Sink marks prediction using three analyses types

[Anonymous]

Hi All,

Apologies for any multiplications on this subject, but I am interested in getting to know the true benefits and limitations of the three types of analyses (midplane, DD, 3D) with respect to "SINK MARKS ESTIMATE [mm]" calculation.

Until now I have not found any literature that discusses the calculation schemes and/or provide reasonable advice which analysis type is better and why when it comes to sink marks estimations. I wish to have enough confidence to be able to choose the most suitable analysis type for future analyses.

My questions are related to the following (let`s assume sufficiently fine mesh and correct thickness interpretation):

- May it be stated the "sink mark estimate [mm]" is best predicted by 3D analysis?
- Is DD analysis sufficiently accurate to predict this depth?
- Are there major differences between Midplane and DD calculation schemes and accuracy?
  E.g. some say midplane and 3D are the "most suitable" and DD should be avoided...?!?!

I kindly would like to hear scientific explanations (e.g. model simplifications made and their effects, ref. to books, etc.),
but experience based opinions would be appreciated too.

Thank you in anticipation!

Martin

[Javier.Jubierre]

Hi @Anonymous,

Great topic that you brought up to our forum.

There are several reasons that my generate sink marks. However, the Insight's solver can only predict sink marks produced on concentrations of mass, such as ribs, bosses, and internal fillets. Usually, voids are associated as additional problems, when sink marks appear. In our help online, you may find the basic troubleshooting in such scenarios.

Because voids happen inside the section of the part, the use of 3D meshes is highly recommended. If only the depresion of the surface is requiered, both meshes, DualDomain and 3D, should return equivalent results.

Here you may find the equation that describes the sink mark indicator, which describes the likely presence and location of a sink mark. I am afraid, we do not have validation documents available.

I hope it helps!

Cheers,

Javier

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[Javier.Jubierre · Accepted solution]

Hi @Anonymous,

Do you have any further question? If you are happy with my reply, please "Accept the solution". If you need further help, just let me know and I will be happy to answer your questions.

Cheers,

Javier

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[PascalGosset]

I'm interested too in this issue.

I took few minutes to compare the results  got with the 3 meshing types.

My model has a T shape. Dimensions: 60 x 40 mm, thickness: 3 mm.

With a 3 mm thick rib, a sink mark must appear.

I did not think I would get the same results but I did.

See the picture joined.

My question is now: when a sink mark becomes visible?

I do not wait a precise answer but, at least, a range of values:

0.01 mm? 0.05 mm? 0.1 mm? 0.5 mm?

Thanks for your replies.

[PascalGosset]

About the voids, I mean holes insides the walls, like this one:

Is it possible to predict them? How? Witch result? What value?

By example in this case:

Many thanks for your replies.

[mppkumar]

Hi @PascalGosset

we follow a thumb rule 

sink mark estimate more than 0.03mm in case of painted or non-textured part will have visible sink

if textured (grained), based on grain depth the value will vary, best practice is any value more than 0.05mm will indicate a visible sink 

if the forum have any other opinion, please post it

[mppkumar]

Hi @PascalGosset

As you said, there is no straight forward result to show voids in the thicker part

but with the interpretation of results like volumetric shrinkage / time to reach ejection temperature / density, we can assume the possibility of voids

[PascalGosset]

Thank you Kumar,

Then, how do you compare volumetric shrinkage / time to reach ejection temperature / density?

When do you say: a void will occur?

In my sample, can you assume if we'll have a void?

[Anonymous]

@PascalGosset 

There is no direct results to measure the Void formation in the part in Moldflow. but with the help of few results we can predict there might be the chance of voids by taking few parts as benchmark.

In most of the cases if we have a part with variable thicknesses (Thick & Thin), we check whether the volumetric shrinkage is uniform or not, if we more Volumetric shrinkage in thicker section, we check how much packing that area gets considering the gate freeze time & area around that thicker section freezing that will definitely tell you where the hottest melt is. Packing plays a vital role on controlling the voids formation in thicker sections. 

Key results you should see --> Volumetric Shrinkage, Time to reach ejection temperature, Temperature & packing condition.

Below is an example case study we did on controlling the voids, picture may help you what i exactly intended to explain you.

[Anonymous]

Hi Javier,

Many thanks for your input and I`m very sorry for getting back to you late. Thanks, the explanations are useful.

regards

Martin