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SolidWorks Plastics Reported Volume Discrepancy versus SW

Question asked by Ken Tenney on May 2, 2014
Latest reply on May 12, 2014 by Ken Tenney

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I have a plastic part in SW 2014 Professional. Using "Mass Properties", the volume is reported as 76.93cm^3.

We have a seat of SolidWorks Plastics. I've run my part through analysis, and when the process is complete, the volume is reported as 77.87cm^3.

Why is there a discrepancy between the two? Is the latter the effective volume of the mesh, and not the original SW definition of part geometry? Regardless, why are they different?

(77.87-76.93)/76.93*100 = 1.22% greater volume in SW Plastics. This doesn't seem insignificant.

Bottom line, which should I use for cost estimating material usage? I've always trusted SW to get this right. SW Plastics has disturbed my trust (for now).

Thanks!

Adrian Ilie

May 7, 2014 10:01 AM

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Hi Ken,

For different mesh size, you will see different Volume results (when you generate the Mesh, you can notice that cylindrical face is more or less 'perfect cylindrical', depends on Mesh Triangle Size). Try with your part with 10 and 2 units for Triangle Size > do the analysis and open Summary and Report to see the Volume. Small number for Triangle Mesh > more accuracy results but time to calculate the analysis is higher.

For Material (Polymer) properties, you can edit it and check the values: RMB on 'Polymer' > Open Database > in right side of Polymer window, you can edit and check any parameter.

Thanks,

Adrian Ilie

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JOHN GEORGE May 2, 2014 12:21 PM
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Ken,

When you do the analysis, it might have added the shrinkage factor to the model (depends on the plastic)
I don't have the plastic module with me, so I cannot confirm.
When a plastic mold is designed, the part is expanded with the shrinkage factor

Meantime welcome to the forum
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- Ken Tenney @ JOHN GEORGE on May 6, 2014 9:10 AM
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  John, no, I don't think it's automatically adding a shrinkage factor (expanding the part). If I use the lowest boundary of a suggested range for a shrinkage factor (1.005) the volume is greater still than SW Plastics reported volume.
  
  I always include the shrinkage in the part design, but only as a separate configuration. This is expressly to keep from inadvertently confusing the cold part geometry from the tooling geometry.
  
  Thanks,
  
  Ken Tenney
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  - David Dinius @ Ken Tenney on May 6, 2014 11:22 AM
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    Ken,
    Take a look at the mass densities from the properties that SW and Plastics are using. There could be slight differences between the data bases or even just a rounding difference between the 2 apps.
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    - Ken Tenney @ David Dinius on May 6, 2014 11:59 AM
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      Dave,
      
      Thank you for the reply. Yes, that's "Part 2" of my general questions about SW Plastics.
      
      First, volume is volume, regardless of the density of the material that occupies that volume. My initial question is only about volume as reported in SW Plastics versus that reported in native SW; why are they different?
      
      Second, I cannot find the value for density used by SW Plastics. It's not reported in the roll-up analysis report. (Such a basic, important number, I find this perplexing.) I went to the text version of the report and found a line that was something like "Density #.### #.### #.###". None of the fields correlated to the would-be density of the material (PC) in US Customary or SI units. So, the application is reporting volume x density = weight in the final report, but what that number is and where it's being stored, I haven't figured out yet. (Maybe someone here has figured out this field in the text report?)
      
      Thanks for your input. I'm relatively new to the Simpoe-derived version of SW Plastics. I used to use old MoldflowXpress and liked that simple package fairly well. I haven't drawn a verdict on SW Plastics yet...
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    - Jason Swackhamer @ David Dinius on May 6, 2014 12:46 PM
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      Ken-
      
      One other thought...does Simpoe add the runner and gate to the shot volume?
      
      -Jason S.
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      - Ken Tenney @ Jason Swackhamer on May 6, 2014 2:07 PM
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        Jason, offhand, I don't think it adds the runner and gate volume, as these could vary tremendously part-to-part or mold-to-mold.
        
        For the part in this discussion, it is direct gated (hot sprue bushing), so there is no runner or appreciable gate material/volume.
        
        At my workplace, we share the workstation that runs SW Plastics, so I need to wait on it, but I've built a simple test model (SW Plastics Test File - 06 May 2014.*) to do some experimenting with. I will get underway with that testing later and report my findings:
        
        1. SW volume vs. SW Plastics volume.
        2. See if I can find the material density in the text version of the report. (I should have said earlier that I did not see it as a field in the polymer properties listing, either.)
        
        It might be interesting to have some others with SW Plastics run the part using generic PC and report their SW volume and SW Plastics volume. I typically perform a manual shell mesh as close to 100k elements as possible. The part is direct-gated on the interior, center (uses reverse-side ejection as a real part).
        
        (This is a generic, but somewhat similar part to my original. That one is proprietary, so I cannot share it.)
        
        Ken Tenney
        Attachments
        SW Plastics Test File - 06 May 2014.x_t.zip3.8 KB
        SW Plastics Test File - 06 May 2014.pdf178.6 KB
        SW Plastics Test File - 06 May 2014.SLDPRT.zip119.2 KB
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        Jason Swackhamer @ Ken Tenney on May 6, 2014 3:26 PM
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        Ken-
        
        Good luck with your investigations! Wish I could help, but we job out our analysis.
        
