usually model the core channels (which don't change) and cavity channel of the last shot, because (i think about how my mold works, rotative-mold) when the mold rotates to the next position it doesn't have the preceding shot cavity channels, and maybe the preceding cooling can also be "deleted" by the next injection at high temperature.

 

Correct me please if i'm wrong.

 

Best regards, 

Matteo

 

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