MoldMaking Technology

AUG 2018

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42 MoldMaking Technology —— AUGUST 2018 MAINTENANCE MATTERS Manifolds: To PM or Not to PM, Part 2 By Steve Johnson The June issue's article on manifolds and preventive mainte- nance (PM) presented opinions on the frequency of hot runner manifold disassembly, cleaning and repair from the manifold manufacturer, a manifold cleaning and repairing company and the molders running these systems. "Environmental factors" were mentioned in the article, as each party noted difficulties with identifying and controlling critical factors, their influence on manifold performance and the resultant costs when they failed to identify and control those factors in an accurate and timely manner. Understanding environmental factors and their potential impact on performance is key to developing accurate and timely PM work instructions, which technicians perform at optimal, scheduled times instead of waiting until the mani- fold starts causing issues or exhibits diminished performance. Diminished performance is a reduction in cavitation that occurs because the cavities freeze off, or they are lost to part quality, heater or thermocouple issues, seal failure or other mechanical problems. The goal of PM is to maintain the hot runner system in a way that is cost-effective while ensuring that it runs consis- tently and reliably, which maximizes performance (in terms of the part quality, cavitation and cycle time) and manifold com- ponent life (in terms of the heaters, thermocouples, valve pins, nozzles and tips). The difficulty with achieving this goal is in a molder's understanding of manifold characteristics and the impact of environmental factors. Facing the Factors Here are three common environmental factors that a molder must understand, monitor and control when running and maintaining a hot runner system. This information will pro- vide the necessary data so that molders can develop accurate and timely PM plans. Resin and Processing Issues Degraded plastic and contamination in the melt stream is the main culprit of a total manifold teardown. The result is black or brown specs or streaks in the molded part, which requires a hot aluminum-oxide sand bath (such as those that Procedyne offers) to remove the residue from the manifold channels com- pletely. The manifold must be removed from the mold frame and completely stripped of components before being inserted into the sand bath. Heat-sensitive resins, incorrect shot size (or residence time), color changes, improper start-up or shut- down procedures cause many contamination issues. Unfortunately, even the necessary in-house equipment and tools will not guarantee success without better troubleshooting for the root cause. Some resins are simply dif- ficult to process, which can cause problems that require the expertise of a resin supplier. For example, a medical company ran an ABS with a white-colorant titanium dioxide (TiO₂) that stuck to the manifold's channel walls and required gun drilling because a hot sand bath would not remove all of the residue. On top of that, the sticking did not occur after a specific num- ber of cycles. It ranged between 350,000 and 2,000,000 cycles, making a solid PM frequency difficult. The company's solu- tion was to buy a second manifold, so operators could quickly swap out the failed one as necessary. This strategy helped solve some production issues, but scheduling would still back up. The company believed that incorrect TiO₂ or colorant levels caused the inconsistency, so the company began comparing the production data (press, run dates and cycles) with the resin lots. This data made it possible to have more meaningful con- versations with the resin supplier. Abrasive and corrosive resins will wear out nozzles, valve tips, valve bushings and gate inserts. As this tooling wears, residue invades piston cups and manifold bushings. As nozzle tips wear, processors are forced to crank up the nozzle tem- peratures to compensate, which further compounds flash and burned material. Operators must perform color changes according to manu- facturer instructions and use maintenance manuals for any required tools or fixtures. Technicians should use nozzle-tip A piston housing installed with too much force created this steel burr and sheared the o-ring. Also, many of the small air passages that actuate the piston or the valve-pin assembly clogged, causing the valve pin to lose pressure and operate sluggishly. The result was a long gate on this part. Images courtesy of MoldTrax.

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