        Looking forward to hearing the explanation for this discrepancy.
        
        Thanks,
        
        -Jason S.
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        Adrian Ilie @ Ken Tenney on May 7, 2014 10:01 AM
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        Hi Ken,
        
        For different mesh size, you will see different Volume results (when you generate the Mesh, you can notice that cylindrical face is more or less 'perfect cylindrical', depends on Mesh Triangle Size). Try with your part with 10 and 2 units for Triangle Size > do the analysis and open Summary and Report to see the Volume. Small number for Triangle Mesh > more accuracy results but time to calculate the analysis is higher.
        For Material (Polymer) properties, you can edit it and check the values: RMB on 'Polymer' > Open Database > in right side of Polymer window, you can edit and check any parameter.
        
        Thanks,
        Adrian Ilie
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        Ken Tenney @ Adrian Ilie on May 7, 2014 10:49 AM
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        Adrian,
        
        Thank you for the reply. That is I thought might be regarding the reported volume being derived from the mesh. That's good to understand that with certainty.
        
        In regard to the Material (Polymer) properties, I'm afraid I still cannot find the value for the material density. I've seen the parameters window, etc. I cannot find a field for density.
        
        By backing out of the final values using Generic PC, the density is 1.317g/cm3. I don't see a value like that in any field. Can you help me see where that is? (This value seems high for unfilled, generic PC. Most typical values are ~1.2g/cm3, but that's another question.)
        
        From my last analysis: 110.24g/83.73cm3 = 1.317gm/cm3
        Attachments
        SW Plastics Test File - 07 May 2014.pdf322.7 KB
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        Adrian Ilie @ Ken Tenney on May 7, 2014 11:50 AM
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        Hi Ken,
        
        Let me try (sorry if I'm wrong, but I'm not an expert in Materials)
        By Default, I cannot see 'Density' filed. I think 'PVT: Modified Tait eqn' values are taking in consideration to calculate Density.
        If you want to specify manually this value for Density, you can copy and past the Material in 'User-defined Databank' > edit Material > activate PVT tab and select '1: Const. Density' > set 1200 Kg/m3 > OK.Then this value is used for Density.
        
        Thanks,
        Adrian Ilie
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        Ken Tenney @ Adrian Ilie on May 9, 2014 4:40 PM
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        This has morphed into another subject, but I'd like to finish what I started. In regard to Density in SW Plastics, I'm still not certain I understand. I see the PVT Section. If I drill down, I then see two different variables with associated values in units m3/Kg.
        
        For Generic PC, those respective values are:
        
        b1l = 0.0008227 m3/Kg (inverse = 1215.5Kg/m3)
        b1s = 0.000817 m3/Kg (inverse = 1224Kg/m3)
        (See attachment for screen capture of parameters for SW Plastics Generic PC.)
        
        Neither equal the derived vaule that SW Plastics is using (1.317g/cm3).
        
        Again, where can I find the density used in the simulation?
        
        This is an essential question regarding the believability and trust of simulation software. If the values on which the simulation is based are not accurate, how can the results be trusted?
        Attachments
        SW Plastics material profile - generic PC.pdf274.3 KB
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        Jerry Steiger @ Ken Tenney on May 10, 2014 1:21 PM
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        Ken,
        
        I know nothing about SW Plastics, but I would not use Generic PC, or Generic anything, as a basis of comparison. I would check the SW Plastics values for a well-known material that you have good data for, such as a Bayer Makrolon or Sabic Lexan.
        
        I don't think I would ever run an analysis for a Generic PC either. The various grades of PC vary too much in important properties to make the analysis useful.
        
        Jerry S.
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        Adrian Ilie @ Ken Tenney on May 12, 2014 3:32 AM
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        Hi Ken,
        
        Please see attached document, with steps how to set manually custom Density value for Plastics material.
        I hope this helps.
        
        Thanks,
        Adrian Ilie
        Attachments
        Set Density for Plastics Material.pdf367.5 KB
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        Ken Tenney @ Adrian Ilie on May 12, 2014 1:22 PM
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        Jerry, the material type is irrelavent to my last question.
        
        Adrian, thanks for the reply, but I think I may have mis-communicated in some regard? I understand how to change the density value per your instructions now and previously for a user-defined material.
        
        My question has been and continues to be about how I can verify what SW Plastics uses as a density value for a given (read "any") material? Backing out of the weight and volume from the SW Plastics report delivers a density that's not to be found in the plastics parameters database. So, where does the density value come from?
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  - Jason Swackhamer @ Ken Tenney on May 6, 2014 11:51 AM
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    Ken-
    
    Yikes—don't like the sound of this. We use the mass properties all the time for weight calculations.
    
    Is it possible that the analysis is using a .004/inch shrink? This would come out 77.86 cm³.
    
    What happens if you fool with the mesh density?
    
    Hoping there's a good explanation for this,
    
    -Jason S.
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SolidWorks Plastics Reported Volume Discrepancy versus SW

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