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## Simulation Moldflow Insight

New Member
Posts: 1
Registered: ‎05-12-2009

# Cross-WLF viscosity model

1144 Views, 4 Replies
11-09-2011 04:31 AM

Hello,

I'm in charge of moldflow analysis of a “speaker grill” made in ABS with melt flow index 5.

The nominal wall thickness is 3 mm, but the effective pass-section between each hole is 1.2 mm (very thin section).

Comparing the results with real molding, there is a big difference between the peak pressure calculated and the one measured on the injection molding machine (the measured pressure is more or less the double of the one calculated). Probably the pressure-dependency of the viscosity has to be considered, but the "D3 parameter" in the "Cross-WLF viscosity model" of the UDB material characterization is set to ZERO.

Does anyone can suggest me a standard/typical value to be assigned to "D3 parameter" for a generic ABS with low fluidity?

Many thanks,

Luka

Valued Contributor
Posts: 62
Registered: ‎08-25-2008

# Re: Cross-WLF viscosity model

11-09-2011 07:50 PM in reply to: luca.scrosta

Well, the D3 coefficient is usually of the same order of magnitude but lower than b6 in the dual-domain Tait model (PVT). As a wild guess, I would start with D3=b6/10 and if it improves the results I would try b6/4 and b6/2. Strictly speaking when putting non-zero D3 we have to modify the other coefficients (especially, decrease n and modify D1) to fit the experimental data. AFAIK you can order Autodesk Moldflow Lab to measure rheology at elevated pressure and fit the pressure-dependent viscosity data for you.

Usually the pressure dpendence of viscosity kicks in after 100MPa injection pressure, if the pressure is below 100MPa then D3 is not important.

How do you model the grill? Do you use midplane, dual-domain or three-D?

New Member
Posts: 2
Registered: ‎03-14-2012

# Re: Cross-WLF viscosity model

03-14-2012 01:40 AM in reply to: luca.scrosta

hi,

I simulated a rubber injection molding process with reactive molding module of moldflow, but the injection pressures calculated by the software has almost -35% deviation from reality. I aim to introduce pressure dependance of viscosity into model, but this is not predicted for reactive molding. according to some researches, these effects are not negligible for injection molding simulations.

Thank you

Valued Contributor
Posts: 62
Registered: ‎08-25-2008

# Re: Cross-WLF viscosity model

I don't have much experience with thermosets/rubber but for thermoplastics the effect of pressure dependence of viscosity is usually small if the maximal pressure is well below 100 MPa. I guess the same rule of thumb is true for thermosets. What was the maximal pressure measured or predicted for the process? I would not bother with the pressure dependence if it is below 100MPa.

There might be many sources of inaccuracies in pressure predictions: material data, geometrical model, processing conditions. As a wild guess: have you included the dwelling time (the curing time in the barrel of the injection molding machine)? If there is some cure there then the pressure of plastic would increase.

New Member
Posts: 2
Registered: ‎03-14-2012

# Re: Cross-WLF viscosity model

03-14-2012 10:27 PM in reply to: luca.scrosta

Hi,

the filling phase of this process is actually pressure-controlled and the hydraulic pressure settings are equal to an injection pressure of about 110 MPa. but the calculated injection pressure (considering the same injection time for simulation and reality) is 65 MPa.

when I run the simulation with full pressure control of filling phase (with hydraulic pressure settings in reality), the filling time calculated by the software is 1.5 sec, while in reality this time is about 9 sec !

the barrel temperature setting is 60 celcius degree, so the rubber induction time would not pass too much when it's ready to be injected. by the way, I will try this possibility.

another question: what is the best method for measuring visosity? I used RPA 2000 from alpha technology and measured viscosity versus frequency with a fixed strain of 15%. Capillary method is used widely for rhelogical data measurements, but I don't think 50% difference between calculated and real pressure rise from viscosity measurment method.