"ActiveFlowBalance": simplifies processing and increases product quality


The machine function "ActiveFlowBalance": simplifies processing and increases product quality.

In order to simplify injection moulding processes, with a smooth transition from injection to hold pressure, Sumitomo (SHI) Demag now offers a new machine function named " activeFlowBalance". This function equalises differing pressure ratios within a cavity or between several cavities of a multi-cavity mould in a gentle manner at the end of the injection procedure. Thus meaning that parts critical to filling, which up to now had to struggle against over-filling, the formation of burrs or burns and required complicated settings, can now be manufactured in a robust process.

Author:Thomas Brettnich, Dipl.-Ing. (FH) - Director of Technological Development


Up to now, there have been two main ways for the fitter to react to the uneven or fluctuating balancing of cavities. Either injection is continued for as much time as is required to fill the very last cavity completely. This means, however, that pressure peaks form in the cavities which have already been filled, which can lead to the formation of burrs. In addition, the various pressure levels generate different conditions in the individual cavities, which can have various consequences, including residual stresses, deformation and varying mould shrinkage of the parts. The melt front speed in the lagging cavities also increases erratically as soon as the first cavity is filled. This often leads to ventilation problems, which can express themselves in burns or incompletely filled parts. An alternative is to switch to hold pressure immediately the volumetric filling of the first cavity has been achieved.

To be sure, this procedure avoids the negative effects cited above, but it does, however, conceal the risk of the cavities being under-filled. In this regard as well, the differing pressure ratios lead to corresponding differences in the texture of the parts

 

The new activeFlowBalance of Sumitomo (SHI) Demag stops the space in front of the screw at the end of the injection procedure and retains the screw in a fixed position for a defined time before building up the hold pressure. During this phase, the melt which has been compromised by the injection pressure relaxes. Partially filled cavities, in which a lower counter-pressure prevails than in the cavities which are already full, then fill more strongly because of this. The fill levels balance out in a natural way. The duration of the pressure balancing is a part of the hold pressure period and does not prolong the cycle.

While it was difficult for the fitter to master such processes fully up to now, activeFlowBalance can be operated extremely easily. In addition to activating the function, just the change-over point and the standstill time of the screw need to be adapted. A ramp can also be used as an option to influence the braking speed of the screw.

The optimal process parameters are established on the basis of a fill study without hold pressure. The choice of the ramp depends on the component. The rule is: the shorter the injection time, the quicker the brake should usually be applied. The point in time is usually chosen in the case of multi-cavity moulds, at which the first cavity is volumetrically filled, even if lagging cavities are only partially filled. In order to establish the optimal duration of time for the balancing of pressure, the activeFlowBalance time is increased in small steps and the weights of the parts from the various cavities are compared. The weights approximate each other more and more as the duration of time increases. The optimal activeFlowBalance time has been achieved if no essential approximation of the weights can be observed any more. Experience has shown that this is the case as regards about 30% of the injection time.

As the pressure reduction already acts as hold pressure during the activeFlowBalance time, the duration of this is to be factored into the hold pressure time. The pressure can also frequently be reduced during the remaining hold pressure time in comparison with the standard process.

The use of activeFlowBalance is currently possible with IntElect Smart all-electric injection moulding machines. The path-finding function, which represents a unique selling point of Sumitomo (SHI) Demag, can also be retrofitted to this machine series without any problem. It is based on the high precision activeDynamics drive technology: the reaction times of the system, its high dynamism and its capability of being reproduced exactly are crucial for regulating the activeFlowBalance function precisely.

activeFlowBalance offers particular advantages when injection moulding with
• Multi-cavity moulds with balancing problems
• Moulds with unstable conditions in the hot runner
• Moulds with ventilation problems
Or of
• Parts with multi-point gates
• Parts with thin-wall areas at the end of the flowpath
• Parts which are very small in relation to the feed weight
• Parts from family moulds and tandem processes.