D-6-Molding Parameters

Here discuss molding parameter settings and output on the IMM which mostly considered during injection molding process.

  1. Barrel Temperature
    The plastic needs to be in a molten state to be injected into the mold. The heaters around the barrel which conducts the heat to the plastic on the inside to aid the melting. Depending on the length of the barrel, there can be several heater bands, and the temperature for each of these will have to be individually set depending on the material that needs to be processed.
  2. Mold Temperature
    The temperature of the mold is set using a controller that circulates is a heat transfer fluid such as water of oil. Usually water is used for achieving mold temperatures below 100 °C, and oil is used for achieving higher temperatures. Sometimes cartridge heaters are also used.
  3. Hot Runner Temperature
    The parameter setting of hot runner system including manifold and hot tip.
  4. Injection Speed (Velocity)
    This is the speed at which the screw moves linearly to inject the melt plastic into the mold. The injection speeds must be set fast enough so that the plastic is still in the molten state until the complete mold is filled to the optimum requirement.
  5. Injection Pressure
    This is the pressure that is applied to the molding screw to maintain the set injection speed. If the viscosity of the plastic increases, the effort or the pressure required to maintain the set speed will also increase so it is important that the speed is kept constant during injection. The machine should always a surplus of pressure available for a robust process.
  6. Hold Pressure
    This is the pressure that is applied to plastic during the hold phase to ensure that the excess plastic does not go into the cavity nor does the melt plastic come back out from the cavity.
  7. Hold Time
    This is the time when the hold pressure is applied.
  8. Screw Speed
    This is the speed of the screw rotation during the picking up of the plastic during the shot build up. It is mainly based on measuring the melt temperature and examining the melt for burning and unmelted particles.
  9. Back Pressure
    This is the pressure that is applied to the back of the screw during re-filling in order to achieve melt homogeneity and a melt free of any volatiles.
  10. Cooling Time
    Set cooling time: The time between after holding to before mold opens, this is the time the mold is kept closed to cool the plastic to the ejection temperature. Once the cooling time has elapsed, the mold opens, and the part is ejected.
    Actual cooling time: The plastic starts to cool off as soon as the plastic touches the mold, therefore, the actual cooling time is equal to the addition of the injection time (fill time), hold and cooling time setting.
  11. Shot Size
    This is also known as the dosage. The zero position of the screw is when the screw is all the way in the forward position. The screw moves back to pick up the melt material. The set distance that the screw moves back to is called the shot size. This is measured in linear distance or in terms of volume. The shot size can be calculated based on the total shot weight and the melt density of the plastic. However, since melt densities are temperature dependent, and it is difficult to estimate the exact temperature, shot size calculations are usually estimations. A correction has to be also provided for a cushion value during holding.
  12. Changeover Position
    The point of transfer from the injection phase to the next phase is called the changeover position. This transfer can also be done via time, hydraulic pressure, or by external means.
  13. Decompression or Suck Back Position
    Because the shot is built up in the presence of back pressure, the plastic that is in front of the screw is compressed and is under pressure. At this time there is an injected shot in the mold. As soon as the mold opens and the molded sprue moves back with the mold, the melt pressure is released. This can cause the plastic to flow out of the machine nozzle. For this reason, at the end of the screw rotation, the screw is pulled back in a linear movement and the pressure is released to decompress the melt. This distance through which the screw is pulled back is called decompression or suck back. These distances are usually very small (less than 1 cm) and can affect the cosmetics of the part.

The molding parameters above are machine inputs. The following are the outputs from the machine, which are a result of the settings of the molding parameters as available on the molding machine screens.

  1. Fill Time
    This is also known as the injection time and is the time the screw takes to move from the shot size to the transfer position. This is also the duration of the fill phase.
  2. Pressure at Changeover
    This is the actual pressure that is required to reach the changeover position.
  3. Peak Pressure
    This is the maximum actual pressure that is reached during the injection phase. This may or may not be equal to the pressure at transfer and depends on the mold and part design.
  4. Cushion Value
    The position of the screw at the end of the holding phase is the cushion value. It is the buffer that is required for the shrinkage compensation. If the cushion value is zero, then there is no pressure being applied to the melt, therefore, the melt is not being injected/filled to the optimum value. In other words, the shrinkage is not being compensated. Cushion values should never be zero.
  5. Re-filling Time
    This is the length of time the screw rotates to pick up the required material for the next shot.
  6. Cycle Time
    This is the time it takes for one complete molding cycle. It can also be described as the time to mold one shot.

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