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INJECTION MOULDING PROCESS

The moulding process refers to shaping of plastic melt inside the mould. This involves

  • Filling phase,
  • Switchover point, peak pressure occur after slight delay.
  • Pressure phase,
  • Cooling phase.

Sequence in the process

  • It is desirable to have highest possible filling speed (Injection Rate) during filling phase.
  • Filling speed is normally limited by limitation of shearing in engineering polymers. Highly thermally stable material - commodity polymers- can be filled at high filling rate with out any fear of degradation of polymers melt.
  • Filling phase covers 85 to 98% of total injection stroke.
  • High speed filling is terminated at Switch-over point and low speed Pressure phase commences.
  • Due to relaxation of compression (10-15%) of melt during injection stroke, melt expands and fills up remaining empty space in the mould. It overfills to raise the cavity pressure as relaxation completes.
  • If we don't leave any space for relaxation of melt then, tremendous amount of pressure generated. This can damage the mould.
  • Since mould surface is maintained at lower temperature (than that of melt), melt cools and hence tries to lower its volume as per the pvT characteristics.
  • At this point the compensation for shrinkage is supplied at lower speed. Since less than 10% of space is to be filled, it does not require high speed filling. The cavity pressure is just to be maintained in such a manner to compensate for void created by the shrinking melt on account of falling melt temperature. The pressure phase has to be maintained till the melt transfer stops.
  • There after the melt has to be cooled further, below heat defection temperature of material. This would prevent deformation of moulding during ejection.

Important note

During filling phase, pressure value should be set 15-25 bar above the actual pressure encountered in the cavity. This is because if the relief valve is actuated during the filling phase, oil would drain and you loss control over the speed. In case of doubt, it is ok to set value at about 80% of maximum pressure for the machine through out the filling phase. The value of maximum hydraulic system pressure is recommended in the manual. To maintain the constant melt front velocity, it is required to adjust the injection speed in stroke dependent multiple steps.

During pressure phase, the filling speed is low and constant through out the pressure phase and pressure can be adjusted in time dependent multiple steps to avoid moulded-in stress.

SETTING POSITION SPEED AND PRESSURE

cavity pressure profile

PRINCIPLE for SETTING OF INJECTION SPEED, PRESSURE AND POSITION

POSITION

SPEED

PRESSURE

FILLING PHASE

Also known as Speed phase.

Select end of 1st step and start of 2nd step.

Set SPEED 1

Try to set high.

Only one pressure setting is required during FILLING PHASE.

Pressure Setting should be more than actual filling pressure. as relief valve should not be actuated. If it is actuated, then speed control will be lost.

Filling pressure depends on resistance to move the melt. It depends on flow ratio and viscosity of melt.

Also known as Speed phase.

There may be no. of steps available on machine.

Select end of 2nd step and start of 3rd step

Set SPEED 2

Lower speed for crossing narrow passage / gate

Steps are position controlled.

Select end of 3rd step and start of SWITCH OVER POINT. This point is at around 80-95% of the injection stroke.

Set SPEED 3

Reduce to lower sink mark / increase to shift weld line.

PRESSURE PHASE-

Holding phase.

There may be no. of steps available on machine.

Timer controls the Holding pressure steps if available on machine.

Holding pressure time for step 1 is set on a timer,

Set SPEED low value say up to 35% not more.

This can be one step of speed for different Holding steps.

Speed set is low, as there is less or no space to move the melt.

Holding pressure 1 set just enough to fill cavity without overpacking.

Steps are timers controlled.

Holding pressure time for step 2 is set on a timer,

Holding pressure 2 set just enough to fill cavity without overpacking.

Holding pressure time for step 3 is set on a timer,

Holding pressure 3 set just enough to fill cavity without overpacking.

Click here to understand pvT diagram.

Click here to know Typical Process Guidelines.

Click here to know Typical Barrel Temperature Settings.

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