Plastics Technology

OCT 2018

Plastics Technology - Dedicated to improving Plastics Processing.

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forward during injec- tion (Fig. 2). They perform the same function, except that the sliding ring does not rotate and the locking ring does rotate during plasti- cation. The locking- ring style appears to dominate the market. My question is, why? My view is that most molders believe that locking rings make a better seal, leak less and last longer. But where is the data? What is the cost? The locking-ring valve shown in Fig. 3 has been in use about 2000 hr. Note the scoring and scratches both circumferentially due to rotation and horizontally. Imagine what the inside of the barrel must look like. Is there any doubt that the barrel needs to be replaced too? What is only partially shown is the carbon buildup on the flights of the screw. Three flights of the metering section were nearly full of carbon and glass fibers. In this case, I contend that the locking teeth are partially blocking the forward flow of plastic. In an article I wrote for the August 2017 issue of Plastics Technology ("Time for Another Look at Non-Return Valves"), I voiced my concerns, and Fig. 3 provides some evidence those concerns are real. Can you find sliding-ring valves this scarred? Probably, but after far more machine time and great abuse. Do sliding-type rings wear? Yes of course, but they are far less likely to cause barrel damage. As for better sealing, the industry needs to evaluate stepped-angle and radiused seats. While both are better than standard near-mating-angle valves, one study I have seen shows an 85% improvement for radiused seats over near-mating-angle valves. Bottom line: I am not a fan of locking-ring check valves and suggest you stay away from them. Granted, we need some head-to-head comparisons with solid data to provide the facts. However, Fig. 3 speaks volumes. How much will it cost to replace the barrel? FIG 3 Used Locking Ring These can include: 1. Unmelted or partially melted pellets blocking proper mating of ring to the seat. 2. Contamination, such as bits of metal and other solids, blocking proper seating or mating of the valve. 3. Valves with large mating areas to their seats. Good seals are not made across a large surface area; small contact areas are superior. 4. Valve wear. This is a high-wear component and wear is not easily evaluated without pulling the screw and tip. 5. Viscosity of the polymer. 6. Diameter of the barrel or size of the screw. How well the check ring functions to prevent backflow deter-mines the consistency of shot size and cushion. Since there are two definitions for cushion, here it is defined as the position of the screw at the end of second stage or pack-and-hold time. How well the non-return valve functions plays a major role in producing consistent parts. Ideally you want to inject the same amount of plastic into every shot. There are quite a few machines in plants today—for making small parts in particular—that have no check valves at all. These are two-stage presses where material melted in a small barrel with a fixed screw is fed to a plunger or shot pot, where a precisely fitted piston acts as a plunger that injects the plastic into the mold. There is one drawback: The nozzle is longer and has a larger pressure loss. If we understand the mechanics of the check valve, we can now evaluate the sliding- vs. locking-ring issue. With a sliding-ring check valve, the ring does not rotate with the screw during plas- tication or screw recovery. It only slides backward during screw rotation and forward during injection (Fig. 1). With the locking-ring valve, the ring rotates with the screw during plastication and moves This check valve has been in use about 2000 hr. Note the scoring and scratches both circumferentially due to rotation and horizontally. Inset shows the dramatic difference between and old and new check valve. The locking-ring style appears to dominate the market. My question is, why? 30 OCTOBER 2018 Plastics Technology PTonline.com K now How ABOUT THE AUTHOR: John Bozzelli is the founder of Injection Molding Solutions (Scientific Molding) in Midland, Mich., a provider of training and consulting services to injection molders, including LIMS, and other special- ties. Contact john@scientificmolding.com; scientificmolding.com.

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