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Uniform freezing of melt in the mould

Originally melt was in cylindrical shape in side the plasticating barrel. On injection - on entering the mould, melt forms streams in different direction and spread inside the mould.

For uniform freezing of melt in the mould, all the melt fronts should spread inside the mould with constant velocity.

The melt stream may divide in to number of streams if there are obstruction in the path of melt stream and move with different velocity.

Velocity can vary because of

  • variation of cross sectional area of melt front on account of obstruction due to core (hole or cut out in the part) or
  • variation in wall thickness
  • variation in surface temperature.

The cross section of melt stream or melt front keeps changing depending on the geometry of melt flow during its passage through the mould. Therefore velocity of melt front changes as the Melt Front Area - MFA- (cross section area of moving melt front) changes during the passage of melt through the mould.

MFV depends on MFA and the set injection speed on machine control. Therefore it is necessary to visualise the melt front movement while setting the injection speed in multi steps.

It can be observed that MFV increases when melt divides around a slot - core - as MFA is reduced while passing around the slot . This increase in MFV causes the higher surface stress and orientation resulting in warpage. Hence it becomes necessary that on reaching the slot (at appropriate stroke), injection speed should be reduced on the control to maintain the uniform MFV in the melt front.

To maintain constant melt stream velocity in the mould, it requires to compute the changing cross sectional area of the melt stream through out the filling phase. It is very tough to manually calculate the variations required in injection linear screw speed. It is best done by MOLDFLOW analysis. It gives optimised injection speed profile.

In case of bucket, the melt moves in balanced manner from the gate located at geometrically central position. Melt spreads in single melt stream and cross sectional area of the stream gradually increases. Therefore for a constant injection speed set on the machine, the melt stream velocity would be reducing gradually as it fills up.

See how freezing of melt takes place while melt spreads in the mould with velocity.

Flow depends on injection speed set on the machine and resistance to flow offered in the mould whereas Freezing depends on cooling rate, a function of cooling circuit design and water flow rate and inlet temperature.

Remember melt freezes while it moves. Freezing is time dependent under the influence of mould cooling. Complete filling of mould depend on adequate (high enough) injection rate to counter the influence of freezing.

Melt flow and freeze simultaneously in the mould

Freezing of melt in the mould is shown in these animation..

Freezing during filling stage:.

Freezing after filling and during cooling stage:.

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