John DeGaspari, senior correspondent at Plastics Machinery Magazine
Husky Injection Molding Systems Ltd., Bolton, Ontario, has introduced a self-cleaning mold system for manufacturers of PET preforms. The company says that the technology, which is an option on Husky's HyPET HPP5 preform molding system, can eliminate hundreds of hours of maintenance per mold per year.
Husky's technology is designed to minimize the need for traditional methods of mold cleaning, which are labor-intensive and time-consuming. Although effective, traditional mold-cleaning processes can involve at least four hours of downtime on a 96-cavity mold.
One traditional process, hand-cleaning, requires a skilled technician to remove, clean, dry and replace mold components, and requires solvents and brushes. Its alternative, dry-ice cleaning, is less labor-intensive, allowing the mold to be cleaned while still in the press. It introduces carbon dioxide pellets into an air stream at high velocity to remove residue. While it does not produce waste, it is expensive, Husky says.
The flash on these PET preforms is intentional, part of Husky Molding Systems’ self-cleaning method for molds.
Using flash as a cleaning mechanism
Husky says that its technology has advantages over both manual processes. It uses what Husky calls controlled flash technology to remove impurities. Controlled flash is the intentional creation of flash in the parts of the tool where dust and debris accumulate.
Roman Pirog, Husky's director of development engineering, explains that the technology is directed at the neck ring area of the PET preform, which is the main area of dust accumulation in the mold cavity. A mold that is equipped with the self-cleaning system can run in either production mode or cleaning mode, which is triggered by the machine operator with the touch of a button on the control panel.
When the cleaning cycle is initiated, the production cycle is paused, allowing actuators in the mold to position the cavity in the tool for the cleaning cycle. Pirog explains that the vents, which allow air to be displaced by the resin during the injection cycle, are in the neck ring of the PET preform. During the normal cycle, the neck rings are closed to prevent flash from forming. During the self-cleaning cycle, the neck rings are partially opened, allowing resin to enter the vent area to clean out impurities.
Precisely controlled gaps ensure that the flash does not travel beyond the target area. During the cleaning cycle, the vents are filled with an overflow of PET, which sticks to the dust in the vents. The preforms, with the dust, are then ejected into a reject tray. After the parts are ejected, the cleaning mechanism is de-activated, allowing the mold to close normally and for the production cycle to resume. The cleaning process takes 10 to 15 seconds.
The neck ring: taking a proactive approach
Pirog says that cleaning the vents in the neck ring area is the first line of defense against dust accumulation. "The vents are the dominant means for air to escape the mold cavity during filling," says Pirog. "Any gases or vapors created by resin in the cavity during the injection process will also exit through these vents. As the vents are cooled and they are the first point of contact for the exiting gas, they become a collection point for condensing residue from the outgoing air." After just one cycle, the effect is imperceptible, but it becomes obvious after tens of thousands of cycles. Variables that contribute to the rate of dust adhesion include resin type, additives and fillers, resin temperature, preform geometries and neck finishes.
"That's why this technology focuses directly on those vents, the first area of accumulation, and does proactive cleaning of this target area before it gets to a blocking or a clogging state," he says. "The approach here is to get ahead of the game, allowing us to avoid more time-consuming cleaning."
Although it may take hundreds of thousands of cycles to accumulate debris, periodic maintenance is not something that should be ignored, says Pirog. In fact, he adds, cleaning becomes even more important as molding presses have become capable of operating with faster cycle times.
Husky’s HyPET HPP5 unit for PET preform production now has a self-cleaning function that trims maintenance times.
Typically, one cleaning cycle is enough to clean the vents. Pirog says one advantage the self-cleaning process has over manual cleaning methods is that it is able to effectively clean all of the cavities uniformly, which is particularly important with a high-cavitation mold.
Nevertheless, while required less frequently, the need for manual cleaning is not eliminated entirely. The self-cleaning technology focuses on the neck area, but not all areas of the mold where dust can accumulate are cleaned.
To date, Husky has received more than a dozen orders for the self-cleaning mold technology from customers around the world. Husky cites the case of one molder that needed to manually clean vents every 50,000 cycles to avoid defects such as top sealing surface shorts, flow lines on the thread finish and shorts in the leading thread. Since implementing the self-cleaning technology, the customer has reached 570,000 cycles and still has no need for dry-ice cleaning. The customer estimates that it has saved 320 hours per year of maintenance downtime.
So far, Pirog says, the feature is only available, at additional cost, on new Hypet HPP5 PET preform tooling. Pirog says retrofits on existing molds could be a possibility in the future.
One of the advantages of the self-cleaning mold system is that it can be initiated on the fly by the machine operator with the touch of a button on the HyPET HPP5 control panel screen. At present, the decision to start self-cleaning is up to the machine operator, based on scheduled mold maintenance or post-process inspection of the parts. "Cleaning cycles are specific to the part itself," Pirog says. "Most molding customers have direct schedules for maintenance, so we have enabled it for the operator." However, Pirog says future upgrades are planned. "Automation is the next obvious next step in terms of scheduling," he says.
