Injection Molding

  • Plasticization at about 280°C
  • Injection temperature at about 280°C
  • Shot and injection cushion as minimum as possible.
  • Screw speed, transfer speed, and machine cycle time as minimum as possible.
  • Back pressure as minimum as possible

By keeping all the above at minimum values and having a good plasticized resin, it is possible to achieve low IV drop and low acetaldehyde generation in the produced preform. Especially for acetaldehyde, its formation rate depends on the melt temperature and the length of the time when PET stays molten. Therefore, the whole process must be set to run as fast as possible without forgetting to minimize the screw “dead time” , which has to be always less than 2 sec, and by injecting the molten PET into the mold at a slower rate to help reduce shear heating.

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    Figure 3

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    Figure 4

Mold Cooling

The limiting factor on cycle time for most injection molding processes is the cooling time in the mold. The target must be to always keep the mold at low temperature by minimizing the mineral deposits that usually occur in the cooling channels if the chilled water is not treated properly. Ambient moisture should not be allowed to accumulate on the mold because it can cause preform defects, which will not become apparent until the blowing process takes place. However, this can be avoided by employing dehumidified air in an enclosed mold.

  • Chilled water temperature at about 7-9°C
  • Total cooling time (hold time + mold cooling time ) should be enough to freeze polymer quickly from 280°C to 75°C
  • The less time the polymer remains in the range between 280 -75°C the more transparent and clearer (not hazy) it will become.

Robot Cooling

  • Robot plate sucks the preforms which are retracted by an ejector stripper just after the mold opening.
  • The robot tubes are cooled off using chilled water in order to decrease the preform temperature below 30°C so as to prevent them from sticking to each other when they fall down to the conveyor or cardboard box.