Plastics Technology - Dedicated to improving Plastics Processing.
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Ever wonder how many shots are injection molded each day? An enormous number, no doubt, yet not all shots make acceptable parts. Shot size, melt uniformity, color mixing, splay, contamination, material changes, and black specks are only a few of the causes for rejects. These part problems cost companies huge sums of time and money. While there are a number of possible causes for these problems, they all share a critical component of a molding machine—the screw. In many ways, it's the forgotten piece of the puzzle when it comes to troubleshooting. Consider some of these demanding functions the reciprocating screw has to do on every cycle: 1. Compress the granules for the transition zone, 2. Drive air and volatiles out the feed throat, 3. Feed resin to the transition zone, 4. U niformly melt various plastics without discoloring or degradation, 5. A ccommodate resin and color changes with the minimum amount of time and wasted resin, High-Pressure Water: A New Concept for Screw Cleaning 6. Build a consistent shot size, 7. Develop consistent backpressure. Each is difcult to accomplish consistently. For this column, I'll explain why a clean, well-maintained screw will perform these func- tions more efciently and provide less hassle. Then I'll address a topic everybody hates: How to mechanically clean a reciprocating screw. Many molders use one or more of the commercially available purging compounds for screw cleaning. That's sensible: Compared with pulling a screw for cleaning, purging a screw is easier, faster, takes less time and is less expensive. That said, there comes a time when screw needs to be pulled and cleaned. It's unavoidable. Pulling out and cleaning a screw is a tough job that drains profts because the machine is not producing parts. It is difcult, hot, dirty, laborious, and time-consuming. Not many enjoy the day when the order comes to pull a screw for cleaning. Typically, these are the steps of disassembly (the order may be diferent depending on manufacturer or type of machine): 1. The barrel is he ated to the last resin's melting temperature; 2. The barrel is emptied of resin; a purging compound may be used to start the cleaning process; 3. The feed block is pulled of; 4. The nozzle plus heater band(s), thermocouples, and end cap are disassembled; 5. The drive mechanism is uncoupled or detached; 6. The screw is pushed/pulled out with everything from gloved hands to hoists to fork lifts and a push bar. Each one of these steps can be a nightmare in itself. The end-cap screws, bolts, wires, etc. can be encrusted with molten or degraded plastic. Bolts can be rusted, seized, or broken. Removing the nozzle tip and check ring is often a signifcant physical experience, especially if someone forgot to use anti-seize or used the wrong type. Each of these steps can be frustrating and time consuming. The nightmare can get worse in the process of cleaning any of these components. The screw can get damaged, for instance, or while cleaning it you may fnd damaged fights or other problems that may result in costly repairs and delays. And let's not forget about the By John Bozzelli Everyone in a molding plant is on the same page for screw cleaning: Avoid pulling it until there is no alternative. Here are ways for dealing with the inevitable, including an altogether new technique. Scratches and carbon buildup on screw fights are detrimental to screw function. 24 DECEMBER 2014 Plastics Technology PTonline.com K now How INJECTION MOLDING