In addition, the preform starts to become oval due to any resoftening. With greater wall thicknesses and ever shorter cycle times, the danger therefore arises that the residual heat management in the preform body will increasingly result in ovalisation.
This means an injection mould with the best cooling properties no longer determines the cycle time alone. On the contrary, the intensive, uninterrupted post-cooling is more indispensable and important than ever.
Netstal has analysed this problem and developed the Calitec system, which it has registered for a patent. Thanks to this innovation, the inside of the preform is subjected to slight pressure during the post-cooling phase with compressed air. This property prevents the moulded part from shrinking away, as it is uniformly pressed onto the cooling sleeve. In addition, cooling is not interrupted and the preform body is simultaneously subjected to a shaping calibration.
The fact that the entire circumference of the preform now constantly contacts the cooling sleeve results in considerably more uniform and intensive cooling – and that with ideal shaping!
The required inside pressure is generated with a hollow-drilled special gripper. This is not only responsible for gripping the preform, it also closes off the preform gently yet securely. Then the gripper routes the low-level compressed air into the preform. This outstanding feature enables the calibration, while intensive cooling is also provided by the now increased contact between the preform and the cooling sleeve. In addition, the calibration pressure slowly increases during the cooling phase in order to counteract increasing shrinking of the plastic.
A continuous endurance test of the Calitec system has been conducted since September 2007. It indicates the sustainability of this new technology. Until now the field test customer has been able to reliably increase its productivity by over 10% – and that while simultaneously improving the preform geometry. In addition, the rubber gripper parts used have exceeded all expectations with regard to their service life. They were replaced after approximately six months of continuous operation as a preventative measure. However, from a mechanical standpoint no complications were found in the entire gripper system.
The injection mould and the machine time are not always the sole deciding factors for reducing the cycle time. On the contrary, post-cooling is more important than ever. For example, uninterrupted cooling on the outer skin of the preform is absolutely necessary. Caution is required in the first cooling phase, in which resoftening of the preform must be avoided. The preform must not be allowed to shrink freely during this process. Thanks to Calitec, this is successfully avoided through specific calibration with compressed air, which replaces the core.
With the development of Calitec, Netstal has optimally succeeded in achieving a major reduction of the cycle time in the preform manufacturing process. And the quality level of the preform is also increased, which of course results in a decisive advantage for customers. Calitec can be ordered as an option on new systems, or can of course be retrofitted on existing PET systems.
Based on Netstal’s informations.
