>> Sunday, 27 February 2011
The purpose of the Secondary Blow air is to make the final shape of the container with all details. The Secondary Blow air should NOT be controlling or influencing the inflation of the preform.
As with the Primary Blow Delay timer, there is no "correct" setting for Secondary Blow Delay. The optimum setting will depend on many other factors upstream of the stretch blow process and will include the setting of the Primary Blow Delay. Therefore, adjusting the Secondary Blow Delay time should be the last processing adjustment to be made when setting up a machine.
As a general rule, the Secondary Blow Delay timer should be set as short as possible to get the best definition in the container, but without upsetting the blow-up of the preform. Having the Secondary Blow Delay set too short is likely to cause off center gates and neck rings.
If there is too much delay between the introduction of the primary air and the secondary air, it may cause pearlescence in the corners of the container.
Before setting up the Secondary Blow Delay time, the Primary Blow Delay setting should be finalized. Then, adjust the Secondary Blow Delay to the earliest possible setting without upsetting the blow-up of the preform.
After the Primary Blow Delay time has been set to give the best blown bottle using Primary Air only, the machine should be re-started.
Reduce the Secondary Blow Delay setting from its long setting to a setting of about 0.5~0.8 seconds.
While blowing bottles, decrease the Secondary Blow Delay setting in steps , this should be about 0.1 second per step. Collect a sample container at each step, mark it and place it on a table. At first, there will appear to be no or very little difference in the containers but on close inspection you should be able to see improvements in the definition of ribs, logos and corners. At some point, the container quality will go bad, typically this will be an off center gate or a neck ring. There is no point in going any further once this happens and it is possible that a preform will split leaving fragments of PET in the mold if you continue.
Once the quality has turned bad, slightly increase the Secondary Blow Delay setting a little until the container quality comes good again. This is the optimum setting for the Secondary Blow Delay timer.