I don't think the team is too keen on sharing their algorithms Sorry.
The current implementation of the wall slip feature is a free slip model (no friction). Slip occurs at a node if the local pressure is below the specified pressure threshold or the local temperature is above the specified temperature threshold. Otherwise, the normal no-slip boundary condition is imposed. Admittedly this is a very simple model. In the future we hope to add a friction coefficient for the slip condition.
Dear AMI engineers,
Could you tell me what 'pure compression molding' is?
what is difference between 'injection-compression' and 'pure compression'?
Thanks in advance,
Pure compression, you open your mold, you place your material (as it is thermosets, you put a block of solid material), and then you start to close the mold.
Injection compression, you start the process with a mold not fully close, you inject liquid polymer in it, and you finish your process by closing the mold(compression phase).
If you look on the Autodesk labs site, you can see 2 movies which shows what is happening in the software.
Thank you very much for the information and I apologize for not responding sooner. Also, you mentioned that if the pressure is below a certain amount, slip is permitted to occur. Conceptually, I would think that if MORE pressure was present, the molecules along the surface would tend to slip more. Would you mind explaining why it is that lower pressures support slip? Thank you for your continued help, Franco!
I think it is necessary to be clear whether we are talking about hydrostatic pressure or pressure drop (or pressure gradient).
When I said that slip occurs below a certain pressure, I was refering to the local hydrostatic pressure. More exactly, we should be talking about the normal stress at the wall. That is, the force which is pushing the polymer onto the cavity wall. I think you can understand that if the normal stress (hydrostatic pressure) is higher, then the adhesion to the wall will be stronger. If there is almost no pressure, then the material can slip easily along the wall.
When you mention pressure, I think you are actually referring to the injection pressure, or the amount of pressure drop from the injection point to the flow front (where the pressure is zero). This pressure drop can also be related as the presssure gradient over the flow length. It is the pressure gradient which determines the wall shear stress and this is what determines the degree of molecular alignment. You are right to mention this, and a friction model with a friction coefficient would multiply the friction coefficient by the wall shear stress to get the traction force at the wall.
The slip model we have implemented so far in Scandium is much simpler that this. It is just a free slip model, which means that we assume zero traction force while slip is occuring (that is a friction coefficient of zero) and it assumes an infinite traction force (no slip) when the conditions for no slip are met. The conditions for no slip are that the local (hydrostatic) pressure is above the pressure threshold, or that the local temperautre is below a temperature thresold. (That is, if the pressure is so high that the polymer is pushed hard onto the cavity wall, or the temperature is so low that the polymer has solidified).
We hope at some stage to add a friction coefficient to the wall slip model in Scandium.
Thanks for the question.
Respected Dr Costa,
I have an question based on you are reply
(That is, if the pressure is so high that the polymer is pushed hard onto the cavity wall, or the temperature is so low that the polymer has solidified)
how to decide the pressure threshold and Temp. threshold values in Free silp assumption?
For the venting analysis how exactly is the air pressure calcuted is it included in the equations during flow and pack solver? may be in furture versions will it be possible to watch where exactly the traped air is going or it is traped inside the cavity?
we are still learning what are appropriate threshold values as we gain experience with this technology preview of wall slip. Threshold values which I have found some success with are to set the pressure threshold to 0.1 MPa while the temperature threshold to value much higher than the melt temperature. These setting mean that slip will occur whereever the local pressure is below 0.1 MPa - while no slip occurs whereever the pressure is higher than 0.1 MPa. (Irrespective of temperature).
Regarding venting. Yes, the calculated air pressure acts is included in the calculation of polymer pressure and movement. The air pressure calculation includes the air flow through the vent - and so depends on the size and length of the vent as well as its position. If a pocket of air is trapped with no access to a vent, then the air pressure will continue to rise. If the vent is too small, the air pressure can also rise.