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Compression Moulding of Micropart

86 Views, 7 Replies
03/18/2018 5:09 PM

Dear Moldflow-Community,

 

i am currently working on a compression moulding simulation of an micropart (2 mm diameter). It seems to me important to let you know, that i have a very small gate to fill the part (0.2 mm). Nevertheless i can fill the part and have some holding pressure at least for less than halfe a second.

 

I have done a lot of trials with the settings for compression moulding but some results just do not make sense to me.

 

1.Question:

When looking at the results for average shrinkage I see no improvement of the quality of the part. Looking at the X-Y-Plot of pressure inside the part i can see that during the compression step no pressure is built up (stays at 0), but movement of the core is visible. Even when the gate is frozen and compression is still going on, there is no increase in pressure

 

2. Question:

When using a huge compression gap, compared to the size of the part, i do not understand where all the addition polymer is going, when the gate is already frozen. The final shrinkage is not negative, so i asume, there is no overpacking. But where is the material going?

 

Would really help me, if someone has idea. Thanks in advance

 

Best Regards

Bernd

 


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Message 1 of 7 ( Views: 86 )

Re: Compression Moulding of Micropart

03/20/2018 4:33 PM

Hi ,

I assume you use Thermoplastic Injection Compression.

 

Now, it depends a bit on how you intend to process.

 

Often the volume is injected, none or very short packing pressure.

 

If not closing gate, material can flow backward.

Either close by valve gate or by end of packing.

 

For 3D mesh types, the injection locations are automatically closed after packing stage.

If you inject material, and set packing time to 0s, and then start compression (Press compression starts "At the end of packing"),

the injected volume should be kept in cavity.

Did you run like this?

 

You also might need to switch over by % volume filled > 100%, and 105-120% volume or so depending on material.

 

Hope this helps.

Regards,

Berndt

 

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Message 2 of 7 ( Views: 70 )

Re: Compression Moulding of Micropart

03/21/2018 11:05 AM

Hi Berndt and all others,

 

first, thank you very much for your reply. I appreciate that.

 

"For 3D mesh types, the injection locations are automatically closed after packing stage.

If you inject material, and set packing time to 0s, and then start compression (Press compression starts "At the end of packing"),

the injected volume should be kept in cavity.

Did you run like this?

 

You also might need to switch over by % volume filled > 100%, and 105-120% volume or so depending on material."

 

Yes, this is exactly what i am doing in my simulation. Reading my first message again, i may have to give you some more details:

 

Attached you will find the screenshots of the process settings. I am using a compression gap of 0.2 mm. Before starting compression I fill the cavity until its completely full (122%). After that i immediately start compressing without packing stage (injection location is closed). What now happens is very weird to me.

The 0.2 mm ist actually much to big for the small part (necessary gap size should be 0.04 mm to compensate shrinkage). In my understanding the additional polymer which was injected can not go out of the cavity as the gate is closed. So the polymer has to be compressed very strong. What i would expect is, that the average shrinkage will be negative, as i have compressed the material to much. But this is not happening for some reason. As you can see the part weight is decreasing. This means some polymer mass must go somewhere else, isn't it?

 

Any idea what i could do wrong?

 

Best Regards

Bernd

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Message 3 of 7 ( Views: 64 )

Re: Compression Moulding of Micropart

03/21/2018 2:37 PM

Hi Bernd,

thank you for the information.

Sounds strange indeed.

 

Do you see any velocity backflow in gate area?

 

Would it be possible for you to share Analysis Log?

 

Open Logs for study > In log text area, right-click and Save As... to a text.file.

Add to topic.

The Analysis Log could shed some light over the issue.

 

Regards,

Berndt

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Message 4 of 7 ( Views: 56 )

Re: Compression Moulding of Micropart

03/21/2018 4:13 PM

Do you see any velocity backflow in gate area?

That was also my first thought, but the small gate freezes very early and the compression is going on much longer. I checked the velocity, but as you can see in the picture there is no velocity in the gate during compression.

 

Attached you will also find the LogFile and the diagram for pressforce during compression. I tried the last days severall alternatives but always get to the same results.

Perhaps its a bug in my old moldflow version (2014)?

 

And what exactly means "The %volume filled at end of compression = 107.0448 %"? Should it not be 100% at the end of compression, as the gap is closed completely?

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Message 5 of 7 ( Views: 50 )

Re: Compression Moulding of Micropart

03/21/2018 7:26 PM

Hi Bernd,

thank you for providing additional information.

 

I am a bit puzzled about the settings.

Seems as mold halves are not completely closed, right?
And press compression force not used, as the press speed does not reach limit.

 

I would suggest following as starting point:

1) Switch over at 105% volume filled
This to be in control of volume injected.
To be changed later, increased, as needed.
To ensure a certain volume is injected.

 

2) Set press distance to, 0.04 mm
" (necessary gap size should be 0.04 mm to compensate shrinkage)"

 

3) Press compression force cap:0.0318 tonne
(Not reached looking in log file, and assume you want to use it?)

 

4) Switch to press force control
From the start of compression.
To ensure the press force is used for compression.

 

5) Compression force after switch to press force control
Absolute value
0.0318 tonne

 

Would such a starting point be useful?
I think so, as you would be in control of:
volume
press distance
press force

 

Does it make sense to you?

 

I think 3D injection compression was released to Insight 2014.
Improvements since then, but not sure if it is related to your issue.

 

Regards,

Berndt

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Message 6 of 7 ( Views: 43 )

Re: Compression Moulding of Micropart

03/23/2018 5:56 PM

Hi Berndt,

 

thanks againg for your feedback.

 

First I would like to explain and summary one more time, what I have tried so fare in general:

1) Speed Control of the whole compression moulding process (No switch to force control)

-Tool can close the whole gap with the speed entered. Problem here: Process needs no or very low force to close the tool. No high pressure inside of the cavity, which leads to shrinkage. And it does not matter how big i define the compression gap, the speed controlled process will close it. Again, here i have always have the problem which I explained earlier: the polymer mass is going somewhere, even when the gate is closed.

 

2) Force Control of the whole compression moulding process (No switch to speed control)

- The force will be built up at the beginning of the process. It will move forward, but it will never reach the end of the compression process (does not matter how high the maximum force is). The problem seems, that the speed is stopping way to early, so the gap can not be closed. The compression process stops before the maximum compression time. I can not see any defined reason why the process stops so early.

 

I have tried out your suggestions. In general it works out to reach the force. Two things i have found out trying different compression speeds (always force control with 0,0318 t =600 bar inside cavity during compression moulding). The compression gap is 0.04 mm and i started with a closing speed of 0,4 mm/s.

 

- If I have a to high compression speed (1 mm/s), the time period is to short to reach the force. In the meantime the compression gap is then already closed. Makes totaly sense to me.

 

- If I have a too low compression speed (0,05 mm/s), the compression force is reached but it will not close the gap (ends at 0,023 mm) because the speed stops for some reason at 0,573 s (same problem as explained in Point 2). I do not know why? I would like to have a forced control compression process for at least 1.5s and higher. But this seems not to be possible. Any idea how to do that? You will find the logfile attached

 

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