PETWORK
01/2017

XTREME: ONE GIANT STEP CLOSER TO THE PERFECT PREFORM

SIPA’s XTREME injection-compression molding technology is like no other in its ability to make high performance thin walled PET bottle preforms. What’s more, it does so in a highly energy-efficient way, and just as quickly as traditional injection molding technology. This is quite remarkable, given that XTREME makes each preform one at a time!
XTREME technology uses a carousel system in which individual molds are fed by a continuously running extruder. The carousel rotates at high speed, with the opening and closing movements of the molds driven by cams.

A typical XTREME preform for a 0.5-L bottle is immediately distinguishable from its injection molded counterpart, since it is noticeably longer. What is not so obvious is the difference in wall thickness. The base of the XTREME preform is 25% thinner than its rival: 1.2 mm versus 1.6 mm. Extra length and lower thickness together provide for a more optimal stretch ratio when the preform is made into a bottle. In fact, L/T ratios achievable with XTREME technology are indisputably the highest in the industry.

A typical preform mold operating on a 300-tonne injection molding machine has 96 cavities. Today, a typical cycle time is around six seconds. To get all those cavities to fill in such a short time, a massive amount of pressure needs to be used – something like 1500 atmospheres when measured at the machine nozzle. That puts an almost incredible amount of stress on the polymer. By contrast, XTREME technology running with the same output requires a clamp force on each mold of just two tonnes, and the melt pressure is only 270 atmospheres – less than 20% as much.

The main reason why it is possible to fill the molds using XTREME technology using such low pressure is the moving –or floating – core. With injection molding, when melt is injected, the molds are already closed and the core position is fixed. So if you want a preform with a base thickness of 1.6 mm, all the melt has to pass through a passage 1.6 mm thick before it can fill the cavity (since, of course, the injection point for the preform is at the base). With XTREME technology, when the melt is injected, the cavity is slightly open and the melt passage is therefore far wider, at 10 mm. Then the core moves into its final position, forcing the melt up the cavity walls.

The net effect is a much more gentle treatment of the melt. All this means that acetaldehyde (AA) levels are reduced by up to 40%, and resin intrinsic viscosity (IV) falls far less: over 70% less in fact. Obviously, this improves the mechanical properties of the bottle.
Improved distribution of material around the base of the preform also enables improved cooling, with no crystallization in the gate area. Furthermore, there is no sign of the “crown” gate typical of injection molded preforms.

Not only do preforms made with XTREME technology have optimal L/T ratios, they also have optimal designs overall, with a much better distribution of material throughout the part than can be achieved with injection molding. So while the base is much thinner, the lower sections of the walls are actually thicker; not by much, just 0.04 mm, but this does have another important influence on the mechanical properties of the finished bottle. In addition, XTREME preforms have thinner walls just below the support ring, where in injection molded versions there is an excess of material. As much as 0.6 g can be shaved off the weight of the preform in this area alone.