Build More Flexibility Into Your PET Preform Molds

High-output PET preform molding systems are very efficient for manufacturing large numbers of identical parts. When more than one preform is required to make different bottles, it may seem that multiple lower-output molding systems and multiple molds are required. But the combined advantages of high-output systems and the liberty to change production from one preform to another can be achieved in some cases by careful planning of mold-change parts, together with the required modifications of the preform designs. The same upfront planning can cut costs and improve results when converting a mold.

Two molds for quite different preforms can share interchangeable parts. For example, changes in the weight of the preform to make different sizes of similarly shaped bottles can be accomplished with a new set of cores, if the original preform design is suitable. Moreover, different neck finishes can be produced with interchangeable neck rings, allowing you to vary the thread finish. This is assuming the preform and mold design permit you to choose the placement of the split line.

But adding modularity should not be an afterthought. Features to permit varying the dimensions of the preform and changing the mold cores, cavities, neck rings and gate inserts should be considered with new tooling acquisitions rather than after the mold has been built.

Plan for flexibility

Designing a mold with interchangeable parts to produce a series of preforms of different designs offers more opportunity to achieve good preform performance and cost-effectiveness than does trying to change the design of an existing perform as an afterthought. Preform design changes are possible in a few different ways. A second set of interchangeable cores is often manufactured to allow a lighter preform to be molded. This approach costs much less than building a second mold. But it may be more appropriate for one-time mold conversions than for flexible molds whose cores will be changed back and forth regularly to produce different preform weights. In the latter case, a changeable core-plate assembly is a much faster way of converting the mold than changing individual cores. This option is also safer and it minimizes the danger of damaging delicate mold components.

In many cases, better results can be achieved by changing the outside dimensions of the preform cavity rather than those of the inside core. A core change would result in the lighter preform having a larger average diameter, whereas a cavity change makes the lighter preform smaller in average diameter. Typical 2-liter carbonated soft-drink (CSD) preforms often cannot be made significantly lighter with a core change at all, because the core is already as large as it can be for the inside dimensions of the thread finish (fig. 1).

Changing cavities and gate inserts (or a cavity-plate assembly) may be the preferred way to change part weight on a high-output molding system. On a conventional system, changing the outside dimensions of the part would also necessitate changing all the take-out tubes in the parts-handling equipment.

Even mixing and matching cores, cavities, or other mold parts to produce more combinations of preform shapes and weights may be possible in some circumstances. But don't neglect to consider this option before finalizing the design of the first preform, if flexibility may be required in the future.

In principle, a second set of neck rings can be used to change from one thread finish to another one, as long as the major dimensions are comparable. For instance, a 28-mm Alcoa finish can be changed to a 28-mm PCO finish with a slight adjustment in the transition region of the preform. Accomplishing this, however, depends on the preform design (especially the inside diameter), the mold design, and the location of parting lines.

Anticipating a neck-finish change (either during the current project or as a future change) will ensure that the preform design is suitably flexible and that the parting line is placed in the appropriate location for all the neck finishes to be used. In many cases, the preferred parting-line location for a given neck finish might be at the support ledge, which would make it impossible to change the neck finish in the future unless the height and support-ledge diameter match (fig. 2). Two molds producing significantly different preforms (for instance, a 21-g preform for a 1-liter water bottle and 40-g preform for 1.5L) may even be able to share sets of neck rings to make different finishes--so long as both preforms and both molds are designed at the same time. A production preform mold with three different sets of neck rings has been designed and produced with this approach.

As with core changes, neck-ring changes are faster and safer if they are part of an entire multi-cavity assembly--in this case a stripper plate that includes the slides that carry the neck rings. The savings in switch-over time and reduced component damage typically makes up for the additional cost of the assembly.

Mold/machine modifications

To achieve this kind of flexibility in preform mold tooling, it must be one of the objectives in the original part-design process. Sometimes existing preform designs need only a minor modification to enable the mold to be constructed with the appropriate change parts. Usually, however, changes in preform length, diameters, and wall thickness must be possible in order to provide true flexibility. Such changes will have a significant impact on the injection and blow molding processes, and on the bottle properties.

Let us consider, for example, a 52-g and a 48-g preform (for 2L and 1.5L soft-drink bottles, respectively) made in the same mold with no more than a core change. It is certainly possible to design the two preforms such that only the inside dimensions change, and thus it is possible to construct the mold with interchangeable sets of cores to mold the two preforms. However, the preforms will not be typical of optimized preforms designed for the two bottle applications. For example, the wall thickness of the heavier part might be quite high, since the core diameter must be small enough to permit 4 g to be removed with only a core change. The injection molding cycle time therefore might be significantly longer than anticipated. This could have such an impact on the output-to-capital ratio that the added cost of separate tooling would be justified.

Keep in mind that the capabilities of the blowing equipment affect whether or not a given preform will make a suitable bottle. In our example above, the heavier preform, with its smaller inside diameter, must be stretched further than usual when blown into the larger bottle. It might be difficult to find a robust blow molding process that consistently makes good quality bottles. And the lighter preform will be larger in diameter than is usual for the bottle dimensions. This results in lower stretch ratios, less orientation, and most likely poorer creep properties. When filled with a carbonated soft drink, the bottle may gradually expand, thus failing to meet the package requirements.

Concurrent prototyping of the preform and manufacturing of the production mold can greatly reduce the amount of time it takes to get a new product to market. A production mold can be released from engineering at the same time as the initial preform design enters the prototyping phase. By leaving extra metal on the molding surfaces of the production tool, changes can be made if determined necessary during prototyping.

And in the case of an existing mold, conversion costs can be reduced by designing the preforms to utilize some of the existing mold components.

Reinhart Dravetz is product manager at the Bottle Development Center (BDC) of Husky Injection Molding Systems Ltd. in Bolton, Ont. Laura Martin is a product specialist at the BDC. Husky offers preform and bottle prototyping services, as well as new preform designs and evaluation.