I received this blog topic request yesterday…
Concerning injection molding, is there a specialized screw used for PET?
Yes, the goal in PET processing is to (1) maximize output, (2) control shear heat, and (3) minimize melt tempertaure… yet ensure the polymer is fully melted and as homogeneous as possible.
(1) The need to maximize output is due to the fact that most PET injection molding processes are for high-cavitation high-speed molding PET preform machines.
(2) The need to control shear heat is due to the fact that a higher temperature polymer requires more time to cool… and may increase the development of semi-crystalline regions. Also, inconsistent melt temperatures will result in inconsistent cooling and crystallinity… resulting in profit losses due to scrap and troubleshooting.
(3) PET also has very poor thermal conductivity… making it great for drink containers, but terrible for part cooling. The goal in processing PET is to process the material as close to the melting point as possible, while ensuring all the pellets are melted and mixed.
To address all these concerns, different machine manufacturers have devised some great machine and screw designs.
Whether it is a reciprocating screw machine, or a two stage ‘shot-pot’ machine, most PET single screw designs incorporate a barrier screw. In such a design, the screw channel depth remains the same from the feed into the transition zone, but the channel width becomes progressively smaller. As this channel width decreases… an adjacent shallow channel, separated by a barrier flight begins. Ultimately, the melted material flows over the barrier flight into the shallow channel until it transitions into the metering zone. Such a design reduces shear heating and allows stingy pellets more time to melt.
In most single screw designs, a distributive mixing element is typically incorporated on the screw or a static mixer is placed within the nozzle to ensure a more homogeneous melt… especially if a colorant is added to the polymer.
Some of the more ambitious machine manufacturers are working with twin screw designs for two stage molding machines. These systems show great promise since a good twin screw design would provide the best control over both shear and melt temperature.
PET processing is one of the more advanced and refined fields of injection molding, making it very competitive. I strongly suggest anyone getting into PET molding do some significant research to ensure your strategy is well suited to take advantage of today’s advances.