Keeping the injection velocity low will reduce the shear that occurs in the material. Shear is a major factor affecting overheating of the material and I.V. reduction, therefore reducing velocity will protect the PET resin from excessive damage.
When working with machines (hot preform method), the injection velocity will also make a significant difference to the material distribution in the finished container. Filling slower means that the preform will be hotter when the mold opens and its temperature balance will also have changed. Typically, the shoulder area will become relatively hotter than the base area giving more stretch at the top of the preform.
Excessive injection velocity can also disturb the alignment of the injection core, especially if the design is long and thin.
Reducing the injection velocity will also have the effect of making the holding time shorter since the V/P time will increase.
For machines fitted with electronic injection control, reduce the velocity percentage value on the injection screen of the electronic injection control system. Minimum usable value is typically around 15~17% but beware of making a short shot at very low settings.
For machines without electronic injection control, reduce the setting of the valve found on the operator side of the injection unit. Beware of making a short shot at very low settings.
In most cases, the five steps of injection control can be set to the same value. Different values may be advantageous in cases of complicated preform design or technically difficult bottles.
Reducing the injection velocity too far may cause other preform defects such as specks of crystal in the gate area.
Raising the injection velocity will reduce the time taken to fill the cavity and it is therefore possible to achieve faster cooling of the preform. However, it will also increase shear in the material. Shear is a major factor affecting overheating of the material, A.A. Generation and I.V. reduction, therefore increasing velocity will damage the PET resin.
When working with machines (hot preform method), the injection velocity will also make a significant difference to the material distribution in the finished container. Filling faster means that the preform will be colder when the mold opens and its temperature balance will also have changed. Typically, the shoulder area will become relatively cooler than the base area giving less stretch at the top of the preform.
For machines fitted with electronic injection control, increase the velocity percentage value on the injection screen of the electronic injection control system. Maximum allowable setting is 99%.
For machines without electronic injection control, increase the setting of the valve found on the operator side of the injection unit.
In most cases, the five steps of injection control can be set to the same value. Different values may be advantageous in cases of complicated preform design or technically difficult bottles.
Increasing the injection velocity too far may cause other preform defects such as lowered I.V., increased A.A., silver streaks and internal sink marks.