Plastics Technology - Dedicated to improving Plastics Processing.
Issue link: http://pty.epubxp.com/i/105397
know-how injection molding john bozzelli Don't ignore nozzle temperature control Most molders consider controlling the temperature of the nozzle tip and body a trivial detail. however, Dr. Deming tried to teach us about the importance of details, and ignoring this one will cost you money. Let's start with some data, another foundation of Dr. Deming's teachings that too many molders tend to ignore. Table 1 shows a difference of over 150° F (65° C) between the setpoint and the actual measured temperature of the nozzle tip and body in a PETG molding operation. Reject rate was near 50%. Granted, this may not be typical, but in my experience it is not rare. And, yes, this was with a PID temperature controller, not just a Variac. Table 2 is more typical, in my experience. But both are unacceptable and need to be fxed for quality and consistent production. Why? One of the primary variables in molding is using the resin within its proper temperature range, and just as important is that the melt be uniform in temperature. With temperature variations you will get rejects. Possible types of rejects include the following: 1. Blush, 2. Gloss differential, 3. Degradation, 4. Halos, 5. Color variation, 6. Texture variation, 7. Inconsistent properties, 8. Marbling, 9. Cold slug in the nozzle tip, 10. Drooling or Stringing, 11. Burn marks at the gate, 12. Asymmetrical flling, 13. Shot-to-shot variations, 14. Swirls. Bottom line: This is a signifcant problem that costs molders tens of thousands of dollars a month, including hours of wasted time trying to adjust a perfectly good process. I do not have a complete fx, but the following will provide melt temperatures much closer to setpoint and signifcantly better consistency between machines. First, perhaps to convince yourself you have a problem, take a walk through the production foor and check where the thermocouple is located on the nozzle body and/or tip on a number 20 February 2013 Plastics technology FIG. 1. A spade or butterfy-type thermocouple, placed between the nozzle body and heater band, can help maintain the setpoint temperature of the nozzle tip and body. Photo: Marathon Heater, Inc. FIG. 2. A thermocouple situated on the heaterband clamp is unlikely to provide accurate temperature control of the nozzle tip and body. Typically, as shown here, there are gobs of plastic hanging from the wire, which is a safety hazard. Table 1 — Molding PETG Setpoint Actual 480 F 625 F before Fix 480 F 465 F after Fix of machines. You will notice a lack of consistency. Sometimes the controlling thermocouple is on the heater band, others have it in the hex of a nozzle body. If the nozzle body is longer than 2 in., the hex is not the place to put it. Still others will have the thermocouple on the clamp around the heater band. You can bet money that one is not giving the temperature controller the right information. In fact, all of the above have signifcant problems. Ideally, we would have a magical thermocouple that we'd put on the outside of the nozzle body and, through ultrasonic sound waves, microwaves, infrared, or some new technology, we would