Effect of tool bending on part deflection in injection molding

Hereby is presented an analysis on possible solutions to be applied  in order to reduce the mismatch introduced by general accepted  approximation of real tool behavior, two different approach have been  investigated. 

Tool Bending Calculation

The first approach proposed consider modelling the whole tool and run a simulation considering the tool itself as an insert allowing the  simulation software to calculate itself the tool bending and the  relative effect on the injected polymer. 

It must be clear since the beginning that within this approach, which is surely more time demanding in terms of computing and pre-processing, the result strongly relies on boundary conditions set for core-shift  calculation and on settings of computation parameters, which influence  the stability of calculation. 

In the presented case the boundary conditions as set as follow: 

-         Fixed condition at interface with back plate 

-         Cylindrical condition on columns 

-         One side moving condition on parting plane 

The Core Shift calculation settings are set considering higher  frequency of updates respect to proposed standard. Calculation is  performed during filling and packing considering 10 nodes tetrahedra  mode.

The result of calculation is reported on the picture below. it is  clearly visible that tool deflects more on the outside areas increasing  significantly the part thickness, whereas it is very stable in the  middle.

This scenario is completely opposite to the experimental observation, in fact in the real case the sensor A, which is positioned in the  middle of the tool, reports the maximum deflection and so the maximum  thickness increase.

The experiment highlights that tool deflection is really difficult to manage due to complexity of interaction between all the tool  components. 

Beside that, the results highlight also another important aspect:  that there’s not any impact of tool deflection neither on pressure  behaviour nor on deflection. This is because in the Autodesk Moldflow®  2017.3 module of core shift calculation the interaction between insert  and plastic component is limited to the freezing time of polymer, after  that time no more interaction is calculated.

Tool Compliance Method

The second approach investigated in order to improve the cavity  pressure evaluation consist in a coupled flow analysis and tool  deflection by activating a back door option in Autodesk Moldflow® 2017.3 proposed and developed by Dr. Franco Costa. 

The method consist in adding a variable thickness ∆t at each time  step of simulation according to the following:   ∆t = Cm * Fclamp 

Were Fclamp represent the clamping force at given time and Cm is the  Mold Compliance, which is measured in mm/tons and represents a  characteristic of each tool. 

The basic idea of the approach is to calculate an average global tool opening according to clamping force value at each time instant by a  constant factor depending of tool stiffness. 

In this specific case Cm value has been calculated by considering the average deflection value measured by force sensor. 

This method has been proved to work and provides an easy way to  consider the spring back effect of tool during the packing phase,  contributing significantly in reducing the mismatch between simulated  and real cavity pressure.

As consequence also the overall deflection prediction results  significantly improved reaching a very good matching in some points.  Some variation are still present but it maybe possible that depends on  other factor than tool deflection and need to be investigated  separately.