The bodyweight of the bottle was first reduced, with special bottle designs compensating for the reduced stiffness of the container. Other approaches ignored this loss of stability altogether, despite the fact that these kinds of bottles result in cheap products. A second lightweighting development phase optimised the design of the neck. CSD (carbonated soft drinks) products first deployed necks (5.8 g) from the BPF (British Plastic Federation), allowing the same caps to be used for glass and plastics, followed by PCO (plastic closure only) 1810 (5.0 g), and later substituted for lighter option PCO 1881 (3.8 g and less). Necks for non-carbonated contents followed suit.
Today’s supermarket shelves are lined with bottles of all qualities. While the lightest bottles are often seen in the low price beverage segment, the high price segment shows strong, well-designed bottles.
However, there is the potential to reduce the quantity of “wasted” raw PET material in both “lightweight” and “heavyweight” bottles without affecting the handling of the bottle. “Wasted” material refers to the material that can be removed from the body without impairing the stiffness of the bottle.
If we halved empty bottles from the neck to the gate, we would see that the bottle base contains a significant amount of raw material that serves no purpose to the application at hand. It is clear that the greater the volume of the container, the higher the potential savings of raw materials.
The following questions still need to be addressed, however: How can we stop material from accumulating in the bottle base? Which options are there for the preform design? Which options are there within and in addition to the injection moulding process? How does removing this material affect the stability of the blowing process and the base and bottle stability itself?
Saving raw material in the bottle base – a new approach
The end product – the PET bottle – is at the centre of all developments that focus on avoiding material that is not absolutely necessary. Several converters have already recognised the potential to reduce material in bottles, but have yet to find a sustainable solution because of the demanding blowing process and high bottle specification requirements. The only way to remove PET material from the centre of the bottle base is by combining an optimal preform design for the bottle at hand with an optimal blowing process. This optimal combination of preform and bottle, however, does not allow any flexibility but requires a narrow process window consisting of a bottle blowing process that is stretched to its limits, which is not recommended under real production conditions.
Since it is not possible to significantly modify the blow moulding process to avoid wasteful use of material in bottle bases, efforts focussed more and more on the preform base design. From the very beginning, it has not been possible to produce preforms with a preform base wall thickness of less than 80% of the wall thickness of the body. Many tests have shown that a thinner wall thickness in the base freezes too fast to maintain the holding pressure for a reasonable time. This means that sensitive areas would shrink too much to deliver the expected quality. Mould makers have generally accepted this fact, but have never stopped working to reduce material in bottle bases.
In 2006, Concordia Developments, driven by Gianfilipo Pagliacci, started with the Cappello Design for the preform base. Husky followed with a similar approach: the EcoBase solution. MHT AG in Germany offered a solution called savebase. In 2009, Netstal Maschinen AG came up with PreBlow, and Mint-Tec was introduced by Creative Packaging Solutions (CPS) in 2012 (see article on page 40).
The aforementioned questions are still highly relevant.
The different solutions seen in the market today will be compared and analysed in the next issue of comPETence:
Part 2: Saving material in bottle bases – preform solutions offered by the industry today