Calitec® - intelligent cycle time reduction

The cycle time reduction is without a doubt one of the most targeted productivity goals since the invention of the injection moulding machine. The reason for this is simple: which processor wouldn't like to utilise its investment-intensive production goods as efficiently as possible? The efforts for reducing cycle times are especially prominent on production systems used for mass production, for example in the packaging sector. These packaging products also include the PET bottle, which is produced from the pre-product, i.e. the preform. Apart from Netstal-Maschinen AG, few manufacturers have succeeded in successfully realising production machines for manufacturing these preforms up until now.

Initially, shorter cycles times were mainly achieved for preform production with faster machine movements. These include rapid robot movements for preform removal and impressive speeds during the clamping process, which by the way are possible despite extremely high mould weights thanks to toggle lever technology. When combined, these two achievements result in lock-to-lock times which specifically strive for the two-second mark. However, from an economical standpoint, the physical limits have already been reached. In addition, it was also possible to reduce the cycle time by shortening the injection time and optimising mould cooling using improved hot runner technology. Parallel to this development, it was naturally also necessary to focus on an increase in the plasticising performance with maximum, homogeneous melt quality.

 

Insufficient cycle times often lead to reheating of the injection moulded part

It is entirely possible for an optimum, intensive cooling of the injection moulded part to result in the generation of additional reductions in the cycle time. However, there are also certain limits here depending on the wall thickness of the preform. For example, with produced wall thicknesses from approx. 2.5 mm, the duration of sufficient cooling is extremely important. For even if the outer skin of the plastic solidifies through proper cooling so that the preform can be demoulded without danger, from this wall thickness the remaining thermal energy is high enough to present a danger of reheating. In the case of a short cooling time, injection moulded parts can therefore result, which soften again after a short time at the points with greater wall thicknesses, and as a result also have a higher thermal energy.

 

Intensive, shaping post-cooling "“ the solution for an additional reduction of cycle times

As already indicated, a preform demoulded after an extremely short cooling time still has a high share of thermal energy. This is particularly the case in areas with greater wall thicknesses - generally in the shaft area. If the intensive cooling of the outer skin is now interrupted, there is a risk of these sections softening again due to the residual heat.

The preform is guided into a cooling sleeve following mould removal for post-cooling, and therefore cools around the entire circumference. However, as the preform shrinks away from the walls of the cooling sleeve and is no longer supported by the mould core, it looses cooling contact to the cooling sleeve around its entire circumference with the exception of the contact surface.

Due to this one-sided separation of the preform from the cooling walls, cooling is interrupted around the circumference. This makes it impossible to obtain a radially uniform thermal image. And although the moulded part is in the first cooling sleeve for less than one cycle, it is marked with a heat profile within this short time which cannot be compensated. 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 indispensible 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.

 

Conclusion

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 (from DSP 4).

 

 

» Download: Foto: PET-LINE
» Download: Foto: Calibration gripper / Kalibriergreifer
» Download: Foto: Field test customer / Feldtestkunde