Hot Runners: System Targets Dust Reduction in PET Preform Molds

Darmstadt University of Applied Sciences in Germany and Mold specialist MHT conducted a publicly funded research project to determine how hot runner design affects preform quality. Specifically they examined thermal degradation and decomposition of PET material in the mold, which can lead to dust that impedes valve needle movement and must be suctioned out. The research project’s goal was to prevent the dust from developing at all.

The researchers said that art of the issue is many manufacturers combine an existing hot half, with a mold pitch of 50 by 140 mm with matching cold sides of different contour. The result is the same hot runner must supply preforms ranging in weight from 6 to 40g and lengths of 30 to 150 mm.

For the project, MHT built a 4-cavity test mold for 12.5g preforms that simulated the conditions found in a 96-cavity tool by using hot runners connected in series. The mold featured pressure and temperature sensors to determine the viscosity of the melt at various points. The mold was run on a KraussMaffei 16-540X injection molding machine outfitted with an online rheometer nozzle to record viscosity during injection.

The researchers sought hot spots in the test mold and simulations of a 96-cavity tool, and they also looked at straight and curved melt channels of different diameters to see what effect channel geometry had on different polymers. Nine channel models were use with 180, 90, and 45 degree angles and diameters of 4, 6, and 8 mm.

The researchers found that dust formation on the 4-cavity experimental mold varied greatly depending on the grade of the material. They determined that the dust was made from oligomers that escaped as a gas from the PET and then condensed on the mold surface.

Ultimately the researchers developed an equation that could calculate whether a disruptive amount of dust would be produced during molding based on the PET material grade, preform weight, maximum residence time and hot runner volume .

Using this, the team designed a new hot runner that aims to transport PET melt rapidly and gently. This was installed in a 96-cavity prototype mold with a  50-by-140-mm pitch that was tested at an MHT customer for one year. The test revealed that dust formation was greatly reduced even though the preforms were made from a material mix featuring 70% recycled resin.

The new two-plate hot runner is undergoing further development and is scheduled to launch at K 2019. It features several standardized elements that are inserted in the manifold block. Melt is fed centrally and the melt runners are naturally balanced, according to MHT. To preload the system, springs are installed at the point where the melt is transferred between the cross manifold and two sprue bushings of the hot runner manifold. This results in a fully sealed and leak-free system, hot or cold. An adapter plate is used to support different component in the piston housings.